Controlling fire to provide heat and light was one of humankind’s first great achievements, and the basic chemistry of combustion—what goes in and what comes out—was established long ago. But a complete quantitative understanding of what happens during the combustion process has remained as elusive as the ever-changing shape of a flame.

Even a simple fuel like methane (CH₄), the principal component of natural gas, burns in a complex sequence of steps. Oxygen atoms gradually replace other atoms in the hydrocarbon molecules, ultimately leaving carbon dioxide and water. Methane oxidation involves about 20 chemical species for releasing energy, as well as many minor species that can become pollutants. Turbulence can distort the distribution of species and the redistribution of thermal energy which are required to keep the flame burning. These turbulence–chemistry interactions can cause the flame to burn faster or slower and to create more or less pollution.

The holy grail of combustion science has been to observe these turbulence–chemistry effects. Over the past few decades amazing progress has been made in observational techniques, including the use of lasers to excite molecules of a given species and produce a picture of the chemical distribution. But laser imaging is limited in the species and concentrations that can be reliably observed, and it is difficult to obtain simultaneous images to correlate different chemical species.

Figure 6. The calculated surface of a turbulent premixed laboratory methane flame.

Because observing the details of combustion is so difficult, progress in combustion science has largely coincided with advances in scientific computing. For example, while basic concepts for solving one-dimensional flat flames originated in the 1950s, it only became possible to solve the 1D flame equations some 30 years later using Cray-1 supercomputers. Those calculations, which are routine on personal computers today, enabled a renaissance in combustion science by allowing chemists to observe the interrelationships among the many hypothesized reaction processes in the flame.

Simulating three-dimensional turbulent flames took 20 more years of advances in applied mathematics, computer science, and computer hardware, particularly massively parallel systems. But the effort has been worth it. New 3D simulations are beginning to provide the kind of detailed information about the structure and dynamics of turbulent flames that will be needed to design new low-emission, fuel-efficient combustion systems.

The first 3D simulation of a laboratory-scale turbulent flame from first principles—the result of a SciDAC-funded collaboration between computational and experimental scientists at Berkeley Lab—was featured on the cover of the July 19, 2005 Proceedings of the National Academy of Sciences (Figure 6).4 The article, written by John Bell, Marc Day, Ian Shepherd, Matthew Johnson, Robert Cheng, Joseph Grcar, Vincent Beckner, and Michael Lijewski, describes the simulation of “a laboratory-scale turbulent rod-stabilized premixed methane V-flame.” This simulation was unprecedented in several aspects—the number of chemical species included, the number of chemical reactions modeled, and the overall size of the flame.

This simulation employed a different mathematical approach than has typically been used for combustion. Most combustion simulations designed for basic research use compressible flow equations that include sound waves, and are calculated with small time steps on very fine, uniform spatial grids—all of which makes them very computationally expensive. Because of limited computer time, such simulations often have been restricted to only two dimensions, to scales less than a centimeter, or to just a few carbon species and reactions.

                 Simulation                                  Experiment
Figure 7. Left: A typical centerline slice of the methane concentration obtained from the simulation. Right: Experimentally, the instantaneous flame location is determined by using the large differences in Mie scattering intensities from the reactants and products to clearly outline the flame. The wrinkling of the flame in the computation and the experiment is of similar size and structure.

In contrast, the Center for Computational Sciences and Engineering (CCSE), under Bell’s leadership, has developed an algorithmic approach that combines low Mach-number equations, which remove sound waves from the computation, with adaptive mesh refinement, which bridges the wide range of spatial scales relevant to a laboratory experiment. This combined methodology strips away relatively unimportant aspects of the simulation and focuses computing resources on the most important processes, thus slashing the computational cost of combustion simulations by a factor of 10,000.

Using this approach, the CCSE team has modeled turbulence and turbulence–chemistry interactions for a three-dimensional flame about 12 cm (4.7 in.) high, including 20 chemical species and 84 fundamental chemical reactions. The simulation was realistic enough to be compared directly with experimental diagnostics.

The simulation captured with remarkable fidelity some major features of the experimental data, such as flame-generated outward deflection in the unburned gases, inward flow convergence, and a centerline flow acceleration in the burned gases (Figure 7). The simulation results were found to match the experimental results within a few percent. This agreement directly validated both the computational method and the chemical model of hydrocarbon reaction and transport kinetics in a turbulent flame.

The results demonstrate that it is possible to simulate a laboratory-scale flame in three dimensions without having to sacrifice a realistic representation of chemical and transport processes. This advance has the potential to greatly increase our understanding of how fuels behave in the complicated environments inside turbulent flames.

Research funding: BES, ASCR, SciDAC
Computational resources: NERSC
This article written by: John Hules, Jon Bashor