DNS of hydrogen auto-ignition under HCCI-like conditions with wall heat transfer; parametric study

Abstract

Two-dimensional direct numerical simulations (DNSs) are performed to study the effect of wall heat transfer on the auto-ignition of lean hydrogen-air mixtures in the presence of temperature stratifications at conditions relevant to homogeneous charge compression ignition (HCCI) engines. The DNS are used to further advance combustion modelling for HCCI combustion previously developed using periodic boundary conditions. The simulations are initialised using the statistics available from a previous DNS study of compression heating under HCCI conditions which considered an idealised engine geometry [1]. Three different wall temperatures, leading to three different levels of temperature stratification, are chosen. The results for these wall-HCCI cases are compared to their counterparts with periodic boundary conditions. The heat release rate (HRR) for the wall-HCCI cases exhibit lower peaks and longer combustion duration compared to the cases with periodic boundaries. Using the DNS data for the wall-HCCI cases, an attempt is made to study the effect of the wall on the HCCI combustion from a modelling point of view, with particular attention to conditional moment closure (CMC). A first-order closure hypothesis with enthalpy (chemical + sensible) as conditioning variable and ignoring the spatial variations of conditional statistics is first tested and found to break down at high levels of stratification. Accounting for the dependence of conditional statistics on the distance from the wall cannot provide a closure either. Second order evaluation of the reaction rates, however, yields satisfactory closure, even without accounting for the spatial dependence of conditional statistics. The threeway interaction between chemistry, turbulence and wall is illustrated using temperature, enthalpy, mass fraction of H2O and the distance from the wall. Some remarks on the challenge of addressing the conditional fluctuations in the vicinity of the wall are made and the scope of future modelling work is outlined.