Cold surface frosting visualization experiment system based on schlieren method and operation method

Abstract

The invention relates to a cold surface frosting visualization experiment system based on a schlieren method and an operation method. The cold surface frosting visualization experiment system comprises a cavity, a data acquisition system, a constant humidity experiment cavity system, a refrigeration cycle system and a schlieren visualization system. The constant humidity experiment cavity system comprises a ceramic heating rod, a water bath tank and an adjustable constant flow source; the refrigeration cycle system comprises a circulating pump, a copper plate, a heat sink, a low-temperature thermostat and a pipeline for refrigerant circulation; the schlieren visualization system comprises optical glass, a reflector, a laser light source and a non-reflex camera, wherein light emitted by the laser light source is imaged in the non-reflex camera after passing through the reflector. Compared with a traditional visualization system, a schlieren system is introduced to analyze the influence of the cold plate surface flow characteristics on the frost layer growth characteristics under the natural convection condition. And meanwhile, characteristic parameters such as flow characteristics of different relative humidity of wet air on the periphery of the cold surface and the growth speed of the frost layer can be analyzed, so that the growth mechanism of the frost layer at the initial stage under different relative humidity is explored.

Description

Technical Field

[0001] The present invention relates to the observation of the growth of a frost layer on a cold surface, in particular to a visualization system for frost formation on the cold surface of an air-source heat pump based on the schlieren method. Technical Background

[0002] Frost formation on a cold surface is a spontaneous phenomenon when the temperature of humid air is lower than the air dew point and the water triple point, and it has received extensive attention in the fields of heating, agriculture, aerospace, and wind power generation. If an effective defrosting system is not equipped on the fan blades, the frost formation on their surfaces will cause a significant loss of power generation capacity; while the frost layer accumulated in the air-source heat pump heat exchanger will significantly increase the overall energy consumption of the heat pump. To ensure the efficient operation of the above systems, it is crucial to take effective anti-frost and defrost measures. Therefore, optimizing the defrosting process based on the growth characteristics of the frost layer and reducing the defrosting energy consumption have become key issues for improving the performance and benefits of related products. Existing research mostly observes the growth law of the local frost layer to clarify the influence mechanism of the frost layer growth by environmental factors. At present, there are a large number of experimental data on the morphology and thickness of the frost layer, but the research on the flow structure near the frosting cold surface is very scarce.

[0003] As a physical process affected by the coupled transport of heat and mass, the solidification of humid air on a cold surface is affected by the coupling of the local temperature distribution and the flow structure. At present, most of the experimental studies on the growth of the frost layer record the morphology of the frost layer on the side and top surfaces of the cold plate through one or more groups of CCD cameras, and the velocity distribution around the cold plate cannot be obtained simultaneously. Therefore, a system that can specifically analyze the velocity characteristics of the cold surface is needed to better explore the growth characteristics of the frost layer and make an important supplement to the research on the initial growth mechanism of the frost layer.

[0004] After retrieving the existing technology, it is found that the patent with the publication number CN109612027A, "A Method and Control System for Analyzing Frost Formation on an Air-Source Heat Pump Based on Microscopic Photography", discloses a method and control system for analyzing frost formation on an air-source heat pump based on microscopic photography, which mainly uses a microscopic camera to analyze the frost formation state on the surface and control the defrosting of the air-source heat pump. This scheme uses image algorithms for defrosting detection. The patent with the publication number CN109520072A, "A Method and System for Dynamic Monitoring of Frost Formation on an Air-Source Heat Pump", discloses a method and system for dynamic monitoring of frost formation on an air-source heat pump, which performs defrosting by judging whether the frost thickness, air temperature, and air humidity meet preset conditions. Both systems judge the specific defrosting time based on conditions such as the surface structure of the frost layer and environmental parameters, but neither can predict the air flow structure around the frost layer. Summary of the Invention

[0005] In view of the deficiencies of the prior art, the present invention proposes a visualization experimental system for cold surface frosting based on the schlieren method. By calculating the velocity vector distribution of humid air during cold surface frosting using the schlieren method, the frost layer growth characteristics of the cold surface are obtained. The convective heat transfer process of the cold surface is visualized using schlieren observation, and then the initial growth mechanism of the frost layer under natural convection conditions is evaluated. The schlieren method is introduced into the observation of the frost layer growth structure. By adjusting the temperature of the low-temperature constant temperature bath and the ceramic rod heating water bath in the system, the present system can observe the frosting behavior of the cold surface at different temperatures and different relative humidities.

[0006] The present invention is realized through the following technical solutions:

[0007] A visualization experimental system for cold surface frosting based on the schlieren method, including a cavity (13), a data acquisition system, a constant humidity experimental cavity system, a refrigeration cycle system, and a schlieren visualization system. Among them, the data acquisition system is used to collect data, including: a hygrometer (1), a thermocouple (2), and a data acquisition instrument (9). The hygrometer (1) and the thermocouple (2) are connected to the data acquisition instrument (9); the constant humidity experimental cavity system is used to keep the humidity in the cavity stable, including: a ceramic heating rod (5), a water bath, and an adjustable constant current source (8); the refrigeration cycle system is used to refrigerate the cavity, including a copper plate (3), a heat sink (4), and a low-temperature thermostat (7); the schlieren visualization system is used to observe the cavity, including: an optical glass (6), a reflector (10), a laser light source (11), a mirrorless camera (12), and an acrylic cavity (13). The light emitted by the laser light source (11) forms an image on the mirrorless camera (12) after passing through the reflector. Preferably, a U-shaped coil fluid passage is opened inside the copper heat sink to enable the refrigerant to fully contact the heat sink, increasing the heat transfer efficiency of the cold quantity. A refrigerant circulation passage is formed with a low-temperature cooling circulation pump through a pipe wrapped with heat-insulating material. The pipes at the inlet and outlet of the U-shaped pipe are extended, and the copper pipe is connected to the rubber pipe through a ferrule quick-connect union to prevent refrigerant leakage.

[0008] Optionally, the reflector is a concave mirror. The reflector (10) includes a first reflector (101) and a second reflector (102). The second reflector is used to receive the reflected light of the first reflector and reflect it to the mirrorless camera. The laser light source, the first reflector (101), the second reflector (102), and the mirrorless camera form a "Z"-shaped double-mirror optical path.

[0009] Optionally, an optical observation window is opened on the cavity, and the light of the "Z"-shaped optical path exits through the optical window. The optical observation window is equipped with an optical glass.

[0010] Optionally, openings are provided on the side wall of the cavity for passing through the water bath, ceramic heating rod, copper plate, heat sink, hygrometer, and thermocouple placed inside the cavity.

[0011] Optionally, the refrigeration cycle system is composed of a copper heat sink, an experimental copper plate, a cryostat, connecting pipelines and valves connected in sequence. Each component is connected to form a cycle system, and the temperature of the experimental copper plate is adjusted by the cryostat.

[0012] Optionally, the refrigeration cycle system further includes pipelines and a cooling pump. A U-shaped pipeline fluid passage is arranged inside the copper heat sink to enable the refrigerant to fully contact the heat sink, increasing the transfer efficiency of the cooling capacity. A refrigerant circulation passage is formed with the low-temperature cooling circulation pump through the pipeline wrapped with heat-insulating materials. The pipeline at the inlet and outlet of the U-shaped pipeline is extended, and the copper pipe is connected to the rubber pipe through a ferrule quick-connect union.

[0013] Optionally, the ceramic heating rod and the water bath form a humidity control system. The ceramic heating rod heats the water bath to keep the relative humidity in the cavity constant. The ceramic heating rod is externally connected to an adjustable constant current source. By adjusting the output voltage of the adjustable constant current source, the evaporation amount in the water bath is changed, and the cavity is an acrylic cavity.

[0014] A running method of a cold surface frosting visualization experiment system includes the steps of: starting the self-circulation of the circulation pump of the cryostat (7) to cool the refrigerant to the set temperature;

[0015] Opening the circulation pump valve to drive the refrigerant to flow into the pipeline to pre-cool the heat sink (4) and the experimental copper plate (3);

[0016] Turning on the laser light source (11) and the data acquisition system (9) to start recording temperature data;

[0017] When the temperature of the experimental copper plate (3) drops and stabilizes to the required temperature, turn on the adjustable constant current source (8) to turn on the ceramic heating rod (5) in the water bath;

[0018] Adjusting the voltage to control the heating power to stabilize the relative humidity in the cavity respectively;

[0019] After the relative humidity in the experimental cavity fluctuates less than 5% within 2 minutes, a polyethylene film is drawn out from one side of the cavity (13) to enable the wet air to directly contact and condense on the cold surface, and start recording schlieren images.

[0020] Optionally, the operation of the cryostat includes two modes: internal and external circulation. In the internal circulation, the ethylene glycol refrigerant is cooled inside the cryostat; when the external rubber hose of the cryostat forms a refrigeration cycle loop with the heat sink, the ethylene glycol is passed into the internal pipeline of the copper heat sink through the refrigerant circulation of the pump to achieve the purpose of cooling the upper experimental copper plate (3). During the experiment, open the valve to make the cryostat in the external circulation mode, and control the temperature of the ethylene glycol liquid in the circulation pipeline by adjusting the compressor frequency of the low-temperature circulation pump, so as to maintain the temperature of the heat sink and the cold surface temperature of the experimental copper plate constant.

[0021] Optionally, a polyethylene film (PE) is used to cover the surface of the cold plate to prevent moisture in the air from condensing prematurely. After the experiment is over, wait for the experimental copper plate (3) to rise to room temperature, clean the residual moisture, repeat the above operation to start the experiment of other temperature conditions, and keep the relative humidity in the experimental chamber stable at the set value, with a fluctuation of less than ±5% RH.

[0022] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows:

[0023] 1. Compared with the traditional visualization system, the cold surface observation experimental system based on the Schlieren method of the present invention utilizes the advantages of the Schlieren method in non-invasive measurement and is extended to the field involving the density change of transparent fluids. With the help of quantitative technology, the Schlieren method can obtain the velocity distribution in the fluid domain, capture the interaction between the air flow movement in the boundary layer and the evolution of the frost layer, and reveal the coupling mechanism of heat and mass transfer. And combined with the growth characteristics of the frost layer, the frosting mechanism is further explored.

[0024] 2. High sensitivity and early detection capability. The Schlieren method is extremely sensitive to small changes in the refractive index of air and can observe air flow throughout the entire period of frost growth, with observation capabilities superior to those of the naked eye or conventional microscopic observations. It can also indirectly reflect the dynamic characteristics of latent heat diffusion by tracking the local temperature gradient caused by the phase change of water vapor during the frosting process, providing data support for the verification of theoretical models.

[0025] 3. The present invention can quantify the driving factors of phase change, deduce the temperature or concentration field distribution through the refractive index gradient, and assist in establishing the quantitative relationship between the frosting rate and the environmental parameters.

[0026] 4. The core components only require a point light source, a slit, a reflector and a camera. Compared with laser interferometers or high-speed micro-CT, the cost is lower and it is easy to deploy in laboratories or industrial sites. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a structural schematic diagram of a cold surface frost visualization experimental system based on the Schlieren method in one embodiment of the present application;

[0028] Figure 2 This is a schematic diagram of the working principle of a Schlieren visualization system of a cold surface frost visualization experimental system based on the Schlieren method in one embodiment of the present application. DETAILED DESCRIPTION

[0029] The present invention will be described in detail below in conjunction with the embodiments and drawings, but the protection scope of the present invention is not limited to the following embodiments. Figure 1As shown in the figure, a visualization experimental system for cold surface frosting based on the schlieren method includes a cavity 13, a data acquisition system, a constant humidity experimental chamber system, a refrigeration cycle system, and a schlieren visualization system. Among them, the data acquisition system is used to collect data, including: a hygrometer 1, a thermocouple 2, and a data acquisition instrument 9. The hygrometer 1 and the thermocouple 2 are connected to the data acquisition instrument 9; the constant humidity experimental chamber system is used to keep the humidity in the chamber stable, including: a ceramic heating rod 5, a water bath 14, and an adjustable constant current source 8; the refrigeration cycle system is used to refrigerate the chamber, including a circulation pump, a copper plate 3, a heat sink 4, a cryostat 7, and a pipeline for refrigerant circulation; the schlieren visualization system is used to observe the chamber, including: an optical glass 6, a mirror 10, a laser light source 11, and a mirrorless camera 12. The light emitted by the laser light source 11 passes through the mirror and forms an image on the mirrorless camera 12.

[0030] In Figure 1 In the embodiment shown, the refrigeration cycle system includes a heat sink 4 located in the experimental cavity 13, a copper plate 3 located on the heat sink 4, and a cryostat 7 located outside the experimental cavity 13. The cryostat 7 is connected to the heat sink 4 through a connecting pipeline 15 and a valve 16, and is used to control and adjust the temperature of the experimental copper plate 3.

[0031] In some embodiments, the connecting pipeline is a rubber hose, and a refrigerant is provided in the connecting pipeline. The cryostat leads the refrigerant to the heat sink through the connecting pipeline to achieve the function of controlling the temperature of the copper plate.

[0032] In one embodiment, the mirror is a concave mirror. The mirror 10 includes a first mirror 101 and a second mirror 102. The second mirror is used to receive the reflected light of the first mirror and reflect it to the mirrorless camera. The laser light source, the first mirror 101, the second mirror 102, and the mirrorless camera form a "Z"-shaped double-mirror optical path.

[0033] In one embodiment, the cavity is an acrylic cavity.

[0034] In one embodiment, an optical observation window is opened on the cavity. The light of the "Z"-shaped optical path exits through the optical window, and the optical observation window is equipped with an optical glass.

[0035] In one embodiment, openings are provided on the side wall of the cavity for passing through the water bath, ceramic heating rod, copper plate, heat sink, hygrometer, and thermocouple placed in the cavity.

[0036] In one embodiment, the refrigeration cycle system is composed of a copper heat sink, an experimental copper plate, a cryostat, a connecting pipeline, and a valve connected in sequence. Each component is connected to form a cycle system, and the temperature of the experimental copper plate is adjusted by the cryostat.

[0037] In one embodiment, the refrigeration cycle system further includes a pipeline and a cooling pump. A U-shaped pipeline fluid passage is arranged inside the copper heat sink, enabling the refrigerant to be in full contact with the heat sink, increasing the heat transfer efficiency of the cold quantity. A refrigerant circulation passage is formed with a low-temperature cooling circulating pump through the pipeline wrapped with heat insulation material. The pipeline at the inlet and outlet of the U-shaped pipeline is extended, and the copper pipe is connected to the rubber pipe through a ferrule quick-connect straight joint to prevent refrigerant leakage.

[0038] In one embodiment, the ceramic heating rod and the water bath form a humidity control system. The ceramic heating rod heats the water bath to control the relative humidity inside the cavity to remain constant. The ceramic heating rod is externally connected to an adjustable constant current source. By adjusting the output voltage of the adjustable constant current source, the evaporation amount in the water bath is changed, and the cavity is an acrylic cavity.

[0039] The hygrometer 1 and the thermocouple 2 together constitute the temperature and humidity measurement system inside the experimental cavity, and are respectively arranged near the side wall of the acrylic cavity 13 and the experimental copper plate 3 to measure the surface temperature of the experimental copper plate and the relative humidity inside the cavity. And the data is collected and displayed by the data acquisition instrument 9.

[0040] The experimental copper plate 3 and the heat sink 4 are cooperated with each other through thermal grease to improve the thermal conductivity. At the same time, the cryostat 7 leads the circulating refrigerant to the heat sink 4 through an external pipeline to achieve the function of controlling the temperature of the copper plate; the ceramic heating rod 5 changes the applied voltage through the adjustable constant current source (8) to heat the water bath, so that the relative humidity inside the acrylic cavity 13 changes accordingly.

[0041] The optical glass 6 is assembled on the front and back sides of the "Z"-type double mirror optical path on the acrylic cavity 13. The structure of the mirror 10 is divided into two first mirrors 101 and a second mirror 102. The light source emitted by the laser light source 11 is reflected by the mirror surface, and finally the schlieren image obtained is photographed and recorded by the mirrorless camera 12.

[0042] In one embodiment, the mirror 10 includes a first mirror 101 and a second mirror 102. The second mirror is used to receive the reflected light of the first mirror and reflect it to the mirrorless camera. The laser light source, the first mirror 101, the second mirror 102, and the mirrorless camera form a "Z"-type double mirror optical path. The "Z"-type double mirror optical path can reduce the off-axis aberration. Compared with the traditional single mirror schlieren optical path, it can effectively reduce the interference of double image ghosting on the observation. The laser light source is a point light source, covering the experimental cavity through the "Z"-type double mirror optical path. The light field is provided by the point light source, and finally the schlieren image obtained is photographed and recorded by the camera to form a schlieren visualization system.

[0043] Preferably, the focal length of the mirrorless camera lens is 56 mm, and the schlieren image is recorded at a resolution of 1920×1080 and a frame rate of 30 FPS.

[0044] Preferably, a 50×50×5 mm experimental copper plate is assembled at the center of the upper surface of a 100×140×25 mm copper heat sink through socket head cap screws, and thermal grease is applied in the assembly gap between the two to improve the thermal conductivity.

[0045] Preferably, the water bath heating tank is a resin water tank with a size of 40×20×35 mm and a wall thickness of 2 mm, and is fixed on the upper side wall of the acrylic cavity. Before the experiment starts, the water volume in the tank is replenished to the scale line through a syringe to keep the water level constant during each experiment.

[0046] Preferably, the hygrometer is a Vaisala temperature and humidity sensor with a measurement accuracy of ±1.5% RH, which can measure the relative humidity value inside the acrylic cavity and is installed on one side wall of the cavity perpendicular to the optical path to monitor the humidity inside the experiment cavity.

[0047] Preferably, the thermocouple is a K-type thermocouple with an accuracy of 0.5℃, which is arranged on the copper side surface and 20 mm above the experimental copper plate to record the temperatures of the copper plate and the gas inside the experiment cavity respectively.

[0048] Preferably, the data acquisition system consists of an Agilent 34972A and a thermocouple scanner, which collect and record the relative humidity and thermocouple readings respectively.

[0049] The present invention also provides an operation method for a cold surface frosting visualization experimental system, including the steps of: starting the self-circulation of the circulating pump of the cryostat (7) to cool the refrigerant to the set temperature;

[0050] Opening the circulating pump valve to drive the refrigerant into the pipeline to pre-cool the heat sink 4 and the experimental copper plate 3;

[0051] Turning on the laser light source 11 and the data acquisition system 9 to start recording temperature data;

[0052] When the temperature of the experimental copper plate 3 drops and stabilizes to the required temperature, turn on the adjustable constant current source 8 to turn on the ceramic heating rod 5 in the water bath;

[0053] Adjusting the voltage to control the heating power to stabilize the relative humidity inside the cavity respectively;

[0054] After the relative humidity fluctuation inside the experiment cavity is less than 5% within 2 minutes, a polyethylene film is drawn out from one side of the cavity 13 to allow the wet air to directly contact and condense on the cold surface, and start recording schlieren images.

[0055] In one embodiment, the operation of the cryostat includes two modes: internal and external circulation. In the internal circulation mode, the ethylene glycol refrigerant is cooled inside the cryostat. When the cryostat is externally connected to a rubber hose to form a refrigeration cycle loop with a heat sink, the ethylene glycol is passed through the internal pipeline of the copper heat sink by the refrigerant circulation of the pump, so as to cool the upper experimental copper plate 3. During the experiment, open the valve to make the cryostat in the external circulation mode, and control the temperature of the ethylene glycol liquid in the circulation pipeline by adjusting the frequency of the cryogenic circulation pump compressor, so as to maintain the temperature of the heat sink and the cold surface temperature of the experimental copper plate constant.

[0056] In one embodiment, the surface of the cold plate is coated with a polyethylene film (PE) to prevent the premature condensation of moisture in the air. After the experiment is completed, wait for the experimental copper plate 3 to rise to room temperature, clean the residual moisture, and repeat the above operations to start the experiments under other temperature conditions, keeping the relative humidity in the experimental chamber stable at the set value with a fluctuation of less than ±5% RH.

[0057] For the above embodiments, the specific process of observing frost formation on the cold surface is as follows:

[0058] Before the experiment starts, fix the experimental copper plate 3 to the center of the upper surface of the heat sink 4 with an Allen screw and adjust the position so that the experimental copper plate 3 is located at the center of the optical path field of view. Subsequently, adjust the positions and deflection angles of the first reflector 101 and the second reflector 102 so that the laser light source 11 and the slit are located at the foci of the two concave reflectors. At the same time, change the imaging sensitivity of the schlieren system by adjusting the height of the slit to clearly observe the schlieren image with distinct light and dark caused by natural convection.

[0059] At the start of the experiment, first start the self-circulation of the circulation pump of the cryostat 7 to cool the refrigerant to the set temperature. Subsequently, open the valve of the circulation pump to drive the refrigerant into the circulation pipeline to pre-cool the heat sink 4 and the experimental copper plate 3. During this period, the surface of the cold plate is coated with a polyethylene film (PE) to prevent the premature condensation of moisture in the air. At the same time, turn on the laser light source 11 and the data acquisition system 9 to start recording temperature data. When the temperature of the cold plate drops and stabilizes at the required temperature, turn on the adjustable constant current source 8 to turn on the ceramic heating rod 5 in the water tank, and control the heating power by adjusting the voltage to stabilize the relative humidity in the chamber respectively. After the relative humidity in the experimental chamber fluctuates less than 5% within 2 minutes, pull out the polyethylene film from one side of the acrylic cavity 13 to allow the humid air to directly contact and condense on the cold surface, and start recording the schlieren image. During the experiment, keep the relative humidity in the experimental chamber stable at the set value with a fluctuation of less than ±5% RH. After the experiment is stopped, close the valves of the heating water tank and the cryogenic circulator 7 pump respectively.

[0060] After the experiment is completed, wait for the experimental copper plate 3 to rise to room temperature, clean the residual moisture, and repeat the above operations to start the experiments under other temperature conditions.

[0061] The velocity vector distribution of wet air when frosting on a cold surface is calculated by the Schlieren method, so as to obtain the frost growth characteristics of the cold surface. The convective heat transfer process of the cold surface is visualized by Schlieren observation, so as to evaluate the initial growth mechanism of the frost layer under natural convection conditions, and introduce the Schlieren method into the observation of the frost growth structure. By adjusting the temperature of the low-temperature thermostatic bath and the ceramic rod heating water bath in the system, this system can observe the frosting behavior of the cold surface at different temperatures and relative humidity.

[0062] Compared with the traditional visualization system, the cold surface observation experimental system based on the Schlieren method of the present invention utilizes the advantages of the Schlieren method in non-invasive measurement and is extended to the field involving the density change of transparent fluid. With the help of quantitative technology, the Schlieren method can obtain the velocity distribution in the fluid domain, capture the interaction between the air flow movement in the boundary layer and the evolution of the frost layer, and reveal the coupling mechanism of heat and mass transfer. And combined with the growth characteristics of the frost layer, the frosting mechanism is further explored.

[0063] It has high sensitivity and early detection capability. The Schlieren method is extremely sensitive to small changes in the refractive index of air and can observe air flow throughout the entire period of frost growth, with observation capabilities superior to those of the naked eye or conventional microscopic observations. It can also indirectly reflect the dynamic characteristics of latent heat diffusion by tracking the local temperature gradient caused by the phase change of water vapor during the frosting process, providing data support for the verification of theoretical models.

[0064] The present invention can quantify the driving factors of phase change, deduce the temperature or concentration field distribution through the refractive index gradient, and assist in establishing the quantitative relationship between the frosting rate and the environmental parameters.

[0065] The core components only require a point light source, a slit, a reflector and a camera. Compared with laser interferometers or high-speed micro-CT, they are less expensive and easier to deploy in laboratories or industrial sites.

[0066] The above description of the embodiments is to facilitate the understanding and use of the invention by those skilled in the art. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative work. Therefore, the present invention is not limited to the above embodiments, and improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the scope of protection of the present invention.

Claims

1. A visualization experimental system for cold surface frosting based on the schlieren method, characterized in that It includes a cavity (13), a data acquisition system, a constant humidity experimental chamber system, a refrigeration cycle system, and a schlieren visualization system. Among them, the data acquisition system is used to collect data, including: a hygrometer (1), a thermocouple (2), and a data acquisition instrument (9). The hygrometer (1) and the thermocouple (2) are connected to the data acquisition instrument (9); the constant humidity experimental chamber system is used to keep the humidity in the chamber stable, including: a ceramic heating rod (5), a water bath, and an adjustable constant current source (8); the refrigeration cycle system is used to refrigerate the chamber, including a circulation pump, a copper plate (3), a heat sink (4), a cryostat (7), and a pipeline for refrigerant circulation; the schlieren visualization system is used to observe the chamber, including: an optical glass (6), a reflector (10), a laser light source (11), and a mirrorless camera (12). The light emitted by the laser light source (11) is imaged on the mirrorless camera (12) after passing through the reflector.

2. The cold surface frosting visualization experimental system based on the schlieren method according to claim 1, characterized in that, The reflector is a concave mirror. The reflector (10) includes a first reflector (101) and a second reflector (102). The second reflector is used to receive the reflected light of the first reflector and reflect it to the mirrorless camera. The laser light source, the first reflector (101), the second reflector (102), and the mirrorless camera form a "Z"-shaped double-mirror optical path.

3. The schlieren method-based cold surface frosting visualization experimental system according to claim 3, characterized in that, An optical observation window is opened on the cavity. The light of the "Z"-shaped optical path exits through the optical window, and the optical window is equipped with an optical glass.

4. The visualization experimental system for cold surface frosting based on the schlieren method according to claim 1, wherein Openings are provided on the side wall of the cavity for passing through the water bath, ceramic heating rod, copper plate, heat sink, hygrometer, and thermocouple placed in the cavity.

5. The cold surface frosting visualization experimental system based on the schlieren method according to claim 1, wherein The refrigeration cycle system is composed of a copper heat sink, an experimental copper plate, a cryostat, connecting pipelines, and valves connected in sequence. Each component is connected to form a cycle system, and the temperature of the experimental copper plate is adjusted by the cryostat.

6. The schlieren method-based cold surface frosting visualization experimental system according to claim 1, wherein The refrigeration cycle system also includes pipelines and a cooling pump. A U-shaped pipeline fluid passage is provided inside the copper heat sink to make the refrigerant fully contact with the heat sink, increasing the transfer efficiency of the cooling capacity. A refrigerant circulation passage is formed with the low-temperature cooling circulation pump through a pipeline wrapped with heat-insulating material. The pipeline at the inlet and outlet of the U-shaped pipeline is extended, and the copper pipe is connected to the rubber pipe through a ferrule quick-connect union.

7. The visualization experimental system for cold surface frosting based on the schlieren method according to claim 1, characterized in that The ceramic heating rod and the water bath form a humidity adjustment system. The ceramic heating rod heats the water bath to control the relative humidity in the chamber to remain constant. The ceramic heating rod is externally connected to an adjustable constant current source. By adjusting the output voltage of the adjustable constant current source, the evaporation amount in the water bath is changed. The cavity is an acrylic cavity.

8. A method for operating the cold surface frosting visualization experiment system according to claim 1, characterized in that, It includes the steps: starting the self-circulation of the circulation pump of the cryostat (7) to cool the refrigerant to the set temperature; Opening the circulation pump valve to drive the refrigerant to flow into the pipeline to pre-cool the heat sink (4) and the experimental copper plate (3); Turning on the laser light source (11) and the data acquisition system (9) to start recording temperature data; When the temperature of the experimental copper plate (3) drops and stabilizes to the required temperature, turn on the adjustable constant current source (8) and turn on the ceramic heating rod (5) in the water bath; Adjusting the voltage to control the heating power to make the relative humidity in the chamber stable respectively; After the relative humidity in the experimental chamber fluctuates less than 5% within 2 minutes, the polyethylene film is taken out from one side of the chamber (13) to allow the moist air to directly contact and condense on the cold surface, and the schlieren images are started to be recorded.

9. The operating method according to claim 8, characterized in that, The operation of the cryostat includes two modes: internal and external circulation. In the internal circulation, the ethylene glycol refrigerant is cooled inside the cryostat; when the cryostat is externally connected to a rubber hose to form a refrigeration cycle circuit with the heat sink, the ethylene glycol is passed into the internal pipeline of the copper heat sink through the refrigerant circulation of the pump to achieve the purpose of cooling the upper experimental copper plate (3). During the experiment, the valve is opened to make the cryostat in the external circulation mode, and the temperature of the ethylene glycol liquid in the circulation pipeline is controlled by adjusting the frequency of the low-temperature circulation pump compressor, thereby maintaining the temperature of the heat sink and the cold surface temperature of the experimental copper plate constant.

10. The operating method according to claim 9, characterized in that, The cold plate surface is covered with a polyethylene film (PE) to prevent the premature condensation of moisture in the air; after the experiment, when the experimental copper plate (3) rises to room temperature, the residual moisture is cleaned, and the above operations are repeated to start the experiments under other temperature conditions, keeping the relative humidity in the experimental chamber stable at the set value with a fluctuation less than ±5%RH.

Images