Ignition delay study of moist hydrogen/oxidizer mixtures using a rapid compression machine

Autoignition of moist hydrogen/oxidizer mixtures has been studied experimentally using a rapid compression machine (RCM). This work investigated the effect of water addition on ignition delays of stoichiometric hydrogen/oxidizer mixtures in the end of compression temperature range of TC = 907–1048 K at three different end of compression pressures viz. PC = 10 bar (1 MPa), 30 bar (3 MPa), and 70 bar (7 MPa). RCM experiments were conducted with 0%, 10%, and 40% molar percentages of water in the reactive mixture. At PC = 30 bar and 70 bar, the presence of 10% and 40% water vapor was shown to promote autoignition. However, at PC = 10 bar, water addition (10%) was seen to retard the reactivity, thereby increasing the ignition delay. Comparison with different reaction kinetic mechanisms reported in literature shows widely different results of simulated ignition delays for the temperature and pressure range studied, although most of the mechanism predictions demonstrate similar trend in ignition delay with water addition. A recent chemical kinetic mechanism, which shows good agreement with the present experiments at higher pressure but some discrepancy at lower pressure, was used for brute force sensitivity analysis in order to identify the important reactions for the dry mixtures in the temperature and pressure window investigated. An important reaction identified was further adjusted within the uncertainty limit as an attempt to improve the results from mechanism prediction for the ignition delay at low pressure (PC = 10 bar) without water addition. In addition, the modification in the reaction rate leads to good agreement between the experiment data and the mechanism prediction for the moist mixtures at varying compressed pressures.

► Water addition effect on H2 autoignition is studied using rapid compression machine. ► Water addition leads to longer ignition delay at lower pressures. ► Water addition leads to shorter ignition delay at higher pressures. ► Experimental results are simulated using detailed reaction mechanism. ► High collision efficiency of water causes the opposing ignition behavior.