Measuring Turbulent Flame Growth by Visualization

High speed schlieren video and pressure trace analyses were used to study the effects of turbulence on burning velocity in a fixed volume combustion chamber. Lean methane-air mixtures of equivalence ratios of 0.76 and 0.96 were ignited at 1 atm and 23°C. Schlieren images of flame growth were recorded on video at 2000 frames per second while combustion chamber pressure was simultaneously recorded. The turbulence intensity at ignition was set at 0 m/s to 4 m/s intensity with integral scale around 7.6 mm by pulling a perforated plate across the chamber prior to ignition. In the analysis, the turbulence parameters were adjusted for the effect of decay and rapid distortion in a closed vessel during combustion.

Results of both video and pressure trace analyses show a linear relationship between turbulent burning velocity and turbulence intensity as expected. Moderate changes in equivalence ratio had a negligible effect on this relationship. In studying the flame growth from the ignition spark up to 55 mm flame radius, it was found that the effectiveness of turbulence increased dramatically as the flame grew. While the relationship of burning velocity to turbulence intensity remained linear, the strength of the proportional constant increased with increasing flame size. It is shown that this can be explained by relating the size of the flame to the turbulence integral scale. A flame must be much larger than the integral scale for the turbulence to be fully effective.