A PCM heat exchanger flow channel selection optimization method and device

Abstract

The application discloses a PCM heat exchanger flow channel type selection optimization method and device, and is implemented according to the following steps: S1, obtaining the logarithmic mean temperature difference of the whole PCM heat exchanger according to the preset plate heat exchanger shape size, import and export air temperature and phase change point temperature of the PCM phase change material, and establishing an energy equation based on the logarithmic mean temperature difference; S2, solving the energy equation by using a numerical iteration method, and obtaining the flow channel design parameters most meeting the conditions by using a difference matrix; S3, obtaining a thermal performance factor according to the flow channel design parameters, repeating the above step S2 for different air flow channel spacings, obtaining multiple groups of air spacing and corresponding PCM heat exchange plate thickness combinations, and obtaining the optimal flow channel design parameters according to a thermal performance factor matrix, in combination with the range limitation of the air spacing and the corresponding PCM heat exchange plate thickness. The application solves the problems of low utilization efficiency of the phase change material and low energy conversion efficiency of the PCM heat exchanger designed by the current design type selection optimization method.

Description

Technical Field

[0001] This invention belongs to the field of air heat exchanger technology, specifically relating to a method and apparatus for optimizing the flow channel selection of a PCM heat exchanger. Background Technology

[0002] Phase change materials (PCMs) are materials that can provide latent heat when their state of matter changes while the temperature remains constant. This is based on the large enthalpy change of PCMs near their phase change point, which allows them to absorb, store, or release large amounts of heat energy within a relatively small temperature range. PCM-based phase change energy storage technology has become a hot research topic in recent years, with broad engineering applications, such as solar energy storage and waste heat recovery, and peak shaving and valley filling in the power grid. For example, in building ventilation systems, PCMs serve as a heat exchange medium, used to cool or heat air through PCM heat exchangers, acting as temperature control devices to replace or partially replace air conditioning, thereby achieving energy savings. The application process involves the solidification (energy charging) and melting (energy releasing) of the PCM. PCMs solidify and release heat at night when temperatures are lower, and melt and absorb heat during the day when temperatures are higher, cooling the building space. Through convective heat transfer, this solidification-melting cycle reduces temperature changes inside the building, indirectly reducing the building's cooling / heating energy consumption.

[0003] Flow channel design and selection is one of the key technologies for PCM heat exchanger applications. Many studies have shown that a suitable PCM heat exchanger flow channel design can significantly improve heat exchange efficiency, reduce phase change time, make the heating / cooling process more stable and efficient, and reduce dependence on materials or external heat pump systems.

[0004] There is considerable research on PCM plate heat exchangers, but few practical applications exist. Research often focuses on the flow heat transfer of simplified units within microchannels and the heat storage and release characteristics of phase change materials, lacking research on the overall design and selection of the flow channels. However, the design and selection of the flow channels determine the performance and energy consumption of the heat exchanger, which is crucial for commercial applications.

[0005] Currently, there is no commercial software or open-source code available to perform this task; in fact, selection tools are a core technology within the industry. Especially in the field of phase change material energy storage, there is almost no information in publicly available literature or patents concerning the design and selection of PCM heat exchangers. Summary of the Invention

[0006] The purpose of this invention is to provide a method and apparatus for optimizing the flow channel selection of a PCM heat exchanger, in order to solve the problems of low utilization efficiency of phase change materials and low energy conversion efficiency in PCM heat exchangers designed by current optimization methods.

[0007] This invention adopts the following technical solution: a method for optimizing the flow channel selection of a PCM heat exchanger, implemented according to the following steps:

[0008] S1. Based on the preset plate heat exchanger dimensions, inlet and outlet air temperatures, and the phase change point temperature of the PCM phase change material, obtain the overall logarithmic average temperature difference of the PCM heat exchanger, and establish an energy equation based on the logarithmic average temperature difference.

[0009] S2. Under the initial guess of the air channel spacing, the energy equation is solved using a numerical iteration method, and the channel design parameters that best meet the conditions are obtained using the difference matrix. The channel design parameters include the number of PCM heat exchange plates 2 and the thickness p of the PCM heat exchange plate 2 corresponding to the air spacing a.

[0010] S3. Based on the flow channel design parameters, obtain the thermal performance factor. Repeat step S2 above for different air flow channel spacings to obtain multiple combinations of air spacing and corresponding PCM heat exchange plate 2 thicknesses, thereby obtaining multiple thermal performance factors and a thermal performance factor matrix. Based on the thermal performance factor matrix, and combined with the range limitation of air spacing and corresponding PCM heat exchange plate 2 thickness, the optimal flow channel design parameters are obtained.

[0011] Furthermore, the logarithmic mean temperature difference in step S1 is obtained by the following formula:

[0012]

[0013] Where, ΔT ap Logarithmic mean temperature difference, T air，i ,T air，o It refers to the inlet and outlet air temperature, while PCT is the phase change point temperature of the phase change material.

[0014] The energy equation based on the logarithmic mean temperature difference is expressed as follows:

[0015]

[0016] Where q is the heat exchange power. It is the inlet air mass flow rate, Cp air It is the average specific heat capacity of the air inside the heat exchanger, T air，o It is the inlet air temperature, T air，i U is the outlet air temperature, U is the overall heat transfer coefficient, and A is the total heat transfer area.

[0017] Furthermore, in step S2, the energy equation based on the logarithmic mean temperature difference is solved iteratively, and the expression for Res[j] in the resulting difference matrix is ​​as follows:

[0018]

[0019] Where j represents the j-th iteration, and q(N) is the heat exchange power when the number of PCM plates is N.

[0020] Furthermore, in step S3, the calculation formula for η[i] in the thermal performance factor matrix is ​​as follows:

[0021]

[0022] Where i represents the i-th set of flow channel design parameters, Nu r f r A r These are the Nusselt number, friction factor, and heat transfer area of ​​the heat exchanger under reference operating conditions.

[0023] The second technical solution adopted in this invention is a PCM heat exchanger flow channel selection optimization device, comprising:

[0024] The design point input module 410 is used for the program to obtain air inlet and outlet temperatures, flow rates, pressures, working temperatures of phase change materials, and overall dimensional constraints of the heat exchanger.

[0025] The physical property query module 420 is used by the program to obtain the physical properties of air under the stated temperature and pressure conditions;

[0026] The flow heat transfer calculation module 430 is used to construct and solve the energy equation of the flow channel based on the operating condition data and physical property data of modules 410 and 420, and obtain the heat exchanger design parameters under the corresponding operating conditions.

[0027] The flow channel optimization module 440 is used to establish a thermal performance factor matrix based on the thermal performance factors corresponding to the heat exchanger design parameters designed by the flow heat transfer calculation module 430, and to optimize the geometric parameters of the heat exchanger to obtain the optimized design parameters.

[0028] The result output module 450 is used to output the optimized design parameters and their corresponding operating conditions and intermediate data generated during the calculation process to the designers.

[0029] The third technical solution adopted in this invention is a PCM heat exchanger flow channel selection optimization device, including a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the computer program, it implements a PCM heat exchanger flow channel selection optimization method.

[0030] The fourth technical solution adopted in this invention is a computer-readable storage medium storing a computer program, which, when executed by a processor, implements a method for optimizing the flow path selection of a PCM heat exchanger.

[0031] The beneficial effects of this invention are as follows: This invention establishes a PCM heat exchanger flow channel selection optimization analysis model based on the logarithmic mean temperature difference method; through numerical iteration, the difference matrix is ​​obtained, and then the design parameters of the internal flow channels of the heat exchanger device are obtained. Furthermore, the thermal performance factor matrix for each operating condition is calculated, and the selection and optimization work is carried out using the thermal performance factor matrix, greatly saving calculation time and design costs. The technical solution proposed in this invention can help engineers quickly obtain the geometric dimensions of the internal flow channels of the heat exchanger and other relevant overall performance parameters of the heat exchanger. The optimization process has low computational load, high efficiency, and is easy to use. This method enriches the design methods of plate heat exchangers, makes up for the deficiencies of current phase change material heat exchanger design and selection optimization methods, and effectively improves the utilization efficiency and energy conversion efficiency of phase change materials. Attached Figure Description

[0032] Figure 1 This is a simplified schematic diagram of the flow channel of the PCM heat exchanger of the present invention;

[0033] Figure 2 This is a flowchart of a PCM heat exchanger flow channel selection optimization method according to the present invention;

[0034] Figure 3 This is a diagram showing the design parameter results in the embodiment;

[0035] Figure 4 This is a schematic diagram of the module connection relationship of a PCM heat exchanger flow channel selection optimization device according to the present invention.

[0036] Among them, 1. outer shell skin; 2. PCM heat exchange plate. Detailed Implementation

[0037] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0038] This invention provides a method for optimizing the flow channel selection of a PCM heat exchanger, such as... Figure 1 The diagram shows a simplified flow path of a PCM heat exchanger. Multiple PCM heat exchange plates 2 are arranged in parallel at intervals inside the PCM heat exchanger. The width of the air flow path is a (m), the thickness of the PCM heat exchange plates 2 is p (m), and the number of PCM heat exchange plates 2 is N. The overall length of the PCM heat exchanger is L (m), the width is W (m), and the height is H (m). Insulated outer shells 1 are provided on the upper and lower sides of the PCM heat exchanger.

[0039] A flowchart of a PCM heat exchanger flow channel selection optimization method according to the present invention is shown below. Figure 2 As shown, its purpose is to determine the overall dimensions of a heat exchanger (length L, width W, height H) and the inlet and outlet air conditions (temperature T). air，i ,T air，oInlet quality flow rate In the case of ), the structural design parameters of the internal flow channel of the heat exchanger are given, namely the thickness p and spacing a of the PCM heat exchange plate 2.

[0040] To achieve the above objectives, the present invention provides a PCM heat exchanger flow channel selection optimization method implemented according to the following steps:

[0041] S1. Based on the preset plate heat exchanger dimensions, inlet and outlet air temperatures, and phase change point temperature of the PCM phase change material, obtain the overall logarithmic average temperature difference of the PCM heat exchanger, and establish an energy equation based on this logarithmic average temperature difference.

[0042] S2. Under the initially assumed airflow channel spacing condition, the energy equation is solved using a numerical iteration method. The difference matrix is ​​used to obtain the most suitable channel design parameters, including the number of PCM heat exchanger plates 2 and the thickness p of the PCM heat exchanger plate 2 corresponding to the airflow spacing a. The initial assumption is a hypothetical initial solution, or an initial solution based on experience. This value is given by the designer and is equivalent to the initial solution of the implicit equation. The more accurate the initial solution, the faster the accurate solution is obtained. The difference matrix is ​​obtained based on the established energy equation.

[0043] This invention uses a numerical iterative method to solve the governing equations, ultimately obtaining the thickness p of the PCM heat exchanger plate 2 corresponding to each air gap a. The calculation flowchart is shown below. Figure 2 As shown.

[0044] The entire calculation process can be summarized as follows: Using existing boundary conditions, such as inlet and outlet temperatures, airflow, PCM phase change point, overall dimensions of the heat exchanger (heat exchanger assembly space), and the thermal properties of the PCM and air, a series of iterative calculations are performed to obtain the number N and thickness p of the PCM heat exchange plates 2 corresponding to each airflow channel spacing 'a', i.e., the key design parameters of the flow channel. During the iteration process, it is necessary to solve for the Reynolds number Re, friction factor f, Nusselt number Nu, and convective heat transfer coefficient h of the flow channel. air The calculation method has been given in detail above. Engineering technicians can use it directly or choose alternative empirical formulas based on the actual engineering situation, but the basic idea is similar.

[0045] The above are the flow channel design parameters calculated assuming an air gap 'a'. This means that under the given heat exchanger dimensions and boundary conditions, there are multiple combinations of 'a' and 'p' that meet the design requirements. Therefore, it is necessary to optimize these multiple solutions, that is, to select the optimal design parameters.

[0046] S3. Based on the flow channel design parameters, obtain the thermal performance factor. Repeat step S2 for different air channel spacings to obtain the thermal performance factor for each air channel spacing. Each round of calculation yields a set of air channel design parameters, which correspond to a thermal performance factor. Multiple combinations of air spacings and corresponding PCM heat exchanger plate 2 thicknesses are obtained, resulting in a thermal performance factor matrix. Based on this thermal performance factor matrix, and combined with the range limitations of the air spacing and the corresponding PCM heat exchanger plate 2 thickness, the optimal flow channel design parameters are obtained.

[0047] The selection process introduces an evaluation index to assess the quality of the flow channel design, namely the target parameters. By comparing the thermal performance factor η corresponding to each set of solutions (a and p), and considering the range constraints of a and p, an optimal solution can be found, representing the best flow channel design parameters. The minimum value of the difference matrix and its corresponding N represent the correct flow channel design dimensions under that operating condition. Based on a and N, the thickness p and thermal performance factor of the corresponding PCM heat exchanger plate 2 are calculated.

[0048] In some embodiments, in step S1, it is first necessary to establish the governing equations of the PCM heat exchanger flow channel selection optimization analysis model, that is, the overall energy equation:

[0049]

[0050] Where q is the heat exchange power (unit: W), It is the inlet air mass flow rate (unit: kg / s), Cp air It is the average specific heat capacity of the air inside the heat exchanger (unit: J / (kg-K)), T air，o It is the inlet air temperature (unit: K), T air，i It is the outlet air temperature (unit: K); U is the overall heat transfer coefficient (unit: W / (m²)). 2 K)) is related to the number N of PCM heat exchange plates 2; A is the total heat exchange area, which is also a function of the number N of PCM heat exchange plates 2.

[0051] Where, ΔT ap (Unit: K) Logarithmic mean temperature difference method (LMTD) is used:

[0052]

[0053] Among them, T air，i ,T air，o It refers to the inlet and outlet air temperature, and PCT is the phase change point temperature of the phase change material (unit: K).

[0054] In some embodiments, in step S2, the energy equation based on the logarithmic mean temperature difference is solved iteratively, and the expression for Res[j] in the resulting difference matrix is ​​as follows:

[0055]

[0056] Where j represents the j-th iteration, and q(N) is the heat transfer power (in W) when the number of PCM plates is N.

[0057] The flow channel design parameters are calculated using a difference matrix: the minimum value of the difference matrix and its corresponding N represent the correct flow channel design dimensions under that operating condition. Based on a and N, the thickness p of the corresponding PCM heat exchanger plate 2 and the thermal performance factor η are calculated.

[0058] In some embodiments, the formula for calculating η[i] in the thermal performance factor matrix in step S3 is as follows:

[0059]

[0060] i represents the i-th group of flow channel design parameters, Nu r f r A r These are the Nusselt number, friction factor, and heat transfer area of ​​the heat exchanger under reference operating conditions (the number of plates is N, where N can be any positive integer).

[0061] The Nusselt number Nu is calculated using the following formula for laminar flow:

[0062] Nu = 7.54

[0063] For turbulence:

[0064]

[0065] Here, Pr is the Prandtl number, taken as a constant of 0.71, and μ... air The average dynamic viscosity (Pa·s) of the air inside the heat exchanger.

[0066] The friction factor f is obtained using the following formula:

[0067] f = (0.79lnRe - 1.64) -2

[0068] The applicable scope is 3000. <Re<5×10 6 Re is the Reynolds number of the flow channel.

[0069] This invention also provides a PCM heat exchanger flow channel selection optimization device, such as... Figure 4As shown, the system includes: a design point input module 410, used by the program to obtain air inlet and outlet temperatures, flow rates, pressures, operating temperatures of the phase change material, and overall dimensional constraints of the heat exchanger; a physical property query module 420, used by the program to obtain the physical properties of the air under the above temperature and pressure conditions, including density, dynamic viscosity, thermal conductivity, specific heat capacity, enthalpy, and specific heat capacity data of the phase change material; a flow heat transfer calculation module 430, which, based on the operating condition data and physical property data from modules 410 and 420, constructs and solves the energy equation of the flow channel to obtain the heat exchanger design parameters under the corresponding operating conditions, namely, the channel spacing, the number of PCM plates, and the PCM plate thickness; a flow channel optimization module 440, which establishes a thermal performance factor matrix based on the thermal performance factors corresponding to the heat exchanger parameters designed by the calculation module 430, optimizes the geometric parameters of the heat exchanger, and obtains the final design parameters; and a result output module 450, which outputs the optimized design parameters and their corresponding operating conditions, as well as intermediate data generated during the calculation process, such as heat transfer coefficient, pressure drop, and total heat transfer area, to the designer.

[0070] The present invention also provides a PCM heat exchanger flow channel selection optimization device, including a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the computer program, it implements a PCM heat exchanger flow channel selection optimization method.

[0071] The present invention also provides a computer-readable storage medium storing a computer program that, when executed by a processor, implements a method for optimizing the flow path selection of a PCM heat exchanger.

[0072] Example

[0073] Operating conditions: Air inlet temperature 27°C, outlet temperature 25°C, PCM phase change point temperature 20°C, overall heat exchanger dimensions (length 0.5m, width 0.3m, height 0.3m), with a total of 3 PCM heat exchangers. Maximum power 3kW (including fan coil units), required to operate within 10%–90% of maximum power (corresponding to airflow of 400–1100 m³ / h). 3 Calculate the dimensions of the internal flow channels of the heat exchanger ( / h) and select the optimal solution.

[0074] The specific calculation steps are as follows:

[0075] Step 1: First, based on the air inlet and outlet temperatures and the PCM phase transition temperature, consult the property table to obtain the average density, specific heat capacity, dynamic viscosity, and thermal conductivity ρ of the air. air =1.18kg / m 3 Cp air =1006.3 J / (kgK), μ air =1.85×10 -5 Pa·s,λ air =0.026322W / (mK), and the density and latent heat ρ of PCM material. PCM =857.34kg / m 3 H PCM =230kJ / kg.

[0076] Step 2, when the inlet flow rate is 400m³ 3 When the air gap is 6 mm and the number of PCM heat exchanger plates 2 is 1, calculate the Reynolds number Re, friction factor f, Nusselt number Nu, and convective heat transfer coefficient h of the flow channel unit. air Substitute the values ​​into the energy control equation, calculate the difference between the left and right sides of the equation, and store the result.

[0077] Step 3: Following the calculation method in Step 2, calculate N iteratively from 1 to 100 to obtain the difference matrix.

[0078] Step 4: The minimum value in the difference matrix and its corresponding N are the values ​​under this operating condition (inlet flow rate of 400 m³ / s). 3 The correct flow channel design dimensions are given (with an air gap of 6 mm). Based on a and N, the thickness p and thermal performance factor η of the corresponding PCM heat exchanger plate 2 are calculated.

[0079] Step 5, when the inlet flow rate is 400m³ 3 / h, the air gap changes from 6mm to 35mm, and the calculation method of steps 2-4 is repeated to obtain the correct flow channel design dimensions and the corresponding thermal performance factor η.

[0080] Step 6: As a preferred option, based on literature and experience, the selection constraints are as follows:

[0081] ① The airflow channel is between 8 and 20 mm;

[0082] ②The thickness of the PCM heat exchange plate 2 is between 2 and 15 mm;

[0083] ③ The thermal performance factor should be as large as possible;

[0084] The above constraints can be determined by technical personnel based on the actual project conditions. Therefore, an inlet flow rate of 400 m³ / h can be selected. 3 Under the condition of / h, the optimal flow channel design dimensions are: air gap a = 15mm, PCM heat exchange plate 2 thickness is 5.357mm, and PCM total mass is 28.935kg.

[0085] Inbound traffic from 400m 3 / h changed to 1100m 3 / h, repeat steps 2-6, from Figure 3It can be seen that the optimal size of the heat exchanger can be obtained under different power conditions.

[0086] This invention establishes an optimization analysis model for PCM heat exchanger flow channel selection based on the logarithmic mean temperature difference method. Through numerical iteration, a difference matrix is ​​obtained, leading to the determination of design parameters for the internal flow channels of the heat exchanger. Furthermore, the thermal performance factor matrix for each operating condition is calculated. Using this matrix for selection and optimization significantly reduces computation time and design costs.

[0087] The technical solution proposed in this invention can help engineers quickly obtain the geometric dimensions of the internal flow channels of a heat exchanger and other relevant overall performance parameters of the heat exchanger. The optimization process involves minimal computation, is highly efficient, and is easy to use. This method enriches the design methods for plate heat exchangers, compensates for the shortcomings of current design and selection optimization methods for phase change material heat exchangers, and effectively improves the utilization efficiency and energy conversion efficiency of phase change materials.

Claims

1. A method for optimizing the flow channel selection of a PCM heat exchanger, characterized in that, Follow these steps: S1. Based on the preset plate heat exchanger dimensions, inlet and outlet air temperatures, and the phase change point temperature of the PCM phase change material, obtain the overall logarithmic average temperature difference of the PCM heat exchanger, and establish an energy equation based on the logarithmic average temperature difference. S2. Under the initially guessed airflow channel spacing condition, the energy equation is solved using a numerical iteration method, and the channel design parameters that best meet the conditions are obtained using the difference matrix. The channel design parameters include the number of PCM heat exchange plates and the thickness p of the PCM heat exchange plate corresponding to the air spacing a. The iterative solution to the energy equation based on the logarithmic mean temperature difference yields the difference matrix. The expression is as follows: ， Where j represents the j-th iteration, and q(N) is the heat transfer power when the number of PCM plates is N. It is the inlet air mass flow rate, It is the average specific heat capacity of the air inside the heat exchanger. It is the outlet air temperature, It is the inlet air temperature; S3. Based on the flow channel design parameters, obtain the thermal performance factor. Repeat step S2 above for different air flow channel spacings to obtain multiple combinations of air spacing and corresponding PCM heat exchange plate thicknesses, thereby obtaining multiple thermal performance factors, and then obtaining a thermal performance factor matrix. Based on the thermal performance factor matrix, and combined with the range limitation of air spacing and corresponding PCM heat exchange plate thickness, the optimal flow channel design parameters are obtained. In the thermal performance factor matrix The calculation formula is as follows: ， Where i represents the i-th set of flow channel design parameters, Nu r f r A r These are the Nusselt number, friction factor, and heat transfer area of ​​the heat exchanger under reference operating conditions.

2. The PCM heat exchanger flow channel selection and optimization method as described in claim 1, characterized in that, The logarithmic mean temperature difference in step S1 is obtained by the following formula: ， in, It is the logarithmic mean temperature difference. , It refers to the inlet and outlet air temperature, while PCT is the phase change point temperature of the phase change material. The energy equation based on the logarithmic mean temperature difference is expressed as follows: ， in, For heat exchange power, It is the inlet air mass flow rate, It is the average specific heat capacity of the air inside the heat exchanger. It is the outlet air temperature, It is the inlet air temperature. It is the overall heat transfer coefficient. It is the total heat exchange area.

3. A PCM heat exchanger flow channel selection and optimization device, characterized in that, The PCM heat exchanger flow channel selection optimization method according to claim 1 includes: The design point input module (410) is used for the program to obtain the air inlet and outlet temperatures, flow rates, pressures, working temperatures of the phase change material, and overall size constraints of the heat exchanger. The physical property query module (420) is used by the program to obtain the physical properties of air under the temperature and pressure conditions. The flow heat transfer calculation module (430) is used to construct and solve the energy equation of the flow channel based on the operating condition data and physical property data of the design point input module (410) and the physical property query module (420), and obtain the heat exchanger design parameters under the corresponding operating conditions. The flow channel optimization module (440) is used to establish a thermal performance factor matrix based on the thermal performance factors corresponding to the heat exchanger design parameters designed by the flow heat transfer calculation module (430), and to optimize the geometric parameters of the heat exchanger to obtain the optimized design parameters. The result output module (450) is used to output the optimized design parameters and their corresponding operating conditions and intermediate data generated during the calculation process to the designers.

4. A PCM heat exchanger flow channel selection optimization device, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that, When the processor executes the computer program, it implements a PCM heat exchanger flow channel selection optimization method as described in any one of claims 1 to 2.

5. A computer-readable storage medium storing a computer program, characterized in that, When the computer program is executed by the processor, it implements the PCM heat exchanger flow channel selection optimization method as described in any one of claims 1 to 2.

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