Spectrum-Line Reversal Measurements of Free-Electron and Coupled N2 Vibrational Temperatures in Expansion Flows

Abstract

Spectrum-line reversal measurements of the excitation temperature of Na atoms in expansion flows of shock-heated Ar and 1% N2+Ar mixtures are described. The measurements were made in a conical nozzle attached to the end of a conventional shock tube. For expansion flows of pure Ar from reservoir temperatures of 3200° to 4200°K and a reservoir pressure of about 37 atm, the measured Na temperatures (≈2200°K) were considerably in excess of the local-translational temperature (≈400°K). These high excitation temperatures are interpreted in terms of the excitation of Na by free electrons produced from ionization of the Na in the reservoir. On this basis, free-electron temperatures are deduced which indicate freezing of the electron thermal energy at values corresponding to those expected at the nozzle throat. This result suggests a slow rate of transfer of electron thermal energy to the Ar translational mode. The addition of 1% N2 to the Ar expansions produced large reductions in the measured Na temperatures. The reduced temperatures are shown to correspond to the frozen N2 vibrational temperatures expected on the basis of previous results. These reductions are explained in terms of an efficient transfer of the excess-electron thermal energy to the N2 vibrational mode, the free-electron and N2 vibrational temperatures being thereby coupled and equilibrated during the expansion. The present results also substantiate the faster vibrational relaxation rates inferred previously from similar expansion-flow studies of pure N2; they further suggest that the simple Landau—Teller rate equation, when used with relaxation times measured in shock-wave flows, is not adequate to describe the vibrational relaxation process under the extreme nonequilibrium conditions associated with nozzle-expansion flows.