Contribution of different turbulent scales to mixing and reaction between unpremixed reactants

The quality of prediction of the reaction rate from models based on the assumption of a single controlling stage for mixing has been tested by comparison with experimental data concerning the neutralization of NaOH with HCl in a tubular reactor with coaxial foods. The reactor was operated both in isokinetic and in jet-flow configuration and it was equipped with a fiber-optic spectrophotometer to measure the axial concentration profile or NaOH. A numerical code based on orthogonal collocation along the axis and finite difference along the radius has been developed for efficient simulations. The models based on the k/epsilon and on the (nu/epsilon)(1/2) time-scales have been found to be usually able to predict the length of the reaction zone and to furnish qualitative agreement with experimental data. In particular, the best results, among the tested relationships, were given by the engulfment model of Baldyga and Bourne. However, if a very accurate description of the reaction field is required, this approach is inadequate. especially for the jet-flow configuration, and more complex closures must be developed that correctly take into account the contribution from all the turbulence scales.