Thrust performances of scramjet engines were compared with theoretical values to quantify the progress in engine performances from Mach (termed as "M") 4 to M8 flight conditions. An engine with a ramp produced a net thrust of 215 N under the M8 tests and a comparison of a theoretical thrust yielded a thrust achievement factor of 51%. By excluding boundary layer, an engine with a thick strut delivered a net thrust of 560 N and showed a thrust factor of 92% and a net thrust factor of 45%. The thrusts were limited by flow separation caused by engine combustion (termed as "engine unstart"). The starting characteristics was improved by boundary layer controls in M6 and M4 conditions. An engine with a thin strut doubled the thrust from 1620 N to 2460 N by the boundary layer bleeding in the M6 tests. The improved thrust factor was 60% at the stoichiometric H 2 condition. Under M4 tests, the net thrust was tripled by the bleed and a two-staged injection of H 2. As results, the thrust factor was raised from 53% to 70%, the net thrust factor was increased from 32% to 55%. Studies required for improving the net performance was addressed. Nomenclature A 1 Inlet area 0.2 m-wide 0.25 m-high) cf wall friction coefficient Cint Internal drag coefficient of engines Dint Internal drag of engines = Cint q 1 A 1) Df Internal friction drag on engines Df0 Minimum friction drag on the internal surface of a rectangular duct Dp Internal pressure drag of engines d1 Displacement thickness of boundary layer Isp Fuel specific impulse (km/s) M Mach number q 1 Freestream dynamic pressure at inlet (kPa) hp Total pressure recovery factor hc Air capture ratio in inlets DF Thrust increment by combustion DFnet Net thrust by combustion (= DF-Dint) hdrag Drag achievement factor (= Df0/Dint) hDF Thrust achievement factor (= DFexp /DF0) hnet Net thrust achievement factor e Geometrical contraction ratio of inlets F H 2 equivalence ratio subscripts exp experimental values 0 baseline values
Airframe-integrated scramjet engines swallow the boundary layers that develop on the forebody of space planes. The scramjet engine easily falls into engine stall (engine unstart) as a result of the boundary-layer separation resulting from combustion. In this study, to investigate the unstart characteristics, numerical simulations of a whole scramjet engine with boundary-layer bleeding are performed by using a reacting flow code, and the physics determining the engine performance is examined. Our computations well reproduce the engine combustion tests results with bleeding. Bleeding of 0.65% in a captured airflow suppresses the separation of the ingested boundary layer and extends the start limit from the fuel equivalence ratio of 0.5 to 1.0. The numerical results predict small discrete circular flames anchored around individual fuel jets near the injectors. These discrete flames merge to form a large envelope diffusion flame in the downstream portion of the combustor, as a result of the secondary flow produced by high pressure of the cowl shock and intensive combustion. This merged structure causes a large mass of unburned fuel and restricts the combustion efficiency and the thrust performance. Nomenclature D add = additive drag in inlet D int = engine internal drag H = engine height Hc = captured height M = Mach number P = static pressure Pw = wall pressure q = dynamic pressure qw = heat flux T = static temperature T w = wall temperature x = streamwise direction Y i = mass fraction y = height-wise direction z = spanwise direction F = thrust increment with combustion t = time step η c = air capture ratio τ w = wall share stress = bulk fuel equivalence ratio Subscript 1 = incoming flow
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