Product Recirculation and Mixing Studies in a Stagnation Point Reverse Flow Combustor

Abstract

Gas turbine combustor design is being driven towards leaner operation to conform to increasingly stringent emissions regulations. The low temperatures reduce NOx emissions but adversely impact the stability of the combustor. A novel combustor design, referred to as a Stagnation-Point Reverse-Flow (SPRF) combustor, was recently developed that is able to operate stably at very lean fuel-air mixtures, over a range of loadings, and with low NOx emissions making it an exciting option for ground power and aircraft gas turbines. Various optical diagnostic techniques are employed to investigate the flame and mixing characteristics of the SPRF combustor operating at atmospheric pressure. These measurements include chemiluminescence imaging, Particle Image Velocimetry (PIV) for velocity fields and Spontaneous Raman Scattering (SRS) for species concentrations. This unique reverse flow design allows hot product gases to mix with and preheat the incoming reactants. The flow reversal also gives rise to a low velocity stagnation zone in the combustor with a high turbulence level facilitating robust flame stabilization over a variety of flow conditions. Product entrainment has been quantified in terms of the recirculation ratio (Kv), defined as the ratio of recirculated mass of product gas to the mass of fuel and air entering the flame zone. Although reactants are found to burn in a highly preheated (1300 K) and turbulent environment due to mixing with hot product gases, the residence times are sufficiently long compared to the ignition timescales such that the reactants do not autoignite. Turbulent flame structure analysis indicates that the flame is primarily in the thin reaction zones regime throughout the combustor. The flame tends to become more flamelet like, however, for increasing distance from the injector. Fuel-air mixing measurements indicate that the fuel is shielded from hot products until it is fully mixed with air, providing almost premixed performance without its associated safety issues. The effect of changing the air injector dimensions on the mixing of fuel and air is also investigated.