Experimental Study on Condensation Heat Transfer Performance of Hydrophilic/Hydrophobic Microstructured

In order to study the condensation and heat transfer characteristics of similar microstructure surfaces, two similar microstructure surfaces, cylindrical and circular, were fabricated by femtosecond laser technology on a 0.5 mm silicon wafer. The cylindrical surface is superhydrophobic when the contact angle is more than 150°, and the circular surface is hydrophilic when the contact angle is less than 90°. The difference in condensation heat transfer characteristics between superhydrophobic and hydrophilic microstructures was analyzed, and a visual condensation experimental platform was built. Experimental research showed that: At the same flow rate, the heat transfer coefficient of the superhydrophobic surface and the hydrophilic surface decreases significantly with the increase of the surface subcooling degree, but the heat transfer coefficient of the cylindrical surface is still much larger than that of the circular surface. In addition, the heat transfer performance of the hydrophobic microstructure surface is better than that of the hydrophilic surface at medium and high-speed cooling water flow rates. Although the surface microstructures are similar in shape, the heat transfer performance of cylindrical microstructures is much better than that of circular microstructures under the same conditions, and the heat flux of cylindrical microstructures is 2.2 times that of circular microstructures.


Introduction
Steam condensation heat transfer has a wide range of applications in petrochemical, power generation, refrigeration, aerospace, and other industries.Improving condensation heat transfer characteristics has great significance for energy saving, consumption reduction, and safe production.The condensation heat transfer coefficient of droplet condensation is larger than that of film condensation.Due to the increasingly perfect preparation process, many droplet condensation surface microstructures have been prepared in the industry and are widely used in all walks of life.Mahapatra [1] coated a wettability pattern on the metal surface and utilized the hydrophilic-hydrophobic pattern surface to generate dropwise condensation and discharge the condensed water at the same time, thereby enhancing the overall condensation heat transfer on the entire pattern surface.
Sangsoo Lee [2] et al. simulated the heat transfer of micro/nanostructures and concluded that micro/nanostructures have a strong influence on heat transfer.The heat transfer efficiency is increased by about 60% after changing the structural thickness of micro/nanotips.Chen [3] et al. fabricated a microstructured surface containing a hydrophobic pyramid structure, which was covered with hydrophilic aqueous nanostructures achieving global superhydrophobicity.Compared with the superhydrophobic surface containing nanostructures alone, the droplet density can be increased by 65%, and the droplet shedding efficiency can be increased by 450% compared with other microstructures.Dong et al. [4] fabricated a layered nanoscale V-groove structure on the copper surface and studied the spontaneous movement of condensed droplets on the superhydrophobic surface of the microgrooves.Long [5] optimized the superhydrophobic condensing surface structure based on the genetic algorithm and obtained that the superhydrophobic surface structure parameters and the diameter of the nanopillars have a significant influence on heat transfer.Kim [6] et al. established a corresponding mathematical model and integrated the heat transfer of a single droplet to establish a heat transfer model of the droplet size distribution.The results showed that a large contact angle is beneficial to the enhancement of condensation heat transfer.Zarei [7] used numerical simulation to simulate droplet condensation on the surface of microstructures such as half cones, half pyramids, cylinders, and cones and studied the effects of parameters such as surface microstructure height, solid phase ratio, roughness coefficient, and condensation base area.Both the condensation radius of a single droplet and the overall heat transfer have a large effect.
In summary, the geometry and distribution of micro/nanostructures have extremely important effects on their surface heat transfer processes.A large number of scholars have conducted research through numerical simulations and mathematical models.Some scholars have prepared some superhydrophobic surfaces with micro/nanostructures, but the superhydrophobic surfaces have poor stability, and the experimental results of condensation heat transfer are not ideal.On the other hand, numerical simulations also have limitations, and the obtained results are difficult to verify experimentally.Therefore, on the basis of the numerical simulation of previous scholars, the condensation of steam on the surface of the microstructure in the presence of non-condensable gas is studied by means of experiments, which can more intuitively and reliably show the effect of the geometry of the microstructure on the heat transfer process.In the experiment, a silicon material with a purity of more than 99.5% was selected to make a silicon wafer with a length and width of 50 × 50 mm and a thickness of 0.5 mm.Micromachining produces microstructured surfaces with cylinders and frustums on the surface of silicon wafers.A visual experimental platform for drop coagulation was built, and the drop coagulation experiment was carried out based on the previous simulation basis.The effects of the structure shape parameters on the condensation of droplets and condensation heat transfer are investigated when the structure heights are the same.

Surface morphology of two microstructures
The surface of the silicon wafer is processed by a femtosecond laser to make its surface have microstructures with different functions.Figure 1 is a scanning electron microscope image (SEM) of a surface with a cylindrical microstructure, and Figure 2 is a scanning electron microscope image of a surface with a frustum microstructure.In Figure 1 and Figures 2, (a) represents the shape of a single cylinder and a truncated cone; (b) represents the local arrangement of the microstructures of the cylinder and the truncated cone, and the arrangement of the cylinder and the truncated cone is square.The depth of the surface microstructure of the cylinder and the truncated cone is 6 μm, the diameter of the cylinder is 4 μm, and the diameter of the top and bottom of the truncated cone is 2 μm and 4 μm, respectively.Here, the diameter of the bottom is processed to 4 μm.The purpose is to form the same variable with the cylinder and compare the coefficient relationship between other variables.The distance between the truncated cone and the cylinder is both 4 μm.Using the OCA25 video optical contact angle measuring instrument at normal atmospheric temperature (ambient temperature: 25°C), the static contact angle on the surface of the cylindrical microstructure was measured to be 158.4°,and over 150° showed superhydrophobicity, as shown in Figure 3 (a).The static contact angle on the surface of the frustum microstructure is 86.4°, which exhibits hydrophilicity below 90°, as shown in Figure 3 (b). .

Experimental platform and steps
The heat transfer performance experiment was carried out on the built experimental platform.The experimental environment was kept at room temperature of 23-27 °C, the atmospheric pressure was kept at 101.325 kPa, and the relative humidity was 77%.The experimental samples with an area of 50 mm × 50 mm were placed in ethanol solution and deionized water for ultrasonic cleaning to remove surface contaminants.After drying, they were installed vertically in a copper condensation chamber, and the measuring bottle was placed at the water outlet.At the bottom of the condensation, water and thermocouples are installed in the cooling water inlet, water outlet, steam port, and condensate water outlet to collect the temperature in real time, and the temperature of the four corners of the sample surface and the temperature of the center point (T1, T2, T3, T4, T5) were measured by a handheld infrared camera (FLIR E6).
The experimental device consists of four parts: a steam generation system, a cooling system, a condensation system, and a data acquisition system.The experimental device is shown in Figure 4. Deionized water is injected into the steam generator, and after heating for a period of time, the steam generator provides supersaturated steam at 129°C (rated steam temperature is 151°C), and then the steam is sent to the condensation chamber through the control valve and the connected steam pipe.During the experiment, the steam will condense when it encounters the pipe wall and the valve due to the temperature difference, so two layers of insulation cotton with a thickness of 10 mm are wrapped on the outer surface of the steam transmission pipe and valve.A steam nozzle and a gas-liquid separator are installed at the steam outlet.The steam nozzle makes the steam evenly distributed in the steam chamber, and the gas-liquid separator completely separates the condensed water from the steam to avoid errors in the collection of the condensed water.Since the experiment uses the steam instantaneous flow rate measured by the vortex flowmeter of the steam generator, the average value of 20 groups of instantaneous flow velocity values is calculated, and the average steam flow rate is 0.255/h.The saturated steam temperature of the condensation chamber was measured by a K-type thermocouple to be 97°C.The cooling water is transported to the condensing chamber through the water cooler, and the nozzle is installed at the water outlet so that the cooling water can be evenly sprayed on the cooling wall surface, so as to fully achieve the effect of uniform cooling.The deionized water in the steam generator is continuously boiled for 20 minutes to remove the influence of the accumulation of non-condensable gases on the experiment.After the experiment is stabilized, the required data are collected.

Theoretical Analysis
Rose et al. [8] established a general expression for the heat flux of a single droplet of any size in a wide range of smooth surfaces, mainly considering three factors: surface curvature, droplet conduction thermal resistance, and vapor-liquid interface thermal resistance.The corresponding thermal resistances are expressed as Tc, Td, and Ti by the temperature difference.
In the formula, Ts is the saturation temperature of the steam;  is the surface tension, hfg is the latent heat of vaporization; v is the density of the condensed water; r is the droplet radius; L is the thermal conductivity of the condensed liquid; K1 is a constant, taking 2/3 [9] ; qs is the thermal conductivity of the condensed water of the hemispherical droplet;  is the contact angle.
Based on adsorption theory and population equilibrium theory, Niu [10] established a droplet condensation heat transfer model considering the influence of liquid-solid interface thermal resistance.In this model, the influence of liquid-solid interface resistance is considered, and the critical nucleation radius and nucleation density are calculated.The temperature difference caused by the thermal resistance effect of the gas-solid interface is: where hls is the liquid-solid interface heat transfer coefficient.
So the total temperature difference between the steam and the microstructured surface due to thermal resistance can be expressed as: According to Fourier's law, the heat flux is determined by: q A   (7)   where  is the heat flow, F=-A•dt/dx=cGTio.In the formula, A is the condensation surface area, c is the specific heat capacity of the cooling water, G is the flow rate of the cooling water delivered by the gear pump, and Tio is the temperature difference between the cooling water inlet and outlet.By Newton's cooling formula: The surface condensation heat transfer coefficient can be obtained: In the formula, Tsw=Ts -Tw, where Tw is the average temperature of five temperature points on the condensation surface measured by the infrared thermal imager.

Stability Analysis of Experimental System
In order to test the reliability of the experimental device, the film condensation experiment was carried out on the condensation surface of the silicon wafer, and compared with the theoretical calculation value of Nusselt, the average heat transfer coefficient of the film condensation was obtained.Based on Nusselt's analytical solution for laminar film condensation of steam, the Nusselt theoretical solution for film condensation of vertical walls in the laminar flow of the liquid film is obtained.The average surface heat transfer coefficient of vertical walls is: The comparison result between the experimental value and the theoretical calculation value is shown in Figure 5.The heat transfer coefficient of the condensing surface of the measured sample is basically consistent with the theoretical heat transfer coefficient of Nusselt, and the maximum deviation is less than 5.3%, so the experimental design meets the requirements.The temperature at the inlet remains basically unchanged, because the cooling water is not recycled, and the temperature of the condensed water at the outlet rises slightly, mainly due to the accumulation of heat released by the condensation of the steam in the condensation chamber.Comparing the temperature of the two water outlets, the temperature of the cylindrical water outlet is about 10℃ higher than that of the circular cone water outlet, indicating that the cylindrical surface takes away more condensation heat than the circular cone surface, and the heat transfer performance is stronger than that of the circular cone surface.Secondly, the temperature fluctuation frequency and amplitude of the cylinder are smaller than those of the truncated cone, showing better stability.
From the heat flux Formula ( 7), it can be seen that the cooling water flow rate is an important factor affecting the heat flux.Based on the low-speed flow rate studied by previous scholars, experiments were carried out under the condition of medium and high flow rates.The flow rate of cooling water was increased to observe the effect of the cooling water flow rate on heat flux.Figure 7 shows the relationship between the heat flux and the degree of subcooling of the two microstructured surfaces under the condition of containing non-condensable gas.It can be seen that the heat flux of both surfaces increases linearly with the increase of subcooling, but the change of the heat flux on the cylindrical surface is much larger than that on the truncated surface.This shows that in the presence of non-condensable gases, the heat transfer performance of superhydrophobic and hydrophilic surfaces is not reduced by the wall thermal resistance caused by the condensate trapped between the microstructures.But due to the microstructures on the surface, the surface of the cylindrical microstructure is much larger than the surface of the frustum microstructure [11] , so the heat dissipation performance of the surface of the cylindrical microstructure is better than that of the surface of the frustum microstructure.When the cooling water flow rate G=500.7 g/min, the maximum heat flux on the cylindrical surface is 380.13 kW/m 2 , while the maximum heat flux on the truncated surface is only 173.29 kW/m 2 ; when the cooling water flow rate G=603.95g/min, the maximum heat flux on the cylindrical surface is 458.44 kW/m 2 , while the maximum heat flux on the truncated surface is only 208.99 kW/m 2 .As the flow rate increases, more heat will be taken away, so the vapor will quickly condense into droplets on the condensing surface, and shorten the period of the droplet growth process, thereby enhancing the heat transfer performance of the microstructured surface.So increasing the cooling water flow rate can increase the heat flux to the microstructured surface.Among the two microstructured surfaces, although the heat flux of the hydrophilic surface is about one-half of that of the superhydrophobic surface, it exhibits better condensation heat transfer performance than the smooth non-microstructured surface.

Condensation heat transfer coefficient of the microstructured surface
The relationship between the water vapor condensation heat transfer coefficient and the surface subcooling degree on the surface of the cylindrical microstructure and the surface of the truncated microstructure under two cooling water flow rates is shown in Figure 8  It can be seen that the condensation heat transfer coefficients on both microstructured surfaces decrease with the increase of surface subcooling.When the subcooling degree is 7 K, the heat transfer coefficient of the truncated cone increases and then decreases because the droplets form film-like condensation on the hydrophilic surface, which affects the shedding of the droplets and causes this phenomenon.However, under the same subcooling degree, the condensation heat transfer coefficient of the cylindrical microstructured surface is more than twice that of the frustum microstructured surface.The growth of droplets is shown in Figure 9.When the cooling water flow rate increases, the condensation circulation rate of water vapor on the surface of the microstructure increases, thereby increasing the heat flux of condensation heat transfer and increasing the condensation heat transfer coefficient.It is mainly because the rate of the merging and shedding cycle process of droplets on the hydrophobic surface is higher than that on the hydrophilic surface.And the droplets formed on the hydrophilic surface are not easy to fall off due to factors such as the tension on the surface of the condensed water.And it is easy to form droplets with larger diameter parameters, which affects the heat transfer from the steam to the microstructure surface, while the droplet diameter on the hydrophobic surface is smaller than that of the hydrophilic surface.The droplet shedding diameter on the water surface has a larger condensation surface and shows better heat dissipation performance.

Conclusion
Two types of microstructured surfaces were fabricated by using femtosecond laser technology: one is a superhydrophobic cylinder and the other is a hydrophilic cone.A visual condensation experiment platform was built, the data stability analysis of the experimental system was carried out, and the factors such as superhydrophobic surface and hydrophilic surface, steam temperature difference, noncondensable gas content, and cooling water flow rate caused by different contact angles were obtained.It has a significant effect on the dropwise condensation heat transfer, and the results show that: (1) Because of the slight difference in the structure of the cylinder and the frustum, the two have different contact angles.The contact angle of the cylindrical microstructure surface is 158.4°, and the contact angle of the frustum microstructure surface is 86.4°, which shows different hydrophobicity and affinity water nature.
(2) The increase of the heat flux on the surface of the cylindrical microstructure with the increase of the degree of subcooling is greater than that of the surface of the truncated microstructure, and the heat flux can reach twice that of the surface of the truncated microstructure, showing better heat dissipation performance.
(3) Because the speed of nucleation, growth, merging, and falling off of droplets on the cylindrical surface is higher than that of the truncated surface, the heat transfer coefficient of the cylindrical microstructure surface is much larger than that of the truncated surface.Therefore, the thermal conductivity of the cylindrical microstructured surface is higher.

Figure 4 .
Figure 4. Schematic diagram of the experimental flow .1088/1742-6596/2636/1/012048 6 where g is the acceleration of gravity, 2 m s ; r is the latent heat of vaporization, J kg ; l  is the mass concentration of deionized water, (kg ∕ m ); l  is the thermal conductivity; l  is the dynamic Viscosity, Pa s ; l is the vertical wall height, m ; V represents the vertical wall.

Figure 5 .
Figure 5.Comparison of the experimental value of film coagulation and Nusselt value

Figure 6 .
Figure 6.Variation of cooling water temperature at the inlet and outlet of the cylinder and truncated cone with time

Figure 9 .
Figure 9. Growth of cylindrical and truncated droplets