Control of Combustion Instabilities by Fuel Spray Modification Using Smart Fuel Injector

Abstract

This paper describes an experimental investigation of suppressing combustion instabilities using a smart fuel injector in a liquid fueled (n-heptane) combustor. The main objective is to determine the effects of primary and secondary swirling air flows generated within the injector upon fuel spray properties and recirculation zones which are responsible for the severity of combustion instabilities. The combustor consisted of a center-mounted, double-staged, air- assist atomizer. The swirl was imparted to both primary (inner stage) and secondary (outer stage) air streams in the counter direction through each set of tangentially oriented orifices. To produce a fuel spray, primary air flow was perpendicularly injected into the fuel stream at the exit of the fuel nozzle. Secondary air flow then impinged upon the generated spray, further developing atomization. Combustion occurred when they were mixed in a recirculation zone generated downstream of the injector face. Results show that the relative mass flow rate ratio of primary to secondary swirling air flows (K) greatly affects the fuel spray properties, flow field and, thus, combustion instabilities. For relatively lower K, huge variation of instability frequency and amplitude against the equivalence ratio was observed and a toroidal recirculation zone appeared in periphery near the inlet of the combustor. This zone was stretched downstream as K was increased. For relatively higher K, the recirculation zone developed along the centerline and the RMS pressure amplitude was nearly constant at about 6% of the combustor's mean pressure over the entire range of investigated equivalence ratios. The results of this study strongly suggest that such smart fuel injectors could be used to prevent the onset of detrimental combustion instabilities in real engines.