Characteristics of flow through orifices in pipes

Abstract

Orifice plates are key components used for flow measurement and control in several industries. For instance, they find applications in gas and liquid circuits of lithography machines, nuclear power plants and aerospace propulsion systems. They are used typically either for measuring flow-rate or to introduce a pressure drop for purposes of flow balancing. The present study focuses on the latter application. It is widely acknowledged in literature, that the turbulent, unsteady nature of the flow issuing through an orifice can also be a source of structural vibration. In order to understand the nature of the vibration source, the present experimental investigations analyze the time-varying flow field by means of unsteady wall-pressure measurements and time resolved, planar, particle-image-velocimetry (PIV). In addition to understanding the dynamics of the flow through a single-hole orifice, this study has assessed the possibility of using multiple-hole orifices as an alternative. It is observed that the overall magnitude and extent of the disturbance levels in the flow are reduced with the multiple-hole orifices, while maintaining similar levels of pressure drop. It is hypothesized that these lower disturbances are a result of the small-scale flow structures associated with the multiple-hole orifice flow (multiple jets as opposed to one). The measurements give detailed insights into the flow behavior downstream of orifice plates. Results indicate a low frequency flapping motion of the single-hole orifice jet, which is sustained by the surrounding large recirculation regions. For the geometries of the sharp-edged single-hole orifices investigated, the flapping frequency was observed to increase with flow speed and is found to occur at a Strouhal number = 0.02 based on the orifice jet velocity and the difference in internal diameters of the pipe and orifice.