Date of Award

Summer 1994

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical & Aerospace Engineering

Program/Concentration

Mechanical Engineering

Committee Director

A. Sidney Roberts, Jr.

Committee Member

E. Gartenberg

Committee Member

G. B. Northam

Committee Member

S. Tiwari

Committee Member

O. Baysal

Abstract

A comparative study of the interaction between wall mounted swept-ramp injectors and injector nozzle shape has been conducted in a constant area duct to explore techniques to enhance mixing in scramjet combustors. The scramjet combustors are currently being developed for propulsion systems applications on the envisioned hypersonic vehicles. Short combustor residence time, a requirement for fuel injection parallel to the main flow in the combustor, and an overall strong sensitivity of the vehicle performance to the propulsion system motivated the investigation. The swept-ramp injector investigated in this study produces vortex shedding and local separation downstream of the injector's nozzle exit, at the ramp's base. Six different injector nozzle inserts for preconditioning the fuel flow were tested to explore the interaction between the preconditioned fuel jet and the vortical flowfield produced by the ramp. The six injector nozzle inserts were: circular nozzle (baseline), nozzle with three downstream facing steps in the divergent section, nozzle with four vortex generators, elliptical nozzle, tapered-slot nozzle, and trapezoidal nozzle, all having equal exit and throat areas. The main flow was air at Mach 2, and the fuel was simulated by air injected at Mach 1.63 and by helium injected at Mach 1.7. The main portion of the research involved a series of experiments conducted at the NASA Langley Research Center in the Mach 2 Traverse Jet Facility. Extensive flow field surveys, combined with Mie and Rayleigh scattering visualization were used to investigate the flow field. An existing three-dimensional Navier-Stokes code was used to conduct a numerical study which closely tracked the experimental effort. The injector performance was evaluated in terms of both the experimental and computational results. The different nozzle inserts showed only minor performance differences, indicating that the injectant/air mixing in the far-field is independent of the injector geometry, molecular weight of the injectant, and the initial convective Mach number. However, the nozzle with vortex generators displayed the highest mixing performance, and there is evidence that the tapered-slot nozzle has a promising mixing performance. In the vicinity of the ramp base, the flow is dominated by the strong vortical flow field generated by the swept-ramp while well downstream of the base ramp the quasi-axisymmetric flow pattern indicates a "loss of memory" of the near-field stirring, the flow being controlled by small-scale turbulence. The flow field characteristics (mixing and losses) of this mixing scheme are determined by the ramp, the injector inner geometry having a very little effect. The injectant penetration into the main flow at an average angle that is equal to the initial injection angle, suggests that the penetration can be controlled by changing the injection angle.

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DOI

10.25777/ab1f-6k55

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