| Summary: | Flow through an experimental model of a U-shaped flow channel is used to investigate the hydrodynamic phenomena that occur within serpentine reactant transport channels of fuel cells. Achieving effective mixing within these channels is crucial for the proper operation of the fuel cell and proper understanding and characterization of the underlying fluid dynamics is required. Particle image velocimetry (PIV) is used to investigate the three-dimensional structure of the flow by analyzing the velocity and associated vorticity field over two perpendicular channel cross-sections. A range of Reynolds numbers, 109 I Re I 872, corresponding to flow rates encountered in a fuel cell operating at low to medium current densities is investigated. The effect of the flow rate is characterized in terms of the instantaneous and time-averaged representations of the velocity vectors, out-of-plane vorticity and the velocity streamlines. At the lowest Reynolds numbers, the flow is steady and is characterized by high vorticity regions associated with shear layers separating from the sharp convex comers of the U-bend and reattaching on downstream surfaces. The flow also exhibits the classical secondary Dean flow pattern with two symmetric circulation zones. Transition takes place in the range 381 I Re I 436 as the two recirculation zones, which originally develop in the U-bend region, merge into one separation region. This transition is accompanied by the generation of additional vortices in the secondary flow plane. The relationship between the flow in both planes and the transition is examined along with properties of the instability including RMS, Reynolds stress, and the oscillation frequency. The quantitative flow visualization results obtained presented here should be useful in guiding numerical models of fuel cells, and indicate that the commonly used assumption of steady laminar flow should be revisited, and alternative models developed.
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