Abstract

Nonlinear combustion phenomenon was investigated through an experiment in a hybrid rocket motor. A poly(methyl methacrylate) (PMMA) / gaseous oxygen (GOx) combination was used with several types of disks equipped in a prechamber with the aim of modifying the local turbulent flow. By allowing this disturbance generated in a prechamber to interact with the shedding vortex inherently produced in the main chamber, a possibility of commonly observed nonlinear combustion feature such as DC-shift was analyzed. In a baseline test, a vortex shedding occurs due to the interaction of a main oxidizer flow with the evaporated fuel stream coming out of the surface during the regression process. Among the several types of disks, it turned out that only the disk4 produced the excitation which subsequently suppressed the vortex shedding phenomenon in the main chamber. This descent interaction was reflected in a sudden pressure drop (which may be described as direct current (DC) shift) of about 10 psi in the time history of the pressure during the nominal combustion. The present result with the disk4 suggests the possibility of phase cancellation between the excitation induced by the disk4 and the shedding vortex but much more work should be conducted to extract more accurate correlation of the phase information. In order to understand the baseline flow physics, a compressible large eddy simulation (LES) was conducted with the prescribed wall blowing boundary condition. The result clearly exhibited the existence of vortex shedding phenomenon with a specified frequency. The fact that important flow features of the present computation are quite similar to those obtained with an incompressible assumption in a flat channel suggests that both compressibility and curvature effects do not dominate in the present flow configuration.

Highlights

  • Hybrid rocket is attracting much attention recently because of its excellent safety features, relatively high performance compared to solid rocket motor, thrust control capability, and low development cost

  • All the data at the given three streamwise locations showed the peak around ω∗ ∼ 8.8. This clearly indicates that the quasi-steady vortex shedding was produced. These vortices were obviously produced in the shear layer lifted away from the wall where the main oxidizer §ow was being mixed with the wall injected §ow

  • Motivated by the fact that the nonlinear combustion in solid rocket motor was frequently caused by the resonance of the shedding vortices with external disturbances near the fuel surface, the present study made an attempt of identifying the role of those large-scale vortices produced in the shear layer near the fuel surface in the hybrid rocket motor

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Summary

INTRODUCTION

Hybrid rocket is attracting much attention recently because of its excellent safety features, relatively high performance compared to solid rocket motor, thrust control capability, and low development cost. The radial injector could induce vortex shedding in the prechamber and generate the periodic unsteady heat release Even with their ¦ndings that the interaction of periodic unsteady heat release with acoustic oscillations through vortex shedding had the triggering mechanism of DC-shift, they did not explain why negative DC-shifts were still observed in the test with certain §ow conditions in terms of vortex shedding and oscillatory §ow near the surface. Reviewed with experimental observations in solid and hybrid rocket combustion, it can be summarized that DC-shift is the result of the sudden increase in the regression rate due to an enhanced heat transfer to the propellant caused by the oscillatory gas motion associated with vorticity formation. Motivated by the necessity of a new scenario, this study put some emphasis on the analysis of the occurrence of speci¦c time-scale of surface §ow oscillation resulting from the §ow instability and the evolution of DC-shift linked with vortex shedding in hybrid rocket combustion. Since a circular duct better represents the real combustion geometry for the measurement tests, LES result is expected to better predict the §ow patterns and oscillatory motions near the surface

NONLINEAR CHARACTERISTICS OF HYBRID ROCKET COMBUSTION
LARGE EDDY SIMULATION OF A CYLINDRICAL DUCT FLOW WITH WALL BLOWING
Findings
SUMMARY
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