Effects of Unstable Thermal Stratification and Mean Shear on Chemical Reaction and Turbulent Mixing in Grid-Generated Turbulence.

Abstract

The effects of unstable thermal stratification and mean shear on chemical reaction and turbulent mixing were experimentally investigated in a liquid mixing-layer downstream of turbulence generating grids with and without a rapid chemical reaction. Experiments were carried out in both an unsheared unstably-stratified flow and a sheared unstratified flow. Instantaneous velocity and concentration were simultaneously measured using a two-component laser Doppler velocimeter (LDV) and a laser-induced fluorescence (LIF) technique. From these turbulence signals, turbulence quantities such as intensities and power spectra of velocity fluctuations, turbulent mass fluxes and the Reynolds stress were estimated. Furthermore, the total amount of reaction product was estimated from the transverse concentration profiles of chemical product. The results show that the turbulent mixing is enhanced at both large-and small-scales by buoyancy under unstably-stratified conditions and that the chemical reaction is also promoted. The mean shear acts to enhance the turbulent mixing mainly at large scales but the chemical reaction rate is not so large compared to the unstably stratified case. The unstable stratification is a good tool to attain the unsheared mixing since the shearing stress acting on the fluid is much weaker in an unstably-stratified flow than in a sheared unstratified flow.