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

为了避免生物反应器和聚合反应器中的生物细胞或聚合物分子被破坏，需要使用免剪切混合技术。因此，这项研究的目的是通过浮力对流等体力来开发一种无剪切混合技术。实验研究了不稳定热层结对化学反应和湍流混合的影响，包括网格生成的湍流和湍流反应混合层。实验是在非剪切中性分层、非剪切不稳定分层和剪切中性分层条件下进行的。使用激光多普勒测速仪和激光诱导荧光技术同时测量瞬时速度和浓度。将基于大涡概率密度函数模型的大涡模拟(LES)应用于相同的热分层流动，结果表明，在非稳定分层条件下，浮力在大尺度和小尺度上增强了湍流混合，从而强烈地促进了化学反应。平均切变主要在大尺度上起增强湍流混合的作用。然而，与具有相同湍流度的非稳定分层流动相比，剪切流动中的化学反应速率并不是很大，这是因为剪切中性分层流动的小尺度混合弱于非剪切非稳定分层流动。在非稳定分层流动中，作用在流体上的剪应力比在剪切流动中弱得多，因此不稳定分层被认为是实现非剪切混合的较好工具。大涡模拟结果较好地反映了层结对湍流混合和化学反应的影响。

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.