Velocity Profiles in the Injection Zone of an EHD Generator and Efficiency Considerations

Electrical measurements taken in a cell modeling the injection zone of an EHD generator showed the existence of two flows regimes. In our wire-two plate geometry, either the electrical forces dominate or the hydrodynamic ones do, depending on the applied voltage and the Reynolds number of the forced flow through the cell. In the former case entrainment of ions out of the injection tion zone is very poor, whereas, in the latter regime, entrainment of the injected space charge is considered complete. We present here a more advanced study based on velocity measurements by laser doppler anemometry. Velocity profiles have been determined in the following three situations: (1) the convective four-roll case obtained with no forced flow; (2) the dominant electric force regime (high applied voltage and low flowrate) and (3) the entrainment regime (low applied voltage and high flowrate). We discuss the experimental observations in the light of what is known about wall jets (case (1)); wakes (case (3)); and EHD migration of space charge (case (2)). The functioning of an= EHD generator is then considered with these measurements in mind and the theoretical one-dimensional model of Stuetzer for the conversion zone is reappraised and modified to give an expression for conversion efficiency. This goes some way to explaining the low efficiency obtained with these machines previously.