Impinging jet flow structure and its impact on combustion instability

Studies of impinging and single jet injectors are presented to elucidate the mechanism controlling combustion instability. Rocket chamber studies of an impinging jet injector demonstrate that atomization is a key mechanism in driving combustion instabilities. Pressure oscillations at frequencies corresponding to the first through third longitudinal modes were driven in a model combustor using electro-mechanically driven impinging jets. The ability to drive these very high-frequency pressure oscillations is consistent with the results from an empirical stability correlation. Studies to investigate the fluid mechanic mechanisms that affect the combustion instability behavior of impinging jet injectors were conducted using a single jet under cold flow conditions using water. Specifically, the effects of downstream impingement on the size of the boundary layer and its turbulent frequency characteristics showed no measurable changes when an impingement plate was introduced. This finding questions whether boundary layer modification represents a potential approach to controlling the combustion instability behavior of impinging jet injectors * Graduate Student f Research Associate, Member AIAA * Professor, Member AIAA § Senior Research Associate, Member AIAA. $ Research Assistant * Research Associate, currently Senior Principal Propulsion Engineer at Orbital Sciences Corporation, Chandler, AZ, Member AIAA. Copyright © 1998 by Robert J. Santoro. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. Introduction The current research effort is motivated by the critical need for an accurate a priori predictive capability for liquid rocket combustors. Liquid rocket engine combustion characteristics (efficiency, thermal compatibility, and combustion instability) are controlled largely by atomization. Of particular interest in the current research is the role of the atomization process in combustion instability and performance for impinging jet injectors. Previous studies conducted in our laboratory have provided a substantial base of fundamental understanding of the processes affecting atomization and its relationship to the initiation and sustenance of combustion instabilities. The role of impact waves formed at the jet impingement point has been argued to be a key process in the establishment of the breakup length, drop size distribution and atomization frequency for these injector elements. However, many questions remain as to how the formation of the impact waves is related to the turbulence characteristics of the flow, the orifice geometry and the response of impact waves to oscillations on the jet imposed upstream of the impingement point. The results presented in this paper describe two areas of current activity. The first area involves combustion studies in which an electro-mechanical device was used to introduce periodic oscillations in the liquid jet flow. The results have revealed that it is possible to initiate and sustain combustion instabilities at characteristic longitudinal frequencies of the rocket chamber in a controlled manner. The second area describes progress made to understand the basic flow structures that control the initiation and sustenance of these combustion instabilities. These studies are focused on the following objectives that seek to determine: 1 American Institute of Aeronautics and Astronautics 1. the correlation between the turbulent structures characterizing the jet and the impact wave formation. 2. the growth rate of the impact waves as a function of the frequency of the imposed upstream oscillations produced by a novel electro-mechanical device that generates well-characterized disturbances on the jet. 3. the effects of orifice geometry on the turbulence field and the impact wave formation process. 4. the manner in which jet turbulence can be altered by geometric effects to produce atomization characteristics that enhance combustion stability. Results obtained regarding the behavior of the boundary layer in circular jets and the effect of jet impingement on its structure will be presented. Preliminary results indicate that the impingement process downstream does not significantly affect the flow structure and turbulent frequency characteristics of the boundary layer in the injector orifice. Thus, control of impact wave formation through affecting boundary layer processes does not appear to be a viable combustion instability control approach based on the current study. Effects in the free jet region as well as the potential for other geometric effects to enhance the instability behavior of impinging jets remain to be studied.