Experimental Investigation of Injector Spacing Effects on Injector Behavior During Transverse Instability

An experimental rocket combustor was designed and tested to investigate the effects of injector element location on transverse combustion instability in ox-rich staged combustion type engines. The combustor consisted of a two-dimensional chamber with a linear array of nine injector elements. The combustor design was based on previous designs that presented self-excited instabilities. We built and tested three injector configurations; a baseline injector pattern with equal spacing between each of the elements. A second configuration to test the effects of reduced spacing between the central three elements, where the velocity fluctuations due to transverse instabilities would be greatest. and a third configuration to measure the effect of a close spacing between the outboard injector elements and the combustor wall. The combustor was fitted with windows to provide access for high-speed imagery of the reacting flows from the center and near-wall elements, and was instrumented extensively with high-frequency pressure transducers to map out the unsteady pressure field. We collected simultaneous measurements of pressure and flame emission to enable the calculation of the flame transfer functions. To provide the best possible measurements of heat release, gaseous methane was used as the fuel and the chamber was operated at a moderate pressure so that large windows with a wide field of view could be used. We successfully tested each of the three chamber configurations while collecting high frequency pressure data and high-speed video. All three configurations exhibited a wide variety of dynamic behavior that could be classified into three broad categories for comparison: steady, intermittent, and high-amplitude behavior. Transitions from lower to higher amplitude behavior were present and allowed the characterization of linear growth of self-excited instabilities. Statistical analysis showed no significant difference between the global pressure dynamics of the three configurations. However the experiments were able to provide information about the effects of injector spacing on individual injector behavior as detailed analysis of the high-speed chemiluminescence video data by Thomas Liu was able to detect differences in the behavior of individual injectors when their relative positions were changed