Handbook of Turbomachinery Second Edition Revised - Ventech!

Government- and university-authored papers usually provide a greater
detail about the calculation but generally for a geometry that is often not
representative of actual components in the industry. Efforts focused on
understanding turbine airfoil heat transfer have traditionally been overshadowed
by the work done on the associated aerodynamics. This was not
due to a lack of appreciation for heat-transfer problems, but rather the
difficulty of the problem that became more prominent as the turbine inlet
temperature increased.
Most conceptual models for heat transfer in a modern 3D airfoil
require significantly more information than is required for aerodynamic
calculations. The computational grid required for the heat-transfer
calculation is much finer than required for the corresponding aerodynamic
calculation. This is because the temperature gradient at the airfoil wall
drives the heat-flux calculation and the grid resolution in the boundary layer
must be significantly finer. The boundary-layer approximation assumes a
constant pressure through the boundary-layer thickness, but the temperature
through the boundary-layer thickness is not constant. The codes have
progressed from boundary-layer codes that have their origin in correlations
of early flat-plate data, through the 2D or quasi-3D steady codes to the full
3D unsteady Navier–Stokes codes.
In addition to analyses of the individual cooled turbine components, it
is very common that turbine cooling designers with expertise in heat transfer
and fluid mechanics are also responsible for secondary flow analysis and
overall engine thermal management. This includes analyses of the cooling
flows and seal leakages, and temperature distribution for most of the hot
section components at steady-state and transient operating conditions. The
cooling designers are also expected to provide design input for transient
behavior of the entire turbine structure, including interaction between the
turbine rotor and stator.
Metal Temperatures and Their Effect on Life of the Turbine
Components
In most of the cases, steady-state temperatures define the life of turbine
components for the selected turbine materials. An exception is the low cycle
fatigue life, which is defined by transient temperature gradients.
A cooling designer starts by either being given or assuming certain
component life requirements based on oxidation/corrosion/erosion, creeprupture,
and thermal (low cycle) fatigue, which dictate acceptable metal
temperature limits. For each component these life and temperature
requirements must be met under given duty cycle conditions. Life of
combustor liners, for example, and, in most of the cases, that of stage one
Copyright © 2003 Marcel Dekker, Inc.