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Experimental and Numerical Investigation of Laminar Flame Characteristics of Isooctane/air Mixtures at High Preheating Temperatures and H2O Dilution Ratios

ABSTRACT Liquid water injection technology is an effective method to improve the fuel economy and reduce the emissions of an internal-combustion engine. In this study, the laminar burning velocity and Markstein length of isooctane/air mixtures at a fixed pressure of 101.3 kPa, H2O dilution ratios of 0–10%, and preheating temperatures of 400–500 K was investigated using a combustion bomb and high-speed schlieren photography. The different effects of water vapor and preheating temperatures on the laminar burning velocity of isooctane/air mixtures were analyzed using the CHEMKIN software. The physical (dilution and thermal) and chemical effects of water vapor on the laminar burning velocity were identified. The results demonstrate that the laminar burning velocity decreases linearly with an increasing H2O dilution ratio and increases exponentially as the preheating temperature increases. The Markstein length increases gradually with the increase of H2O dilution ratio. The laminar burning velocity decreases by 0.13 m/s-0.17 m/s with 1% H2O dilution ratio at a preheating temperature of 400 K. The addition of water vapor decreases the concentration of free radicals. The physical and chemical effects increase linearly as the H2O dilution ratio increases. The physical effects are the main reason for the decrease in the laminar burning velocity. The chemical effects reduce the mole fraction of free radicals (H and O) and increase that of OH, which increases the net reaction rate of the reaction O + H2O = 2OH.