Ongoing CCSE research includes study of a rod-stabilized V-flame experiment in LBNL's EETD laboratory. Simulation issues related to turbulent premixed flames are discussed here, and background on the EETD laboratory flames is discussed here. The rod-stabilized V-flame is created by pushing premixed fuel though a 5-cm cylindrical nozzle that has been fitted with a perforated turbulence grid 7.5 cm or more upstream of the fuel exit plane. A thin rod is placed across the nozzle exit, creating a small flame-stabilizing recirculation zone in the flow. In the lab, the flame is ignited, and is anchored quite robustly at the rod for a large range of fuel flow rates.
The figures below show a photograph of the laboratory V-flame, and a respresentative calculation under identical conditions, which used 20 chemical species and 84 fundamental reactions. The computations were performed over a 12 cm cubic domain centered over the exit plane of the burner nozzle. In the photo on the left, the bluish color results from the fluorescence of exited flame radicals, and indicates the location of the primary reaction zone. Due to the finite shutter speed of the camera, the surface of this turbulent flame appears smeared out over a "flame brush thickness". On the right, the gradient of the simulation temperature field provides a convenient marker for the computed flame surface. The surface exhibits large-scale wrinkling of the instantaneous flame surface, and when averaged over time, shows remarkable agreement with the laboratory photos, in terms of brush thickness, spreading and growth rates.
In the experiment, the instantaneous location of the flame may be visualized using particle image velocimetry (PIV). Here, inert particles are distributed in the unburned gas with a uniform density. As the gas expands through the flame, the density of tracer particles decreases. The location of the abrupt change in particle density, as captured in the photograph below (left) indicates the instantaneous flame position. The photo may be compared to a representative planar slice of the simulated fuel concentration (right), since fuel is consumed at the flame front. The still images demonstrate exceptional agreement, both in terms of overall flame shape and brush growth characteristics, but also in terms of fine-scale wrinkling of the flame surface. Currently, we are working with EETD researchers Robert Cheng and Ian Shepherd to develop statistical measures of the simulation and experimental data so that we can obtain a more quantitative comparison. We are also working to understand the volumes of new simulation data in order to quantitively characterize how fuel-stream turbulence affects the detailed combustion process.
The adaptive low Mach number simulation code, an extension of CCSE code IAMR for incompressible flows, is not presently available for release. For more information about the adaptive methodology for low Mach number combustion modeling applications, or about these calculations, contact Marc Day or John Bell of CCSE.