Experimental investigations of the three-dimensional flow in a large deflection turbine cascade with tip clearance

Abstract

Results obtained from an experimental study of the three-dimensional flow survey within and exit of a large deflection linear turbine cascade are presented for a tip clearance levels of 0.08, 1.5, 3.0 percent of chord and compared with the help of boundary layer probes and that within and exit of a blade passage was done with a miniaturised five hole probe. End wall and blade tip surface static pressures were also obtained, in addition to flow visualisation studies. A strong horse-shoe vortex forms in front of the leading edge for zero clearance whereas this vortex does not appear for 3 percent clearance indicating that for large clearance the pressure forces have dominating influence than the viscous forces. In addition to normally known clearance vortex, a small tip separation vortex was noticed on the blade tip surface inside the tip gap. Due to the area contraction caused by the tip separation vortex, the fluid moving towards the tip gap from the pressure side is accelerated. Downstream of the vortex, the endwall pressure increases due to flow mixing. Both vortices increase in size and strength along the chord. The mixing is incomplete in the aft portion of the blade. The tip gap velocity profiles exhibit wake like characteristics especially at axial positions where the mixing is incomplete. The passage vortex in the present investigations did not diminish with increase in clearance. The discharge coefficient and the total pressure loss coefficient within the tip gap show similar tendency with lower values near the leading and trailing edge regions. K e y w o r d s : t u r b i n e c a s c a d e , t i p c l e a r a n c e v o r t e x , t i p s e p a r a t i o n v o r t e x , h o r s e -s h o e v o rt e x ,

Study of Sweep and Induced Dihedral Effects in Subsonic Axial Flow Compressor Passages—Part II: Detailed Study of the Effects on Tip Leakage Phenomena

“…Streamlines crossing the tip gap through the low static pressure zone inside the tip have highest leakage velocity. This figure admits that tip leakage flow is fully 3D in nature, which is neither normal to the camber line [8] nor parallel to the rotation (tangential). They flow approximately tangentially near the blade TE, where the pressure differentials between PS and SS are the lowest on the chord.…”

“…They interpreted the flow downstream of venacontracta of the separation bubble as a sudden expansion in flow area, with a consequent loss in total pressure being experienced by the fluid in passing through the tip clearance. However, Govardhan et al [8] observed incomplete mixing inside the tip gap (t/τ = 10) in a high loaded turbine cascade with a significant loss occurring within the tip gap. Mixing within the tiny tip gap is conclusively a source of loss due to entropy generation and loss in fraction of total pressure, which is typically observed to occur with a t/τ ratio exceeding 6.…”