CN107679314B

CN107679314B - Method and system for establishing computational fluid mechanics model for simulating cigarette combustion

Info

Publication number: CN107679314B
Application number: CN201710891571.2A
Authority: CN
Inventors: 李巧灵; 李跃锋; 刘泽春; 陈国钦; 陈河祥; 黄朝章; 张建平; 黄惠贞; 林艳; 谢卫
Original assignee: China Tobacco Fujian Industrial Co Ltd
Current assignee: China Tobacco Fujian Industrial Co Ltd
Priority date: 2017-09-27
Filing date: 2017-09-27
Publication date: 2021-03-12
Anticipated expiration: 2037-09-27
Also published as: CN107679314A

Abstract

The invention relates to a method and a system for establishing a computational fluid dynamics model for simulating a temperature field and/or a substance concentration field during cigarette combustion. The method comprises the following steps: a) establishing a geometric model for simulating a cigarette combustion scene; b) establishing an equation for simulating physical and/or chemical reactions in a cigarette combustion scene, wherein the equation comprises the following steps: establishing a tobacco shred pyrolysis reaction kinetic equation; establishing a combustion reaction kinetic equation of tobacco shred pyrolysis products corresponding to different oxygen concentrations; and c) loading the geometric model and the equation established in the steps by adopting solving software of computational fluid dynamics software, and establishing a computational fluid dynamics model for simulating a temperature field and/or a substance concentration field during cigarette combustion. The accuracy of the model established by the method is high.

Description

Method and system for establishing computational fluid mechanics model for simulating cigarette combustion

Technical Field

The invention belongs to the field of cigarettes, and particularly relates to a method and a system for establishing a computational fluid dynamics model for simulating cigarette combustion.

Background

In the article for research progress of cigarette combustion models (Chinese tobacco journal, 2013(2):115-122.), prucalor et al reviewed the research progress of cigarette combustion models. This article describes the negative and flame retardant models reported in the current literature. For smoldering processes, the flow rate of the gas is determined, so the reaction of the process is in the control range of stoichiometry and dynamics, and therefore, the smoldering model of the cigarette is relatively much researched.

In the numerical simulation article (tobacco science and technology, 2014(6)) of the cigarette smoldering process, prucalor et al integrates cigarette smoldering models reported in the literature, and a relatively perfect cigarette smoldering model is established by utilizing Fluent software, and the model can simulate temperature field distribution at different moments in the cigarette burning process and concentration field distribution of oxygen, carbon monoxide, carbon dioxide and water vapor in cigarette smoke, and compare smoldering linear burning speed and the simulation value of the highest temperature inside the cigarette with experimental values.

Disclosure of Invention

The inventor finds that in the cigarette combustion process, the oxygen concentration in different areas in the cigarette is different, and different combustion reactions of tobacco pyrolysis products occur due to different oxygen concentrations. The inventor further finds that a combustion reaction kinetic equation of the tobacco pyrolysis product corresponding to different oxygen concentrations is established, and a computational fluid mechanics model for simulating a temperature field and a substance concentration field during cigarette combustion more accurately can be obtained.

The invention provides a method for establishing a computational fluid dynamics model for simulating a temperature field and/or a substance concentration field during cigarette combustion, which comprises the following steps:

a) establishing a geometric model for simulating a cigarette combustion scene;

b) establishing an equation for simulating physical and/or chemical reactions in a cigarette combustion scene, wherein the equation comprises the following steps:

establishing a tobacco shred pyrolysis reaction kinetic equation; and

establishing a combustion reaction kinetic equation of tobacco shred pyrolysis products corresponding to different oxygen concentrations; and

c) and (3) loading the geometric models and the equations established above by using solving software (such as Fluent, CFX, Phoenics, Flow3d and Flowmaster) of computational fluid dynamics software to establish a computational fluid dynamics model for simulating the temperature field and/or the substance concentration field during the cigarette combustion.

By establishing a combustion reaction kinetic equation of the tobacco shred pyrolysis products corresponding to different oxygen concentrations, the combustion state of the tobacco shred pyrolysis products can be simulated more accurately, and a more accurate model is obtained.

In a further aspect, the present invention provides a system for creating a computational fluid dynamics model for simulating a temperature field and/or a material concentration field during combustion of a cigarette, comprising:

the geometric model building module (such as Gambit software) is used for building a geometric model for simulating a tobacco combustion scene;

the cigarette combustion scene physical and/or chemical reaction equation establishment module comprises

A tobacco shred pyrolysis reaction kinetic equation establishing module;

the tobacco shred pyrolysis product combustion reaction kinetic equation establishing module is used for establishing tobacco shred pyrolysis product combustion reaction kinetic equations corresponding to different oxygen concentrations;

and the computational fluid dynamics model establishing module is used for establishing a computational fluid dynamics model for simulating a temperature field and/or a substance concentration field during cigarette combustion by adopting a geometric model established by the computational fluid dynamics software geometric model establishing module and an equation established by the cigarette combustion scene physical and/or chemical reaction equation establishing module.

In one embodiment, step b) includes establishing a first tobacco shred pyrolysis product combustion reaction kinetic equation corresponding to a first oxygen concentration; and establishing a second tobacco shred pyrolysis product combustion reaction kinetic equation corresponding to the second oxygen concentration, and optionally establishing a third tobacco shred pyrolysis product combustion reaction kinetic equation corresponding to the third oxygen concentration.

In one embodiment, the oxygen concentration is distributed in the range of 0 to 20 vol%.

In one embodiment, the oxygen concentration comprises 1%, 2%, 3%, 5%, 10%, 15%, 20%.

In one embodiment, the first oxygen concentration is 0 to 2.5 vol%, the second oxygen concentration is 2.5 to 7.5 vol%, and the third oxygen concentration is 7.5 vol% or more.

In one embodiment, in step b), the method for establishing the combustion reaction kinetic equation of the tobacco pyrolysis product comprises the following steps:

heating the tobacco shred pyrolysis product under the oxygen-containing atmosphere with s oxygen concentrations, detecting the mass change of the tobacco shred pyrolysis product by adopting a thermogravimetric analysis method, and respectively measuring differential thermogravimetric curves of the s tobacco shred pyrolysis products when being heated, wherein s is a positive integer (preferably, s is more than or equal to 2);

fitting the s differential thermogravimetric curves to the kinetic equations of the following reaction k, respectively:

k is a positive integer;

α_c,kin order to obtain the conversion of reaction k,

conversion a for reaction k_c,kDerivative with respect to time T, T_cFor heating temperature, R is the ideal gas constant, E_c,kTo react the activation energy of k, n_c,kNumber of reaction stages for reaction k,. rho₂Is the oxygen density;

a_c,kand T is obtained from the differential thermogravimetric curve, A_c、E_c,kAnd n_c,kObtained by fitting.

In one embodiment, the concentration of s oxygen gases is in the range of 1 to 25 vol%;

in one embodiment, two adjacent oxygen concentrations differ by at least 2 vol%;

in one embodiment, s ≧ k ≧ 2, s and k are positive integers;

in one embodiment, k is 2, 3, 4, 5, 6, 7 or 8;

in one embodiment, s is 2, 3, 4, 5, 6, 7 or 8;

in one embodiment, the s oxygen concentrations include the following 7 oxygen concentrations: 0.8-1.2 vol%, 1.5-2.5 vol%, 2.8-3.5 vol%, 4-6 vol%, 8-12 vol%, 16-18 vol%, 19-22 vol%;

preferably, the oxygen-containing atmosphere is a mixed atmosphere of oxygen and nitrogen;

preferably, the tobacco shred pyrolysis product refers to a product obtained after tobacco shreds are heated and decomposed in a non-oxidizing atmosphere;

preferably, the tobacco shred pyrolysis product is a product obtained by thermally decomposing the tobacco shred at 800-1000K.

In one embodiment, step b) further comprises establishing an equation for the permeability of the wrapper as a function of temperature comprising:

setting a first temperature for the cigarette paper to have a first permeability;

setting the cigarette paper at the second temperature to have a second permeability;

setting a third temperature for the cigarette paper to have a third permeability;

preferably, the first and second electrodes are formed of a metal,

the first temperature is below a K, and the first permeability is 0.5 × 10^-15～5×10^-15m²；

The second temperature is a-b K, and the second permeability is 1 × 10^-9～6×10^-9m²(e.g., 1.5X 10)^-9～4.5×10^-9m²)；

The third temperature is above b K, and the third permeability is 0.5 × 10⁵～5×10⁵m²；

a＝450～500K，b＝600～650K；

Preferably, the permeability of the wrapper is set to vary unidirectionally, increasing only without decreasing.

In one embodiment, step b) further comprises establishing cigarette paper combustion reaction kinetics equations comprising:

heating the cigarette paper sample in an oxygen-containing atmosphere, detecting the mass change of the cigarette paper sample by adopting a thermogravimetric analysis method, and measuring a differential thermogravimetric curve when the cigarette paper is heated;

-performing a peak fitting on said differential thermogravimetric curve, dividing the curve into m unimodal curves;

fitting the m unimodal curves to the kinetic equations of m reactions j:

the kinetic equation for each reaction j is as follows:

j is 1 to m, and j and m are positive integers;

a_z,jthe conversion in reaction j,

The derivative of the conversion of reaction j with time, T_zFor heating the temperature of the cigarette paper, R is the ideal gas constant, A_z,jTo reflect the pre-exponential factor of j, E_z,jTo react the activation energy of j, n_z,jThe reaction order of reaction j;

a_z,j、

T_zobtained from the differential thermogravimetric curve of the cigarette paper when heated, A_z,j、E_z,jAnd n_z,jObtained by fitting.

In one embodiment, the method for establishing the tobacco shred pyrolysis reaction kinetic equation comprises the following steps:

-heating the cut tobacco sample in a non-oxidizing gas atmosphere (e.g. nitrogen atmosphere), detecting the mass change of the cut tobacco sample by thermogravimetric analysis, and drawing a differential thermogravimetric curve of the cut tobacco sample;

-performing a peak fitting on said differential thermogravimetric curve, dividing the curve into n unimodal curves;

fitting n unimodal curves to the kinetic equations of n reactions i:

i is 1, 2 … n, n and i are integers;

a_v,ithe conversion rate for reaction i,

Derivative of the conversion rate over time, T, for reaction i_vThe temperature for heating the tobacco shred sample, beta is the heating rate for heating the tobacco shred sample, R is an ideal gas constant, A_v,iIn response to a pre-exponential factor of i, m_v,iCorrection of the parameters for the rate of rise of reaction i, E_v,iTo react the activation energy of i, n_v,iThe reaction grade number of the reaction i;

T_vbeta is obtained from the differential thermogravimetric curve of a tobacco sample, A_v,i、m_v,i、E_v,iAnd n_v,iObtained by fitting.

In one embodiment, the method of creating a computational fluid dynamics model further comprises creating an equation relating the amount of harmful components produced to temperature for different oxygen concentrations,

preferably, the method for establishing the relation equation of the release amount of the harmful components corresponding to different oxygen concentrations and the temperature comprises the following steps:

heating tobacco shred samples under the atmosphere with different oxygen concentrations;

detecting the relationship between the release amount of harmful components and the heating temperature when the tobacco shred sample is heated;

fitting a relation equation of harmful component release amount and temperature corresponding to different oxygen concentrations according to the detection result of the previous step;

preferably, the harmful ingredient is CO or tar.

In one embodiment, the method of creating a computational fluid dynamics model further comprises the step of creating an equation for the filter rod retention rate of tar:

preferably, the retention rate equation of the filter stick to the tar comprises:

e_s＝E_IN+E_IM+E_D+E_ID；

wherein E is_IN、E_IM、E_D、E_IDIs an intermediate parameter;

J＝(29.6-28ω^0.62)R_L ^2.8；

e is the retention rate of the filter stick to tar, e_sRetention rate of tar by single fiber tow of filter stick, L_fIs the length of the filter stick, d_fIs the diameter of the fiber tow of the filter rod, omega is the volume fraction of the fiber tow in the filter rod, D_tIs the total denier of the filter stick, D_sIs the single denier of the filter stick, C_fiberIs the ratio of crimped fibers in the filter rod, S_filterIs the cross-sectional area of the filter rod, p_gIs the gas density, μ_gViscosity of gas, v_iIs the gas velocity in the i direction, d_cIs the diameter of the aerosol particles of cigarette smoke, d_fIs the diameter of the filter rod fiber tow, D_kIs the diffusion coefficient of the particles, K_BIs the Botzmann constant, T_filterIs the temperature of the filter rod.

In one embodiment, the method of creating a computational fluid dynamics model, step d) further comprises the step of loading a cigarette smoking program with solving software for the computational fluid dynamics software;

preferably, the cigarette smoking program is set as follows: setting the position of the cigarette area to be sucked as a speed inlet and other positions as a pressure outlet, and performing a suction period every 60s, wherein the suction period of the cigarette suction gas flow rate comprises the following a) and b):

a) smoldering 58s, v ═ 0;

b) the suction is carried out for 2s,

units of vIn ml/s.

In one embodiment, a method of creating a geometric model of cigarette combustion comprises:

establishing a geometric model of a cigarette combustion scene by adopting preprocessing software (such as Gambit and ICEM CFD) of computational fluid dynamics software, wherein the geometric model comprises a gas area and a cigarette area, the cigarette area is positioned in the gas area, and the cigarette area comprises a tobacco shred area and a cigarette paper area;

preferably, the cigarette area further comprises a filter rod area;

preferably, the geometric model of the cigarette combustion scene is a two-dimensional model.

In one embodiment, the method of creating a computational fluid dynamics model further comprises the step of loading the following parameters with solving software of the computational fluid dynamics software:

-physical or chemical parameters of the cut tobacco;

-physical or chemical parameters of the cigarette paper;

-a physical or chemical parameter of the gas;

-a mass transfer equation;

-a momentum transfer equation;

-an energy transfer equation.

In one embodiment, the species concentration field comprises one or more of:

an oxygen concentration field;

a tobacco shred pyrolysis product concentration field;

a cigarette paper combustion product concentration field;

field of concentration of harmful components.

In one embodiment, the temperature field refers to the temperature field of the tobacco shred area.

the geometric model establishing module is used for establishing a geometric model for simulating a tobacco combustion scene;

A tobacco shred pyrolysis reaction kinetic equation establishing module;

In one embodiment, the system for establishing the computational fluid dynamics model includes a tobacco pyrolysis product combustion reaction kinetic equation establishing module for establishing a first tobacco pyrolysis product combustion reaction kinetic equation corresponding to a first oxygen concentration.

In one embodiment, the system for establishing the computational fluid dynamics model includes a tobacco pyrolysis product combustion reaction kinetic equation establishing module for establishing a second tobacco pyrolysis product combustion reaction kinetic equation corresponding to a second oxygen concentration.

In one embodiment, the system for establishing the computational fluid dynamics model includes a tobacco pyrolysis product combustion reaction kinetic equation establishing module configured to establish a third tobacco pyrolysis product combustion reaction kinetic equation corresponding to a third oxygen concentration.

In one embodiment, the cigarette paper combustion reaction thermogravimetric analysis module is configured to:

heating the tobacco shred pyrolysis product under the oxygen-containing atmosphere with s oxygen concentrations, detecting the mass change of the tobacco shred pyrolysis product by adopting a thermogravimetric analysis method, and respectively measuring differential thermogravimetric curves of the s tobacco shred pyrolysis products when being heated, wherein s is a positive integer (preferably k is more than or equal to 2);

in one embodiment, the tobacco pyrolysis product combustion reaction kinetic equation establishing module is configured to:

the s differential thermogravimetric curves were fitted to the kinetic equation of the following reaction k, respectively:

k is a positive integer;

α_c,kin order to obtain the conversion of reaction k,

In one embodiment, s ≧ k ≧ 2, s and k are positive integers;

in one embodiment, k is 2, 3, 4, 5, 6, 7 or 8;

in one embodiment, s is 2, 3, 4, 5, 6, 7 or 8;

In one embodiment, the system for establishing the computational fluid dynamics model further comprises a tobacco shred pyrolysis reaction thermogravimetric analysis module for performing thermogravimetric analysis on the tobacco shred in an oxygen-free atmosphere and outputting data to the tobacco shred pyrolysis reaction kinetic equation establishing module.

In one embodiment, the system for establishing the computational fluid dynamics model further comprises a tobacco shred pyrolysis product combustion reaction thermogravimetric analysis module, which is used for performing thermogravimetric analysis on the tobacco shred pyrolysis product in an oxygen-containing atmosphere and adding data to output to the tobacco shred pyrolysis product combustion reaction kinetic equation establishing module.

In one embodiment, the system for establishing the computational fluid dynamics model further comprises a tobacco shred pyrolysis product combustion reaction thermogravimetric analysis module, which is used for performing thermogravimetric analysis on the tobacco shred pyrolysis product in an oxygen-containing atmosphere with a first oxygen concentration, and adding data to output to the tobacco shred pyrolysis product combustion reaction kinetic equation establishing module.

In one embodiment, the system for establishing the computational fluid dynamics model further includes a tobacco shred pyrolysis product combustion reaction thermogravimetric analysis module, which is configured to perform thermogravimetric analysis on the tobacco shred pyrolysis product in an oxygen-containing atmosphere with a second oxygen concentration, and add data to output to the tobacco shred pyrolysis product combustion reaction kinetic equation establishing module.

In one embodiment, the system for establishing the computational fluid dynamics model further comprises a tobacco shred pyrolysis product combustion reaction thermogravimetric analysis module, which is used for performing thermogravimetric analysis on the tobacco shred pyrolysis product in a third oxygen-containing atmosphere, and adding data to output to the tobacco shred pyrolysis product combustion reaction kinetic equation establishing module.

In one embodiment, the system for establishing the computational fluid dynamics model, the cigarette combustion scenario physical and/or chemical reaction equation establishing module further comprises a cigarette paper combustion reaction kinetic equation establishing module.

In one embodiment, the system for establishing the computational fluid dynamics model further comprises a cigarette paper combustion reaction thermogravimetric analysis module for performing a thermogravimetric analysis of the cigarette paper combustion reaction in an oxygen-containing atmosphere and outputting data to the cigarette paper combustion reaction kinetics equation establishment module.

In one embodiment, the system for establishing the computational fluid dynamics model further comprises an equation establishing module for cigarette paper permeability as a function of temperature.

In one embodiment, in the system for establishing the computational fluid dynamics model, the cigarette combustion scene physical and/or chemical reaction equation establishing module further includes a tobacco harmful component release amount equation establishing module, which is used for establishing a relationship equation between tobacco harmful component release amounts corresponding to different oxygen concentrations and temperature.

In one embodiment, the system for establishing the computational fluid dynamics model further comprises a tobacco harmful component emission amount detection module for:

and outputting the detection result to a tobacco shred harmful component release amount equation establishing module.

In one embodiment, the system for establishing the computational fluid dynamics model, the cigarette combustion scene physical and/or chemical reaction equation establishing module further comprises a filter stick to tar retention rate equation establishing module;

the filter stick tar retention rate equation establishing module comprises the following steps of establishing a filter stick tar retention rate equation according to the following equation:

e_s＝E_IN+E_IM+E_D+E_ID；

wherein E is_IN、E_IM、E_D、E_IDIs an intermediate parameter;

J＝(29.6-28ω^0.62)R_L ^2.8；

Interpretation of terms

The Fluent software is, for example, Computational Fluid Dynamics (CFD) software of ANSYS corporation.

Unless otherwise specified, the temperature is in K.

Unless otherwise specified,% is vol%.

The beneficial technical effects are as follows:

1. performing thermogravimetric analysis experiment of tobacco shred pyrolysis by adopting a temperature rise rate of 300-800K/min;

2. parameters are introduced into a tobacco shred pyrolysis reaction kinetic equation:

wherein beta is the rate of temperature rise, m_v,iCorrecting parameters for the rate of temperature rise;

3. performing thermogravimetric analysis experiments on combustion of tobacco shred pyrolysis products under different oxygen concentrations, and fitting according to obtained experimental data to obtain a combustion reaction kinetic equation of the tobacco shred pyrolysis products;

4. establishing the pyrolysis combustion dynamics of the cigarette paper, and introducing permeability parameters of the cigarette paper at different temperatures;

5. according to the fact that the release amount of CO and tar is only related to the temperature and the oxygen content when the cut tobacco is heated, the release amount of the CO and the tar is established;

6. and a tar entrapment model of the filter stick is established, and the tar entrapment rate of the filter stick is predicted.

Drawings

The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the invention without limiting the invention. In the drawings:

FIG. 1 is a division of four regions in a cigarette combustion model;

FIG. 2 is a DTG curve (heating rate 300 K.min.) of a cut tobacco powder sample of trade mark 1^-1)；

FIG. 3 shows the cut tobacco of grade 1 at 300Kmin^-1～800K·min^-1Comparing the experimental value and the fitting value of the pyrolysis DTG curve at the temperature rise rate;

FIG. 4 is a graph comparing the DTG curve experimental value and the calculated value of combustion of tobacco pyrolysis products under different oxygen concentrations;

FIG. 5 is a graph comparing the experimental value and the calculated value of a DTG curve of the pyrolysis combustion of the cigarette paper;

FIG. 6 is a schematic view of a tobacco shred pyrolysis combustion platform;

FIG. 7 is a graph showing the variation of CO release amount of tobacco shreds with temperature under different oxygen concentrations;

FIG. 8 is a curve of tar release amount of tobacco shreds with temperature under different oxygen concentrations

FIG. 9 is a photograph of a filter for a tar release experiment of cut tobacco under different oxygen concentrations;

FIG. 10 is a graph of tobacco pyrolysis product concentration (top) and cigarette paper burn line (bottom) fitted to a cigarette combustion model for brand 1 cigarettes at 60s and 120 s;

FIG. 11 is a graph of tobacco pyrolysis product concentration (top) and cigarette paper burn line (bottom) fitted to a cigarette combustion model for brand 1 cigarettes at 180s and 240 s;

FIG. 12 is a graph of tobacco pyrolysis product concentration (top) and cigarette paper burn line (bottom) fitted to a cigarette combustion model for 300s and 360s for a brand 1 cigarette;

FIG. 13 is a graph showing the variation of the tobacco pyrolysis product concentration and the variation of the combustion line of the cigarette paper for a cigarette of the brand No. 1 in a single puff;

FIG. 14 is the experimental and predicted values of the temperature profile at 22mm, 24mm, 26mm, 28mm, 30mm, 32mm, 34mm, 36mm for the brand 1 cigarette

FIG. 15 shows the tar release amount and tar retention rate at the inlet and outlet of the cigarette filter stick of No. 1 during one-time suction (180-182 s)

FIG. 16 is an air flow velocity field plot for a cigarette at 180s for a brand 1 cigarette;

FIG. 17 is a plot of the air flow velocity field for a cigarette with brand 1 cigarette at 181 s;

figure 18 is a plot of the air flow velocity field for a cigarette at 182s for a brand 1 cigarette.

Detailed Description

Materials and instruments

Cigarette tobacco (Fujian tobacco industry, Limited liability company, brand No. 1, brand No. 2, brand No. 3). A proper amount of cigarette tobacco shreds are taken and placed in a constant temperature and humidity box with the temperature of 22 +/-1 ℃ and the relative humidity of 60 +/-2% for 48 hours for standby.

STA 449F3TG-DTA/DSC synchronous thermal analyzer (Netzsch, Germany); TF-M100 cigarette burning and smoking gas phase temperature field analyzer (Beijing Zidong science and technology Co., Ltd.); a three-hole smoke sucking machine; XP205 electronic balance (induction 0.00001g, Mettler-Toledo, Switzerland); GM200 mill (Retsch, germany); KBF720 constant temperature and humidity cabinet (Binder, Germany). J2KN multifunctional smoke analyzer (Germany rbr company)

Establishing cigarette combustion (smoking) model based on Fluent

1. Establishing two-dimensional geometric body model in Fluent software

1.1 partitioning grid regions

Four computational domains are divided in the two-dimensional model, as shown in FIG. 1: a tobacco shred area 101; a cigarette paper region 102; a filter rod region 104; a gas region 103. The cigarette area comprises a tobacco shred area, a cigarette paper area and a filter stick area.

Shredded tobacco region 101 is rectangular. The cigarette paper area 102 comprises two rectangles, the cigarette paper area 102 being the same length as the tobacco shred area. The cigarette paper area 102 is located outside the tobacco shred area 101 and one long side of the cigarette paper area 102 coincides with the long side of the tobacco shred area 102. The filter stick region 104 is rectangular, and the length of the short side of the filter stick region 104 is the same as the length of the short side of the tobacco shred region 101. The filter plug region 104 is located outside the tobacco shred region 101, and one short side of the filter plug region 104 coincides with the short side of the tobacco shred region 101.

Wherein

regions

101, 102, 103 are porous media and region 104 is a gas flow field. The initial composition of the gas zone was 23 vol% oxygen, with the balance being nitrogen. Performing simulation operation on the pyrolysis and combustion processes of the tobacco shreds in the tobacco shred area of the cigarette; performing simulation operation on the combustion process of the cigarette paper and the permeability change of the cigarette paper in the cigarette paper area; and performing simulation operation on the retention rate of the filter stick to the tar in the filter tip area.

The tobacco shred area in the cigarette combustion model is 0.058m (x axis) long and 0.008m (y axis) wide. The filter area was 0.028m long (x-axis) and 0.008m wide (y-axis). The length of the cigarette paper area is 0.058m, the width is 0.05mm, and the length of the air area is 0.115m, and the width is 0.04 m.

Boundary conditions: the suction position is a speed inlet, and the speed value is controlled by a suction curve equation; the remaining boundaries are pressure inlets, and the gauge pressure is 0.

1.2 setting of physicochemical parameters of the regions

The materials of each region are set according to the physicochemical properties of each cigarette, and are shown in Table 1.

TABLE 1

The gas composition of the gas region was set to 23% oxygen, with the balance being nitrogen. The density, specific heat capacity and heat conductivity of the gas are assumed to be kept unchanged in the smoking and burning process of the cigarette.

The initial temperature is set to 300K, and the initial mass fractions of carbon monoxide, carbon dioxide and water vapor in the flue gas are all set to 0.

The equations in the following steps 2-9 are all programmed by using UDF self-definition (UDF is a user-defined function which is a user-programmed program and can be set individually for Fluent. the Fluent solver can dynamically load the UDF. the above equations are solved by Fluent 6.3.26, a separation solver is adopted, the pressure-speed coupling algorithm adopts the SIMPLE format, the equations are linearized and solved by using the implicit format, and the discretization adopts the first-order windward format to calculate the flux.

2. Establishing tobacco shred pyrolysis and combustion equation

The combustion process of the tobacco shreds in the tobacco shred area is assumed to be divided into two stages, namely a tobacco shred pyrolysis stage and a tobacco shred pyrolysis product combustion stage. The thermogravimetric analysis experiment is carried out on the tobacco shred (cigarette in Fujian, brand 1), and a tobacco shred pyrolysis equation and a tobacco shred pyrolysis product combustion equation are established according to experimental data.

2.1 establishing the equation for the pyrolysis of tobacco

2.1.1 tobacco shred pyrolysis thermogravimetric analysis

Taking a proper amount of tobacco shreds, and placing in a constant temperature and humidity box with temperature (22 + -1) deg.C and relative humidity (60 + -2)% for 48 h. Grinding the cut tobacco into powder, sieving and selecting the tobacco powder with the grain size of 198-165 mu m (80-100 meshes) as a cut tobacco sample.

Weighing 9.50mg of the tobacco shred sample, paving the tobacco shred sample in a tray of a synchronous thermal analyzer, heating the thermal analyzer to 873K from room temperature, and then cooling to room temperature to obtain a tobacco shred pyrolysis product. The experimental carrier gas was nitrogen (N)₂) The flow rate of the carrier gas is 50 mL/min^-1. Respectively using different heating rates (300 K.min)^-1、400K·min^-1、 500K·min^-1、600K·min^-1、700K·min^-1And 800 K.min^-1) Multiple thermal analysis experiments were performed.

2.1.2 tobacco shred pyrolysis equation fitting

Recording physical quantities such as time (min), temperature (K), mass (mg) and the like through thermogravimetric experiments to obtain the derivative of the sample mass loss rate m (namely the mass percentage of the lost mass in the initial mass) to the temperature T

Curve of variation with temperature T (K), i.e. differential weight loss curve (differential thermogravimetric curve)。

FIG. 2 is a differential thermogravimetric curve (temperature rise rate 300 K.min) of a tobacco shred powder sample when the tobacco shred powder sample is heated^-1). The differential thermogravimetric curve was divided into five valleys (R1, R2, R3, R4 and R5) using Origin software. Suppose that five valleys correspond to five main components i and their respective independent parallel pyrolysis reactions i (i ═ 1 to 5). The first valley (R1) represents, for example, evaporation of moisture; the second to fifth valleys, for example, represent pyrolysis of different pyrolysis precursors, respectively. The second valley (R2) represents, for example, the pyrolysis of sugars, nicotine, pectin and some other volatile species. The third valley (R3) and the fourth valley (R4) represent, for example, the pyrolysis of hemicellulose and cellulose. The fifth valley (R5) represents, for example, pyrolysis with lignin as the main component.

The peak areas of the five valleys R1, R2, R3, R4, and R5 of the differential thermogravimetric curve of fig. 2 were measured as a percentage of 9.52%, 17.71%, 18.04%, 13.58%, and 41.16%, respectively. Based on the ratio, the temperature of the mixture is raised to other temperature raising rates beta (400 K.min)^-1、500K·min^-1、600K·min^-1、700K·min^-1And 800 K.min^-1) The obtained pyrolysis differential thermogravimetry curve is subjected to peak separation.

Assuming that five valleys correspond to five independent parallel reactions i (i-1 to 5) of five types of components i (i ═ 1 to 5), respectively, of tobacco shred samples

Can be represented by the following formula:

in the equation (2.1) above,

the derivative of the percentage mass loss of each type of component i on the tobacco shred temperature T is expressed.

Conversion a of the pyrolysis reaction of each type of component i_i(%) is represented by the following formula (2.2):

in equation (2.2), m_i0And m_i∞Refers to the mass of each type of component i at the beginning and end of the pyrolysis reaction. T is₀And T_i∞Refers to the temperature of the tobacco shreds at the beginning and end of the pyrolysis reaction for each type of component i.

The formulation of equation (2.2) yields:

for reaction i, its kinetic equation f (a)_v,i) Can be defined as:

in the formula (2.4), α_v,iIs the conversion of reaction i, A_v,i(min^-1) Is a pre-exponential factor of reaction i, beta (K/min) is the rate of temperature rise of the heated tobacco shreds, m_v,iIs a temperature rise rate correction parameter of reaction i, E_v,i(J/mol) is the activation energy of component i, R (J/mol/K) is the ideal gas constant, T_v(K) Is the temperature at which the tobacco is heated.

Suppose f (α)_v,i)＝(1-α_v,i)^n,iThe fitting equations of the pyrolysis reactions of the 5 tobacco shreds are respectively shown as the following formula (2.5):

in the case of the formula (2.5),

a_v,i、T_vcan be obtained from differential thermogravimetric curve by Origin softwareThe global fitting method in the linear fitting function fits the data of 5 valleys simultaneously to find the best solution of the parameters. During the fitting process, Origin adopts a chi-square minimization method to determine the optimal kinetic parameters: activation energy of pyrolysis reaction E_v,iThe tobacco shred pyrolysis reaction pre-exponential factor A_v,iThe number of reaction stages n_v,iAnd a temperature rise rate correction parameter m_v,iI.e. to minimize the deviation of the theoretical curve from the experimental curve.

The resulting pyrolysis reaction kinetic parameters are shown in table 2. In table f_v,iRepresenting the content of each component, and is obtained by the peak area ratio of Gaussian partial peaks.

TABLE 2

FIG. 3 shows the red wolf tobacco shred at 300Kmin^-1～800K·min^-1Graph comparing the experimental curve and the fitted curve of pyrolytic DTG at the temperature increase rate. As shown in fig. 3, at 300Kmin^-1、400Kmin^-1、500Kmin^-1、 600Kmin^-1、700Kmin^-1、800Kmin^-1The goodness of fit of the experimental curve and the fitting curve is high.

2.2 establishing the Combustion equation of the tobacco pyrolysis products

2.2.1 thermogravimetric experiments on the combustion of tobacco pyrolysis products

After the tobacco shreds are pyrolyzed, the pyrolysis products of the tobacco shreds are subjected to oxidation reaction in oxygen, namely oxidation/combustion of the pyrolysis products of the tobacco shreds.

To establish the combustion equation of the pyrolysis products of tobacco shreds, the following experiment was performed. Taking a proper amount of red wolf tobacco shreds, and placing in a constant temperature and humidity box with the temperature of 22 +/-1 ℃ and the relative humidity of 60 +/-2% for 48 h. Grinding the cut tobacco into powder, sieving and selecting the tobacco powder with the particle size of 198-165 mu m (80-100 meshes) as an experimental sample. Accurately weighing 9.50mg sample, spreading in a tray of a synchronous thermal analyzer, heating the temperature of the thermal analyzer from room temperature to 873K, and heating at a rate of 10K min^-1The experimental carrier gas is nitrogen, and the flow rate is 50 mL/min^-1At the moment, the tobacco pyrolysis product is obtained, when the temperature is reduced to the normal temperature, the concentration (1%, 2%, 3%, 5%, 10%, 15%, 20% O) of oxygen in the carrier gas is changed according to the requirement₂The balance being N₂) Flow rate 50 mL/min^-1The temperature of the thermal analyzer is increased to 873K from room temperature, and the temperature rising rate is 10 K.min^-1And obtaining the quality loss of the tobacco shred pyrolysis product under different oxygen concentrations.

Fig. 4 shows experimental and fitted value curves of combustion DTG of pyrolysis products of cut tobacco of No. 1 cigarette at different oxygen concentrations. 1%, 2%, 3%, 5%, 10%, 15%, 20% O₂The experimental values and the fitted value curve are very close under the oxygen concentration.

2.2.2 tobacco shred pyrolysis product combustion equation fitting

And fitting the combustion differential thermogravimetric curve of the pyrolysis product into an equation by referring to the tobacco shred pyrolysis equation fitting process.

The oxidation reaction of the tobacco shred pyrolysis product is assumed to be a grade 1 reaction. Describing the combustion behavior of the tobacco shred pyrolysis product by using an independent parallel reaction model, wherein the combustion equation of the tobacco shred pyrolysis product is shown as the formula (2.6):

in the formula (2.6), the variable ρ o₂(kg·m^-3) Represents O₂Density (0.28 kg. m)^-3) T is the heating temperature, a_cIn order to achieve the conversion rate of the combustion reaction,

variable T, a being the derivative of combustion reaction conversion with time t_c、

Can be obtained by DTG experiments, E_cFor oxidation activation energy, A_cIndicating a pre-factor for the oxidation reaction, n_cAre correlation coefficients corresponding to different oxygen concentrations.

For equation 2.6, the global fitting method in the Origin software nonlinear fitting function is used, and in the fitting process, Origin adopts the chi-square minimization method to determine the optimal kinetic parameters: oxidation activation energy E_cThe oxidation reaction is referred to as pro-factor A_cCorrelation coefficient n corresponding to different oxygen concentrations_c,iI.e. minimizing the deviation of the theoretical curve from the experimental points, the results are shown in table 3.

TABLE 3

Fig. 4 shows an experimental value curve of combustion DTG of the pyrolysis product of cut tobacco of cigarette No. 1 at an oxygen concentration of 1% to 20%.

When writing UDF parameters, A_c、E_cAnd n_cThe value of (A) is changed along with the change of the oxygen concentration, and when the oxygen concentration is 0-2.5 percent, A is_c＝1.48E+07min^-1，E_c＝91.04KJ/mol，n_c1.09; when the oxygen concentration is 2.5-7.5%, A_c＝4.26E+07min^-1，E_c＝111.20KJ/mol，n_c0.957; when the oxygen concentration is more than 7.5 percent, A_c＝8.30E+07min^-1，E_c＝116.31KJ/mol，n_c＝0.36。

Establishing a cigarette paper pyrolysis combustion equation:

the cigarette paper is peeled from the cigarette with the brand number 1, 20.0mg of a sample is accurately weighed and placed in a tray of a synchronous thermal analyzer, the temperature is raised to 773K from the room temperature, and the temperature raising rate is 10 K.min < -1 >. The experimental carrier gas was air (23 v% oxygen, balance nitrogen) at a flow rate of 50mL min^-1。

A change curve of the sample mass loss rate (wt%) along with the temperature (K), namely a weight loss curve TG, is obtained by recording physical quantities such as time (min), temperature (K), mass (mg) and the like by using a synchronous thermal analyzer. The TG curve is derived to obtain the weight loss rate (%. min)^-1) And a differential weight loss curve DTG. Figure 5 is a TG and differential thermogravimetric plot of a sample of cigarette paper pyrolyzed in an air atmosphere. The differential thermogravimetric curve was divided into 3 valleys (R1, R2, R3) with Origin software, representing 3 independent parallel pyrolysis reactions.

2.3 cigarette paper pyrolysis combustion equation fitting

Referring to the tobacco shred pyrolysis equation fitting process, 3 independent parallel reaction models are used for describing the pyrolysis combustion behavior of each component of the cigarette paper, and the kinetic equation of the pyrolysis combustion of each component of the cigarette paper can be defined as:

equation (3.1) variable a_z,j、T_z、

Can be obtained from the differential thermogravimetric curve of the pyrolysis combustion of the cigarette paper A_z,j(min^-1) Is a pre-factor of the pyrolysis combustion reaction of the cigarette paper E_z,j(kJ/mol) is activation energy of pyrolysis combustion reaction of the cigarette paper, R (J/mol/K) is ideal gas constant, n_z,jIs the reaction grade of the cigarette paper pyrolysis combustion reaction.

The data of 3 valleys were fitted simultaneously using the global fitting method in the Origin software nonlinear fitting function to find the optimal solution for the parameters. During the fitting process, Origin adopts a chi-square minimization method to determine the optimal kinetic parameters: activation energy E of pyrolysis combustion reaction of cigarette paper_z,iThe cigarette paper pyrolytic combustion reaction indicates a pre-factor A_z,iAnd the number of reaction stages n_z,iI.e. minimizing the deviation of the theoretical curve from the experimental points, the kinetic parameters of each of the 3 reactions described above were determined by fitting and the results are given in table 4.

TABLE 4

Pyrolysis combustion of cigarette paper	R1	R2	R3
				f_z,j(％)	79.8	11.5	8.7
A_z,j(min^-1)	2.01E+08	1.031E+26	1.35E+45
				E_z,j(kJ/mol)	98.19	347.67	614.06
n_z,j	1.42	1.52	1.14
				R²	0.997	0.999	0.990

3. Equation of mass transfer

3.1 diffusion coefficient of oxygen in gas phase

The diffusion coefficient D of oxygen in the gas phase is related to the gas phase temperature:

D＝D₀(T_g/273)^1.75 (4.1)

D₀is O₂Reference value of diffusion coefficient in porous medium at 273K at 1 atmosphere, T_gIs the gas phase temperature (calculated from the energy equation).

D₀Value of (D) and tobacco shred porosity

In relation to the porosity of the tobacco

Wherein D is_gIs the unlimited diffusion coefficient of oxygen. Diffusion coefficient of oxygen in nitrogen atmosphere

D_g＝2×10^-5m²s^-1。

3.2 oxygen Mass transfer equation Source terms

Assuming that the oxidation reaction occurs at the surface of the solid phase, oxygen and gaseous products enter the gas phase. Neglecting the boundary layer resistance of the gas from the solid phase into the gas phase. Source term of oxygen

(kg·m^‐3·s^‐1) Comprises the following steps:

in the formula (4.3), the metal oxide,

is the stoichiometric coefficient of oxygen in the oxidation reaction of 1.65, and other parameters includeSee table 3 for definitions and values.

3.3 solid phase Mass transfer equation Source terms

3.3.1 tobacco shred pyrolysis model

The tobacco shreds are assumed to be subjected to pyrolysis reactions respectively in five pyrolysis precursors (component i in 2.1.2 is 1-5), and the pyrolysis reaction equation can be expressed as follows:

where ρ is_v,iIs the density (kg/m) of the pyrolysis precursor component i³)，ρ_vIs the total density, rho, of the five pyrolysis precursor components_v,0The initial total density of the five pyrolysis precursor components, namely the initial density of the cut tobacco is 740 kg.m^-3； T_s(K) Is the solid phase temperature (calculated according to the energy equation below), A_v,i(min^-1) Is a leading factor of the pyrolytic reaction of the pyrolytic precursor i, E_v,i(J/mol) is the activation energy for the pyrolysis reaction of the pyrolysis precursor i, R (J/mol/K) is the ideal gas constant, f_v,i(%) is the mass fraction of pyrolysis component i, n_viThe number of reaction stages is shown in Table 2.

3.3.2 pyrolysis product combustion model:

according to the fact that the pyrolysis product is a pyrolysis product of pyrolysis precursor component i (i ═ 3, 4 and 5), the combustion reaction equation of the pyrolysis product is as follows:

first part of the equation to the right

Representing pyrolysis producing pyrolysis products, a second fraction

Represents the oxidative combustion of pyrolysis products; wherein f is_cMass conversion ratio coefficient for conversion of pyrolysis precursor to pyrolysis product, f_c＝0.3083；ρ_cIs the density of the pyrolysis product, kg.m^-3； ρo₂Is the oxygen density.

A_c(min^-1) Is a leading factor of the combustion reaction of the pyrolysis products, E_c(J/mol) is the pyrolysis product combustion reaction activation energy, n_cOxygen concentration correlation coefficient, see table 3.

3.3.3 soot model

According to the mass fraction of the residual ash after the combustion of the pyrolysis product of 13.01 percent, the generation equation of the ash is expressed as follows:

in equation (4.9), f_ashMass conversion ratio coefficient for conversion of pyrolysis products into soot, f_ash＝0.1301。

3.3.4 cigarette paper pyrolysis combustion model

The cigarette paper pyrolysis combustion reaction equation can be expressed as:

where ρ is_z,j(kg·m^-3) Is the density of cigarette paper component j; rho_z(kg·m^-3) Is the total density of the cigarette paper; rho_z0The initial density of the cigarette paper is 548.2 kg.m^-3。A_z,j，E_z,j，n_z,jSee table 4 for values.

4. Equation of energy

4.1 effective thermal conductivity

When the temperature exceeds 1000K during the combustion of the cut tobacco, the influence of radiation is great. In the solid phase, the effect of radiation on the temperature equation was modeled using the Rosseland approximation method. In the gas phase, the radiation effect is neglected.

Effective thermal conductivity (k) in the gas phase_g,eff) Comprises the following steps:

solid phase effective thermal conductivity (k)_s,eff) Comprises the following steps:

k_gis the thermal conductivity of air, k_g＝0.0242W·m^-1·K^-1，

Is the porosity of the tobacco, epsilon is the radiation coefficient of the tobacco, k_sIs the heat conductivity coefficient of tobacco shred, d_pThe pore diameter of the cut tobacco is shown in the table 1. Sigma is Stefan-Boltzmann constant, sigma 5.67X 10^-8W/(m²·K⁴)。

Source term of energy equation

Solid phase energy equation in porous media:

gas phase energy equation in porous media:

in equations (5.3) and (5.4),

C_p,s(J·kg^-1·K^-1) Is the specific heat capacity of the tobacco shred, 1043 J.kg^-1·K^-1；

C_p,g(J·kg^-1·K^-1) Is a specific heat capacity of gas, 1004 J.kg^-1·K^-1；

ρ_s(kg/m³) Is the total density of all solids, including pyrolysis precursors, pyrolysis products and soot ρ_s＝ρ_v+ρ_c+ρ_z+ρ_ashOne variable thereof;

ρ_g(kg/m³) Is the density of gas, 1.225 kg.m^-3；

T_sIs the tobacco shred temperature, K;

T_gis the gas phase temperature, K.

Porous medium specific surface equation:

solid energy source term equation:

wherein, Δ H_i(kJ/kg) Heat of oxidation released by combustion of tobacco pyrolysis products (. DELTA.H), respectively_c，17570 kJ·kg^-1) Heat of evaporation reaction with moisture in tobacco shreds (Δ Η)_w，-2257kJ·kg^-1) The values are shown in Table 1. The heat of pyrolysis reaction is assumed to be small and negligible.

Heat transfer coefficient h (W.m) between gas and solid^-2.K^-1) It can be calculated by the following equation:

wherein mu_gIs the gas viscosity, kg.s^-1·m^-1；v_iIs the velocity in the i direction, m.s^-1(ii) a Re is Reynolds number; pr is the prandtl number; nu is the nuschel number.

The radiation of the surface of the cigarette cylinder in the model is simplified into the calculation of the cylindrical solid energy source term, and the calculation formula is as follows:

wherein S is_r(J/m³s) is an energy source term, σ is Stefan-Boltzmann constant; r' (m) is the radius of the cylinder of the cigarette and is 0.004 m; l (m) is the length of the cylinder of the cigarette, 0.058 m.

5. Equation of momentum transfer

6.1 when the air flow enters from the front end of the cigarette combustion cone

At this time, the source term of the momentum equation is composed of viscous resistance and inertial resistance:

wherein v is_iIs the velocity of the gas in the i direction, m.s^-1(ii) a K is the tobacco shred permeability, m²(ii) a C is an empirical constant of the inertia term.

K and C are solved using the euro-root equation,

wherein d is_pIs the aperture of the tobacco shred,

is the tobacco shred porosity.

K is the tobacco shred permeability

During the combustion process of the cut tobacco, the cut tobacco permeability K changes. It is assumed that the tobacco permeability varies linearly with the density of the unburned tobacco:

K＝K_u(1-g)+K_bg (5.4)

K_upermeability of unburned tobacco, 5X 10^-10m²，K_bIs the burnt tobacco shred permeability 10⁵m². g is an insertion factor. Rho_sIs the total mass concentration of solids, including pyrolysis precursors, pyrolysis products, and soot, which is a variable. Rho_v,0Is the starting density of the cut tobacco (see table 1).

6.2 when the air flow enters from the rear end of the cigarette burning line

At this time, the source terms of the momentum equation are composed of:

wherein K_zIs the permeability of the cigarette paper, m². The inventors have found that the permeability of the wrapper is temperature dependent. Therefore, the permeability K of the cigarette paper in the temperature range of 473-623K is set_z,m＝1.5×10^-9m². Permeability K of unburned cigarette paper_z,u＝10^-15m²Permeability K of the wrapper in the burnt zone_z,b＝10⁵m²。

Equation for CO and Tar Release

1.0g of cut tobacco sample of cigarette is weighed and loaded into a pyrolysis combustion experimental platform of CN 104267140A. FIG. 6 is a schematic diagram of a pyrolysis combustion experimental platform. Wherein: the method comprises the following steps of 1-a gas source, 2-a mass flow controller, 3-a computer, 4-a temperature control system, 5-a smoke analysis device, 6-a Cambridge filter, 7-an infrared lamp tube, 8-a tobacco sample, 9-a thermocouple, 10-a heat insulation layer, 11-a cavity and 12-a quartz glass tube. The contents of CN104267140A are incorporated herein in its entirety.

The tobacco shred sample is filled in the middle of a quartz glass tube and is placed in a rapid tube type heating furnace, different atmospheres are introduced to the left side, the gas flow is set to be 2.1L/min, after 3min of gas introduction, the heating rate is controlled to be 20K/s, the temperature is raised to the target temperature and is balanced for 10min, the Cambridge filter is used for capturing tar in the smoke, and the CO in the gas phase substances penetrating through the Cambridge filter is monitored on line by a smoke analyzer. Each experimental condition was repeated 3 times and averaged.

7.1 relationship between CO release amount in flue gas and changes of temperature and oxygen concentration

FIG. 7 shows the cut tobacco in 2% O₂+98％N₂Mixed gas, 10% O₂+90％N₂Mixture and in air (23% O)₂+67％N₂) And the ordinate is the quality of CO generated by pyrolysis of average per gram of cut tobacco.

7.2 relationship between the amount of tar released in Smoke and the variation of temperature and oxygen concentration

FIG. 8 shows the amount of tar released at different temperatures and different oxygen concentrations. When the temperature is low 513K, the tar release amount is rapidly increased to 90-100 mg along with the temperature rise, and then the temperature continues to be increased, so that the tar release amount is not greatly changed.

FIG. 9 shows tar-like behavior collected from different conditions of pyrolytic combustion of Cambridge filter. In general, the color of the cambridge filter is similar in the three atmospheres, and the cambridge filter under the 423K condition is white, which indicates that the tar generated at the temperature is very little; the Cambridge filter sheet under 483K condition is light yellow, which indicates that tar begins to slowly increase; and after the temperature is higher than 513K, the Cambridge filter is brownish yellow, a large amount of tar is generated, the temperature is increased to 663K, the Cambridge filter is brown, the temperature is continuously increased, and the color difference of the Cambridge filter is small.

7.3 equation for CO and Tar Release

Depending on the CO and tar release, only the oxygen concentration and temperature are relevant. An equation of the trade mark 1 is established for the change of CO and tar released by a unit tobacco shred along with the temperature under different oxygen concentrations, and the equation is shown in a table 5. And inputting the mathematical relations into a cigarette combustion mathematical model of Fluent in the form of UDF, and predicting the generation conditions of tar and CO under different oxygen concentrations and different temperatures.

TABLE 5

Therefore, the source terms (kg. m) of tar and CO at different temperatures and oxygen concentrations^-3·s^-1) Can be expressed as:

where ρ is_sRefers to the total density of all solids, including the total density of the five pyrolysis precursors, coke and soot, which is a variable.

7. Filter rod tar filtering model

The tar belongs to particulate matter in the smoke of the cigarette and is expressed as aerosol particles. When the cigarette smoke passes through the filter stick, one part of aerosol particles penetrate through the filter stick, and the other part of aerosol particles are intercepted by the filter stick. The collection capacity of the filter rod for the aerosol of the smoke of the cigarette is expressed by the retention efficiency e, which is defined as the fraction of particles entering the filter rod and retained by the filter rod and is expressed as follows:

in the formula m_inAnd m_outIndicating the quality of the entering and exiting tar, respectively.

The aerosol particles are deposited on the surface of the fiber tows of the filter stick, and the main action mechanism comprises an interception effect E_INInertia effect E_IMDiffusion effect E_DAnd diffusion-interception interaction E_ID. Interception efficiency e of single fiber tows of filter stick_sCan be expressed as:

e_s＝E_IN+E_IM+E_D+E_ID (8.2)

the retention rate e of the cigarette smoke aerosol on the filter stick is expressed by a fan model filtering theory:

wherein L is_fIs the length of the filter stick, 0.028 m; d_fIs the diameter of the fiber tows of the filter stick, 2.51 multiplied by 10^-5m; omega is the volume fraction of the fiber tows in the filter stick, and the expression of omega is as follows:

wherein D_tIs the total denier of the filter stick, 35000, d_fIs the diameter of the fiber tows of the filter stick, 2.51 multiplied by 10^-5m， D_sThe denier of the filter stick is 3.0; c_fiberIs the ratio of crimped fibers in the filter rod, 0.17; s_filterIs the cross section area of the filter stick, 5.024 multiplied by 10^-5m²。

Source item S of tar in cigarette combustion process_tar(kg·m^-3·s^-1) Comprises the following steps:

according to the source term equation, the tar flow (kg/s) of all grids on the inlet face of the filter stick can be obtained in the Fluent model, and the airflow flow (kg/s) of all grids on the inlet face of the filter stick can be obtained in the Fluent model according to the momentum conservation equation. Taking the ratio of the two as the mass fraction f of tar in the smoke on the inlet surface of the filter stick_tar (％)。

The rejection rate of the filter stick at different positions can be expressed as:

wherein L is_xAnd the x axial length of the filter stick from different positions to the front end of the filter stick is shown.

When smoke passes through the filter tip and part of tar is deposited on the surface of the fiber tow of the filter tip, the quality of the tar is reduced, and therefore the source item S 'of the tar in the filter tip'_tar(kg·m^-3·s^-1) A negative value at this time can be expressed as:

according to the equation, the tar release amount (mg) of all grids on the inlet face of the filter stick before 0s of suction and the tar release amount (mg) of all grids on the outlet face of the filter stick after 2s of suction can be calculated in the Fluent model, and the retention rate e of the filter stick to tar can be calculated according to the equation (8.1).

8.1 interception Effect E_IN

When the airflow is bent, particles enter from the streamline of the airflow right to a position which is less than the radius of the particles from the surface of the filter stick fiber, the particles are possibly collected, the effect is the interception effect, and the interception efficiency is as follows:

the interception efficiency depends on a dimensionless interception parameter R_L：

Wherein d is_cOf aerosol particles of cigarette smokeDiameter, assuming that the aerosol particles are all 4.4X 10 in diameter^-7m，d_fIs the diameter of the fiber tows of the filter stick, 2.51 multiplied by 10^-5m。

K′_uThe mulberry prime dynamic factor represents the influence of air flow bending caused by the existence of other filter stick fiber tows, and the expression is as follows:

8.2 inertial Effect E_IM

The cigarette smoke can bend when passing through the filter stick, aerosol particles with certain mass can not completely move along with the bending, and due to the inertia effect, the aerosol particles can not be turned in time and impact on the fibers of the filter stick to be trapped, and the benefit is called as inertia effect. Formula of rejection rate due to inertial effects:

J＝(29.6-28ω^0.62)R_L ^2.8 (8.10)

stk is the Stokes number, the inertial effect depends on the Stk number, and the expression is as follows:

where ρ is_gIs the gas density, μ_gViscosity of gas, v_iIs the gas velocity in the i direction, d_cIs the diameter of the aerosol particles of cigarette smoke, d_fIs the diameter of the filter rod fiber tows.

8.3 diffusion Effect E_D

The small particles in the cigarette smoke do not move completely with the airflow and usually leave the airflow due to brownian motion. The particles impinge on the surface of the filter rod fibers, so that the concentration of particles near the surface is reduced to zero. The concentration difference near the surface will promote the diffusive deposition of particles at the surface, an effect known as the diffusion effect. The collection efficiency due to diffusion is:

wherein Pe is the Bekley number and represents the interception efficiency of the monofilament fiber, and the expression is as follows:

wherein D is_kIs the diffusion coefficient of the particles, m²·s^-1；K_BIs the Beziman constant, 1.38X 10^-23J· K^-1Wherein T is_filterIs the temperature of the filter rod, 313K.

8.4 mutual diffusion-interception Effect E_ID

The retention efficiency resulting from diffusion and interception interactions is expressed as:

8. cigarette lighting and smoking program

The 5mm area of the lighting end was set to 1000K and lit for 8 seconds.

According to GB/T19609-containing 2004, the cigarette is smoked according to the ISO mode, the smoking capacity is 35ml, one puff is smoked every 60s, namely 2s smoking, 58s smoldering smoking, 2s smoking and 58s smoldering smoking … are circulated until the cigarette burnout is extinguished.

Within 0-2 s of the pumping,

v denotes the suction capacity per port.

Compiling 1-8 parameters into a User Defined Function (UDF), loading the UDF into Fluent software, establishing a computational fluid mechanics model of cigarette combustion (smoking combustion), and operating the model to accurately simulate a temperature field and a substance concentration field in the cigarette combustion process so as to obtain parameters such as the temperature field, the tobacco shred pyrolysis product concentration, an oxygen concentration field and the release amount of harmful components during cigarette combustion.

The control equation is dispersed through a finite volume method, a separation solver is used for solving, and a hidden mode is selected for carrying out linearization and solving on the control equation. All terms in the governing equation are discretized in a first-order windward format. The SIMPLE algorithm is used for pressure-velocity coupling. Convergence criterion was set to 10^-3. The iteration interval time was 0.001 s.

Secondly, comparing the temperature prediction result of the cigarette smoking and burning model with the experimental value

1. Pyrolysis product density field and cigarette paper combustion line (burnt cigarette paper and unburned cigarette paper interface)

FIGS. 10, 11, 12 show the variation of the density field of the pyrolysis products as the cigarette is combusted at different times (60s, 120s, 180s, 240s, 300s and 360 s). The combustion cone had formed at 60s, and as combustion progressed, the combustion center shape was gradually tapered and moved backward, indicating that cigarette combustion continued. The tail end of the burning cone is flush with the burning line of the cigarette paper. This shows that the cigarette combustion calculation fluid mechanics model of the embodiment has accurate prediction results.

2. Moving distance of combustion line in 2s of suction

FIG. 13 is a graph showing the change in the position of the combustion line of the cigarette paper at the end of the combustion cone at 2s (180-182 s) of suction. The backward migration of the combustion line of the cigarette paper in the observation experiment is close to 3-4 mm, and the migration of the combustion line predicted by the model is also close to 3-4 mm through the change of the combustion position of the cigarette paper in the cigarette combustion model, which shows that the model is close to the amount of the tobacco actually participating in pyrolysis combustion in 2s of cigarette smoking.

From the ignition to the front end of the filter stick at 3mm (taking the combustion of the cigarette paper in the model to 3mm as reference), the cigarette smoking model sucks 7 mouths totally, and the actual cigarette sucks 6.2 mouths, which indicates that the combustion speed of the cigarette model is basically consistent with the combustion speed of the actual cigarette.

3. Predicted value and actual value of cigarette combustion temperature

In order to test the accuracy of the cigarette combustion temperature predicted by the model and the actual temperature, a TF-M100 cigarette combustion and smoking gas-phase temperature field analyzer and a three-hole-channel smoking machine are utilized, a micro thermocouple is inserted into a specific position in a cigarette support, the cigarette is placed in a cigarette holder of the smoking machine, and temperature acquisition software is operated to acquire the temperature data of smoldering and smoking of the cigarette.

And (3) positioning the real-time temperature of the depth of the center of the cigarette from the centers of the cigarettes 22mm, 24mm, 26mm, 28mm, 30mm, 32mm, 34mm and 36mm to the front end of the cigarette, and only 1 cigarette is smoked when the burning line of the cigarette paper is observed to be smoked at the position of 26mm in the experimental process, so that the smoldering state of the cigarette is kept before. The temperature at the 26mm position at the time of aspiration was found to be 933K by experiment. In the cigarette combustion model, the real-time temperatures of the centers of the cigarettes 22mm, 24mm, 26mm, 28mm, 30mm, 32mm, 34mm and 36mm away from the front end of the cigarette are monitored according to experimental fixed points, and when the temperature of the center position of 26mm reaches 933K, the cigarette is smoked, only 1 mouth is smoked, and the smoldering state of the cigarette is kept before. FIG. 10 shows experimental and model fit values for temperatures at various locations from 0 to 350 s. As can be seen from fig. 14, the trends of the temperature curves of the experimental values and the fitted values are consistent, and the overall temperature deviation is small.

The standard root mean square error of the experimental temperature and the model predicted temperature of different position points during cigarette combustion is shown as the following table:

position (mm)	22	24	26	28	30	32	34	36
									Standard root mean square error (%)	16.0	17.5	11.0	8.5	13.9	16.0	15.8	11.9

The error of each measuring point is below 18 percent, which indicates that the predicted value basically accords with the experimental value.

Third, CO and tar release amount prediction result and experimental value

And (3) building a cigarette burning computational fluid mechanics model of the brand number 1 by referring to the steps. And establishing cigarette burning computational fluid mechanics models of the brand 2 and the brand 3 in the same way.

The release amounts of CO and tar in the mainstream smoke after smoking each cigarette were calculated and compared with the release amounts of CO and tar in the mainstream smoke measured in the experiment, and the results are shown in Table 6. As can be seen from Table 6, the model predicts that the number of puffs for 3 brand cigarettes is very close to the experimental value.

The release amount of tar and CO of each mouth is calculated to obtain that the predicted tar release amount of a total 7-mouth cigarette smoking model is 11.4 mg/cigarette, the actual tar release amount of the cigarette is 11.2 mg/cigarette, and the relative deviation is 1.8%; the predicted CO release amount of the cigarette model is 14.1 mg/cigarette, the actual CO release amount of the cigarette is 13.2 mg/cigarette, and the relative deviation is 6.4%.

The cigarettes of the

brand numbers

2 and 3 were also predicted according to the method of the brand number 1 cigarette, and the results are shown in table 6, with little relative deviation.

TABLE 6

Fourthly, prediction result and experimental value of tar retention rate

The release amount of the total tar of 6 smoked mouths of the cigarette with the brand number of 1 at the filter stick inlet is calculated to be 21.1 mg/cigarette, the release amount of the tar at the filter stick outlet is calculated to be 11.4 mg/cigarette, and the retention rate of the tar is calculated to be 46.0%.

Since nicotine is mainly present in tar, the retention rate of nicotine is detected in the experiment to represent the retention rate of tar. The retention rate of the filter stick to nicotine measured by the experiment is 44.5%. Therefore, the relative deviation of the predicted value from the experimental value was 3.4%.

FIG. 15 shows the tar release amount and tar retention rate at the inlet and outlet of the cigarette filter stick of No. 1 during one-time smoking (180-182 s). And calculating the release amount of tar at the inlet of the filter stick in 180-182 s in a single mouth to be 3.31 mg/mouth, the release amount of tar at the outlet of the filter stick to be 1.82 mg/mouth and the retention rate of the tar to be 45.0%. And calculating the release amount of the total tar at the inlet of the filter stick of 7 mouths sucked by the cigarette to be 21.1 mg/cigarette, the release amount of the tar at the outlet of the filter stick to be 11.4 mg/cigarette and the retention rate of the tar to be 46.0%.

Because the experiment determines that the retention rate of the tar in the filter stick adopts a weighing method, the experimental error is larger, and because the nicotine mainly exists in the tar, the embodiment adopts the measurement value of the retention rate of the nicotine as the retention rate of the tar. The retention rate of the filter stick to nicotine measured by experiments is 44.5 percent, and the relative deviation is 3.4 percent

Fifth, the influence of the permeability of the cigarette paper on the prediction of the harmful ingredient result

The permeability of the cigarette paper in the temperature range of 473-

TABLE 7

As can be seen from the above comparative experiments, the permeability of the wrapping paper with 473-623K is set to 1.5X 10^-9m²The established tobacco combustion computational fluid mechanics model is more accurate, and the obtained result of the release amount of the harmful ingredients is closer to the experimental measurement value.

Sixth, air velocity field

Figures 16, 17 and 18 show the variation in the airflow velocity field for a cigarette in puffs 180s, 181s and 182s (i.e. within 2s of a puff), respectively.

At 180s, the air flow speed is low when the cigarette is smoldered, and the maximum air flow speed is only 0.00195m & s^-1When the air is sucked for 180-181 s, the air flow speed is increased, and the central area of the combustion coneThe air flow speed is high, and the maximum air flow speed is 1.032 m.s^-1When the air flow is sucked for 181-182 s, the air flow speed is reduced, but the air flow speed in the area close to the cigarette paper is higher than that in the central area of the combustion cone, because the permeability of the yellowing area of the cigarette paper at the rear end of the combustion line is increased, so that the air flow enters from the area close to the cigarette paper.

Claims

1. A method of creating a computational fluid dynamics model that models the temperature field and/or the material concentration field of a cigarette as it burns, comprising:

a) establishing a geometric model for simulating a cigarette combustion scene;

establishing a tobacco shred pyrolysis reaction kinetic equation;

c) loading the geometric model and the equation established in the above steps by adopting solving software of computational fluid dynamics software, and establishing a computational fluid dynamics model for simulating a temperature field and/or a substance concentration field during cigarette combustion;

in the step b), the method for establishing the combustion reaction kinetic equation of the tobacco shred pyrolysis product comprises the following steps:

heating the tobacco shred pyrolysis product under the oxygen-containing atmosphere with s oxygen concentrations, detecting the mass change of the tobacco shred pyrolysis product by adopting a thermogravimetric analysis method, and respectively measuring differential thermogravimetric curves of the s tobacco shred pyrolysis products when being heated, wherein s is a positive integer;

k is a positive integer;

α_c，kin order to obtain the conversion of reaction k,

conversion a for reaction k_c，kDerivative with respect to time T, T_cFor heating temperature, R is the ideal gas constant, E_c，kTo react the activation energy of k, n_c，kThe number of reaction stages in reaction k,

is the oxygen density;

a_c，kand T_cObtained from a differential thermogravimetric curve, A_c、E_c，kAnd n_c，kObtaining through fitting;

s is more than or equal to k and more than or equal to 2, and s and k are positive integers.

2. A method according to claim 1, step b) further comprising establishing an equation for the permeability of the cigarette paper as a function of temperature, comprising:

the cigarette paper set to the third temperature has a third permeability.

3. The method of claim 2, wherein the first temperature is aK or less and the first permeability is 0.5 x 10^-15～5×10^-15m²；

The second temperature is a-bK, and the second permeability is 1.5 multiplied by 10^-9～6×10^-9m²；

A third temperature bK or higher and a third permeability of 0.5X 10⁵～5×10⁵m²；

The value range of a is 450-500K, and the value range of b is 600-650K.

4. A method according to claim 2, wherein the permeability of the wrapper is set to vary unidirectionally, increasing without decreasing.

5. A method according to claim 1, step b) further comprising establishing cigarette paper combustion reaction kinetics equations comprising:

fitting the m unimodal curves to the kinetic equations of m reactions j:

the kinetic equation for each reaction j is as follows:

j ranges from 1 to m, m is the number of fitting reactions, and j and m are positive integers;

a_z，jthe conversion in reaction j,

The derivative of the conversion of reaction j with time, T_zFor heating the temperature of the cigarette paper, R is an ideal gas constant, A_z，jTo reflect the pre-exponential factor of j, E_z，jTo react the activation energy of j, n_z，jThe reaction order of reaction j;

a_z，j、

T_zobtained from the differential thermogravimetric curve of the cigarette paper when heated, A_z，j、E_z，jAnd n_z，jObtained by fitting.

6. The method according to claim 1, wherein in the step b), the method for establishing the pyrolysis reaction kinetic equation of the cut tobacco comprises the following steps:

-heating the tobacco shred sample in a non-oxidizing gas atmosphere, detecting the mass change of the tobacco shred sample by adopting a thermogravimetric analysis method, and drawing a differential thermogravimetric curve of the tobacco shred sample;

fitting n unimodal curves to the kinetic equations of n reactions i:

the value range of i is 1, 2.. n, n and i are integers, and n is the number of fitting reactions;

a_v，ithe conversion rate for reaction i,

Derivative of the conversion rate over time, T, for reaction i_vThe temperature for heating the tobacco shred sample, beta is the heating rate of the tobacco shred sample, R is an ideal gas constant, A_v，iTo reflect the pre-exponential factor of i, m_v，iCorrection of the parameters for the rate of rise of reaction i, E_v，iTo react the activation energy of i, n_v，iThe reaction grade number of the reaction i;

T_vbeta is obtained from the differential thermogravimetric curve of a tobacco sample, A_v，i、m_v，i、E_v，iAnd n_v，iObtained by fitting.

7. The method of claim 1, wherein step b) further comprises establishing an equation relating the amount of the harmful components produced to the temperature for different oxygen concentrations.

8. The method of claim 7, wherein the step of establishing an equation relating harmful component emissions to temperature for different oxygen concentrations comprises:

and fitting a relation equation of the release amount of the harmful components corresponding to different oxygen concentrations and the temperature according to the detection result of the last step.

9. The method according to claim 1, step b) further comprising the step of establishing a filter rod tar retention equation comprising:

e_s＝E_IN+E_IM+E_D+E_ID；

wherein E is_IN、E_IM、E_D、E_IDIs an intermediate parameter;

J＝(29.6-28ω^0.62)R_L ^2.8；

e is the retention rate of the filter stick to tar, e_sRetention rate of tar by single fiber tow of filter stick, L_fIs the length of the filter stick, d_fIs the diameter of the fiber tow of the filter rod, omega is the volume fraction of the fiber tow in the filter rod, D_tIs the total denier of the filter stick, D_sIs the single denier of the filter stick, C_fiberIs the ratio of crimped fibers in the filter rod, S_filterIs the cross-sectional area of the filter rod, p_gIs the gas density, μ_gViscosity of gas, v_iIs the gas velocity in the i direction, d_cIs the diameter of the aerosol particles of cigarette smoke, D_kIs the diffusion coefficient of the particles, K_BIs the Botzmann constant, T_filterIs the temperature of the filter rod.

10. The method of claim 1, step d) further comprising the step of loading the cigarette smoking program with software for solving computational fluid dynamics software.

11. The method according to any one of claims 1 to 10, wherein the step a) of establishing a geometric model of cigarette combustion comprises:

the method comprises the steps of establishing a geometric model of a cigarette combustion scene by adopting preprocessing software of computational fluid dynamics software, wherein the geometric model comprises a gas area and a cigarette area, the cigarette area is located in the gas area, and the cigarette area comprises a tobacco shred area and a cigarette paper area.

12. The method of claim 1, the species concentration field comprising one or more of:

an oxygen concentration field;

a tobacco shred pyrolysis product concentration field;

a cigarette paper combustion product concentration field;

field of concentration of harmful components.

13. A method for predicting the temperature and/or the substance concentration of a cigarette during combustion, wherein the computational fluid dynamics model is established according to the method of any one of claims 1 to 12, the model is operated, and the temperature and/or the substance concentration of the cigarette during combustion are obtained according to the simulation result of the model.

14. A system for creating a computational fluid dynamics model that models the temperature field and/or the material concentration field of a cigarette as it burns, comprising:

the cigarette combustion scene physical and/or chemical reaction equation building module comprises:

the tobacco shred pyrolysis reaction kinetic equation establishing module is used for establishing a tobacco shred pyrolysis reaction kinetic equation;

the computational fluid dynamics model building module adopts a geometric model built by the computational fluid dynamics software loading geometric model building module and an equation built by the cigarette combustion scene physical and/or chemical reaction equation building module to build a computational fluid dynamics model for simulating a temperature field and/or a substance concentration field during cigarette combustion

The tobacco shred pyrolysis reaction kinetic equation establishing module is used for executing the following operations:

k is a positive integer;

α_c，kin order to obtain the conversion of reaction k,

is the oxygen density;

15. The system of claim 14, having one or more of the following features:

the system further comprises a tobacco shred pyrolysis reaction thermogravimetric analysis module, which is used for performing thermogravimetric analysis on the tobacco shreds in an oxygen-free atmosphere and outputting data to the tobacco shred pyrolysis reaction kinetic equation establishing module;

the system further comprises a tobacco shred pyrolysis product combustion reaction thermogravimetric analysis module, which is used for performing thermogravimetric analysis on the tobacco shred pyrolysis product in an oxygen-containing atmosphere and adding data to output to the tobacco shred pyrolysis product combustion reaction kinetic equation establishing module;

the system further comprises a cigarette combustion scene physical and/or chemical reaction equation establishing module and a cigarette paper combustion reaction kinetic equation establishing module;

the system further comprises a cigarette paper combustion reaction thermogravimetric analysis module, which is used for carrying out thermogravimetric analysis on the cigarette paper combustion reaction in an oxygen-containing atmosphere and outputting data to the cigarette paper combustion reaction kinetic equation establishing module;

-the system further comprises an equation building block of permeability of the cigarette paper as a function of temperature;

the system also comprises a cigarette combustion scene physical and/or chemical reaction equation establishing module and a tobacco harmful component release amount equation establishing module, wherein the tobacco harmful component release amount equation establishing module is used for establishing a relation equation between tobacco harmful component release amounts corresponding to different oxygen concentrations and temperature;

-the system further comprises a tobacco harmful component emission amount detection module for:

outputting the detection result to a tobacco shred harmful component release amount equation establishing module;

-the system further comprises a filter rod to tar retention equation establishing module;

e_s＝E_IN+E_IM+E_D+E_ID；

wherein E is_IN、E_IM、E_D、E_IDIs an intermediate parameter;

J＝(29.6-28ω^0.62)R_L ^2.8；