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增强沸腾传热特性的被动技术，并进行了实验研究。颗粒被表面产生的蒸汽气泡流化，并起到增强传热的作用。此外，它们起到去除可能在表面上形成的污垢层的作用。将直径0.88mm的圆柱形加热器设置在床上，床上的颗粒在试验液体中自由移动，测量了成核沸腾传热特性。用于颗粒的材料有立方氧化铝、玻璃氧化铝和多孔氧化铝，直径从0.3mm到2.5mm。测试的液体是水和乙醇。传热放大比随着热流密度的增大而增大，在中等热流密度条件下达到最大。所获得的最大增广率约为液体增广率的10倍。最大热流密度随颗粒床表面下加热器深度的变化而变化。另一组实验是在一个平坦的受热面上进行沸腾传热的。金属颗粒，如铝，以及氧化铝和玻璃颗粒进行检查。同时还考察了粒子几何形状的影响。在有限圆柱形粒子的实验中得到了很好的效果。在这种情况下获得的最大放大比达到液体的7倍左右。用NaCl溶液对受热面污垢层的清洗效果进行了验证。

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.