Spectral analysis of atomic argon radiation in an expanding hypersonic flow

Entering the atmosphere of a planet is one of the key phases of many space missions, being significant for both interplanetary travel and for returning home to Earth, and remains one of the major challenges of interplanetary missions. Convective and radiative heat loads pose a significant threat to equipment and personnel during this process. Thermal protection systems are employed to mitigate the thermal loads, but are expensive in both cost and mass of the vehicle. Knowledge of these thermal heat loads requires accurate understanding of the flow-field that will develop around the vehicle. Experimental measurements are needed due to significant uncertainty in current aerothermodynamic models. Measurements in actual flight tests are difficult, due to the massive cost and the complexity of the flight tests themselves. This makes ground tests, conducted in hypersonic wind tunnel facilities, an essential practical alternative.Aerodynamic data for interplanetary flight can be generated in specialised wind tunnel facilities. Impulse facilities, specifically, have the ability to replicate planetary entry conditions. Due to the hostile nature of hypersonic test flows, sensors on the body of the test model may suffer damage from repeated use. Instrumentation attached to the test model can also influence the flow-field, impacting on the accuracy of measurements. Spectroscopic measurements are a non-intrusive method of extracting properties of the hot gas during testing, by measuring the radiation emitted by the gas, which have the advantage of not interfering with the gas flow during measurement.This thesis reports on the use of spectroscopic techniques to probe a radiating hypersonic flow-field, extracting population densities of excited states, and then determining the thermochemical state of the gas. A simplified two-dimensional wedge geometry was used to generate an oblique shock wave followed by an expansion. The aim of the project was to investigate the potential non-equilibrium flow effects during the expansion, an afterbody effect which remains poorly understood. Argon was chosen for the test gas as it is a monatomic gas with chemistry limited to ionisation and only two thermal modes. Such a simplified gas potentially allows for more direct and quantifiable determination of equilibrium or non-equilibrium of the gas as it expands, serving as a good basis for development of fundamental understanding and model development.Two experimental campaigns were conducted using the X2 expansion tube at The University of Queensland. The first campaign examined a relatively low-density argon flow which was expected to contain thermochemical non-equilibrium regions, due to the low gas density. The second campaign examined a higher density argon flow which was expected to be closer to thermochemical equilibrium. A structured analytical framework was developed to analyse the spectral data, which was coded into the application SELA.The populations of excited electronic energy levels were determined by examining the spectral emission of the gas. An optical thickness coefficient was calculated based on the Einstein coefficients to account for absorption and stimulated emission. The determined excited state populations were corrected for using this optical thickness coefficient. Temperatures and densities were then determined from the excited state populations. Excitation temperature was resolved along a line starting upstream of the model, in the free stream, then passing through the shock, over the model, and through the expansion fan. These temperatures and densities were compared to numerical simulations using Eilmer4, UQ’s in-house Navier-Stokes compressible flow code, and to the NASA state-to-state equilibrium gas solver CEA. Densities determined by considering optical thickness in tandem with measurements of excited energy level populations agreed with Eilmer4 and CEA for the low-density campaign.Significant radiation was detected upstream of the model in the free stream of both campaigns. It was postulated that the hot argon flow above the model was emitting radiation which was exciting the incoming flow. It is likely that this effect was also occurring in the reverse direction and acted on the downstream expansion region of the gas.Larger than expected ionisation was measured and compared to equilibrium calculations from CEA. It was found that the low and high density argon conditions had ionisation percentages of 45% to 80% and 35% to 46% respectively. This was supported by density, continuum, argon line radiance and optical thickness measurements.In both campaigns, significant excitation temperatures were detected before the shock and were higher in the expansion region than the post shock region. This is thought to be due to radiation from the hot gas post-shock, pre-expansion, influencing the excitation temperatures in both regions. It suggests that, for any flow-field with significant radiation, it may not be appropriate to use a single temperature to represent excitation temperature. For simulations without radiation coupling, it may be possible to account for precursor radiation by increasing the electron number density and the electron temperature in the freestream. A modelling strategy is not obvious for the expansion region of the gas, as the level of influence is unknown