The Investigation of Entrainment Limit in Operating Heat Pipes

Abstract

A heat pipe is a two-phase flow heat transfer device that utilizes the latent heat of vaporization of a fluid. It is a sealed container that is lined with a wick structure which is saturated with a selected working fluid. Heat input at one end raises the temperature, which creates an axial pressure gradient and causes the vapor to flow to the opposite end, where heat is rejected and the vapor recondenses. The capillary pumping action provided by the wick structure maintains the circulation of the fluid. At steady-state, the operation of the heat pipe is limited by the occurrence of several phenomena--boiling in the wick, the capillary limit, the sonic limit or the choking of the vapor flow, and the entrainment of liquid droplets in the vapor flow. Of these four limits, the entrainment limit is the least understood. The analytical studies available to date yielded four different equations that predict significantly different maximum heat transfer rates at the onset of entrainment. Therefore, it was the objective of this investigation to compare experimental results to the entrainment limits predicted by these four equations. In this experiment, liquid droplet entrainment was monitored using a laser beam travelling down the length of the vapor channel to investigate qualitatively the occurrence of entrainment. A graph of the results indicated that the heat flux at the entrainment limit may be at least as great as the lowest value predicted by the four analytical equations. More tests are being conducted to determine the actual entrainment limit in the heat pipe. The occurrence of slugging affected the heat pipe's operation. Further studies to analytically determine the slugging limit will help in understanding this behavior in an operating heat pipe.