Development of an unsheared mixing technology by means of buoyant convection and ultrasound

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The unsheared mixing technology is required to avoid the destruction of the biocell or polymer molecules in bio- and polymerization reactors. The purpose of.this study is, therefore, to develop an unsheared mixing technology by means of body forces such as buoyant convection. The effects of unstable thermal-stratification on the chemical reaction and turbulent mixing were experimentally investigated both in reacting grid-generated turbulence and in turbulent reacting mixing-layer. Experiments were carried out in unsheared neutrally-stratified, unsheared unstably stratified and sheared neutrally-stratified conditions. Instantaneous velocity and concentration were simultaneously measured using both a laser Doppler velocimeter and a laser-induced fluorescence technique. The Large-eddy simulation (LES) based on the large-eddy probability-density function model was applied to the same thermally stratified flows.The results show that the turbulent mixing is enhanced at both large and small scales by buoyancy under unstably stratified conditions and therefore the chemical reaction is strongly promoted. The mean shear acts to enhance the turbulent mixing mainly at large scales. However, the chemical reaction rate in the sheared flow is not so large compared to the unstably stratified case with the same turbulence level, since the mixing at small scales in the sheared neutrally stratified flow is weaker than that in the unsheared unstably-stratified flow. The unstable stratification is regarded as a better tool to attain the unsheared mixing since the shearing stress acting on the fluid is much weaker in the unstably stratified flow than in the sheared flow. The effects of the stratification on the turbulent mixing and chemical reaction were well estimated by the LES.