The absolute radiation measurements obtained in the electric arc-driven shock-tube facility at NASA Ames Research Center were analyzed to test the collisional-radiative model developed at Ecole Centrale Paris. Two conditions representative of Earth reentry at 10.54 and 11.17  km/s were investigated in the vacuum ultraviolet and infrared spectral ranges. For each of the conditions, the corresponding charge-coupled device images were analyzed. The electron number density was inferred from Stark-broadened nitrogen and Hα lines. Comparisons with the predicted electron number density profiles enabled us to validate the ionization rate constant model implemented in the flowfield solver and to accurately locate the shock front. For both freestream conditions and all the spectral ranges, the predictions of the initial intensity rises were improved when the total spatial smearing (due to the shock motion, the optics, and the camera) was taken into account. The nonequilibrium intensities observed in the vacuum ultr...

Prediction of Nonequilibrium Air Plasma Radiation Behind a Shock Wave

Detailed spectrally and spatially resolved radiance has been measured in the Electric Arc Shock Tube (EAST) facility for conditions relevant to high speed entry into a variety of atmospheres, including Earth, Venus, Titan, Mars and the Outer Planets. The tests that measured radiation relevant for Earth re-entry are the focus of this work and are taken from campaigns 47, 50, 52 and 57. These tests covered conditions from 8 km/s to 15.5 km/s at initial pressures ranging from 0.05 Torr to 1 Torr, of which shots at 0.1 and 0.2 Torr are analyzed in this paper. These conditions cover a range of points of interest for potential fight missions, including return from Low Earth Orbit, the Moon and Mars. The large volume of testing available from EAST is useful for statistical analysis of radiation data, but is problematic for identifying representative experiments for performing detailed analysis. Therefore, the intent of this paper is to select a subset of benchmark test data that can be considered for further detailed study. These benchmark shots are intended to provide more accessible data sets for future code validation studies and facility-to-facility comparisons. The shots that have been selected as benchmark data are the ones in closest agreement to a line of best fit through all of the EAST results, whilst also showing the best experimental characteristics, such as test time and convergence to equilibrium. The EAST data are presented in different formats for analysis. These data include the spectral radiance at equilibrium, the spatial dependence of radiance over defined wavelength ranges and the mean non-equilibrium spectral radiance (so-called 'spectral non-equilibrium metric'). All the information needed to simulate each experimental trace, including free-stream conditions, shock time of arrival (i.e. x-t) relation, and the spectral and spatial resolution functions, are provided.

Benchmark Shock Tube Experiments for Radiative Heating Relevant to Earth Re-Entry

A vibration-specific approach is employed to describe vibrational relaxation and reactive processes in shock-heated air. Models are implemented that include results from recent theoretical calculat...

State-Specific Modeling of Vibrational Relaxation and Nitric Oxide Formation in Shock-Heated Air

The formation of electronically excited atomic oxygen was studied behind reflected shock waves using cavity-enhanced absorption spectroscopy. Mixtures of 1% O2/Ar were shock-heated to 5400−7500 K and two distributed-feedback diode lasers near 777.2 and 844.6 nm were used to measure time-resolved populations of atomic oxygen’s 5S° and 3S° electronic states, respectively. Measurements were compared with simulated population time-histories obtained using two different kinetic models that accounted for thermal nonequilibrium effects: (1) a multi-temperature model and (2) a reduced collisional-radiative model. The former assumed a Boltzmann distribution of electronic energy whereas the latter allowed for non-Boltzmann populations by treating the probed electronic states as pseudo-species and accounting for dominant electronic excitation/de-excitation processes. The effects of heavy-particle collisions were investigated and found to play a major role in the kinetics of O-atom electronic excitation at the condit...

Kinetics of Excited Oxygen Formation in Shock-Heated O2-Ar Mixtures.

In the present study, three different electronic state-to-state methods are proposed to analyze nonequilibrium air plasma flows behind a strong shock wave. In the first approach representing the conventional method, a two-temperature model combined with the electronic quasi-steady-state assumption is adopted. In the second and the third methods, atomic and molecular electronic master equations are coupled with a conservation equation to describe the electronic state-to-state kinetics. State-of-the-art electronic transition rates for atmospheric gas species are compiled with comparisons of existing data. A prediction of the measured nonequilibrium radiation is made for the flow conditions of recent electric-arc shock tube experiments. In a comparison with the measured spectrum, the present electronic master equation coupling methods are more accurate than the conventional approach when used to estimate the initial rising rate and peak value of the diatomic intensity and small amounts of atomic radiation when the diatomic nonequilibrium condition is dominant. Moreover, the spatial distributions of the intensity and electron number density are more accurately predicted by the present methods when the flow fields are dominated by atomic nonequilibrium.

Electronic-state-resolved analysis of high-enthalpy air plasma flows.

This paper reviews the electron- and heavy-particle impact processes governing the population/depletion of the states of N, O, N2, and N2+, which can be significantly affected by nonequilibrium conditions while strongly radiating in lunar-return shock layers The collisional-radiative model developed in this work includes electron-impact excitation, ionization, heavy-particle impact excitation, vibration–electron, vibration–translation, dissociation and bound–bound radiative mechanisms The available experimental and theoretical reaction rates governing these processes were critically reviewed to produce a reliable set of rate constants The collisional-radiative model was then applied to typical nonequilibrium thermodynamic conditions encountered during lunar-return Earth reentry The effect of heavy-particle impact processes is discussed

E. Raynaud

Papers

Prediction of Nonequilibrium Air Plasma Radiation Behind a Shock Wave

Air Collisional-Radiative Modeling with Heavy-Particle Impact Excitation Processes