The flow field in a rectangular channel with a transversely placed spiral coil insert is investigated by Particle Image Velocimetry (PIV) measurements in both transverse and longitudinal sections.The experimental results show that multi-longitudinal vortices are induced downstream of the spiral coil and are distributed as a symmetrical vortex array along the horizontal central line of the transverse section.Along the mainstream,due to the spiral motion of the longitudinal vortices,the velocity fluctuates in a manner of the damped sinusoidal curve and the velocity component in the wall normal direction is improved in the channel.Compared with the flow field in a smooth channel it is found that the movement of the longitudinal vortices can cause a continuous disturbance near the channel walls and thus enhances the fluid velocity in the near wall region,which consequently leads to the reduction of the velocity gradient and a more uniform velocity distribution.With the increase of the Reynolds number,the induced longitudinal vortices gain strength and become straighter and closer to the channel walls,thus the turbulence intensity is furthcr enhanced in this area.