Handbook of Turbomachinery Second Edition Revised - Ventech!

NONUNIFORM INLET FLOW
All gas turbines operate with a combustor that supplies the energy to the
flow through the addition of the enthalpy (heat) of combustion. Because
combustor space and weight generally must be kept to a minimum, designs
tend to have features that present a more or less nonuniform flow to the
turbine, i.e., hot and cold streaks relative to the average flow temperature. In
addition, cooling gas may be introduced through the combustor and vane
walls to protect them from the hottest gases. Inertia forces separate this
mixture of hotter and cooler flow as it is accelerated and turned through the
vanes and blades. This process sets up secondary flows in the blade passages;
that is, gas movement not directly aligned with the center of the passage
because of the density differences between the hot and cold flows. This is
often called the Kerrebrock–Mikolaczak effect after a study by these
authors of similar phenomena in compressor inlets [47].
One of the first experimental studies of nonuniform inlet flow effects
was by Roback and Dring [48]. These authors used a large-scale rotating rig
(Fig. 21), with amixture of air and carbon dioxide to simulate the density
difference between typical hot and cold flows. They found that the
‘‘rectification’’ process, as they define it, caused hot gas to accumulate on
the pressure sides of rotating blades (an often-observed phenomenon
revealed by higher rates of heat transfer on the pressure sides) and cold gas
to accumulate on the suction sides. In cooled turbines, stator coolant
injected upstream can add to the accumulating flow on the rotor suction
side. Other parts of the rectification process observed defined the effect of
the ratios of temperature (density) between the hot and cold streaks in radial
movement toward the tips and end walls.
FREE-STREAM TURBULENCE
Free-stream turbulence (FST) is known to have very significant effects on
transition and separation, and therefore on heat transfer. This may be above
and beyond the FST effect on the magnitude of the heat transfer in a
boundary that has not yet separated. Some of the first work on FST effects
is that of Kestin et al. [49], which was followed by several others and
continues today. Figure 22, from [50], demonstrates the sensitivity of airfoil
heat transfer to lower levels of FST. Many more recent studies have been
done with higher levels of FST that are expected to exist at the combustor
outlet. A good summary of the earlier work is found in Thole and Bogard
[51], where a correlation is presented, given in Fig. 23, based on data
provided from Hancock and Bradshaw [52] and others. The correlation
Copyright © 2003 Marcel Dekker, Inc.