Experimental and Numerical Studies on High-pressure Trapped Vortex Combustion

Abstract

Recent climate change issues demand stringent emission standards for natural gas-fired power generation applications. The Trapped Vortex Combustor (TVC) is a relatively new concept where the flame is stabilized using a physical cavity with direct injection of reactants. This configuration offers superior performance and emission benefits over conventional gas turbine combustors. The present thesis involves design and development of a unique high-pressure combustor with optical access to enable visualization and detailed experiments. Experimental studies on this high-pressure combustor are complemented by Reynolds Averaged Navier-Stokes (RANS) numerical simulations. The first part of the work involves numerical analysis of the TVC to understand the flame stabilization methods in the cavity and main flow of the combustor. The cavity flame can be stabilized in the upper vortex, lower vortex, or between the vortices based on the jet momentum flux ratio. It is observed from the simulations that the flame stabilization and pollutant emissions are related to the jet momentum flux ratio and the height of the main duct. These parameters regulate the penetration of the cavity flow into the main flow and entrainment of the main flow into the cavity. Insight from these numerical simulations is used in the design of the experimental facility. The second part of the thesis involves experimental studies at atmospheric pressure which bring out the effect of main flow velocity, jet momentum flux ratio, and cavity/main flow equivalence ratios on the static stability of the flame, pattern factor, combustion efficiency, and pollutant emissions. Efficient operation of the TVC requires higher values of jet momentum flux ratio, but the blockage of the main flow is the drawback. The cavity equivalence ratio is observed to be limited by the lean and rich blow-out limits. It is further observed that the fueling of the main flow is mandatory to achieve low pollutant emissions by preventing the quenching of the cavity flow reactants. The optimum control parameters derived from this phase of the study are used to investigate the performance of the TVC at high pressures in the third part of the thesis. An increase in the combustor efficiency and NOx emission is observed with a noticeable increase in combustor noise. It is observed that control of cavity equivalence ratio and decrease in jet momentum flux ratio are crucial for reducing the combustor noise at high pressure. A detailed experimental investigation is performed at 5 bar (145 kW) to understand the effect of fuel stratification on the performance of the combustor. The variation of the stratification ratio by fixing the flow parameters and overall equivalence ratio results in two modes of TVC operation, lean premixed (LP) and rich-burn quick-mix lean-burn (RQL). An optimum operating condition is identified for each operating mode of the TVC by comparing pattern factor, combustion efficiency, NOx emission and combustor noise level. In the last part of the thesis, the dynamic stability of the combustion process is studied by analyzing the pressure time-series data and high-speed chemiluminescence images of the OH* radical. Large amplitude pressure oscillations (2% of mean combustor pressure) are observed when the cavity equivalence ratio transitions from the LP to the RQL regime. The OH* images exhibit oscillations near the cavity bottom for cavity equivalence ratio near 1.4 leading to limit cycle oscillations (LCO). A Spectral Proper Orthogonal Decomposition (SPOD) analysis is conducted to understand the dominant mechanism of the oscillations for optimum LP and RQL operations and the LCO condition. The shear layer oscillation on the top of the cavity is observed to be the dominant mechanism in all conditions except for the LCO. The high-pressure TVC is optimized to operate in both LP and RQL regimes. The LP operation offers ultra-low NOx emission (3 ppm), whereas the RQL operation leads to relatively lower combustor noise. Overall, the results can be used to design and choose optimum conditions for the TVC operating on natural gas fuel.