Project Report
2011 MVK160 Heat and Mass Transport
May 19, 2011, Lund, Sweden
Internal Blade Cooling Technology on Gas Turbines
Le Gloanec Erwan
Dept. of Energy Sciences, Faculty of Engineering,
Lund University, Box 118, 22100 Lund, Sweden
ABSTRACT
Cooling technologies are a major issue in gas turbine
devices. Certain parts of them undergo high variations of
temperature and, by a consequence, need to be efficiently
cooled in order to avoid serious damages.
This project will focus on turbine blades. Several cooling
technologies are available to guarantee a safe running
environment. They are divided into two main parts:
external and internal cooling. Much more material would
be needed to present both of them. Thus, this work will
only develop two common technologies which take part
of the internal cooling section, the pin fins and the
turbulence promoters.
INTRODUCTION
Cooling technologies on gas turbines are still a major
concern. Thus, gas turbine manufactures don’t present
their own technology, even if the basic principles are
similar from one to another.
For a modern gas turbine, the blade cooling airflow is
about 10 to 15% of the total compressor flow. A major
challenge in achieving high turbine efficiency is to
minimize turbine cooling airflow with the best utilization
of its cooling potential.
PROBLEM STATEMENT
The following illustration presents a section of a turbine
blade. The inlet cooling air comes from the bottom of the
blade and then, follows different channel networks: this is
an internal cooling.
Figure 1 - Turbine Blade section
The inlet airflow is divided into three parts. The first one,
on the right of the blade which corresponds to its thinnest
part, goes directly through small holes which are
distributed all along the downstream part of the blade:
these are the “Pin fins”.
The second flow, in the middle part of the blade, is forced
to pass through a complex network of pipes which
contain obstacles whose role is to generate turbulences
and consequently, to increase cooling effects: this is the
reason why they are called “Turbulence promoters”.
The third flow, on the left part of the blade, goes either
through turbulence promoters or, thanks to internal
connections, to a region marked “Impingement cooling”
on the previous picture. The role of the latter one is to
Copyright © 2011 by Erwan Le Gloanec
create an air cooling film on the upstream part of the
blade.
Most of the time, pin fins are not used in a single row.
Those small cylinders are placed on several series, along
a straight line or a shift line as illustrated in the following
picture:
LITERATURE SURVEY
A previous study was realized on blade cooling
technologies, entitled “Refroidissement des turbines à gaz
– Techniques et efficacité” by Bruno Facchini and Luca
Innocenti. It presents both internal and external cooling
technologies used on gas turbine devices. Moreover, it
introduces the way of conceiving a turbine blade by using
CFD calculations.
Another survey has been used for this work, entitled
« Simulation d’un couche limite perturbée par un
obstacle » by Abdelkader Lahcene, Miloud Aminallah
and Samir Laouedj. The purpose of this project was to
study the effects on pressure and the thermal exchange
coefficient of the presence of obstacles inside pipes,
which are subject to a turbulent air stream flow.
Using shift lines increases the turbulence effects on the
airflow which, as we mentioned previously, also increases
the thermal exchange coefficient. From the root of the
blade, the latter is relatively limited on the first lines of
pin fins. Then, it progressively increases at each stage of
pin fin lines because of wakes and turbulences generated
by the upstream pin fins. This phenomenon stabilizes
from the fourth or the fifth stage.
Most of studies about this subject use pin fins which have
the same height in order to simplify computations. But in
reality, pin fins inside the downstream part of the blade
have a variable height within the pipe. The acceleration of
the stream tends to decrease the thermal exchange
coefficient by reducing the Nusselt number dependency
on the Reynolds number.
PROJECT DESCRIPTION
Pin Fins
Aerodynamic losses on airfoils depend on
the thickness of the downstream part of the
blade. For this reason, the latter has to be as
thin as possible. Consequently, multi-pass
channels are not used on this part of the
blade to avoid the thickening of the
downstream part of the airfoil. Thus, small
cylinders (the so-called “pin fins”) are
introduced in a narrow channel from the
root of the blade. They are placed in an
orthogonal way compared with the airflow
in order to considerably increase the
turbulence effects: this phenomenon leads
to increase the thermal exchange coefficient
which improves the cooling effect.
Figure 3 - Two different placements of pin fins
Metzger developed a relation between the Nusselt number
and the Reynolds number for 10 pin fins:
𝑋 −0.34
𝑁𝑢𝑑 = 0.135 ∗ 𝑅𝑒𝐷0.69 ∗ ( )
𝑑
Figure 2 - Pin
fins along a
turbine blade
The previous relation is valid for short pin fins with
1.5<X/d<5, Y/d=2.5 and h/d=1. It can be applied to other
number of pin fins by modifying the Nusselt number, as
illustrated in the following graphic:
Besides, depending on the ratio between the height and
the length of the pin fin (h/d), the thermal exchange area
may arise. Pin fins have a ratio 0.5<h/d<4. If h/d<2, the
pin fins are qualified as short ones: in this case, the heat is
mainly exchanged by the lateral area and the overall
exchange area is reduced by the presence of pin fins.
Copyright © 2011 by Erwan Le Gloanec
Airflow
Figure 6 - Removed ribs
Figure 4 - Evolution of the Nusselt number
The Nusselt number represents the ratio between the
thermal transfer by convection and the thermal transfer by
conduction. The convective part represents the heat
transfer through the airflow along pin fins. The ratio
𝑁𝑢 ⁄̅̅̅̅
𝑁𝑢 (which is the ratio between the local Nusselt
number and the average one) tends to stabilize around one
as the number of lines of pin fins increases. This means
heat transfer occurs both thanks to the convective and
conductive way. The maximum value of the ratio is
obtained for four lines of pin fins: since its value is larger
than one, the convective part is a little bit more significant
than the conductive part.
If the ribs are stuck on the inner surface of the pipe, a
recirculated airflow is going to be created just after each
rib, which will decrease the thermal exchange coefficient
in this specific area. On the contrary, if the ribs are
removed from the inner surface of the pipe, the airflow
will be separated into two parts, on both side of each rib.
Therefore, because it doesn’t generate a secondary and
stationary airflow, the cooling effect will be more
efficient in this case due to a higher thermal exchange
coefficient.
This situation is illustrated through the following graphic:
it represents the thermal exchange coefficient as a
function of x which is the airflow direction indicated on
the two previous sketches.
Specific pipes to increase turbulence
effects
Certain blades are fitted with specific pipes: instead of
being slick, they contained bumps whose role is to
enhance the turbulence phenomenon (those bumps are
also called ribs). Indeed, using ribs considerably increases
the level of turbulence and, at the same time, the value of
the thermal exchange coefficient. Thus, by using limited
airflow, the cooling efficiency is better than the one
observed with slick pipes. However, because it is
technically more complex to settle, using ribs inside pipes
is more expensive.
Two types of ribs can be used: they are either stuck on the
inner surface of the pipe or removed from it.
Airflow
Figure 5 - Stuck ribs
Figure 7 - Evolution of the thermal exchange coefficient
Due to the recirculated and stationary airflow, the thermal
exchange coefficient presents strong variations after each
rib. This is not the case with removed ribs and that is
probably the reason why it is more efficient in terms of
cooling. Blades undergo both convective and conductive
phenomenon which mainly determine their running
temperature. To ensure a high efficiency, the area close to
the combustion chamber imposes really high temperature
to the closest compressor and turbine blades. Those
temperatures are not so far from the melting point of the
blade material: thus, it is extremely important not to have
undesired variations of temperature due to strong
evolutions of the thermal exchange coefficient. As a
consequence, removed ribs seem to be more used than
stuck ribs.
As shown on the illustration on the first page, turbulence
promoters are often used in multi-pass devices. The shape
Copyright © 2011 by Erwan Le Gloanec
of the heat exchanger is likely to be a streamer which
contains several pipes whose axis is radial. Alternately
crossed by centrifugal and centripetal airflow, those pipes
are connected thanks to bends, most of the time slick
ones. Streamers are located in the central part of the
blade, where the thickness is not a crucial factor and
thermal solicitations are less important than the ones on
the front part of the blade.
In the case of multi-pass streamers, the effects of the
rotation modify the thermal exchange mechanism.
Because of the presence of both centrifugal and
centripetal pipes, the Coriolis force alternately increases
thermal exchanges on the side of the pipe which is turned
up (the leading surface) and on the one which is turned
down (the trailing surface) as illustrated in the following
picture:
𝑁𝑢 = 𝑎 ∗ 𝑅𝑒 −𝑏 ∗ 𝑃𝑟 −𝑐
The previous relation is valid for a slick pipe. In the
presence of ribs, the “b” exponent on the Reynolds
number has to be lower of approximately 4% and the “a”
coefficient has to be multiplied by 2.7.
CONCLUSIONS
The use of pin fins is one of the only technologies
available to cool the downstream part of the blade since it
needs to be really thin.
Concerning the turbulence promoters, using removed ribs
seems to be a better solution than stuck ribs in order to
limit the variations of the thermal exchange coefficient.
REFERENCES
[1]
Klaus Brun and Rainer Kurz, 1999, Introduction
to Gas Turbine Theory – An overview of
fundamental concepts. Book
[2]
Bruno
Facchini
and
Luca
Innocenti,
Refroidissement des turbines à gaz – Techniques
et efficacité, Techniques de l'Ingénieur editions,
TI-bm4566
[3]
Abdelkader Lahcene, Miloud Aminallah and
Samir Laouedj, 2005, Simulation d’une couche
limite perturbée par un obstacle. Paper in
Conference Proceedings
Available on the website http://web.univubs.fr/limatb/EG2M/Disc_Seminaire/CFM2005/a
rticles/157.pdf
[4]
www.wikipedia.com
[5]
Course Material
Figure 8 - Multi-pass streamers
Those types of devices have a great cooling efficiency
and the coolant fluid undergoes high variations of
temperature. Here, there are not any analytical methods to
calculate the thermal exchange coefficient for a turbulent
flow. Indeed, the presence of ribs generates flow
movement really complex like airstream separations or
recirculation of fluid. Thus, most of surveys use thermal
exchange coefficient determined from semi-empiric
correlation which results from known geometries.
In 1988, J.C. Han and J.S. Park developed such
correlations to predict the behavior of a rectangular pipe
which contained ribs on both side of the inner surface and
placed orthogonally to the inlet airstream, in a fully
developed turbulence flow. The thermal exchange
coefficient is computed after having determined the
Nusselt number:
Copyright © 2011 by Erwan Le Gloanec