Bubble Jet Loop Heat Pipe 내부 유동 가시화

Abstract

Heat pipe (closed heat transport device to use the phase change) transfer a large amount of heat with a small temperature difference between the evaporating and condensing section. Performance when operating in the horizontal orientation is significantly degraded because circulation of the working fluid is affected by the gravity. Bubble Jet Loop Heat Pipe (BJLHP) was designed for the purpose of heat transport in the horizontal orientation is easy to manufacture because it has no wick. BJLHP is operated by nucleate boiling in the evaporating sections, and the boiling heat rate is transported to the condensing section by bubble oscillation and circulation. So it is possible to overcome low efficiency of heat pipe in the horizontal orientation. Because the effect of nucleate boiling on BJLHP performance is very large, nucleate boiling enhancement is essential for high performance BJLHP. So it has been applied to the device of floor heating, root heating of green house farms, electronics cooling, heat recovery and defrosting. However, research on the working principle of BJLHP is hardly performed. In order to understand its application in various areas, basic research about BJLHP (such as internal flow and operating principles) are necessary. To improve efficiency of BJLHP, heat transfer surface have to enhance and clearance between the evaporating section and heater have to change. In case of the heater within a tube, heat transfer coefficient is changed depending on clearance from the same heat flux. Also the enhanced nucleate boiling surface in the evaporating section of BJLHP improved the performance of BJLHP. So first, this study investigates the liquid-return mechanism in the horizontal orientation of BJLHP by internal flow visualization. In the evaporating section, nucleate boiling in the annular gap generates bubbles. In the condensing section, high velocity vapor flows in the upper part of the tube toward the end of the condensing section and relatively low velocity liquid flows along the lower part of the tube as a counter flow (toward evaporating section). At the end of condensing section, liquid waves bump against each other and then move into the evaporating section. By this mechanism, liquid returns from the condensing section to the evaporating section. BJLHP operates continuously in the horizontal orientation. Second, I conducted visualization about nucleate boiling flow phenomenon of working fluid in accordance with the clearance(2mm, 2.5mm, 3mm) between the evaporating section and heater and the configuration of heat transfer surface(smooth surface heater and sanded surface heater). Through this, we want to gain the basic design data of high-performance BJLHP. And I measured super heat degree about heater in annular gap.