Numerical simulation of spray autoignition by the first-order conditional moment closure model

Abstract

The ignition delay time of a n-heptane spray is calculated by the first-order conditional moment closure (CMC) model. At each time step the mixing field is calculated with the spray models in KIVA3. The CMC equations are then solved by the fractional step method, which sequentially considers transport and reaction terms. The evaporation terms in the variance equation of mixture fraction are treated in three different ways: no source, the model by Holman and Gutheil, and the one-droplet model. They all show similar spatial distributions with differences in the ignition delay time within 0.5%. The evaporation terms in the CMC equations are also treated in three different ways: no source, as a boundary flux on the fuel side, and the one-droplet model. They do not have any noticeable influence on the conditional profiles near the stoichiometric mixture fraction, where most reactions occur. A parametric study is performed to investigate the influence of initial temperature, drop size distribution, spray angle, and injected fuel quantity. The computed ignition delay times show a reasonable comparison with the measurements under different initial temperatures.