利用能量耗散率计算管流的平均剪切率

CALCULATION OF AVERAGE SHEAR RATE IN PIPE FLOW BASED ON ENERGY DISSIPATION RATE

含蜡原油的流变性与其所经历的剪切历史有关。以往用截面平均剪切率或湍流时的管壁剪切率衡量流体管流的剪切历史 ,有不当之处。根据能量耗散率与剪切率的关系 ,提出了按流体的体积平均能量耗散率计算流体平均剪切率的方法 ,即可根据流体的密度、平均流速、管道直径、流动的摩擦因子或摩阻压降以及流变参数计算牛顿流体和幂律流体管流的平均剪切率。对于湍流 ,按本方法计算的平均剪切率能合理地度量流体的平均剪切历史。在原油管道正常运行的雷诺数范围内 ,按截面平均方法计算的湍流剪切率比按体积平均能量耗散率计算的结果约偏低 2 0 %～ 40 %。

Flow properties of a waxy crude are influenced by shear history.The cross_sectional average shear rate or the shear rate at the pipe wall in turbulent flow was unreasonably used to evaluate shear history in pipe flow.According to the relationship between the shear rate and the energy dissipation rate,a method for calculating the average shear rate based on the volumetrically average energy dissipation rate is presented.This method may be easily used to calculate the average shear rate in pipe flow of the Newtonian and the power law fluids according to the fluid density,average velocity,pipe diameter,the frictional factor or frictional pressure drop,and the rheological parameters.The average shear rate obtained with this method is a more reasonable measurement of the average shear history of fluid in turbulent pipe flow.Within the range of the Reynolds number commonly encountered in crude oil pipelines,the cross_sectional average shear rate of pipeflow is less by 20%～40% than that calculated by using the presented method.