Aft Wall Research Articles

Numerical study on self-sustained oscillation characteristics of cavity flameholders in a supersonic flow

Self-sustained oscillation characteristics of two-dimensional cavity-based flameholders in a cold supersonic flow were investigated in the present numerical study. The unsteady flowfields were calculated using hybrid Reynolds-averaged Navier—Stokes/large eddy simulation (RANS/LES) method. The key cavity parameters that were varied include cavity aft wall angle and cavity length to depth ratio. The hybrid model favourably captured the large scale unsteady characteristics and revealed the evolution process of cavity free shear layer. Pressure fluctuation spectra agreed well with theoretical predictions. The hybrid RANS/LES data indicate that cavity shape and size both have significant effects on the oscillation characteristics. As the cavity length to depth ratio decreases, oscillation frequency corresponding to maximum oscillation amplitude shifts to higher values. Most of the oscillation modes are greatly suppressed as the cavity aft wall angle decreases. Contrast to the steep aft wall cavity, only higher frequency modes remain in the oscillation of the cavity with small aft wall angle.

Visualization of flameholding mechanisms in a supersonic combustor using PLIF

Laser-induced fluorescence of OH and CH 2O was imaged to investigate the flame stabilization mechanism in a flameholder with a Mach 2.4 free stream. Ethylene was burned in a rectangular cavity with two points of injection: the aft wall and the cavity floor. When injected from the aft wall, the fuel came into immediate contact with hot combustion products from the reaction zone under the shear layer. Primary combustion occurred under the shear layer and in the aft region of the cavity volume. In contrast, when fuel was injected from the floor, a jet-driven recirculation zone of hot products near the upstream wall of the cavity served as a flameholder. The reaction then occurred on the underside of the shear layer. In conditions near lean blowout, significant changes in the flameholding mechanisms were observed. Improved CH 2O fluorescence signal was obtained by taking advantage of the long fluorescence lifetime at low pressures and delaying the camera gate to reduce the background signal.

Neural Network Modeling of Deterministic Unsteadiness Source Terms

A neural-network-based lumped deterministic source term technique is presented that results in the prediction of an approximate time-average solution when used to modify a steady-state solver. Three different neural networks are developed for simple cavity flows using Mach number, cavity length-to-depth ratio, and aft wall translation as parameters. The results indicate that axial force data can be reproduced with less than 15% error as compared to the time average of a fully unsteady calculation. Computation times for the resultant neural network lumped deterministic source term approach were up to two orders of magnitude less than the comparable unsteady solution and were essentially identical to that of a steady-state calculation, although it must be noted that a database of unsteady calculations is required to develop the technique. The lumped deterministic source terms did not appear to affect the robustness of the steady-state solver adversely.

Stability limits of cavity-stabilized flames in supersonic flow

Experiments were performed to examine the stability of hydrocarbon-fueled flames in cavity flameholders in supersonic airflows. Methane and ethylene were burned in two different cavity configurations having aft walls ramped at 22.5° and 90°. Air stagnation temperatures were 590 K at Mach 2 and 640 K at Mach 3. Lean blowout limits showed dependence on the air mass flowrates, cavity geometry, fuel injection scheme, Mach number, and fuel type. Large differences were noted between cavity floor and cavity ramp injection schemes. Visual observations, planar laser-induced fluorescence of nitric oxide, and shadowgraph imaging were used to investigate these phenomena. Cavity ramp injection provided better performance near the lean blowout limit, whereas injection from the cavity floor resulted in more stable flames near the rich limit. Ethylene flames have a wider range of stable operations than methane in all conditions. Lean blowout limits were not significantly different between the Mach 2 and Mach 3 cases at the lean limit; however, variation in Mach number had a measurable effect near the rich limit. Fuel flowrates at ignition were much greater than the lean blowout limit, but showed similar dependence on air mass flowrate.

Vortical structures over rectangular cavities at low speed

Experiments have been performed to acquire qualitative and quantitative flow-field data for an open cavity flow using the particle image velocimetry technique. The study focuses on the time-mean and instantaneous development of the turbulent flow structures in the cavity shear layer. The effects of geometry (length-to-depth ratio) and flow speed on these structures are investigated. The shear layers are found to be characterized by coherent vortical structures whose size and rate of growth vary with geometry. The smaller scales of the flow are investigated using a large-eddy decomposition method. Results show these stochastic structures to predominate primarily in the shear layer and aft wall regions.

Large-eddy simulation of a compressible flow in a three-dimensional open cavity at high Reynolds number

Large-eddy simulations of a subsonic three-dimensional cavity flow with self-sustaining oscillations are carried out for a Reynolds number based on the length of the cavity equal to . Meticulous comparisons with available experimental data corresponding to the same configuration demonstrate a high level of accuracy. Special attention is paid to the mixing layer that develops over the cavity and two different zones are identified. The first one is dominated by Kelvin–Helmholtz instability, and the linear as well as quadratic energy transfers leading to the filling of velocity spectra are described. The Kelvin–Helmholtz instability also appears to be forced near the origin of the layer, and it is postulated that the small recirculation bubble located in this area is responsible for the forcing. Downstream of the first zone and up to the vicinity of the aft wall, the layer behaves very similarly to a free mixing layer by exhibiting a linear spreading. An influence of the recirculating flow inside the cavity upon the growth of the layer is nevertheless observed at downstream stations. Analysis of the pressure on the floor of the cavity reveals that the self-sustaining oscillation-related pressure modes (Rossiter modes) are independent of their spanwise location inside the cavity. On the contrary, Rossiter modes exhibit streamwise modulations and it is demonstrated that a very simple two-wave model is able to reproduce the spatial shape of the modes. Nonlinear interactions between Rossiter modes are encountered, as well as nonlinear interactions with low-frequency components. A joint time–frequency analysis shows a temporal modulation of the Rossiter mode levels at similar low frequencies, resulting in a special form of intermittency with competitive energy exchanges between modes.

Large-Scale Disturbances and Their Mitigation Downstream of Shallow Cavities Covered by a Perforated Lid

Flow past cavities covered by perforated lids pose a challenging problem for design engineers. Kelvin–Helmholtz waves appear early in the separated shear layers above the perforations that quickly mature into large-scale coherent structures far downstream. This evolution is sustained by a hydrodynamic feedback mechanism within the cavity even when its aft wall is far removed from the lid. Herein, the results from large-eddy simulations show analogous fundamental characteristics between open and perforated-cover cavities. Both adequately scale the fundamental frequency of the large-scale disturbance using the freestream velocity and the cavity width (or lid length). Moreover, the dimensionless frequencies jump to higher modes at equivalent length scales. Unlike the open cavity, one can invoke certain conditions that instigate the instability above the perforations but not a simultaneous long-term feedback mechanism necessary to fully sustain the periodic oscillation. The lid itself offers options for mitigating (or even eliminating) the instability. Results (for laminar separation) show the perforation spacing as the key factor. While maintaining the same fundamental frequency, one can easily dampen its spectral peak to complete disappearance by extending the perforation spacing.

Numerical study on supersonic combustion with cavity-based fuel injection

The present study describes the numerical investigations concerning the combustion enhancement when a cavity is used for the hydrogen fuel injection through a transverse slot nozzle into a supersonic hot air stream. The cavity is of interest because recirculation flow in cavity would provide a stable flame holding while enhancing the rate of mixing or combustion. Several inclined cavities with various aft wall angle, offset ratio and length are evaluated for reactive flow characteristics. The cavity effect is discussed from a viewpoint of total pressure loss and combustion efficiency. The combustor with cavity is found to enhance mixing and combustion while increasing the pressure loss, compared with the case without cavity. But it is noted that there exists an appropriate length of cavity regarding the combustion efficiency and total pressure loss.

Investigation of Kerosene Combustion Characteristics with Pilot Hydrogen in Model Supersonic Combustors

Experimental investigations on the ignition and combustion stabilization of kerosene with pilot hydrogen in Mach 2.5 airflows were conducted using two test combustors, with cross sections of 30.5 x 30 and 51 x 70 mm, respectively. Various integrated modules, including the combinations of different pilot injection schemes and recessed cavity flameholders with different geometries, were designed and tested. The stagnation pressure of vitiated air varied within the range of 1.1-1.8 NiPa, while the stagnation temperature varied from 1500 to 1900 K. Specifically, effects of the pilot hydrogen injection scheme, cavity geometry, and combustor scaling on the minimally required pilot hydrogen equivalence ratio were systematically examined. Results indicated that the cavity depth and length had significant effects on the ignition and flameholding, whereas the slanted angle of the aft wall was relatively less important. Two cavities in tandem were shown to be a more effective flameholding mechanism than that with a single cavity. The minimally required pilot hydrogen equivalence ratio for kerosene ignition and stable combustion was found to be as low as 0.02. Furthermore, combustion efficiency of 80% was demonstrated to be achievable for kerosene with the simultaneous use of pilot hydrogen and a recessed cavity to promote the ignition and global burning.

Fundamental Studies of Cavity-Based Flameholder Concepts for Supersonic Combustors

Experimentalandcomputationalinvestigationsofthee owe eldassociatedwithseveralcavity-basede ameholders in a nonreacting supersonic e ow are described. All cavity e ows were of the open type, that is, length-to-depth ratio L/D<10. Two values of L/D were studied with several offset ratios (OR) and aft ramp angles µ. Results indicate that the aft ramp angle plays an important role in determining the character of the shear layer that spans the cavity. For a rectangular cavity with OR=1 and µ=90 deg, a compression wave forms as the e ow separates from the cavity’ s upstream corner. A strong recompression occurs at the aft wall, and the e ow is visibly unsteady. The pressure on the cavity fore wall decreases steadily and the recompression process occurs more gradually with decreasing aftrampangle.Higherdrag coefe cientsandshorterresidencetimesarefoundin cavitieswithshallower ramp angles.

Algebraic turbulence model simulations of supersonic open-cavity flow physics

A time-accurate double thin-layer Navier-Stokes computation is performed for an unsteady supersonic open cavity with a length-to-depth ratio of 2. The results are used to determine the flow-physics mechanisms responsible for the cavity oscillation cycle. A new cycle is described and compared to previous descriptions. It is found that a shed vortex impinges on the cavity aft lip and forms a pressure pulse that augments or forces, at the vortex shedding frequency, an internal upstream moving wave that has been reflected from the aft corner. This upstream moving wave eventually reflects off the cavity forward wall and forces the shedding of a new vortex. It was found, however, that the reflected wave dissipates before it reaches the aft wall. Instead, a second wave forms beneath the shed vortex and eventually reflects from the aft corner and is forced at the shedding frequency by the shed vortex wave, completing the cycle.