Experimental and Numerical Study on Flame Stabilization in a Supersonic Combustor with Hydrogen Injection Upstream of Cavity Flameholders

This study investigates an enhanced passive control strategy for suppressing combustion oscillations in scramjet engines by introducing an auxiliary small cavity upstream of the primary flameholder. Experimental tests conducted on a direct-connect rig with Mach 3 inflow demonstrate that the staged-cavity design achieves remarkable stability improvements across equivalence ratios (Φ = 0.34–1.2). Key findings include: (1) 48.9% and 47.2% reduction in pressure oscillations at Φ = 1.2 and 0.6, respectively, with the mean normalized pressure increasing by approximately 8.57%; (2) Expansion of flammability limits (lean ignition at Φ = 0.34 vs baseline Φ = 0.48); (3) 81.2% suppression of flame front fluctuations through standing flame anchoring. Mechanistic analysis reveals five synergistic effects: (i) pre-combustion in the auxiliary cavity decentralizes heat release, (ii) the small cavity's shear layer sustains a persistent pilot flame without additional injectors, (iii) boundary layer stabilization prevents separation-induced oscillations, (iv) enhanced fuel–air mixing creates localized premixed zones within the diffusion flame, and (v) shock restructuring maintains flame anchoring despite pressure variations. High-speed imaging and schlieren visualization confirm the design's ability to maintain stable jet-like flames while eliminating chaotic combustion modes observed in conventional configurations. The auxiliary cavity approach demonstrates simultaneous improvements in combustion stability (oscillation suppression), performance (thrust increase), and operational envelope (extended ignition limits), providing a practical passive control solution for scramjet combustors. Residual low-frequency fluctuations (4%–5% amplitude) suggest future optimization opportunities in cavity-flowfield integration. These findings establish staged cavity combustion as an effective strategy for next-generation hypersonic propulsion systems.

Phenomenology of methane flame propagation into compositionally stratified, gradually richer mixtures

Fuel–air mixing characteristics in an axisymmetric cavity-based scramjet combustor were investigated using the 3-pentanone tracer planar laser-induced fluorescence (PLIF) technique. The optically accessible scramjet combustor was deployed in a direct-connected rig at Mach 6 flight enthalpy. The mixture containing the air and the 3-pentanone vapor was used as a simulated gas fuel, and the fuel intensity distribution in streamwise and spanwise directions was visualized by the 3-pentanone PLIF technique. Distributions of the fuel edge were calculated and analyzed according to their variation due to the increase in fuel injection pressure. It was observed that most of the fuel was convected inside the cavity region by the vortex interaction of the jet with the cavity shear layer and the cavity recirculation zone. The extracted penetration heights based on the mean PLIF images were then observed to increase with the jet momentum flux ratio, and this function was expressed by a fitting curve. 3-pentanone PLIF results in three spanwise sections showed that the radial centroids of the ring-like PLIF profile were located inside the cavity region, and they approached the centerline gradually with increasing fuel injection pressures. The large eddy simulation results showed that counter-rotating vortex structures and the cavity recirculation zone enhanced the fuel–air mixing. The vortex pair induced by the fuel jet was found to improve the turbulent fluctuations inside the fuel plume and significantly alter the shape of the plume, whereas the cavity recirculation zone tended to transport more fuel from the shear layer inside the cavity region.

An experimental study on flame propagation through an interface between layered methane and air, propane and air, and methane and oxygen in inert baths has been made with a combustor having a length of 10 cm. Detailed flame structure was investigated by means of schlieren photography and a hot wire anemometer. The premixed flame, the diffusion flame, and the convection flame just like those propagating in line appeared in the limited combustor space, but one after another with some time lags in between. The diffusion flame appeared about 80 msec after the premixed flame passed, which equivalently corresponds to about 10 cm of space existing between the premixed flame and the forefront of the diffusion flame. An anomalous structure was observed in the premixed flame. That is, a post burning occurs immediately behind the conventional flame front which forms double flame fronts. The temperature in between these is flat, and the separated gap width increases towards lean and rich mixtures. Steady fla...

Optical and laser diagnostic investigation of flame stabilization in a novel, ultra-lean, non-premixed model GT burner

Influence of velocity on the transient auto-ignition of ethylene jet in a hot coflow

In order to clarify the unsteady behavior and the quenching mechanism of the hydrogen-air premixed flame at a small circular hole made in the cooled plate put in the large container, the numerical calculation was carried out by using the detailed chemical kinetics. In this calculation, the stationary premixed gas is filled in the container at the start, and then the mixture is ignited at the end face and the flame propagates to the hole. At the hole, the flow is induced by the flame propagation and the heat is taken from the cooled wall, so that the flame front changes unsteadily and has the curvature. When the flame front has the curvature, the effects of non-unity Lewis number of the hydrogen fuel lean or rich mixtures appear in the quenching process. The relationship between the quenching diameter and the equivalence ratio are shown for the different thickness of the cooled plate. Moreover, the unsteady flame behavior and the critical quenching process are shown for the fuel lean and rich mixtures, and the quenching mechanism is examined so that the effects of the cooling and the Lewis number are charified.

Characteristics of laser ignition and spark discharge ignition in a cavity-based supersonic combustor

High-Repetition-Rate OH Planar Laser-Induced Fluorescence of a Cavity Flameholder

For the detailed understanding of transient combustion processes, in particular, of auto-ignition, quantitative measurements with high spatio-temporal resolution are desirable. These can, for instance, serve as validation data for time-resolved numerical simulations and in particular for the combustion models used in those simulations. In the current study, a jet-in-hot-coflow (JHC) burner, developed at the German Aerospace Center (DLR), the DLR JHC, was used to inject a turbulent methane jet into the hot exhaust gas of a lean hydrogen/air flame, and a steady state jet flame was established. In addition, fuel could be injected in a transient manner. Here, an auto-igniting jet was observed. The flame stabilization of the steady state jet flame and the auto-ignition during transient fuel injection were studied using high-speed laser-based and optical measurements. A strategy for quantifying high-speed OH planar laser-induced fluorescence is presented, and the measurement uncertainties are evaluated. The flame stabilization mechanism in steady state jet flames was assessed using probability density functions of the OH concentration at different axial and radial locations. The formation of auto-ignition kernels during transient fuel injection is evaluated based on time series of the OH concentration. It is shown how the OH concentration levels and PDF shapes can be used to characterize the chemical state of the reacting flow and to distinguish between auto-ignition and flame propagation.

Premixed methane-air mixture is introduced at the center of two parallel circular quartz plates separated by a millimeter scale distance (<or= 5mm). Two plates are heated with an external heater to create a positive temperature gradient condition along the flow direction. The combustion characteristics of lean methane-air mixtures are investigated at phi=0.67 and phi=0.85 in the laminar flow regime (Re approximately 500-1800). Contrary to the general perception of a stable premixed flame front at a radial location, a large variety of unstable and dynamic flame front patterns are observed at phi=0.67. At phi=0.85, stable flame propagation mode dominated the regime diagram and unstable flame propagation modes are observed under limited conditions. In these unstable flame propagation modes, single and multiple flame fronts rotate around the center at a rate of 15-50 Hz.

Due to increasing environmental concerns, clean technology has become a key feature in industrial gas turbines. Swirler design is directly associated with the combustion performance for its roles in fuel distribution and flame stability. In this study, the development process of three new conceptual swirlers from Samsung Techwin is presented. Each swirler has unique features to enhance fuel-to-air mixing; Swirler 1 uses tangential air-bypass, Swirler 2 minimizes pressure loss using impeller-like design, and Swirler 3 has combined flow characteristics of axial and radial swirlers. Using extensive computational fluid dynamics (CFD) analysis, lead time and cost in manufacturing the prototypes were significantly reduced. The numerical methods were verified with a lab-scale combustion test; particle image velocimetry (PIV) measurement of cold flow, direct flame images, and OH planar laser induced fluorescence (PLIF) images were compared with result of large-eddy simulation (LES), and they showed good agreement. After design optimization using CFD, full-scale combustion tests were performed for all three swirlers. Flame from each swirler was visualized using a cylindrical quartz liner; direct images and OH chemiluminescence images of flames were obtained. Flame stability and blow-off limit at various air load were examined by gradually lowering the equivalence ratio. NOx and CO concentration were measured at the exhaust. All three swirlers satisfied low NOx and CO levels at the design conditions. The performance maps bounded by the NOx and CO limits and blow-off limit were obtained for all swirlers. Further efforts to maximize the combustors performance will be made.

Evaluation of flamelet/progress variable model for the applications in supersonic combustion using hybrid RANS/LES approach

The effects of SiCladdition on flame structures have been studied in flame hydrolysis deposition (FHD) processes using Coherent anti-Stokes Raman spectroscopy (CARS) and planar laser induced fluorescence (PLIF) to measure temperatures and OH concentrations, respectively. The results demonstrate that even a small amount of SiCl addition can change thermal and chemical structures of H/O diffusion flames. When SiCl is added to a flame temperature decreases in non-reacting zone due to the increases in both specific heat and density of the gas mixture, while flame temperature increase in particle formation zone due to the heat release through hydrolysis and oxidation reactions of SiCl. It is also found that OH concentration decreases dramatically in particle formation zone where temperatures increase. This can be attributed to consumption of oxidative species and generation of HCl during silica formation.

Flame stabilization mechanism in reacting jets in swirling vitiated crossflow