Handbook of Turbomachinery Second Edition Revised - Ventech!

Some studies showed that the cooling effectiveness can be increased by 60%
at a realistic pressure ratio of 1.03, if the hole is made at an angle of 208
versus being normal to the wall surface. It is clear that practical
implementation of angled effusion cooling is highly dependent on an ability
to accurately, consistently, and economically manufacture large numbers of
oblique holes of very small diameter. Advances in laser drilling have made
this possible, and this cooling method is now regarded as a viable and
economically acceptable technique. At the present time, the lower limit on a
hole diameter is about 0.4 mm, whereas the lowest attainable hole angle is
just below 208. Andrews [110–113] presents a number of papers that are
widely used for advanced liner cooling design. Full coverage discrete hole
impingement cooling and effusion film cooling are extensively used in gas
turbine blade and combustor wall cooling. However, most applications and
most experimental investigations for these cooling techniques are used
separately. The combination of impingement and effusion cooling offers a
good means of improving the overall cooling effectiveness of both turbine
blades and combustor walls and of minimizing the coolant flow required to
achieve the desired cooling effectiveness. Combined impingement/effusion
cooling with equal numbers of holes, but the main pressure loss at the
impingement holes, has very good internal wall heat-transfer characteristics
with increases of 45% and 30% found for two designs, relative to the
impingement-only situation. Studies of combined impingement–effusion
cooling yielded the following conclusions:
The combined impingement–effusion cooling heat transfer was not
greatly influenced by the effusion wall design for the effusion–
impingement hole diameter ratios.
The measured combined impingement–effusion heat-transfer coefficients
were lower than the sum of the separate impingement and
effusion wall heat transfer by approximately 15–20% for the two
designs tested. This indicates that there was an interaction between
the two heat-transfer modes, which reduced the net heat transfer.
The overall cooling effectiveness results demonstrated the large
benefits to be obtained from the addition of impingement cooling
to effusion cooling. However, the film-cooling part of the process,
which was strongly dependent on the effusion hole size, had a strong
influence on the wall impingement–effusion heat-transfer coefficient.
Cost, increased weight, durability, and ability to repair the angled effusioncooled
liners are the main concerns that limit their application. These issues
can only be fully resolved by extensive service experience.
Future developments in angled effusion cooling will tend to focus on
the optimization of hole geometry. A diffuser-shaped expansion at the exit
Copyright © 2003 Marcel Dekker, Inc.