Micro-nano structures surface preparation and its heat transfer characteristics of aluminum-based microchannel in heat exchangers

Micro-channel enhanced heat transfer research is a fundamental research of micro-channel heat exchangers. Micro-channel heat exchangers, as a new type of heat exchanger, have extensive applications in high heat flux equipment and micro-devices. In agricultural engineering, solar energy has a wide application in agricultural energy. However, there are many problems in the heat dissipation of solar cells. The use of micro-channel heat exchangers can effectively control the temperature of the solar cells, prolong the service life and improve the power generation efficiency. In addition, micro-channel technology can also be used in heat pump air conditioners. Heat pump air conditioners have many applications in animal husbandry. New heat pump air conditioners have adopted micro-channel condensers, which can significantly increase heat exchange efficiency and save energy. Heat pipe technology can be used in temperature control of grain storage, drying system of agricultural products, etc. With the improvement of manufacturing and processing technology, micro-channel heat pipes have become a research hotspot as a new type of heat pipe. In order to study the effect of micro-nano structures surface on the heat transfer characteristics of micro-channel flow boiling, the CuCl2 solution was used to etch the surface of aluminum micro-channels, and the copper particles deposited on the surface were removed by ultrasonic cleaning to obtain micro-nano structures surface. The contact angle of the surface was measured, droplets were dropped on the surface and quickly spread, the contact angle was approximately 0°, and super hydrophilicity was exhibited. The surface of the micro-nano structures was modified with a fluorosilane solution for a certain period of time, and the surface contact angle was measured to be 160.2°, showing super-hydrophobicity. Only by sanding treatment was smooth surface, and the surface contact angle was measured to be 67.2°, exhibiting hydrophilicity. Flow boiling experiments were performed using three different surface micro-channels. The experimental refrigerant was R141b and the operating pressure was 142 kPa. The whole experimental section can be divided into an aluminum base, a micro-channel, a quartz glass plate and an aluminum cover plate (with a visible window on the aluminum cover plate). At the side of the aluminum base, there were thermocouple measuring holes in the entrance section, the exit section and the middle section. In the middle section was a pair of upper and lower temperature measuring holes with four pairs. There was a temperature measurement hole at the entrance and exit, and a total of 10 thermometer holes. Pressure sensors were installed on the other side of the base entrance and outlet respectively for monitoring inlet and outlet pressure. The micro-channel was composed of 12 parallel rectangular single channels. The width and height of a single channel were 1 and 2 mm, respectively, the interval between channels was 2 mm, and the length of the micro-channel was 240 mm. The mass flow rates were 312.6 and 505.2 kg/(m2•s), and the heat fluxes were 3.42-34.6 kW/m2. The effects of micro-nano structures surfaces on the heat transfer characteristics at different mass flow rates and heat fluxes were investigated. The experimental results showed that the super-hydrophobic surface had the best heat transfer characteristics under low heat flux, and the minimum superheat required for the onset of nucleate boiling was the lowest, followed by the super-hydrophilic surface. For the overall average heat transfer coefficient, the super-hydrophobic surface maximum increased the heat transfer coefficient by 31.6% relative to the smooth surface under the condition of low heat flux, and the heat transfer coefficient of the super-hydrophilic surface was the largest when the heat flux was raised to a certain value, the maximum heat transfer coefficient was 20.6% higher than the smooth surface heat transfer coefficient. Compared with the super-hydrophobic surface, the super-hydrophilic surface heat transfer coefficient had a smaller proportion of increase, mainly because the super-hydrophilic surface showed the best heat transfer characteristics in the saturated boiling section, and the saturated boiling section had a much larger heat transfer coefficient than single phase flow and sub-cooled boiling section. The super-hydrophilic and super-hydrophobic surfaces had the same micro-nano structures surface. The micro-nano structures surface increased the vaporization cores and the contact area between the refrigerant and the wall surface. The difference was that the super-hydrophobic surface was modified by the fluorosilane solution. Hydrophilic and hydrophobic properties, which affected the detachment of bubbles, making the heat transfer characteristics show different characteristics: the heat transfer characteristics of the super-hydrophobic surface in the region of low heat flux was the best, and super-hydrophilic surface in the region of high heat flux had the best heat transfer characteristics. To continue to increase the heating power, a large number of bubbles were generated in the micro-channel under high heat flux density. Due to the hydrophobic feature of the super-hydrophobic surface, bubbles did not easily separate, the heat transfer begins to deteriorate, and the heat transfer coefficient gradually decreases, even lower than smooth surface.