Device and method for measuring surface thermal mass transfer coefficient of liquid drop

Abstract

The invention discloses a liquid drop surface heat and mass transfer coefficient measuring device and method, and belongs to the technical field of liquid drop surface heat and mass transfer coefficient measurement. Nitrogen passes through a liquid nitrogen container from a high-pressure nitrogen cylinder and then is introduced into an experiment cavity at a certain temperature, and parameter regulation and control are achieved in cooperation with a heating pipe, a flow meter and the like; a laser displacement sensor is used for measuring droplet quality, a thermocouple is used for measuring temperature, a camera records form, and a data collector and a computer synchronously process data; the method comprises the steps that low-temperature nitrogen drives liquid drops to freeze, a nitrogen purging window is used for preventing fog, and the mass transfer coefficient and the heat exchange coefficient are calculated by measuring the temperature and flow of nitrogen flow, the temperature of the liquid drops and the deflection of nickel-chromium alloy wires and combining the Fick law and energy conservation. According to the liquid drop surface heat and mass transfer coefficient measuring device and method, transient changes are captured through high-frequency data collection, a zero-humidity environment is achieved through nitrogen displacement, form observation is ensured through air curtain antifogging, heat loss is reduced through heat preservation design, and the measuring precision and reliability are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of measuring heat and mass transfer coefficients on droplet surfaces, and in particular to a device and method for measuring heat and mass transfer coefficients on droplet surfaces. Background Art

[0002] Measuring the heat and mass transfer coefficient (HTTC) on a droplet surface involves quantifying the efficiency of heat and mass transfer between the droplet surface and its surroundings through experimental or simulation methods. The HTC represents the heat transfer capacity driven by temperature differences, while the MTC reflects the diffusion rate driven by concentration differences. The HTC and MTC are coupled during processes such as droplet evaporation and condensation, and are influenced by interfacial effects such as surface tension and latent heat of phase change. This measurement is of great significance: in industrial applications, it can optimize spray cooling systems to improve the heat dissipation efficiency of electronic devices, improve the combustion characteristics of fuel droplets to enhance engine efficiency and reduce pollutant generation, and control the quality of spray drying and crystallization processes in the chemical and pharmaceutical industries. It also provides data support for research on environmental issues such as cloud droplet formation and pollutant mass transfer in the atmosphere. Academically, it helps to shed light on the mechanisms of multiphase flow and the theory of phase change heat and mass transfer, laying the foundation for the development of micro- and nanoscale technologies such as microfluidic chips. Furthermore, in emerging fields such as fuel cell water management, hydrogen energy storage safety control, and aerospace anti-icing system design, HTC measurement is a key driver of technological innovation, serving as a vital bridge between theoretical models and engineering practice.

[0003] In the existing technology for measuring heat and mass exchange between droplets and the external environment, the collection of key parameters of the freezing process has the problem of insufficient data due to the short duration of the process, which affects the accuracy and reliability of the measurement results. In terms of measuring the mass transfer coefficient of the droplet's outer surface, some technologies do not fully consider the verification of the uniformity of the temperature distribution inside the droplet, which may lead to deviations in the calculation of the surface saturated vapor pressure; and the treatment of the water vapor partial pressure in the chamber environment is not rigorous enough, affecting the accuracy of the mass transfer coefficient measurement. When measuring the heat transfer coefficient of the droplet's outer surface, some devices do not effectively eliminate the influence of heat loss, or the approximate treatment of the droplet surface temperature lacks theoretical basis and experimental verification, resulting in errors in the calculation results of the heat transfer coefficient. When observing the freezing of droplets, the overall experimental device may have the problem of fogging the observation window due to low temperature, affecting the real-time observation and recording of changes in droplet morphology. Summary of the Invention

[0004] The purpose of the present invention is to provide a device and method for measuring the heat and mass transfer coefficient of a droplet surface, thereby ensuring a stable experimental environment, accurately capturing short-term parameter changes during the freezing process, using cooled nitrogen to completely remove the air in the chamber to reduce the relative humidity to 0, simplifying the calculation of the mass transfer equation, effectively avoiding low-temperature fogging, and ensuring clear observation and recording of morphological changes during the droplet freezing process; and accurately obtaining changes in droplet mass to provide reliable data for the calculation of the mass transfer coefficient.

[0005] To achieve the above objectives, the present invention provides a device for measuring the heat and mass transfer coefficient of a droplet surface, comprising a high-pressure nitrogen cylinder, the high-pressure nitrogen cylinder being connected to a liquid nitrogen container, a rotor flowmeter and a valve being provided between the high-pressure nitrogen cylinder and the liquid nitrogen container, a spiral copper tube being provided on the outside of the liquid nitrogen container, the outlet of the liquid nitrogen container being connected to the inlet of an experimental chamber, a heating tube, an inlet flowmeter, and an inlet temperature probe being provided in sequence between the liquid nitrogen container and the experimental chamber, the outside of the experimental chamber being covered with an insulation layer, the inside of the experimental chamber being provided with a screen and a thermocouple, two sets of thermocouples being provided, one for measuring the center temperature of the droplet and the temperature of the air near the droplet, respectively, the outlet of the experimental chamber being provided with an outlet flowmeter and an outlet temperature probe, the droplet being suspended by a nickel-chromium alloy wire, a laser displacement sensor being provided below the nickel-chromium alloy wire; the measuring device as a whole being provided with a data collector and a computer, and the experimental chamber being made of a transparent material and provided with a camera.

[0006] The present invention also provides a method for measuring the heat and mass transfer coefficient of a droplet surface, comprising the following steps:

[0007] S1: Nitrogen gas is cooled in a liquid nitrogen container and introduced into the experimental chamber at a certain temperature to drive the droplets to freeze. A heating tube is used to control the temperature of the nitrogen gas according to the experimental conditions. The heat-exchanged nitrogen gas is blown through the experimental chamber window at a flow rate of 5 m / s, forming an air curtain to prevent fogging at low temperatures and ensure that the camera can clearly capture the droplet morphology throughout the entire process.

[0008] S2. Obtain the nitrogen temperature T at the inlet of the experimental chamber through the inlet flow meter and inlet temperature probe. in and flow Q in The nitrogen temperature T at the outlet of the experimental chamber is obtained by the outlet flow meter and the outlet temperature probe. out and flow Q out , the droplet center temperature T is measured by thermocouple d and the air temperature near the droplet T en The mass m(t) of the droplet at different time periods is obtained by measuring the deflection of the nickel-chromium alloy wire with a laser displacement sensor. The surface area of ​​the droplet is obtained by taking an image of the droplet with a camera and calculating the equivalent diameter d based on the spherical assumption.

[0009] S3. Calculate the droplet surface mass transfer coefficient h m ;

[0010] S4. Calculate the droplet surface heat transfer coefficient h.

[0011] Preferably, in S2, the deflection δ of the nickel-chromium alloy wire is x Acquisition involves the following steps:

[0012] S21, the laser displacement sensor emits a red laser beam with a wavelength of 650nm and focuses it on the surface of the nickel-chromium alloy wire to form a light spot with a diameter of 5μm;

[0013] S22, the reflected light is focused on the CMOS detector through the convex lens to form a light spot position signal;

[0014] S23, when the nickel-chromium alloy wire bends downward due to the mass of the droplet, the position of the light spot on the CMOS detector shifts;

[0015] S24. Calculate the deflection value in real time through geometric trigonometric relationships.

[0016] Preferably, in S2, the specific calculation method of the droplet surface area A is: use a camera to capture the droplet shape, fit the droplet equivalent diameter d through an image recognition algorithm, consider non-spherical deformation, fit the major and minor axes of the ellipse, and calculate the surface area A=πd based on the spherical assumption 2 , correcting the error of ignoring morphological changes in traditional methods.

[0017] Preferably, S3 includes the following steps:

[0018] S31. Based on Fick's law and the conservation of mass at the phase interface, the change in droplet mass is dominated by surface water vapor diffusion, and the mass transfer equation is: in is the droplet mass change rate;

[0019] S32. Combined deflection-mass relationship Calculate the droplet mass m(t) and find the rate of change by the difference method

[0020] S33. Calculate the surface saturated water vapor concentration C S and environmental concentration C ∞ , specifically: C S The droplet center temperature T d Calculate the saturated vapor pressure P using the Antoine equation S , and then converted to concentration where R g is the water vapor gas constant; when the droplet diameter is ≤200μm, the Biot number where K S is the droplet thermal conductivity coefficient. The internal conduction thermal resistance of the droplet is much smaller than the surface convection thermal resistance. Assuming that the droplet temperature is uniform, T d It also represents the droplet surface temperature;

[0021] Before the experiment, nitrogen was introduced into the experimental chamber for 5 minutes. The relative humidity in the experimental chamber was 0. The relative humidity was monitored in real time by a humidity sensor. The experiment was started after it was confirmed to be stable. Therefore, C ∞ =0, simplified to:

[0022] Preferably, S4 includes the following steps:

[0023] S41. Based on the law of energy conservation, the enthalpy difference between nitrogen entering and leaving the experimental chamber is equal to the sum of the heat exchange of the droplets and the heat loss of the experimental chamber, that is, Q = Q conv +Q dis ; Where Q is the total heat exchange, Q conv is the convective heat transfer of the droplets, Q dis is the heat loss of the experimental chamber;

[0024] S42. The total heat transfer Q is calculated from the inlet and outlet parameters of nitrogen entering and leaving the experimental chamber, Q = m N2 c p (T out -T in ), where m N2 is the nitrogen mass flow rate, which is converted after measuring the volume flow rate by the rotor flowmeter, c p is the specific heat capacity of nitrogen at constant pressure, specifically 1040 J / (kg·K);

[0025] S43, droplet convection heat transfer Q conv It is expressed as Q through Newton's cooling formula conv =h·A·(Ts-T en ), where Ts≈T is based on the assumption that Bi<0.1 d ;

[0026] S44, experimental chamber heat loss Q dis Calibrate through a blank experiment, specifically, introduce the same flow rate of nitrogen when there is no droplet, and record the enthalpy difference of nitrogen entering and leaving the experimental chamber, which is Q dis , eliminating environmental interference of heat dissipation in the experimental cavity and heat conduction in the window;

[0027] S45, solve the heat transfer coefficient h, and combine S41-S44 to get

[0028] Therefore, the present invention adopts the above-mentioned device and method for measuring the heat and mass transfer coefficient of a droplet surface, which has the following beneficial effects:

[0029] 1) High-frequency data acquisition using thermocouples and cameras accurately captures transient parameter changes during the freezing process. Biot number analysis of the droplet is used to approximate the uniform temperature distribution within the droplet, improving the accuracy of surface temperature and saturated vapor pressure calculations. Cooling nitrogen is used to completely expel air from the chamber, reducing relative humidity to zero, simplifying mass transfer equations and reducing errors.

[0030] 2) The design of nitrogen purge on the experimental chamber surface effectively avoids fogging at low temperatures, ensuring clear observation and recording of morphological changes during droplet freezing;

[0031] 3) The experimental chamber is insulated to reduce heat leakage and ensure a stable experimental environment. The nickel-chromium alloy wire suspension droplet combined with laser deflection measurement can accurately obtain the change in droplet mass, providing reliable data for mass transfer coefficient calculation.

[0032] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 The present invention is a schematic diagram of the structure of a device for measuring the heat and mass transfer coefficient of a droplet surface and a device for measuring the heat and mass transfer coefficient of a droplet surface according to an embodiment of the present invention.

[0034] Reference numerals

[0035] 1. High-pressure nitrogen cylinder; 2. Rotor flowmeter; 3. Valve; 4. Liquid nitrogen container; 5. Spiral copper tube; 6. Heating tube; 7. Screen; 8. Thermocouple; 9. Inlet temperature probe; 10. Inlet flowmeter; 11. Experimental chamber; 12. Outlet flowmeter; 13. Outlet temperature probe; 14. Data acquisition device; 15. Computer. DETAILED DESCRIPTION

[0036] The technical solution of the present invention is further described below with reference to the accompanying drawings and embodiments.

[0037] Unless otherwise defined, the technical or scientific terms used in the present invention shall have the usual meanings understood by persons of ordinary skill in the field to which the present invention belongs. The words "first", "second" and similar terms used in the present invention do not indicate any order, quantity or importance, but are only used to distinguish different components. Words such as "include" or "comprise" mean that the elements or objects preceding the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects. Words such as "connect" or "connected" are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. "Up", "down", "left", "right" and the like are only used to indicate relative positional relationships. When the absolute position of the object being described changes, the relative positional relationship may also change accordingly.

[0038] Example 1

[0039] The present invention provides a device for measuring the heat and mass transfer coefficient of a droplet surface. The basic structure of the device is as follows: Figure 1As shown, it includes a high-pressure nitrogen cylinder 1, which is connected to a liquid nitrogen container 4. A rotor flowmeter 2 and a valve 3 are provided between the high-pressure nitrogen cylinder 1 and the liquid nitrogen container 4. A spiral copper tube 5 is provided on the outside of the liquid nitrogen container 4. The outlet of the liquid nitrogen container 4 is connected to the inlet of the experimental chamber 11. A heating tube 6, an inlet flowmeter 10 and an inlet temperature probe 9 are provided in sequence between the liquid nitrogen container 4 and the experimental chamber 11. The outside of the experimental chamber 11 is covered with an insulation layer to avoid heat leakage, reduce external thermal interference, and maintain a steady-state low-temperature environment in the experimental chamber 11. A screen 7 and a thermocouple 8 are provided inside. Two sets of thermocouples 8 are provided, one for measuring the temperature at the center of the droplet and the temperature of the air near the droplet. The diameter of the thermocouple 8 is ≤20μm to minimize interference with the droplet morphology. The outlet of the experimental chamber 11 is equipped with an outlet flowmeter 12 and an outlet temperature probe 13. The droplets are suspended within the experimental chamber 11 by a horizontal nickel-chromium alloy wire. A laser displacement sensor is installed below the nickel-chromium alloy wire. The laser displacement sensor measures deflection non-contact, avoiding the volume interference and low-temperature drift problems of traditional weighing sensors. The entire measurement device is equipped with a data acquisition device 14 and a computer 15. The experimental chamber 11 is made of transparent material and is equipped with a camera to observe the changes in the droplet morphology during the freezing stage. After the nitrogen gas completes the heat exchange with the droplet, it is guided through a pipe to blow onto the surface of the experimental chamber 11 to prevent low-temperature fogging. The droplet freezing process is recorded by the camera. In this embodiment, the nickel-chromium alloy wire has a diameter of 50 μm and a deflection measurement accuracy of ±1 μm, capable of capturing minute changes in droplet mass. Thermocouple 8 is a T-type thermocouple with a response time of <5 ms and an accuracy of ±0.1 K. The inlet temperature probe 9 and outlet temperature probe 13 are PT100 sensors with an accuracy of ±0.5 K. The insulation layer is 5 cm thick and has a thermal conductivity of ≤0.04 W / (m·K). Nitrogen gas passes through valve 3 and exchanges heat in liquid nitrogen container 4, causing the temperature to drop. The nitrogen flow rate can be controlled by valve 3 of high-pressure nitrogen cylinder 1. The cooled nitrogen gas is then introduced into experimental chamber 11 to exchange heat with the droplets, causing the droplet temperature to drop and solidify. Thermocouple 8, inlet flowmeter 10, inlet temperature probe 9, outlet flowmeter 12, and outlet temperature probe 13 are all connected to a data acquisition device 14. The droplet morphology data captured by the camera is stored in computer 15 via host computer software.

[0040] When the measuring device described in this embodiment is used, the following steps are included:

[0041] S1. Nitrogen is cooled by liquid nitrogen container 4 and introduced into experimental chamber 11 at a certain temperature to drive the freezing of droplets. The temperature of the nitrogen can be controlled by a heating tube according to the experimental conditions. The heat-exchanged nitrogen is blown through the window of experimental chamber 11 at a flow rate of 5m / s, forming an air curtain to prevent low-temperature fogging and ensure that the camera can clearly capture the droplet morphology throughout the process.

[0042] S2, obtain the nitrogen temperature T at the inlet of the experimental chamber 11 through the inlet flow meter 10 and the inlet temperature probe 9.in and flow Q in The nitrogen temperature T at the outlet of the experimental chamber 11 is obtained by the outlet flow meter 12 and the outlet temperature probe 13 out and flow Q out The droplet center temperature T is measured by thermocouple 8 d and the air temperature near the droplet T en The mass m(t) of the droplet at different time periods is obtained by measuring the deflection of the nickel-chromium alloy wire with a laser displacement sensor, and the surface area of ​​the droplet is calculated based on the spherical assumption according to the equivalent diameter d by taking the droplet image with a camera.

[0043] Deflection δ of nickel-chromium alloy wire x Acquisition involves the following steps:

[0044] S21, the laser displacement sensor emits a red laser beam with a wavelength of 650nm and focuses it on the surface of the nickel-chromium alloy wire to form a light spot with a diameter of 5μm;

[0045] S22, the reflected light is focused on the CMOS detector through the convex lens to form a light spot position signal;

[0046] S23, when the nickel-chromium alloy wire bends downward due to the mass of the droplet, the position of the light spot on the CMOS detector shifts;

[0047] S24. Calculate the deflection value in real time through geometric trigonometric relationships.

[0048] The specific calculation method of the droplet surface area A is as follows: use a camera to capture the droplet shape, fit the droplet equivalent diameter d through the image recognition algorithm, consider the non-spherical deformation, fit the major and minor axes of the ellipse, and calculate the surface area A = πd based on the spherical assumption. 2 , correcting the error of ignoring morphological changes in traditional methods.

[0049] S3. Calculate the droplet surface mass transfer coefficient h m ; Specifically including the following steps:

[0050] S31. Based on Fick's law and the conservation of mass at the phase interface, the change in droplet mass is dominated by surface water vapor diffusion, and the mass transfer equation is: in is the droplet mass change rate;

[0051] S32. Combined deflection-mass relationship Calculate the droplet mass m(t) and find the rate of change by the difference method

[0052] S33. Calculate the surface saturated water vapor concentration C S and environmental concentration C ∞ , specifically: CS The droplet center temperature T d Calculate the saturated vapor pressure P using the Antoine equation S , and then converted to concentration where R g is the water vapor gas constant; when the droplet diameter is ≤200μm, the Biot number where K S is the droplet thermal conductivity coefficient. The internal conduction thermal resistance of the droplet is much smaller than the surface convection thermal resistance. Assuming that the droplet temperature is uniform, T d represents the surface temperature;

[0053] Before the experiment, nitrogen was introduced into the experimental chamber 11 for 5 minutes. The relative humidity in the experimental chamber 11 was 0. The relative humidity was monitored in real time by the humidity sensor. The experiment was started after it was confirmed to be stable. Therefore, C ∞ =0, simplified to:

[0054] S4, calculating the droplet surface heat transfer coefficient h; comprising the following steps:

[0055] S41. Based on the law of energy conservation, the enthalpy difference between the nitrogen entering and leaving the experimental chamber 11 is equal to the sum of the heat exchange of the droplets and the heat loss of the experimental chamber 11, that is, Q = Q conv +Q dis ; Where Q is the total heat exchange, Q conv is the convective heat transfer of the droplets, Q dis is the heat loss of the experimental chamber 11;

[0056] S42. The total heat exchange Q is calculated from the inlet and outlet parameters of nitrogen entering and leaving the experimental chamber 11, Q = m N2 c p (T out -T in ), where m N2 is the nitrogen mass flow rate, which is converted after measuring the volume flow rate by the rotor flowmeter, c p is the specific heat capacity of nitrogen at constant pressure, specifically 1040 J / (kg·K);

[0057] S43, droplet convection heat transfer Q conv It is expressed as Q through Newton's cooling formula conv =h·A·(Ts-T en ), where Ts≈T is based on the assumption that Bi<0.1 d ;

[0058] S44, heat loss Q of experimental chamber 11 dis Calibrate through a blank experiment, specifically, introduce the same flow rate of nitrogen when there is no droplet, and record the enthalpy difference of nitrogen entering and leaving the experimental chamber 11, which is Q dis , eliminating environmental interference of heat dissipation in the experimental cavity and heat conduction in the window;

[0059] S45, solve the heat transfer coefficient h, and combine S41-S44 to get

[0060] In this embodiment, multi-parameter synchronous measurement is used. Temperature, flow rate, and deflection are collected synchronously at 100 Hz. The camera captures one frame every 1 ms. All data are aligned by time stamp to construct a millisecond-level transient dataset containing mass, temperature, and morphology. This captures the dynamic coupled changes of droplet surface parameters during the freezing process, such as the corresponding relationship between the sudden drop in mass and the temperature plateau.

[0061] Therefore, the present invention adopts the above-mentioned device and method for measuring the heat and mass transfer coefficient of the droplet surface, and performs high-frequency data acquisition through thermocouples and cameras, which can accurately capture short-term parameter changes during the freezing process; utilizes droplet Biot number analysis to reasonably approximate the uniform temperature distribution inside the droplet, thereby improving the accuracy of surface temperature and saturated vapor pressure calculation; uses cooling nitrogen to completely exclude the air in the chamber, so that the relative humidity is 0, simplifies the mass transfer equation calculation, and reduces errors; the design of nitrogen purges the surface of the experimental chamber effectively avoids low-temperature fogging, ensuring clear observation and recording of morphological changes during droplet freezing; the experimental chamber is insulated to reduce heat leakage and ensure a stable experimental environment; the nickel-chromium alloy wire suspends droplets combined with laser deflection measurement to accurately obtain droplet mass changes and provide reliable data for mass transfer coefficient calculation.

[0062] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the same. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solutions of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A device for measuring heat and mass transfer coefficient on a droplet surface, characterized by: The apparatus comprises a high-pressure nitrogen cylinder, which is connected to a liquid nitrogen container. A rotor flowmeter and a valve are provided between the high-pressure nitrogen cylinder and the liquid nitrogen container. A spiral copper tube is provided on the outside of the liquid nitrogen container. The outlet of the liquid nitrogen container is connected to the inlet of an experimental chamber. A heating tube, an inlet flowmeter, and an inlet temperature probe are sequentially provided between the liquid nitrogen container and the experimental chamber. The outside of the experimental chamber is covered with an insulation layer. A screen and a thermocouple are provided inside the experimental chamber. Two sets of thermocouples are provided, which are respectively used to measure the center temperature of the droplet and the temperature of the air near the droplet. An outlet flowmeter and an outlet temperature probe are provided at the outlet of the experimental chamber. The droplet is suspended by a nickel-chromium alloy wire. A laser displacement sensor is provided below the nickel-chromium alloy wire. The measuring device as a whole is provided with a data collector and a computer. The experimental chamber is made of transparent material and is provided with a camera.

2. A method for measuring the heat and mass transfer coefficient of a droplet surface, characterized in that: The following steps are involved: S1: Nitrogen gas is cooled in a liquid nitrogen container and introduced into the experimental chamber at a certain temperature to drive the droplets to freeze. A heating tube is used to control the temperature of the nitrogen gas according to the experimental conditions. The heat-exchanged nitrogen gas is blown through the experimental chamber window at a flow rate of 5 m / s, forming an air curtain to prevent fogging at low temperatures and ensure that the camera can clearly capture the droplet morphology throughout the entire process. S2. Obtain the nitrogen temperature T at the inlet of the experimental chamber through the inlet flow meter and inlet temperature probe. in and flow Q in The nitrogen temperature T at the outlet of the experimental chamber is obtained by the outlet flow meter and the outlet temperature probe. out and flow Q out , the droplet center temperature T is measured by thermocouple d and the air temperature near the droplet T en The mass m(t) of the droplet at different time periods is obtained by measuring the deflection of the nickel-chromium alloy wire with a laser displacement sensor. The surface area of ​​the droplet is obtained by taking an image of the droplet with a camera and calculating the equivalent diameter d based on the spherical assumption. S3. Calculate the droplet surface mass transfer coefficient h m ; S4. Calculate the droplet surface heat transfer coefficient h.

3. The method for measuring the heat and mass transfer coefficient of a droplet surface according to claim 2, characterized in that: In S2, the deflection δ of the nickel-chromium alloy wire x Acquisition involves the following steps: S21, the laser displacement sensor emits a red laser beam with a wavelength of 650nm and focuses it on the surface of the nickel-chromium alloy wire to form a light spot with a diameter of 5μm; S22, the reflected light is focused on the CMOS detector through the convex lens to form a light spot position signal; S23, when the nickel-chromium alloy wire bends downward due to the mass of the droplet, the position of the light spot on the CMOS detector shifts; S24. Calculate the deflection value in real time through geometric trigonometric relationships.

4. The method for measuring the heat and mass transfer coefficient of a droplet surface according to claim 2, characterized in that: In S2, the specific calculation method of the droplet surface area A is as follows: use a camera to capture the droplet shape, fit the droplet equivalent diameter d through the image recognition algorithm, consider the non-spherical deformation, fit the major and minor axes of the ellipse, and calculate the surface area A = πd based on the spherical assumption. 2 , correcting the error of ignoring morphological changes in traditional methods.

5. The method for measuring the heat and mass transfer coefficient of a droplet surface according to claim 4, characterized in that: S3 includes the following steps: S31. Based on Fick's law and the conservation of mass at the phase interface, the change in droplet mass is dominated by surface water vapor diffusion, and the mass transfer equation is: in is the droplet mass change rate; S32. Combined deflection-mass relationship Calculate the droplet mass m(t) and find the rate of change by the difference method S33. Calculate the surface saturated water vapor concentration C S and environmental concentration C ∞ , specifically: C S The droplet center temperature T d Calculate the saturated vapor pressure P using the Antoine equation S , and then converted to concentration where R g is the water vapor gas constant; when the droplet diameter is ≤200μm, the Biot number where K S is the droplet thermal conductivity coefficient. The internal conduction thermal resistance of the droplet is much smaller than the surface convection thermal resistance. Assuming that the droplet temperature is uniform, T d It also represents the droplet surface temperature; Before the experiment, nitrogen was introduced into the experimental chamber for 5 minutes. The relative humidity in the experimental chamber was 0. The relative humidity was monitored in real time by a humidity sensor. The experiment was started after it was confirmed to be stable. Therefore, C ∞ =0, simplified to:

6. The method for measuring the heat and mass transfer coefficient of a droplet surface according to claim 4, characterized in that: S4 includes the following steps: S41. Based on the law of energy conservation, the enthalpy difference between nitrogen entering and leaving the experimental chamber is equal to the sum of the heat exchange of the droplets and the heat loss of the experimental chamber, that is, Q = Q conv +Q dis ; Where Q is the total heat exchange, Q conv is the convective heat transfer of the droplets, Q dis is the heat loss of the experimental chamber; S42. The total heat transfer Q is calculated from the inlet and outlet parameters of nitrogen entering and leaving the experimental chamber, Q = m N2 c p (T out -T in ), where m N2 is the nitrogen mass flow rate, which is converted after measuring the volume flow rate by the rotor flowmeter, c p is the specific heat capacity of nitrogen at constant pressure, specifically 1040 J / (kg·K); S43, droplet convection heat transfer Q conv It is expressed as Q through Newton's cooling formula conv =h·A·(Ts-T en ), where Ts≈T is based on the assumption that Bi<0.1 d ; S44, experimental chamber heat loss Q dis Calibrate through a blank experiment, specifically, introduce the same flow rate of nitrogen when there is no droplet, and record the enthalpy difference of nitrogen entering and leaving the experimental chamber, which is Q dis , eliminating environmental interference of heat dissipation in the experimental cavity and heat conduction in the window; S45, solve the heat transfer coefficient h, and combine S41-S44 to get

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