Turbulent Chemical Reactions : Application to Atmospheric Chemistry

The general problem of predicting the rates of chemical reactions in turbulent fluid has been studied. Particular attention has been given to bimolecular (second-order) chemical reactions occurring isothermally. The problem of predicting the rates of turbulent chemical reactions has been formulated in both Eulerian and Lagrangian frameworks. The Eulerian formulation leads to a closure problem in the equations for the moments of concentrations of the reacting species. The Lagrangian formulation, while circumventing the usual closure problem, is based on a transition probability density function of particle displacements which can only be predicted in the simplest of cases. The principal result of the work is the development of a new closure method for second-order chemical reactions in turbulence. The method is referred to as the diffusion zone model. The essential idea of the model is that one defines the diffusion zone as that region of the fluid in which the reactive species would coexist if they were inert. Then, chemical reaction is presumed to occur only within the diffusion zone; outside the diffusion zone the concentrations of the reactive species are identical to those if the species were inert. The diffusion zone model is tested extensively on available laboratory data and gives good agreement. The diffusion zone closure method is adapted for predicting the rates of atmospheric chemical reactions occurring in plumes. The effect of inhomogeneous mixing on the reaction rates is explicitly accounted for in the formulation. In assessing this effect, a numerical planetary boundary layer velocity field is employed as a source of the necessary turbulence data. The model is applied to predicting the rates of nitric oxide oxidation in a plume from the Morgantown power plant in Maryland. Predictions are in good qualitative agreement with available measurements on the plume.