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.