High-speed tomographic PIV measurements of strain rate intermittency and clustering in turbulent partially-premixed jet flames

The effects of combustion on the strain rate field in turbulent jets were studied using 10 kHz tomographic particle image velocimetry (TPIV). Measurements were performed in three turbulent jets: a well-studied, piloted partially-premixed methane/air jet flame, Sandia flame C, with low probability of localized extinction; a second piloted jet flame, analogous to flame C but with a reduced pilot flow rate and a high probability of localized extinction; and a non-reacting air jet. Since the jet exit Reynolds number of approximately 13000 was nearly identical in the three jets, differences in the strain rate fields were attributed to the effects of combustion. Spatiotemporal characteristics of the strain rate field were analyzed. Overall, the strain rate norm was larger in the flames than in the non-reacting jet with the most stable flame having the largest values. In all three jets, the compressive strain rate was on average the largest of the three principal strain rates. At high strain rates, the ratios of the compressive and extensive strain rate to the intermediate strain rate were similar to those found in isotropic incompressible turbulent flows. The three-dimensional velocity measurements were used to analyze the spatial distribution of strain rate clusters, defined as singly-connected groups of voxels where the strain rate magnitude exceeded a threshold value. The presence of a stable flame significantly attenuated the number of clusters of intermediate strain rate. Strain rate bursts, corresponding to sudden increases in the number of clusters, were identified in the three jets. Bursts in the non-reacting jet and the unstable flame contained up to twice as many clusters as in the stable flame. The temporal intermittency of intense strain rate clusters was analyzed using the time-series measurements. Clusters with strain rates greater than five times the standard deviation of the strain rate norm were highly intermittent.