Within the fuel cell community interest is increasing concerning the understanding of the complicated fluid mechanical phenomena in the small channels of the flow field. Different surface-structures and wetting-properties (e.g. channel walls and gas diffusion layer) together with the gaseous and liquid phases due to the electro-chemical reactions form a complex fluid mechanical system which is not fully understood. However, it is known that flow field design still can be improved. Until now there was no technique available that allows for in-situ-measurements of the flow inside the micro-channels under real conditions and for simultaneous cell-power measurements.In general flows in micro-structures can be measured and analyzed using micro-Particle-Image-Velocimetry (µPIV). This method is based on the optical detection of small tracer-particles within the fluid. Those particles need to follow the flow precisely. The scenery is illuminated by laser-light and the particle shift within the measurement plane is detected by a camera. The particle movement is transferred into a planar velocity distribution by means of cross-correlation schemes. µPIV is established for measurements of liquid flows.It was one goal of the project to develop a µPIV-method that enables local velocity measurements of a gas-flow in micro-channels. As a second goal the new method should be applied in the difficult environment of an operated fuel cell.To reach these goals tracer particles have to be generated in sufficient number, with adequate size and fluorescence intensity. Different methods and materials were systematically and successfully tested. Means to effectively couple the particle-generation system to the test-cell were developed and the corresponding procedures for successful µPIV-measurements were identified (see figure). Measurements of microchannel flows were performed under various predefined conditions to validate the measurement-principle and accuracy. Additionally, the newly developed µPIV-technique was successfully applied in fuel cell models and in operated fuel cells under real conditions. The results were used to validate numerical simulations and to optimize flow field designs.Above all the in-situ µPIV-measurements with simultaneous cell-power monitoring revealed new, unknown phenomena of the occurrence and effects of the two-phase flows. It could be shown that the channel blockage due to CO2-bubbles at the anode side of a DMFC leads to an increase of the cell-power. At the cathode side liquid water accumulates at the channel walls forming films. Channel blockage at the cathode side was not observed (see figure). It has to be stated that the connection between cell-power and the formation of two-phase flows in fuel cells is not understood in detail. But the newly developed µPIV-technique may serve as a tool to further analyze those aspects.

Publications related to the project:

• final report (German only)
• La Houille Blanche, n°4, 2011, p. 55-61 
• tm - Technisches Messen, Vol. 78, No. 5, p 253-259 (6)
• PIV as service of ZBT