Operational regimes in a confined pulsatory two-phase thermosyphon

Abstract

The major loopholes in existing criteria used for the classification of various flow characteristics/patterns in the confined two-phase closed thermosyphons (TPCTs), and the additional accountable parameters have been recognized. The phenomenology of flow regimes in confined TPCTs, and the influence of confinement on their heat transfer characteristics need to be understood better. Also, the mitigative solutions to enhance the operational limits of a confined TPCT need to be developed. So, a visual and detailed study of the thermo-hydrodynamics of TPCTs is essential. The current research presents a confined pulsatory two-phase thermosyphon (PTPT), which is a hybrid cooling device derived by combining the ideologies of a pulsating heat pipe (PHP) and a TPCT. The functioning of PTPT involves thermosyphon phenomena in its evaporator coupled with oscillations of a liquid–vapor interface in its condenser. An experimental study of different flow regimes, viz. bubbly, bubbly/slug, geyser, slug, churn and annular flows, and associated heat transfer mechanisms observed in the PTPT at different applied thermal boundary conditions has been performed. The two-phase flow in PTPT has been rigorously mapped and an empirical flow regime map has been developed using the confinement and Jakob numbers for regime characterization. Contrary to the conventional notions, highly confined as well as unconfined flow patterns have been observed within a narrow range of the confinement number, with unconfined ones at the highest confinement number. The film-flow along with the corresponding instabilities, nucleation and growth of vapor bubbles, inter-regime transitional states, and associated pressure fluctuations have been studied in detail.