High-Fidelity Simulation and Analysis of Ignition Regimes and Mixing Characteristics for Low Temperature Combustion Engine Applications.

Abstract

Computational singular perturbation (CSP) technique is applied as an automated diagnostic tool to classify ignition regimes in low temperature combustion (LTC) engine environments. Various problems representing LTC are simulated using high-fidelity computation with detailed chemistry for hydrogen-air, and the simulation data are then analyzed by CSP. The active reaction zones are first identified by the locus of minimum number of fast exhausted time scales. Subsequently, the relative importance of transport and chemistry is determined in the region ahead of the reaction zone. A new index I T , defined as the sum of the absolute values of the importance indices of diffusion and convection of temperature to the slow dynamics of temperature, serves as a criterion to differentiate spontaneous ignition from deflagration regimes.

The same strategy is then used to classify ignition regimes in n-heptane air mixtures. Parametric studies are conducted using high-fidelity simulations with detailed chemistry and transport. The mixture at non-NTC conditions shows initially a deflagration front which is subsequently transitioned into a spontaneous ignition front. For the mixtures at the NTC conditions which exhibit two-stage ignition behavior, the 1st stage ignition front is found to be more likely in the deflagration regime. On the other hand, the 2nd stage ignition front occurs almost always in the spontaneous regime because the upstream mixture contains active radical species produced by the preceding 1st stage ignition front. The effects of differently correlated equivalence ratio stratification are also considered and the results are shown to be consistent with previous findings. 2D turbulent auto-ignition problems corresponding to NTC and non-NTC chemistry yield similar qualitative results.

Finally, we look into the modeling of turbulent mixing, in particular, the scalar dissipation rate, in the context of flamelet approach. This involves a number of aspects: (i) probability density functions, (ii) mean scalar dissipation rates, and (iii) conditional scalar dissipation rates, for mixture fraction (Z) and total enthalpy (H). The validity of existing models both in the RANS and LES contexts is assessed, and alternative models are proposed to improve on the above three aspects.