Film thickness control method for surface film of heat dissipation structure based on thin film evaporation

Abstract

The application discloses a surface film thickness control method of a heat dissipation structure based on film evaporation, and comprises the following steps: obtaining an initial thickness of a water film at an entrance on a top of a wall surface of the heat dissipation structure; obtaining dynamic thicknesses of the water film at various positions along a flowing direction of the wall surface; determining a minimum water film thickness based on a mass flow of the water film, combining dynamic force balance and stability analysis of water film flow; reversely deducing a maximum water film thickness dominated by evaporation heat dissipation based on a heat transfer coefficient threshold; and if the initial thickness or the dynamic thickness exceeds a safety range between the minimum water film thickness and the maximum water film thickness, adjusting a wind speed or a water flow to make the initial thickness or the dynamic thickness within the safety range. The application realizes dynamic control of the water film thickness between the minimum water film thickness and the maximum water film thickness by real-time acquisition of the initial thickness and the dynamic thickness of the water film, combining minimum and maximum film thickness limits, so that the heat dissipation is always dominated by latent heat phase change of the water film evaporation, and the heat dissipation efficiency is improved.

Description

Technical Field

[0001] This invention relates to the field of heat dissipation and cooling technology, and in particular to a method for controlling the surface film thickness of a heat dissipation structure based on thin film evaporation. Background Technology

[0002] Traditional heat dissipation equipment, such as air-cooled and water-cooled condensers, has the following limitations: Air-cooled condensers rely on forced convection heat transfer, resulting in low heat transfer coefficients, a sharp drop in efficiency at high temperatures, and a cooling limit of only 10°C above ambient temperature. Water-cooled condensers require a cooling tower, leading to complex systems, large footprints, and a cooling temperature difference of 3-5°C. Both are limited by sensible heat exchange and cannot overcome the temperature limits of the cooling medium. Evaporative cooling technology, however, utilizes the principle of water evaporation and heat absorption, replacing sensible heat exchange with latent heat of phase change, significantly improving heat dissipation efficiency. In existing evaporative cooling equipment, the control of water film thickness is unstable, easily leading to decreased evaporative cooling performance due to localized drying or excessive film thickness. Especially when airflow passes over the water film surface, the interaction between water and airflow (such as co-current or counter-current flow) increases the complexity of dynamic water film control. Micro-nano structured hydrophilic surfaces can enhance water film spreadability, but how to precisely control the water film thickness to optimize the heat and mass transfer process remains a technical challenge. Therefore, a design method that integrates heat and mass transfer models and real-time control strategies is needed to ensure that the water film operates stably within a safe range.

[0003] The patent document "A Pump-Driven Thin-Film Evaporation Heat Dissipation Device and Method for High Heat Flux Density Electronic Devices," published in Chinese patent literature (CN111146167A, published on 2020-05-12), includes a liquid storage device, a piezoelectric micropump, an evaporator, a micropump, a condenser, and pipes and valves connecting the various components. The cooling medium is stored in the liquid storage device and enters the evaporator via the piezoelectric micropump. The generated gaseous cooling medium is driven by the micropump and cooled to a liquid state by the condenser before finally entering the liquid storage device. Excess liquid cooling medium in the evaporator also enters the liquid storage device. In the evaporator, the liquid cooling medium is vertically replenished onto the microstructure surface by a replenisher. Through the combined action of the replenisher, the nanoporous membrane, the microstructure surface, and the liquid channels, a uniform and continuous micro-liquid film is formed between the nanoporous membrane and the microstructure surface, enabling heat dissipation at ultra-high heat flux densities. However, this technology only ensures the presence of a water film on the heat dissipation surface by replenishing the liquid, and cannot stably control the thickness of the water film on the heat dissipation surface. It cannot accurately control the thickness of the water film to optimize the heat and mass transfer process, and is prone to a decrease in evaporative heat dissipation performance due to local drying or excessive film thickness. Summary of the Invention

[0004] The present invention aims to overcome the problem in the prior art that it is impossible to accurately control the thickness of the water film to optimize the heat and mass transfer process, and that the evaporative heat dissipation performance is easily reduced due to local drying or excessive film thickness. The invention provides a method for controlling the surface film thickness of a heat dissipation structure based on thin film evaporation.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: A method for controlling the surface film thickness of a heat dissipation structure based on thin film evaporation, comprising: Obtain the initial thickness of the water film at the top inlet of the wall of the heat dissipation structure; Obtain the dynamic thickness of the water film at various positions along the flow direction along the wall; Based on the mass flow rate of the water film, combined with dynamic force balance and stability analysis of the water film flow, the minimum water film thickness is determined. The maximum water film thickness dominated by evaporative heat dissipation is calculated by back-calculating the heat transfer coefficient threshold. If the initial thickness or dynamic thickness exceeds the safe range between the minimum and maximum water film thickness, adjust the wind speed or water flow rate to bring the initial thickness or dynamic thickness within the safe range.

[0006] This invention achieves dynamic control of the water film thickness by acquiring the initial and dynamic thickness of the water film in real time, combined with minimum and maximum film thickness limits, thereby improving heat dissipation efficiency and equipment reliability. It breaks through the traditional limits of air / water cooling heat transfer by utilizing the latent heat of water film evaporation to replace sensible heat exchange, enhancing the equipment's heat transfer coefficient. Simultaneously, by controlling the water film thickness between the minimum and maximum thickness, heat dissipation is always dominated by the latent heat phase change of water film evaporation, avoiding water film drying or sensible heat dominating heat dissipation, thus greatly improving heat dissipation efficiency. Furthermore, the surface shear force generated by surface wind speed can be used to strengthen the evaporation driving force in the saturated region of the gas-liquid interface, allowing the final cooling effect to approach the ambient wet-bulb temperature.

[0007] Preferably, the process of adjusting the wind speed or water flow rate to keep the initial thickness or dynamic thickness within a safe range includes: If the initial thickness or dynamic thickness is less than the minimum water film thickness, then at least one of the following adjustment methods should be selected: increasing the water flow rate or increasing the wind speed in the downstream direction. If the initial thickness or dynamic thickness is greater than the maximum water film thickness, then at least one adjustment method should be selected: reducing the water flow rate or increasing the wind speed in the counter-current direction.

[0008] Preferably, the process of determining the minimum water film thickness includes: Based on the critical mass flow rate of the water film and the balance between interfacial tension, gravity, interfacial shear force, and wall hydrophilicity, the minimum water film thickness is calculated. The minimum water film thickness is directly proportional to the third power of the interfacial tension, inversely proportional to the third power of the water density, inversely proportional to the square third power of the interfacial shear force, and directly proportional to the third power of the difference between 1 and the cosine of the contact angle.

[0009] Preferably, the process of determining the maximum water film thickness includes: the maximum water film thickness is inversely proportional to the heat transfer coefficient threshold; The maximum water film thickness is obtained by dividing the thermal conductivity of water by the heat transfer coefficient. The numerator of the heat transfer coefficient includes convective heat dissipation, evaporative heat dissipation, and sensible heat transport, while the denominator is the difference between the wall temperature and the interface temperature at the water film-air phase interface.

[0010] Preferably, the process of obtaining the initial thickness includes: Establish the force balance equations for the water film in the direction of flow along the wall, and calculate the velocity distribution inside the water film; The average flow velocity of the water film is calculated based on the velocity distribution, and the mass flow rate of the water film per unit width is further calculated; the direction of the unit width is perpendicular to the wall surface and also perpendicular to the thickness direction of the water film. The initial thickness is calculated based on the mass flow rate expression and the Reynolds number definition of the water film.

[0011] Preferably, the calculation of the velocity distribution inside the water film includes: The force balance equation is established based on the equilibrium between the viscous resistance and gravity inside the water flow in the direction of flow along the wall. The velocity distribution is solved by applying boundary conditions of no wall slip and continuous interfacial shear force. The velocity distribution includes a gravity-driven term and a shear-driven term. The gravity-driven term exhibits a parabolic distribution, while the shear-driven term exhibits a linear distribution.

[0012] Preferably, the process of obtaining the dynamic thickness includes: An evaporation mass model was established, and the evaporation mass loss of the water film along the wall flow direction was calculated based on the law of mass conservation. A heat dissipation model was established, and the heat dissipation of the water film along the wall flow direction was calculated based on energy balance. Based on the initial thickness, and combining the evaporation mass model and the heat dissipation model, the dynamic film thickness after evaporation loss along the flow direction along the wall is calculated.

[0013] Preferably, the establishment of the evaporation mass model includes: Establish a balance between the rate of change of mass flow rate along the wall and the evaporation rate; The evaporation rate is the product of the mass transfer coefficient and the mass transfer driving force. The mass transfer driving force is the difference between the saturated water vapor density at the water film-gas phase interface and the ambient water vapor density. The mass transfer coefficient is calculated using the Chilton-Colburn analogy.

[0014] Preferably, the establishment of the heat dissipation model includes: Establish an energy balance equation between the sum of convective heat dissipation, evaporative heat dissipation, and sensible heat transfer terms and the wall conduction term; The wall thermal conductivity term is the heat flux density transferred from the wall to the water film through thermal conduction. The convective heat dissipation term is the heat flux density transferred to the air by convective heat transfer at the water film interface. The evaporative heat dissipation term is the latent heat flux density of phase change absorbed by the evaporation at the water film interface. The sensible heat transfer term is the rate of change of sensible heat carried by the water film flow in the flow direction, which is reflected by the rate of change of the average temperature of the water film.

[0015] Preferably, the process of obtaining the average temperature of the water film includes: The temperature distribution along the thickness direction within the water film is obtained. The average temperature of the water film is obtained by dividing the integral of the water film flow rate and the temperature distribution along the thickness direction by the mass flow rate. The average temperature of the water film is the superposition of the average temperature based on a linear distribution, the temperature drop caused by evaporative cooling, and the temperature change caused by the flow of the water film.

[0016] This invention has the following beneficial effects: By establishing a water film thickness solution model, an evaporation mass model, and a heat dissipation model, combined with minimum and maximum film thickness constraints, dynamic control of the water film thickness is achieved, thereby improving heat dissipation efficiency and equipment reliability; it breaks through the traditional air-cooled / water-cooled heat transfer limit, utilizing the latent heat of phase change of water film evaporation to replace sensible heat exchange, enhancing the equipment's heat transfer coefficient, while controlling the water film thickness between the minimum and maximum film thickness, ensuring that heat dissipation is always dominated by the latent heat phase change of water film evaporation, avoiding the drying of the water film or sensible heat dominating heat dissipation, greatly improving heat dissipation efficiency; at the same time, the surface shear force generated by surface wind speed can be used to strengthen the evaporation driving force in the saturated zone of the gas-liquid interface, so that the final cooling effect can approach the ambient wet-bulb temperature. Attached Figure Description

[0017] Figure 1 This is a flowchart of the surface film thickness control method for heat dissipation structure based on thin film evaporation in this invention.

[0018] Figure 2 This is a schematic diagram of the water film flowing downwards along a vertical wall in this invention.

[0019] Figure 3 This is a schematic diagram of the force balance of the micro-element liquid in this invention.

[0020] Figure 4 This is a schematic diagram of the micro-element liquid energy balance in this invention.

[0021] Figure 5 This is a schematic diagram of the water film flowing downwards along the inclined wall in this invention.

[0022] Figure 6 This is a schematic diagram of the water film flowing along the surface of the circular tube in this invention. Detailed Implementation

[0023] The present invention will now be further described with reference to the accompanying drawings and specific embodiments.

[0024] like Figure 1 As shown, a method for controlling the surface film thickness of a heat dissipation structure based on thin film evaporation includes: Obtain the initial thickness of the water film at the top inlet of the wall of the heat dissipation structure; Obtain the dynamic thickness of the water film at various positions along the flow direction along the wall; Based on the mass flow rate of the water film, combined with dynamic force balance and stability analysis of the water film flow, the minimum water film thickness is determined. The maximum water film thickness dominated by evaporative heat dissipation is calculated by back-calculating the heat transfer coefficient threshold. If the initial thickness or dynamic thickness exceeds the safe range between the minimum and maximum water film thickness, adjust the wind speed or water flow rate to bring the initial thickness or dynamic thickness within the safe range.

[0025] This invention achieves dynamic control of the water film thickness by acquiring the initial and dynamic thickness of the water film in real time, combined with minimum and maximum film thickness limits, thereby improving heat dissipation efficiency and equipment reliability. It breaks through the traditional limits of air / water cooling heat transfer by utilizing the latent heat of water film evaporation to replace sensible heat exchange, enhancing the equipment's heat transfer coefficient. Simultaneously, by controlling the water film thickness between the minimum and maximum thickness, heat dissipation is always dominated by the latent heat phase change of water film evaporation, avoiding water film drying or sensible heat dominating heat dissipation, thus greatly improving heat dissipation efficiency. Furthermore, the surface shear force generated by surface wind speed can be used to strengthen the evaporation driving force in the saturated region of the gas-liquid interface, allowing the final cooling effect to approach the ambient wet-bulb temperature.

[0026] It should be noted that the main purpose of the heat dissipation structure wall surface in this invention is to form a stable water film on the wall surface. This film can be a circular tube surface formed by stacking several heat dissipation pipes (the diameter of the heat dissipation pipes is relatively small compared to the overall size, so the outer surface formed by the stacked heat dissipation pipes can be considered a single unit), or it can be a flat plate surface formed directly from a flat plate. As long as the outer surface of the heat dissipation structure can form a wall surface that allows the water film to extend and distribute, the specific heat dissipation structure inside does not affect the specific process of controlling the water film thickness in this invention. To better form a water film of a certain thickness on the wall surface, the outer surface of the heat dissipation structure can be treated to form a micro / nano structure, making the outer surface of the heat dissipation structure hydrophilic. This hydrophilicity makes it easier for water to form a water film of a certain thickness on the outer surface of the heat dissipation structure; the thickness and uniformity of the water film are determined by the hydrophilicity.

[0027] It's worth noting that for flat surfaces, they can be categorized into vertical walls (90 degrees) and inclined walls based on the angle between the flat surface and the horizontal ground. For inclined walls, the effect of the inclination angle can be eliminated by multiplying the gravitational acceleration by the sine of the angle. For circular tube surfaces, control is achieved by adjusting the friction length (x=2r), mass flow rate (mass flow rate of the flat surface multiplied by 2η), and evaporation mass (evaporation mass of the flat surface multiplied by 2η). Here, r is the radius of the circular tube, and the 2 in the adjustment formulas for mass flow rate and evaporation mass refers to the circular tube surface; a semicircle can be equivalent to a vertical flat wall, and a circular tube has two semicircles, i.e., the vertical flat wall multiplied by 2. η refers to the error generated by the equivalent process, which can be obtained through experimental data.

[0028] Specifically, water enters from the top inlet of the heat dissipation structure. Due to the hydrophilicity of the wall's micro-nano structure, a water film can form on the surface. The thickness of the water film has a significant impact on the actual evaporative heat dissipation efficiency. If the water film is too thin, it will easily break and dry out. If the water film is too thick, sensible heat transfer will replace latent heat phase transformation as the dominant heat dissipation factor, both of which will reduce the heat dissipation efficiency. Therefore, the water film at the top inlet must first be within a safe range. If it is not within a safe range, the initial thickness of the water film can be brought within a safe range by adjusting the water flow rate and / or wind speed.

[0029] If the initial thickness is within the safe range, the water flow will extend downwards along the wall, forming a water film covering the entire wall surface. Similarly, the water film thickness at any location below the top inlet must also be within the safe range, i.e., the dynamic film thickness must be within the safe range. If it is not within the safe range, the water flow rate and / or air velocity will be adjusted to bring the dynamic film thickness within the safe range. When both the initial and dynamic film thicknesses are within the safe range, the corresponding water flow rate and air velocity parameters will be maintained for stable operation until either the initial or dynamic film thickness is detected to exceed the safe range.

[0030] Determining the safe range first involves identifying the minimum and maximum water film thicknesses, then using the minimum water film thickness as the lower limit and the maximum water film thickness as the upper limit to define the safe range interval. The minimum water film thickness is determined based on the water film's mass flow rate, combined with the balance between interfacial tension, gravity, interfacial shear force, and the wall's hydrophilicity. The maximum water film thickness is calculated using the expression for the heat transfer coefficient with respect to the water film thickness, and by inversely calculating the corresponding maximum water film thickness based on the lower limit threshold of the heat transfer coefficient. The calculation of the minimum and maximum water film thicknesses requires real-time acquisition of parameters from the heat dissipation structure and the environment for dynamic updates to the safe range. These parameters include, but are not limited to, the temperature of the refrigerant in the heat dissipation structure, the wind speed and direction blowing across the wall, the water flow rate entering from the top inlet of the wall, and the ambient temperature and humidity. The safe range is dynamically updated based on these parameters, allowing for real-time monitoring and adjustment of the water film thickness.

[0031] This invention considers the control of water film thickness under the interaction of airflow and water flow during water film evaporation. At the interface between the water film formed on the outer surface of the heat dissipation structure and the air, there exists a saturated water vapor layer. When ambient air is drawn across the water film surface by a fan, the relative humidity of the ambient air is not saturated. Due to the concentration difference, the saturated water vapor at the gas-liquid interface enters the ambient air, breaking the saturation of the water vapor layer. Water evaporates from inside the water film, replenishing the water vapor layer and forming a new saturated water vapor layer.

[0032] Under the influence of surface wind speed, ambient air continuously flows over the surface of the heat dissipation structure, carrying away a large amount of water vapor. In order to maintain the stability of the saturated water vapor layer at the gas-liquid interface, a large number of water molecules evaporate from inside the water film to replenish it. The evaporation process of water molecules generates heat transfer phenomena. The evaporation phase change absorbs heat and carries away a large amount of heat, which is much greater than the heat carried away by traditional air cooling / water cooling convection.

[0033] The refrigerant flows through channels formed on the inner surface of the heat dissipation structure. The high-temperature refrigerant contacts the inner surface and transfers heat to the outer wall surface, where it is finally carried away by the evaporation of a water film. The high-temperature refrigerant continues to flow over the inner surface of the heat dissipation structure, and the water film continues to evaporate, carrying away heat and causing the high-temperature refrigerant to cool down rapidly (the heat absorbed by evaporation is much greater than the heat dissipation by conventional sensible convection).

[0034] The evaporation of the water film is caused by the difference in relative humidity between the ambient air and the water vapor concentration at the gas-liquid interface of the water film. This results in a cooling method primarily based on evaporation, unaffected by sensible heat factors such as the working fluid temperature and ambient temperature (in conventional cooling equipment, the cooling effect is affected by the temperature difference between the working fluid and the cooling medium, i.e., the lower the cooling medium temperature, the better the effect; however, the working fluid temperature after cooling has a limit, i.e., it must be higher than the cooling medium temperature). This method can achieve near-wet-bulb temperature cooling, breaking the limit that the working fluid temperature cannot be lower than the cooling medium temperature after cooling. Even if the working fluid temperature, water film temperature, and ambient temperature are all equal, as long as the relative humidity of the ambient air flowing over the water film surface is not saturated, water vapor transfer and evaporation heat absorption will still occur, further reducing the working fluid temperature until the relative humidity of the ambient air becomes saturated. At this point, the working fluid temperature will be lower than the ambient temperature, approaching the ambient wet-bulb temperature.

[0035] The water film formed on the outer surface of the heat dissipation structure becomes thinner and thinner under the action of evaporation. By replenishing water from the top inlet of the wall, the water film thickness is controlled to be maintained within a certain range. If the water film is too thin, it will dry out easily. If it is too thick, the water film flow rate will increase, sensible heat convection will dominate, and the evaporation effect will be weakened.

[0036] In this invention, the initial film thickness and dynamic film thickness can be obtained in various ways. For example, the film thickness at each position can be obtained by acquiring images of the water film covering the wall surface in real time from the side of the wall, or it can be calculated based on actual parameters combined with a theoretical model. Any method that can obtain the initial film thickness and dynamic film thickness can be applied to the film thickness control method of this invention.

[0037] As a specific embodiment, the process of adjusting the wind speed or water flow rate to keep the initial thickness or dynamic thickness within a safe range includes: If the initial thickness or dynamic thickness is less than the minimum water film thickness, then at least one of the following adjustment methods should be selected: increasing the water flow rate or increasing the wind speed in the downstream direction. If the initial thickness or dynamic thickness is greater than the maximum water film thickness, then at least one adjustment method should be selected: reducing the water flow rate or increasing the wind speed in the counter-current direction.

[0038] Specifically, for the water film thickness at any location on the wall, it is determined whether the water film thickness is greater than the minimum water film thickness. If the water film thickness is greater than the minimum water film thickness, the current water flow rate and wind speed parameters (including wind direction, with the direction in the same direction as the water flow being positive and the opposite direction being negative) remain unchanged. If the water film thickness is less than or equal to the minimum water film thickness, adjustments need to be made to ensure that the water film thickness is greater than the minimum water film thickness to prevent the water film from breaking or drying out. Therefore, the water film thickness can be increased, the minimum water film thickness can be decreased, or both can be adjusted simultaneously. The adjustment method to increase the water film thickness is to increase the water flow rate, and the adjustment method to decrease the minimum water film thickness is to increase the wind speed in the same direction as the water flow.

[0039] For any point on the wall surface, determine if the water film thickness is less than the maximum water film thickness. If it is, keep the current water flow rate and wind speed parameters (including wind direction, with the direction in the same direction as the water flow being positive and the opposite direction being negative) unchanged. If the water film thickness is greater than or equal to the minimum water film thickness, to avoid sensible heat transfer becoming the primary source of inefficient heat dissipation, adjustments need to be made to make the water film thickness less than the maximum. This can be achieved by decreasing the water film thickness, increasing the maximum water film thickness, or both. Decreasing the water film thickness involves decreasing the water flow rate, while increasing the maximum water film thickness involves increasing the wind speed in the opposite direction.

[0040] Optionally, the process of determining the minimum water film thickness includes: Based on the critical mass flow rate of the water film and the balance between interfacial tension, gravity, interfacial shear force, and wall hydrophilicity, the minimum water film thickness is calculated. The minimum water film thickness is directly proportional to the third power of the interfacial tension, inversely proportional to the third power of the water density, inversely proportional to the square of the third of the interfacial shear force, and directly proportional to the third power of the difference between 1 and the cosine of the contact angle.

[0041] In the process of obtaining the minimum water film thickness, the key to preventing localized drying of the wall surface is to ensure that the water film thickness δ is greater than the critical minimum value δ at any location. min This critical value is based on the stability analysis of water film flow, taking into account interfacial tension γ, gravity, and interfacial shear force τ. i The dynamic balance between the water film and the wall's hydrophilicity (contact angle β) is crucial. When the water film is too thin, the interfacial tension cannot resist the combined effects of flow inertia and interfacial shear forces (especially the upward pull from the opposite wind direction), causing the water film to break or form dry spots. The core of the anti-drying hypothesis is determining the minimum thickness (critical film thickness δ) required for the water film to maintain stable flow. minThis method aims to prevent the wall surface from drying out due to water film rupture. Its derivation is based on dynamic force balance and interfacial stability analysis, and considers the influence of wind direction on shear force. When the water film thickness decreases to a critical value, the interfacial tension cannot resist the combined effect of flow inertia and interfacial shear force, leading to water film rupture.

[0042] The critical film thickness δ can be determined based on the expression for the mass flow rate of the water film at the critical point. min The solution (the critical point is the critical film thickness corresponding to the minimum mass flow rate) is obtained by combining force balance and interfacial tension effects, and the formula for the minimum film thickness is derived.

[0043] Minimum water film thickness δ min It is the coefficient 1.82 multiplied by the power of one-third of the first film thickness parameter, and then multiplied by the power of the square of three of the second film thickness parameter.

[0044] The first film thickness parameter is calculated by subtracting the product of the difference in the cosine of the contact angle β and the interfacial tension, then dividing by the density of water. The second film thickness parameter is the dynamic viscosity of water, μ. i Divide by the interfacial shear force. The interfacial tension is calculated using Harkins' empirical formula: 75.796 - (0.145 multiplied by the interfacial temperature T of the water-gas phase interface). i ) - (0.00024 multiplied by the interface temperature T) i The unit is millinewtons per meter (mN / m). When the wind speed is downstream, the interfacial shear force is greater than 0, and this shear force boosts the net driving force, decreasing the minimum water film thickness. When the wind speed is upstream, the interfacial shear force is less than 0, and this shear force resists, decreasing the net driving force and increasing the minimum water film thickness.

[0045] As a better option, in order to leave a margin in engineering and prevent drying and breakage at the critical state of minimum water film thickness, the calculated minimum water film thickness can be amplified to a certain extent to obtain the final minimum water film thickness; for example, the calculated minimum water film thickness can be multiplied by 1.5 to obtain the final minimum water film thickness within the safe range.

[0046] Optionally, the process of determining the maximum water film thickness includes: The maximum water film thickness is inversely proportional to the heat transfer coefficient threshold; The maximum water film thickness is obtained by dividing the thermal conductivity of water by the heat transfer coefficient. The numerator of the heat transfer coefficient includes convective heat dissipation, evaporative heat dissipation, and sensible heat transport, while the denominator is the difference between the wall temperature and the interface temperature at the water film-air phase interface.

[0047] The core of the maximum water film thickness is to ensure that the heat dissipation equipment is dominated by evaporation. The larger the water film thickness, the greater the mass flow rate and the greater the water flow velocity. The heat transfer will gradually change to convection heat transfer (similar to the heat transfer form of traditional water-cooled condensers, and the cooling effect is also similar). The heat transfer coefficient, i.e. the cooling effect, will be greatly reduced. Therefore, the water film thickness needs to be controlled within a certain range so that it is not too large, and the latent heat phase change is always the dominant factor in heat dissipation.

[0048] Maximum water film thickness δ max The thermal conductivity k of water is less than or equal to that of water. l Divide by the minimum threshold of the heat transfer coefficient α (usually set to 2000 W / (m²)). 2 The heat transfer coefficient is equal to the total heat dissipation divided by the wall temperature T. w The interfacial temperature T at the water film-gas interface i The difference, the total heat dissipation includes convective heat dissipation, evaporative heat dissipation and sensible heat transport.

[0049] As a better option, in order to avoid approaching the limit of the maximum water film thickness and to ensure the stable operation of the heat dissipation process dominated by latent heat phase change, the calculated maximum water film thickness can be reduced to a certain extent to obtain the final maximum water film thickness; for example, the calculated maximum water film thickness can be multiplied by 0.9 to obtain the final maximum water film thickness within the safe range.

[0050] As a specific embodiment, in order to more accurately obtain the thickness of the water film covering the wall surface, this invention combines actually collected parameters and theoretical models to calculate the initial thickness and dynamic thickness, and then dynamically controls the water film thickness according to a safe range. Figure 2-6 The diagram shows a schematic representation of the analysis of the water film on the wall surface. Figure 2-4 This is a schematic diagram for analyzing a vertical wall. Figure 5 This is a schematic diagram of an inclined wall. Figure 6 This is a schematic diagram of the surface of a circular tube. The specific analysis methods for the three types of walls are the same, but the specific gravity parameters, mass flow rate, and evaporation rate (evaporation mass) will be modified according to different application scenarios.

[0051] δ0 represents the initial film thickness, δ is the dynamic film thickness at any position along the flow direction on the wall, and T w T is the wall temperature. l T represents the average temperature of the water film. i For the interface temperature, T a Let u represent the ambient temperature, ρ represent the velocity of the water film, θ represent the density of water, and θ represent the angle between the wall and the horizontal plane. A coordinate system is established with the direction of flow along the wall as the positive x-axis and the direction perpendicular to the wall and pointing towards the air as the positive y-axis (representing the direction of water film thickness) for subsequent calculations.

[0052] The process of obtaining the initial thickness includes: establishing the force balance equation of the water film in the direction of flow along the wall, and calculating the velocity distribution inside the water film; establishing the force balance equation based on the equilibrium between the viscous resistance and gravity inside the water flow in the flow direction; and solving for the velocity distribution by applying the boundary conditions of no slippage on the wall and continuous interfacial shear force. The velocity distribution includes gravity-driven terms and shear-driven terms, with the gravity-driven terms exhibiting a parabolic distribution and the shear-driven terms exhibiting a linear distribution.

[0053] The average velocity of the water film is calculated based on the velocity distribution, and the mass flow rate of the water film per unit width is further calculated; the direction of the unit width is perpendicular to the wall surface and also perpendicular to the thickness direction of the water film. The initial thickness is calculated based on the expression for the mass flow rate of the water film and the definition of the Reynolds number of the water film.

[0054] Specifically, the force equilibrium equations for the water film are first established, and the boundary conditions are determined. Considering a steady-state laminar water film on the wall (y-axis perpendicular to the wall, x-axis along the flow direction), neglecting the influence of fluid inertia, the force equilibrium equation (momentum equation) for the water film in the x-direction is that the sum of the viscous drag inside the water film and the gravity acting on the water flow is 0. This equation describes the equilibrium relationship of the velocity distribution u(y) inside the water film. Under the assumption of steady-state laminar flow, the viscous drag and gravity are in equilibrium in the flow direction.

[0055] The viscous resistance inside the water film is given by the dynamic viscosity of water, μ. i Multiply by the second derivative of the water film velocity u(y) in the y-direction (thickness direction). The gravity of the water flow can be expressed as the water density ρ multiplied by the gravitational acceleration g and then multiplied by the sine of the included angle θ (when the wall is perpendicular to the horizontal plane, the sine of the included angle θ is 1). The dynamic viscosity of water, in Pa·s, represents the internal viscous resistance of the water. The water film velocity, in meters per second, is a function of the y-direction. The density of water, in kilograms per cubic meter. The gravitational acceleration, in meters per second squared.

[0056] The corresponding boundary conditions include two conditions: no slip on the wall and continuous interfacial shear force at the water film-gas phase interface.

[0057] The boundary condition for no slip on the wall can be expressed as follows: when y is 0, the velocity of the water film is 0, that is, the velocity of the water film at the contact point between the water film and the wall is 0.

[0058] The boundary condition for continuous interfacial shear force can be expressed as follows: at the water-gas interface (where y=δ, and δ is the water film thickness), the viscous drag inside the water film is equal to the force exerted by the wind speed on the water-gas interface (i.e., interfacial shear force). Specifically, the dynamic viscosity of water multiplied by the first derivative of the water film velocity in the thickness direction (at y=δ) equals the interfacial shear force. The direction of the interfacial shear force is defined as follows: when the airflow is in the same direction as the water film (co-current), the interfacial shear force is positive, indicating that the wind speed exerts a downward drag force on the water film; when the airflow is in the opposite direction to the water film (counter-current), the interfacial shear force is negative, indicating that the wind speed exerts an upward resistance force on the water film.

[0059] Secondly, after completing the force equilibrium equations for the water film and determining the boundary conditions, the interfacial shear force is calculated. The interfacial shear force is determined by the airflow conditions. The local friction coefficient C is calculated using the Blasius solution for the laminar boundary layer. f 0.664 divided by the local Reynolds number Re a The square root of C. f It is the local friction coefficient generated when air flows over a wall. The local Reynolds number Re is also relevant. a air density and wind speed u a The product of the absolute value of the velocity and the distance x along the direction of water film flow, divided by the dynamic viscosity of air μ a (take wind speed u) a The absolute value guarantees a positive definite Reynolds number.

[0060] Because of wind speed u a Much greater than the water film velocity u l Therefore, the interfacial shear force τ i Directly using wind speed for calculation, i.e., the interfacial shear force is determined by the magnitude of the air velocity and the sign function sign(u). a The product of ) and . Where the shear force amplitude is half multiplied by the local friction coefficient C. f Then multiply by the air density, and then multiply by the square of the wind speed. The sign function is used to reflect the influence of the airflow direction on the direction of the interface shear force (+1 for downstream and -1 for upstream).

[0061] Then, the velocity distribution inside the water film is solved. The force balance equation (i.e., the momentum equation) of the water film is first integrated once and substituted into the boundary condition of continuous interfacial shear force to obtain the integration constant C1. Then, a second integration is performed and substituted into the boundary condition of no slippage on the wall to obtain the final velocity distribution u of the water film. l (y).

[0062] The distribution of water film velocity along the y-direction is composed of two superpositions: a gravity-driven term and a shear-driven term.

[0063] The gravity-driven term represents the flow caused by gravity, exhibiting a parabolic distribution. Its expression is the product of the gravity coefficient and the thickness function. The gravity coefficient is calculated by multiplying the density of water by the gravitational acceleration, then multiplying by the sine of the included angle θ, and dividing by the dynamic viscosity of water. The thickness function is the product of the water film thickness δ and y, minus the square of half of y.

[0064] The shear force driving term represents the flow caused by gas-phase shear force and exhibits a linear distribution. Its expression is the interfacial shear force divided by the dynamic viscosity of water, then multiplied by y; the dynamic viscosity of water is μ. l The sign of the term determines whether it increases speed (downstream) or decreases speed (upstream).

[0065] Then, the average velocity and mass flow rate of the water film are calculated based on the velocity distribution of the water film.

[0066] The average velocity of the water film is the average value of the velocity distribution of the water film along its thickness direction, and is the reciprocal of the water film thickness δ multiplied by the integral of the velocity in the y-direction from zero to δ. Its final expression is the sum of a first velocity parameter and a second velocity parameter. The first velocity parameter is the product of the water density, gravitational acceleration, the sine of the included angle θ, and the square of the water film thickness, divided by three times the dynamic viscosity of water; the second velocity parameter is the interfacial shear force multiplied by the water film thickness, divided by twice the dynamic viscosity of water.

[0067] The mass flow rate Γ of the water film represents the mass of water flowing through a specific location on the x-axis per unit time and unit width (perpendicular to the xy-plane). The mass flow rate of the water film is calculated by multiplying the water density by the water film thickness and then by the average flow velocity of the water film. Its final expression is the sum of a first mass flow rate parameter and a second mass flow rate parameter. The first mass flow rate parameter is the product of the square of the water density, the gravitational acceleration, the sine of the angle θ, and the cube of the water film thickness, divided by three times the dynamic viscosity of water. The second mass flow rate parameter is the product of the water density, the interfacial shear stress, the square of the water film thickness, divided by twice the dynamic viscosity of water.

[0068] The above expression for the mass flow rate of the water film is for a flat plate surface. For a circular pipe surface, it needs to be corrected by multiplying the mass flow rate of the flat plate surface by 2η, i.e., Γ. tube =2ηΓ.

[0069] Finally, the water film thickness δ is calculated. The water film thickness δ satisfies the equation: the product of the cube of four times the density of water, the gravitational acceleration, the sine of the included angle θ, and the cube of the water film thickness, plus the square of six times the density of water multiplied by the interfacial shear force multiplied by the square of the water film thickness, minus the square of three times the Reynolds number of the water film multiplied by the dynamic viscosity of water, equals zero. The initial film thickness is then calculated using this equation.

[0070] Reynolds number of water film lEqual to four times the mass flow rate of the water film divided by the dynamic viscosity of water, used to characterize the flow state of the water film (Re for laminar flow). l (Less than 20). When solving for the water film thickness, the influence of wind speed direction is considered (the interfacial shear force is positive when flowing downstream, and the water film thickness increases; the water film thickness decreases when flowing upstream).

[0071] As a specific embodiment, the process of obtaining the dynamic thickness based on the calculated initial film thickness includes: An evaporation mass model was established, and the evaporation mass loss of the water film along the wall flow direction was calculated based on the law of mass conservation. Establish a heat dissipation model and calculate the heat dissipation of the water film along the wall flow direction based on energy balance; Based on the initial thickness, and combining the evaporation mass model and the heat dissipation model, the dynamic film thickness after evaporation loss along the flow direction of the water film on the wall is calculated.

[0072] First, establishing the evaporation mass model includes: Establish a balance between the rate of change of mass flow rate along the wall and the evaporation rate; The evaporation rate is the product of the mass transfer coefficient and the mass transfer driving force. The mass transfer driving force is the difference between the saturated water vapor density at the water film-gas phase interface and the ambient water vapor density. The mass transfer coefficient is calculated using the Chilton-Colburn analogy.

[0073] Specifically, due to evaporation from the water film surface, the mass flow rate Γ per unit width of the water film gradually decreases along the positive x-direction (downward). Therefore, a mass conservation equation is established: the rate of change of mass flow rate along the positive x-axis is equal to the negative evaporation rate, and the negative sign indicates the mass loss caused by evaporation.

[0074] Evaporation rate is determined by mass transfer coefficient h m The mass transfer is determined by both the water film and the mass transfer driving force (concentration difference). The driving force for mass transfer from the water film to the air is the difference between the saturated water vapor density at the water film-gas interface and the ambient water vapor density, i.e., ρ. sat (T i )-H a ρ sat (T a ), where ρ sat (T) represents the saturated water vapor density (kg / m³). 3 ), is a function of temperature.

[0075] The above evaporation rate corresponds to the evaporation rate expression for a flat plate surface. In the case of a circular tube surface, the evaporation rate needs to be corrected by multiplying the evaporation rate of the flat plate surface by 2η.

[0076] Mass transfer coefficient h m For the local Sherwood number Sh xMultiply by the water vapor diffusion coefficient D in air, then divide by the distance x the water film travels along the wall surface. Local Sherwood number Sh x The intensity of convective mass transfer is represented by 0.664 multiplied by the local Reynolds number of air, Re. a The diffusivity of water vapor in air, D, is calculated by multiplying the first half of the diffusivity by the third power of the Schmidt number, Sc. The unit of the diffusivity D is meters squared per second. The Schmidt number Sc represents the ratio of momentum diffusivity to mass diffusivity, and is the aerodynamic viscosity μ. a Divide by air density and then by diffusion coefficient D. Local Reynolds number Re a The calculation takes into account the influence of airflow direction and uses the relative velocity between air and water film. For downstream flow, the relative velocity is the air velocity minus the average velocity of the water film; for upstream flow, it is the air velocity plus the average velocity of the water film. When the air velocity (i.e., wind speed) is much greater than the water film velocity, only the air velocity u can be used. a The calculation involves multiplying the air density by the wind speed by the distance the water film travels on the wall (x), and then dividing by the dynamic viscosity of the air (μ). a .

[0077] Secondly, establishing the heat dissipation model includes: Establish an energy balance equation between the sum of convective heat dissipation, evaporative heat dissipation, and sensible heat transfer terms and the wall conduction term; The wall thermal conductivity term is the heat flux density transferred from the wall to the water film through thermal conduction. The convective heat dissipation term is the heat flux density transferred to the air by convective heat transfer at the water film interface. The evaporative heat dissipation term is the latent heat flux density of phase change absorbed by the evaporation at the water film interface. The sensible heat transfer term is the rate of change of sensible heat carried by the water film flow in the flow direction, which is reflected by the rate of change of the average temperature of the water film.

[0078] Specifically, based on the energy conservation of any infinitesimal element in the water film, the wall heat conduction term equals the convective heat dissipation term plus the evaporative heat dissipation term plus the sensible heat transfer term.

[0079] The wall thermal conductivity term is the thermal conductivity k of water. l Multiply by the wall temperature T w With interface temperature T i The difference is then divided by the water film thickness. The convective heat dissipation term is used to calculate air convective heat dissipation, and is the convective heat transfer coefficient h multiplied by the interface temperature T. i With ambient temperature T a The difference is as follows. The evaporative heat dissipation term is used to calculate the heat absorbed during the evaporation phase change, and is the evaporation rate multiplied by the latent heat of vaporization of water, r, which is approximately 2260 kJ / kg. The sensible heat transfer term is used to calculate the change in heat carried by the water flow, and is the specific heat at constant pressure of water, c. pl Multiply by mass flow rate and then multiply by the average water film temperature T.i The derivative with respect to x.

[0080] After establishing the heat dissipation model, further calculations are needed to determine the average temperature of the water film in order to obtain the calculation results. The process of obtaining the average water film temperature includes: The temperature distribution along the thickness direction within the water film is obtained. The average temperature of the water film is obtained by dividing the integral of the water film flow rate and the temperature distribution along the thickness direction by the mass flow rate. The average temperature of the water film is the superposition of the average temperature based on a linear distribution, the temperature drop caused by evaporative cooling, and the temperature change caused by the flow of the water film.

[0081] Specifically, the temperature distribution and its average value T inside the water film are solved using a layered approximation method. i .

[0082] Assuming a linear temperature distribution in the water film (ignoring convection; ignoring heat conduction in the x-direction; no heat source in steady state within the water film; under laminar flow, low evaporation rate, and small amplitude conditions, the water film temperature simplifies to a linear distribution), its average temperature is approximately the sum of the wall temperature and the interface temperature, divided by two. The basic linear distribution expression is the wall temperature T. w Add a gradient temperature, where the gradient temperature is the interface temperature T. i Subtract wall temperature T w The difference is multiplied by the ratio of distance y to water film thickness.

[0083] Meanwhile, in the presence of both evaporation and convection, especially when evaporation is dominant, the water film temperature distribution is nonlinear. This temperature distribution then includes the sum of the fundamental linear term, the evaporation distribution term, and the convection distribution term.

[0084] The evaporation distribution term satisfies the pure heat conduction equation, meaning the product of the second derivative of the evaporation distribution term with respect to y and the thermal conductivity of water is zero. Solving this equation yields the result for the evaporation distribution term. This correction results in the maximum temperature decrease at the interface and zero at the wall, exhibiting a parabolic distribution. It is used to describe the temperature distribution under a pure heat conduction mechanism (ignoring convection effects). Since evaporative cooling is a local interfacial phenomenon, this equation assumes that normal heat conduction dominates the temperature change caused by evaporative heat absorption.

[0085] The convection distribution term is used to indicate that the water film flow carries heat along the positive x-axis, thus affecting the temperature distribution. Specifically, the product of the second derivative of the convection distribution term with respect to y and the thermal conductivity of water equals the average temperature T of the water film. l The product of the derivative of d, the density of water, the specific heat at constant pressure of water, and the water film velocity. Solving this product yields the convection distribution term. This correction reflects the effect of the flow on the temperature distribution. It is used to describe the balance between convective and normal heat transfer in the x-direction.

[0086] The average temperature of the water film is the result of integrating the water film velocity and temperature distribution along the y-direction and dividing by the mass flow rate. Its final form can be expressed as the superposition of the average temperature of the basic linear distribution, the temperature drop due to evaporative cooling, and the temperature change caused by the water film flow.

[0087] The average temperature of the basic linear distribution is the sum of the wall temperature and the interface temperature divided by two. The temperature drop caused by evaporative cooling is the evaporation rate multiplied by the latent heat of vaporization multiplied by the water film thickness, divided by twelve times the thermal conductivity of water. The temperature change caused by water film flow is the density of water multiplied by the specific heat at constant pressure of water divided by the thermal conductivity of water, then multiplied by the rate of change of the average water film temperature along the x-direction (the direction of water film flow), and then multiplied by the convection thickness parameter; the convection thickness parameter is the density of water multiplied by the gravitational acceleration multiplied by the sine of the included angle θ multiplied by the fifth power of the water film thickness divided by forty-five times the dynamic viscosity of water, plus the interfacial shear force multiplied by the fourth power of the water film thickness divided by twenty-four times the dynamic viscosity of water.

[0088] The calculated expression for the average water film temperature is substituted into the sensible heat transfer term of the energy balance equation for expansion, thus decomposing it into the heat carried by the change in water flow temperature and the heat removed due to evaporation loss from the mass flow rate; these are then recombined into the energy balance equation to obtain the final energy equation.

[0089] The left side of this energy equation is the thermal conductivity k of water. l Multiply by the wall temperature T w With interface temperature T i The difference is then divided by the water film thickness. The right side includes heat dissipation from air convection, heat dissipation from evaporation phase change (including latent heat and sensible heat from the evaporating water flow), and sensible heat transport by the water flow. Air convection heat dissipation is the convective heat transfer coefficient h multiplied by the interface temperature T. i With ambient temperature T a The difference is as follows: Evaporation phase change heat dissipation is the evaporation rate multiplied by the latent heat of vaporization of water (r), minus the product of the evaporation rate, the isobaric specific heat of water, and the average temperature of the water film. Sensible heat transport in water flow is the isobaric specific heat of water (c). pl Multiply by mass flow rate and then multiply by the average water film temperature T. i The derivative with respect to x.

[0090] Based on the above energy equation expression, the convective heat transfer coefficient h needs to be determined. The Chilton-Colburn analogy and relative velocity can be used to calculate the convective heat transfer coefficient, which is the local Nusselt number Nu. x Multiply by the thermal conductivity of air, k a Then divide by the distance x. Local Nusselt number Nu xThe intensity of convective heat transfer is represented by 0.664 multiplied by half the local Reynolds number of air, and then by one-third of the Prandtl number Pr. The Prandtl number Pr represents the ratio of momentum diffusivity to thermal diffusivity, and is calculated by multiplying the dynamic viscosity of air by its isobaric specific heat c. pa Divide by the thermal conductivity of air, k a .

[0091] The overall heat transfer performance of a heat dissipation device (from the wall to the environment) can be characterized by the heat transfer coefficient α. This is achieved by dividing both sides of the energy equation by the wall temperature T. w With interface temperature T i The difference between these two values ​​allows us to derive the expression for the heat transfer coefficient. It comprehensively reflects the overall heat transfer capacity of the wall through the water film, the convective / evaporative heat dissipation from the water film to the air, and the sensible heat transport by the water flow; it is a key indicator for evaluating radiator performance.

[0092] The above embodiments are further elaborations and descriptions of the present invention to facilitate understanding, and are not intended to limit the present invention in any way. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for controlling the surface film thickness of a heat dissipation structure based on thin-film evaporation, characterized in that, include: Obtain the initial thickness of the water film at the top inlet of the wall of the heat dissipation structure; Obtain the dynamic thickness of the water film at various positions along the flow direction along the wall; Based on the mass flow rate of the water film, combined with dynamic force balance and stability analysis of the water film flow, the minimum water film thickness is determined. The maximum water film thickness dominated by evaporative heat dissipation is calculated by back-calculating the heat transfer coefficient threshold. If the initial thickness or dynamic thickness exceeds the safe range between the minimum and maximum water film thickness, adjust the wind speed or water flow rate to bring the initial thickness or dynamic thickness within the safe range.

2. The method for controlling the surface film thickness of a heat dissipation structure based on thin-film evaporation according to claim 1, characterized in that, The process of adjusting the wind speed or water flow to keep the initial or dynamic thickness within a safe range includes: If the initial thickness or dynamic thickness is less than the minimum water film thickness, then at least one of the following adjustment methods should be selected: increasing the water flow rate or increasing the wind speed in the downstream direction. If the initial thickness or dynamic thickness is greater than the maximum water film thickness, then at least one adjustment method should be selected: reducing the water flow rate or increasing the wind speed in the counter-current direction.

3. A method for controlling the surface film thickness of a heat dissipation structure based on thin-film evaporation according to claim 1 or 2, characterized in that, The process of determining the minimum water film thickness includes: Based on the critical mass flow rate of the water film and the balance between interfacial tension, gravity, interfacial shear force, and wall hydrophilicity, the minimum water film thickness is calculated. The minimum water film thickness is directly proportional to the third power of the interfacial tension, inversely proportional to the third power of the water density, inversely proportional to the square third power of the interfacial shear force, and directly proportional to the third power of the difference between 1 and the cosine of the contact angle.

4. A method for controlling the surface film thickness of a heat dissipation structure based on thin-film evaporation according to claim 1 or 2, characterized in that, The process of determining the maximum water film thickness includes: the maximum water film thickness is inversely proportional to the heat transfer coefficient threshold; The maximum water film thickness is obtained by dividing the thermal conductivity of water by the heat transfer coefficient. The numerator of the heat transfer coefficient includes convective heat dissipation, evaporative heat dissipation, and sensible heat transport, while the denominator is the difference between the wall temperature and the interface temperature at the water film-air phase interface.

5. A method for controlling the surface film thickness of a heat dissipation structure based on thin-film evaporation according to claim 1 or 2, characterized in that, The process of obtaining the initial thickness includes: Establish the force balance equations for the water film in the direction of flow along the wall, and calculate the velocity distribution inside the water film; The average flow velocity of the water film is calculated based on the velocity distribution, and the mass flow rate of the water film per unit width is further calculated; the direction of the unit width is perpendicular to the wall surface and also perpendicular to the thickness direction of the water film. The initial thickness is calculated based on the mass flow rate expression and the Reynolds number definition of the water film.

6. The method for controlling the surface film thickness of a heat dissipation structure based on thin-film evaporation according to claim 5, characterized in that, The calculation of the velocity distribution inside the water film includes: The force balance equation is established based on the equilibrium between the viscous resistance and gravity inside the water flow in the direction of flow along the wall. The velocity distribution is solved by applying boundary conditions of no wall slip and continuous interfacial shear force. The velocity distribution includes a gravity-driven term and a shear-driven term. The gravity-driven term exhibits a parabolic distribution, while the shear-driven term exhibits a linear distribution.

7. A method for controlling the surface film thickness of a heat dissipation structure based on thin-film evaporation according to claim 1 or 2, characterized in that, The process of obtaining the dynamic thickness includes: An evaporation mass model was established, and the evaporation mass loss of the water film along the wall flow direction was calculated based on the law of mass conservation. Establish a heat dissipation model and calculate the heat dissipation of the water film along the wall flow direction based on energy balance; Based on the initial thickness, and combining the evaporation mass model and the heat dissipation model, the dynamic film thickness after evaporation loss along the flow direction of the water film on the wall is calculated.

8. The method for controlling the surface film thickness of a heat dissipation structure based on thin-film evaporation according to claim 7, characterized in that, The establishment of the evaporation mass model includes: Establish a balance between the rate of change of mass flow rate along the wall and the evaporation rate; The evaporation rate is the product of the mass transfer coefficient and the mass transfer driving force. The mass transfer driving force is the difference between the saturated water vapor density at the water film-gas phase interface and the ambient water vapor density. The mass transfer coefficient is calculated using the Chilton-Colburn analogy.

9. The method for controlling the surface film thickness of a heat dissipation structure based on thin-film evaporation according to claim 7, characterized in that, The establishment of the heat dissipation model includes: Establish an energy balance equation between the sum of convective heat dissipation, evaporative heat dissipation, and sensible heat transfer terms and the wall conduction term; The wall thermal conductivity term is the heat flux density transferred from the wall to the water film through thermal conduction. The convective heat dissipation term is the heat flux density transferred to the air by convective heat transfer at the water film interface. The evaporative heat dissipation term is the latent heat flux density of phase change absorbed by the evaporation at the water film interface. The sensible heat transfer term is the rate of change of sensible heat carried by the water film flow in the flow direction, which is reflected by the rate of change of the average temperature of the water film.

10. The method for controlling the surface film thickness of a heat dissipation structure based on thin-film evaporation according to claim 9, characterized in that, The process of obtaining the average temperature of the water film includes: The temperature distribution along the thickness direction within the water film is obtained. The average temperature of the water film is obtained by dividing the integral of the water film flow rate and the temperature distribution along the thickness direction by the mass flow rate. The average temperature of the water film is the superposition of the average temperature based on a linear distribution, the temperature drop caused by evaporative cooling, and the temperature change caused by the flow of the water film.

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