CN112052578B - Method and system for calculating resistance of ventilation air conditioner pipeline system based on cut-off method - Google Patents
Method and system for calculating resistance of ventilation air conditioner pipeline system based on cut-off method Download PDFInfo
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Abstract
The invention belongs to the field of ventilation air conditioning engineering, and discloses a ventilation air conditioning pipeline system resistance calculation method and system based on a cut-off method, wherein the method comprises the following steps: step 1: acquiring a most adverse loop of the piping system, the most adverse loop comprising a plurality of local components; step 2: selecting a part component outlet of the pipeline system for cutting to obtain a plurality of part components comprising an upstream straight pipe section and a downstream straight pipe section; step 3: calculating the cut-off distance of each upstream straight pipe section and each downstream straight pipe section; step 4: and obtaining the resistance value of the upstream straight pipe section and the downstream straight pipe section of each local component by using a resistance calculation model according to the cut-off distance of the upstream straight pipe section and the downstream straight pipe section, and obtaining the resistance value of each local component by using a pressure drop method to obtain the total resistance value of the pipeline system. According to the invention, the resistance is accurately analyzed by the cut-off method, and compared with the resistance obtained by the traditional method, the resistance is lower, so that the fan shape selection is influenced, and the purpose of reducing the electric energy consumption is achieved.
Description
Technical Field
The invention belongs to the field of ventilation air conditioning engineering, and particularly relates to a resistance calculation method and a resistance calculation system for a ventilation air conditioning pipeline system based on a cut-off method.
Background
For decades, traditional research on pipe resistance has been largely based on the premise of fully developing flow. However, in the field of ventilation air-conditioning ducts, the resistance variation caused by underdeveloped flow is significant. In the former conventional study, due to the need for simplifying the study, it is assumed that the distance between the pipe-local fittings is "far enough" that the flow fields in the fittings are not interfered with each other, that is, the local fitting inlet is assumed to be "fully developed flow". We can refer to the subassembly at this time temporarily as a "single subassembly" (also known as a "non-coupling condition", "a subassembly in a non-composite condition"). Under this condition, the flow field variation inside the partial fitting and the resistance problem caused by it are only affected by the deformation of the partial fitting itself; and the flow state in the straight pipe section is the fully developed flow. However, in the field of construction, the actual distance between the fittings is "sufficiently close" that the flow fields between adjacent fittings are mutually interactive, i.e. the pipe inlet and outlet boundary condition is "under-developed flow". We can refer to this condition temporarily as a "neighboring influence condition" (also called a "coupling condition", "recombination condition"). Some previous studies have shown that the length of the vortex generated by the local fittings is 5-7 pipe diameters on the upstream flow field and 30-50 pipe diameters on the downstream flow field. The common hydraulic diameter 1m of the ventilation air conditioner is used for estimation, the action distance of adjacent influences is about 35-57 m, and in the practical engineering, most of the installation distances of pipeline local fittings are smaller than the range due to the limitation of building structures. That is, in the field of construction, adjacent influences are virtually ubiquitous. However, the resistance calculation method for the underdeveloped flow at the present stage is still lacking.
With the resistance calculation method, since there is no flow for the underdeveloped flow, there are two methods at the present stage to calculate the local resistance under the underdeveloped flow. The first is to simply stack the local resistance of a single fitting by looking up the manual and looking up the local resistance of each fitting. In existing manuals, such as ASHRAE manual, CIBSE guide, idelchik manual, the local resistance is summarized from a number of experimental works, most of which were formulated according to ASHRAE standards, 2017ASHRAE hand book mentions that the calculation of the local resistance for the coupling fitting is assumed to be isolated for each fitting and not interacted with any other fitting, in that the conventional practice of adding the local resistance of individual fittings gives a conservative (upper) estimate. However, the upper limit is not true, and an excessively high estimated resistance value may cause the fan to be selected to be large and deviate from the stable operating region, and may also cause unnecessary power consumption. Then, extensive research is needed on how to perform hydraulic calculation of the ventilation air-conditioning duct system in the state of insufficiently developed flow.
In addition, the second approach is to derive the resistance under underdeveloped flow directly through experimental and design methods. Many studies have shown that the error between the local drag coefficients of a simple superposition of coupled and uncoupled fittings is large. However, a general set of methods is not summarized and only individual analysis can be performed for each case.
Disclosure of Invention
The invention aims to provide a resistance design method and a resistance design system for a ventilation air-conditioning pipeline system based on a cut-off method, which are used for solving the equivalent problems in the prior art.
In order to realize the tasks, the invention adopts the following technical scheme:
A resistance calculation method of a ventilation air-conditioning pipeline system based on a cut-off method comprises the following steps:
Step 1: acquiring a most unfavorable loop of the pipeline system, wherein the most unfavorable loop comprises a plurality of local components;
Step 2: cutting off the outlet of each partial component to obtain a plurality of partial components comprising an upstream straight pipe section and a downstream straight pipe section, wherein the upstream straight pipe section is a straight pipe section before the inlet of each partial component, and the downstream straight pipe section is a straight pipe section after the outlet of each partial component;
Step 3: calculating the cutoff distance of the upstream straight pipe section of each local component and the cutoff distance of the downstream straight pipe section of each local component, wherein the cutoff distance of the upstream straight pipe section refers to the full development flow distance of the straight pipe section minus the length of the upstream straight pipe section, and the cutoff distance of the downstream straight pipe section refers to the full development flow distance of the straight pipe section minus the length of the downstream straight pipe section;
Step 4: obtaining a resistance value of the upstream straight pipe section and a resistance value of the downstream straight pipe section of each local component by using a resistance calculation model according to the cut-off distance of the upstream straight pipe section of each local component and the cut-off distance of the downstream straight pipe section of each local component, and obtaining the resistance value of each local component by a pressure drop method;
A total resistance value of the tubing is obtained, the total resistance value comprising a resistance value of the upstream straight tube section of each partial member and a resistance value of the downstream straight tube section of each partial member and a resistance value of each partial member itself.
Further, the partial member includes a three-way member, a reducing member, and an elbow member.
Further, in the resistance calculation model, the upstream straight pipe section is fitted by a straight line, the downstream straight pipe sections of the reducing component and the elbow component are fitted by a double Gaussian curve, and the downstream straight pipe section of the three-way component is fitted by an exponential function.
The resistance calculation system of the ventilation air-conditioning pipeline system based on the cut-off method comprises a most unfavorable loop calculation module, a cut-off position selection module, a cut-off distance calculation module and a resistance calculation module;
The most adverse loop calculation module is used for calculating a most adverse loop of the pipeline system, and the most adverse loop comprises a plurality of local components;
The cut-off position selection module is used for cutting off the outlet of each local component to obtain a plurality of local components comprising an upstream straight pipe section and a downstream straight pipe section, wherein the upstream straight pipe section is a straight pipe section before the inlet of each local component, and the downstream straight pipe section is a straight pipe section after the outlet of each local component;
the cut-off distance calculation module is used for calculating the cut-off distance of an upstream straight pipe section of each local component and the cut-off distance of a downstream straight pipe section of each local component, wherein the cut-off distance of the upstream straight pipe section refers to the full development flow distance of the straight pipe section minus the length of the upstream straight pipe section, and the cut-off distance of the downstream straight pipe section refers to the full development flow distance of the straight pipe section minus the length of the downstream straight pipe section;
The resistance calculation module is used for calculating the total resistance value of the pipeline system, the total resistance value comprises the resistance value of an upstream straight pipe section of each local component, the resistance value of a downstream straight pipe section of each local component and the resistance value of each local component, the resistance value of the upstream straight pipe section and the resistance value of the downstream straight pipe section of each local component are obtained by using a resistance calculation model according to the cut-off distance of each upstream straight pipe section and the cut-off distance of the downstream straight pipe section respectively, and the resistance value of each local component is obtained according to a pressure drop method.
Further, the partial member includes a three-way member, a reducing member, and an elbow member.
Further, in the resistance calculation model, the upstream straight pipe section is fitted by a straight line, the downstream straight pipe sections of the reducing component and the elbow component are fitted by a double Gaussian curve, and the downstream straight pipe section of the three-way component is fitted by an exponential function.
Compared with the prior art, the invention has the following technical characteristics:
(1) The invention relates to an engineering design calculation method-a cut-off method suitable for calculating the resistance of a ventilation air conditioner pipeline under the boundary condition of insufficient development flow, and the functional relation between the local resistance and the cut-off distance of a tee joint, a reducing joint and an elbow is obtained on the premise of finding out the action mechanism of the resistance of the ventilation air conditioner pipeline. The truncation method can calculate local resistance according to the truncation distance, and resistance superposition is performed on the basis. The method design calculation of the resistance in the ventilation air-conditioning field under the condition of insufficient development flow is provided for the first time, the general design steps are provided, and the method is suitable for the resistance calculation under different conditions of engineering.
(2) According to the invention, the resistance is accurately analyzed by the truncation method, the resistance value of the local component is obtained by the truncation distance, and the superposition calculation result is closer to the resistance true value under the coupling condition, so that the error is small and the accuracy is higher.
(3) According to the invention, the resistance is accurately analyzed by the cut-off method, the resistance value of the local component is obtained by the cut-off distance, and the result of superposition calculation is lower than the resistance value obtained by the traditional method, so that the fan shape selection is affected, and the purpose of reducing the electric energy consumption is achieved.
Drawings
FIG. 1 is a schematic diagram of the present invention;
FIG. 2 is a view of four cutoff positions of the present invention;
FIG. 3 is a graph of upstream and downstream drag versus ratio statistics for different pipe diameters for different parts at the same speed and six sets of aspect ratios; wherein fig. 3 (a) is the pressure drop caused by elbow resistance at different aspect ratios, fig. 3 (b) is the pressure drop caused by reducing resistance at different aspect ratios, fig. 3 (c) is the pressure drop caused by tee straight pipe resistance at different aspect ratios, and fig. 3 (d) is the pressure drop caused by tee resistance at different aspect ratios;
FIG. 4 shows the correlation between different cut-off distances of an elbow, a variable diameter tee and a tee joint and corresponding resistance values under the condition of fully developed flow (10 meters in each of front and rear straight pipe sections); wherein FIG. 4 (a) is a correlation of upstream resistance and cutoff distance, and FIG. 4 (b) is a correlation of downstream resistance and cutoff distance;
FIG. 5 is a graph showing the error between the resistance value calculated by the present invention and the resistance value calculated by the conventional method;
FIG. 6 is a schematic diagram of a ventilation air conditioning duct system in an embodiment;
fig. 7 is a graph comparing actual resistance values with resistance values obtained by the present invention.
Detailed Description
First, technical words appearing in the present invention are explained:
Pipeline system: the pipeline system comprises a plurality of ventilation air-conditioning pipelines and is also called a pipeline network, wherein the pipelines are connected through local components.
A partial member: also known as local resistance members, means members for connection between different adjacent straight pipe sections of a ventilation air conditioning duct, and generally includes three-way members, reducing members and elbow members. The partial member comprises an inlet and an outlet, which inlet and outlet are connected to different straight pipe sections, respectively.
The most disadvantageous loop: and a loop with the largest total resistance value in the ventilation air conditioning pipeline system.
Fully developing flow distance: assuming that the distance between the pipe partial fittings is "far enough" that the flow fields within the fittings do not interfere with each other, the partial fitting inlets are "fully developed flow" and the distance between the two partial fittings satisfies the fully developed flow distance.
Pressure drop method: the pressure values of the two surfaces of the inlet and the outlet of the local component are obtained through the result of software Fluent simulation, and the pressure drop is obtained by further making difference.
Straight pipe section: the pipe connected between each two partial members is called a straight pipe section.
The embodiment discloses a ventilation air conditioner pipeline system resistance design method based on a cut-off method, which comprises the following steps:
Step 1: acquiring a most unfavorable loop of the pipeline system, wherein the most unfavorable loop comprises a plurality of local components;
Step 2: cutting off the outlet of each partial component to obtain a plurality of partial components comprising an upstream straight pipe section and a downstream straight pipe section, wherein the upstream straight pipe section is a straight pipe section before the inlet of each partial component, and the downstream straight pipe section is a straight pipe section after the outlet of each partial component;
Step 3: calculating the cutoff distance of the upstream straight pipe section of each local component and the cutoff distance of the downstream straight pipe section of each local component, wherein the cutoff distance of the upstream straight pipe section refers to the full development flow distance of the straight pipe section minus the length of the upstream straight pipe section, and the cutoff distance of the downstream straight pipe section refers to the full development flow distance of the straight pipe section minus the length of the downstream straight pipe section;
Step 4: obtaining a resistance value of the upstream straight pipe section and a resistance value of the downstream straight pipe section of each local component by using a resistance calculation model according to the cut-off distance of the upstream straight pipe section of each local component and the cut-off distance of the downstream straight pipe section of each local component, and obtaining the resistance value of each local component by a pressure drop method;
A total resistance value of the tubing is obtained, the total resistance value comprising a resistance value of the upstream straight tube section of each partial member and a resistance value of the downstream straight tube section of each partial member and a resistance value of each partial member itself.
Specifically, the local component comprises a tee component, a reducing component and an elbow component.
Specifically, in the resistance calculation model, the upstream straight pipe section is fitted by a straight line, the downstream straight pipe sections of the reducing component and the elbow component are fitted by a double Gaussian curve, and the downstream straight pipe section of the three-way component is fitted by an exponential function.
Wherein, double Gaussian curve: Exponential curve: /(I) In the double Gaussian curve, y 0 represents the drag coefficient at 1 to 2 hydraulic radii downstream of the fitting; x c represents the cutoff distance at 1-2 hydraulic radii downstream of the fitting; h represents the steepness of the peak; w 1 represents a constant, which does not vary with speed. In the exponential curve, y 0 represents the resistance value at zero cutoff, and a represents the steepness of the curve.
Specifically, the cutting position can be selected as the distance between the inlet of each partial component and the straight pipe section and the pipe diameter of one time of the outlet of each partial component, however, the outlet of each partial component has the best effect, and the downstream resistance value is relatively large after analysis, so that the downstream resistance value is cut off, and the method is described with reference to fig. 3 and 5.
In particular, the fully developed flow distance is set to be 30-50 times of the hydraulic radius, and the flow distance is set to be 10m.
Specifically, the obtaining of the resistance value of each local component according to the pressure drop method means obtaining the pressure values of the inlet and outlet surfaces of the local component through the result of software Fluent simulation, and obtaining the pressure drop by making a difference.
The embodiment also discloses a resistance design system of the ventilation air-conditioning pipeline system based on the cut-off method, which comprises a most unfavorable loop calculation module, a cut-off position selection module, a cut-off distance calculation module and a resistance calculation module;
The most adverse loop calculation module is used for calculating a most adverse loop of the pipeline system, and the most adverse loop comprises a plurality of local components;
The cut-off position selection module is used for cutting off the outlet of each local component to obtain a plurality of local components comprising an upstream straight pipe section and a downstream straight pipe section, wherein the upstream straight pipe section is a straight pipe section before the inlet of each local component, and the downstream straight pipe section is a straight pipe section after the outlet of each local component;
the cut-off distance calculation module is used for calculating the cut-off distance of an upstream straight pipe section of each local component and the cut-off distance of a downstream straight pipe section of each local component, wherein the cut-off distance of the upstream straight pipe section refers to the full development flow distance of the straight pipe section minus the length of the upstream straight pipe section, and the cut-off distance of the downstream straight pipe section refers to the full development flow distance of the straight pipe section minus the length of the downstream straight pipe section;
The resistance calculation module is used for calculating the total resistance value of the pipeline system, the total resistance value comprises the resistance value of an upstream straight pipe section of each local component, the resistance value of a downstream straight pipe section of each local component and the resistance value of each local component, the resistance value of the upstream straight pipe section and the resistance value of the downstream straight pipe section of each local component are obtained by using a resistance calculation model according to the cut-off distance of each upstream straight pipe section and the cut-off distance of the downstream straight pipe section respectively, and the resistance value of each local component is obtained according to a pressure drop method.
Specifically, the local component comprises a tee component, a reducing component and an elbow component.
Specifically, in the resistance calculation model, the upstream straight pipe section is fitted by a straight line, the downstream straight pipe sections of the reducing component and the elbow component are fitted by a double Gaussian curve, and the downstream straight pipe section of the three-way component is fitted by an exponential function.
Example 1
The embodiment discloses a resistance design method of a ventilation air-conditioning pipeline system based on a cut-off method, which is a system formed by coupling a most common local accessory in the ventilation air-conditioning pipeline system, namely an elbow, a reducing part and a tee. The three-way branch in the local fitting is divided into a straight-through direction and a bypass direction, so that the system is divided into four branches, namely:
(1)1-2-3-4-5-6；(2)1-2-3-4-5-7-8-9；(3)1-2-3-4-5-7-8-10-11-12；(4)1-2-3-4-5-7-8-10-11-13。
Wherein the total resistance of the system is determined from the most disadvantageous loop, i.e. from the loop with the greatest resistance. Assuming that the most disadvantageous loop in the above system is the third branch, i.e. 1-2-3-4-5-7-8-10-11-12, the calculation of the resistance of this branch is of paramount importance. In fact, in the present system, the third branch is indeed the most disadvantageous loop.
In the present embodiment, the resistance calculation value table:
In addition, in this embodiment, as can be seen from fig. 5, we determine that the truncation is calculated at the truncation position 2 (i.e. truncating the downstream resistance value PDR), and find that for the most unfavorable loop branch 3, the truncation method has obvious advantages in terms of both accuracy and low resistance compared to the conventional resistance value.
In this embodiment, by calculation, the resistance of the upstream straight tube section of each partial member in the total resistance value is directly taken as Y value in the graph according to classification as shown in fig. 4 (a), and the resistance of the downstream straight tube section is as shown in fig. 4 (b), conforming to the following table:
Wherein, the value of X is the cut-off distance, and Y is the calculated resistance value.
As shown in fig. 7, the invention adopts full-size experiments to verify the local resistance when all the local fittings are coupled together, and simultaneously compares the simulation results, the local resistance coefficient experiment results of the most unfavorable loop (third branch) under different wind speeds, and the simulation comparison under the same condition verifies the rationality of the simulation. Meanwhile, compared with the traditional local resistance calculation method, the true value is obviously smaller than the traditional value, and the reasonability of the cut-off method is reflected from the side.
Claims (2)
1. The method for calculating the resistance of the ventilation air conditioning pipeline system based on the cut-off method is characterized by comprising the following steps of:
Step 1: acquiring a most unfavorable loop of the pipeline system, wherein the most unfavorable loop comprises a plurality of local components;
the most disadvantageous loop: a loop with the largest total resistance value in the ventilation air-conditioning pipeline system;
the partial member: the device is also called a local resistance component and refers to a component used for connecting different adjacent straight pipe sections of a ventilation air-conditioning pipeline, and comprises a tee component, a reducing component and an elbow component; the local component comprises an inlet and an outlet, and the inlet and the outlet are respectively connected with different straight pipe sections;
Step 2: cutting off the outlet of each partial component to obtain a plurality of partial components comprising an upstream straight pipe section and a downstream straight pipe section, wherein the upstream straight pipe section is a straight pipe section before the inlet of each partial component, and the downstream straight pipe section is a straight pipe section after the outlet of each partial component;
The straight pipe section comprises: the pipeline connected between every two partial components is called a straight pipe section;
Step 3: calculating the cutoff distance of the upstream straight pipe section of each local component and the cutoff distance of the downstream straight pipe section of each local component, wherein the cutoff distance of the upstream straight pipe section refers to the full development flow distance of the straight pipe section minus the length of the upstream straight pipe section, and the cutoff distance of the downstream straight pipe section refers to the full development flow distance of the straight pipe section minus the length of the downstream straight pipe section;
the fully developed flow distance: assuming that the distance between the pipeline local fittings is 'far enough', and the flow fields in the local fittings are not interfered with each other, at the moment, the inlets of the local fittings are 'fully developed flow', and the distance between the two local fittings meets the fully developed flow distance;
Step 4: obtaining a resistance value of the upstream straight pipe section and a resistance value of the downstream straight pipe section of each local component by using a resistance calculation model according to the cut-off distance of the upstream straight pipe section of each local component and the cut-off distance of the downstream straight pipe section of each local component, and obtaining the resistance value of each local component by a pressure drop method;
Obtaining a total resistance value of the pipeline system, wherein the total resistance value comprises a resistance value of an upstream straight pipe section of each local component, a resistance value of a downstream straight pipe section of each local component and a resistance value of each local component;
The pressure drop method: obtaining pressure values of two surfaces of an inlet and an outlet of a local component through a software Fluent simulation result, and obtaining pressure drop by difference;
in the resistance calculation model, the upstream straight pipe section is fitted by adopting a straight line, the downstream straight pipe sections of the reducing component and the elbow component are fitted by adopting a double Gaussian curve, and the downstream straight pipe section of the three-way component is fitted by adopting an exponential function; wherein, double Gaussian curve: Exponential curve: /(I) In the double Gaussian curve, y 0 represents the drag coefficient at 1 to 2 hydraulic radii downstream of the fitting; x c represents the cutoff distance at 1-2 hydraulic radii downstream of the fitting; h represents the steepness of the peak; w 1 represents a constant, which does not vary with speed; in the exponential curve, y 0 represents the resistance value at zero cutoff, and a represents the steepness of the curve.
2. The resistance calculation system of the ventilation air-conditioning pipeline system based on the cut-off method is characterized by comprising a least favorable loop calculation module, a cut-off position selection module, a cut-off distance calculation module and a resistance calculation module;
The most adverse loop calculation module is used for calculating a most adverse loop of the pipeline system, and the most adverse loop comprises a plurality of local components;
the most disadvantageous loop: a loop with the largest total resistance value in the ventilation air-conditioning pipeline system;
the partial member: the device is also called a local resistance component and refers to a component used for connecting different adjacent straight pipe sections of a ventilation air-conditioning pipeline, and comprises a tee component, a reducing component and an elbow component; the local component comprises an inlet and an outlet, and the inlet and the outlet are respectively connected with different straight pipe sections;
The cut-off position selection module is used for cutting off the outlet of each local component to obtain a plurality of local components comprising an upstream straight pipe section and a downstream straight pipe section, wherein the upstream straight pipe section is a straight pipe section before the inlet of each local component, and the downstream straight pipe section is a straight pipe section after the outlet of each local component;
The straight pipe section comprises: the pipeline connected between every two partial components is called a straight pipe section;
the cut-off distance calculation module is used for calculating the cut-off distance of an upstream straight pipe section of each local component and the cut-off distance of a downstream straight pipe section of each local component, wherein the cut-off distance of the upstream straight pipe section refers to the full development flow distance of the straight pipe section minus the length of the upstream straight pipe section, and the cut-off distance of the downstream straight pipe section refers to the full development flow distance of the straight pipe section minus the length of the downstream straight pipe section;
the fully developed flow distance: assuming that the distance between the pipeline local fittings is 'far enough', and the flow fields in the local fittings are not interfered with each other, at the moment, the inlets of the local fittings are 'fully developed flow', and the distance between the two local fittings meets the fully developed flow distance;
The resistance calculation module is used for calculating a total resistance value of the pipeline system, wherein the total resistance value comprises a resistance value of an upstream straight pipe section of each local component, a resistance value of a downstream straight pipe section of each local component and a resistance value of each local component, the resistance value of the upstream straight pipe section and the resistance value of the downstream straight pipe section of each local component are obtained by using a resistance calculation model according to the cut-off distance of each upstream straight pipe section and the cut-off distance of the downstream straight pipe section respectively, and the resistance value of each local component is obtained according to a pressure drop method;
The pressure drop method: obtaining pressure values of two surfaces of an inlet and an outlet of a local component through a software Fluent simulation result, and obtaining pressure drop by difference;
in the resistance calculation model, the upstream straight pipe section is fitted by adopting a straight line, the downstream straight pipe sections of the reducing component and the elbow component are fitted by adopting a double Gaussian curve, and the downstream straight pipe section of the three-way component is fitted by adopting an exponential function; wherein, double Gaussian curve: Exponential curve: /(I) In the double Gaussian curve, y 0 represents the drag coefficient at 1 to 2 hydraulic radii downstream of the fitting; x c represents the cutoff distance at 1-2 hydraulic radii downstream of the fitting; h represents the steepness of the peak; w 1 represents a constant, which does not vary with speed; in the exponential curve, y 0 represents the resistance value at zero cutoff, and a represents the steepness of the curve.
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