Experimental Study on Flow Boiling Heat Transfer Characteristic in Top- Connected Microchannels with a Ni/Ag Micro/nano Composite Structure

Microchannel heat exchangers have a large specific surface area, which endows them with high heat and mass transfer efficiency and broad application prospects in fields such as chemical engineering and energy. Aiming at enhancing the flow boiling heat transfer in microchannels, a top-connected microchannel heat exchanger with a Ni/Ag micro/nano composite structure surface was designed. This top-connected microchannel consists of 11 parallel microchannels. The cross - section of each microchannel is a square with a size of 400 μm×400 μm, and the height of the connected space above the parallel channels is also 400 μm. The Ni/Ag micro/nano composite structure was prepared on the surface of the top-connected microchannel by the brush - plating technique. Using anhydrous ethanol as the working fluid, a comparative experimental study on flow boiling heat transfer in regular parallel microchannels (Regular Microchannel - RMC), top-connected microchannels (Top-connected Microchannel-TCMC), and top-connected microchannels with a micro/nano composite structure surface (TCMC-Ni/Ag) was carried out. The results show that the maximum local heat transfer coefficient of the TCMC-Ni/Ag surface reaches 179.84 kW/m²·K, which is 4.1 times higher than that of the RMC. Visualization studies have found that for TCMC-Ni/Ag, the strongly hydrophilic micro/nano composite structure surface simultaneously increases the nucleation density and nucleation frequency. Under medium and low heat flux conditions, a flow pattern structure is formed where the gas phase converges in the top-connected region while a large number of bubbles still form on the microchannel surface. Under high heat flux density conditions, the capillary liquid absorption effect of the strongly hydrophilic micro/nano composite structure leads to a thin-liquid -film convective evaporation heat transfer mode in the channel, which is the main mechanism for the significant improvement of its heat transfer performance.