Studies in expanded shock tube flows II. Interpretation of ion density profiles

Abstract

Ion density profiles have been obtained in expanded shock tube flows from incident shock waves in argon and in air. The rate of increase of ion density and the ‘plateau’ level of ion density have been measured for shock waves in argon over the temperature range, 3670 to 10730°K, and in air over the range 2260 to 3790°K. The Arrhenius temperature coefficients obtained are much smaller than the ionization potentials of the major species involved and the results are interpreted in terms of a multi-step process involving impurity ionization. The changes in the physical properties associated with the flow through the expansion and their effect on the ‘freezing’ condition are also discussed. Evidence is also presented for vibra­tional equilibration during the expansion process.