Nonthermal Ionization Caused by Aerodynamic Discontinuities in Charged Aerosol Jets

Significant space-charge accumulations caused by thermodynamic changes are produced whenever an electrical corona discharge of the streamer type is maintained in a region of rapid and sustained mass density variation. The experimential investigation uses a highly charged supersonic aerosol jet (0.03 C/m3) at temperatures below 300°K and pressures below 3 atm. The aerosol emerges from an underexpanded nozzle producing a characteristic series of expansion and compression regions. The charge density of the aerosol is sufficiently high so that electrical breakdown is induced at a grounded needle placed near or within the jet. It is shown that the ionization produced within the supersonic region, where rapid mass density changes occur, liberates charge in excess of the charge carried by the aerosol and that the current to the induced corona needle reproduces density variations in the underexpanded jet. A model is presented whereby changes in electrical conductivity, produced by the streamers propagating through the jet, result in space charge accumulations. Computation shows that the charge accumulation on shocks and compression and expansion waves is of sufficient magnitude to be responsible for the excess ionization and the visual appearance of the corona discharge.