MICROMIXING IN GRID-GENERATED TURBULENCE - THEORETICAL-ANALYSIS AND EXPERIMENTAL-STUDY

The properties of the decaying turbulence downstream from a grid depend upon its design (diameter and spacing of the rods). Over a certain range the turbulent energy dissipation rate decays as the inverse square of the distance from the grid. The local rate of micromixing, which is due to the engulfing action of energy-dissipating vortices, then decreases linearly with increasing distance. It is first shown how micromixing in this inhomogeneous field can be calculated. Semi-batch reactor operation with a single, concentric feed tube, situated in grid turbulence, is considered. Modeling encompasses micromixing, self-engulfment and radial turbulent dispersion of the feed. The product distribution of four competitive-consecutive reactions is calculated by integrating three ordinary differential equations. Experimental results (30 runs), using various flow rates and flow rate ratios, feed locations, concentrations and feed pipes, are reported and compared with the model. The principal conclusion is that the feed tube was a source of additional turbulence, which also decays rapidly. A correlation, which includes the grid Reynolds number, the feed point location and the bore of the feed tube, accounting for this additional turbulence was established.