Laminar Mixing, Heat Transfer and Pressure Losses in a Chaotic Mini-Channel: Application to the Fuel Cell Cooling

Currently, the heat exchangers allowing the cooling of the low temperature fuel cells (PEMFC) are integrated in the bipolar plates and constituted of a network of straight channels. The flow regime is laminar, and thus, unfavorable to an intense convective heat transfer. In order to increase the power density of the fuel cells, the use of chaotic geometries in the cooling system is envisaged to intensify high convective heat transfer. In this numerical study, several chaotic three-dimensional mini-channels of rectangular section (2 millimeters × 1 millimeter) are evaluated in terms of heat transfer efficiency, mixing properties and pressure losses. Their performances are compared to those of a straight channel geometry currently used in the cooling systems of the PEMFC, and a serpentine 2-D channel. Hydrodynamic and thermal performances of these geometries are computed using the commercial CFD code Fluent©. At the inlet section, the velocity profile is hydrodynamically established. The thermophysical properties of the fluid are constant and equal to those of water at 300 K. The Nusselt number is evaluated for a Reynolds number equal to 200 and with a uniform density flux imposed on the walls and equal to 10,000 W/m². For the calculation of the mixing rate, a condition of adiabatic wall is imposed. The inlet section is horizontally divided into two parts. Water in the higher part is at the temperature of 320K and in the lower part is at the temperature of 300K. The calculation of the mixing rate is made for Reynolds numbers equal to 100 and 200. The present study shows that a 3-D chaotic channel geometry significantly improves the convective heat transfer compared to regular straight or serpentine channels. Among all the studied geometries, one of them induces the higher heat transfer intensification (mean Nusselt number equal to 20) with a strong pressure loss. With an alternative geometry, we obtained a better compromise between high heat transfer and reduced pressure loss. However, all the chaotic geometries present similar mixing rate for the two studied Reynolds number. To confirm the performances of the selected geometries, an experimental study is currently undertaken. The final aim is to realize and test a prototype of chaotic heat exchanger in a bipolar plate of PEMFC.