CFD and Flow Network Analysis of Manifolding in a PEMFC

Abstract

Near-uniform flow distribution in a fuel cell stack is essential to stack performance and overall system efficiency. The gradients induced by the non-uniformity of the flow within each of the unit cells also have a significant impact on stack durability. In typical configurations, the oxidant and fuel are fed into a stack through manifolds and then enter each unit cell through secondary inlet port. After flowing through the unit cells, the spent gases as well as possible liquid water then enter the outlet header to leave the stack. The objective of this paper is to develop a practical model to predict cell-to-cell flow distribution in a proton exchange membrane fuel cell (PEMFC) stack. The flow distribution is first simulated using a computational fluid dynamics (CFD) tool, CFD-ACE+, in a 3D computational domain for single-phase gas flows. The simulations use a domain encompassing the flow from the inlet header through an array of unit cells to the outlet header. The CFD simulations show that in the outlet header, the flow injected from the unit cells to the header changes the flow pattern considerably, which results in a reduced cross section area for the flow in the axial direction. A circulation zone is seen near the low velocity end of the header, which may potentially become a region where liquid water accumulates. Increasing static pressure along the flow direction is observed in the inlet header. The simulated results are validated and found to be in good agreement with experimentally measured pressures in a fuel cell stack. Based on the observations in the CFD simulations, a flow network model is developed to provide quick estimates of the flow distribution as a function of stack dimensions including header and unit cell geometry. In essence, the flow network model solves for the pressure at each junction of the unit cell and the header. Three fitting parameters are introduced to account for effects of surface roughness of the headers, reduced effective header area in the outlet header, and pressure drop in the unit cell. The flow network model is shown to capture the characteristics of pressure variation and flow distribution obtained in the CFD simulations. The flow network model can effectively match experimental data and be used as a fast tool for initial design of a PEMFC stack.