EXPERIMENTAL-STUDY OF WALL SHEAR RATES IN THE ENTRY REGION OF A CURVED TUBE

Local wall shear rates in steady flow in the entry region of a curved tube have been measured by the electrochemical limiting current method. A semi-circular rigid tube of circular cross-section with radius ratio 1/7 has been employed for a range of Dean number between 139 and 2868. The circumferential and axial distributions of the wall shear rates have been measured at 20° circumferential increments at five different sections of the entry region. The experimental wall shear distributions show a valley in the circumferential wall shear profile and a region of non-monotonic variation of the average wall shear rates with downstream distance, both of which are prominent features of the entry flow in a curved tube. The existence of the valley suggests a vortex structure with several pairs of vortices in a developing curved tube flow in contrast with one pair in a fully developed flow. The circumferential distribution of wall shear also suggest that a potential vortex flow is formed in the upstream core. The wall shear is lowest at the innermost part of the bend and reaches a maximum in the mid-circumference region. The wall shear is significantly enhanced by secondary flows, as much as eight times greater in the curved tube entry flow than in fully developed Poiseuille flow in the range of Dean number between 139 and 2868. The cross-over in shear maximum from the inner part of the bend to the outer part occurs at about 1·9 cross-section radii from the inlet and seems to decrease with Dean number. © 1979, Cambridge University Press