Effect of Nonlocal Vacuum Ultraviolet Radiation on a Hypersonic Nonequilibrium Flow

Abstract

During high Mach number reentry into Earth's atmosphere, spacecraft experience hypersonic nonequilibrium flow conditions where molecules are dissociated and atoms are weakly ionized. Since the electronic levels of atomic species are strongly excited under these high Mach number conditions, the radiative contribution to the total heat load can be significant. The modeling of radiative transport is further complicated by radiative transitions occurring during the excitation process of the same radiating gas species. This interaction affects the distribution of electronic state populations and, in turn, the radiative transport. The radiative transition rate in the excitation/deexcitation processes and the radiative transport equation must be coupled to account for nonlocal effects. A quasi-steady-state model is presented to predict the electronic state populations of radiating gas species taking into account nonlocal radiation. The definition of the escape factor that is dependent on the incoming radiative intensity from over all directions is presented. To perform accurate and efficient analyses of radiation from a chemically reacting flowfield, the direct simulation Monte Carlo and the fully three-dimensional photon Monte Carlo radiative transport methods are used. The effect of the escape factor on the distribution of electronic state populations of the atomic N and O radiating species is examined in a highly nonequilibrium flow condition, and the corresponding change of the radiative heat flux due to the nonlocal radiation is also investigated. It is found that inclusion of the escape factor increases the upper electronic state populations by about a factor of 2 with a similar increase in the radiative heat flux to the vehicle surface. Nomenclature Ai; j = Einstein coefficient for spontaneous emission from state i to state j, s 1 Ai; c = Einstein coefficient for photoionization from state i to continuum state, s 1 Bi; j = Einstein coefficient for stimulated emission and absorption, cm 3 m=J-s c = speed of light, 2:9979 10 10 cm s 1 d = distance of a traveling photon, cm D surf = distance from vehicle surface, m e = electron charge, 4:8030 10 10 statcoul E i = electronic term energy for atomic level i, J E emis = total emission energy, W=m 3 G = incoming radiative intensity integrated over all directions, w=cm 2 -m g = degeneracy, dimensionless h = Planck's constant, 6:6262 10 34 Js I = radiative intensity, w=cm 2 -m-sr i, j = index of electronic state, dimensionless K e i; j = excitation rate coefficient of collisional transition from state i to state j by electron impact, cm 3 s 1 Ki; c = excitation rate coefficient of collisional transition from state i to continuum state, cm 3 s 1 K n;1 = freestream Knudsen number, dimensionless k = Boltzmann's constant, 1:3806 10 23 JK 1 L = nose radius of the Stardust blunt body, m l = number of electronic states for bound-bound transition, dimensionless N i = number density of electronic state i, m 3 n = number density, m 3 n a = atom number density, m 3 n e = electron num...