FLUID FLOW BEHAVIOR INSIDE A CIRCULAR HELICOIDAL PIPE USING A LASER DOPPLER ANEMOMETRY/VELOCIMETRY (LDA/LDV) SYSTEM

Abstract

ABSTRACT An experimental study using Laser Doppler Anemometry/Velocimetry (LDA/LDV) 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, D c /d, and the pitch ratio of the centerline of the coil diameter to the pitch of the coil, D c /b, are 13.6 and 4.0, respectively. The uncertainties for velocity, the friction factor, the Reynolds number, and the Dean number were found to be under 1.52%, 3.41%, 2.37% and 2.60%, respectively. In the present investigation, the friction factor for the helicoidal pipe was measured and it was found that the flow remains laminar until Re ≈ 8,000, which is much higher than the critical Reynolds number for a straight pipe (Re cr ≈ 2,500). In addition, the ratio of friction factor of the current helicoidal pipe (f c ) to the friction factor of the straight pipe (f s ) has been compared with previous investigations, and good agreement has been obtained. Furthermore, the axial velocity distributions at two different cross sections of a helicoidal pipe for different Reynolds numbers (1,360, 2,500, 3,100, 4,400 and 5,900, corresponding to Dean numbers of 369,678, 841, 1,193 and 1,600, respectively), have been measured and reported herein. One component of the secondary velocity perpendicular to the axial velocity and the measuring line has also been measured on the entire cross section. The secondary velocity measurement confirms that two vortices are superimposed on the axial 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 as axial velocity becomes fully developed, the secondary flow also acquire a similar flow pattern. Inspection of 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. Thus, it can be concluded that the torsion effect for the pitch ratio (D c /b=4.0) in the present investigation is very small compared to the curvature effect, but it is evident.