Investigations on the flow inside pumps by means of 2D particle image velocimetry

This work is concerned with the sampling, evaluating and critically interpreting of fuid dynamics phenomena inside two radically different pumps. Large scale flow structures are investigated in the vaned diffuser of a centrifugal pump and in the rotor passage of a water jet axial pump by means of two-dimensional particle image velocimetry (2DPIV). In the first part of the work, a centrifugal pump is run at various capacities to derive information about the flow around the diffuser vanes. Preliminarily, time resolved pressure measurements have indicated the presence of very large scale non-periodic flow instability at very low flow-rates. Phase-averaged PIV measurements are performed at midspan over the blade to blade plane in the diffuser to describe both the channel flow and the wakes, which indeed dominate the flow in the diffuser. The flow is separated at the convex side of the diffuser vanes at reduced capacities, i.e. below the best effciency point (BEP). Wakes shed from the impeller are convected in the di user passages and distorted because of the different transport velocity at the convex and concave sides of the vane. The measurements have been performed at Laboratorio di Fluidodinamica delle Macchine (LFM) at Politecnico di Milano, Milan, Italy. In the second part, the tip leakage flow (TLF), tip leakage vortex (TLV) and the vorticity shed by the blade tip are investigated in the meridional plane of an axial pump to shed light over the inner structure of the vortices that develop in the passage, as well as the production of vorticity at the casing end-wall, induced by the presence of the TLV. Turbulence inside the TLV is also investigated. The rolling up of the TLV induces the detachment of the end-wall boundary layer vorticity, the detached flow rearranges in vortices - counter rotating respect with the TLV - that orbit around the TLV centerline and are interlaced with the TLV vorticity until its bursting. The detachment of the end-wall vorticity is unsteady and high level of in plane production of turbulent kinetic energy is found there. The vorticity layer that connects the blade and the TLV is also unsteady and source of large scale unsteadiness; the core of the TLV is unsteady but in plane production of turbulence is almost absent there. The experiments have been performed at the Department of Mechanical Engineering, The Johns Hopkins University (JHU), Baltimore, MD, USA.