Investigation of non-equilibrium thermochemistry in expanding flows

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This project falls within the EPSRC: Fluid dynamics and aerodynamicsAt hypersonic velocities, the behaviour of gases manifests non-equilibrium phenomena, representing the most intricate aspect of understanding and predicting hypersonic flows. The major difficulty in modelling these phenomena lies in accounting for the multitude of degrees of freedom involved throughout the flow history, such as the various gas species and their excitation states. Furthermore, the available thermodynamic and chemical data are inadequately characterized, and existing non-equilibrium models remain marked by uncertainties. However, non-equilibrium phenomena hold critical implications for high-speed engineering applications, particularly in the design and safety of atmospheric entry vehicles. In fact, these phenomena significantly affect vehicle aerodynamics, thermal loads, and propulsion-system efficiency. A notable example is the heating experienced by spacecraft upon entry into planetary atmospheres. This heating predominantly arises from the expansion of plasma around the vehicle's outer edge generating radiation from gases in excited states, far from thermodynamic equilibrium.It was generally assumed that afterbody radiation for Earth entry was negligible in comparison to convective heating, as indicated by the minimal readings from Apollo's afterbody radiometers. However, subsequent findings revealed that the radiometers were calibrated for the wrong wavelength [1], casting doubt on the efficacy of the existing two-temperature model for non-equilibrium flows in the wake of vehicles entering Earth's atmosphere. The understanding of non-equilibrium phenomena is even more limited for atmospheres containing carbon species, such as Mars or Venus, where the non-equilibrium wake largely contributes to afterbody heating. Consequently, the analysis of afterbody heating for vehicles designed for such planets often relies on numerical models whose validity remains unverified [2]. Moreover, with planned missions to ice giants in this decade by space agencies like NASA and ESA [3], the pressing need for an enhanced comprehension of non-equilibrium flow in expanding configurations becomes evident.The objective of this project is to investigate non-equilibrium flows to attain a comprehensive understanding of the microscopic state of the gas and encapsulate this knowledge within commonly used two-temperature equation numerical models in engineering computations. Specifically, the expanding flow, generated around the edge of a spacecraft, can be replicated through unsteady expansions in expansion tubes. During the expansion process, a reduction in density and translational temperature effectively decelerates non-equilibrium thermochemical processes, allowing the observation of these phenomena within the short time frame available in short-duration facilities through the utilization of spectroscopy. The project's primary objective is thus to develop a numerical model for steady expanding flows that is capable of representing the internal degrees of freedom to an arbitrary detail. This model will be instrumental in analysing spectroscopic data gathered in expansion tube test campaigns. The innovative numerical model will be rooted in the Navier-Stokes equations for a non-equilibrium mixture of excited and ionized species, incorporating terms to represent streamline divergence at the centreline of a test setup designed to produce steady expansion waves and will be based on the axisymmetric Navier-Stokes solver known as FRamework for Overset Simulation of Shock Tubes (FROSST) and the LAgrangian Shock Tube Analysis (LASTA) code [4].[1] Johnston, Christopher O., and Aaron M. Brandis. "Features of afterbody radiative heating for earth entry." Journal of Spacecraft and Rockets 52.1 (2015): 105-119.[2] Edquist, Karl, et al. "Aerothermodynamic design of the Mars Science Laboratory hea