James F. Driscoll University of Michigan
Understanding the physics of how a turbulent flame extinguishes and blows out is an important and yet unsolved barrier that limits progress in the area of combustion science. It is desirable to extend the flame blowout limits in order to achieve lower nitric oxide pollutant levels and higher energy efficiency from stationary energy sources and from engines, but first the basic physics must be understood. At present, computational design codes cannot predict flame blowout limits because of the need for critical experimental data. This research project will obtain new insights into the flame blowout problem by making measurements using new laser diagnostics recently developed by the PI on a previous NSF project. High-speed movies will measure the local gas velocity field and the fuel and air concentration levels within a highly turbulent flame. Laser light sheets pass through the turbulent flame and the fluorescence is recorded at 20,000 frames per second by high-speed cameras. The images provide measurements of fundamental quantities that are needed by the computational models, including the local flame turbulent propagation speed, the aerodynamic strain rate and the flame index.
The novel aspects of the work are that the new laser diagnostics provide measurements of the true, three-dimensional flame propagation speed. They also provide information about the type of combustion that occurs, such as the locations were the fuel and air have been properly premixed and where they burn in a non-premixed manner. The high-speed movies will help to explain the dynamic, unsteady processes that previously could not be measured. The approach is to systematically vary the non-dimensional governing parameters and then attempt to collapse the data set to fit it to a set of simple scaling relations (formulas) that will be of general use. The high-speed movies will record fluoresecence from formaldehyde, acetone and nitrogen dioxide in order to image the concentrations of fuel, air and intermediate chemical species. Three dimensional measurements will be achieved by rapidly sweeping the laser light sheets.
The success of the work will lead to the 3-D data for the first time for understanding flame anchoring/blowout in turbulent flames spanning the whole spectrum of burning regimes premixed, partially premixed, and non-premixed. This is critical for developing codes for designing combustion devices. The study would further enable low-emission combustion (e.g., NOx), because once the flame anchoring/blowout mechanism is understood, the combustion conditions can be pushed toward as close to the lean limit conditions as possible, resulting in low temperatures and low levels of NOx.
