Time sequence showing autoignition of hydrogen/air mixtures in a turbulent field. Isocontours of HO2 mass fractions at 1/2, 1, 3/2, and 2 autoignition induction times (from top left to lower right). The color ranges correspond to mass fractions of 0 for blue to maximum values of 0.001 for red. The color ranges and the corresponding scales are the same for all times, except the induction-phase image, where HO2 mass fractions are multiplied by 100. Ignition kernels A, B, and D show transition of the kernels to propagating fronts, while kernel C undergoes extinction due to high mixing rates.

Jacqueline Chen, Scott Mason, and Tarek Echekki, Sandia National Laboratories
Reddy Raghumara, Pittsburgh Supercomputing Center, Carnegie Mellon University

Research Objectives
This project seeks to further develop and apply direct numerical simulation (DNS) to understand the influence of turbulence on compression ignition, flame propagation, and burnout in compression ignition engine environments. With the insights obtained from these simulations, control strategies for the combustion timing and burn rate will be devised and optimized such that improved efficiencies and lower pollutant generation (especially NOx) will be achieved.

Computational Approach
DNS is used to solve the compressible turbulent reacting governing equations along with boundary and initial conditions. Higher-order temporal and spatial discretization is used (8th order in space, 5th order in time) along with error monitoring. The method is fully explicit, and the code has been written in MPI for scaleable parallelism. The code scales nearly linearly on Cray T3E, IBM SP3, SGI Origin 2000, and Compaq cluster platforms up to as many as 2048 processors.

Accomplishments
Interaction of premixed turbulent flames: The global burning rate response of twin premixed hydrogen/air flames embedded in homogeneous turbulence was determined with DNS with detailed chemistry. Superequilibrium radical bursts appear during mutual flame annihilation for fuel-rich mixtures, resulting in enhanced postflame burning rates that contribute to the global response.

Autoignition of inhomogeneous premixed hydrogen-air flames: The effect of turbulent mixing on autoignition of hydrogen/air mixtures in the second ignition limit were determined by DNS with detailed chemistry. Unlike in previous studies, it was shown that the scalar dissipation history influences the radical pool history during the induction phase, and also determines the rate of thermal runaway.

Significance
A homogeneous charge compression ignition (HCCI) engine operates on the principle of compressing a dilute premixed charge until it autoignites volumetrically. This is a relatively new realm of combustion, wherein the mixture is often sufficiently dilute so as to prevent flame propagation—hence, combustion occurs volumetrically. However, in practice, it is seldom the case that mixtures are truly homogeneous. In fact, mixture inhomogeneities are desirable to spread out the pressure rise in time so as to avoid engine knock. HCCI engines are an attractive alternative to diesel engines, offering the potential for diesel-like efficiencies, while concurrently 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.

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 Symposium (International) on Combustion, (Edinburgh, Scotland, July 30-August 4, 2000).

T. Echekki and J. H. Chen, "Direct numerical simulation of autoignition in inhomogeneous hydrogen-air mixtures," Paper 184, in Proceedings of the Second Joint Meeting of the U.S. Sections of the Combustion Institute (2001).

H. G. Im and J. H. Chen, "Effects of flow transients on the burning velocity of hydrogen-air premixed flames," in Proceedings of the Twenty-Eighth Symposium (International) on Combustion, (Edinburgh, Scotland, July 30-August 4, 2000).