Handbook of Turbomachinery Second Edition Revised - Ventech!

impingement of air jets are generally considerably higher than those
obtainable by other conventional methods.
Jet impingement cooling, which has been proven to be highly effective
for turbine nozzles, and where spent air is removed toward the trailing edge
through the passage between airfoil inner wall and insert, is usually not
practical for the blades without immediate film discharge of the spent air.
Without this, the presence of cross flow results in a significant reduction of
the impingement cooling effectiveness (Kercher and Tabakoff) [82].
An important advantage of impingement cooling, particularly if the
design uses sheet metal inserts, is that it is usually relatively easy to modify
the geometry for engine temperature uprates or if shortfalls are
encountered in a design. However, it must be noted that an impingement
design can add complexity to a part, especially if the impingement holes are
a cast-in feature. Difficulties in the internal casting core design, core yield
rate, and control of impingement hole size can become limiting factors.
This method of cooling has been recognized for some time as being highly
effective and, therefore, has received much attention from researchers. As a
result, a considerable amount of experimental data and correlations exist
for impingement cooling heat transfer. With such an extensive collection of
data, it simplifies matters to classify the data by geometry. The first
classification relates to the impinged surface, which can be flat or curved
(concave). In the context of turbine airfoil cooling, the leading-edge
internal area forms a concave surface, but further back in the midchord
region the impinged surface is nearly flat. The second classification relates
to the impinging jet and this can be either two-dimensional (slot) or
axisymmetric (hole). Then there is the possibility of a single jet, a row of
jets, or an array of jets. With an array of jets there is the added
complication of spent air cross flow, which interferes with the jets and
reduces their effectiveness.
The first and most important task for the designer is to identify the
impingement geometry most suited to the particular design under
consideration and select the data or correlations that most closely relate
to that geometry. It is quite possible that a cooling design may embody
several types of flow geometry at various locations. A considerable amount
of data on impingement heat transfer is available in the literature for various
impingement configurations. The local impingement heat-transfer coefficient
data of Chupp et al. [83] and the average heat-transfer coefficient data
of Metzger et al. [84] were obtained with test models simulating turbine
airfoil leading-edge configurations and are therefore particularly useful in
turbine airfoil leading-edge cooling design applications.
For most practical turbine airfoil-cooling applications, there are two
main geometries of interest (Fig. 20). The first is impingement from an array
Copyright © 2003 Marcel Dekker, Inc.