Analysis of Rotational Non-equilibrium in Rarefied Gas Flows by REMPI

The rotational nonequilibrium along the centerline of supersonic free molecular flows of nitrogen, is analyzed, measuring the rotational energy distribution by REMPI. As a result, the rotational temperatures obtained from the Boltzmann plot using small rotational quantum numbers coincide with the Marrone's data obtained by electron beam flourescence technique. However, a straight line cannot fit all data points, especially at large rotational quautum numbers. This means that the rotational nonequilibrium occurs in the rotational energy distribution, that is, the rotational temperature cannot be defined. The transition of the rotational nonequilibrium phenomena is also discussed by showing the dependence of the rotational energy distribution on the distance from the nozzle exit in the flows.