Boiling heat transfer enhancement of double-tube heat pipe for high power devices

Using a double-tube heat pipe to cool high power devices such as inverter devices, narrow space of the currently applied boiling cooling method will not only improve the cooling performance but also contribute to a lightweight device and will be maintenance-free. Considering the structure of inner tube of evaporator section of a heat pipe, we focused on the previous study which found using narrow space in the double-tube evaporator promoted nucleate boiling. We have conducted an experiment using an evaporator with a double-tube structure under pool boiling state. As a result, narrow space enhanced working fluid launching effect and the heat transfer performance. Based on these results, the purpose of this research is to observe boiling behavior generated in narrow space and to build database for putting a double-tube heat pipe to commercial application. We made three experimental apparatus that haves already simulated double-tube heat pipe's evaporator. These experiments were conducted changing the location of the inner tube against the heating surface. The experimental parameters were as follows: the gap (inner tube diameter), water level, inner tube height and heat flux. We conclude that at higher inner tube height and narrower space, caused the launching effect and keep the upper part wetness and the degree of superheat became low, and enhanced heat transfer coefficient. In particular, using d=15mm off-axial inner tube, the larger heat transfer coefficient was obtained than that without inner tube. A the outer tube inside diameter D=24mm and inner tube diameter d=24mm is the limit, height of capillary force and the effect of double-tube heat pipe.