Gasdynamic Instrumentation of High Enthalpy Flows

Techniques for making measurements of gas properties in supersonic plasmas generated by continuous thermal arc heaters and crossed field accelerators are described: Specifically, measurements of static pressure, total pressure, flow velocity, local mass flow, and local total enthalpy. The use of these measurements together with the relationships between the gasdynamic parameters such as temperature, pressure, density, specific heat, sound speed, Mach number, and degree of dissociation and ionization are discussed. When the gas is out of equilibrium, as it is in crossed field accelerator plumes produced to date and in many thermal arc jets, the number of independent measurements required to determine the gas properties unambiguously is three plus one for each non-equilibrium energy reservoir. Previously described diagnostic techniques for plasma jets, have relied upon known reservoir conditions and upon the use of an effective specific heat ratio. However these assumptions cannot be made for crossed field accelerator plumes because reservoir conditions are not known and because large fractions of the gas energy may be invested in non-equilibrium energy reservoirs. For this reason the analytical treatment of the measurements is important. Results obtained in argon plumes using velocity, static pressure, total pressure, local mass flow, and local total enthalpy probes are presented. Gas properties determined from these measurements are given, and the degree of reliability of the measurements is ascertained by cross-checks wherever possible.