Thermo-Hydrodynamics in a Closed Loop Pulsating Heat Pipe

Abstract

Pulsating heat pipes (PHPs) represent a promising solution for passive on-chip, two-phase cooling of micro-electronics, providing advantages such as a simple construction and operation in any gravitational orientation. Unfortunately, the unique coupling of thermodynamics, hydrodynamics and heat transfer responsible for their operation has so far eluded comprehensive description or accurate prediction. The complexity of the self-sustained two-phase flow in PHPs presents many challenges to the understanding on the physical phenomena taking place. It is important to evaluate the heat and mass transfer mechanisms occurring during their operation in order to better describe their performance as a function of the operating conditions. In the present study, a new facility at the Laboratory of Heat and Mass Transfer (LTCM) was built to allow the synchronized thermal and visual investigation of a Closed Loop Pulsating Heat Pipe (CLPHP). A single-turn channel CLPHP was investigated using R245fa as the working fluid. The tests were carried out at filling ratios from 10 to 90 % and heat inputs from 2 to 60 W, for vertical and inclined orientation. Flow visualization was attained via the transparent front side of the test section which provided full optical access to the flow inside of the CLPHP channels. A novel time-strip image processing technique was applied to the high speed videos to extract qualitative details of the flow regimes and quantitative flow data concerning the liquid/vapor interface dynamics. Local temperature oscillations were also measured and their frequency spectra further helped in characterizing the self-sustained two-phase flow. Thermal resistance measurements were used to qualitatively and quantitatively assess the effect of the flow dynamics on the system thermal performance, which are presented as operational maps. The dynamics governing the PHP operation during oscillating and circulating flows and their effects on the system thermal performance were investigated and insight gained. Four distinct flow regimes and their thermal and flow-dynamics characteristics were identified, suggesting the strong coupling between the two-phase flow pattern and the system thermal behavior. Thin film evaporation was observed to be the most dominant thermal mechanism while heat transfer into the oscillating liquid slug was of second importance, together with localized nucleate boiling. Moreover, the net forces acting on the system could be identified through the novel time-strip technique, revealing new details on the mechanisms producing self-sustained two-phase flow oscillation and circulation, and the two-phase flow pattern transition. The role of gravity for the operation of this single-turn CLPHP was also assessed.