Nonequilibrium thermochemical modelling for aeroshell flows

Abstract

The design of planetary reentry vehicles is challenged by large aerodynamic heating, a direct outcome of travelling at extreme hypersonic velocities. It is of particular interest to obtain accurate estimates of the heating load on a blunt body due to radiative and convective heating as a result of hot gas present in a bow shock layer region.This thesis aims to demonstrate that improvements in modelling of this aerodynamic situation can be achieved through a computational fluid dynamics (CFD) framework. Eilmer is one of many CFD tools that can be used to model transient compressible gas flows [1]. Eilmer currently has the infrastructure necessary to model complex chemical and thermal nonequilibrium effects which are important in accurately describing the flow state around a blunt body at hypersonic speeds. However, to date, limited models are implemented that describe these flow regimes. This thesis is one of many small steps towards a larger goal of sophisticated CFD simulations that include coupled thermochemical nonequilibrium effects. This thesis compares simulation results from different gas models that consider chemical and thermal nonequilibrium in isolation. A chemical nonequilibrium model for dissociation based on an ideal gas assumption is compared to a model where the gas is assumed to be thermally perfect. The two models were used to simulate flow of nitrogen over a blunt plate, and it is shown that the thermally perfect reacting gas model provides a better estimate for flow conditions that contain dissociation of nitrogen. These results provide justification for the use of non-ideal gas assumptions in reentry situations containing finite rate chemical reactions. Additionally, this thesis implements a new gas model into Eilmer which considers the effects of thermal nonequilibrium through vibrational kinetics modelling. This model is able to take into account non-Boltzmann behaviour of gas, where the population distributions across quantum vibrational energy levels does not follow a Boltzmann Distribution. The new model is used to simulate a validation case of nitrogen flow past an infinite cylinder. Comparison to a model where the vibrational energy levels follow a Boltzmann Distribution provides insight into the importance and applicability of the vibrational kinetics model. By demonstrating that these gas models can be implemented into CFD programs, and showing that the results obtained match those found in literature, a step has been taken towards more accurate thermochemical nonequilibrium modelling. Looking forward, this implementation provides some basis for improving heating estimates on aeroshells during atmospheric-entry.