电火花改性表面池沸腾换热特性

通过电火花成型加工技术在铜基换热表面制备微纳结构改性表面，以自制换热表面性能测试装置进行改性表面的池沸腾换热性能实验。改性表面随加工电流改变而具有不同粗糙度、孔隙率和粗糙度因子，表面接触角范围在117.4°~133.5°。实验结果表明，改性表面的微纳结构提高换热面的池沸腾换热效果，临界热流密度较光滑铜表面提高了26%~87.8%，最大传热系数提高了48.1%~213%。改性表面的传热系数随着粗糙度增大而减小，而临界热流密度则是先增大后减小；孔隙率的增大使得改性表面的传热系数也随之增大，临界热流密度则是随着孔隙率的增大而先增大后减小；临界热流密度随着粗糙度因子的增大而降低，传热系数则是先增大后降低。粗糙度对沸腾换热的强化效果较小，孔隙率和粗糙度因子是强化池沸腾换热的关键，孔隙率和粗糙度因子分别影响了气泡核化密度和实际接触面积，提高了气泡脱离频率，带走更多的热量，但两者间存在互相制约的平衡关系。

Study on pool boiling heat transfer on modified surface fabricated via electrical discharge machining

Copper based heat exchange surfaces were modified by electrical discharge machining technology. The pool heat exchange properties of modified surfaces were investigated by self-made test device of heat exchange property for heat exchange surface. Due to the change of the machining currents, the roughness, porosity and roughness ratio of the modified surfaces were changed respectively. The contact angles of the modified were 117.4°-133.5°. The results of experiments indicated that the micro and nano structures of modified surfaces enhanced the properties of pool boiling heat exchange. The critical heat flux of modified surfaces was improved 26%-87.8% than that of polished copper surface. The maximum heat transfer coefficients were improved 48.1%-213% than that of polished copper surface. The heat transfer coefficient of modified surfaces decreased with the increase of roughness, and the critical heat flux increased first then decrease later. The increase of porosity of modified surfaces made the heat transfer coefficient increase, but the critical heat flux went up first and then down. The critical heat flux of modified surfaces decreased with the increase of roughness ratio, and the heat transfer coefficient went up and then down later. The effect of the roughness for enhancing pool boil heat transfer was poor. The porosity and roughness ratios were the key facts to enhance the property of pool boiling heat exchange. The porosity affected the bubble nucleation density and the roughness ratio affected actual contact area, which would improve the bubble departure frequency, and more heat flux was taken away from the modified surfaces by the departure bubbles. There was a mutually restrictive balance between porosity and roughness ratio.