Reaction and Mixing-Controlled Combustion in Scramjet Engines

Abstract

H 2 -fueled scramjet engines were tested under Mach 4 (M4) to Mach 8 (M8) ight conditions, and the local equivalence ratio and combustion ef ciency were measured by gas sampling at the engine exit. Correlation between the local values of equivalence ratio and combustion ef ciency showed that the M4 combustion was principally reaction controlled and the reaction and the mixing-controlled combustion coexisted in the M6 condition. The M8 combustion in the engines was rate controlled by the mixing of H 2 . Comparison of engine performance in the air, supplied by a combustion heater and a storage heater, indicated that the performance strongly depended on the air-heating methods. The dependence of engine performance on air-heating methods could be explained by the nding that the M6 engine combustion was partially reaction controlled. The wall-heating rate and pressure distribution in the M6 tests also supported the shift from the partially reaction-controlled to the mixing-controlled combustion as the fuel rate was increased. The mixing-controlled combustion suggested weak facility dependence in the M8 condition. Because reaction in the M8 condition is not suf ciently fast, the main combustion region might be blown downstream in the engine. NomenclatureH = height of the engine (250 mm) k 1 = reaction rate of the reaction H C O 2 ! OH C H M = Mach number n = pressure exponent in Eq. (3) (order of overall reactions) P b = typical burning (static) pressure weight-averaged with n D 1:6 P i = typical ignition (static) pressure weight-averaged with n D 1 P w = wall pressure S = heated by a storage air heater T r = recovery temperature in engines t b = burning time of H 2 in engines (assumed to be t 95 ) t ow = residence time in the burning length, x e t ig = ignition time de ned as 10 £ t 1 t s = typical gas sampling time (1.5 s) t 1 = characteristic time de ned as (k 1 [O 2 ]) ¡1 t 95 = reaction time to raise the temperature from 5 to 95% of the equilibrium temperature rise U = typical ow velocity in engines V = heated by a vitiation air heater x b = characteristic length required to burn fuel x e = burning length in engines (1.3 m) x ig = characteristic length required to ignite fuel 8 = bulk fuel equivalence ratio supplied to engines Á = local fuel equivalence ratio (time-averaged, if not speci ed) c = local combustion ef ciency (time-averaged, if not speci ed) -= time-or space-averaged values

Mechanism and Control of Combustion-Mode Transition in a Scramjet Engine

“…The flame index and the OH radical distributions in our simulations indicates that a reaction rate controlled combustion in the weak mode, and a mixing rate controlled combustion in the intensive mode. Our simulations qualitatively reproduce the gas sampling measurement at the exit plane [4]. Thus, our simulation captured not only the mode transition point, but also the flame structures at each combustion mode.…”

“…Mitani et al [4] conducted the gas sampling measurement at the exit of the engine for each of the modes. The gas sampling data showed that a reaction rate controlled combustion in the weak mode, and a mixing rate of fuel controlled combustion in the intensive mode.…”

“…Although Thomas and Guy [2] reported a sudden increase in thrust under M6 flight conditions, they did not mention its cause in open literature. Our group has experimentally investigated the transition mechanisms [3,4]. These mechanisms, however, have not yet been fully clarified.…”

“…One was based on the formation of the mergedrecirculation region when the fuel equivalence ratio (i.e., fuel injection pressure) increased [3]. The other focused on the upstream propagation of the combustion region in the weak mode [4].…”

Supersonic Combustion Using a Stinger-Shaped Fuel Injector

“…We sampled gas at the exit of the combustor using a water-cooled sampling probe to evaluate the local equivalence ratio and combustion efficiency. The probe ensured reaction quenching during sampling [28]. There were 63 sampling points with an interval of 10 mm (9 points in the height direction x 7 in the width direction).…”