Handbook of Turbomachinery Second Edition Revised - Ventech!

the cavity region and that vortices persist throughout the length of the
cavity. Their modeling of this flow suggests that one of the vortices is the
result of separation off the pressure side of the tip edge and that this vortex
rolls over the cavity pressure side wall. The second vortex is the result of a
flow separation again at the tip edge on the blade suction side. There
appears to be a dividing line where the main stream starts rolling into the
gap from the pressure side of the blade. Flow separation occurs because of
the edge of the tip. Strong secondary flows can be expected to be present in
the gas path. This can have the effect of bringing very hot portions of the
mainstream gas to the vicinity of the clearance gap, downstream of the blade
leading-edge lip. This lip acts like a rearward-facing step with resulting flow
separation and attachment in the recessed cavity. As the flow emerges from
the suction side of the gap, it usually observed to roll into a vortex as it
meets the oncoming shroud wall flow.
A significant portion of the literature relevant to experimental studies
of tip gap flows has dealt with flat tip blades. Distributions of blade-tip
static pressure were measured in a 2D cascade by Bindon [94]. He included
the effects of clearance gap and pressure-side edge radius on a flat tip.
Special attention was given to the separation bubble formed at the tip entry
along the pressure side. The same study addressed the effects of profiling of
tip geometry in both stationary and rotating cascades using a pressure-side
edge radius, suction-side squealer, or full squealer. The study also focused
on effect of leakages on losses and stage performance. It concluded that for
all tested configurations the flat tip performed best, and relative motion was
of importance.
A recent study (Bunker 2001 [95]) of the effects of squealer cavity
depth on tip cavity heat-transfer distribution has used a stationary blade-tip
cascade, representing a high-pressure turbine blade with actual aerodynamic
pressure distribution. The effect of cavity depth was found to be not
uniform over the tip cavity surface. The general observation is that a deeper
cavity produces lower heat-transfer coefficients. A shallow squealer cavity is
shown to be very close to the flat tip behavior. A depth often found in
practical design tip cavity is shown to reduce overall heat load by 50%.
Several researchers have investigated ways of reducing performance
losses by controlling tip leakage. Mixing of the leakage flow with the rotor
passage flow causes total pressure loss and reduces turbine stage efficiency.
The losses originate during the formation of a leakage vortex and its
interaction with the passage vortex. Recently published [96, 97] (Camci, and
Harvey) studies use the concept of a tip platform extension that is a very
short ‘‘winglet’’ obtained by slightly extending the tip platform in the
tangential direction. The use of a pressure-side tip extension can
significantly affect the local aerodynamic field by weakening the leakage
Copyright © 2003 Marcel Dekker, Inc.