Influence of thermochemical nonequilibrium on expansion tube air test conditions : a numerical study

Abstract

Using a Lagrangian solver, thermochemical nonequilibrium simulations are performed for the entire range of practical operating conditions of expansion tubes to isolate the influence of nonequilibrium and identify key features in large-scale facilities. Particular attention is given not only to the influence of the nonequilibrium unsteady expansion but also to the influences of the nonequilibrium region behind the primary shock and non-ideal secondary diaphragm rupture. The nonequilibrium unsteady expansion is found to be the most influential process in the test flow - it can significantly influence the flow properties and cause significant temporal variations in the properties during the test time. The nonequilibrium unsteady expansion is also found to accelerate the secondary shock and contact surface. The non-ideal secondary diaphragm rupture is found to increase the amount of nonequilibrium in the test flow due to the generation of a reflected shock. The nonequilibrium region behind the primary shock may be considered negligible in most conditions. Regarding the creation of thermochemical equilibrium test conditions, important factors for achieving this include having a high acceleration tube fill pressure, large-scale facility, and high total enthalpy. The combined effects of viscosity and nonequilibrium are postulated, and the results are supported by experimental works that report consistent findings. To provide an idea of the sensitivity of the numerical configuration, simulations of fixed-volume reactors at various de-excitation conditions are performed using different nonequilibrium models.