Convective heat transfer can be enhanced effectively by longitudinal vortices

thus it is meaningful to study the swirling flow induced by longitudinal vortex generator in exploring the heat transfer enhancement mechanism.The structure of the flow field and the flow characteristics in a tube with trapezoid winglets embedded were investigated by performing particle image velocimetry(PIV)measurement in both transverse and longitudinal sections.The trapezoid winglets were uniformly distributed over the circumference upstream placed at an angle of 135° to the tube wall.The experimental results showed that multi-longitudinal vortices flow was induced downstream of the winglets with four symmetrical counter-rotating vortex pairs.Two different flow patterns were in the near wall region

one was the flow towards the wall and the other was the flow away from the wall

both of which effectively increased the component of the transverse velocity perpendicular to the mainstream.In a certain range of Reynolds number involved in the experiment

it could be drawn that the maximum value of transverse velocity and the radial velocity in the sections downstream the winglets could reach 27% and 20% of the averaged mainstream velocity respectively.The continuous disturbance to the flow induced by the longitudinal vortices proceeded downstream along the tube wall

which increased the velocity of the flow near the wall significantly

by 1.0-3.6 times of that in the smooth tube.The fields of velocity and vorticity showed that the disturbance turned out to be more powerful with the increase of Reynolds number.Compared with the flow field in the smooth tube

it was found that the flow mixing generated by the winglets enhanced mass transfer between the mainstream and the fluid in the near wall region due to its enhancement of radial velocity component perpendicular to the wall

which was beneficial for convective heat transfer enhancement in the tube.