J. Fluid Mech. zyxwvutsrq (1991), vol. 233, pp. 211-242 Printed in Great Britain zyxwvutsr 21 1 Reaction in a scalar mixing layer By R. W. BILGER, L. R. SAETRANt AND L. V. KRISHNAMOORTHY Department of Mechanical Engineering, University of Sydney, NSW 2006, Australia (Received 18 July 1990 and in revised form 21 May 1991) zyx Reaction in a scalar mixing layer in grid-generated turbulence is studied experimentally by doping half of the flow with nitric oxide and the other half with ozone. The flow conditions and concentrations are such that the chemical reaction is passive and the flow and chemical timescales are of the same order. Conserved scalar theory for such flows is outlined and further developed; it is used as a basis for presentation of the experimental results. Continuous measurements of concentration are limited in their spatial and temporal resolution but capture sufficient of their spectra for adequate second-order correlations to be made. Two components of velocity have been measured simultaneously with hot-wire anemometry. Conserved scalar mixing results, deduced from reacting and non-reacting measurements of concentration, show the independence of concentration level and concentration ratio expected for passive reacting flow. The results are subject to several limitations due to the necessary experimental compromises, but they agree generally with measurements made in thermal mixing layers. Reactive scalar statistics are consistent with the realizability constraints obtainable from conserved scalar theory where such constraints apply, and otherwise are generally found to lie between the conserved scalar theory limits for frozen and very fast chemistry. It is suggested that Toor’s (1969) closure for the mean chemical reaction rate could be improved by interpolating between the frozen and equilibrium values for the covariance. The turbulent fluxes of the reactive scalars are found to approximately obey the gradient model but the value of the diffusivity is found to depend on the Damkohler number. 1. Introduction The turbulent flow behind a grid, which has at some upstream location a step change in temperature in the direction transverse to the mean flow direction, has been termed the thermal mixing layer. It has been studied experimentally by Watt z & Baines (1973)’ Keffer, 01sen & Kawall (1977), LaRue & Libby (1981), LaRue, Libby & Seshadri (1981), Ma & Warhaft (1986) and Gibson, Jones & Kanellopoulos (1989), and theoretically by Libby (1975), Durbin (1980), Wu & O’Brien (1982), Lumley (1986) and Gibson zyxwv et zyxwvutsrq al. (1989). Durbin’s stochastic model is presented in terms of species concentrations, but since heat and species are transported in a similar fashion in a turbulent flow at low Mach number the problem is analogous. We can speak in general terms of a scalar mixing layer. For small temperature differences and dilute species, variations in the fluid density and viscosity will be small and the effect of the scalars on the flow can be negligible. The scalars are then said to be passive. Study of the passive scalar mixing layer will help to elucidate many aspects t Present address: Division of Hydro and Gas Dynamics, NTH, University of Trondheim, Norway.