The rapid mixing of two miscible fluids is a precursor to promoting fluid-phase reactions. The configuration of a turbulent jet in a crossflow or tee-mixer is the most efficient passive design for rapid mixing. Here the Damkohler number has been defined from available tee-mixer data where the reactions are slow to fast compared with mixing times. For the available data the Damkohler number has been found to be of secondary importance for optimum design. The numerical simulation of jets in a tubular reactor suggests that large jet-to-tube momentum ratios with no backmixing are superior. Moreover the optimum design of maximum jet entrainment is shown to be synonymous with either a minimum in the relative standard deviation or the skewness. A formula relating the optimum diameter ratio to the jet-to-tube flow ratio has also been derived assuming geometrically similar jet trajectories, and the result correlates both numerical and experimental data.