Two-Phase Flow in Square Mini-Channels: Flow Pattern Transition

For the internal fluid distribution in a fuel cell, channels of about 1 mm are usually used. The type of two-phase flow (reactive gases, liquid and vapor water) in these mini-channels has an important influence on the fuel cell performances. Thus it is important to optimize this two-phase flow to avoid the flooding of channels and electrodes and the instabilities in the fuel cell. To understand the type of two-phase flow, depending of the mini-channel size and section, and for different fluid velocities, several flow visualizations have been done, in channels of circular (2 mm diameter) and square (1 and 4 mm²) cross-section for different channel slope. Two main classes of flow patterns have been identified: separated flow (annular flow with or without waves, stratified flow), and intermittent flow (plug and slug flow). It has been also observed that the pressure gradient presents a specific behavior near the transition between separated flow and intermittent flow. A model for predicting the flow pattern transition is proposed.