Toshinori Kouchi

Large-Eddy / Reynolds-Averaged Navier-Stokes Simulations of a Dual-Mode Scramjet Combustor

Numerical simulations of reacting and non-reacting flows within a scramjet combustor configuration experimentally mapped at the University of Virginia s Scramjet Combustion Facility (operating with Configuration A ) are described in this paper. Reynolds-Averaged Navier-Stokes (RANS) and hybrid Large Eddy Simulation / Reynolds-Averaged Navier-Stokes (LES / RANS) methods are utilized, with the intent of comparing essentially blind predictions with results from non-intrusive flow-field measurement methods including coherent anti-Stokes Raman spectroscopy (CARS), hydroxyl radical planar laser-induced fluorescence (OH-PLIF), stereoscopic particle image velocimetry (SPIV), wavelength modulation spectroscopy (WMS), and focusing Schlieren. NC States REACTMB solver was used both for RANS and LES / RANS, along with a 9-species, 19- reaction H2-air kinetics mechanism by Jachimowski. Inviscid fluxes were evaluated using Edwards LDFSS flux-splitting scheme, and the Menter BSL turbulence model was utilized in both full-domain RANS simulations and as the unsteady RANS portion of the LES / RANS closure. Simulations were executed and compared with experiment at two equivalence ratios, PHI = 0.17 and PHI = 0.34. Results show that the PHI = 0.17 flame is hotter near the injector while the PHI = 0.34 flame is displaced further downstream in the combustor, though it is still anchored to the injector. Reactant mixing was predicted to be much better at the lower equivalence ratio. The LES / RANS model appears to predict lower overall heat release compared to RANS (at least for PHI = 0.17), and its capability to capture the direct effects of larger turbulent eddies leads to much better predictions of reactant mixing and combustion in the flame stabilization region downstream of the fuel injector. Numerical results from the LES/RANS model also show very good agreement with OH-PLIF and SPIV measurements. An un-damped long-wave oscillation of the pre-combustion shock train, which caused convergence problems in some RANS simulations, was also captured in LES / RANS simulations, which were able to accommodate its effects accurately.

Collaborative Experimental and Computational Study of a Dual-Mode Scramjet Combustor

Advanced computational models of hypersonic air-breathing combustion processes are being developed to better understand and predict the complex flows within a dual-mode scramjet combustor. However, the accuracy of these models can only be quantified through comparison to experimental databases. Moreover, the quality of computational results is dependent on accurate and detailed knowledge of the combustor inflow and boundary conditions. Toward these ends, this paper describes results from a collaboration of experimental and computational investigators. Detailed computational fluid dynamics and finite element analyses were performed throughout the design and implementation of experiments involving a direct-connect scramjet combustor operating at steady state during long duration testing. The test section hardware was designed to provide substantial access for optical laser diagnostics. Measurement locations included the inflow plane and several locations downstream of fuel injection. A suite of advanced in-...

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

A sidewall compression scramjet engine operated in two combustion modes underMach 6 flight condition, weakand intensive-combustionmodes.Theweakmode occurredbelow the overall fuel equivalence ratio ( ) of around0.4. Transition from the weak mode to the intensive mode occurred at 0:4, accompanied by a sudden increase in thrust. Mechanisms of the transition were numerically investigated in this study. Simulations captured the sudden increase in thrust at the mode transition. In the weak mode, combustion occurred in only a region near the topwall where an igniter was installed. The combustion region expanded toward the cowl with boundary-layer separation at the mode transition. Simulations demonstrated that low ignition capability resulted in the weak mode. This study demonstrated that the presence of additional igniters on the sidewalls improved the ignition capability and achieved the intensive mode in the entire range.

Large-Eddy Simulation of Jet in Supersonic Crossflow with Different Injectant Species

Nomenclature C = molar concentration, mol=m cp;k = specific heat at constant pressure for species k, J= kg K D = injector diameter, m Dk = diffusion coefficient for species k, m =s E = total energy per unit mass, J=kg f = frequency, 1=s H = total enthalpy per unit mass, J=kg h = enthalpy per unit mass, J=kg hk = enthalpy per unit mass for species k, J=kg H j = subgrid-scale total enthalpy flux vector, J= m s h = subgrid-scale species mass fraction-enthalpy correlation, J=kg J = jet-to-crossflow momentum flux ratio k = turbulent kinetic energy, m=s k = subgrid-scale kinetic energy, m=s M = Mach number m = mass flow rate, kg=s Mw = molecular weight, kg=mol p = static pressure, Pa Prt = turbulent Prandtl number pt = total pressure, Pa qj = heat flux vector, J= m s q j = subgrid-scale energy diffusion due to species diffusion, J= m s R = mixture’s gas constant, J= kg K Rk = gas constant for species k, J= kg K ReD = Reynolds number based on D rs = spatial correlation coefficient rts = time–space correlation coefficient Sij = rate of strain tensor, 1=s Sct = turbulent Schmidt number T = static temperature, K t = time, s Tt = total temperature, K T = subgrid-scale mixture gas constant-temperature correlation, J=kg T = reference temperature, K U = velocity magnitude, m=s u, v, w = velocity components in x, y, and z directions, m=s Uc = convection velocity, m=s ui = velocity component in xi direction, m=s Vj;k = species diffusion velocity vector for species k, m=s x, y, z = streamwise, transverse, and spanwise direction distances in Cartesian coordinates, m xi = Cartesian coordinates, m Yk = mass fraction for species k Y j;k = subgrid-scale species diffusion vector, kg= m s = specific heat ratio hf;k = standard heat of formation at T , J=kg in = mean boundary-layer thickness at inlet, m m = mixing efficiency sgs j;k = subgrid-scale species mass fraction-diffusion velocity correlation, kg= m s = mixture’s thermal conductivity, J= m K s = mixture’s molecular viscosity, kg= m s t = subgrid-scale eddy viscosity, m =s = density, kg=m ij = viscous stress tensor, Pa sgs j = subgrid-scale viscous work, J= m s sgs ij = subgrid-scale stress tensor, Pa ’ = local equivalence ratio

Characteristics of Hydrogen Jets in Supersonic Crossflow: Large-Eddy Simulation Study

The characteristics of hydrogen jets transversely injected into a supersonic crossflow under four different injection and crossflow conditions were investigated by large-eddy simulation. The effects of the jet-to-crossflow momentum flux ratio and crossflow velocity were studied. The jet trajectory in the averaged field was controlled by the value of the square root of the jet-to-crossflow momentum flux ratio irrespective of the crossflow conditions. When the crossflow conditions were fixed many jet characteristics were similar in the space normalized using the square root of jet-to-crossflow momentum flux ratio. On the other hand, the crossflow conditions had a strong impact on the jet characteristics. Although the turbulent intensity around the jet was not affected to a great extent, the shape and convection velocity of the large-scale structures appearing on the windward side of the jet plume depended on the crossflow conditions. With a higher crossflow velocity the convection velocity was higher, the j...

Penetration Characteristics of Pulsed Injection into Supersonic Crossflow

This report experimentally and numerically investigated the penetration characteristics of the pulsed injection into a supersonic crossflow. In the experiment, we injected helium gas with the pulse rise time of 250 μs and the pulse width of 750 μs into Mach 2 supersonic crossflow-air. One hundred time-series schlieren images captured the pulsed jet motion and investigated its penetration performance. The images revealed the pulsed jet penetration in a rising phase of injection pressure was higher than that in a declining phase of injection pressure at a certain value of injection pressure. Two-dimensional unsteady RANS simulations injecting air well captured the trends on the jet penetration observed in the experiment. They revealed the hysteresis of pulsed jet penetration was owing to the upward flow produced by the pulsed jet like a slug which inclined toward the injection wall. A counter-rotating vortex pair was generated in front of the slug and induced the upward flow. The upward flow deformed the jet shapes as it traveled downstream. As the result, the hysteresis of pulsed jet penetration occurred in a low regime of pulsation frequency below 5 kHz. The size and strength of vortex pair in the pulsed jet were greatly depended on the pulsation frequency. The vortex rings was generated in each pulse cycle with the pulsation frequency above 10 kHz. They remarkably increased the jet penetration and promoted mixing of the injectant with crossflow-air, compared with the steady mode at the peak injection pressure of the pulse mode. For further higher pulsation frequency, the jet penetration rapidly decreased with increasing the pulsation frequency. The penetration height approached to that in the steady mode of the average injection pressure in a pulse cycle, because an interval between the neighboring vortex rings became close to weaken the upward flow.

Pulsed Transverse Injection Applied to a Supersonic Flow

*, † ‡ # An experimental investigation was conducted to reveal jet penetration and mixing performance of pulsed injection in a Mach 2.5 crossflow. Helium and nitrogen gas were injected perpendicularly through flush-mounted circular sonic orifice. Probing techniques including species composition sampling and high speed framing schlieren were employed to determine the penetration and mixing performance at several downstream locations. Our investigation consisted essentially of two parts. The first part was an investigation of the continuous jet. The performance of the continuous jet was mainly controlled by effective velocity ratio (r) as the square root of the momentum-flux ratio and the orifice diameter (d). The centerline trajectories of the jets and the maximum concentration decay were collapsed by the rd scale and the ratio of oncoming boundary-layer thickness to the injector diameter. The second part was an comparison of the performance between the pulse and continuous jets. The penetration of the pulse jet was adjustable by changing the pulse duty cycle at a condition of fixed injectant mass flow rate. Even at a condition of fixed injection pressure, the pulsed injection showed better mixing performance and the higher penetration, due to the fluctuation of the large-scale eddies in the jet associated with the fluctuation of the bow shock in front of the jet.

Matched Pressure Injections into a Supersonic Crossflow through Diamond-Shaped Orifices

Matched pressure injections through diamond-shaped injectors were applied to a Mach 2.5 supersonic crossflow, and penetration and mixing characteristics of the injected plume were experimentally investigated. In determining injection conditions, the effective backpressure to the injectant plume was assumed to be equal to pressure on a solid-wedge surface with the identical wedge angle to the injector orifice at a designed flow rate. Both subsonic and supersonic injections were introduced to attain the required low plume pressure at a high supply pressure, ensuring a stable injectant flow rate in reacting flows with high backpressures. The matched pressure injections through the diamond-shaped orifices resulted in little jet-airflow interaction. With the supersonic injection, the plume floated from the injection wall, and the best penetration height was attained, whereas the benefit of matched pressure supersonic injection over the matched pressure sonic injection was not as remarkable as the circular injector case. The penetration height increased at an overexpanded condition, while the maximum mass fraction decay was insensitive to the injection pressure. In the case with the subsonic injection, the plume shape was similar to a pillar, and a certain fraction of the injectant was left within the boundary layer region. The penetration height as well as the maximum mass fraction decay was found to be insensitive to the injection pressure.

Supersonic Combustion Using a Stinger-Shaped Fuel Injector

The authors developed a stinger-shaped injector (stinger injector) for supersonic combustors in cold-flow experiments. The stinger injector has a port geometry with a sharp leading edge in front of a streamwise slit. This injector produced higher jet penetration at a lower jet-to-crossflow momentum flux ratio J than a conventional circular injector. We applied the injector in a Mach 2.44 combustion test at a stagnation temperature of 2060xa0K. At a low fuel-equivalence ratio Φ regime (i.e., low J regime), the injector produced 10% higher pressure thrust than the circular injector because of high jet penetration as expected from the cold-flow experiments. Even at a moderate Φ regime, the stinger injector produced higher pressure thrust than the circular injector. At moderate Φ, the stinger injector held the flame around the injector and generated a precombustion shock wave in front of the injector. The presence of the precombustion shock wave decreased the momentum flux of the crossflow air and diminished th...

Correlation between Hypermixer and Fuel Injection Locations

An experiment was conducted to understand the relation between a wall-mounted alternating-ramp-wedge type hyper mixer and transverse injectors with two different injection locations in a supersonic flow. Three experimental techniques, such as a schlieren visualization, a gas-sampling method, and a stereoscopic particle image velocimetry, were employed to study flowfield having velocity components and vortex structures induced by the interaction between the hyper mixer and the transverse injections, and also to compare the difference of mixing performance of the hyper mixer driven by the different injection location. For normal 1 injection case, an injection hole is located under the compression wedge, so injected helium is immediately impinged on the wedge and widely spread downstream, leading to enhancing mixing performance and holding helium in the mixing layer. On the other hand, for normal 2 injection case, helium is injected downstream of the hyper mixer, thus two flow structures created from the hyper mixer and the transverse injection interact with each other; as momentum flux ratio is increased, the flow structure from the transverse injection plays a significant role on the mixing region, and plenty of helium is penetrated into the supersonic flow.