Numerical Investigation of a 3-D Chemically Reacting Scramjet Engine at High Altitudes Using JP8 - Air Mixtures

A generic dual-plane compression configuration is employed to obtain insight into the flowpath dynamics of a Mach 8 scramjet utilizing a hydrocarbon fuel at 50K feet altitude. The geometry considered includes dual-plane compression and is designed to highlight various phenomena of interest. A nonaxisymmetric configuration modified from a prior effortPP is adopted as a baseline. Particular emphasis is placed on examining three-dimensional shock-boundary layer and shock-shock interactions and their impact on the combustor downstream. In this region, a reacting stoichiometric fuel-air mixture is employed to characterize the kinematic and dynamic structures resulting from interactions between shocks and chemical reactions. The numerical procedure solves the full 3-D Navier-Stokes equations supplemented with a two-step chemical mechanism of 13 species and 20 reactions. Liquid jet fuel is introduced into the duct upstream of the conductor through rounded injectors positioned perpendicularly into the air flow. Three different combustor configurations are studied, each incorporating a commonly used single cavity combustor (SCC, also referred to here as a trap vortex combustor or TVC) system to provide flame holding and to augment combustor efficiency. PP The first two designs include 1) one injector on the top wall with a 4 degree expansion ramp on the floor and 2) one injector on the top and another on the bottom wall, which is not ramped. These calculations are performed using an LES turbulence-chemistry model to capture the breakup of fuel droplets, unsteadiness of the spray injectors and burning process,. A hydrocarbon liquid fuel (CB12 BH B23B: JP8) is considered with specified initial conditions, including spray droplets of assumed size, distribution, temperature, injection location, velocity magnitude and directions. Based on these studies, a third design consisting of a top row of 10 injectors and a bottom row of 9 injectors with a constant area is applied to the full scramjet engine and directly coupled with a two equation k-ω turbulence model. Details of the vortex residence times, entrainment of the cavity flow into the free stream, and pumping effects for inboard and outboard cavities in the presence of swirl are described. Finally, the steady state solutions are analyzed to extract insight into the various processes that determine performance, and comparisons are made with respect to phenomenological models. 43rd AIAA Aerospace Sciences Meeting and Exhibit 10 13 January 2005, Reno, Nevada AIAA 2005-1435 This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. Introduction and Background Efforts to develop sustained hypersonic flight capability and associated technologies have been pursued since the late 1950s. During this time, substantial advancements have been made in lightweight, high temperature structural materials and thermal protection systems; rocket, turbine, ramjet and scramjet propulsion; computational fluid dynamics and structural analysis; and multidisciplinary optimization. Taken together, these technologies are ever closer to helping realize practical, long-range hypersonic transport and routine, affordable space access. The advantages of supersonic combustion devices for high-speed propulsion are well known and a number of recent theoretical, experimental and numerical efforts have proposed and examined various designs. Despite advances in scramjet analysis, many uncertainties persist in predicting and understanding three-dimensional, supersonic chemically reacting flows, and, significant challenges need to be overcome. Challenges include mitigation of high heat loads and improvement of observed low combustion efficiency stemming from problems in accomplishing mixing at high-speeds. Simulation methods can complement difficult to perform experiments and thus play a major role in developing a comprehensive understanding of the key phenomena that dominate performance. This paper describes a numerical effort to explore the fluid dynamics of a scramjet internal flowpath with two main goals: 1) Understand the three-dimensional interplay between viscous/inviscid interactions including shock -boundary layer and shock-shock interactions in the inlet at 50K feet altitude and 2) Investigate the effects due to combustion of a reacting stoichiometric fuel-air mixture with emphasis on the kinematics and dynamics of complex structures that result from interactions between shocks, chemical reactions and the expansion in the nozzle which ultimately yields thrust. Intrusive fuel injectors and flame-holders are difficult to maintain inside the extremely harsh environment of a scramjet combustor and often yield very complex features with severe internal drag penalties. In addition, external ignition aids are commonly employed at low flight Mach numbers (i.e., around M = 4) and may require scramjet-based systems to carry potentially heavy generators. These devices reduce available payload for fuel at the low speed takeover point. These techniques include flushwall fuel injection, wall-mounted flame holding, and devices to enhance the atomization and vaporization characteristics of liquid fuels. Significant attention has been paid to analyzing combustor performance using a wide array of conventional and advanced diagnostic techniques. Plasma igniters also hold promise for future supersonic combustion applications. With their ability to produce combustion-enhancing radicals at relatively low power and their robust performance, they are ideal candidates to overcome many of the obstacles faced in scramjet combustor design. The present paper considers a fuel injector and flame-holder concept that incorporates flush wall fuel injection upstream of a wall cavity. Combustor inlet flow properties simulate flight conditions between Mach 4 and 5 at dynamic pressures of 1000 psf. In the last few years, wall cavities have gained the attention of the scramjet community as a promising flame-holding device, owing to results obtained in flight tests and to feasibility demonstrations in laboratory scale supersonic combustors. However, comprehensive studies are needed to determine the optimal configuration which will yield the most effective performance. Numerous geometries have been tested to improve the mixing characteristics of fuel injection ports during the NASP program and in recent years. One interesting concept initiates detonation with supersonic jets recessed in a small cavity. By focusing these towards the center of the cavity, a region of high energy density can be produced to cause onset of detonation. The enhanced pressures and temperatures within the volume of the cavity result in fast turbulent mixing. Figure 1 depicts the scramjet configuration considered in the present study. The design is comprised of a dual-plane compression inlet system followed by a constant-area combustor and a short 15 expansion nozzle. Since this study is focused principally on the inlet and combustor, the nozzle is truncated. The simplified design is well-suited to explore the generic three-dimensional features likely to be encountered in a scramjet but isolated from the overwhelming geometrical complexity of an actual device.