Abstract
Transverse injection into supersonic flow after rearward-facing step combines a simple way to improve mixing and flameholding in Scramjet engines. In this essay, large-eddy simulation, NPLS, and PIV techniques are applied to study the flow characteristics of a sonic jet with different flow rates after rearward-facing step interacting with supersonic turbulent flow. Results show that since a big recirculation zone is generated by rearward-facing step before the jet, the barrel shock in the windward side is not so obviously compressed so that jet fluid enters the recirculation zone providing a flammable region for ignition and combustion stabilization. Besides, jet fluid bypassing the Mach disk comes directly into interaction with the turbulent boundary layer separated from the rearward-facing step to enhance vortices development in the further downstream. The comparisons of average boundaries abstracted from NPLS images with empirical penetration height equations show that mixing enhancement of transverse injection into supersonic flow after rearward-facing step has a close relationship with the jet to free-stream momentum. In small jet to free-stream momentum case, the mixing shear layer grows mainly in the near field of the jet, while in big jet to free-stream momentum case, the growth of the mixing shear layer could extend to the far-field wake flow zone of the transverse jet.
Highlights
The flow field inside a supersonic combustor is highly complex and presents a considerable challenge in developing a combustor geometry and a fuel injection control method which promote sufficient mixing of the air and fuel
In region around the jet exit, that is, region III, a bow shock is observed ahead of the jet, barrel shocks are observed at its periphery, the jet fluid forms a Mach disk as it expands into and bends toward the crossflow, and the interface or shear layer in the near-field jet region is confronted with the flow behind the bow shock
Region IV is the near-field wake flow zone of the transverse jet, where jet fluid bypassing the upstream of the Mach disk comes into interaction with the incoming turbulent boundary layer to prominent large scale coherent vortices in high shear downstream and the nanoparticles transported from the upstream reflect the wake flow structures
Summary
The flow field inside a supersonic combustor is highly complex and presents a considerable challenge in developing a combustor geometry and a fuel injection control method which promote sufficient mixing of the air and fuel. Transverse fuel injection behind a rearward-facing step is a flow configuration that has been suggested as a means of enhancing mixing and combustion completeness (the jet flow direction is vertical to the main flow direction). For the case of supersonic upstream flow, injecting fuel transversely behind a rearward-facing step enables the jet to penetrate well into the (locally lowspeed) flow before being turned by supersonic cross-flow. Several experimental and numerical investigations have been conducted to understand the mechanisms of the jet mixing and flameholding behind a rearward-facing step. Karagozian et al [1, 2] combined experimental data of transverse gas injection behind a rearward-facing step with theoretical analysis to develop a model for jet trajectories prediction. Aider and Danet [3] conducted large-eddy simulation to investigate the influence of upstream boundary conditions on the development of a rearward-facing step flow. Behrens et al [5] utilised reacting flow PIV to investigate flame anchorability behind a rearward-facing step by varying the equivalence ratio and near-field counterflow
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