Experimental Study of the Transient Flow in a Coiled Pipe Using PIV

Abstract

A comprehensive knowledge of energy dissipation during the accelerations and decelerations in transient flow is essential to develop robust and accurate hydraulic transient solvers. Currently, most transient solvers are not accurate enough to describe the physical phenomenon, tending to underestimate observed energy dissipation. The current paper aims to contribute to a better understanding of transient flow by measuring instantaneous velocities using 2D particle image velocimetry (PIV) and discussing key features of the mean and of the turbulent flow. Measurements were carried out at the middle section of a coiled copper pipe for Reynolds and Dean numbers of 7,000 and 1,040, respectively. In steady-state flow, the time-averaged velocity profiles have shown a strong asymmetry, with maximum axial velocity in the outer bend region of the pipe, caused by the centrifugal force. During transient flow, the ensemble-averaged velocity profiles have clearly shown regions with strong gradients near the pipe wall and reverse flow. The direction of flow is firstly reversed in the inner bend region of the pipe. The unsteady wall shear stress is different for the outer and inner wall, and is lower than in the steady state, confirming that energy dissipation cannot be estimated based on steady-state formulae for the wall shear stress.