Cavity Recirculation Research Articles

Experimental and numerical investigation on initial flame kernel blow-out in a supersonic combustor with a rear-wall-expansion cavity flameholder

The initial flame kernel blow-out process in an ethylene fueled scramjet combustor with a rear-wall-expansion cavity was investigated experimentally and numerically. In the experiment, it was observed that the initial flame kernel was blew off quickly after ignition under 5 different global equivalence ratios. A high precision numerical method was then adopted and the method was validated by comparing numerical results with available experimental data. It is found that the physical process from ignition to flame complete blow-out is well simulated by the numerical solver. Based on the numerical results, the blow-out process is divided into 6 stages and the characteristics of each stage are concluded. It is analyzed that the combined effect of the cavity recirculation zone and the cavity shear layer as well as the upper mainflow reduces the retention time of the initial flame kernel before it can develop to a steady flame base, which is the reason for the unsuccessful ignition in the rear-wall-expansion cavity.

Cavity enhanced jet interactions in a scramjet combustor

The shock structure around a fuel jet drives most of the initial mixing in a scramjet combustor, making it of interest for mixing enhancement studies. The effect of a cavity placed upstream of a fuel injector on the jet interaction of a transverse jet in a supersonic crossflow was examined numerically in this work. The cavity was found to significantly alter the structure of the typical supersonic cross-flow jet interaction. The typical horseshoe vortices were found to be absent and the barrel shock was found to be larger and more upright than in the no-cavity case. This was caused by the cavity recirculation shielding the fuel jet. The shock structure around the cavity was found to decrease the strength of the bow shock, reducing total pressure loss in the flowfield close to the injector. A small region of fluid above the cavity circulation was found to be the origin of vortical structures in the jet interaction, as opposed to the wall boundary layer in the conventional jet interaction. The presence of the fuel jet was found to alter the flow behaviour inside the cavity from closed cavity flow to open cavity flow, with the recirculation rising out of the cavity. This transition from closed to open cavity flow was found to be turbulence model dependent, however the main flow features and behaviour were shown to be maintained across turbulence models. Fuel was entrained in the cavity for the configuration under investigation, however this was dependent on fuel injection pressure, with no fuel entering the cavity at lower injection pressures.

The ignition characteristics of the close dual-point laser ignition in a cavity based scramjet combustor

The ignition processes of single-point laser ignition (LI) and dual-point LI were experimentally investigated in a cavity-based Mach 2.52 scramjet combustor fueled with ethylene. The stagnation temperature and total pressure of the supersonic inflow were 1486 K and 1.6 MPa, respectively. The results indicate that the ignition performance of dual-point LI with a plasma distance of 3 mm is the same as the single-point LI. However, further increasing the plasma distance to 7 mm degrades the ignition performance of dual-point LI. Therefore, dual-point LI is not superior to single-point LI in the scramjet combustor, which is contradictory to the investigations performed in quiescent premixed combustible mixtures. The reason is that the high turbulence intensity and strain rate in the cavity recirculation flow (∼100 m/s) lead to serious plasma and flame quenching instead of enhancing the flame propagation. Therefore, expanding the spatial distribution of the plasma by dual-point LI method, which might result in significant radical loss and heat loss, is not recommended for igniting a scramjet combustor.

Effect of cavity geometry on fuel transport and mixing processes in a scramjet combustor

This paper reports a numerical investigation on the effect of cavity geometry on fuel transport and mixing processes in a scramjet combustor with a single rear-wall-expansion cavity. The numerical solver and the LES methods were validated against available experimental data and the numerical results were shown in good agreement with the experiments. Effect of the cavity rear wall height on the non-reacting flow fields was then investigated. It was found that the vertical flow velocity of the region located right after the expansion wave starting from the cavity leading edge was increased significantly towards the cavity when lowering the rear wall height, leading to an enhanced fuel entrainment into the cavity. Subsequently, at a larger cavity expansion ratio, the mixture inside the cavity became more fuel-rich, giving rise to a deteriorated mixing environment. In addition, the cavity recirculation zone was further compressed and the turbulent flow and scalar dissipation inside the cavity would be enhanced, which were likely the reason causing the ignition failures and poor flame stabilizations. An optimal cavity expansion ratio for the maximum fuel entrainment was found in the present rig.

Entrainment characteristics of cavity shear layers in supersonic flows

Two-dimensional large eddy simulations have been carried out to investigate entrainment characteristics of cavity shear layers in supersonic flows, which is crucial to the understanding and modeling of cavity-stabilized combustion for scramjet applications. Effects of free-stream Mach number and cavity configuration are investigated. The stability and growth rate of the cavity shear layers are found to strongly depend on the compressibility effects, similar to those observed in the free shear layers. However, the growth rate of the cavity shear layers is much higher under low supersonic conditions and decreases more rapidly with increasing Mach number when compared to that of free shear layers. Cavities with larger length-to-depth ratio or larger aft angle appear to have greater shear-layer growth rate. Entrainment of the free-stream fluids into the cavity recirculation zone is to a large extent determined by the interactions between the shear layer and the cavity aft wall. The mass flux entrainment ratio of the cavity shear layers appears lower than that of free shear layers and basically ranges from 0.6 to 1.7 under the present conditions.

Hybrid Reynolds-Averaged/Large-Eddy Simulation of a Cavity Flameholder: Modeling Sensitivities

Steady-state and scale-resolving simulations have been performed for flow in and around a model scramjet combustor flameholder. The cases simulated corresponded to those used to examine this flowfield experimentally using particle image velocimetry. A variety of turbulence models were used for the steady-state Reynolds-averaged simulations, which included both linear and nonlinear eddy viscosity models. The scale-resolving simulations used a hybrid Reynolds-averaged/large-eddy simulation strategy that is designed to be a large-eddy simulation everywhere except in the inner portion (log layer and below) of the boundary layer. Hence, this formulation can be regarded as a wall-modeled large-eddy simulation. This effort was undertaken to formally assess the performance of the hybrid Reynolds-averaged/large-eddy simulation modeling approach in a flowfield of interest to the scramjet research community. The numerical errors were quantified for both the steady-state and scale-resolving simulations before making any claims of predictive accuracy relative to the measurements. The hybrid Reynolds-averaged/large-eddy simulation results were also carefully scrutinized to ensure that even the coarsest grid had an acceptable level of resolution to meet accepted guidelines for large-eddy simulation and that the time-averaged statistics were acceptably accurate. The autocorrelation and its Fourier transform were the primary tools used for this assessment. Both simulation strategies accurately predicted the mean streamwise velocity distribution within the cavity, although the Reynolds-averaged simulations that used a linear eddy viscosity model tended to overpredict the strength of the primary cavity recirculation zone. Second-order moments of the velocity field were found to be highly sensitive to the turbulence model chosen for the Reynolds-averaged simulations, with all models overpredicting the intensity of the velocity fluctuations within the cavity flameholder. The hybrid Reynolds-averaged/large-eddy simulation results also overpredicted the velocity variances and covariances, unless a filtering operation was applied using a filter size that matched the control volume used to process the particle image velocimetry measurements. This observation suggests that a significant fraction of the turbulence energy was not resolved by the measurements. Taking this uncertainty into account, the second-order statistics extracted from the hybrid simulation strategy could not be shown to be any more accurate than the “best” Reynolds-averaged result.

Gabion Stepped Spillway: Interactions between Free-Surface, Cavity, and Seepage Flows

AbstractOn a gabion stepped chute, the steps contribute to the dissipation of turbulent kinetic energy, free-surface aeration may be intense, and there are complex interactions between the free-surface flow and seepage motion. Detailed measurements were conducted in a relatively large gabion stepped spillway model. Using a combination of high-speed movies and phase-detection probe measurements, the air–water flow properties in the step cavities and in the gabions were documented. Strong air–water exchanges between seepage and stepped cavity flows were observed. The data showed a complex bubbly seepage motion in the gabions associated with a high level of interactions between seepage and free-surface flows, leading to a modification of the step cavity recirculation and lesser flow resistance.

Investigation on Ignition Enhancement Mechanism in a Scramjet Combustor with Dual Cavity

The spark ignition and flame stabilization of kerosene in a dual-cavity scramjet combustor is investigated with the isolator entrance Mach number of 2.52, total pressure of 1.6 MPa, and stagnation temperature of 1486 K, which corresponds to flight condition. The effects of dual-cavity configuration on ignition and flameholding were analyzed. Two distinct cavity-organized flame regimes were found: cavity flame and cavity shear-layer flame. The driving force of the transition of flame stabilization mode from cavity flame to cavity shear-layer flame is the high pressure resulted from combustion, which occurs in the cavity. The dual-cavity flameholding scheme can promote the ignition ability. First, the downstream cavity could stabilize the flame convected from the upstream cavity. Second, compared with the upstream cavity, the downstream cavity is a more suitable ignition location because the fuel has a longer distance for evaporation and mixing. The boundary-layer separation and upstream moving of precombustion shock train are responsible for the contraflow flame propagation process. Also, an unstable boundary-layer/cavity-stabilized combustion is found during the flame transition process. This phenomenon results from the interaction between the cavity recirculation zones and the separation zone immediately upstream of T1 cavity, which is caused by the strong shock/boundary-layer interactions, as the shock train is pushed upstream.

Interaction between free-surface aeration and total pressure on a stepped chute

Stepped chutes have been used as flood release facilities for several centuries. Key features are the intense free-surface aeration of both prototype and laboratory systems and the macro-roughness caused by the stepped cavities. Herein the air bubble entrainment and turbulence were investigated in a stepped spillway model, to characterise the interplay between air bubble entrainment and turbulence, and the complicated interactions between mainstream flow and cavity recirculation motion. New experiments were conducted in a large steep stepped chute (θ=45°, h=0.10m, W=0.985m). Detailed two-phase flow measurements were conducted for a range of discharges corresponding to Reynolds numbers between 2×105 and 9×105. The total pressure, air–water flow and turbulence properties were documented systematically in the mainstream and cavity flows. Energy calculations showed an overall energy dissipation of about 50% regardless of the discharge. Overall the data indicated that the bottom roughness (i.e. stepped profile) was a determining factor on the energy dissipation performance of the stepped structure, as well as on the longitudinal changes in air–water flow properties. Comparative results showed that the cavity aspect ratio, hence the slope, has a marked effect on the residual energy.

Numerical simulation on ignition transients of hydrogen flame in a supersonic combustor with dual-cavity

Abstract Consequent flame stabilization in a practical combustor is achieved via ignition transients. Given the background of supersonic combustion in a scramjet engine, the ignition transient phenomena is of millisecond time scale and will present complex characteristics. In the present paper, a hybrid Reynolds-Averaged Navier–Stokes (RANS)/Large Eddy Simulation (LES) approach is adopted to investigate the ignition transient process in the supersonic combustors with hydrogen jet upstream of parallel and tandem dual-cavity. Flame luminosity images are also obtained by using high speed camera to experimentally observe the dynamic behavior of the combustion. For both parallel and tandem case, simulations reasonably reproduce the spatiotemporal evolution of the hydrogen flame structures during the period from the moment after the ignition deactivation to a quasi-steady combusting state. In the transients of parallel dual-cavity, significant flame transformation in the combustor is performed, with the flame front moving upstream toward the jet exit region. Positive feedbacks are believed to function among the sub-processes: the strong heat release and hot products generate around the jet mixing region and cavity shear layer, the major vortex within the cavity recirculation zone transfers the active radicals, and high-pressurized combustion zone extends upstream and further compresses the incoming core flow. Regarding the tandem case, the overall flame development is dominated by the rapid growth of combustion from the downstream cavity to the upstream region, along with the concentrated heat release spreading against the main stream. Collaboration of the downstream combustion and the upstream flame packets provides appropriate temperature, pressure and velocity field in the middle section between the two cavities, which should be responsible for the flame transients.

Numerical and experimental study on flame structure characteristics in a supersonic combustor with dual-cavity

Combined numerical and experimental approaches have been implemented to investigate the quasi-steady flame characteristics of supersonic combustion in tandem and parallel dual-cavity. In simulation, a hybrid Large Eddy Simulation (LES)/assumed sub-grid Probability Density Function (PDF) closure model was carried out. Comparison of calculation and experiment as well as comparison of the two configurations are qualitatively and quantitatively performed regarding the flame structure and other flowfield features. Simulation shows a good level of agreement with experimental observation and measurement in terms of instantaneous and time-averaged results. Given the same fuel equivalence ratio, the parallel dual-cavity with the two opposite injections gathers the major combustion around the cavities, thus leading to the concentrated heat release, the greatly extended recirculation zones and the converging–diverging core flow path. Only intermittent stray flame packets can be found in the downstream region. Flame in the combustor with tandem dual-cavity appears to be stabilized by the upstream cavity shear layer and grows gradually to the second cavity, peaking its most intensity in the middle section between the two cavities. For both dual-cavity configurations, the strongest reaction takes place in near chemistry stoichiometric region around the flame edge, and is mainly confined in the supersonic region supported by the inner subsonic combustion. The coexistence of three parts plays a vital role in flame stabilization in the parallel and tandem dual-cavity: a reacting reservoir transferring hot products and activated radicals within the cavity recirculation zone, the hydrogen-rich premixed flame in the jet mixing region, and the downstream diffusion flames supported by the upstream premixed combustion region. In addition, for the parallel dual-cavity under the given condition, significant reaction are present near jet exit upstream the cavity leading edge.

Large eddy simulation of a hydrogen-fueled scramjet combustor with dual cavity

Large eddy simulation (LES) has been carried out to investigate a hydrogen-fueled scramjet combustor with dual cavity, where a Reynolds-Averaged Navier–Stokes (RANS) model is used for near-wall treatment. The recycling/rescaling method is adopted to generate unsteady turbulent inflow conditions for the LES. Experimentally-observed flow and combustion structures are reasonably well captured and explained by the simulation. The results show that the intersection of the bow shock waves and the concentrated heat release generate a high-pressure region between the cavities, which induces great pressure gradients as well as evident flows in the transverse direction, pushing the fuel jets towards the combustor walls. Consequently, strong interactions occur between the fuel jets and the cavity aft walls, promoting the fuel transport into the cavity. Meanwhile, the cavity recirculation regions are considerably extended and distorted, and the mass exchange between the fluids in and out of the cavities may be greatly enhanced. In contrary, these flow structures support the concentrated heat release around the cavity by enhancing the fuel–air mixing and increasing the residence time of the combustible. Then, a positive feedback loop is formed by this close coupling of flow and heat release. It is also observed that the combustion downstream of the cavity is confined within narrow regions near the combustor walls due to the decreased fuel jet penetration in the farfield.

Velocity field and parametric analysis of a subsonic, medium-Reynolds number cavity flow

Cavity flows are a class of flows bounded by material structures, where a recirculation region is present, and they are found in many practical applications. In the present study, the interaction between a boundary layer and an open parallelepipedic cavity develops a Kelvin–Helmholtz-like instability coupled with the cavity recirculation. PIV measurements of the flow are carried out in two orthogonal planes inside the cavity, for different aspect ratios, incompressible flow conditions, and Reynolds numbers in the range 1,900–12,000. Mean velocity and second-order moments of velocity fluctuations reveal the flow morphology. For particular conditions, centrifugal instabilities appear that are induced by flow curvature due to wall confinement. The use of an identification criterion indicates the presence of pairs of counter-rotating vortices winded around the recirculation. A parametric analysis is conducted, and the inviscid Rayleigh discriminant provides the potentially unstable flow regions inside the cavity. Finally, a stability parameter considering the ratio between centrifugal destabilizing effects and stabilizing viscous effects is carried out and gives thresholds for the emergence of the centrifugal instability. The study draws to an end with a comparison with a well-documented lid-driven cavity flow.

Numerical Investigation of the Interaction between Mainstream and Tip Shroud Leakage Flow in a 2-Stage Low Pressure Turbine

The pressing demand for future advanced gas turbine requires to identify the losses in a turbine and to understand the physical mechanisms producing them. In low pressure turbines with shrouded blades, a large portion of these losses is generated by tip shroud leakage flow and associated interaction. For this reason, shroud leakage losses are generally grouped into the losses of leakage flow itself and the losses caused by the interaction between leakage flow and mainstream. In order to evaluate the influence of shroud leakage flow and related losses on turbine performance, computational investigations for a 2-stage low pressure turbine is presented and discussed in this paper. Three dimensional steady multistage calculations using mixing plane approach were performed including detailed tip shroud geometry. Results showed that turbines with shrouded blades have an obvious advantage over unshrouded ones in terms of aerodynamic performance. A loss mechanism breakdown analysis demonstrated that the leakage loss is the main contributor in the first stage while mixing loss dominates in the second stage. Due to the blade-to-blade pressure gradient, both inlet and exit cavity present non-uniform leakage injection and extraction. The flow in the exit cavity is filled with cavity vortex, leakage jet attached to the cavity wall and recirculation zone induced by main flow ingestion. Furthermore, radial gap and exit cavity size of tip shroud have a major effect on the yaw angle near the tip region in the main flow. Therefore, a full calculation of shroud leakage flow is necessary in turbine performance analysis and the shroud geometric features need to be considered during turbine design process.

Air–water flow properties in step cavity down a stepped chute

For the last three decades, the research into skimming flows down stepped chutes was driven by needs for better design guidelines. The skimming flow is characterised by some momentum transfer from the main stream to the recirculation zones in the shear layer developing downstream of each step edge. In the present study some physical modelling was conducted in a relatively large facility and detailed air–water flow measurements were conducted at several locations along a triangular cavity. The data implied some self-similarity of the main flow properties in the upper flow region, at step edges as well as at all locations along the step cavity. In the developing shear layer and cavity region (i.e. y/ h < 0.3), the air–water flow properties presented some specific features highlighting the development of the mixing layer downstream of the step edge and the strong interactions between cavity recirculation and mainstream skimming flows. Both void fraction and bubble count rate data showed a local maximum in the developing shear layer, although the local maximum void fraction was always located below the local maximum bubble count rate. The velocity profiles had the same shape as the classical mono-phase flow data. The air–water flow properties highlighted some intense turbulence in the mixing layer that would be associated with large shear stresses and bubble–turbulence interactions.

Three-Dimensional Analysis of a Supersonic CombustorCoupled to Innovative Inward-Turning Inlets

Three-dimensional simulations are employed to examine the effect of inlet distortion and model fidelity with Reynolds averaged Navier-Stokes or large eddy simulation approaches for a generic circular cross-sectional supersonic combustor at a flight condition of Mach 6 and an altitude of about 24.2 km. To examine inlet distortion effects on combustion, frozen and finite rate chemistry simulations are performed on combustors connected to two different types of streamline-traced inlets (denoted and Jaws) with the Wilcox k-ω turbulence model. For comparison, uniform inflow boundary condition to the combustor is also simulated. The metrics employed include qualitative assessments related to flow structure as well as quantitative values offuel combustion efficiency and thrust ratios. The results indicate a complex overall effect of distortion due to inlet design. For Jaws, the increased pressure loss associated with distortion is mitigated slightly by improved combustion efficiency and better thrust performance. The Scoop inlet has lower distortion and better recovery, but the combustion coefficient is lower than Jaws. In the second part ofthis study, finite rate chemistry results with unsteady Reynolds averaged Navier-Stokes are compared with those from large eddy simulation with an uniform inflow profile. Differences in transient processes include the manner in which large- and small-scale structures originate and evolve in the cavity recirculation and downstream regions. The finer details observed with the large eddy simulation model have significant consequences on the overall field, including the unsteady position of the shock structure arising from the interaction of the incoming flow with the cavity shear layer, combustion processes, and injection jet interactions.

Turbulence and cavity recirculation in air–water skimming flows

Current expertise in air–water turbulent flows on stepped chutes is limited mostly to laboratory experiments at low to moderate Reynolds numbers on chutes with flat horizontal steps. In this study, highly turbulent air–water flows skimming down a large-size stepped chute were investigated with a 1V:2.5H slope. For some experiments, the cavity recirculation was controlled using triangular vanes, or longitudinal ribs, to enhance the interactions between the skimming flow and cavity recirculating region. New experiments were performed with seven configurations. The results demonstrated the strong influence of the vanes on the cavity recirculation patterns and on the air–water flow properties. An increase in flow resistance was observed consistently with maximum rate of energy dissipation achieved with vanes placed in a zigzag pattern.

Turbulence manipulation in air–water flows on a stepped chute: An experimental study

In a stepped channel operating with large flow rates, the flow skims over the pseudo-bottom formed by the step edges as a coherent stream. Intense three-dimensional recirculation is maintained by shear stress transmission from the mainstream to the step cavities, while significant free-surface aeration takes place. The interactions between free-surface aeration and cavity recirculation are investigated herein with seven step cavity configurations. The experiments were conducted in a large stepped channel operating at large Reynolds numbers. For some experiments, triangular vanes, or longitudinal ribs, were placed across the step cavities to manipulate the flow turbulence to enhance the interactions between the mainstream flow and the cavity recirculation region. The results showed a strong influence of the vanes on the air–water flow properties in both free-stream and cavity flows. The findings demonstrate some passive turbulence manipulation in highly turbulent air–water flows.

Effects of plume buoyancy and momentum on the near-wake flow structure and dispersion behind an idealized building

Dispersion simulations of buoyant and neutral plume releases within the recirculation cavity behind a cubical building were performed using a commercially available CFD code and the RNG k– ε turbulence model. Plume buoyancy was observed to affect the size and shape of the cavity region and the flow structure and concentration profiles within. Source momentum of a neutral plume release had similar effects on the flow structure and the cavity region to that caused by plume buoyancy. However, the effects of momentum on the concentration profiles were noticeably different from that caused by plume buoyancy. Plumes released immediately downwind of a cubical building appear to alter the flow field and dispersion characteristics of the cavity recirculation region due to their inherent momentum and buoyancy. A greater fraction of a plume was captured inside the wake as the plume became increasingly buoyant. Contrarily, greater plume momentum resulted in smaller plume fractions captured inside the wake. Inclusion of these effects in the downwash algorithms would improve the accuracy of modeling results for far-field concentration distributions and would be mandatory in accident assessments where accurate predictions of short-term, near-field concentration fluctuations near source releases are required.

Numerical simulation of air-water two-phase flow over stepped spillways

Stepped spillways for significant energy dissipation along the chute have gained interest and popularity among researchers and dam engineers. Due to the complexity of air-water two-phase flow over stepped spillways, the finite volume computational fluid dynamics module of the FLUENT software was used to simulate the main characteristics of the flow. Adopting the RNG k - e turbulence model, the mixture flow model for air-water two-phase flow was used to simulate the flow field over stepped spillway with the PISO arithmetic technique. The numerical result successfully reproduced the complex flow over a stepped spillway of an experiment case, including the interaction between entrained air bubbles and cavity recirculation in the skimming flow regime, velocity distribution and the pressure profiles on the step surface as well. The result is helpful for understanding the detailed information about energy dissipation over stepped spillways.