Isocontour plots of H2 consumption rate for fuel-air equivalence ratios (a) 0.6 and (b) 0.4. The inset denoted h-r in (a) corresponds to heat release rate.

Jacqueline H. Chen, Sandia National Laboratories, Livermore
Hong G. Im, University of Michigan
Ravi Subramanya and Reddy Raghumara,
Pittsburgh Supercomputing Center

Research Objectives
We are implementing novel computational and modeling tools to understand the effect of mixture composition and temperature variations in homogeneous charge compression ignition (HCCI) engines. A novel turbulence model coupled with autoignition chemistry will be developed and validated against direct numerical simulation (DNS) of turbulent autoignition and flame propagation. The research focus is on the creation of several DNS benchmark databases that will reveal the influence of unsteady stretch effects on turbulent premixed flame propagation and autoignition.

Computational Approach
DNS is used to solve the compressible turbulent reacting governing equations along with boundary and initial conditions. Detailed chemistry and transport models are incorporated including hydrogen-air and hydrocarbon-air mechanisms. Higher-order temporal integration and spatial discretization are used (eighth-order in space, fifth-order in time) along with error monitoring.

Accomplishments
Five parallel runs were made on the Cray T3E at NERSC. Two runs were made to investigate the effects of unsteady stretch and preferential diffusion on flame propagation in a turbulent premixed hydrogen/air flame. Three runs were made to simulate the effect of turbulent mixing on ignition delay times in a hydrogen/air scalar mixing layer.

We found that strong stretch/preferential diffusion interactions exist in a turbulent flame with computed Markstein numbers varying between –5.34 and 2.85 and area-weighted mean burning velocities 2.4 times the laminar speed. Markstein numbers derived from the DNS data compare favorably with experimental data. We observed that, over a broad range of mixture conditions, as the ratio of the characteristic turbulence to flame time decreases, the flame response to stretch is attenuated. This is consistent with theoretical predictions and recent unsteady counterflow computations.

Autoignition in a hydrogen/air scalar mixing layer in homogeneous turbulence with an initial counterflow of unmixed nitrogen-diluted hydrogen and heated air (1100 °K) showed that away from the steady state ignition turning point, the variation in ignition delay due to variations in turbulence intensity is small. However, for turbulence intensities near the steady ignition limit, ignition delay is sensitive to variations in strain rate.

Significance
HCCI engines are an attractive alternative to diesel engines, offering the potential for diesel-like efficiencies, while producing extremely low emissions without expensive aftertreatment. The primary technical challenge of HCCI is to control the in-cylinder fluid motion to obtain the desired heat release time profile across the load-speed map of the engine. The computational tools and DNS databases developed in this project will provide a significant jump start to HCCI design efforts.

Publications
J. H. Chen and H. G. Im, “Stretch effects on the burning velocity of turbulent premixed hydrogen-air flames,” in Proceedings of the Twenty-Eighth International Symposium on Combustion (The Combustion Institute, Pittsburgh, PA, 2000).

H. G. Im and J. H. Chen, “Structure and propagation of triple flames in partially-premixed hydrogen/air mixtures,” Combustion and Flame 119, 436 (1999).