Cooling of PEMFC with pentane boiling in minichannels : A study of flow instabilities using neutron radiography visualization

Compared to other heat transfer systems, Polymer Electrolyte Fuel Cells are characterized by very low heat flux. In order to analyse two-phase pentane cooling of bipolar plates, high speed neutron visualizations have been performed in a circular steel tube (Di=1.1 mm, Do=2 mm) at the Institut Laue-Langevin in Grenoble, France. The flow pattern is clearly visible and can be synchronized with time resolved temperature measurements. At low mass flow rates, the pipe wall is high above the saturation temperature and the pipe filled with vapour and liquid slugs. At higher flow rates, the wall is superheated when filled with liquid and at saturation temperature when exposed to a liquid-vapour mixture. An irregular switching between these two states was observed. Superheated liquid exists in both stable and time dependent states. The ?-radiation produced by the neutrons is shown to have an important effect on the boiling process. Simplified steady and time dependent models are proposed to explain the measured wall temperature instabilities and superheat.