Handbook of Turbomachinery Second Edition Revised - Ventech!

Because of difficulties with measurements in operating combustors,
designers have often guessed what the appropriate turbulence intensity and
scale might be. Techniques to incorporate combustor-generated turbulence
into heat-transfer design systems have been under development for many
years, but with limited results. A significant portion of the experimental
information used for this purpose has come from programs utilizing flat
plates as the test article. It has been often assumed that the turbulence
intensity values generated for these measurements are representative of
typical engine combustors. Seldom for these studies has there been an
attempt to simultaneously duplicate the ‘‘thought to be’’ turbulence
intensity and scale of the turbine free-stream inlet turbulence. In addition,
a significant obstacle for the turbine heat-transfer design community has
been that flat-plate flows have not been demonstrated to be representative of
the unsteady flow-field environment associated with a turbine stage. There
are many coupled phenomena associated with the unsteady turbine stage
flow field, and recent experimental results suggest that the influences of freestream
turbulence, wakes, fluid injection or ejection, and shock interaction
may not be individually separable.
Very limited data of the flow and thermal fields usually exist for an
actual gas turbine engine due to the extremely harsh operating conditions in
the engine. Consequently, development of computational models for
predicting heat transfer in the turbine section has relied on experimental
simulations of the turbine environment. Simulations of film-cooled airfoils
have become much more sophisticated over the last decade, including highdensity
coolants, curved and rough walls, and geometrically more
complicated airfoils. Over the past 10 years, much effort has been put
into correctly simulating the main-stream turbulence conditions characteristic
of the flow entering the turbine section. Studies have shown that very
high turbulence levels can cause dramatic increases in heat transfer and
rapid diffusion of film-cooling jets. Many heat-transfer-related papers begin
by making the assertion that the flow leaving the combustor and entering
the turbine has very high turbulence intensity with numbers cited ranging
from 6 to 20%. Seldom do these publications cite an available reference with
a measurement of turbulence intensity or turbulence length scale for an
operating combustor. For this reason, a comprehensive understanding of
the effects of highly turbulent main stream is still lacking, particularly in
regard to the influence of the length scale of the turbulent eddies. All these
factors explain a limited ability for reliable computational modeling of these
flows and the associated heat transfer.
It is well known that the flow exiting the combustor and entering the
turbine has significant radial and circumferential temperature distortion or
‘‘hot streaks/hot spots.’’ It is not surprising that the number of hot streaks at
Copyright © 2003 Marcel Dekker, Inc.