(307f) Cross-Gradient Scalar Transport in Turbulent Shear Flows

Experimental studies of turbulent mass transport and mixing using simultaneous particle-image velocimetry (PIV) and planar-laser induced fluorescence (PLIF) were carried out for a confined-jet flow in an enclosed water channel and in the wake generated by splitter plates in the channel. Not only can the turbulent Schmidt number be measured for these flows, but results show that the downstream turbulent mass flux is greater than the cross-stream flux, despite the fact that the mean concentration gradient is cross-stream; in other words the directions of the turbulent mass flux and mean concentration gradient are not aligned. These results are consistent with earlier measurements of turbulent heat flux in a homogeneous turbulent shear flow by Tavoularis & Corrsin (1982). The jet flow and wake measurements also agree with large eddy simulations made for the same conditions. A simple model explains the results that the eddy diffusivity for turbulent mass transfer is not a diagonal tensor quantity in a shear flow.

Similar measurements using stereo-PIV and PLIF were made for a different geometry—swirling flow in a multi-inlet vortex reactor, scaled up from one developed for a flash-nanoprecipitation process. The model reactor is a short cylinder with four tangential inlets and a single withdrawal tube along the axis of the cylinder. A correlation between turbulent flux and vortex strength was determined, but no conclusion has been reached yet regarding alignment of flux and mean concentration gradient.