Experimental study of vorticity-strain rate interaction in turbulent partially premixed jet flames using tomographic particle image velocimetry

摘要

In turbulent flows, the interaction between vorticity, omega, and strain rate, s, is considered a primary mechanism for the transfer of energy from large to small scales through vortex stretching. The omega-s coupling in turbulent jet flames is investigated using tomographic particle image velocimetry (TPIV). TPIV provides a direct measurement of the three-dimensional velocity field from which omega and s are determined. The effects of combustion and mean shear on the omega-s interaction are investigated in turbulent partially premixed methane/air jet flames with high and low probabilities of localized extinction as well as in a non-reacting isothermal air jet with Reynolds number of approximately 13 000. Results show that combustion causes structures of high vorticity and strain rate to agglomerate in highly correlated, elongated layers that span the height of the probe volume. In the non-reacting jet, these structures have a more varied morphology, greater fragmentation, and are not as well correlated. The enhanced spatiotemporal correlation of vorticity and strain rate in the stable flame results in stronger omega-s interaction characterized by increased enstrophy and strain-rate production rates via vortex stretching and straining, respectively. The probability of preferential local alignment between omega and the eigenvector of the intermediate principal strain rate, s(2), which is intrinsic to the omega-s coupling in turbulent flows, is larger in the flames and increases with the flame stability. The larger mean shear in the flame imposes a preferential orientation of omega and s(2) tangential to the shear layer. The extensive and compressive principal strain rates, s(1) and s(3), respectively, are preferentially oriented at approximately 45 degrees with respect to the jet axis. The production rates of strain and vorticity tend to be dominated by instances in which omega is parallel to the (s(1)) over bar-(s(2)) over bar plane and orthogonal to (s(3)) over bar. (C) 2016 AIP Publishing LLC.