The effects of water vapor dilution on autoignition and detonation development induced by an ignition spot with thermal non-uniformity in an n-C7H16/air mixture are numerically investigated. Zero-dimensional homogeneous ignition under constant-volume conditions is studied first. It is found that excitation time increases, whereas total heat release decreases with a H2O vapor mole fraction. Moreover, the role of H2O vapor diluents as a third body considerably influences the critical temperature gradient. One-dimensional autoignition and detonation development caused by temperature gradients in ignition spots is then studied. Three different autoignition modes are identified: (I) supersonic deflagrative wave, (II) detonative wave, and (III) subsonic deflagrative wave. It is found that H2O dilution has a slightly better performance on detonation suppression than CO2 dilution. The chemistry–acoustics interactions during autoignition development are weakened when the H2O mole fraction is increased. Besides, H2O vapor dilution can delay the detonation initiation and reduce detonation intensity. Furthermore, typical autoignition processes induced by a hotspot and the chemical effects of water vapor diluent are discussed. It is seen that the chemical effects of H2O dilution do not affect the lower limits of detonation development curves. Besides, the third body effect from the H2O vapor diluent is important in suppressing the detonation development for the investigated ignition spot size. Finally, the effects of equivalence ratios and ignition spot sizes on the autoignition modes of n-C7H16/air/H2O mixtures are studied. It is observed that the water vapor diluted mixtures with the fuel-lean condition are advantageous in inhibiting detonation from localized thermal non-uniformity.

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