The effects of unstable stratification and mean shear on the chemical reaction in grid turbulence

The effects of unstable thermal stratification and mean shear on chemical r eaction and turbulent mixing were experimentally investigated in reacting a nd non-reacting liquid mixing-layer flows downstream of a turbulence-genera ting grid. Experiments were carried out under three conditions: unsheared n eutrally stratified, unsheared unstably stratified and sheared neutrally st ratified. Instantaneous velocity and concentration were simultaneously meas ured using the combination of a laser-Doppler velocimeter and a laser-induc ed fluorescence technique. The results show that the turbulent mixing is en hanced at both large and small scales by buoyancy under unstably stratified conditions and therefore the chemical reaction is strongly promoted. The m ean shear acts to enhance the turbulent mixing mainly at large scales. Howe ver, the chemical reaction rate in the sheared flow is not as large as in t he unstably stratified case with the same turbulence level, since the mixin g at small scales in the sheared neutrally stratified flow is weaker than t hat in the unsheared unstably stratified flow. The unstable stratification is regarded as a better tool to attain unsheared mixing since the shearing stress acting on the fluid is much weaker in the unstably stratified flow t han in the sheared flow.