Mixing Characteristics of Inclined Fuel Injection via Various Geometries for Upstream-Fuel-Injected Scramjets

Abstract

Efficient fuel/air mixing plays a crucial role in successful operation of hypersonic airbreathing engines, particularly scramjets, where fuel must be injected into high-speed crossflow and mixed with air at an extremely short timescale. This paper presents the results of a numerical study that investigates the effects of various orifice shapes on fuel mixing characteristics into hypersonic airflow at Mach 5, aiming at the application to scramjet operation with upstream fuel injection at Mach 10. The performance of the injectors at an inclination angle of 45 deg are assessed with respect to various criteria such as the mixing efficiency, streamwise circulation, total pressure recovery, fuel penetration, and spread. Streamwise slot injectors have been found to yield higher mixing efficiency than the other injectors tested (namely, square, circular, diamond, and triangular injectors), owing to the buffering effects. Apparent higher total pressure recovery has been obtained with these rectangular injectors, but their advantages have diminished significantly with the alignment of the trailing-edge position. The highest vertical penetration has been achieved by the square injector, whereas the performances of other injectors with a sharp leading edge have been found to be affected considerably by the axis switch phenomenon due to inclined injection at higher injection pressure. A major influence of the fore and aft shapes of the orifice has been observed on the three-dimensional bow shock formation and wake recirculation, respectively. Nomenclature A = cross section at a streamwise station AR = aspect ratio (streamwise/spanwise) A j = injector area on floor, m 2 A n j = injector area normal to jet, m 2 c = mass fraction c H 2 = hydrogen mass fraction c O 2 = oxygen mass fraction c s = stoichiometric mass fraction D = effective injector diameter, m H = height of computational domain, m h f = fuel penetration height J = jet-to-freestream momentum flux ratio L = length of computational domain, m L 0 = distance to upstream end of injector, m _ m = mass flow rate, kg∕s p = static pressure, Pa p 0 = total pressure, Pa p 02 = pitot pressure, Pa U = crossflow velocity, m∕s W = domain width (injector spacing), m w f = lateral fuel spread x = streamwise direction, m y = spanwise direction, m z = vertical direction, m α j = injection angle, deg Γ = streamwise circulation γ= specific heat ratio Δp 0 = total pressure loss, Pa η m = mixing efficiency Φ = fuel/air equivalence ratio χ m = overall mesh refining factor χ m xy = refining factor in x and y directions χ m z = refining factor in z direction ω x = streamwise vorticity, ∕s Subscripts j = jet property STA = stream-thrust-averaged value ∞ = freestream property

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