Turbulence Effects on Developing Turbulent Flames in a Constant Volume Combustion Chamber 930867
High speed Schlieren video and pressure trace analyses were used to study the turbulence effects on burning velocities in a constant volume combustion chamber. Propane-air and methane-air mixtures of equivalence ratios between 0.75 and 0.96 were ignited at 101 kPa and 296 K. Schlieren images of flame growth were recorded on video at 2000 frames per second while combustion chamber pressure was simultaneously recorded. Turbulence at ignition was up to 7 m/s intensity with 2 mm or 8 mm integral scale, produced by pulling a perforated plate across the chamber prior to ignition. In the analysis, the turbulence parameters during combustion were adjusted for the effect of decay and rapid distortion in a closed chamber.
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 from 0.75 to 0.96 and change of fuel from propane to methane had negligible effects on this relationship when normalized by laminar burning velocity. 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. The relationship of turbulent burning velocity to turbulence intensity remained linear, but the linear coefficient increased proportionally with flame size. This linear coefficient can be expressed as a linear function of flame radius. The linear function remained consistent for a given integral scale, regardless of fuel type and equivalence ratio changes. However, increasing the turbulence integral scale makes the turbulence less effective for a given flame size. In short, small scale turbulence (Λ ≈ 2 mm) is more effective in enhancing the burning velocity of a growing flame than larger scale turbulence (Λ ≈ 8 mm) at the same intensity.
For small flame kernels, the increasing turbulence effectiveness has been attributed to the flame kernel growing large enough to be affected by all turbulence scales. For flames much larger than the turbulence scale, the continued increase is attributable to the increasing flame surface corrugation. Larger flames have a longer history of turbulence effect and a greater surface to volume ratio. It is clear that great care must be taken to account for changes in turbulence intensity, turbulence scale, and flame size during experiments to produce meaningful and consistent results.