Augmentation of Boiling Heat Transfer by Free Particles Bed

提出了一种通过在受热面周围预置可自由移动的固体颗粒来强化沸腾换热的被动技术，并进行了实验研究。颗粒被表面产生的蒸汽气泡流态化，并起到增强热传递的作用。在床上设置了直径为0.88 mm的圆柱形加热器，颗粒在试验液中自由移动，测量了沸腾换热特性。用于颗粒的材料有立方氧化铝、玻璃和多孔氧化铝，直径从0.3 mm到2.5 mm不等。测试的液体是水和乙醇。换热强化比随着热流密度的增大而增大，在中等热流密度下达到最大。得到的最大增强率约为液体中的10倍。测量了颗粒床下加热器的最大热流密度与加热器深度的关系。对平坦受热面的沸腾换热进行了一系列实验。对铝等金属颗粒以及氧化铝和玻璃颗粒进行了检查。此外，还考察了颗粒几何形状的影响。在有限圆柱颗粒的实验中取得了很好的效果，其最大增强率约为液体中的7倍。在使用氯化钠溶液的实验中，对受热面污垢的清洗效果得到了认可。

A passive technique to augment boiling heat transfer characteristics by freely movable solid particles preset around the heating surface is proposed and studied experimentally. The particles are fluidized by vapor bubbles generated on the surface and act to augment heat transfer. Further, they act to remove the fouling layer which may be formed on the surface.A cylindrical heater of 0.88mm in diameter is set in a bed whose particles are freely movable in test liquid and nucleate boiling heat transfer characteristics are measured. Materials used for particles are cubic alumina, glass and porous alumina and the diameter ranges from 0.3mm to 2.5mm. Liquids tested are water and ethanol. Augmentation ratio of heat transfer becomes large with the increase of heat flux and reaches the maximum at a medium heat flux condition. The maximum augmentation ratio obtained was about 10 times of those in liquid. The maximum heat flux is also measured versus the depth of heater under the surface of particles bed.Another series of experiments is made for boiling heat transfer on a flat heating surface. Metal particles such as aluminium are examined as well as alumina and glass particles. Effect of particles geometry is also examined. Much effect is obtained in experiments with a finite cylindrical particles.The Maximum augmentation ratio obtained in this case reaches about 7 times of those in liquid. Cleaning effect of the fouling layer on the heating surface is recognized in experiments using NaCl solution.