Rotational Nonequilibrium in Low Density Heated Free Jet Expansions of Nitrogen.

Rotational temperatures have been measured in rarefied, nonequilibrium heated free jet expansions of nitrogen using the electron beam fluorescence technique at the University of California at Berkeley Low Density Wind Tunnel facility. Spectroscopic measurements of the (0,0) band of the first negative system of nitrogen reveal the nonequilibrium behavior in the flow field upstream of, and through the Mach disk, as well as farther downstream. Values of P_0D vary between 270 to 494 Torr-mm (N_0D values of 2.6 to 6.6 times 10^{17 } cm^{-2}) at the exit of the thin plate orifices of diameters 0.8125 and 0.9652 mm. Stagnation temperatures are between 700 and 1000 K. Results compare well with previous free jet expansion data and computations regarding location of the Mach disk and terminal rotational temperature in the expansion. Measurements are also presented for shock thickness based on the rotational temperature changes in the flow. Thickening shock layers, departures of rotational temperature from equilibrium in the expansion region, and downstream rotational temperature recovery much below that of an isentropic normal shock provide indications of nonequilibrium flow behavior. The data obtained are used to infer rotational-relaxation collision numbers from 3 to 9 for the various flow conditions. Collision numbers are seen as not fully descriptive of the rarefied free jet flows due to the high degree of nonequilibrium coupled with the large temperature changes occurring during the expansions and recompressions. Data from this investigation can be used synergistically with direct simulation Monte Carlo (DSMC) code computations to further the understanding of molecular momentum and energy exchanges between internal degrees of freedom in rarefied gas dynamic flows. Such validated computer modelling tools can then be used to help design the next generation of space transportation systems.