A device and method for measuring the heat and mass transfer coefficient of a droplet surface

Abstract

This invention discloses a device and method for measuring the heat and mass transfer coefficient of a droplet surface, belonging to the technical field of droplet surface heat and mass transfer coefficient measurement. Nitrogen gas is introduced into the experimental chamber at a certain temperature from a high-pressure nitrogen cylinder, through a liquid nitrogen container, and then used in conjunction with a heating tube and flow meter to achieve parameter control. A laser displacement sensor measures the droplet mass, a thermocouple measures the temperature, and a camera records the morphology. A data acquisition unit and a computer process the data synchronously. The method includes freezing the droplet with cryogenic nitrogen gas, purging the window with nitrogen gas to prevent fogging, and calculating the mass transfer coefficient and heat transfer coefficient by measuring the nitrogen gas flow temperature and flow rate, the droplet temperature, and the deflection of the nickel-chromium alloy wire, combined with Fick's law and energy conservation. This invention uses the above-mentioned device and method for measuring the heat and mass transfer coefficient of a droplet surface, captures transient changes through high-frequency data acquisition, achieves a zero-humidity environment through nitrogen gas displacement, ensures morphological observation through an air curtain to prevent fogging, and reduces heat loss through insulation design, thereby improving measurement accuracy and reliability.

Description

Technical Field

[0001] This invention relates to the field of measuring the heat and mass transfer coefficient of droplet surfaces, and in particular to a device and method for measuring the heat and mass transfer coefficient of droplet surfaces. Background Technology

[0002] The measurement of the heat and mass transfer coefficient of a droplet surface refers to the quantification of the efficiency of heat and mass transfer between the droplet surface and its surrounding environment through experiments or simulations. The heat transfer coefficient characterizes the heat transfer capacity driven by temperature difference, while the mass transfer coefficient reflects the diffusion rate of matter driven by concentration difference. These two coefficients are coupled during droplet evaporation and condensation processes 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. At the academic level, it helps to reveal the mechanism of multiphase flow and the theory of phase change heat and mass transfer, laying the foundation for the development of micro-nano scale 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, the measurement of the heat and mass transfer coefficient is a key link in promoting technological innovation and an important bridge connecting theoretical models and engineering practice.

[0003] Existing techniques for measuring heat and mass exchange between droplets and the external environment suffer from insufficient data acquisition for key parameters during the freezing process due to its short duration, affecting the accuracy and reliability of measurement results. Regarding the measurement of mass transfer coefficients on the outer surface of droplets, some techniques fail to adequately verify the uniformity of temperature distribution within the droplet, potentially leading to deviations in the calculation of surface saturated vapor pressure. Furthermore, the handling of water vapor partial pressure within the chamber environment is not rigorous enough, impacting the accuracy of mass transfer coefficient measurements. In measuring the heat transfer coefficient of the outer surface of droplets, some devices fail to effectively eliminate the influence of heat loss, or the approximate treatment of droplet surface temperature lacks theoretical basis and experimental verification, resulting in errors in the calculated heat transfer coefficient. Finally, during the observation of droplet freezing, the overall experimental setup may experience fogging of the observation window due to low temperatures, affecting the real-time observation and recording of changes in droplet morphology. Summary of the Invention

[0004] The purpose of this invention is to provide a device and method for measuring the heat and mass transfer coefficient of a droplet surface, ensuring a stable experimental environment, accurately capturing transient parameter changes during freezing, using cooling nitrogen to completely remove air from the chamber to achieve a relative humidity of 0, simplifying the calculation of the mass transfer equation, effectively avoiding low-temperature fogging, ensuring clear observation and recording of morphological changes during droplet freezing, and accurately acquiring droplet mass changes to provide reliable data for mass transfer coefficient calculation.

[0005] To achieve the above objectives, the present invention provides a device for measuring the surface heat and mass transfer coefficient of a droplet, comprising a high-pressure nitrogen cylinder connected to a liquid nitrogen container, a rotor flow meter and a valve disposed between the high-pressure nitrogen cylinder and the liquid nitrogen container, a spiral copper tube disposed on the outside of the liquid nitrogen container, an outlet of the liquid nitrogen container connected to the inlet of an experimental chamber, a heating tube, an inlet flow meter and an inlet temperature probe disposed sequentially between the liquid nitrogen container and the experimental chamber, the experimental chamber being covered with an insulation layer, and a sieve and thermocouples disposed inside the experimental chamber, two sets of thermocouples being disposed for measuring the temperature at the center of the droplet and the temperature of the air near the droplet, respectively, an outlet flow meter and an outlet temperature probe being disposed at the outlet of the experimental chamber, the droplet being suspended by a nichrome alloy wire, and a laser displacement sensor disposed below the nichrome alloy wire; the measuring device as a whole includes a data acquisition unit and a computer, and the experimental chamber is made of transparent material and is equipped 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 by liquid nitrogen container and introduced into the experimental chamber at a certain temperature to drive the droplets to freeze. The temperature of nitrogen gas is controlled by heating tube according to the experimental conditions. After heat exchange, the nitrogen gas is blown through the window of the experimental chamber at a flow rate of 5 m / s to form an air curtain to prevent low-temperature fogging and ensure that the camera can clearly capture the droplet shape throughout the process.

[0008] S2. Obtain the nitrogen temperature T at the inlet of the experimental chamber using an inlet flow meter and an inlet temperature probe. in and traffic Q in The nitrogen temperature T at the outlet of the experimental chamber was obtained using an outlet flow meter and an outlet temperature probe. out and traffic Q out The temperature T at the center of the droplet was measured using a thermocouple. d and the air temperature T near the droplet en The mass m(t) of the droplet at different time periods is obtained by measuring the deflection of the nickel-chromium alloy wire using a laser displacement sensor. The surface area of ​​the droplet is obtained by capturing images 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 heat transfer coefficient h of the droplet surface.

[0011] Preferably, in S2, the deflection δ of the nickel-chromium alloy wire is... x The process includes the following steps:

[0012] S21, The laser displacement sensor emits a red laser beam with a wavelength of 650nm, which is focused 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 onto the CMOS detector by a convex lens to form a light spot position signal;

[0014] S23. When the nickel-chromium alloy wire bends downwards 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 using geometric trigonometric relationships.

[0016] Preferably, in S2, the specific method for calculating the droplet surface area A is as follows: The droplet shape is captured by a camera, and the equivalent diameter d of the droplet is fitted using an image recognition algorithm. Non-spherical deformation needs to be considered. The major and minor axes of an ellipse are fitted, and the surface area A = πd is calculated based on the spherical assumption. 2 This corrects the error of neglecting 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 The rate of change of droplet mass;

[0019] S32, Combining Deflection-Mass Relationship Calculate the droplet mass m(t) and obtain the rate of change using the finite difference method.

[0020] S33. Calculate the surface saturated water vapor concentration C S With environmental concentration C ∞ Specifically: C S From the droplet center temperature T d Calculate the saturated vapor pressure P using the Antoine equation. S Then convert to concentration Where R g The gas constant for water vapor; when the droplet diameter is ≤200μm, the Biot number is... Where K S Let T be the thermal conductivity of the droplet. The internal thermal resistance of the droplet is much smaller than the surface convective thermal resistance. Assuming the droplet temperature is uniform, T... d It also represents the surface temperature of the droplet;

[0021] Before the experiment, the experimental chamber was purged with nitrogen for 5 minutes to achieve a relative humidity of 0%. The relative humidity was monitored in real time using a humidity sensor. The experiment began after the humidity was confirmed to be stable. Therefore, C ∞ =0, simplified to:

[0022] Preferably, S4 includes the following steps:

[0023] S41. Based on the law of conservation of energy, the enthalpy difference between the nitrogen entering and exiting the experimental chamber is equal to the sum of the heat exchanged by the droplets and the heat loss of the experimental chamber, i.e., Q = Q conv +Q dis Where Q is the total heat exchange, Q conv For heat transfer via droplet convection, Q dis For heat loss in the experimental chamber;

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

[0025] S43, Droplet convection heat transfer Q conv Expressed by Newton's law of cooling as Q conv =h·A·(Ts-T) en ), where Ts≈T under the assumption that Bi<0.1. d ;

[0026] S44, Heat loss of the experimental chamber Q dis The experiment was calibrated using a blank test, specifically by introducing the same flow rate of nitrogen gas without any droplets and recording the enthalpy difference between the nitrogen entering and leaving the experimental chamber, which is Q. dis Eliminate environmental interference from heat dissipation in the experimental chamber and heat conduction through the windows;

[0027] S45. Solving for the heat transfer coefficient h, by simultaneously solving S41-S44, we get...

[0028] Therefore, the present invention employs 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 can accurately capture transient parameter changes during freezing; By using droplet Biot number analysis, the uniform temperature distribution inside the droplet can be reasonably approximated, improving the accuracy of surface temperature and saturated vapor pressure calculations; Cooling nitrogen is used to completely remove air from the chamber, making the relative humidity 0, simplifying the mass transfer equation calculation and reducing errors.

[0030] 2) The design of purging the surface of the experimental chamber with nitrogen effectively avoids low-temperature fogging, ensuring clear observation and recording of morphological changes during the freezing process of droplets;

[0031] 3) The experimental chamber is insulated to reduce heat leakage and ensure a stable experimental environment; the method of suspending the droplet with a nickel-chromium alloy wire and measuring the deflection with laser can accurately obtain the droplet mass change and provide reliable data for the calculation of the mass transfer coefficient.

[0032] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

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

[0034] Figure Labels

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

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

[0037] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms "first," "second," and similar terms used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed following the word and their equivalents, without excluding other elements or objects. Terms such as "connected" or "linked" are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. Terms such as "upper," "lower," "left," and "right" are used only to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0038] Example 1

[0039] This 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, the apparatus includes a high-pressure nitrogen cylinder 1, connected to a liquid nitrogen container 4. A rotor flowmeter 2 and a valve 3 are installed between the high-pressure nitrogen cylinder 1 and the liquid nitrogen container 4. A spiral copper tube 5 is installed on the outside of the liquid nitrogen container 4. The outlet of the liquid nitrogen container 4 is connected to the inlet of an experimental chamber 11. A heating tube 6, an inlet flowmeter 10, and an inlet temperature probe 9 are sequentially installed between the liquid nitrogen container 4 and the experimental chamber 11. The experimental chamber 11 is covered with an insulation layer to prevent heat leakage, reduce external thermal interference, and maintain a stable low-temperature environment inside the experimental chamber 11. A sieve 7 and thermocouples 8 are installed inside. Two sets of thermocouples 8 are installed, used to measure the temperature at the center of the droplet and the temperature of the air near the droplet, respectively. The diameter of the thermocouples 8 is ≤20μm to minimize interference with the droplet morphology. An outlet flow meter 12 and an outlet temperature probe 13 are installed at the outlet of experimental chamber 11. The droplet is suspended inside experimental chamber 11 by a horizontally arranged nichrome alloy wire. A laser displacement sensor is installed below the nichrome alloy wire. The laser displacement sensor measures deflection non-contactly, avoiding the volume interference and low-temperature drift problems of traditional weighing sensors. The measuring device is equipped with a data acquisition unit 14 and a computer 15. Experimental chamber 11 is made of transparent material and is equipped with a camera. The camera is used to observe the changes in droplet morphology during the freezing stage. After exchanging heat with the droplet, nitrogen gas is guided through a pipe to blow onto the surface of experimental chamber 11 to avoid low-temperature fogging. The droplet freezing process is recorded by the camera. In this embodiment, the nichrome alloy wire has a diameter of 50 μm and a deflection measurement accuracy of ±1 μm, enabling it to capture 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 has a thickness of 5 cm and a thermal conductivity of ≤0.04 W / (m·K). Nitrogen gas undergoes heat exchange in the liquid nitrogen container 4 through valve 3, causing its temperature to drop. The nitrogen flow rate can be controlled by valve 3 of the high-pressure nitrogen cylinder 1. The cooled nitrogen gas is introduced into the experimental chamber 11 to exchange heat with the droplets, causing the droplets to cool and solidify. Thermocouple 8, inlet flow meter 10, inlet temperature probe 9, outlet flow meter 12, and outlet temperature probe 13 are all connected to the data acquisition unit 14. The droplet morphology data captured by the camera is stored in the 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 gas is cooled by liquid nitrogen container 4 and introduced into experimental chamber 11 at a certain temperature to drive the droplets to freeze. The temperature of nitrogen gas can be controlled by heating tubes according to experimental conditions. After heat exchange, nitrogen gas is blown through the window of experimental chamber 11 at a flow rate of 5 m / s to form an air curtain to prevent low-temperature fogging and ensure that the camera can clearly capture the droplet shape throughout the process.

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

[0043] The deflection δ of nickel-chromium alloy wire x The process includes the following steps:

[0044] S21, The laser displacement sensor emits a red laser beam with a wavelength of 650nm, which is focused 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 onto the CMOS detector by a convex lens to form a light spot position signal;

[0046] S23. When the nickel-chromium alloy wire bends downwards 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 using geometric trigonometric relationships.

[0048] The specific method for calculating the surface area A of a droplet is as follows: The droplet shape is captured by a camera, and the equivalent diameter d of the droplet is fitted using an image recognition algorithm. Non-spherical deformation needs to be considered. The major and minor axes of an ellipse are fitted, and the surface area A = πd is calculated based on the spherical assumption. 2 This corrects the error of neglecting morphological changes in traditional methods.

[0049] S3. Calculate the droplet surface mass transfer coefficient h. m Specifically, it includes 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 The rate of change of droplet mass;

[0051] S32, Combining Deflection-Mass Relationship Calculate the droplet mass m(t) and obtain the rate of change using the finite difference method.

[0052] S33. Calculate the surface saturated water vapor concentration C S With environmental concentration C ∞ Specifically: CS From the droplet center temperature T d Calculate the saturated vapor pressure P using the Antoine equation. S Then convert to concentration Where R g The gas constant for water vapor; when the droplet diameter is ≤200μm, the Biot number is... Where K S Let T be the thermal conductivity of the droplet. The internal thermal resistance of the droplet is much smaller than the surface convective thermal resistance. Assuming the droplet temperature is uniform, T... d Represents surface temperature;

[0053] Before the experiment, nitrogen gas was purged into experimental chamber 11 for 5 minutes, and the relative humidity inside chamber 11 was 0. The relative humidity was monitored in real time using a humidity sensor. The experiment began after the humidity was confirmed to be stable. Therefore, C ∞ =0, simplified to:

[0054] S4. Calculate the droplet surface heat transfer coefficient h; including the following steps:

[0055] S41. Based on the law of conservation of energy, the enthalpy difference between the nitrogen entering and exiting experimental chamber 11 is equal to the sum of the heat exchanged by the droplets and the heat loss of experimental chamber 11, i.e., Q = Q conv +Q dis Where Q is the total heat exchange, Q conv For heat transfer via droplet convection, Q dis For heat loss in experimental chamber 11;

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

[0057] S43, Droplet convection heat transfer Q conv Expressed by Newton's law of cooling as Q conv =h·A·(Ts-T) en ), where Ts≈T under the assumption that Bi<0.1. d ;

[0058] S44, Heat loss Q of experimental chamber 11 dis The experiment was calibrated using a blank test, specifically by introducing the same flow rate of nitrogen gas into the experimental chamber 11 without any liquid droplets, and recording the enthalpy difference between the nitrogen gas entering and exiting chamber 11 as Q. dis Eliminate environmental interference from heat dissipation in the experimental chamber and heat conduction through the windows;

[0059] S45. Solving for the heat transfer coefficient h, by simultaneously solving S41-S44, we get...

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

[0061] Therefore, this invention employs the aforementioned device and method for measuring the surface heat and mass transfer coefficient of a droplet. High-frequency data acquisition via thermocouples and a camera accurately captures transient parameter changes during the freezing process. By utilizing the Droplet Biot number analysis, a reasonable approximation of the uniform temperature distribution within the droplet is made, improving the accuracy of surface temperature and saturated vapor pressure calculations. The use of cooling nitrogen to completely eliminate air from the chamber, achieving a relative humidity of 0, simplifies the mass transfer equation calculation and reduces errors. The nitrogen purging design of the experimental chamber surface effectively prevents low-temperature fogging, ensuring clear observation and recording of morphological changes during droplet freezing. Insulation of the experimental chamber reduces heat leakage and ensures a stable experimental environment. The method of suspending the droplet with a nickel-chromium alloy wire combined with laser measurement of deflection accurately obtains changes in droplet mass, providing reliable data for mass transfer coefficient calculations.

[0062] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions 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 the heat and mass transfer coefficient of a droplet surface, characterized in that: The device includes a high-pressure nitrogen cylinder connected to a liquid nitrogen container. A rotor flow meter and a valve are installed between the high-pressure nitrogen cylinder and the liquid nitrogen container. A spiral copper tube is installed 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 flow meter, and an inlet temperature probe are sequentially installed between the liquid nitrogen container and the experimental chamber. The heating tube is used to regulate the temperature of the nitrogen gas. The nitrogen gas after heat exchange is used to purge the window of the experimental chamber, forming an air curtain to prevent low-temperature fogging and ensure that the camera can clearly capture the droplet morphology throughout the process. The experimental chamber is covered with an insulation layer. The interior of the experimental chamber is equipped with a screen, thermocouples, and a humidity sensor. The humidity sensor monitors the relative humidity in real time. Two sets of thermocouples are installed, one for measuring the temperature at the center of the droplet and the other for measuring the temperature of the air near the droplet. An outlet flow meter and an outlet temperature probe are installed at the outlet of the experimental chamber. The droplet is suspended by a nichrome alloy wire, and a laser displacement sensor is installed below the nichrome alloy wire. The measuring device as a whole includes a data acquisition unit and a computer. The experimental chamber is made of transparent material and is equipped with a camera.

2. A method for measuring the heat and mass transfer coefficient of a droplet surface based on the measuring device described in claim 1, characterized in that, Includes the following steps: S1. Nitrogen gas is cooled by liquid nitrogen container and introduced into the experimental chamber at a certain temperature to drive the droplets to freeze. The temperature of nitrogen gas is controlled by heating tube according to the experimental conditions. After heat exchange, the nitrogen gas is blown through the window of the experimental chamber at a flow rate of 5 m / s to form an air curtain to prevent low-temperature fogging and ensure that the camera can clearly capture the droplet shape throughout the process. S2. Obtain the nitrogen temperature T at the inlet of the experimental chamber using an inlet flow meter and an inlet temperature probe. in and traffic Q in The nitrogen temperature T at the outlet of the experimental chamber was obtained using an outlet flow meter and an outlet temperature probe. out and traffic Q out The temperature T at the center of the droplet was measured using a thermocouple. d and the air temperature T near the droplet en The mass m(t) of the droplet at different time periods is obtained by measuring the deflection of the nickel-chromium alloy wire using a laser displacement sensor. The surface area of ​​the droplet is obtained by capturing images 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 ; Specifically, the following steps are included: 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 The rate of change of droplet mass; S32, Combining Deflection-Mass Relationship Calculate the droplet mass m(t) and obtain the rate of change using the finite difference method. S33. Calculate the surface saturated water vapor concentration C S With environmental concentration C ∞ Specifically: C S From the droplet center temperature T d Calculate the saturated vapor pressure P using the Antoine equation. S Then convert to concentration Where R g The gas constant for water vapor; when the droplet diameter is ≤200μm, the Biot number is... Where K S Let T be the thermal conductivity of the droplet. The internal thermal resistance of the droplet is much smaller than the surface convective thermal resistance. Assuming the droplet temperature is uniform, T... d It also represents the surface temperature of the droplet; Before the experiment, the experimental chamber was purged with nitrogen for 5 minutes to achieve a relative humidity of 0%. The relative humidity was monitored in real time using a humidity sensor. The experiment began after the humidity was confirmed to be stable. Therefore, C ∞ =0, simplified to: S4. Calculate the heat transfer coefficient h of the droplet surface; specifically, this includes the following steps: S41. Based on the law of conservation of energy, the enthalpy difference between the nitrogen entering and exiting the experimental chamber is equal to the sum of the heat exchanged by the droplets and the heat loss of the experimental chamber, i.e., Q = Q conv +Q dis Where Q is the total heat exchange, Q conv For heat transfer via droplet convection, Q dis For heat loss in the experimental chamber; S42. The total heat exchange Q is calculated from the inlet and outlet parameters of nitrogen entering and exiting the experimental chamber, Q = m N2 c p (T out -T in ), where m N2 The nitrogen mass flow rate is converted after measuring the volumetric flow rate using a rotor flow meter. p The specific heat capacity of nitrogen at constant pressure is 1040 J / (kg·K); S43, Droplet convection heat transfer Q conv Expressed by Newton's law of cooling as Q conv =h·A·(Ts-T) en ), where Ts≈T under the assumption that Bi<0.

1. d ; S44, Heat loss of the experimental chamber Q dis The experiment was calibrated using a blank test, specifically by introducing the same flow rate of nitrogen gas without any droplets and recording the enthalpy difference between the nitrogen entering and leaving the experimental chamber, which is Q. dis Eliminate environmental interference from heat dissipation in the experimental chamber and heat conduction through the windows; S45. Solving for the heat transfer coefficient h, by simultaneously solving S41-S44, we get...

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 The process includes the following steps: S21, The laser displacement sensor emits a red laser beam with a wavelength of 650nm, which is focused 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 onto the CMOS detector by a convex lens to form a light spot position signal; S23. When the nickel-chromium alloy wire bends downwards 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 using 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 method for calculating the droplet surface area A is as follows: The droplet shape is captured by a camera, and the equivalent diameter d of the droplet is fitted using an image recognition algorithm. Non-spherical deformation needs to be considered. The major and minor axes of an ellipse are fitted, and the surface area A = πd is calculated based on the spherical assumption. 2 This corrects the error of neglecting morphological changes in traditional methods.

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