Spectrometric Characterization Of The Reaction Zone In An Unstable Liquid Fueled Combustor

Abstract

This paper describes an experimental investigation of suppressing combustion instabilities using a fuel injector in a liquid fueled (n-heptane and jet-A) combustor. The main objective of this study was to demonstrate that by varying the fuel spray characteristics it is possible to change the characteristics of the combustion process, thereby decoupling the combustion heat release and combustor pressure oscillations and preventing combustion instabilities. Spectrometric methods for characterizing the combustion process are developed and compared for both heptane and jet-A fuels; these methods are then used to correlate properties of the combustion process with instabilities in the combustor. The smart fuel injector used in this study consisted of a double-staged, air-assist atomizer. Counter swirling, primary (inner stage) and secondary (outer stage) air streams were supplied to the injector through separate sets of tangentially oriented orifices. To atomize the fuel, the primary air flow was injected next to the exit of the fuel nozzle where it impinged upon the injected fuel jet. The secondary air flow was injected into an annular, external volume, downstream of the fuel injection point where it further atomized the fuel. The combustion process was stabilized downstream of the secondary air injection point. This study shows that the ratio of the primary to secondary swirling air flow rates (K) strongly affects the fuel spray properties, flow field and, thus, combustion instabilities. At relatively low values of K, the instability frequency and amplitude strongly depends upon the equivalence ratio and a toroidal recirculation zone is stabilized near the wall at the combustor inlet. This zone extends further downstream as K increases. For relatively high values of K, the recirculation zone develops along the centerline and the RMS pressure amplitude remains nearly constant at about 6% of the combustor's mean pressure over the entire range of investigated equivalence ratios. The results of this study demonstrate the usefulness of the spectrometric methods for characterizing the combustion process and strongly suggest that smart fuel injectors could be used to prevent the onset of detrimental combustion instabilities in real engines.