CN113420514A - Method for simulating internal flow characteristic numerical value of multistage pressure reduction regulating valve - Google Patents

Method for simulating internal flow characteristic numerical value of multistage pressure reduction regulating valve Download PDF

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CN113420514A
CN113420514A CN202110789002.3A CN202110789002A CN113420514A CN 113420514 A CN113420514 A CN 113420514A CN 202110789002 A CN202110789002 A CN 202110789002A CN 113420514 A CN113420514 A CN 113420514A
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方毅芳
唐春娥
赵婉琳
刘刚
杜孟新
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Instrumentation Technology And Economy Institute P R China
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Abstract

The invention relates to a numerical simulation method for flow characteristics in a multistage pressure reduction regulating valve, which comprises the following steps: step 1, establishing a coupling calculation model of an internal flow field of the multistage pressure reduction regulating valve based on a numerical calculation method of an IDDES turbulence model, a texture two-phase flow model and a Schnerr-Sauer cavitation model; step 2, carrying out numerical simulation calculation on a plurality of groups of flow parameters of working media in the sleeve of the multi-stage sleeve type pressure reducing valve under different opening degrees and different pressure difference working conditions based on the coupling calculation model to obtain the flow characteristics in the regulating valve and the pressure reducing stage pressure reducing characteristics under different pressure differences and different valve opening degrees; the different opening degrees include 20%, 50%, 75%, 100%. According to the invention, an effective numerical calculation method is established according to the flowing characteristics of the multi-stage pressure reduction regulating valve, and the calculation result can reflect the change rule of the flowing inside the valve, so that theoretical basis and reference are provided for the reasonable design of the pressure reduction valve.

Description

Method for simulating internal flow characteristic numerical value of multistage pressure reduction regulating valve
Technical Field
The invention relates to the technical field of pressure reducing valves, in particular to a numerical simulation method for flow characteristics in a multi-stage pressure reducing regulating valve.
Background
At present, a high-pressure regulating valve is widely applied to a plurality of industrial fields, plays a key role in a whole industrial pipeline control system, and is one of important devices for ensuring the safe and economic operation of the system. With the development of large-scale and fine industrial production process, higher requirements are also put forward on the high-pressure regulating valve. Under the high pressure difference, the common regulating valve is difficult to meet the requirements of regulating pressure and flow, and phenomena such as flash evaporation, cavitation, strong vibration, high noise and the like can occur when high-speed fluid flows through the valve, so that the working performance and the safe service life of the regulating valve are seriously influenced. Therefore, understanding the internal flow characteristics of high pressure regulator valves is a prerequisite for efficient application of the regulator valves.
The multistage pressure reduction regulating valve adopts a multistage sleeve type structure, in the valve structure, a certain pressure drop can be generated when high-pressure fluid passes through a throttling section, the pressure mutation in the single-stage valve is converted into pressure gradual change, the pressure drop is shared by multistage throttling elements, and the occurrence of flash evaporation and cavitation can be prevented for incompressible fluid, so that the effects of noise reduction and vibration reduction can be achieved.
The internal flow of the pressure-reducing regulating valve is a typical high-speed turbulent flow, severe pressure pulsation is generated when fluid passes through a throttling element, fluid excitation is induced and high noise is generated, and relevant researches are carried out by domestic and foreign researchers aiming at the flow characteristics of the pressure-reducing regulating valve. The cavitation phenomenon in a typical element is analyzed by using water as a working medium and adopting a multi-dimensional CFD method in the foreign FRANZONI and the like, and the influence of the shape of a valve seat on cavitation generation, pressure distribution and flow coefficient is discussed. Bernead et al performed numerical simulation of the flow inside the valve using a CFD method, first performed research on single-phase flow (liquid), quantitatively described the swirl flow inside the valve, and second performed two-phase flow (cavitation) research using a full-cavitation model. Zaryankin et al give data on pressure pulsations in the valve box, downstream of the diffuser seat and in the subsequent steam line, and research results show that the pulsation level depends mainly on the geometry of the steam inlet flow path. Shin establishes a numerical model of the pressure control valve and a connecting pipe behind the pressure control valve, performs unsteady numerical calculation on the basis, and analyzes transient flow characteristics of pressure, temperature and speed changes in the valve and the pipeline.
The specific method for eliminating the cavitation of the regulating valve of the power plant by using the multi-stage depressurization method and the field use condition are introduced by Yuen et al, which shows that the possibility of the cavitation can be obviously reduced by using the multi-stage sleeve, when the valve is fully opened, the cavitation can be avoided by using the one-stage or two-stage sleeve, and when the opening degree of the valve is smaller, the cavitation is eliminated by using the three-stage or more-stage sleeve. The application of a multi-stage pressure reduction high-pressure difference structure in a emptying environment is introduced, and the effects of tightly closing, cutting off fluid, reducing noise and reducing pressure can be achieved through the design of the multi-stage pressure reduction structure. The characteristics of different sleeve regulating valves are analyzed in view of result comparison. The queen swallow and the like perform numerical simulation calculation on the internal flow of each sleeve structure regulating valve by using Fluent, and properly increasing the interstage clearance or reducing the aperture size on the sleeve is favorable for the noise elimination and vibration reduction of the high differential pressure regulating valve. The research results show that the multi-stage silencing throttling sleeve adopted in the high-pressure drop regulating valve can effectively achieve the purposes of reducing pressure step by step, limiting flow speed and suppressing noise. Penjian and the like carry out simulation research on the cavitation phenomenon in the valve by adopting a numerical calculation method, find that the valve is easy to generate cavitation under small opening degree, and the cavitation part is mainly positioned at a valve core and a valve seat of a throttling hole. The numerical research of flow-induced vibration and flow-induced noise of the plum tree gazelle and the like aiming at the multi-stage sleeve type pressure-reducing drain valve is carried out, and research data shows that the vibration characteristics of the valve under different opening degrees are mainly determined by the structure of a throttling part in the valve, the influence of the opening degrees is small, and the vibration of the valve under the medium opening degree is minimum; the fluid pressure pulsation in the throttling area in the sleeve is strongest, and the noise frequency spectrum of the regulating valve with different sleeve structure parameters shows obvious broadband characteristics.
In general, research is mostly focused on single-stage or two-stage sleeve pressure reduction regulating valves at present, and the research on the internal flow characteristics of the pressure reduction regulating valve with a multi-stage sleeve structure under different opening degrees is less.
Disclosure of Invention
The invention aims to provide a numerical simulation method for internal flow characteristics of a multi-stage pressure reduction regulating valve, which takes the multi-stage pressure reduction regulating valve as a research object, establishes a three-dimensional calculation model of the regulating valve, calculates the internal flow of the valve under the conditions of different pressure differences and different opening degrees by adopting a CFD numerical method, analyzes the pressure drop of each stage of sleeve and valve seat under different pressure differences and different opening degrees and the influence of the pressure drop on cavitation in the valve, and provides theoretical basis and reference for reasonable design of the pressure reduction valve.
The invention provides a numerical simulation method for internal flow characteristics of a multistage pressure reduction regulating valve, which comprises the following steps:
step 1, establishing a coupling calculation model of an internal flow field of the multistage pressure reduction regulating valve based on a numerical calculation method of an IDDES turbulence model, a texture two-phase flow model and a Schnerr-Sauer cavitation model;
step 2, carrying out numerical simulation calculation on a plurality of groups of flow parameters of working media in the sleeve of the multi-stage sleeve type pressure reducing valve under different opening degrees and different pressure difference working conditions based on the coupling calculation model to obtain the flow characteristics in the regulating valve and the pressure reducing stage pressure reducing characteristics under different pressure differences and different valve opening degrees; the different opening degrees include 20%, 50%, 75%, 100%.
Further, the multi-stage depressurization regulating valve in the step 1 comprises a multi-stage sleeve and an outlet section single-stage depressurization channel.
Further, the section of the guide groove of the multistage depressurization regulating valve in the step 1 is circular, and the diameter of the guide groove is 6-9 mm.
Further, the numerical calculation method of the IDDES turbulence model in step 2 is a coupling algorithm of DDES and wall model LES method WMLES, and when the incoming flow condition turbulence is strong and the grid scale can resolve the vortex structure dominated by the boundary layer, the model is switched to the RANS and LES mixed mode, specifically implemented as follows:
IWMLES＝fB(1+fe)IRANS+(1-fB)ILES (1)
in the formula IWMLESIs the length scale of the wall model, IRANSAnd ILESLength scales of RANS and LES, respectively, fBAnd feAn empirical mixing function and a lifting function, respectively.
Empirical mixing function fBObtained by the following formula:
fB＝min[2exp(-9α2),1] (2)
wherein the empirical coefficient alpha is 0.25-dw/hmax，dwDistance of local grid to wall, hmaxIs the maximum size of the local grid in three spatial directions;
empirical function feThe stress attenuation device is used for avoiding the excessive attenuation of Reynolds stress in an RANS and LES interface area; reynolds stresses are ignored when the RANS and LES interface is very close to the wall surface;
fe＝fe2·max[(fe1-1),0]Ψ (3)
Figure BDA0003160140900000031
fe2＝1-max[ft,fl] (5)
in the formula (f)e1And fe2Are all control functions, parameter ftAnd flControlling the strength of the RANS model part in the model, the size of which is also controlled by empirical parameters; when in the boundary layer, f is ensured by selecting appropriate parameterstOr flIs close to 1, so that fe2And feClose to 0, so that RANS and LES can switch smoothly with each other near the boundary layer;
the length scale of the DDES model is modified as shown below:
Figure BDA0003160140900000041
in the formula:
Figure BDA0003160140900000042
is a length scale of the DDES model,
Figure BDA0003160140900000043
is a mixing function. When the incoming flow contains a turbulent component,
Figure BDA0003160140900000044
and IWMLESClose, numerical simulations resolve boundary layer dominated turbulent flow.
Further, the texture two-phase flow model in step 1 simulates fluid or particle phases by solving momentum, continuity and energy equations of the Mixture, volume fraction equations of the second phase and algebraic expressions of relative velocities;
the continuous equation in the texture two-phase flow model is:
Figure BDA0003160140900000045
in the formula (I), the compound is shown in the specification,
Figure BDA0003160140900000046
is the mass mean velocity, pmIs the mixed density;
the momentum equation of the texture two-phase flow model is obtained by summing the respective momentum equations of all phases, and is expressed as:
Figure BDA0003160140900000047
in the formula, n is a phase sequence number,
Figure BDA0003160140900000048
is volume force, mumIn order to obtain the viscosity of the mixture,
Figure BDA0003160140900000049
is the acceleration of gravity, p is the pressure, alphakVolume fraction of the k-th phase, pkOf the k-th phaseThe density of the mixture is higher than the density of the mixture,
Figure BDA00031601409000000410
is the drift velocity of phase k.
Further, the Schnerr-Sauer cavitation model in step 1 uses a method similar to that in the Singhal model to derive an accurate expression of net mass transport from liquid to vapor;
the equation for the vapor volume fraction is:
Figure BDA00031601409000000411
wherein α is the volume fraction of the vapor phase, ρvIn order to be the density of the steam,
Figure BDA00031601409000000412
for the velocity of the vapor, the net mass source term R is calculated as follows:
Figure BDA00031601409000000413
in the formula: ρ is the density of the mixture, ρlFor liquid phase density, the following expression relates vapor volume fraction to the number of bubbles per unit volume of liquid:
Figure BDA0003160140900000051
analogizing to the Singhal method, the following equation is derived:
Figure BDA0003160140900000052
Figure BDA0003160140900000053
wherein R is mass transport rate, RBIs the bubble radius, PvThe saturated vapor pressure, P is the pressure of the bubbles, and n is the number density of the bubbles.
Further, the step 1 comprises:
on the basis of establishing a coupling calculation model of an internal flow field of the multistage pressure reduction regulating valve, the multistage pressure reduction regulating valve is subjected to grid division, the whole multistage pressure reduction regulating valve is divided by tetrahedral grids, and the number of the grids used for numerical calculation is 1200 ten thousand.
By means of the scheme, a calculation model of the internal flow field of the multistage pressure reduction regulating valve is established by a numerical simulation method of the internal flow characteristic of the multistage pressure reduction regulating valve and a numerical calculation method based on a texture two-phase flow model and a Schnerr-Sauer cavitation model, the flow characteristic of the internal flow of the multistage pressure reduction regulating valve and the pressure reduction characteristics of various pressure reduction stages under different pressure differences and different valve openings are obtained through numerical simulation calculation, and theoretical basis and reference value are provided for reasonable design of the pressure reduction valve.
The foregoing description is only an overview of the technical solutions of the present invention, and in order to make the technical solutions of the present invention more clearly understood and to implement them in accordance with the contents of the description, the following detailed description is given with reference to the preferred embodiments of the present invention and the accompanying drawings.
Drawings
FIG. 1 is a cross-sectional view of a multi-stage pressure reducing regulator valve in accordance with an embodiment of the present invention;
FIG. 2 is a schematic diagram of a multi-stage pressure reducing regulator valve fluid domain model according to an embodiment of the present disclosure;
FIG. 3 is a schematic diagram of the overall meshing of a multi-stage pressure reducing control valve in accordance with an embodiment of the present invention;
FIG. 4 is a schematic diagram of partial meshing of a multi-stage pressure reducing control valve in accordance with an embodiment of the present invention;
FIG. 5 is a graph of the pressure profile on the centerline of a multi-valve in one embodiment of the present invention;
FIG. 6 is a pressure distribution (case1) across the middle section of a valve according to one embodiment of the present invention;
FIG. 7 is a velocity profile (case1) of a valve mid-section according to an embodiment of the present invention;
fig. 8 shows the vorticity distribution (Q ═ 1) inside the valve according to an embodiment of the present invention;
FIG. 9 is a gas distribution (case1) of a valve mid-section according to an embodiment of the present invention;
FIG. 10 is a graph of valve inlet flow as a function of opening in accordance with an embodiment of the present invention (case 1);
FIG. 11 is a graph of pressure drop variation for different opening degrees in one embodiment of the present invention;
FIG. 12 is a cloud of static pressure profiles (25% open) of case1 according to an embodiment of the present invention;
FIG. 13 is a cloud of case1 static pressure profiles in accordance with an embodiment of the present invention;
FIG. 14 is a graph of the pressure drop variation (case1) for different opening step-down pressure stages in an embodiment of the present invention;
FIG. 15 is a cloud of case1 speed profiles in accordance with an embodiment of the present invention;
FIG. 16 is a graph illustrating the pressure drop variation (case4) for different opening step-down pressure stages in an embodiment of the present invention;
FIG. 17 is a plot of the velocity profile of stage 1-3 sleeves along the flow direction center line in accordance with one embodiment of the present invention;
FIG. 18 is a static pressure profile along the flow direction centerline for stage 1-3 sleeves in accordance with an embodiment of the present invention.
Detailed Description
The following detailed description of embodiments of the present invention is provided in connection with the accompanying drawings and examples. The following examples are intended to illustrate the invention but are not intended to limit the scope of the invention.
The embodiment provides a method for simulating the internal flow characteristic of a multistage pressure reduction regulating valve, which comprises the following steps:
step 1, establishing a coupling calculation model of an internal flow field of the multistage pressure reduction regulating valve based on a numerical calculation method of an IDDES turbulence model, a texture two-phase flow model and a Schnerr-Sauer cavitation model;
step 2, carrying out numerical simulation calculation on a plurality of groups of flow parameters of working media in the sleeve of the multi-stage sleeve type pressure reducing valve under different opening degrees and different pressure difference working conditions based on the coupling calculation model to obtain the flow characteristics in the regulating valve and the pressure reducing stage pressure reducing characteristics under different pressure differences and different valve opening degrees; the different opening degrees include 20%, 50%, 75%, 100%.
The method for simulating the flow characteristic value in the multistage pressure reduction regulating valve provides theoretical basis and reference value for reasonable design of the pressure reduction valve.
The present invention is described in further detail below.
Numerical calculation model and method
The multi-stage pressure reduction regulating valve has a complex internal structure and large inlet-outlet pressure difference, so that severe pressure pulsation is easily generated in the high-speed flowing process of fluid in the valve, and further the cavitation phenomenon occurs in the flowing inside the valve body. Common experimental designs can only measure statistical parameters such as inlet and outlet pressure differences and flow of the valve, and the complex process of the internal flow of the valve is difficult to quantitatively describe and study. Therefore, the advantage of numerical calculation in this embodiment is embodied: through valve body modeling and grid division, the flow characteristics and cavitation characteristics of the interior of the multistage depressurization regulating valve under different boundary conditions can be calculated and obtained.
1. Numerical calculation model
1) Turbulence model
With the deep development of computational fluid mechanics, the current turbulence computation model can be mainly divided into the following 4 types: direct Numerical Simulation (DNS), reynolds average NS equation (RANS), large vortex simulation (LES), and separation vortex simulation (DES). Vortices with large scale difference exist in the turbulent flow field, and complex interaction exists among the vortices with different scales, so that the RANS method is difficult to better simulate the complex flow inside the pressure reducing valve. Although the DNS and LES methods can capture vortex structures well, the calculation amount is too large, and the calculation time is too long. In the embodiment, the improved delayed separation vortex (IDDES) in the DES method is finally selected to carry out numerical simulation, and the model can meet the engineering requirements on calculation accuracy and efficiency.
In the DES method, the boundary layer uses an unsteady RANS model, and the separation region uses LES processing. The LES region is typically associated with a core turbulent region where a larger unsteady turbulent scale dominates. In the above area, the DES model employs an LES method similar to the submesh model. Whereas in the near-wall region the corresponding RANS model is restored.
The IDDES method is a coupling algorithm of DDES and wall model LES method (WMLES). Similar to DES, the k equation of the BSL/SST model is modified, and information of local grid spacing is added. When the incoming flow condition turbulence is strong and the grid scale can distinguish the vortex structure dominated by the boundary layer, the model is switched to the RANS and LES mixed mode, and the specific implementation method is as follows:
IWMLES＝fB(1+fe)IRANS+(1-fB)ILES (1)
in the formula (f)BAs an empirical mixing function, it can be obtained by:
fB＝min[2exp(-9α2),1] (2)
wherein the empirical coefficient alpha is 0.25-dw/hmax，dwDistance of local grid to wall, hmaxThe maximum size of the local grid in three spatial directions.
Empirical function feThe method is used for avoiding the excessive attenuation of the Reynolds stress of the RANS and LES junction area and solving the problem of logarithmic layer mismatching. Reynolds stresses are negligible when the RANS and LES interface is very close to the wall surface.
fe＝max[(fe1-1),0]Ψfe2 (3)
Figure BDA0003160140900000081
fe2＝1-max[ft,fl] (5)
In the formula, the parameter ftAnd flThe strength of the portions of the RANS model in the model can be controlled, the size of which is also controlled by empirical parameters. When in the boundary layer, f is ensured by selecting appropriate parameterstOr flIs close to 1, so that fe2And feClose to 0 so that the RANS and LES can switch smoothly with each other near the boundary layer.
In order to mix the length scales of the DDES model and the WMLES model, the length scale of the DDES model needs to be modified as shown in the following formula:
Figure BDA0003160140900000082
when the incoming flow contains turbulent components, IDDESAnd IWMLESClose, numerical simulations can resolve boundary layer dominated turbulent flow.
2) Two-phase flow model
The mixture model in the two-phase flow model is a simplified multiphase model which can be used for simulating multiphase flows moving at different speeds, local balance is assumed on a short space length scale, the model can be used for simulating multiphase flows with strong coupling and the same moving speed, and finally, the non-Newtonian viscosity is calculated according to a mixing model.
The Mixture model can model fluid or particle phases by solving momentum, continuity and energy equations of the Mixture, volume fraction equations of the second phase, and algebraic expressions of relative velocities.
The continuous equation in the texture model is:
Figure BDA0003160140900000083
in the formula (I), the compound is shown in the specification,
Figure BDA0003160140900000084
is the mass mean velocity, pmIs the mixed density.
The momentum equation for mix can be obtained by summing the respective momentum equations for all phases, and is usually expressed as:
Figure BDA0003160140900000091
in the formula, n is a phase sequence number,
Figure BDA0003160140900000092
is volume force, mumIn order to obtain the viscosity of the mixture,
Figure BDA0003160140900000093
is the drift velocity of the secondary phase k.
3) Cavitation model
Schnerr and Sauer used methods similar to those in Singhal et al to derive accurate expressions for net mass transport from liquid to vapor. The equation for the vapor volume fraction is generally of the form:
Figure BDA0003160140900000094
in the formula, the net mass source term R is as follows:
Figure BDA0003160140900000095
the following expression can relate vapor volume fraction to the number of bubbles per unit volume of liquid:
Figure BDA0003160140900000096
by analogy to the method of Singhal et al, the following equation can be derived:
Figure BDA0003160140900000097
Figure BDA0003160140900000098
wherein R is mass transport rate, RBIs the bubble radius.
2. The object and boundary conditions are computed.
The calculation target of this embodiment is a certain multi-stage pressure reducing regulating valve, and the cross-sectional view is shown in fig. 1, in which 1 is a stage 1 sleeve, 2 is a stage 2 sleeve, 3 is a stage 3 sleeve, and 4 is a valve seat (stage 4). The number of stages of the sleeve and the valve seat from the left inlet to the right outlet is sequentially defined as i ═ 1,2,3 and 4, and the sleeve and the valve seat flowing through each stage can generate a certain pressure drop to the working medium, so that the stage is defined as the ith pressure reduction stage.
Extracting a three-dimensional fluid domain model of the depressurization regulating valve by using CAD software, wherein the three-dimensional fluid domain calculation model is shown in FIG. 2, a left side section is a valve inlet, and the boundary condition is set as pressure inlet; the right side section is a valve outlet, and the boundary condition is set as pressure outlet; the middle area is a valve body wall surface, the boundary condition is set to wall, and the boundary condition of the internal fluid calculation domain is set to the inlet and outlet pressure (gauge pressure) and the differential pressure of the fluid multistage pressure reduction regulating valve as shown in table 1.
TABLE 1 Inlet and outlet pressures and differential pressures of the valve under different working conditions
Figure BDA0003160140900000101
3. Computational grid and computational method
On the basis of establishing a structural model of the multi-stage pressure reduction regulating valve, the multi-stage pressure reduction regulating valve is subjected to grid division, the sizes of an inlet/outlet flow passage and a flow passage in the middle of a sleeve in the multi-stage pressure reduction regulating valve and the pore diameters of small holes of the sleeves at all levels are large, and the multi-stage sleeve structure is complex, so that the whole multi-stage pressure reduction regulating valve is divided by adopting tetrahedral grids, the grid density degree of different positions in the valve can be conveniently controlled while the grid quality is ensured, and the number of the grids used for numerical calculation is finally determined to be 1200 ten thousand. Fig. 3 and 4 respectively show the whole and partial gridding of the multi-stage pressure reduction control valve structure.
In order to analyze the flow characteristics inside the multi-stage pressure-reducing regulating valve, the numerical calculation method adopted in the present embodiment was determined, as shown in table 2.
TABLE 2 Key value calculation method
Figure BDA0003160140900000102
In numerical simulation, the size and number of computational grids can affect the accuracy of numerical computation results. When the number of grids is small, a large calculation deviation may exist between a calculation result and an accurate solution, and when the number of grids is continuously increased to a certain number, a numerical calculation result gradually tends to be stable and relatively reliable, but required calculation resources are also continuously increased. In order to determine the appropriate grid number, grid numbers with grid numbers of 732.6 ten thousand, 1203 ten thousand and 1758 ten thousand are selected for grid independence verification, valve mass flow rates corresponding to different grid numbers are shown in table 3, the valve flow rates gradually tend to be stable along with the increase of the grid numbers, fig. 5 shows pressure distribution on valve center lines under different grid numbers, it can be seen from fig. 5 that differences among calculation results of different grid numbers are small, deviation of calculation results of M3 and M2 is small, on the premise that accuracy deviation is acceptable, factors such as calculation time and efficiency are comprehensively considered, a scheme including 1200 ten thousand grid units is finally selected, and a flow process in the valve can be well captured.
TABLE 3 mesh categories
Figure BDA0003160140900000111
Second, numerical calculation results
1. Internal flow field distribution of regulating valve
On the basis of establishing an integer value calculation model and a numerical value calculation method, through numerical value simulation calculation, the pressure and the speed distribution of the middle section of the multi-stage depressurization regulating valve when the multi-stage depressurization regulating valve is in a half-open state under the case1 working condition are respectively shown in fig. 6 and 7, when fluid flows through the sleeves and the valve seats at all stages, obvious pressure difference occurs due to the throttling effect of small holes, the sleeves and the valve seats at all stages have different structural sizes, and therefore the pressure drop of each depressurization stage has certain difference. In the embodiment, the working medium is a non-compressible fluid, and as seen in fig. 6, the fluid at the small hole of the sleeve accelerates, the flow rate is much higher than the flow rate at other positions of the valve, the fluid flows into a space with a larger flow area after passing through the small hole, and the high-speed fluid impacts the wall surface of the sleeve to cause violent collision between the sleeves, so that complex turbulent flow is formed. When fluid enters the middle valve core along small holes distributed on the circumference of the third-stage sleeve, a plurality of strands of fluid are intensively mixed and sheared, meanwhile, the flow direction needs to be changed to enter the valve seat, a similar phenomenon also occurs when high-speed fluid flows through the valve seat, a large amount of vortexes are formed in the multistage pressure reduction regulating valve due to the flow characteristics, and the vibration and noise of the valve can be induced by turbulence pulsation impact in the multistage pressure reduction regulating valve. The swirl distribution inside the valve is shown in fig. 8, and it can be seen from fig. 8 that the swirl at the inlet section of the valve is less, the swirl after passing through the sleeves and the valve seats at each stage is increased rapidly, different fluid micelles are mixed with each other to dissipate the fluid energy, and the swirl formed after the valve seat is further gradually developed into the outlet section at the downstream of the valve.
In fig. 6, under the condition of 50% opening, the pressure drop at the third-stage sleeve is close to 6.0MPa, the highest flow velocity at the small hole reaches 120m/s, and cavitation is easily generated under the condition of high-speed flow caused by high pressure difference, fig. 9 is gas phase distribution at the middle section of the multistage pressure-reducing regulating valve, obvious cavitation occurs at the small hole of the third-stage sleeve and the small hole at the valve seat part, and the gas volume at the small hole at the lower side of the third-stage sleeve is larger, which indicates that the cavitation degree at the small hole is stronger; cavitation occurs in almost all the orifices at the valve seat, but the gas phase volume fraction at the surface of each orifice is small, and overall, the valve seat is more susceptible to cavitation than the sleeve, but the degree of cavitation is weaker.
Fig. 10 shows that the inlet mass flow of the multi-stage pressure-reducing regulating valve changes with different opening degrees under the case1 working condition, the inlet mass flow of the valve almost linearly increases in a small flow range with a large throttling, and the increasing trend of the mass flow gradually decreases with the gradual increase of the opening degrees.
2. Regulating valve flow behavior analysis at different pressures
The flow characteristics of the multi-stage pressure reduction regulating valve can be changed to a certain extent under the conditions of different inlet and outlet pressures of the valve. For the pressure reduction regulating valve models with different opening degrees, the pressure drop of the working medium flowing through each pressure reduction stage under different pressure difference working conditions is shown in fig. 11. Fig. 11(a) shows a 100% opening, fig. 11(b) shows a 75% opening, fig. 11(c) shows a 50% opening, and fig. 11(d) shows a 25% opening.
As can be seen from fig. 11(a), under the working condition that the opening degree is 100%, the pressure drop oscillation of the working medium flowing through each pressure reduction stage is low, and the variation trends are substantially consistent. The greater the pressure differential, the greater the pressure drop of the working fluid through the 1 st, 2 nd and 3 rd pressure reduction stages. For case2, case3 and case4, the inlet pressure is unchanged but the back pressure is reduced under three conditions, and it can be seen that as the back pressure is reduced, the pressure drop at the first 3 pressure reduction stages is increased instead, and the pressure drop at the 4 th pressure reduction stage is reduced.
Under the working condition that the opening degree of the valve is 75%, the pressure drop curves of all the pressure reduction stages have the same change trend, the pressure reduction stage is the highest in the 3 rd pressure reduction stage, the pressure reduction stage is the second of the 1 st pressure reduction stage, and the pressure reduction stages 2 and 4 are lower. At 75% opening of the valve, the pressure drops at the 1 st and 2 nd depressurization stages coincide, i.e., where the pressure drop is greatly affected by the inlet pressure and hardly affected by the back pressure, for case2, case3, and case 4.
When the opening degree is further reduced to 50%, as seen from fig. 11(c), the pressure drop of all the working conditions at the 2 nd and 4 th pressure reduction stages is almost the same, and the inlet-outlet pressure difference of the valve at the case1 working condition is smaller, so that the pressure drop of the valve at the 1 st and 3 rd pressure reduction stages is smaller than that of other working conditions.
When the opening is 25%, it can be seen from fig. 11(d) that the pressure drop at the 1 st, 2 nd and 4 th pressure reduction stages is almost uniform and extremely low in all the conditions. The pressure drop of each working condition at the 3 rd pressure reduction stage is governed by the inlet-outlet pressure difference, and as can be seen from the graph in fig. 12, at this opening, the working medium pressure drop under the case1 working condition mainly occurs at the 3 rd pressure reduction stage, and it can be inferred from the pressure drop curve that the working conditions are similar in other working conditions.
3. Analysis of flow characteristics of regulating valve at different opening degrees
Under different opening degrees, the static pressure distribution of the multi-stage pressure reduction regulating valve between the pressure reduction stages generates larger difference. Fig. 13 shows a cloud of pressure distributions at the middle section of the valve at different opening degrees, fig. 13(a) at 100% opening degree, fig. 13(b) at 75% opening degree, fig. 13(c) at 50% opening degree, and fig. 13(d) at 25% opening degree.
As shown in fig. 13(a), when the valve opening is 100%, the static pressure is drastically reduced in the vicinity of the 1 st pressure reduction stage; without significant change near the 2 nd buck stage; near the 3 rd buck stage, a dark blue low-pressure region appears, so the pressure drop should be increased; near the 4 th buck stage, a small drop in static pressure occurs. The above rule can also be obtained from the curve of fig. 13 under the condition of 100% opening.
When the opening degree is 75%, as shown in fig. 13(b), the static pressure at the stage subsequent to the 1 st pressure-reducing stage is higher than the corresponding position in fig. 13(a), and thus the stage pressure drop is smaller than the case of the opening degree of 100%; because the row of small holes of the 3 rd voltage reduction stage are blocked, the flow area of the working medium passing through the stage is reduced, and therefore the following can be obtained according to a continuous equation: the working medium is accelerated when flowing through the 3 rd pressure reduction stage, so that the static pressure is reduced more severely; as can be seen from the 75% opening curve in fig. 14, the 75% opening pressure drop is greater than the 100% opening pressure drop at the 3 rd pressure reduction stage; as can be seen from fig. 15(a), the high speed region of the valve at 100% opening degree exists in all of the 1 st, 2 nd and 3 rd pressure reduction stages, and as can be seen from fig. 15(b), the working medium of the valve at 75% opening degree accelerates sharply only in the vicinity of the 3 rd pressure reduction stage.
As can be seen from fig. 13(c) and (d), as the opening degree is further reduced, the static pressure in the vicinity of the 1 st and 2 nd pressure reduction stages is gradually increased, while the static pressure at the outlet section of the 3 rd pressure reduction stage is greatly reduced, which leads to the increase of the pressure drop of the stage, and a similar conclusion can be obtained by observing fig. 14.
Similar conclusions can be drawn for pressure drop variations of different opening step-down pressure levels for other inlet and outlet pressure conditions of the present embodiment, as shown in fig. 16.
For stage 1,2 and 3 pressure let-down valve sleeves, this example intercepts the in-line pressure and velocity profile at the centerline of the cylindrical orifice of the stage 1-3 sleeve, as shown in FIGS. 17 and 18.
As can be seen from FIG. 17, the speeds along the way at each opening degree are distributed in a step shape, when the working medium flows through the position near the front section of the small hole of the first-stage sleeve, the speed is steeply increased, then the working medium smoothly flows through the small hole of the first-stage sleeve, and the process is repeated. The smaller the valve opening, the greater the velocity increase of the working fluid through the stage 1 and 2 sleeves, and the lesser the velocity increase through the stage 3 sleeve, as compared to the large opening.
In contrast to the velocity profile of fig. 17, fig. 18 reflects the corresponding on-way static pressure profile. The on-way static pressure under each opening degree presents a step-shaped distribution, when the working medium flows through the position near the front section of the first-stage sleeve hole, the pressure is steeply reduced due to the corresponding speed increase, then the working medium smoothly flows through the small hole, and the process is repeated. Compared with a large opening degree, the smaller the opening degree is, the smaller the pressure drop of the working medium flowing through the 1-stage sleeve and the 2-stage sleeve is relatively, and the larger the pressure drop of the working medium flowing through the 3-stage sleeve is relatively.
The invention adopts a texture two-phase flow model, a Schnerr-Sauer cavitation model and an IDDES turbulence model to carry out research on the influence of the flow process inside the multistage pressure reduction regulating valve, different pressure differences and different opening degrees on the pressure reduction stage pressure drop, and the main conclusion is as follows:
(1) a large amount of swirl exists downstream of the sleeve and valve seat due to the intense mixing and shearing of the multiple streams as the fluid flows through the throttling element in the valve; the structural sizes of the sleeves and the valve seats at all levels are different, the pressure drop of each pressure reduction level is different, the cavitation degree at the sleeve at the third level is maximum, and the cavitation is easier to occur at the valve seat;
(2) the change trends of the working medium along the pressure drop of each pressure reduction stage under different pressure differences are the same under the same opening degree of the valve;
(3) with the gradual reduction of the opening degree of the valve, the maximum pressure drop is changed from the 1 st pressure reduction stage to the 3 rd pressure reduction stage, the average pressure drop of the third pressure reduction stage of the valve reaches 8MPa under the opening degree of 25 percent, the pressure drops of the 1 st pressure reduction stage and the 2 nd pressure reduction stage are gradually weakened under the dominant action of inlet pressure, and the dominant action of the 4 th pressure reduction stage under back pressure is also gradually weakened;
(4) the area of the outlet of the 3 rd pressure reduction stage is continuously reduced along with the reduction of the opening degree of the valve, the pressure drop of the pressure reduction stage is increased along with the reduction of the opening degree of the valve, and the speed of the working medium in the small hole of the pressure reduction stage is increased under the same flow.
According to the invention, an effective numerical calculation method is established according to the flowing characteristics of the multi-stage pressure reduction regulating valve, and the calculation result can reflect the change rule of the flowing inside the valve, so that theoretical basis and reference are provided for the reasonable design of the pressure reduction valve.
The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, it should be noted that, for those skilled in the art, many modifications and variations can be made without departing from the technical principle of the present invention, and these modifications and variations should also be regarded as the protection scope of the present invention.

Claims (7)

1. A method for simulating the internal flow characteristic of a multistage pressure reduction regulating valve is characterized by comprising the following steps:
step 1, establishing a coupling calculation model of an internal flow field of the multistage pressure reduction regulating valve based on a numerical calculation method of an IDDES turbulence model, a texture two-phase flow model and a Schnerr-Sauer cavitation model;
step 2, carrying out numerical simulation calculation on a plurality of groups of flow parameters of working media in the sleeve of the multi-stage sleeve type pressure reducing valve under different opening degrees and different pressure difference working conditions based on the coupling calculation model to obtain the flow characteristics in the regulating valve and the pressure reducing stage pressure reducing characteristics under different pressure differences and different valve opening degrees; the different opening degrees include 20%, 50%, 75%, 100%.
2. The method of numerical simulation of internal flow characteristics of a multi-stage pressure-reducing regulator valve according to claim 1, wherein the multi-stage pressure-reducing regulator valve in step 1 comprises a multi-stage sleeve and an outlet section single-stage pressure-reducing passage.
3. The method of claim 2, wherein the cross-section of the guiding groove of the multi-stage pressure-reducing regulating valve in step 1 is circular and has a diameter of 6-9 mm.
4. The method of numerical simulation of internal flow characteristics of a multi-stage pressure-reducing regulator valve according to claim 1,
the numerical calculation method of the IDDES turbulence model in the step 2 is a coupling algorithm of a DDES and wall model LES method WMLES, when the turbulence of the incoming flow condition is strong and the grid scale can distinguish the vortex structure dominated by the boundary layer, the model is switched to an RANS and LES mixed mode, and the specific implementation method is as follows:
IWMLES＝fB(1+fe)IRANS+(1-fB)ILES (1)
in the formula IWMLESIs the length scale of the wall model, IRANSAnd ILESLength scales of RANS and LES, respectively, fBAnd feAn empirical mixing function and a lifting function, respectively.
Empirical mixing function fBObtained by the following formula:
fB＝min[2exp(-9α2),1] (2)
wherein the empirical coefficient alpha is 0.25-dw/hmax，dwDistance of local grid to wall, hmaxIs the maximum size of the local grid in three spatial directions;
empirical function feThe stress attenuation device is used for avoiding the excessive attenuation of Reynolds stress in an RANS and LES interface area; reynolds stresses are ignored when the RANS and LES interface is very close to the wall surface;
fe＝fe2·max[(fe1-1),0]Ψ (3)
Figure FDA0003160140890000021
fe2＝1-max[ft,fl] (5)
in the formula (f)e1And fe2Are all control functions, parameter ftAnd flControlling the strength of the RANS model part in the model, the size of which is also controlled by empirical parameters; when in the boundary layer, f is ensured by selecting appropriate parameterstOr flIs close to 1, so that fe2And feClose to 0, so that RANS and LES can switch smoothly with each other near the boundary layer;
the length scale of the DDES model is modified as shown below:
Figure FDA0003160140890000022
in the formula:
Figure FDA0003160140890000023
is a length scale of the DDES model,
Figure FDA0003160140890000024
is a mixing function. When the incoming flow contains a turbulent component,
Figure FDA0003160140890000025
and IWMLESClose, numerical simulations resolve boundary layer dominated turbulent flow.
5. The method of numerical simulation of internal flow characteristics of a multi-stage pressure-reducing regulator valve according to claim 4,
the texture two-phase flow model in the step 1 simulates a fluid or particle phase by solving momentum, continuity and energy equations of a Mixture, volume fraction equations of a second phase and algebraic expressions of relative speeds;
the continuous equation in the texture two-phase flow model is:
Figure FDA0003160140890000026
in the formula (I), the compound is shown in the specification,
Figure FDA0003160140890000027
is the mass mean velocity, pmIs the mixed density;
the momentum equation of the texture two-phase flow model is obtained by summing the respective momentum equations of all phases, and is expressed as:
Figure FDA0003160140890000028
in which n is a phaseThe serial number of the serial number,
Figure FDA0003160140890000029
is volume force, mumIn order to obtain the viscosity of the mixture,
Figure FDA00031601408900000210
is the acceleration of gravity, p is the pressure, alphakVolume fraction of the k-th phase, pkIs the density of the k-th phase,
Figure FDA00031601408900000211
is the drift velocity of phase k.
6. The method of numerical simulation of internal flow characteristics of a multi-stage pressure-reducing regulator valve according to claim 5,
the Schnerr-Sauer cavitation model in the step 1 adopts a method similar to that in a Singhal model to deduce an accurate expression of net mass transport from liquid to steam;
the equation for the vapor volume fraction is:
Figure FDA0003160140890000031
wherein α is the volume fraction of the vapor phase, ρvIn order to be the density of the steam,
Figure FDA0003160140890000032
for the velocity of the vapor, the net mass source term R is calculated as follows:
Figure FDA0003160140890000033
in the formula: ρ is the density of the mixture, ρlFor liquid phase density, the following expression relates vapor volume fraction to the number of bubbles per unit volume of liquid:
Figure FDA0003160140890000034
analogizing to the Singhal method, the following equation is derived:
Figure FDA0003160140890000035
Figure FDA0003160140890000036
wherein R is mass transport rate, RBIs the bubble radius, PvThe saturated vapor pressure, P is the pressure of the bubbles, and n is the number density of the bubbles.
7. The method of numerical simulation of internal flow characteristics of a multi-stage pressure-reducing regulator valve according to claim 1, wherein the step 1 comprises:
on the basis of establishing a coupling calculation model of an internal flow field of the multistage pressure reduction regulating valve, the multistage pressure reduction regulating valve is subjected to grid division, the whole multistage pressure reduction regulating valve is divided by tetrahedral grids, and the number of the grids used for numerical calculation is 1200 ten thousand.
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