Handbook of Turbomachinery Second Edition Revised - Ventech!

Positioning the film-cooling jets upstream of the leading edge is the
most beneficial for overall stage performance. However, this approach has
to overcome certain challenges imposed by the secondary flows. Over the
last few years, a number of numerical and experimental studies (Friedrichs,
1996, 1997) [52, 53] have addressed this approach with a goal to effectively
cool nozzle end walls relying primarily on the upstream film jets. Some of
the recently published data by Friedrichs and also by Simon [54, 55] clearly
show that properly positioned film jets discharged at optimized blowing
ratio can provide effective end-wall cooling all the way to the nozzle throat
without premature sweeping of the film toward the suction side. Although it
might require a significant amount of the film flow, the overall stage
performance improvement can be achieved by suppression of the secondary
flow and associated performance losses. Coolant distribution is strongly
influenced by the momentum flux ratio. At a lower momentum flux, there is
strong migration toward the suction side due to the cross flow. As the
momentum flux ratio increases, there is better pressure-side cooling, as some
coolant accumulates in the vicinity of the pressure surface. Comparing a
single-slot injection case with a double-slot injection case, it was observed
that uniformity of coolant distribution across the leading edge is better for
the single-slot injection case, due to the higher momentum flux ratio. Direct
comparison of single- and double-slot injection for the same momentum flux
ratio shows that the distributions of cooling effectiveness for both cases are
similar in shape but different in magnitude, providing higher values for the
double-slot injection due to the higher coolant mass flow rate. Other very
important factors affecting secondary flow and film effectiveness are the
shape and degree of converging contouring of the end wall. Improved
numerical prediction techniques [56, 57, 58] might be very useful in
optimizing design of the film-cooled end wall.
TURBINE BLADE COOLING
In addition to high thermal and aerodynamic loads turbine blades operate at
extremely high inertial stresses. Creep and rupture life of the blade material
at certain temperatures often defines the life of the entire engine. The
criticality of a blade failure explains the level of attention paid to the bladecooling
design that is required to support the constantly increasing turbine
rotor inlet temperatures (TRIT). Similarly to nozzle vanes, the shape of a
turbine blade that is selected for the best aerodynamic performance often
has to be compromised to be acceptable from a cooling design standpoint.
Constraints associated with the cooling of modern blades, which are highly
loaded and have a large main-stream turning angle, might require
Copyright © 2003 Marcel Dekker, Inc.