Liquid Distribution in the Axial Direction of an Inclined Triangular Micro Heat Pipe

Abstract

Micro heat pipes (MHPs) are, essentially, miniature heat transfer devices which use phase change to transfer thermal energy. In recent years, there have been numerous proposals for their applications in cooling electronic devices. In this paper, the axial liquid distribution of a triangular MHP is investigated for the case of inclined orientation. The study is limited to the case of positive inclination, whereby the condenser section is elevated from horizontal position. In this case, the inclusion of gravity renders the governing equation unsolvable analytically, and the 4th order Runge-Kutta method has been selected to solve it numerically. The results show that for a horizontally oriented MHP, so that the effect of gravity can be neglected, the liquid distribution along the axial direction increases monotonically from the minimum value at the evaporator end to the maximum at the condenser end. However, if the MHP is positively inclined, the axial distribution of the liquid phase is changed qualitatively. While the liquid distribution still increases monotonically starting from the evaporator end, it reaches its maximum value not at the condenser end but at a certain point in the condenser section, beyond which the liquid distribution decreases monotonically. Moreover, as the angle of inclination is increased, the maximum-distribution point moves further away from the condenser end. This maximum point, where potentially flooding will first take place, results from the balance between the effects of gravity and the heat load on the MHP, the former having the propensity to move all the liquid from the condenser towards the evaporator while the latter the tendency to place more liquid in the condenser section. As the liquid distribution assumes its greatest value at the maximum point, a throat like formation appears there. This formation is detrimental to the performance of an MHP, because it hinders, and at worst may block, the axial flow of the vapor phase.