Measurements of Rotational Temperatures in a Low Density Wind Tunnel

Abstract

The fluorescence of a 30-kV electron beam has been used as a probe to measure the rotational distribution function of flowing nitrogen in a low-density wind tunnel. The fluorescence consists predominantly of the so-called first negative system of nitrogen, a series of bands originating from the B2Σ+ state of N2+. Measurements of the intensities of the rotational structure of the (0–0) vibrational band have been used to calculate, through a theoretical model, the original rotational distribution function of N2 before excitation. This technique was originally proposed by Muntz, who also carried out a series of experiments demonstrating the validity of the theoretical model. A series of measurements was performed in the undisturbed flows of a subsonic nozzle, a Mach 4 nozzle, and free-jet expansions from sonic orifices at Mach numbers ranging up to 19. The rotational temperature measurements are shown to be about 3% high at 280 °K, with a progressively increasing error at lower temperatures. These measurements also indicated no measurable lag in rotational temperature in the free-jet expansions.