Combustion in cavities and accelerating flows

Thermodynamic analyses of gas turbine engine cycles have shown that adding energy in the turbine stage improves performance for both aero and stationary gas turbine systems. A curving and contracting test section with a cavity for flame holding was designed to mimic a turbine stator passage. A cavity was used for flame holding because it provided a low speed zone for mixing the fuel and air and allowed for the injection of liquid fuel into the test section. Combustion with a deep cavity (length/depth = 1) showed that for fixed fuel flow rates, as the air flow rate is increased, the combustion goes through three regimes. For very low flow rates, the combustion was confined to the shear layer and for high air flow rates, combustion was distributed through out the cavity and the shear layer and for intermediate flow rates, combustion was intermittent. A simple predictive model showed that the ratio of cavity velocity and the main flow velocity scaled as the square root of Reynolds number based on the momentum thickness. This result was corroborated by experiments. Combustion in a shallow cavity (length/depth = 2) showed that the interaction between the main flow and cavity was enhanced compared to deep cavities. This enhancement was seen as a increases in the shear layer spreading rate and the fluctuation of the shear layer and as an improvement in the temperature pattern factor at the exit compared to deep cavities. Configuration changes such as injection location and direction, and cavity location had very little effect on the combustion inside the cavity. Liquid fuel combustion was qualitatively similar to the gaseous fuel case. This suggested that the controlling factor in both cases was the mixing of the fuel and air. A simple time-scale analysis which compared the mixing in the cavity and the mixing in the shear layer showed that the ratio of these time scales varies significantly for the three regimes. A novel image processing technique based on the scale invariant features in images was used to determine velocities in reacting flows where flow seeding was not possible.