Steam dilution effect on laminar flame characteristics of hydrogen-enriched oxy-combustion

Hydrogen-enriched oxy-combustion gas-turbine technology has been actively studied to meet the challenge of global decarbonization, zero emission, and pursuing higher energy efficiency. Motivated by the development of the Graz cycle, the objective of this work is to investigate the fundamental characteristics of hydrogen-blending oxy-combustion in a steam-diluted environment. The laminar flame speed and ignition delay time of H2/CH4/O2 with steam dilution are numerically studied over a wide range of H2O dilution ratios (up to 70%) and H2 contents (0-1) at atmospheric pressure. The results show that steam dilution leads to a reduction in the laminar flame speed and an increase in the ignition delay time. At low dilution ratios, the laminar flame speed decreases almost linearly, except for cases with high H2 contents. Conversely, at high dilution ratios, the decreasing trend of laminar flame speed diminishes and deviates from linearity. Furthermore, we find that the effect of H2 addition on the laminar flame speed can be categorized into three zones: linear hydrogen-promoting zone, transition zone, and linear methane-inhibited zone. With increasing dilution ratio, the linear hydrogen-promoting zone expands and the linear methane-inhibited zone reduces. A virtual H2O molecule is proposed to identify the thermal and chemical effects of steam dilution at different H2 contents. Generally, the thermal effect is the major role of steam dilution, and the chemical effect further reduces the laminar flame speed and increases the ignition delay time. The thermal effect on the laminar flame speed increases approximately linearly with dilution ratio, while the chemical effect exhibits a strong nonlinear variation which first increases and then decreases with increasing dilution ratio. Interestingly, the chemical effect of a small amount of H2O at high H2 content even promotes the laminar flame speed. For the ignition delay time, the chemical effect of H2O dilution consistently increases with dilution ratio. Moreover, the rate of production analysis indicates that the chemical effect of H2O dilution alters the radical pool structure and heat release rate, primarily attributed to the participation of H2O in reactions involving crucial radicals. Sensitivity analysis also shows that steam dilution and increasing H2 content amplify the sensitivity coefficients of rate constants of the elementary reactions.