Handbook of Turbomachinery Second Edition Revised - Ventech!

a phase changing liquid as a heat sink coupled with a condenser to remove
the absorbed heat.
Although the following section is devoted primarily to the blades,
many features that are common to both vanes and blades can be found.
Smooth Internal Passages
In general, a blade convection cooling techniques can be subdivided into:
Schemes employing straight-through radial holes or channels that may
be connected to form multipass, or serpentine, systems in which the
air passes through the blade/vane several times before exiting
Schemes involving enhanced heat transfer by means of ribs, dimples,
matrixes, pin-fins, or similar devices to extend the cooled surface
and promote turbulence
Schemes using impingement cooling to more closely match the cooling
to external thermal loads
Swirl cooling schemes utilizing heat-transfer augmentation due to
centrifugal force field in a strong vortex that is induced inside the
blade, primarily for leading edges
Since the level of internal convection cooling depends on the internal cooling
passage surface area and the convection heat-transfer coefficient acting over
this area, both of these parameters must be maximized for an efficient
cooling design. For all cooling systems covered by first two groups, custom
tailoring of the cooling passage cross-sectional flow area provides an
effective means of controlling the local Reynolds number, and thereby the
local heat-transfer coefficient. A convectively cooled airfoil usually uses air
that is brought in at the root or tip (for vanes only) of the airfoil and then
discharged at the other end of the airfoil or through its trailing edge.
Properly designed internal cooling is quite efficient and is usually preferred
where it can satisfy the required cooling effectiveness.
The simplest form of internal cooling system used in airfoil designs
generally consists of smooth radial passages of circular, elliptical, or
triangular cross section. Matching locally of the cooling capacity to the
external airfoil heat load is achieved through appropriate combinations of
cooling flow rate and passage cross-sectional area, which affect the cooling
air velocity and hence the internal heat-transfer coefficient, and cooling
surface area.
The flow in typical airfoil cooling passages is usually assumed to be
turbulent. For this condition the fully developed smooth wall heat-transfer
Copyright © 2003 Marcel Dekker, Inc.