Images of dissipation layers to quantify mixing within a turbulent jet

Images were obtained that visualize the structure of the mixture fraction field and the structure of the scalar dissipation layers that exist near the base of a turbulent jet. To characterize the local mixing rate, measured profiles of the mean dissipation rate were compared with a general scaling analysis. In addition, the joint probability density function (zeta, chi) was measured. Some effects of adding coaxial air and swirl are discussed. The especially Large values of the mean dissipation rate (up to 30 s(-1)) and the instantaneous dissipation rate (up to 175 s(-1)) that were measured in the base region are due to the targe local values of the mean velocity gradient in this region. It was found that the dissipation layers in the base region have a unique structure; the layers tend to be aligned at approximately a 45-deg angle to the flow and thus differ from layers in the far field, which tend to be oriented in an isotropic pattern, as was shown previously. Strong dissipation layers exist at the boundary of the entrained air, and the observed 45-deg alignment in the base region is believed to be due to the ordered vortex pattern in the shear Layer. Dissipation layers are typically 0.3 mm thick. Dissipation rates varied in a manner consistent with the general scaling analysis, and the proper seating constant was measured to be 9300.