Computational and experimental study of nonequilibrium chemistry in hypersonic flows.

摘要

The accurate prediction of hypersonic flows requires models for thermochemical processes which occur in the flowfield. The gas around a hypersonic vehicle will be hot enough to cause chemical reactions and thermal nonequilibrium. This dissertation is concerned with the coupling between the dissociation rate and the vibrational relaxation process. Because of the large flow speeds, the time scales of the dissociation and vibrational relaxation are the same as the fluid motion time scale. Therefore, the gas will be in thermo-chemical nonequilibrium and these processes must be modeled in coupled manner.; There are many proposed models to account for vibration-dissociation coupling effects, but because of a lack of appropriate experimental data, none of the models have been validated. The purpose of this work is to obtain experimental data from the T5 Shock Tunnel at the California Institute of Technology that can be used to evaluate vibration-dissociation coupling models.; The numerical method and basic thermo-chemical models used were validated by reproducing existing T5 data for blunt body geometries with little vibrational nonequilibrium. Computational fluid dynamics was used to design new experiments. Double-wedge and double-cone geometries were identified as producing flowfields that were very sensitive to vibration-dissociation coupling effects. The mechanism responsible for producing this sensitivity was a shock-wave/boundary-layer interaction that is also present in perfect gas flows. This discovery led to a set of perfect gas double-cone experiments which were used to validate the numerical method for high Mach number flows with large separation zones.; Double-wedge and double-cone models were then tested in T5. Mach-Zehnder interferograms were taken along with surface pressure and heat transfer rate measurements. It was found that computations were unable to reproduce the experimental results, even for flowfields insensitive to vibration-dissociation coupling effects. An analysis of the results showed that the reason for the disagreement is uncertainty in both the equilibrium and nonequilibrium dissociation rates. The double-wedge and double-cone flowfields are a much more sensitive test of the temperature dependence of the dissociation rates than blunt body flowfields, and these results indicate that equilibrium dissociation rates for nitrogen are not as well known as has been believed.