Flow Regime Map and Gravity Suppression Mechanism of a Top-Heated Pulsating Heat Pipe for Battery Thermal Management

Pulsating heat pipes (PHPs) beneath a battery cold plate operate with the evaporator above the condenser, so that vapour expands directly against gravity. This orientation is imposed by vehicle packaging, yet its internal flow has rarely been resolved. A closed-loop borosilicate glass PHP of 2 mm/3 mm bore charged with methanol was visualized at 960 frames per second at 17, 22 and 27 W and charged volume fractions of 8.5, 17.0 and 25.6 %, and modelled with a one-dimensional force balance. Eleven flow regimes are grouped into three families-evaporation froth (A), oscillation confined to the condenser and adiabatic sections (B) and turn-to-turn migration (C)-and mapped in the power/filling-ratio plane. Vapour pressure dominates the force balance at 0.12 N, twice the wall friction, while gravity contributes less than 5 %. The model reproduces the measured plug amplitude to within 0.013 m at low filling ratio. Thermal resistance rises from 1.13-1.43 K/W at 8.5 % to 2.11-3.85 K/W at 25.6 %, so underfilling is favourable here. The map allows dry-out and flooding to be diagnosed from the observed regime in gravity-suppressed PHP battery modules.