Hydrogen piloted energy for supersonic combustion of liquid fuels (1995)

minimum piloted energy required to maintain a stable flame, and (c) an evaluation of the liquid fuel utilization efficiency. Ignition and stability of liquid hydrocarbon fuel-flames with a hydrogen pilot in a supersonic air stream at Mach 2.5 were investigated to determine the minimum piloted energy required to maintain stable combustion of the liquid fuel. Preliminary experiments were conducted in a generic combustor configuration employing a rearward-facing step (the step height, H , was 10 mm) as the main flame-holding mechanism. Introduction The need to increase the energyhnit volume of hydrogen fueled vehicles for hypersonic flight has long been recognized'. Liquid hydrocarbons are attractive Toluene was used as a representative liquid fuel and was injected normal to the flow at 5 step heights downstream from the step. The pilot flame is produced by injection of hydrogen from the base of the step parallel to the main flow. The piloted energy added by the hydrogen flow is parametrically modified to produce the local thermodynamic conditions that assure ignition and stable combustion of the liquid fuel. The supersonic-combustion facility includes, as major components, a hydrogen based vitiated heater and a two-dimensional test section with an entrance of 2.54 cm x 2.54 cm. The experiments were performed at stagnation temperatures of 300 and 1000 K; the pilot flame was limited to an equivalence ratio of 0.05 which sustained stable combustion of toluene to equivalence ratios to 0.13. The study addressed the following issues: (a) determination of the piloted energy required to initiate the liquid-fuel combustion, (b) determination of the LJ candidates for the low range of the hypersonic flight regime due to their higher volumetric energy content and the relative simplicity of .operational logistics. Although the enerpyhass density of liquid hydrocarbons is lower than that of hydrogen, some of the mass increase would be recovered by a smaller and lighter structure of the vehicle. Liquid hydrocarbon fuels require substantial residence time to achieve vaporization and complete exothermic reactions and such time is unlikely to be available in reasonable sized supersonic combustors. A substantial reduction of these times are achievable if the fuel, consequent to cooling components of the airframe and engine, is injected into the combustion region at temperatures sufficiently high to ensure flash vaporization (instantaneous vaporization at the injection site pressure). Even then, the chemical kinetics of hydrocarbons are slow in comparison with hydrogen, and for many realistic operational conditions 'Graduate Research Student, Member AIAA. Graduate Research Student, currently Propulsion Systems Engineer, Pratt&Whitney, Member AIAA. 'Assistant Professor, Member AIAA. Copyright@ 1995 by University of Florida. Published by the American Institute of Aeronautics and Astronautics, Inc. with permision. d 1 exothermic reactions can not be achieved within the available residence time (the case of large Damkohler, Da, numbers). Use of a pilot flame with fast kinetics (e.g., gaseous hydrogen), can provide locally at the liquid injection location the conditions necessary to accelerate the hydrocarbon reactions rates and reduce the Damkohler number, Le., high temperature and low fluid velocities. For such a system it is necessary to verify a) the level of piloted energy required to ignite and maintain stable combustion of the liquid fuel and, b) the interaction between the pilot flame and the heat sink represented by the injected hydrocarbon. Previous studies of hydrocarbon combustion in supersonic flows included both gaseous' and liquid fuels4,'. Several methods were used to ignite and sustain the liquid hydrocarbons (JP-7 or kerosene) combustion: in a dual ramjet-scramjet mode (see ref.4) a small part of the air was decelerated to subsonic speed to generate a pilot flame. The liquid fuel was injected both upstream (primary) and downstream of a pilot flame (secondary). .The primary fuel flow, being injected before the pilot combustion region, had the time to undergo a certain degree of mixing and heating prior to ignition. Overall test equivalence raiios were up to 0.07 with very high combustion efficiencies (to 100%) when the liquid, JP-7, was heated prior to injection. Injection upstream of the pilot re,' Oion was also employed in reference 5 which tested a broad range of geometrical configurations. More stable flames were obtained when the mixing length was increased and combined with several flameholder configurations including struts or/and rearward facing steps. With these configurations and at 1500 K air stagnation temperature hydrogen pilot equivalence ratios of 0.1 were able to maintain kerosene combustion to equivalence ratios of 0.6. The present study was conducted with injection of the liquid in the hydl-ogenbased pilot flame, downstream of the step, to determine the minimum hydrogen-pilot equivalence ratio, &, required to maintain a stable flame of the liquid hydrocarbon. Previous estimates6 showed that a Mach 5 cruise vehicle can be effectively cooled using an endothermic fuel, for example, Methycyclohexane (MCH) and, thus, this fuel is a potential candidate for the-low range of hypersonic regime. The study in ref. 6 indicates that the fuel, at the engine injection station, will consist of a mixture of catalytically broken MCH into it's constituents, Le., toluene (C6H,CH,) and hydrogen. Toluene was selected as the liquid fuel for the present study. The liquid fuel was injected into a Mach 2.5 flow at room temperature being vaporized by absorbing heat from the surroundings. The injection pressures were limited by the maximum flowrate allowed before the liquid quenched the pilot flame. Liquid fuel heating is desirable (see discussion in ref. 4) and will be used in a following study. LJ