Abstract:
Many studies have been conducted into steady flows through orifices and sudden constrictions in pipes and hence the flow and loss characteristics are very well defined and well understood. However, the flow and loss characteristics of unsteady flows and especially oscillatory flows through orifices and sudden constrictions are still yet to be fully understood and defined. This study aims to understand oscillatory (sinusoidal) flow through an orifice at low frequencies using air as the working fluid. As part of the study, experiments have been conducted at a range of oscillating frequencies and Reynolds numbers. Numerical CFD techniques have also been utilised for flow visualisation purposes. The experimental rig essentially consists of a driving mechanism and the test section; the driving mechanism consists of a scotch yoke mechanism with flywheels of two different sizes attached to a piston through a slot. The rotation of the flywheel allows the piston oscillation. The flywheel is attached to a DC motor with a variable speed controller. The experimental parameter such as the oscillation frequency can be varied by varying the speed of the DC motor and the oscillation amplitude can be varied by setting the piston at different diameters on the flywheel. Using this arrangement, the following range of oscillation frequencies and oscillating Reynolds numbers in the orifice have been tested: ( ): 0.5 − 3.14; Re : 1500 – 6000. The test section consists of two acrylic tubes of 14mm I.D. and a length of 763mm each. An orifice with a beta ratio of 0.5 is sandwiched between these two sections. The pressure data upstream and downstream of the orifice is collected by using an 8 channel pressure transducer system. In addition to the experiments, transient CFD simulations using the ANSYS CFX package were setup for flow visualisation. Multiple domains were set up for the simulation. The first included a full 3D domain which was an exact match of the experimental test section. In order to reduce the size of the mesh, input and results files, and an axisymmetric domain was used with a symmetry plane. The domains were meshed using ICEM CFD and consisted of hexahedral elements with approximately 2.5million nodes. The boundary conditions consisted of a sinusoidal velocity condition at the inlet, a no slip, smooth wall condition at the walls and an opening condition at the outlet. From the experimental runs of oscillatory flow through constrictions such as an orifice, a large variation was found in the pressure loss coefficient, where for low Reynolds numbers, the coefficient was found to be as high as 104 and was found to asymptote to a value of approximately 1 for high oscillatory Reynolds number. From the numerical simulations, no vortices were found to be shed from the orifice, likely due to the very large stroke length of the fluid oscillation. As expected a 3D model provided a much more accurate prediction of pressure and velocity data at the centre of the orifice.