You have just had a good exposure to the various
fundamental heat transfer science, and
an indication how that science of heat transfer
is used for axial flow turbine cooling. In
today's lecture, we will be looking at the
technology of gas turbine cooling. Now, cooling
of axial flow turbines especially in aero
engines have been in above for nearly 50 years
now. So, the cooling technologies, indeed
50 years, it did not start with cooling
technology axial flow turbines in the early
era were without cooling technology.
However, for nearly 50 years now, various
kinds of cooling technology are being used
and we will look at these entire generations
of cooling technologies in today's lecture.
In
fact, all kinds of those cooling technologies
are still in use today, because you need some
of those cooling technologies are even today
in simple gas turbine engines, whereas
more modern or more ambitious, high performance
gas turbine engine may require more
advanced cooling system.
So, if you are creating a simple gas turbine
engine, old fashioned cooling technology
would still be useful, and hence, those technologies
are still in use in some form or other
in many gas turbine engines especially in
aero engines even today. So, we will look
at all
those technologies that have developed over
last 50 years. Now, as I mentioned the
cooling technology is based primarily on the
science of heat transfer.
The idea of cooling actually came from the
gas turbine thermodynamic cycle analysis,
which very clearly showed that, if you can
increase the turbine into temperature, you
would get a substantial benefit in terms of
the work done by the cycle, which of course,
in terms of the gas turbine engine means more
work done by the turbine, more work
available either for creation of thrust for
aero engines or for running a propeller which
creates thrust, or for land based application,
it simply means that you can get more power
output out of the single gas turbine engine.
So, those fundamental science of thermodynamics
finally bore down on the fact that you
need to have cooling, because the material
science was unable to go beyond a certain
level to provide material or alloys, metal
alloys that could withstand temperature beyond
a certain limit which is roughly of the order
of 1,000 degree key. Now, beyond that, it
became very apparent that, ordinary material
technology is not going to really, be really
greatly helpful, and in which case, additional
techniques are required if you want the
turbine entry temperature to go even higher.
And that is when the cooling technology came
in and of course, the entire field of heat
transfer had to be utilized. So, in today's
class, we will look at the technology. In
the last
class, you had exposure to the science of
heat transfer and we have done all the
aerodynamic that is necessary on axial flow
turbines. So, in today's class, let us take
a
look at various kinds of axial flow turbine
cooling technologies.
Now, much of these cooling technology does
in indeed involved a lot of engineering.
You will see that lot of things are engineered
in science of heat transfer creates the basic
fundamental basis. As I mentioned, a thermodynamics
showed as the way that you need
to have high turbine entry temperature, which
then requires cooling, which uses the
science of heat transfer, and then, there
is a whole lot of engineering the thermal
engineering, the mechanical engineering, the
manufacturing technology, all that comes
into picture and we will have a look at some
of these technologies in today's lecture one
by one. Let us take a look at what are the
fundamental issues involved and how the
whole thing has developed over little more
than 50 years.
If you look at the graph which is available
in any books probably, easily see that in
the
early era in 1950 or so, most of the blades
were actually on cool blades. So, but they
could take up to about 1000 degree centigrade
or so, and then from there onwards if you
are an aspiring toward higher turbine into
temperature, you have to resort to some kind
of cooling technology beyond; otherwise, the
turbine in inlet temperature could not be
improved upon, and irrespective of what material
used; the material technology was not
quite in your helping the matters.
So, are simple cooling technology was used
in which certain amount of cold air was
indeed passed through the blade. This provided
a little bit of cooling of the order of 2530
degrees to begin with and that was quite of
insufficient to raise the temperature by 50
degrees or there about, and slowly the temperature
started rising as the cooling
technology improved upon their it, its, is
basically in engineering method coupled with
the heat transfer science of heat transfer,
and as a result of which, the temperature
could
go up to near about 1400 degree also, and
then of course, people realized that you need
more complex cooling technology to go higher
up, and hence, more complicated cooling
technology came which we will be talking about
in today's lecture, and hence, more
sophisticated cooling system came in which
involve film cooling and then injected
impingement cooling so on and so forth, which
has taken the turbine entry temperature to
near about 2000 degrees.
So, very modern gas turbines do have turbine
into temperature very close to1900 or 2000
K. However, the material, as I mentioned the
material technology has not really helped
matters a great deal over here, and the projected
trend that was shown earlier and by now
we should have reached values of the order
of 2200 or 2300 degree k, that is indeed
actually not happen, and one of the reasons
is some of the cooling technologies that were
projected and we will be talking about it.
For example, transpiration cooling, those
kind
of cooling technologies have not matured.
One of the reasons again is the material
technology.
The new material, the porous material which
was to facilitate transpiration cooling has
not really happen, and as a result of which,
the cooling technology did not project
beyond what is shown here and this projected
development not really quite matured.
There is a lot of research still going on
and we are not quite as yet crossing the 2200
k
mark in actual commercial applications.
So, that is where we are. We are near about
2000 or so and that is good enough for us
actually to use in most of the modern gas
turbine engines, and we shall see that since
the
pressure ratio has gone up, the efficiency
of the turbine has been actually upgraded
hugely through up gradation of compression
ratio, and this up gradation of turbine entry
temperature coupled with a high compression
ratio has given gas turbine engines huge
efficiency boost, and in terms of fuel saving,
there is a huge fuel saving in the modern
engines. So, turbine entry temperature boost
actually is known from fundamental
thermodynamics to give better and more efficient
engines and provide more power.
Now, if we simply summarize the whole thing,
in the fifties the blades were uncooled the
early era of gas turbine engines, and as a
result, the temperature is were some or around
1000 till 100 K. And then of course, the cooling
technology came which were internal 1
or 2 pass cooling. We will look at this one
to pass cooling a very soon today, and that
and the temperature could go up to about 1200
to 1400 k that is about 150 degree more
where possible with cooling. Later on the
cooling technology developed into internal
convection cooling which allowed lot more
coolant to be used and not more elaborate
cooling system, and temperature could be pushed
to 1300 to 1500 K, and then, came of
course, the film cooling towards the end of
seventies which existed along with the
internal convection cooling.
So, film in cooling plus internal convection
cooling pushed the turbine inlet temperature
to higher than 1600 k, and then of course,
the impingement cooling matured. The idea
which has been around for a long time and
has pushed that temperature is now too close
to 2000 K. There about are the whole thing
is kind of a stagnated a little because the
transpiration cooling has not quite matured.
It has been as an idea, it has been around
for
nearly 50 years it involved micro channel
cooling methodology. The science of features
a well known, but the technology is not quite
available as of today, and the porous
material that can withstand the stresses in
a turbine blade has also not quite matured
as of
today to the best of our knowledge. So, the
turbine technology has matured hugely over
a
period of 50 years and as resulted in engines
that are more powerful and of course, which
are indeed smaller in size.
Now, will be talk about turbine cooling. The
first thing that comes to mind is the water
you cooling. You see that turbine is, is,
processing gas which is hot and of course,
high
pressure, but the blade feels at temperature,
the feeling of the blade here is the important
issue. The blade feels a temperature which
is not same as the turbine entry temperature.
Say the turbine entry temperature T 0 1 and
the blade exit temperature is T 0 2. What
the
blade would feel is an average of the 2 minus
the rotating kinetic head which is shown as
u mean square by twice c p of the gas which
is passing through plus the degree of
reaction factored in here. Now, this gives
rise to the situation that other Blade
temperature has felt on the blade surface.
Average blade temperature is something quite
different from the gas temperature.
So, gas temperature is not what the blade
feels. The blade feels a different temperature
arguably and as is quite obviously it will
feel at temperature lower than a gas
temperature. So, the requirement for cooling
is not with respect to the gas temperature;
it
is more with respect to what the blade actually
feels. So, that is a temperature that has
to
be cooled through cooling technology. The
science of the transfer that have done in
the
last class; we can simply bring in here.
The fact that the heat transfer coefficient
is equal to the quantity of heat to be transferred
at any local point on the blade, on the blade
surface in at the location of the blade which
is normally given core wise in terms of often
x y c on the core, and in the local surface
area over there, the temperature differential
between the hot gas and the surface as we
have just seen. The blade surface temperature
is indeed less than hot gas at temperature
and it will vary from the local point to point
on the blade surface, and then of course,
the
time that is required to affect the heat transfer.
Quite often the heat transfer is expressed
as we have done in the last class in some
detail,
in terms of Nusselts number and they are known
to be proportional to Reynolds number
Pand prandtl number. This is also an semi
empirical relationship which has been
developed essentially for simple systems like
flat plate, and Nusselts number is a directly
relatable to Reynolds number and Prandtl number.
For simple systems through some
constant terms which are used to create this
semi empirical relationship. Now, this is
the
kind of science or technology that one would
need to bring forth in the turbine cooling
business.
Now, let us look at the technology. What happens
is, when you have ah blade, this is a
typical turbine blade and you are looking
at a diagram which probably you had at look
at
in the last lecture also, but let us discuss
this again. The flow comes in here, and at
this
point, you have the leading end which is the
stagnation point.
Now, what happens at stagnation point? The
flow comes to a halt. When the flow comes
to a halt, thermally that place the total
temperature is equal to the static temperature,
which means this point of the blade. The blade
feels the entire total temperature of the
gas flow; that means a static temperature
plus the kinetic head that the gas is carrying.
The entire thing is felt over here at the
stagnation point.
So, the stagnation point is indeed the Hot
Spot, the hottest spot on the blade surface.
As
the blade picks up speed, it actually drops
of the temperature, and then, through the
transition from laminar to a turbulent flow
as the, it flows over the gas blade surface,
the
gas actually accelerates. As it accelerates,
the local temperature, the static temperature.
Remember is the static temperature that blade
would be indeed stealing. So, the local
static temperature starts falling off. So,
as you have expansion or acceleration over
the
blade surface, the static temperature indeed
falls of; however, as you are a where
aerodynamically towards a trailing edge, there
is an another possible stagnation point
depending on the flow situation, and summer
on that stagnation point he would again
feel Hot Spot.
So, the flow again would feel the full temperature
field over, there that is, static plus
kinetic at that point same thing happens over
the pressure surface. So, the temperature
there is a continuous variation of temperature.
So, just like pressure, you can see that as
the pressure varies over the blade surfaces;
the temperature also would vary over the
blade surfaces and inner turbine. This variation
is a huge unlike in a compressor where
the pressure varies hugely, but that temperature
does not probably vary so much in
turbine of the temperature would vary hugely.
Now, as you have done in the last class as
the flow transits from laminar to turbulent
flow through the transition. The head transfer
from the hot gas to the body of the blade
is
facilitated, which means more transfer across
the body of the blade as a result, the blade
would actually be getting more heated towards
the rear part of the blade where the static
temperature may be falling off. Here, the
blade is probably somewhat shielded by the
laminar layers, and as a result of which,
the blade feels lower temperature even though
the temperature outside is actually much hotter.
So, near the leading edge, temperature outside
is much hotter but the blade feels less, but
later on, even if the temperature is falling
off, the blade actually is made to feel much
higher temperature on the blade surface because
the boundary layer outside is turbulent
boundary layer. So, with this knowledge, if,
we can move towards the technology.
Now, what happens is as we were discussing,
we have a Hot Spot on the leading edge.
You have a Hot Spot at the trailing edge;
a simple aerofoil is being shown here and
you
have a huge temperature gradient on the chord
wise direction on the blade surface, on
both the surfaces. In a rear turbine, two
surfaces and unequal what is simply done as
you
create blade passages, internal passages through
which cold air is passed.
Let us say from one end of the blade to another
which is from hub to tip. Now, the first
row of Blades is of course, the stator which
is fixed at both ends. So, you can indeed
pass
air from hub to tip or a tip to hub cold air,
and then, that cold air would convict away,
carry away heat. That heat has to first conduct
itself through the blade solid body from
the surface into this cold air and then the
cold air would carry away the heat on a
continuous basis. So, there is a continuous
conduction across the solid body of the blade
surface, and then, continuous convection by
the cooling air which is passing through the
passages on a continuous basis. So, there
is a continuous requirement of cooling air
when
the gas turbine is operational. So, this is
a simple method by which the cooling
technology was inducted into the gas turbine
cooling.
If we look at a little more elaborate system
this is the CFD plus and this simulation and
it
gives an idea about the kind of temperature
profile you have in a typical gas turbine
Blade, and what you do required to do cooling?
You have a Hot Spot here; you have Hot
Spot around here. The trailing edges very
thin and is very difficult to provide cooling
over there air in terms of internal passages.
So, these are the internal passages as you
can see here, the shapes have evolved to not
only allow more passages in a distributed
manner. So, you have more distributed
passages, also you have more control over
the amount of cool and passing through these
passages. So, you have more control over what
is happening. So, this is one passage
which is let us say cooling mode near the
leading edge area, which as be discussed is
area and then they are distributed coolant
passages so that the cooling is distributed
over
the Blade surfaces from both the surfaces.
So, this is a rear Blade that has been fabricated,
and one can see that these are the various
kind of passages that have been created by
through modern fabrication technology in
high temperature material. Remember, turbine
uses high temperature material. So,
making such passages is requires very modern
manufacturing technology. The upper
plate that you see actually is a forged Blade
were these passages were created and the
Blade was made by forging and the passages
which were before forging looks something
like this, and once the Blade is forged into
the shade, the passages take shapes. However,
this Blade is fabricated; that means these
passages and machine in high temperature
material which requires very high modern fabrication
technology.
Now, what we see is that the blade temperature
may vary along the blade surface from
leading edge to trailing edge by almost 200
to 300 k and that is a very large variation
to
carted to, which means that temperature over
here or over here may be 200 to 300 more
than the temperature over here and the temperature
is very all along the Blade surface.
Now, this requires a cooling technology which
would create to the amount of cooling
required here, the amount of cooling required
here, here and here. So, that is what we
discussing that the amount of coolant to be
used here, over here. over here, and then,
the
other trailing edge over here would indeed
vary substantially depending on the Blade
shape, depending on the temperature at which
is an operating, and of course, depending
on the cooling air that is made available.
Now, all this needs to be calculated very
accurately. This accurate calculation is where
engineering and heat transfer science comes
in very strongly. You need to calculated
very accurately what is the amount of air
going to each of those channels, and as you
can
see the shape of the channels, size of the
channel varies depending on where the channel
is located on the Blade.
Now, this requires that you have a very accurate
method of calculating how much
cooling air is required where. Now as we can
see here the temperature and the heat
requirement cooling requirement varies substantially.
So, this needs to be very accurately
calculated. So, and then of course, as you
can see, you have to technology in which these
shapes are created. So, now, the shapes carry
a certain amount of cooling air, and as you
can see here, this has been created. Finally,
it will exit through the trailing edge. As
I was
mentioning trailing edge cooling is a possible
problem, where as the trailing edge in
know is a Hot Spot.
So, the modern as has created facilitated
a process by which the cooling air finally
may
exit through the trailing edge, cooling the
trailing edge itself. So, those are the various
technologies that have to be brought into
in the turbine cooling business. So, as we
are
way of saying that the Blade temperature also
may vary from root to the tip of a rotor.
Stator by design as we have seen may be kept
and twisted, but the rotor may have a little
bit of twist and that will give rise to temperature
gradient from root to tip.
So, that again needs to be factored into this
very accurate cooling technology that we
need to incorporate in modern the actual flow
turbines. Now, maximum temperature is
felt at the leading edge of the first stator
as the flow just comes from the combustion
chamber. So, the first rotor in his first
stator is what requires the best of the cooling
technology if you increase the turbine entry
temperature.
Now, we will talk about more modern cooling
technology as we go along today. The HP
turbine Blades both the stator and the rotor
face maximum temperature across the rotor
and the stator. So, almost all modern aero
engine HP turbine Blade stator and rotor are
cooled. In the earlier nineties only the hp
stator was cooled, but later on both rotor
and
stator are cool, and in today's modern aero
engine, HP turbine first stage as well as
second stage are most likely to be both rotor
and stator are likely to be cooled. Now,
there is on other issue here - the Blades
are thermally loaded in cycles of operation.
Now, this essentially means that you have
a situation when that gas turbine starts
operating. That are while gets heated up,
it goes to full temperature, and then, it
operates
at full temperature during let us say when
the aircraft is flying from takeoff to climb
and
then it goes to cruise. Now, when it goes
to cruise, as you know that are while is actually
brought down to lower temperature level, lower
speed level lower are rpm rotating, and
hence, its operating at a lower temperature.
Then after some time, the turbine engine is
brought down to a lower in operating speed,
slowly the aircraft comes down, it touches
down and it comes to halt; the engine is close
down. So, the turbine is heated up; then it
is cool down, operates in slightly less heat
condition through the cruise and then is shut
up.
So, the turbine is continuously going through
this cycle of temperature. For long period
of time, when the engine is not operational,
it is a cold turbine; it is just static. When
the
engine is operational, it goes through heat
cycles of the very height cycles. If its military
engine, it is going through very short transients.
Very high temperature sometimes not so
high temperature some other times. So, it
is going through cycles of temperatures, and
then, again when the engine is grounded, the
aircraft is grounded, it is cold.
So, the material of the turbine is continuously
being treated to high temperature, and
then, for long period of time to cold state
of static in operation. So, this gives rise
to the
fact that the turbine is treated to a cycles
of temperature. So, many of the turbine failures
occurred in this fatigue failure which is
due to this cyclic loading of the Blades.
Now,
remember, turbine is also a rotor is especially
getting highly loaded due to the
aerodynamic loading; the aerodynamic load
is huge when the gas is flowing pass the
turbine Blades.
So, this aerodynamic load is now compounded
with the thermal loading which as we are
just seeing changes from leading edge to trailing
edge, from root to tip. There is a
continuous variation of temperature on the
Blade surface over the entire Blade of the
turbine. Now, this guide gives rise to temperature
gradient and loading pattern changes,
and when coupled with aerodynamic loading,
it gives rise to a very complicated loading
pattern and very complicated stress pattern,
which as we see now actually goes through
cycles of operation depending on whether the
turbine is operating or it is grounded and
static. The result is that turbine failure
of often occurs in creep mainly due to the,
this
thermal cycles.
So, creep is the fatigue failure due to cyclic
operation of the gas turbine engines. So,
the
failure of the gas turbine often occurs in
creep which is the thermal fatigue kind of
compounded stress at occurs mainly due to
the thermal fatigue. The compressor Blades
for example, do not fail in creep, they fail
only in a marlin loads during operation, those
Blades of course a much thinner. The turbine
Blades are much thicker as we have seen,
but the thermal load indeed creates a lot
of problem.
So, even though turbine blades are thicker,
they are made of high temperature material
even than the life of a turbine Blade is indeed
often much less than that of a typical
compressor Blade. So, this is a huge problem
in terms of life of turbine blades, and of
course, the cooling technology helps a matter.
There are many blades where if you do not
apply cooling, actually the blade would get
chart in a matter of few minutes, whereas
if
you apply cooling, the blade would last for
1000s of hours of operation.
So, the difference between cooling and not
cooling a modern gas turbine makes a
difference in life of turbine blade by 1000
of hours. If a gas turbine into temperature
is of
the order of 1600 or 1700 degree k, and if
that Blade is not cold, rest assured that
blade
will get chart in a matter of few seconds.
So, that is the different between on cool
blade
and a cool Blade in modern gas turbine applications.
So, that is rather cooling comes in.
Let us take a look at the situation. If you
have a typical turbine Blade and if you would
have this kind of let say very a simple cooling
passages so many of them, you know, 1 to
16 of them, they have to run through the entire
length of the blade, of a turbine Blade
from root to the tip. So, typically the air
would be brought in from compressor.
Compressed air to be brought in through the
root system. You have the effort root fixing
system here, it comes in here, and then, it
gets into these passages.
You can have so many passages or you can 4
to 5 passages as we have seen in the earlier
slides, and they all then go through this
entire length cooling the Blade at different
places
in a differential manner. On the right inside,
you just see research output of a internal
temperatures as captured in a French laboratory,
and you can see the kind of temperature
profile that you get some of the internal
temperatures are very high. Here, the internal
temperature is somewhat less, because as we
have seen the flow around here is laminar
in nature, and as a result, the Blades do
not feel so much. Some are over here the Blades
become the outer surface, the flow becomes
turbulent.
Once it becomes turbulent, the gas, hot gas
temperature flows inside very quickly and
then the Blades start getting hugely heated
up. So, the cooling requirement indeed here
is
more even though the temperature near the
leading edges actually more. So, this
aerodynamics actually facilitates the heating
of the Blades around this area, and this is
where you indeed require more cooling, and
that is why most of the cooling technology
in the early era in 60’s and 70’s and
indeed in 80’s the cooling was mostly being
done in
this area, but of course, we know and we will
see today that more and more modern
cooling is available near the leading edge
as well as near the trailing edge. As the
temperature has gone up, you need to indeed
cool the entire Blade.
Let us take a look at the cooling technologies.
The fundamentals of the cooling
technologies that we are talking about. We
mentioned that the cooling is done by internal
convection cooling which essentially means
that you have so many passages, and
through them, the cooling air is passed. This
is let us say the blade surface or the solid
body of the blade. Inside of the solid bodies,
you have are so many cooling passages
through which cooling air as we passed on
a continuous basis and outside you have the
hot gas.
A variant of this is that the cooling air
is blown into the inside surface of the outer
shell
of the blade. So, the Blade is made of a shell
which is a solid body of course. Inside, you
have another shell; inside of which you have
the cooling passage may be a common
cooling passage which has holds. Now, these
holes allow this cooling air to come out an
impinge on the entire inner surface of the
outer shell, and this is called internal
impingement cooling, and if you do that, the
internal entire internal surface of the outer
shell is cooled on a continuous basis through
internal cooling system. So, this is simply
called internal impingement cooling, and we
shall see this is how the leading edges of
most of the modern turbine Blades are actually
cooled.
(Refer Slide Time: 36: 12)
The other method is simply called film cooling.
Now, the kind of matter that the sorts of
the internal cooling has their limits. Once
those limits are reached, you need more
explicit and more active cooling system. So,
the cooling air itself is now brought out;
its
not inside any more. You need to bring it
out, and then, this cooling air is brought
out
through a holes and these holes (( )) out
the air and then create a film on the blade
surface.
So, on the blade surface, you create a film
of cold air, and then, this cold air provides
cold film, which means it submerges inside
the boundary layer on the outside of the
blade surface. So, you have a cold boundary
layer on the blade surface. If you provide
a
distributed holes on the, inside the blade
so that through various holes the inside internal
cold air is brought out and is injected out
onto the surface to create films of cold air
on a
continuous basis over the entire surface of
the blade.
Now, if you do that, you have a continuous
cold air. Now, what happens is, if you are
for
example, brought out cold air over here in
the first cold, by the time it reaches some
distance, that cold air would get mixed with
the hot air would become hot. So, you need
to bring out cold air again, which would again
then become hot after as little distance.
So, you need to bring out cold air every short
distance over the blade surface to keep the
continuously flowing cold boundary layer on
the blade surface.
Now, remember, we want a cold boundary layer.
We do not want this cold air to eject
out like a jet into the hot gas. That would
interfere with the turbine operation and would
adversely affect the turbine working. So,
the work done from the turbine would go down
very sharply, because the basic aerodynamics
that we have discussed in detail would
then be badly affected by this cold air.
We do not want that. We do not want the cold
air to eject out of the those cooling holes
in a stream of jets and get into the hot air
and interfere with the hot air operation which
is
a different operation. The hot air is flowing
over the turbine; it is giving up energy to
the
rotating turbine blades is doing work; transferring
work onto the blades. The blades are
rotating and creating mechanical energy. All
that would be adversely affected if you
allow this cold air to eject out like a jet.
We do not want jets; we do not want cool jets,
we want cold air to just come out very
gently through the holes and create a film
on the blade surface. Then we have a film
cooling. So, the boundary layer which we saw
was becoming turbulent boundary layer
subsumed inside the boundary layer would be
this cold air, and this cold air would create
some kind of a insulation between the hot
gas and the body of the solid body of the
blade. So, this is how the film cooling technology
actually works. If you can do that
correctly, you have a very good cooling system.
If you cannot do it correctly, you are
going to be affected by a turbine work.
Now, the transpiration cooling which people
have been talking about for 50 years now
actually involves that, you have a porous
solid body or outer shell of the turbine Blade.
So, you have cold air oozing out like perspiration
through the solid body of the blades to
create continuous film cooling.
Now, this is of course, the ultimate ideal
of film cooling. You just allow the in air
to go
inside the turbine blades, inside the blade
passage internal passage of the blades with
some pressure and that pressure will drive
the flow out through this force. So, it is
a
porous body, and through which, it will create
just is goes out, again there is no jet to
be
brought out. It will just goes out and create
a film on the surface to create a cold film
on
a continuous basis over the entire surface.
This is much the same way we actually
perspire and chord body sort of gets cold
when the perspiration actually evaporates
from
our skin.
So, it is same concept; however, what is happened
is, over the years, this porous
technology, porous medium that is required
to make the turbine blades has not quite
mature to date. It cannot take the strength.
As we have just discussed, turbine blades
need to be very strong withstand dynamic loads,
to withstand the thermal loads;
otherwise, the life of the turbine as we have
seen can be up in, in, few seconds.
So, the porous material the we need to be
strong enough to withstand all those cycles
of
temperatures and gas loading and work for
1000s of hours preferably ten 10 15 20 1000
hours. That has not happened, that material
has not come through, and as a result, the
transpiration cooling has not quite mature
to date.
So, we have the film cooling in which, you
need to very accurately calculate through
each hole what should be the amount of a coming
out through this hole. What should be
the dimension of each of these holes depending
on where the hole is on the blade, and
then of course, the pressure ratio across
this particular hole, because the pressure
ratio
will drive the flow from inside to outside
and it should be just sufficient for the air
to
come out and create a film.
If the pressure ratio is too high, that means,
if the internal pressure by some chance is
much higher than the outside pressure, pressure,
just over here, the air will inject out like
a jet, and as we just discussed, that is most
unwanted. So, we need to create a pressure
ratio at each of these holes exactly as much
as required for the air to come out at this
particular location and just create a film.
As I mentioned, this requires very accurate
calculation and very accurate estimation of
what is happening gas dynamically and
thermally over the turbine blades .
Now, let us look at some of the details of
the technology. As we can see here, we have
a
ah system. Let us look at the bottom you have
one single channel through which, the air
is a ah internal cold air is being brought
in mainly from the compressors, compressed
air,
and then, this common air flow channel is
indeed making air coming out through various
holes in; it cools inside surface; it cools
entire surface, and finally, it comes out
from the
trailing edge, cooling the trailing edge as
well. So, it cools the insight surface by
impingement, internal impingement cooling.
So, the entire inside surface is been cooled
by internal impingement cooling, whereas,
here we have a holes. So, impingement cooling
is used at the leading edge to cool the
leading edge, which has been mentioned, it
can be a Hot Spot, and then, of course, it
cools the other surfaces through internal
cooling system, and then again here, we allow
the air to come out through that trailing
edge.
In the process, cooling the trailing edge
also you remember. After it cools internal
surfaces, the, the cooling itself gets a little
hot; however, as we have just seen the trailing
edge is very hot, and compared to that, this
internal air is still cold. So, all you require
is
air that is colder, substantially colder than
the outer hot gas and it still can do a little
bit
of cooling. You do not need very cold air
here; you need air that is a little colder
than
what is outside.
Now, this is a picture of a full bleed film
cooling. We have so many passages, separate
passages through which air is a brought in.
Here, in this picture, you have two large
passages. In the picture over here, you have
number of passages. Each of which caters to
one hole or two holes, and through which,
air is brought out, and each of these holes,
then create a little better of film over here;
it is a continuous process. As we just saw,
the
early air gets heated up surface bring in
more air is on a continuous basis cold air
is
brought out to create film on the blades have
faced on both the surfaces really and
finally, of course, certain amount of air
may be brought out near the trailing edge
to cool
the trailing edge area.
So, you may bring out the trailing edge air
through the trailing edge or through some
holes near the trailing edge which creates
film over the trailing edge to cool the trailing
edge area here. On the surface, on the blade
over here, which of course shows a threedimensional
picture, you have two large internal passages
- one passage over here,
another passage over here, which of course,
exits a flow through the trailing edge. Many
of the modern turbine trailing edges are indeed
not rounded, but truncated like this or
sometimes straight away blunt to allow this
cold air to come out.
So, this is something which requires very
high manufacturing technology typical for
gas
turbine a manufacturing, and this you shown
over here. You can see the discrete holes
that are made over here. On the blades surface,
which allows this internal air to come out
from inside onto the blades are faced and
create the films on both the surfaces. Thereby,
cooling this area which we are mention many
times that, it is area that gets terribly
heated up.
So, you are very elaborate cooling system
including film cooling that comes out over
here and creates film cold film over the surface.
Then you have another row of blades
holes coming out through the blades, which
create on other row of another round of film
cooling. Then, you have another again row
of holes in the through which, cold air comes
out and a final row of a holes near the trailing
edge. So, they have calculated very
accurately how many rows of blades are required
and where on the blades are faced in
the chord base position to create appropriate
film cooling on this particular blade. So,
this is how you create the cold film over
the entire blade surface in a very accurate
manner to affect effective cooling over the
entire blade.
This is a picture of a, typical picture of
a turbine blade of modern turbine blade. This
is
the stator; this is the rotor, and as you
can see, both the stator and the rotor have
elaborate holes over the surfaces which loose
out the internal cold air on the surface and
create the cold film on the necessary on the
surface to affect the cooling. As you can
see,
the stator has much more elaborate cooling
technology. The modern Blades also have,
the rotors also have an elaborate cooling
technology on the rotor blades, rotating blades.
So, this is the elaborate cooling technology
that you need to do. Manufacturing these
Blades is extremely costly affair and each
single turbine Blade is an hugely costly affair.
It is entirely possible that one turbine blades,
one single turbine blade here could be
costlier than making a whole set of compressor
blades so that that is how costly the
turbine blades are because they have embedded
cooling technology inside those blades.
Transpiration cooling as we have seen requires.
This is an attempt at a transpiration
cooling as we discussed. You have the porous
outer shell; you require that porous outer
shell, and then, you have an elaborate internal
cooling passage which has a cold and
compressed air and those would be hoozing
out through the pores to create film cooling.
So, you need a porous outer shell or a sheet
to affect transpiration cooling, and this
air of
course, radially flows from hub to tip or
trip to hub across the entire length of the
blade
to affect cooling over the entire length of
the blade. Now, this is what I mention that,
to
the best of our knowledge, such porous technology,
material technology has not yet
matured, and to the best of our knowledge,
that has not yet been applied commercially
even though lots of research is still going
on in this area.
A summary of the kind of technology that has
been used over the years as you can see
here. In the early era, when it was only convection
cooling, the advancement of
compression ratio are actually a required
that you can do with less cooling. So, as
the
compression ratio went up to forty first 20
and then 40, you can see that you can do with
less of temperature, because the relative
cooling flow available from high pressure
compressor allows you to put in more air flow,
and as you can put in more air flow at
high pressure, you know you, you, can effectively
do a lot of cooling; however, as the
advance cooling has been extended with film
cooling, film and convection combine
cooling, you can cool any blade from a pressure
ratio of 5 to 40.
40 is going, you know, high value of compression
ratio. So, up to a pressure ratio of 40,
you can keep on using the combined cooling
system, and it can take you to temperature
up to 20 200 transpiration cooling can give
you even higher temperatures at a lower
coolant film flow.
So, the more and more advance cooling technology,
you can deploy, you can use less
and less coolant flow because higher and higher
compression ratio is now available, and
then, at each of those cooling passages, you
can pump in air at a higher pressure, and
one
you you have that availability of high pressure
air, you can afford to do with less and less
mass of air cold air to affect effective cooling.
So, over the years as the pressure ratio has
gone up, the cooling has indeed been
facilitated by high compression ratio, and
this is one of the advantages of the fact
that
compression ratios gone up; the turbine technology
has also effectively used higher
turbine entry temperature. So, this is what
has happened over the years the two of them
have gone up together.
Let us look at what is happening when the
surface temperature as felt the film cooling
only gives so much of relief. When you have
convection cooling only, it gives so much
relief, but once you have a combine film and
convection cooling, the surface temperature
comes down substantially and this is on the
blade surface. So, it, it, tells us that,
on the
blades surface, the temperature on the blade
surface can be substantially lower if you
have combined cooling system.
So, only film cooling or only convection cooling
actually does not give so much relief by
themselves, but when you have combined cooling
system gives much more relief, and as
we have seen in the earlier the slide, you
can actually do that even actually with less
amount of air because high pressure air is
now available in modern aero engines.
This is a picture again of a typical stator
or nozzle cooling system. As you can see here,
cooling air is brought in from here; it goes
through these channels; it takes a turn over
here, and then, it takes another turn over
here and then it goes out through the tips.
The
other cooling air which is brought in near
the leading edge goes straight out, because
that
is where cooling effectiveness is more and
then certain amount of air is impinge cooling
on the leading edge surface itself.
As we have discussed this impingement cooling
is going on that air is finally also let out
to the tip, and then, certain amount of air
is let out through a holes near the trailing
edge
to affect, trailing edge cooling which again
could be a Hot Spot. The inside surfaces of
these cooling passages actually have turbulent
promoters. The surfaces could be actually,
you know, having small ribs or small bumps
as you have like speed breakers on a road,
those are you can probably see them here a
little those ribs on the surface, that is,
to
promote turbulent so that the flow through
these actually are actually promoted, and
as a
result of which, the flow is facilitated;
otherwise, the flow may get impeded and may
hot
and may not be able to take this as quick
as a root to go through the entire blade passage.
So, that needs to be facilitated through these
ribs which are on the inside surface. You
can probably see them a little bit over here
so that, that is the kind of you know passage
that typically turbine blades have, and then,
they finally come out through the tips over
here which also does a little bit of tip cooling.
The tips need to be cooled also along with
the cross flow over the tip. So, it is a elaborate
cooling technology that is often used in
modern hot gas blades. You, this was the stator
Blade.
We can have a look at a rotor blade in which,
the cooling technology is even more
elaborate because you need to have the blades
are rotating. Remember, the flow inside
will be feeling the centrifugal action. So,
by centrifugal action, the flow will get thrown
out outwards from here. As it comes in from
here, and then, that would facilitate the,
that
will give enough impetus to the flow to go
through this channel, and then finally, come
out through the tip. Same over here, it comes
in over here, gets thrown out by centrifugal
action. It will be promoted through ribs inside,
and then through the passage, it comes
out, and finally, goes out.
So, this is the up and down flow. It goes
up here; it goes down there, and then, he
goes
up here and goes down there, and then, finally,
comes out through these slots to on the
tips too. So, the amount of air for example,
which is going in here is .35 percent the
amount that is coming out could be as low
as 0.03 percent. So, the total amount of air
going is 0.76 percent of the main flow that
is flowing over the gas; over the whole
turbine, the entire gas flow.
So, less than 1 percent of the main gas flow
in terms of air mass is required for this
elaborate cooling system, and if you have
less than 1 percent in the early year of cooling,
the amount of air required used to be of the
order of 3 to 4 percent. But now, you can
do
with less than 1 percent. So, that much has
been the advancement of cooling technology,
and as a result of which, because high pressure
available because of this entire
manufacturing technology, the entire engineering
that has gone into the turbine cooling
technology has facilitated this; entirely
fascinating field of turbine cooling. It is
a field by
itself. People, some people spend their whole
life on turbine cooling technology.
It is a fascinating field, there is no question
about it and it involves heat transfer science
or heat transfer. It involves aerodynamics,
lots of fluid mechanics, extremely high
technology in manufacturing and fabrication,
and then, all of it put together, best of
all of
them put together gives you a cool turbine
blade and that is what you require to do a
modern axial flow gas turbine blades. With
this, we come to the end of our blade cooling
technology discussion. You now know that lot
of technology is required to create a
modern gas turbine blade.
We will try to keep this in mind. When we
go into the next class in which, we will be
discussing overall actual turbine blade design.
We will bring in the blade design
methodology into the next class in which,
we will keep in mind that certain cooling
technologies are available today, and with
this, we will see what kind of blades are
being
created for the modern gas turbines. They
could be a subsonic; they could be transonic
or
we will see that we could even have supersonic
blades for modern gas turbines. So, in
the next class, we will be looking at the
design methodology of modern axial flow
turbines.
