An experimental study using Laser Doppler Anemometry (LDA) has been carried out to investigate the fully developed Newtonian laminar flow inside a helicoidal pipe. In the helicoidal pipe test section, the curvature ratio of the centerline of the coil diameter to the pipe diameter, Dc/d, and the pitch ratio of the centerline of the coil diameter to the pitch of the coil, Dc/b, are 5.77 and 3.43, respectively. The uncertainties for velocity, the Reynolds number, and the Dean number were found to be under 1.52%, 2.37% and 2.60%, respectively. In the present investigation, the secondary velocity vectors have been measured on an entire cross section for the first time. The measured cross section is located 540° (1 1/2 turns) from the entrance of the helicoidal pipe for three different Reynolds numbers (1,000, 1,800 and 2,500, corresponding to Dean numbers of 416, 749 and 1,041, respectively). The corresponding axial velocity distributions have also been measured at the same cross section. The secondary velocity measurement confirms that two vortices are superimposed on the axial pipe flow, and that the magnitude of the secondary flow is proportional to the axial flow or Reynolds number. Upon inspection of the velocity contours, it was observed that the fully developed axial velocity profile shifts toward the outer wall of the helicoidal pipe as a result of the centrifugal force acting on the fluid flow in the helicoidal pipe. It is experimentally illustrated that the axial velocities and the secondary velocity vectors have a similar flow pattern for all three Reynolds (or Dean) numbers. Inspection of the results reveals that the symmetry of the axial velocity distributions and the secondary flow pattern is slightly altered due to an effect of the finite pitch or torsion in the present experiments.
