Handbook of Turbomachinery Second Edition Revised - Ventech!

of arrays of circular cylinders embedded inside a turbine airfoil cooling
passage such that they are oriented normal to the flow direction. The two
most common arrangements are the in-line and staggered arrays. They are
typically, although not exclusively, employed in narrow channels where the
castability of trip-strips (ribs) is in question. The majority of data available
on pin fins is for the staggered array geometry, and unless otherwise stated
all correlations are given for this geometry. This is generally the geometry of
choice for a designer.
The heat transfer in turbine airfoil pin fin banks combines the pin heat
transfer with the pin end-wall heat transfer. Pin fin heat transfer averaged
over the array appears to vary with Reynolds number to a power of between
0.6 and 0.7, depending on the pin height. The local heat transfer in rows of
pin fins increases for the first three to five rows for arrays with nonrestricted
entrances; in-line array heat-transfer exhibits a fully developed behavior
after the third row, while staggered arrays exhibit a peak in the third to fifth
rows, followed by a slight decay in the succeeding downstream rows.
The majority of turbine pin fin applications are to be found in the
trailing-edge region of airfoils. Here, aerodynamic considerations demand a
small wedge angle for the trailing edge, and as a result the internal passages
become so narrow that the choice of cooling scheme is limited. Pin fins in the
trailing-edge region also serve a structural purpose by holding the suction
and pressure surfaces together. It is not always possible to manufacture pins
of optimum length for heat-transfer purposes in the trailing-edge region due
to limits in casting technology, which is not capable of reliably producing
pins much smaller than about 0.02 in. in diameter. Typically in this region,
pin fins have height-to-diameter ratios between 4 and 1, or even lower. In
these ‘‘short’’ pin fin arrays, the heat transfer has been shown to be lower
than from longer pin fins. A few studies showed that for height-to-diameter
ratios less than 3, there is no effect of the ratio on array averaged heat
transfer. However, for the ratio greater than 3, the heat transfer was found
to increase significantly with length. For short pin fins the cylinder/end-wall
interaction becomes important, and the array total heat transfer must
include heat transfer from both the cylinder surface and the uncovered
region on the end wall. This presents a fundamental difference from arrays
having long cylinders in which the end-wall effects become insignificant.
Also, the addition of short pin fin arrays to a cooling passage may actually
cover up more wall surface area than they add in pin surface area. Thus, the
term ‘‘extended surface’’ does not really apply to this configuration.
Therefore, in this situation, the predominant mechanism of heat-transfer
augmentation lies in promoting flow turbulence rather than the addition of
heat-transfer area.
Copyright © 2003 Marcel Dekker, Inc.