The flow physics of inlet-fueled, low-compression scramjets

Abstract

In this thesis, the flow physics in low-compression scramjets, in particular inlet-fueled radical farming scramjets, are investigated using high fidelity numerical simulations. The two-dimensional inlet-fueled radical farming scramjet engine employed by McGuire provides the basis for this thesis. It is used to numerically investigate the flow physics that govern the mixing, ignition and combustion processes. The focus lies in particular on the effect that unsteady flow features and turbulent structures have on the scramjet performance. Therefore, a high fidelity numerical method, wall-modeled large-eddy simulation WMLES, is employed to resolve those effects. For comparison, the Reynolds-averaged Navier-Stokes RANS approach, which is commonly used for large scale scramjet simulations, is employed as well. Furthermore, the scramjet simulations performed here include finite rate chemistry, using the modified JetSurf 2.0 model, and thermal non-equilibrium effects, using the two-temperature model. This thesis represents a contribution to the knowledge of scramjet processes with particular new understanding of: the flow physics governing the enhanced mixing process in inlet-fueled scramjets; the combustion regimes present in radical farming scramjets; the effects of thermal non-equilibrium on scramjet flow structures and combustion performance. This thesis presents a detailed investigation of governing flow physics relevant to inlet injection. Experimental campaigns have shown that inlet-injection significantly improves the scramjet performance, a result of its mixing enhancing capabilities. In the past, researchers argued that increased mixing lengths on the inlet cause the mixing enhancement. More recently, it was shown numerically that fuel plume/flow structure interactions at the entrance of the combustor are responsible for the changing mixing process. However, no study has been conducted that explores the details of those interactions and their sensitivity to other scramjet related flow features. Therefore, a detailed investigation is performed that analyses the governing flow physical processes that are responsible for mixing enhancement. It will be shown that for maximized mixing enhancement, flow structures within the engine have to interact with the inlet injected fuel plume in such a way that the plume splits in half as it is compressed towards the combustor wall, thus increasing the effective mixing area and with it the rate of mixing. Utilizing such an approach, it is shown that the fuel-air mixing rate can be increased by a factor of up to five as the flow passes through the structures at the combustor entrance. Furthermore, the ignition and combustion process within the scramjet engine is investigated in detail. It was found that radicals produced around the fuel jets aid the ignition process. The gas mixture ignites initially at the combustor entrance near the sidewall. Further downstream, with the second shock impingement at the lower combustor wall the gas mixture ignites around the unburned fuel plumes near the scramjet center. As the combustion process proceeds, it becomes mixing limited towards the back of the combustor, where mixing and combustion efficiencies for this specific configuration reach values of 71\% and 61\%, respectively, at the combustor exit. Identifying combustion regimes present in radical farming engines provides valuable information to the scientific community as it will aid the development of turbulence-chemistry interaction models, which are necessary to accurately model combustion processes in turbulent flows. It will be shown that a wide spread of combustion regimes is relevant for this type of engine. In fact, it will be shown that future turbulence-chemistry interaction models should have the capability to accurately represent partially-premixed and non-premixed gas mixtures, whose combustion regimes range from distributed reaction zones to thin reaction sheets. The effect of thermal non-equilibrium on flow structures and the combustion process is analyzed as well, which is of particular interest for shock tunnel testing as the nozzle outflow is in a state of thermal non-equilibrium. It will be shown that the thermal state of the scramjet inflow has only a weak influence on shock structures in the engine, while thermal non-equilibrium modeling within the scramjet has a larger effect. Temperature distributions in hot regions that develop near the scramjet wall are, however, strongly influenced by thermal non-equilibrium effects, in particular for the un-fueled engine. For hydrogen fueled scramjets thermal non-equilibrium effects become negligible downstream of the combustor entrance, as the relaxation process between air and hydrogen is drastically enhanced compared to un-fueled simulations. Nevertheless, thermal non-equilibrium affects the radical production around the jet plumes on the inlet, which influences the ignition process further downstream. The insights provided by this thesis increases the understanding of flow physics in radical farming engines significantly, which allows us to improve future scramjet designs. Furthermore, the relevance of certain flow phenomena is now better understood, thus providing more information to assess the numerical modeling of such phenomena.