Two-phase flow pattern recognition system based on ERT and heat transfer coefficient model construction method

Abstract

The invention discloses an ERT-based two-phase flow pattern recognition system and a heat transfer coefficient model construction method. The ERT-based two-phase flow pattern recognition system comprises a refrigerant liquid storage tank, a semiconductor chilling plate, a heating diaphragm, a transparent polytetrafluoroethylene tube, a subcooler, a gear pump, a buffer, a filter, a flowmeter, a preheating section inlet pressure sensor, a preheating section inlet thermocouple, a preheating section, an inlet copper tube, an experiment section inlet thermocouple, an experiment section and an experiment section thermocouple. The device comprises an experiment section inlet pressure sensor, a pressure difference sensor, an outlet copper pipe, an experiment section outlet thermocouple, a signal transmission cable, a data acquisition and processing unit, an image reconstruction unit, a post-processing computer, a condenser, an ice-water mixing barrel, a water pipe, a water pump and a three-way valve. The flow pattern distribution condition in the opaque copper pipe can be monitored and displayed in real time, the displayed flow pattern image has high resolution, and the heat transfer coefficient prediction method constructed based on the flow pattern in the opaque copper pipe is simple and high in applicability.

Description

Technical Field

[0001] The present invention belongs to the technical field of thermal management of airborne electronic equipment, and in particular relates to a method for identifying two-phase flow patterns (such as gas-liquid) inside an opaque copper tube and a heat transfer coefficient prediction model constructed based on the flow pattern. Background Art

[0002] As the thermal management system of modern airborne electronic equipment faces the challenge of large heat flux density and small heat dissipation space, microchannel (D h <3mm) has shown significant advantages in the aviation field. In the boiling flow process, two-phase flow phenomenon is extremely common. Accurate monitoring of its internal flow pattern structure and the construction of a heat transfer coefficient prediction model are of great significance for optimizing process control. However, traditional detection methods can hardly meet the needs of real-time monitoring of flow patterns in opaque microchannels. In recent years, electrical resistance tomography (ERT), as an emerging detection technology, provides a new solution to this problem by analyzing the conductivity distribution of the sensitive field to obtain flow pattern distribution information. For example, the invention patent "An adaptive multiphase flow measurement device based on ERT sensor" with application number CN201710976992.5 proposes a method for constructing an image of the flow pattern inside a conductor pipe using an adaptively adjusted ERT sensor structure. However, this method has two main limitations: one is that the electrodes installed on the inner wall of the pipe will change the original flow state; the other is that the structure is only applicable to flow pattern measurements of insulated conductor pipes. Another invention patent with application number CN201410328267.3, "Multiphase flow visualization test method based on ultrasonic Doppler and electrical multi-sensor", is applicable to insulating material pipes, but due to the use of an open-loop control system, the accuracy of its reconstructed flow pattern image is relatively low. In addition, there are no patent reports on the construction of a heat transfer coefficient prediction model based on flow patterns.

[0003] Therefore, developing a system that can monitor the flow pattern in opaque copper tubes in real time and a method to predict the heat transfer coefficient based on the flow pattern information is crucial to optimizing the design of cooling systems for airborne electronic equipment. Summary of the invention

[0004] In view of the shortcomings of the above-mentioned prior art, the present invention aims to provide a two-phase flow pattern identification system based on ERT and a method for constructing a heat transfer coefficient model. This method can not only meet the requirements of online real-time reconstruction of the flow pattern image in the opaque copper tube and high resolution of the reconstructed graphics, but also proposes a technical solution for constructing a heat transfer coefficient prediction model based on flow pattern characteristics, which helps to optimize the design of the cooling system of airborne electronic equipment and improve the thermal management efficiency and reliability of the system.

[0005] To achieve the above object, the technical solution adopted by the present invention is as follows: a two-phase flow pattern identification system based on ERT, including a refrigerant circulation loop and a cooling water circulation loop;

[0006] The refrigerant circulation loop comprises a refrigerant storage tank (1), a refrigerant inlet and a refrigerant outlet on a subcooler (5), a gear pump (6), a flow meter (9), a preheating section (12), an inlet copper tube (13), a test section (15), an outlet copper tube (19), and a condenser (24), which are connected end to end in sequence through a transparent polytetrafluoroethylene tube (4); the preheating section (12) is provided with a preheating section inlet pressure sensor (10) and a preheating section inlet thermocouple (11); the inlet copper tube (13) is provided with an test section inlet thermocouple (14); the test section (15) is provided with an test section thermocouple (16), a test section inlet pressure sensor (17), and a differential pressure sensor (18); the outlet copper tube (19) is provided with an test section outlet thermocouple (20); the test section (15), a data acquisition and processing unit (22), and an image reconstruction unit are electrically connected to a post-processing computer (23) through a signal transmission cable (21);

[0007] The cooling water circulation loop comprises an ice-water mixing barrel (25), a water pump (27), a cooling water inlet and a cooling water outlet on a supercooler (5), a three-way valve (28) and a condenser (24) which are connected end to end in sequence through a water pipe (26), and a bypass path of the three-way valve (28) is connected to the ice-water mixing barrel (25).

[0008] On the basis of the above solution, preferably, a buffer (7) for alleviating vibration caused by the gear pump (6) is provided between the gear pump (6) and the flow meter (9).

[0009] Based on the above solution, preferably, a filter (8) is provided between the buffer (7) and the flow meter (9) for filtering any particulate matter that may potentially damage the gears of the flow meter (9).

[0010] On the basis of the above scheme, preferably, the semiconductor cooling sheet (2) and the heating membrane (3) are respectively attached to the top and bottom of the refrigerant liquid storage tank (1), and the two are located on opposite sides of the refrigerant liquid storage tank (1), and are used for fine adjustment of the fluid pressure in the experimental section within a small range.

[0011] Based on the above scheme, as a preferred embodiment, the pin of the inlet thermocouple (14) of the experimental section is in contact with the outer wall of the inlet copper tube (13) and the outlet thermocouple (20) of the experimental section is in contact with the outer wall of the outlet copper tube (19) and is bonded by an insulating tape.

[0012] On the basis of the above scheme, as a preference, the experimental section (15) uses an opaque copper tube (1501) as a substrate, which is wrapped with a thermal conductive tape (1502), a Ni-Cr wire (1503), a thermal conductive tape (1502), a polyimide film (1504) and a rubber-plastic insulation tube (1505) in sequence from the inside to the outside.

[0013] On the basis of the above scheme, preferably, the electrode array (1506) of the experimental section (15) is composed of 6 groups of electrodes (1507) evenly distributed in the axial direction of the opaque copper tube (1501), and each group of electrodes (1507) is composed of 8 independent electrodes (1507) evenly distributed along the circumference of the opaque copper tube (1501); the electrode array (1506) is electrically connected to the electrode selection control module (2202).

[0014] Based on the above scheme, preferably, the electrode array (1506) is tightly connected to the opaque copper tube (1501) through an insulating seal (1508); the height of each electrode (1507) in the electrode array (1506) is consistent with the wall thickness of the opaque copper tube (1501).

[0015] A method for constructing a heat transfer coefficient model based on a two-phase flow pattern identification system comprises the following steps:

[0016] ①Measure the heat transfer coefficient of the experimental section under different working conditions and organize the measured data into a database;

[0017] ② Based on the existing heat transfer coefficient model with excellent prediction performance, the dimensionless parameters with significant correlation with the heat transfer coefficient are screened out;

[0018] ③ Using the dimensionless parameters selected in step ②, construct a preliminary form of the heat transfer coefficient prediction model;

[0019] ④ Use the least squares method and error analysis method to evaluate the accuracy of the initially constructed heat transfer coefficient prediction model and select the model with the best prediction performance;

[0020] ⑤ Repeatedly apply the least squares method and error analysis method to adjust or eliminate the dimensionless parameters selected in step ②, further optimize the accuracy of the heat transfer coefficient prediction model, and finally determine the heat transfer coefficient model with the best prediction performance.

[0021] Beneficial effects of the present invention:

[0022] 1. A complete monitoring system was designed to identify the flow pattern of boiling flow in opaque thermal conductive copper tubes, which can realize online real-time reconstruction of the flow pattern image in the copper tube.

[0023] 2. The data acquisition and processing unit used can not only effectively amplify the weak signal output by the ERT sensor, improve the signal-to-noise ratio, and ensure the accuracy and integrity of the signal, but also effectively suppress the artifacts that may be generated during the image reconstruction process, thereby greatly improving the spatial resolution and contrast of the image.

[0024] 3. Based on the flow pattern images reconstructed by the ERT system and the measured heat transfer coefficient database, a method for predicting the heat transfer coefficient in opaque copper tubes based on flow pattern characteristics was developed. This method is simple and has strong applicability. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a schematic diagram of the two-phase flow pattern identification system based on ERT of the present invention;

[0026] Figure 2 It is a schematic diagram of the experimental section structure of the present invention;

[0027] Figure 3 It is a schematic diagram of the electrode operation of the present invention;

[0028] Figure 4 It is a working principle diagram of the ERT system of the present invention;

[0029] Figure 5 It is a schematic diagram of the definition of the dryness angle of the present invention.

[0030] In the figure: refrigerant liquid storage tank 1, semiconductor refrigeration sheet 2, heating diaphragm 3, transparent polytetrafluoroethylene tube 4, supercooler 5, gear pump 6, buffer 7, filter 8, flow meter 9, preheating section inlet pressure sensor 10, preheating section inlet thermocouple 11, preheating section 12, inlet copper tube 13, experimental section inlet thermocouple 14, experimental section 15, experimental section thermocouple 16, experimental section inlet pressure sensor 17, differential pressure sensor 18, outlet copper tube 19, experimental section outlet thermocouple 20, signal transmission cable 21, data acquisition and processing unit 22, image reconstruction unit and post-processing computer 23, condenser 24, ice-water mixing barrel 25, water pipe 26, water pump 27, three-way valve 28. DETAILED DESCRIPTION

[0031] In order to facilitate the understanding of those skilled in the art, the present invention is further described below in conjunction with embodiments and drawings. The contents mentioned in the implementation modes are not intended to limit the present invention.

[0032] The present invention is a two-phase flow pattern identification system based on ERT, such as Figure 1 and 2As shown, the semiconductor cooling sheet 2 and the heating diaphragm 3 are respectively attached to the upper and lower parts of the refrigerant storage tank 1. The two are located on opposite sides of the refrigerant storage tank 1, and can realize fine adjustment of the fluid pressure in the experimental section in a small range. Both are powered by a DC power supply with a maximum of 12V. By adjusting the size of the power supply voltage, the cooling or heating rate of the refrigerant storage tank 1 can be flexibly controlled. Specifically, the semiconductor cooling sheet installed above the refrigerant storage tank 1 can cause the gas in the upper part of the tank to condense on the wall surface, thereby reducing the pressure of the refrigerant in the tank; while the heating diaphragm installed below the refrigerant storage tank 1 can cause the liquid refrigerant in the tank to be slightly gasified, thereby increasing the pressure of the refrigerant in the tank. The refrigerant storage tank 1, the supercooler 5, the gear pump 6, the buffer 7, the filter 8, the flow meter 9, the preheating section 12, the inlet copper tube 13, the experimental section 15, the outlet copper tube 19 and the condenser 24 components are interconnected by a transparent polytetrafluoroethylene tube 4. The refrigerant in the refrigerant storage tank 1 is first cooled to about 5°C by the subcooler 5 to prevent the refrigerant entering the gear pump 6 from being locally vaporized between the teeth. Subsequently, the gear pump 6 pushes the completely cooled refrigerant to the flow meter 9, the preheating section 12, the experimental section 15, the condenser 24 in sequence, and finally returns to the refrigerant storage tank 1 to complete a cycle. The buffer 7 is used to alleviate the vibration caused by the gear pump 6. The filter 8 installed in front of the flow meter 9 is used to filter any particulate matter that may damage the gears of the flow meter 9. The liquid refrigerant measured by the flow meter 9 is heated to a specific temperature in the preheating section 12, and then enters the experimental section 15 with a certain degree of subcooling for secondary heating. Subsequently, the refrigerant enters the condenser 24 in a gas-liquid two-phase state, is completely condensed through heat exchange with the cooling water, and then flows back to the refrigerant storage tank 1. The flow rate of the cooling water is controlled by adjusting the opening of the three-way valve 28 in the system, thereby realizing a wide range of system pressure regulation.

[0033] The pins of the inlet thermocouple 14 of the experimental section and the outer wall of the inlet copper tube 13, and the outlet thermocouple 20 of the experimental section and the outer wall of the outlet copper tube 19 are all bonded by thermal insulation tape; the six experimental section thermocouples 16 are evenly distributed along the axial direction of the experimental section 15, and their pins are filled and bonded with the grooves pre-opened on the outer wall of the experimental section 15; the experimental section adopts indirect heating, that is, uniform heating is achieved by evenly winding Ni-Cr wire 1503 on the outer surface of the opaque copper tube 1501. The experimental section is wrapped with thermal conductive tape 1502, Ni-Cr wire 1503, thermal conductive tape 1502, polyimide film 1504 and rubber-plastic insulation tube 1505 from the inside to the outside. Since the surface of the copper tube is smooth, it is easy to cause incomplete winding and misalignment when the Ni-Cr wire is directly wound on its surface, resulting in increased contact thermal resistance. First, the thermal conductive tape 1502 is wound on its surface to increase the friction force, which is convenient for winding and then winding the Ni-Cr wire. At the same time, it also ensures the electrical insulation between the Ni-Cr wire 1503 and the opaque copper tube 1501, reducing the contact thermal resistance and improving the thermal conductivity. The polyimide film 1504 is used to achieve the electrical insulation of the experimental section 15. The rubber-plastic insulation tube 1505 serves as an insulation layer to help reduce heat loss. The pins of the experimental section thermocouple 16 avoid the Ni-Cr wire 1503 and are close to the bottom of the groove pre-opened on the outer wall of the opaque copper tube 1501. In order to further reduce the contact thermal resistance between the pins of the experimental section thermocouple 16 and the opaque copper tube 1501, gallium-indium (Ga-In) liquid metal is injected into the gap of the groove.

[0034] The ice-water mixing barrel 25, water pump 27, supercooler 5, three-way valve 28 and condenser 24 are connected by water pipe 26. In the cooling water circulation loop, an ice-water mixture (consisting of tap water and bottled ice cubes) is placed in the ice-water mixing barrel 25 in advance to ensure that the temperature of the liquid water delivered to the supercooler 5 by the water pump 27 is maintained in a low temperature range of 1 to 5°C. By adjusting the opening of the three-way valve 28, cooling water enters the condenser 24 at a constant flow rate and acts as a cold source to exchange heat with the gas-liquid two-phase refrigerant. Excess cooling water is directly returned to the ice-water mixing barrel 25 through the bypass path of the three-way valve 28, thereby realizing circulation.

[0035] like Figures 2 to 4As shown, the electrode array 1506 of the experimental section 15 is composed of 6 groups of electrodes 1507 evenly distributed in the axial direction of the opaque copper tube 1501, and each group of electrodes 1507 is composed of 8 independent electrodes 1507 evenly distributed along the circumference of the opaque copper tube 1501; the electrode array 1506 is electrically connected to the electrode gating control module 2202. The electrode array 1506 is tightly connected to the opaque copper tube 1501 through an insulating seal 1508, which not only ensures absolute insulation between the two, but also effectively prevents fluid leakage; the height of each electrode 1507 in the electrode array 1506 is consistent with the wall thickness of the opaque copper tube 1501, ensuring that the flow state of the opaque copper tube 1501 is not disturbed.

[0036] After the system is started, the image reconstruction unit and the post-processing computer 23 send control instructions through the digital I / O module 2205 to set the various parameters of the signal generation module 2201. The signal generated by the signal generation module 2201 is converted into a current signal through bandpass filtering and noise removal and voltage-controlled current source (VCCS). Then, the electrode gating control module 2202 selects any pair of electrodes 1507 to apply an excitation current, thereby forming a sensitive field inside the test object. The electrode gating control module 2202 sequentially replaces the electrode pairs and cyclically measures the potential difference between adjacent electrodes 1507 to obtain multi-angle conductivity distribution information. These instantaneous signals are converted into digital signals by the A / D conversion module 2204 after passing through the signal conditioning module 2203 (differential amplification, demodulation and filtering processing), and transmitted to the image reconstruction unit and the post-processing computer 23 to reconstruct the grayscale distribution in the field (i.e., the actual conductivity distribution) and generate a three-dimensional image of the time series.

[0037] The present invention provides a method for constructing a heat transfer coefficient model based on flow pattern, which is based on the flow pattern image reconstructed by the ERT system. By considering the heating length L of the experimental section heat , the inner diameter D and outer diameter D of the test section o , the average heating power Q applied to the experimental section ave , thermal conductivity of the experimental section λ test , experimental section inlet temperature t in , the outlet temperature of the experimental section t out , the outer wall temperature of the experimental section t w,o , the inner wall temperature of the experimental section t w and the fluid temperature t l , the actual heat transfer coefficient of the experimental section can be calculated using formulas (1) to (4).

[0038]

[0039]

[0040] By measuring the heat transfer coefficient of the experimental section under different working conditions and collating these data into a database, combined with the ERT system, it can be observed that different heat transfer coefficients correspond to specific flow patterns.

[0041] These observations provide important basic data for building a heat transfer coefficient prediction model. Specifically, the ERT system can intuitively display the flow characteristics under different heat transfer coefficient conditions, providing a reliable basis for model establishment and optimization.

[0042] The general form of the heat transfer coefficient model based on flow pattern characteristics is as follows:

[0043]

[0044] In the formula, θ dry is the dryness angle, which is defined as the arc angle occupied by the steam part in the flow channel cross section, such as Figure 5 As shown, its value is zero (annular flow) and stratified flow stratification angle θ strat Changes between; relative heat transfer coefficient h sp,g It can be calculated using formulas (6) to (9).

[0045]

[0046] In the formula, Re g , Pr g , g , μ g , g , l and ρ tp They represent gas phase Reynolds number, gas phase Prandtl number, gas phase thermal conductivity, gas phase dynamic viscosity, gas phase density, liquid phase density and two-phase flow reduced density respectively; x, G and α represent dryness, mass flow rate and cavitation fraction respectively.

[0047] Heat transfer coefficient of wetted perimeter h wet A preliminary form of can be expressed based on an asymptotic model of boiling flow:

[0048] h wet =[(S·h nb ) n +(h cb ) n ] 1 / n (10)

[0049] In the formula, n is the index to be fitted; S, h nb and h cb They represent the nucleate boiling suppression coefficient, the nucleate boiling heat transfer coefficient and the convection boiling heat transfer coefficient respectively, and their determination methods are as follows:

[0050]

[0051] Where P R Represents the comparative pressure, which is defined as the ratio of the system pressure to the critical pressure; M, Pr l and λ l Represent the relative molecular mass of the refrigerant, the liquid Prandtl number and the liquid thermal conductivity; Re δ and δ represent the liquid film Reynolds number and liquid film thickness respectively, and their calculation formulas are as follows:

[0052]

[0053] If the flow pattern is bubbly flow, slug flow, intermittent flow or annular flow, then θ dry = 0. For stratified wavy flow, θ dry In (0, θ strat ) changes. Delamination angle θ strat The calculation formula is as follows:

[0054]

[0055] In order to determine the best form of the heat transfer coefficient prediction model, the measured heat transfer coefficients in the database are matched one by one with the constructed heat transfer coefficient model. The model parameters are preliminarily estimated based on the least squares method (LSM), especially the exponent n in the model is fitted. The least squares method finds the best fitting parameters by minimizing the sum of the squared differences between the observed values ​​and the model predicted values. According to the results of the error analysis, the model structure or parameter range is adjusted to further optimize the model performance, and finally the model form with the best prediction performance is determined.

[0056] The present invention has many specific application paths. The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements can be made without departing from the principle of the present invention. These improvements should also be regarded as the protection scope of the present invention.

Claims

1. A two-phase flow pattern identification system based on ERT, characterized in that: It includes a refrigerant circulation loop and a cooling water circulation loop; The refrigerant circulation loop comprises a refrigerant storage tank (1), a refrigerant inlet and a refrigerant outlet on a subcooler (5), a gear pump (6), a flow meter (9), a preheating section (12), an inlet copper tube (13), a test section (15), an outlet copper tube (19), and a condenser (24), which are connected end to end in sequence through a transparent polytetrafluoroethylene tube (4); the preheating section (12) is provided with a preheating section inlet pressure sensor (10) and a preheating section inlet thermocouple (11); the inlet copper tube (13) is provided with an test section inlet thermocouple (14); the test section (15) is provided with an test section thermocouple (16), a test section inlet pressure sensor (17), and a differential pressure sensor (18); the outlet copper tube (19) is provided with an test section outlet thermocouple (20); the test section (15), a data acquisition and processing unit (22), and an image reconstruction unit are electrically connected to a post-processing computer (23) through a signal transmission cable (21); The cooling water circulation loop comprises an ice-water mixing barrel (25), a water pump (27), a cooling water inlet and a cooling water outlet on a subcooler (5), a three-way valve (28), and a condenser (24) which are connected end to end in sequence via a water pipe (26), and a bypass path of the three-way valve (28) is connected to the ice-water mixing barrel (25).

2. The two-phase flow pattern identification system based on ERT according to claim 1 is characterized in that: A buffer (7) is provided between the gear pump (6) and the flow meter (9) for alleviating vibration caused by the gear pump (6).

3. The two-phase flow pattern identification system based on ERT according to claim 2 is characterized in that: A filter (8) is provided between the buffer (7) and the flow meter (9) for filtering any particles that could potentially damage the gears of the flow meter (9).

4. The two-phase flow pattern identification system based on ERT according to claim 1, characterized in that: The semiconductor cooling sheet (2) and the heating membrane (3) are respectively attached to the upper and lower sides of the refrigerant liquid storage tank (1). The two are located on opposite sides of the refrigerant liquid storage tank (1) and are used to finely adjust the fluid pressure in the experimental section within a small range.

5. The two-phase flow pattern identification system based on ERT according to claim 1, characterized in that: The pin of the experimental section inlet thermocouple (14) is in contact with the outer wall surface of the inlet copper tube (13) and the pin of the experimental section outlet thermocouple (20) is in contact with the outer wall surface of the outlet copper tube (19) and are bonded by means of thermal insulation tape.

6. The two-phase flow pattern identification system based on ERT according to claim 1, characterized in that: The experimental section (15) uses an opaque copper tube (1501) as a substrate, which is wrapped with a thermal conductive tape (1502), a Ni-Cr wire (1503), a thermal conductive tape (1502), a polyimide film (1504) and a rubber-plastic insulation tube (1505) in sequence from the inside to the outside.

7. The two-phase flow pattern identification system based on ERT according to claim 6, characterized in that: The electrode array (1506) of the experimental section (15) is composed of 6 groups of electrodes (1507) evenly distributed along the axial direction of the opaque copper tube (1501), and each group of electrodes (1507) is composed of 8 independent electrodes (1507) evenly distributed along the circumference of the opaque copper tube (1501); the electrode array (1506) is electrically connected to the electrode gating control module (2202).

8. The two-phase flow pattern identification system based on ERT according to claim 7, characterized in that: The electrode array (1506) is tightly connected to the opaque copper tube (1501) via an insulating seal (1508); the height of each electrode (1507) in the electrode array (1506) is consistent with the wall thickness of the opaque copper tube (1501).

9. A method for constructing a heat transfer coefficient model based on a two-phase flow pattern identification system, characterized in that: The following steps are involved: ①Measure the heat transfer coefficient of the experimental section under different working conditions and organize the measured data into a database; ② Based on the existing heat transfer coefficient model with excellent prediction performance, the dimensionless parameters with significant correlation with the heat transfer coefficient are screened out; ③ Using the dimensionless parameters selected in step ②, construct a preliminary form of the heat transfer coefficient prediction model; ④ Use the least squares method and error analysis method to evaluate the accuracy of the initially constructed heat transfer coefficient prediction model and select the model with the best prediction performance; ⑤ Repeatedly apply the least squares method and error analysis method to adjust or eliminate the dimensionless parameters selected in step ②, further optimize the accuracy of the heat transfer coefficient prediction model, and finally determine the heat transfer coefficient model with the best prediction performance.

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