Center for Computational Sciences and Engineering, NERSC,
Lawrence Berkeley National Laboratory

Research Objectives

Our objective is to develop and validate high-fidelity numerical models that can accurately represent both the chemical and fluid-mechanical behavior of combusting hydrocarbons in a turbulent environment.

Computational Approach

The principal computational tool for this project is the low Mach number adaptive mesh refinement algorithm developed by the CCSE at NERSC. This methodology provides an accurate and efficient approach for modeling reacting flows in the regime that is appropriate for engineering applications. The algorithm uses a fractional step discretization that easily facilitates the inclusion of complex kinetics mechanisms. The methodology uses a block-structured refinement approach that allows computational effort to be focused in regimes of the flow where it is required. The structured refinement approach provides a natural coarse-grained parallelism that has demonstrated excellent performance and scalability on distributed memory architectures.

Accomplishments

During FY99, we have made substantial improvements to our methodology in two areas. In the algorithmic area, we have completed the parallelization of both the compressible and low Mach number versions of our adaptive methodology for distributed memory architectures. Computations using this methodology show that the data distribution and load balancing mechanisms we have developed provide an efficient scalable implementation of our adaptive algorithms in a framework that isolates the parallel implementation from the core physics modules for a particular application.

We have also generalized the low Mach number combustion methodology to allow for arbitrarily complex chemical kinetics and transport packages using an interface to CHEMKIN. The new methodology supports complex reaction mechanisms and differential diffusion in a low Mach number formulation that conserves both species and enthalpy while maintaining second-order accuracy of the overall discretization.

Mole fraction of CH in a premixed methane flame. The flame, initially flat, is perturbed by a counter-rotating vortex pair in the reactant stream. (The gas flows upward here, and only the left half of the symmetric vortex/flame interaction is shown.) The vortex pair modifies the gas composition ahead of the flame, and in fuel-rich cases, significantly alters the diffusion and chemical pathways that lead to CH formation. The computed results are in good agreement with experimental observation.

Significance

The modeling of turbulent fluid flow in realistic engineering geometries, even in the non-reacting case, remains one of the great scientific challenges. For realistic combustion scenarios, the picture becomes more complex because small-scale turbulent fluctuations modify the physical processes such as kinetics and multiphase behavior. These processes, in turn, couple the small scales back to the larger fluid-dynamical scales as chemical constituents react. As a result of this coupling, we must capture the structure of the subgrid fluctuations to make predictions. The use of average quantities as inputs to physical processes will generate large errors through interaction of these models. Developing techniques that accurately reflect the role of small-scale fluctuations on the overall macroscopic dynamics would represent a major scientific breakthrough.

The range of length scales involved in practical engineering devices precludes the possibility of a direct numerical simulation in which all the relevant length scales are resolved. Consequently, any attempt to model realistic devices such as engines and furnaces requires some type of turbulent combustion model that represents the subgrid interplay between turbulence and kinetics. The goal of this project is to develop these types of models.

Publications

C. A. Rendleman, V. E. Beckner, M. Lijewski, W. Y. Crutchfield, and J. B. Bell, "Parallelization of structured, hierarchical adaptive mesh refinement algorithms," Computing and Visualization in Science (in press); LBNL-43154 (1999).

M. S. Day and J. B. Bell, "Numerical simulation of laminar reacting flows with complex chemistry," Combust. Theory Modelling (submitted); LBNL-44682 (1999).

J. B. Bell, N. J. Brown, M. S. Day, M. Frenklach, J. F. Grcar, and S. R. Tonse, "The effect of stoichiometry on vortex flame interactions," 28th Symp. (International) on Combustion (submitted); LBNL-44730 (1999).

http://www.seesar.lbl.gov/ccse/