Ni/Ag微纳结构强化顶部连通型微通道沸腾换热

微通道换热器较大的比表面积使其具有较高的热质传输效率，在化工、能源等领域具有广泛的应用前景。针对微通道流动沸腾换热强化，本文设计了一种具有Ni/Ag微纳复合结构表面的顶部连通型微通道换热器，该顶部连通型微通道由11条并联微通道组成，微通道的截面为400μm×400μm的正方形，并联通道上方连通空间的高度也为400μm；采用电刷镀技术在顶部连通型微通道表面制备了Ni/Ag微纳米复合结构，以无水乙醇为工质，开展了普通并联微通道（regular microchannel, RMC）、顶部连通型微通道（top-connected microchannel，TCMC）以及具有微纳复合结构表面的顶部连通型微通道（TCMC-Ni/Ag）内流动沸腾换热对比实验研究。结果表明：TCMC-Ni/Ag表面的最大局部换热系数达179.84kW/(m²·K)，较RMC的最大局部换热系数提高了4.1倍。可视化研究发现，对于TCMC-Ni/Ag，强亲水性的微纳复合结构表面同时提高了核化密度和核化频率，中低热流条件下形成气相汇聚于顶部连通区域，微通道表面仍然产生大量气泡的流型结构，在高热流密度条件下，强亲水性微纳复合结构的毛细吸液作用使得通道内产生了薄液膜对流蒸发换热模式，是其换热性能大幅提高的主要机理。

Enhancement of flow boiling heat transfer by using Ni/Ag micro/nano structures in a top-connected microchannel

Microchannel heat exchanger has a broad application potential due to its advantage of high heat and mass transfer efficiency induced by its large surface to volume ratio. In order to enhance flow boiling heat transfer in a micro evaporator, a novel top-connected microchannel (TCMC) evaporator with micro/nano coatings on the microchannel surfaces was designed and fabricated by using electric brush plating method. The microchannel evaporator consists of eleven parallel microchannels with a cross section of 400μm×400μm. The height of the top connected zone is 400μm. An experimental investigation was conducted comparatively to find the heat transfer performance of the three test sections, including regular microchannel (RMC), top-connected microchannels (TCMC) and top connected microchannels with Ni/Ag compound coatings (TCMC-Ni/Ag). It indicated that the TCMC-Ni/Ag had the largest local heat transfer coefficient of 179.84kW/(m²·K), which was 4.1 times that of the RMC. It disclosed that the highly hydrophilic surfaces with Ni/Ag micro/nano compound coatings enhanced not only the bubble nucleation density but the bubble generation frequency by high-speed flow visualization. In relatively low and medium heat fluxes, the vapor phase formed by the coalescence of detached bubble tended to aggregate in the top connected zone, while the bubble nucleation was still maintained on the heated microchannel surface. Under high heat fluxes, the strong capillary suction of the highly hydrophilic surfaces (TCMC-Ni/Ag) resulted in the heat transfer mode of thin liquid film convective evaporation, which was the main mechanism for the contribution to the great improvement of heat transfer performance.