轴流桨搅拌槽内完全湍流的研究

A Study on Fully Turbulent Flow in an Axially Stirred Tank

为研究轴流桨搅拌槽内完全湍流状况,采用相位多普勒粒子分析仪(PDPA)对MK和ZHX搅拌器进行流场测试,得到不同工况下的时均速度场分布。应用渐近不变性方法,选取适当的特征尺度,给出挡板处壁面射流的轴向速度相似剖面,确定用于评定槽内完全湍流界限的轴向速度分布曲线,建立了搅拌雷诺数与搅拌槽内完全湍流流动达到的高度之间的线性关系。结果表明,非全槽完全湍流状态下,槽上部会出现过渡流区;随雷诺数的增大,搅拌槽内完全湍流流动达到的高度增大;不同型式搅拌器的完全湍流流场所需的雷诺数不同,单层桨搅拌槽内达到全槽完全湍流需要很大的搅拌功率。

The single phase turbulent flow in a fully baffled, flat bottomed cylindrical tank of diameter 300 mm equal to the liquid height, stirred with two different axial flow impellers, a ZHX propeller and a 45°pitched blade turbine impeller, was investigated by using Phase Doppler Particle Analyzer （PDPA） measurement technique. Mean flow velocities at many points in front of the baffle were obtained under different operating conditions. The limits of fully developed turbulence in the bulk of a stirred tank were examined by using dimensional analysis and asymptotic invariance. The flow field was made dimensionless by using a characteristic length scale and a characteristic velocity scale. Similarity profiles of axial velocity were examined in the wall jet at the baffle. An accurate regression curve was carried out by using enough data points from various experimental configurations. If the dimensionless profile of axial velocity deviates from the regression cure, the traverse was classified as transitional. There was a linear relationship between the Re and the turbulent height of the fluid in a stirred tank. The results indicated that the flow regime at the top region could drop into the transitional flow regime when the flow in the bulk of a stirred tank was not full turbulent. The height of the fully turbulent flow region in the stirred tank increased clearly with the increase of Re. The Re value required to attain fully turbulent flow was different for different axial agitators, and a very large amount of power was needed to maintain fully turbulence throughout the tank in the single impeller system.