Handbook of Turbomachinery Second Edition Revised - Ventech!

to its original position. This unsteady behavior of the flow resulted in large
fluctuations in blade lift and cascade loss.
Heat transfer is another extremely important consideration in turbine
flow fields, due to the exposure of the blade rows to the hot exhaust gases
from the combustion process.
Blair [32] studied the heat-transfer distribution on the blade surfaces
and the hub end wall in an axial turbine rotor passage. The investigation
focused on the effects of hub and tip secondary flows, tip leakage, and the
leading-edge horseshoe vortex system. All of these three-dimensional flow
structures were found to produce local regions of substantially enhanced
heat transfer.
On the suction surface, the hub and tip secondary flows were
responsible for heat-transfer rates up to 60% higher than in the midspan
region of the rotor. However, the highest heat-transfer rates on the suction
surface, almost double the midspan values, were observed in a highly
localized region near the tip, where the tip leakage flow rolled up into a
vortex.
The leading-edge horseshoe vortex caused enhanced heat transfer in
the region near the intersection of the rotor leading edge and the hub end
wall. Heat-transfer rates in this region were almost double the values seen on
the end wall at the leading-edge midpitch.
Also of importance in a highly three-dimensional turbine flow field is
the behavior of hot streaks from the combustor exit and cooling flows from
upstream blade rows. Roback and Dring [33, 34] used a large-scale rotating
rig with a 1 1
2-stage turbine model to examine the impact of a nonuniform
turbine inlet temperature profile due to combustor-generated hot and cold
streaks, and also to investigate the effect of ‘‘phantom cooling,’’ which
occurs when cooling air discharged from the trailing edge of the upstream
stator collects in a nonuniform manner on the downstream rotor. Their
investigation revealed the locations on the turbine rotor where hot and cold
streak accumulation and phantom cooling were most likely to be strongest.
The experimental study revealed that the hot streaks tended to
accumulate on the pressure surface of the rotor, while cold streaks
accumulated on the suction surface. This difference in behavior resulted
from the difference in densities of the two streams. It was also observed that
the streaks could migrate to the hub and tip regions of the rotor, depending
on the spanwise location at which they were introduced. Thus, the streaks
could alter temperatures at both end walls and the rotor tip. However, hub
and tip secondary flows on the rotor acted to minimize the accumulation of
cold streaks near the end walls, by preventing them from reaching the
suction surface.
Copyright © 2003 Marcel Dekker, Inc.