Handbook of Turbomachinery Second Edition Revised - Ventech!

vane trailing edges usually does not undergo uniform circumferential mixing
and is often skewed toward the tip of a blade at the suction side. Actual
measurements in engines and analytical predictions show that inlet radial
temperature profile along the gas path is usually further skewed toward the
outer radius, creating additional challenges for cooling the tip sections of
blades toward trailing edges.
Effects of Unsteadiness
Unsteady interaction between stator and downstream rotor has a significant
effect on heat transfer. Heat-transfer measurements for a full turbine rotor
stage have first been reported by Dunn et al [69]. Their study showed that
the wakes from upstream vanes cause an intermittent increase in the heat
transfer on the rotor blade surfaces. The highest enhancement of heat
transfer could be seen mainly near the leading edge and just around the first
part of the suction surface. Further downstream on the suction surface the
enhancement of heat transfer decays quickly. On the pressure surface the
increase in heat transfer persists all the way back to the trailing edge,
although the enhancement near the leading-edge region up to 20% axial
chord is not as high as that on the suction surface.
Later aerodynamic and heat-transfer measurements on a rotating
turbine made by Hodson [70] showed that the rotor blade boundary layer
undergoes transition from laminar to turbulent flow as the wake from
upstream vanes pass through the rotor blade passage. This results in
increased shear stress and aerodynamic loss. The increased shear stress is
also expected to increase the heat transfer. A number of studies have shown
that there is an intermittent increase in heat transfer over the suction and
pressure sides of a blade. Measurements that were made even at a low
turbulence level (4%) showed that the moving turbulence simulating grid
induces wakes and results in increased heat transfer. Usually the increase on
the pressure side is greater than that on the suction side. In a similar
experiment Dullenkoph et al. [71] have also demonstrated similar effects on
the time-averaged heat transfer for upstream wakes on a rotor blade
cascade. Strong increases in heat transfer are shown by Johnson et al. [72]
for the case of shock waves from upstream vanes impinging on the rotor
blade surface. This phenomenon has been analyzed more recently by Abhari
et al. [73] and Rao et al. [74] using comprehensive numerical models.
Internal Blade-Cooling Techniques
Because of its high thermal load combined with large inertial and dynamic
forces, the turbine blade is the most critical component for engine durability.
Copyright © 2003 Marcel Dekker, Inc.