多孔球层内沸腾现象与传热特性研究

Boiling Phenomenon and Heat Transfer in Bead-packed Porous Structure

采用池式沸腾实验系统,在常压底部加热条件下分别对由直径4、6、8mm玻璃球构建的多孔结构内沸腾过程进行了可视化研究。结果表明,过冷沸腾时,加热壁面上产生孤立汽泡,小汽泡可聚合为主汽泡,主汽泡脱离频率较低,汽相以分散的小汽泡为主;饱和沸腾初期,汽泡生长变快,主汽泡体积变大,连续汽相范围广阔;主汽泡形成频率随热流密度增加而增加;膜态沸腾时,底面被汽膜包围,液相占据球层空间。球体直径越大,产生同类现象需要的热流密度越大,传热系数的极值越大。饱和沸腾存在传热强化区和抑制区。直径4、8mm玻璃球构建的多孔介质传热系数随热流密度的增加而增加,6mm多孔介质则相反。

A visual study on pool boiling behavior and phase distribution was conducted on the porous structures made of staggered glass beads at atmospheric pressure. The bead-packed structure was heated on the bottom. The investigations were carried out respectively at different glass bead diameters which were 4mm, 6mm and 8mm. The results show that during subcooled boiling, small isolated bubbles are formed on the heated surface and combine into main-bubbles, the dispersion frequency of the main-bubbles is low and the small bubbles scatter in the bead-packed porous structures. At the initial stage of saturated boiling, the bubble growth rate, the volume of main-bubbles and the range of continuous vapor phase increase. The dispersion frequency of main-bubbles increases with the increasing of heat flux. During film boiling, the heated surface is absolutely covered with vapor film and the porous structure is full of liquid. The larger the diameter of beads is, the higher heat flux is needed for the same phenomenon, and the higher maximum value of heat transfer coefficient will be. During the whole saturated boiling, and the heat transfer enhanced firstly and then weakened. Being opposite to that of the diameters of 4mm and 8mm, the heat transfer coefficient in the 6mm-bead-packed porous structure decreases with the increasing of the heat flux.