MHD Flow Control and Power Generation in Low-Temperature Supersonic Flows

The paper presents results of cold MHD flow deceleration and MHD power generation experiments using repetitively pulsed, short pulse duration, high voltage discharge to produce ionization in M=3 nitrogen and air flows. MHD effect on the flow is detected from the flow static pressure measurements. Retarding Lorentz force applied to the flow produces a static pressure increase of up to 17-20%, while accelerating force of the same magnitude results in static pressure increase of up to 5-7%. No discharge polarity effect on the static pressure was detected in the absence of the magnetic field. The fraction of the discharge input power going into Joule heat in nitrogen and dry air, inferred from the present experiments, is low, α=0.1, primarily because energy remains frozen in the vibrational energy mode of nitrogen. Comparison of the experimental results with the modeling calculations shows that the retarding Lorentz force increases the static pressure rise produced by Joule heating of the flow, while the accelerating Lorentz force reduces the pressure rise. This result provides first direct evidence of cold supersonic flow deceleration by Lorentz force. The experiments have also shown that at the present conditions, electric current in the MHD power generation regime is very low, of the order of 1 mA. This is entirely due to the bottleneck effect of the non-self-sustained discharge cathode layer, at the conditions when the MHD open voltage is significantly lower than the cathode voltage fall. Adding an easily ionizable species to the flow (C9H15N at 0.1% level) did not result in the flow conductivity increase, both because of seed condensation in the cold supersonic flow and a low seed fraction. Kinetic modeling calculations of the pulser-sustainer MHD discharge demonstrate that (i) low open voltages reduce MHD currents by more than two orders of magnitude, and (ii) this effect cannot be circumvented by seeding the flow at feasible levels or by using electrodes with high secondary emission coefficient. Based on the results of the present experiments and modeling calculations, the only feasible approach to extracting MHD power at low flow temperatures is to increase the MHD open voltage to the value comparable with the discharge cathode voltage fall.