Flow visualisation and heat/mass transfer in a linear turbine cascade with tip clearance

Abstract

Flow visualisation and heat/mass transfer measurements in a linear turbine cascade with tip/casing clearances ranging from 0.60% to 6.90% of chord show the influence of blade tip geometry for a standard flat tip, a squealer tip and a geometry with a winglet on the pressure side and a squealer on the suction side of the blade. Oil-lampblack and oil-dot techniques show the surface flow on the blade endwall and tip while a laser light-sheet and a smoke wire indicate the three-dimensional flow in the tip clearance and blade passages. Local heat/mass transfer from the blade tips is measured using the naphthalene sublimation technique. Experiments with the standard flat tip blade clearly show that a separation bubble exists at the pressure side edge of the tip surface at all tip clearance levels. The effect of the tip leakage vortex on the secondary flows in the blade passage is also evident: with an increase of tip clearance, the horseshoe vortices disappear while the passage vortex is weakened and pushed away from the suction surface by a strengthened leakage vortex. The flow in the gap changes significantly for the squealer tip and the winglet-squealer tip cases. The surface flow visualisation identifies reattachment and recirculation regions on the blade tips. Photographic images, compared to detailed convective heat/mass transfer measurements, provide an interpretation of the heat/mass transfer results.