Flow patterns and corresponding local heat transfer coefficients in a pulsating heat pipe

A Pulsating Heat Pipe (PHP) is a passive two-phase heat transfer device for handling moderate to high heat fluxes, typically suited for power electronics and similar applications. It usually consists of a meandering capillary tube, closed end-to-end in a loop, evacuated and then partially filled with a working fluid. The internal flow patterns in a PHP are a function of the applied heat flux. At low heat flux levels, an oscillating slug flow pattern is prevalent but once the heat power increases, semi-annular and annular flow patterns gradually appear; in addition, a net fluid circulation in the entire loop may occur. In order to estimatethe local heat transfer coefficient for the randomly oscillating/circulating two-phase flow, a PHP experimental apparatus has been designed and built. Relevant parameters, i.e., local fluid and wall temperatures and corresponding internal pressure fluctuations, have been recorded and the ensuing internal two-phase flow patterns have been visualized. The local heat transfer coefficient in the evaporator zone has been estimated at different heat inputs and related to the corresponding flow patterns.