DNS of the effects of thermal stratification and turbulent mixing onH2/air ignition in a constant volume, and comparison with themulti-zone model

摘要

The inuence of thermal stratification on auto-ignition at constant volume and high pressure is studied by DirectNumerical Simulation (DNS) with complex H2/air chemistry with a view to providing better understandingof combustion processes in homogeneous charge compression ignition engines. In particular the dependence ofoverall ignition progress on initial mixture conditions is determined. The propagation speed of ignition frontsthat emanate from "hot spots" given by a temperature spectrum is monitored by using the displacement velocityof a scalar that tracks the location of maximum heat release. The evolution of the front velocity is comparedfor different initial temperature distributions and the role of scalar dissipation of heat and mass is identified.It is observed that both deagrative as well as spontaneous ignition front propagation occur depending uponthe local temperature gradient. It is found that the ratio of the instantaneous front speed to the deagrativespeed is a good measure of the local mode of propagation. This is verified by examining the energy and speciesbalances. A parametric study in the amplitudes of the initial temperature uctuation is performed and showsthat this parameter has a significant inuence on the observed combustion mode. Higher levels of stratficationlead to more front-like structures. Predictions of the multi-zone model are presented and explained using thediagnostics developed.