Time-strip visualization and thermo-hydrodynamics in a Closed Loop Pulsating Heat Pipe

New trends in the microprocessor industry are leading not only to miniaturization and integration but also to increases in power dissipation rates which now require advanced cooling solutions to prevent thermal damage to the devices. Pulsating heat pipes (PHPs) represent a promising solution for passive on-chip, two-phase cooling of such 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. This paper reports on flow visualization experiments in a Closed Loop Pulsating Heat Pipe (CLPHP)-charged with R245fa-operating over a range of test conditions. A novel time-strip image processing technique has been applied to the flow videos to extract qualitative details of flow regimes and quantitative flow data concerning the liquid/vapor interface dynamics. The latter can be coupled with thermal data to reveal new details regarding flow characteristics, such as two-phase flow pattern and its oscillation. Four distinct flow regimes and their steady thermal oscillation characteristics have been identified and discussed. - Highlights: • A time-strip image processing technique is applied to two-phase flow videos in a CLPHP. • Quantitative flow data from the time strips are compared with synchronized temperatures. • Temperature and displacement signal share the same frequency components. • Oscillating slug-plug regime is produced by nucleation and bubble expansion. • For circulating regime 'local flow direction switch' is observed