Effects of electrode and boundary-layer temperatures on MHD generator performance

Experiments were performed in a combustion-driven MHD generator to determine the effects of electrode temperature and boundary-layer temperature on its performance. The generator was a single electrode pair guarded to avoid end effects by two electrode pairs both upstream and downstream. However, end effects were found to be small, so the tests can be interpreted in terms of a five-electrode-pair generator. The electrodes were held at three temperature levels--775°, 1475°, and 1685°K--by controlling individual cooling circuits. Two electrode boundary-layer temperature profiles were investigated: one resulting from a hot (2400°K) upstream wall, and the other resulting from a cold (750°K) water-cooled plate that replaced the normal upstream electrode wall. Probes were installed in the insulator side wall between the center pair of electrodes to measure the voltage profile in the gas. Visual and photographic observations of the electrodes were made. It was found that the generator with hot electrodes downstream of the hot walls produced about seven times the power of the generator with cold dectrodes downstream of cold walls. Electrode voltage drops, obtained from the side wall probe data, were found to vary with load current, electrode temperature, and upstream wall temperature. Cathode voltage drops were higher than anode voltage drops, but the difference decreased with increasing electrode temperature and upstream wall temperature. An analytical model was postulated to interpret the data. The ohmic resistance of the boundary layer was calculated by integrating an equilibrium conductivity profile through the boundary layer to a point one Debye length from the electrode surface. The calculated values agree with resistance of the anode boundary layer obtained from probe data. The difference between resistances of cathode and anode boundary layers was attributed to surface and/or sheath effects at the cathode.