Cinema particle imaging velocimetry time history of the propagation velocity of the base of a lifted turbulent jet flame

Several basic assumptions in the theories of the liftoff of non-premixed turbulent jet flames were investigated experimentally. A new cinema particle imaging velocimetry (CPIV) technique was used to record the time evolution of the gas velocity field and the flame boundary. The time resolution (8000 CPIV images/s) was sufficient to obtain time histories of gas, flame, and propagation velocities conditionally measured at the rapidly moving flame base. Conditionally averaged flame propagation velocities (0.7 S-L and 1.2 S-L) are similar to the laminar flame speed S-L in the test cases considered (Reynolds numbers 4200 and 8500, respectively). The propagation velocities vary nearly linearly with conditionally measured turbulence levels, consistent with theory. The relatively low propagation velocities could be explained by strong flame-flow interactions that are not considered by theory. The gas and flame velocities are highly correlated in time. Average gas velocities ahead of the flame are reduced by factors of 3 to 4 compared to the non-reacting cases. Flow reversals (upstream gas velocities) ahead of the flame are observed. The dilatation velocity field induced by heat release influences the flow similar to a bluff body, reducing or reversing gas velocities ahead of the flame as it moves upstream. Some dynamics of flame stabilization are seen; individual vortices cause roll-up of the flame base and move it radially inward, subjecting it to larger gas velocities. Time histories show the appearance of isolated islands of flame upstream of the previous base location, suggesting azimuthal wrinkling of the flame.