螺旋管内对流传质场协同强化模拟

Simulation on field synergy enhancement for convective mass transfer in helical tube

通过计算流体力学方法对层流直管内浓度场的模拟,发现大部分区域浓度梯度在径向比轴向高2个数量级。为了强化传质,应降低浓度梯度矢量和速度矢量的夹角,在径向产生一定的流动。对螺旋管内浓度场和速度场的计算表明,二次流横穿浓度等值线,提高了两场的协同。两种螺旋结构的比较表明,二次流随着Reyn-olds数的增大而增强,传质增强效果也越显著,正是二次流显著提高了速度场和浓度场的协同。所研究的螺旋结构在Re=1000～2400时,平均二次流速度最大达到主流速度的6.5%～6.8%,Sherwood数增大了4.99～6.43倍。

The field synergy principle for convective mass transfer indicates that Sherwood number depends not only on Reynolds number and the Schmidt number but also on the synergy of concentration field and velocity field.The numerical simulations are performed for laminar convective mass transfer and fluid flow characteristics in the straight tube,and the results show that concentration gradient in radial direction is 2 orders of magnitude larger than that in axial direction when Sc is 1.2,which means that the synergy of concentration field and velocity field is not good.In order to strengthen the mass transfer,the angle of velocity and concentration gradient should be decreased,in another word,the radial flow of fluid should be generated.From the observation and quantitative calculation of the velocity field and concentration field in the helical tube,it is found that the secondary flow crosses the equi-concentration line,and the presence of secondary flows improves the synergy of the two fields.For the same Re,the secondary flow is more intense,the effect of mass transfer is more significant.For the same helical structure,the secondary flow increases with Re.The secondary flows promote the synergy of concentration and velocity fields,which is the reason for helical tube to improve the mass transfer.The maximum of area-average secondary velocity reaches 6.5%—6.8% of bulk velocity,and Sh increases 4.99—6.43 folds when Re=1000—2400 for helical structure in the paper.It is shown that the field synergy principle well explains the mechanism of mass transfer enhancement in the helical tube.