Handbook of Turbomachinery Second Edition Revised - Ventech!

in the section on blade cooling that follows, where application of film
cooling is often somewhat limited compared to nozzles.
Film-cooling application for turbine airfoils is very common in
advanced gas turbines. For this reason the following section considers most
of the factors that influence film cooling. A comprehensive early guide to the
methodology of film cooling was given by Goldstein (1971) [42]. However,
these generic early data have very limited application for the modern airfoil
shapes and realistic main-stream velocity distribution. The most recent
bibliography of the film cooling can be found in [43, 44].
With film cooling the heat flux q 00
f is given by the following relation:
q 00
f ¼ hfðTaw TwÞ; the heat-transfer coefficient, hf, is often assumed to be
that which would exist without film injection, although it may deviate
significantly from this value in the near-hole region. The adiabatic wall
temperature Taw is the temperature assumed at the wall in the absence of
heat flow (without film cooling, the adiabatic wall temperature for low
Mach number is equal to the main-stream gas temperature), and Tw is the
actual wall temperature. In the presence of film cooling, the adiabatic wall
temperature is defined by a dimensionless wall temperature called filmcooling
effectiveness generally designated by Z and defined as
where
Z ¼ðTg TfÞ=ðTg TcÞ
Tg and Tc are the gas stream and coolant exit temperatures,
respectively.
Tf is the film temperature (adiabatic wall temperature resulting from
film injection).
The film temperature is a function of distance downstream from the
injection point and must be experimentally determined for a given filmcooling
geometry. Film temperature is generally correlated in terms of local
film effectiveness as
Tf ¼ Tg ZðTg TcÞ
There are two main types of geometry used in film cooling: slots and
rows of discrete holes. However, slots are not often used in airfoils because
of thermal stress and mechanical design considerations. High thermal
stresses are caused by the fact that the airfoil region just upstream of a slot is
hot, while just downstream of a slot it is cold. Provision of slots causes
difficulty in holding the airfoil together at the slot, especially in the presence
of the high thermal stresses. Replacing the slot with a row of holes tends to
reduce these problems considerably because the metal between the holes
Copyright © 2003 Marcel Dekker, Inc.