Propagation speeds and stretch rates measured along wrinkled flames to assess the theory of flame stretch

Local propagation speeds and stretch rates were measured along a premixed flame that undergoes unsteady wrinkling in order to see if these two quantities correlate in the manner that is predicted by the theory of flame stretch. The Markstein number, which relates these two quantities, also was measured. Previous studies had considered the simple geometries of counterflow or spherical flames, but in this case a complex geometry was generated by interacting a flame with a vortex, such that both the strain and curvature components of the stretch rate are present. The diagnostics used were shadowgraph movies and simultaneous particle imaging velocimetry and OH planar laser-induced fluorescence. The overall conclusion is that the theory of flame stretch remains valid for these unsteady complex conditions, because the measured trends are found to be in agreement with trends predicted by the theory. That is, propagation speeds decrease at locations where positive stretch is applied to stable (lean propane-air and rich methane-air) flames. Conversely, propagation speeds increase where positive stretch was applied to unstable (lean methane-air) flames. The shape of the profiles of propagation speed along stable flames is opposite to that of unstable flames, as is predicted by the theory. However, values of Markstein number show large variations and are much larger than that of an outwardly propagating spherical flame. Negative strain regions are of particular interest because they previously had not been studied experimentally; these regions yield the largest propagation speeds for the stable cases and some negative speeds for the unstable cases.