Abstract:

To investigate the evaporation characteristics of porous media with different porosity, 2 pressed slice samples composed of nickel powder are manufactured with porosities 0.85, 0.75 and 0.60 (named sample A,B and C). The slice samples are long and thin that the wetting frontier movement can be clearly visible during suction. The samples were firstly used in suction experiments in which ethanol penetration was recorded by a high-speed camera. The results show that the sample with larger porosity has higher suction rate. It is because that porous media with larger porosity have larger pore sizes, which can promote the suction rate according to theoretical analysis. Next the samples were heated with constant heat load to test their cooling ability and research their heat transfer efficiency by evaporation. Heating experiments were separated into 3 parts:steady state-cooling, dry-cooling and wet-cooling. Steady state-cooling results show that larger porosity sample has higher evaporation rate mainly due to its higher suction ability. Dry-cooling is aimed to compare samples' transient cooling ability. The results indicate that the highest heat flux caused by evaporation belongs to sample A while its cooling time is the shortest. Although sample C keeps cooling for the longest, its limited evaporation heat flux constrains its cooling degree. Sample B, with both modest evaporation heat flux and cooling time decreases the heater temperature most. In wet-cooling test (on purpose of testing samples' performance in dry-out condition), sample A keeps itself wet for the longest time and has the highest dry-out temperature, representing the best ability to avoid dry-out phenomenon. Sample B shows relative low superheat degree and the longest time in intensive evaporation, resulting in its overall working temperature being the lowest.

Key words: porous media, microchannels, porosity, evaporation, dryout, interfacial tension

摘要：

制作了具有3种不同孔隙率（0.85、0.75和0.60）的多孔镍粉样本，并进行了毛细抽吸实验和蒸发冷却实验（包括稳态冷却实验，初始状态为干态和湿态的冷却实验），以探究多孔介质抽吸和蒸发过程规律。毛细抽吸实验发现孔隙率大的镍粉样本抽吸速率更快，且根据毛细单管抽吸实验和毛细抽吸理论分析结果表明：孔隙率大的镍粉样本具有更大孔径是其抽吸速率更快的根本原因。稳态冷却实验结果表明孔隙率0.85的镍粉样本具有最快的蒸发速率，0.60镍粉样本蒸发速率最慢，这是受到多孔样本抽吸速率主导的结果。在初始干态冷却实验中，孔隙率为0.85的镍粉样本具有最大的瞬态冷却热通量，但要达到最大冷却温度幅度须取得瞬态冷却热流与冷却时间的平衡，因此孔隙率0.75样本冷却温度幅度最大。在初始湿态实验中，孔隙率为0.85的镍粉样本具有最高的烧干温度和最长的烧干时间，表现出最强的抗烧干性能，但是孔隙率0.75样本剧烈蒸发过热度最小且剧烈蒸发时间最长，最能有效抑制样本热端温度升高。

关键词: 多孔介质, 微通道, 孔隙率, 蒸发, 烧干, 表面张力