Flow Structure and Turbulence in the Tip Region of a Turbomachine Rotor Blade

Abstract

The flow structure and turbulence in the tip region of a rotor blade operating downstream of a row of Inlet Guide Vanes (IGVs) are investigated experimentally in a refractive index matched facility that provides unobstructed view of the entire flow field. Stereo-PIV measurements are performed in closely spaced radial planes near the blade tip in a region extending from (slightly upstream of) the blade trailing edge to about 40% of the chord downstream of it. The data enable calculations of all the components of the phase-averaged velocity and vorticity vectors, as well as the strain rate, Reynolds stress, and turbulent diffusion tensors. Each rotor blade is confined between two tip-leakage vortices, a right hand vortex (RHV), generated by the subject blade and propagating along its right hand side, and a left hand vortex (LHV), generated by the previous blade in the same row and propagating along the left hand side of the subject blade. In addition, a trailing edge vortex (TEV) lays underneath the LHV and is subject to intense shearing/deformation by the LHV. RHV-induced radial gradients of radial phase-averaged velocity cause negative turbulence production, P, along the RHV-axis, and formation of a region of low P in the gap between the RHV and the blade suction surface. Trends of turbulent kinetic energy k and P within the RHV do not agree due to the effects of advection by the phase-averaged flow. To the left of the blade, shearing of the TEV by the LHV enhances turbulence production in the region between the two vortices and the rotor wake. Trends of turbulent kinetic energy and its production rate are in good agreement and peaks of k and P occur at the same location. As the TEV migrates away from the LHV, shearing effects become weaker and the dominant contributors to production are terms containing vortex-induced radial gradients of axial and radial velocities. Turbulent diffusion is a minor contributor to the evolution of turbulent kinetic energy in the tip region. It is also shown that the tip-leakage flow/vortex deteriorates the rotor blade performance, causing a ∼66% increase in shaft power input (per unit mass flow-rate) in the tip region in comparison with midspan.