Combustion Chambers Part 2 - Aircraft Gas Turbine Engines #09
Summary
TLDRThis script discusses methods for eliminating excess fuel in gas turbine engines, including fuel drain systems and evaporation techniques. It explores the annular combustion chamber system, highlighting its advantages over multiple chamber systems. The script also covers the importance of the correct air-fuel ratio for efficient combustion, the concept of self-sustaining speed, and the challenges of relight after a flameout. Additionally, it delves into combustion efficiency, the role of fuel spray nozzles, and various fuel atomization methods, such as air spray systems and vaporizing tubes, crucial for engine start and operation.
Takeaways
- 🔧 Two primary methods are used to eliminate excess fuel in gas turbine engines: the fuel drain system and evaporation via a blowout cycle.
- 💧 The fuel drain system works by utilizing drain tubes that connect the lowest part of each combustion chamber to the next chamber below, allowing excess fuel to exit the engine.
- 🌀 A blowout cycle involves motoring the engine with the starter motor, without fuel and ignition, to evaporate any remaining fuel traces within the engine.
- 🔥 The turbo-annular combustion chamber system is more compact than the multiple combustion chamber system and does not have individual air casings for each flame tube.
- 📏 The annular combustion chamber system is shorter and more efficient than the tubo-annular system, with no flame propagation issues and less cooling air required.
- 🌡️ Combustion in gas turbine engines occurs at a nearly constant pressure, with a slight pressure loss due to turbulence and mixing within the combustion chamber.
- 🔥 The chemically correct air-fuel ratio of 15:1 is used in gas turbine engines without causing detonation or dissociation, unlike in piston engines.
- 🔄 Combustion stability is maintained over a wide range of air/fuel ratios and air mass flows, with the stability range narrowing as air mass flow increases.
- ✈️ Relight, or restarting the engine in flight after a flameout, may require slowing down or descending to achieve the right conditions for successful ignition.
- 💯 Modern gas turbine engines achieve very high combustion efficiency, nearing 99% at high power and maintaining around 95% at idle, thanks to effective fuel spray nozzles.
Q & A
What are the two methods mentioned for getting rid of excess fuel in a gas turbine engine?
-The two methods are the fuel drain system and a method of evaporating the remaining traces of fuel from the combustion chambers, the turbine, and the jet pipe.
How does the fuel drain system work in a gas turbine engine?
-The fuel drain system utilizes drain tubes that connect the lowest part of each combustion chamber with the next chamber below. After a wet start, excess fuel flows from the top of the engine to the bottom chamber and exits the engine.
What is a blowout cycle in the context of gas turbine engines?
-A blowout cycle is when the engine is turned over by the starter motor without high-pressure fuel and the ignition system deselected, allowing air to flow through the engine to assist in evaporating any remaining fuel.
What is the difference between a tuber annular combustion chamber system and a multiple combustion chamber system?
-The tuber annular combustion chamber system is more compact as it does not have individual air casings for each flame tube. Instead, a number of flame tubes are fitted within an inner and an outer air casing.
What are the advantages of the annular combustion chamber system over the multiple combustion chamber system?
-The annular system has a shorter length, no flame propagation problems, requires less cooling air, has higher combustion efficiency, and provides a more even load on the turbine.
What is the chemically correct air-fuel ratio for maximum heat release in a gas turbine engine?
-The chemically correct air-fuel ratio for maximum heat release in a gas turbine engine is 15 units of air to one unit of fuel by weight.
What is the self-sustaining speed of a gas turbine engine?
-The self-sustaining speed is the speed at which the engine can accelerate without the assistance of the starter motor after start.
What is a flameout in a gas turbine engine, and how can it be caused?
-A flameout is the extinction of the flame due to various unusual occurrences, such as the ingestion of large quantities of water into the engine intakes during takeoff.
What is the significance of combustion stability in a gas turbine engine?
-Combustion stability refers to the engine's ability to maintain smooth burning over a large range of air/fuel ratios and air mass flows, which is crucial for reliable engine performance.
How does the combustion efficiency of a modern gas turbine engine compare at high power and idle conditions?
-At high power operating conditions, combustion efficiencies as great as 99% are achievable, while at idle, the system can still provide up to 95% efficiency.
What challenges do fuel spray nozzles face in gas turbine engines, and how are they addressed?
-Fuel spray nozzles face challenges such as the high velocity of the air stream and the short distance for combustion. They address this by atomizing or vaporizing the fuel. At low engine speeds and pressures, the spray pattern may form a 'bubble', which is insufficient for ignition. As the engine accelerates, the pump builds up pressure to form a 'tulip' shape, which is still not optimal. Eventually, the pump produces a spray that is finely atomized enough to ensure rapid burning.
Outlines
🔥 Fuel Management in Gas Turbine Engines
This paragraph discusses two primary methods for managing excess fuel in gas turbine engines: the fuel drain system and evaporation. The fuel drain system uses drain tubes to remove excess fuel from the combustion chambers, allowing it to flow from the top to the bottom chamber and exit the engine. Evaporation is necessary for any remaining fuel traces and is achieved by motoring the engine on a blowout cycle, which involves rotating the engine with the starter motor without fuel and ignition, facilitating the evaporation of residual fuel. The paragraph also introduces the annular combustion chamber system, which is more compact and efficient than the multiple combustion chamber system, and highlights its advantages such as better pressure distribution, higher combustion efficiency, and the elimination of flame propagation issues.
🌀 Combustion Stability and Efficiency in Gas Turbines
The second paragraph delves into the concept of self-sustaining speed, which is the point at which the engine can continue running without the starter motor's assistance. It also addresses the importance of combustion stability, which is the engine's ability to maintain a smooth burn over a wide range of air/fuel ratios and air mass flows. The text explains that combustion stability narrows as air mass flow increases and discusses the challenges of relighting the engine after a flameout, which may require adjusting the flight conditions. The paragraph further explores the high combustion efficiency of modern gas turbine engines, which can reach up to 99% during high power operations and still maintain around 95% at idle. The use of fuel spray nozzles is credited for this efficiency, as they atomize the fuel effectively even at high air stream velocities and within the limited combustion chamber space.
💧 Advanced Fuel Atomization Techniques
The final paragraph focuses on the challenges and solutions related to fuel atomization at engine start, particularly when fuel pressures are low. It describes the bubble spray pattern that occurs at low pressures, which is insufficient for combustion. As the engine accelerates, the pump builds up pressure, leading to a more effective spray pattern. The paragraph introduces various methods to ensure proper atomization, such as the air spray system, which uses a high-velocity airstream to break up the fuel flow, and the duplex system, which employs a variable orifice size at low fuel pressures. Additionally, the vaporizing tube method is discussed, where fuel is sprayed into tubes that are heated by combustion, vaporizing the fuel before it enters the flame tube. These techniques are crucial for achieving efficient and stable combustion in gas turbine engines.
Mindmap
Keywords
💡Fuel Drain System
💡Evaporation
💡Combustion Chambers
💡Turbine
💡Jet Pipe
💡Annular Combustion Chamber System
💡Air-Fuel Ratio
💡Self-Sustaining Speed
💡Flameout
💡Combustion Efficiency
💡Atomization
Highlights
Two methods for fuel removal: fuel drain system and evaporation of remaining fuel.
Fuel drain system uses drain tubes to remove excess fuel from engine chambers.
Evaporation of remaining fuel is achieved through a blowout cycle with starter motor.
Annular combustion chamber system described, differing from multiple chamber systems.
Advantages of the annular combustion chamber system over other types.
Chemically correct air-fuel ratio of 15 to 1 is essential for maximum heat release.
Gas turbine engines do not face the same detonation issues as piston engines at optimal air-fuel ratios.
Combustion efficiency can reach up to 99% in modern gas turbine engines.
The importance of fuel spray nozzles for efficient combustion in gas turbine engines.
Challenges in atomizing fuel at low pressures and high air stream velocities.
Different types of fuel pumps and their roles in providing fuel pressure for atomization.
The bubble spray pattern at low engine RPM and its inability to support combustion.
The transition from bubble to chu'lak spray pattern as engine RPM increases.
The use of air spray systems for better fuel atomization at low fuel pressures.
Duplex system's variable orifice size for efficient fuel atomization at different pressures.
Vaporizing tube method for fuel atomization using primary air and heat from combustion.
Combustion stability is crucial and is influenced by air/fuel ratios and air mass flows.
Relight envelope concept for gas turbine engines and the conditions for successful relight.
Transcripts
two means of getting rid of the fuel are
open to us first the fuel drain system
and second a method of evaporating the
remaining traces of fuel from the
combustion chambers the turbine and the
jet pipe
the fuel drain system utilizes the drain
tubes which connect the lowest part of
each chamber with the next chamber below
it
after a wet start we'll attempt to find
its own level by flowing from the top of
the engine to the bottom chamber
once in the bottom chamber the excess
fuel exits the engine
any remaining traces of fuel within the
engine must be evaporated to this end
the engine must be motored over on what
is termed a blowout cycle
during a blowout cycle the engine is
turned over by using the starter motor
it's rotated for the time normally
allocated to a full start cycle but with
the high-pressure fuel shut and the
ignition system deselected
air from the compressor will flow
through the combustion chambers the
turbine and the exhaust system and
assist in the evaporation of any fuel
still remaining within
the to bow annular combustion chamber
system shown here is sometimes also
called the cannula or can annular system
the turbo annular combustion chamber
system differs from the multiple
combustion chamber system insofar as it
does not have individual air casings for
each of the frame tubes
a number of flame tubes are fitted
within an inner and an outer air casing
which makes the system a more compact
unit
notice the position of the igniter plug
this illustration is of a typical
example of an annular combustion chamber
system it has only one flame tube
inner and outer air casing
the annular system has several
advantages over the multiple combustion
chamber system and the tuber annular
system from which it was developed they
are
well the same power output the length of
the annular chamber is only 75 percent
that of a chuubo annular system of the
same diameter
no flame propagation problems
compared to a chuubo annular system the
air casing area is less consequently
less cooling air is required
the combustion efficiency is raised to
the point where unburned fuel is
virtually eliminated allowing the
oxidization of carbon monoxide to non
toxic carbon dioxide
there is a much better pressure
distribution of the gases impinging on
the turbine so it has a more even load
placed upon it
we stated at the beginning of this
lesson that we had to obtain the maximum
heat release from burning the mixture of
fuel and air in the combustion chambers
to do this we must use the chemically
correct air-fuel ratio of 15 to 1
whereas in the piston engine the use of
the chemically correct air-fuel ratio of
15 to 1 would cause detonation and
dissociation to occur in the gas turbine
engine it poses no such problem because
there are no peaks of pressure to assist
in their generation
the fuel and air are therefore mixed and
burnt in the primary zone of the
combustion chamber in the ratio of
approximately 15 units of air to one
unit of fuel by weight the addition of
secondary and tertiary air will however
dilute the mixture to the extent that
the overall ratio may vary from between
45212 as weekers 130 to 1
in the introduction lesson we stated
that combustion theoretically occurs at
a constant pressure in fact as is shown
here a small loss in pressure does in
reality occur as the gas passes from the
compressor end of the combustion chamber
to the turbine end
this loss of pressure is caused by
having to provide adequate turbulence
and mixing losses varies from three to
eight percent of the pressure at the
entrance to the combustion chamber
during normal engine running conditions
combustion is self-supporting the
system is actually switched off as soon
as the engine has attained
self-sustaining sweet self-sustaining
speed is the speed at which after start
the engine can accelerate without the
assistance of the starter motor
however be certain engine operating
conditions which do require the use of
the ignition system
for instance just such a condition would
occur following a flameout which is
extinction of the flame due to various
unusual occurrences such as the
ingestion of large quantities of water
thrown up from the nose wheels into the
engine intakes during takeoff from a
heavily contaminated runway
stability means smooth burning of the
mixture coupled with the ability to
remain alight over a large range of
air/fuel ratios and air mass flows this
graph shows the limits of those air/fuel
ratios and air mass flows within which
combustion will remain stable
the graph shows that combustion
stability will occur only between a
narrower and narrower range of air/fuel
ratios as the air mass flow through the
engine increases
level of a mass flow the flame is
extinguished
outside the ignition loop which lies
within the stable region it is more
difficult to start combustion than it is
to sustain it once it has started
restarting the engine in the air while
it's wind milling is called a relight
a consequence of this is that should the
engine flameout at high speed or high
altitude it may be necessary to slow
down and/or descend before the engine
can be successfully real it
this graph illustrates a relight
envelope for an imaginary engine showing
the flight conditions under which it
would be guaranteed to relight if it was
fully serviceable
the airflow through the engine will
cause it to rotate or windmill so the
compressor is supplying sufficient air
to support combustion
all that is then required is the opening
of the high-pressure fuel to
deliver a fuel supply and operation of
the ignition system to add the final
ingredient a spark
operation of the ignition system to
supply the spark is achieved by
selection of the relight switch the
electrical power to the relight ignition
circuit functions independently from
that which feeds the normal start
circuit
combustion efficiency is the efficiency
with which the combustor assembly
extracts the potential heat actually
contained in the fuel
this graph shows the combustion
efficiency of a modern gas turbine
engine across the range of air/fuel
ratios which occur during normal
operating conditions
modern gas turbine
a very efficient combustion cycle at
high power operating conditions
combustion efficiencies as great as 99%
are achievable
and at idle the systems will still give
as much as 95%
the very high combustion efficiency
attained
gas turbine engines is due in no small
part to the fuel spray nozzles which are
used in them these nozzles have the task
of atomizing or vaporizing the fuel to
ensure that it is completely burnt
this is no easy undertaking considering
the velocity of the air stream from the
compressor
and the small distance available within
the chamber for combustion to occur
other difficulties occur as a result of
the relatively low pressures attainable
by the engine-driven high pressure fuel
pump at engine start
the pumps can be of the plunger type
or the gear type
the pumps are fitted to the high speed
gearbox which is driven by the
high-pressure compressor shaft
the pumps are only rotating at a minimal
speed during initial engine start and
are incapable at that speed of providing
the high pressures 1,500 to 2,000 pounds
per square inch
required to give a good spray pattern
at engine start when there is low engine
rpm and low fuel pressure the fuel spray
pattern forms what is known as a bubble
this spray pattern is unable to atomize
a fuel jet sufficiently for ignition to
occur
as the engine accelerates during the
star sequence the pump rotates faster
and builds up more pressure until the
spray pattern forms a chu'lak shape
atomization is still not sufficient to
support combustion
eventually the pump produces Saphir
to shorten the tulip until it touches
the orifice only now is the fuel jet
atomized sufficiently to ensure rapid
burning
we've demonstrated then that a small
orifice of fixed size will provide a
finely atomized spray at high fuel
pressures
however when higher volume fuel flows
are needed through larger bore nozzles
the precious required from the pumps to
provide that finely atomized spray
become unattainable
thus for that type of situation other
methods must be found to sufficiently
atomize the fuel that engine start when
fuel pressures are low
the air spray system uses a
high-velocity Airstream to break up the
flow of fuel it requires only relatively
low fuel pressures and can therefore
operate using the gear type pump which
is much lighter than the more
sophisticated plunger type pump
the duplex system and shown here the
duple spray nozzle effectively used an
orifice of variable size at low fuel
pressures a pressurizing valve closes
off the main fuel feed to the nozzle the
only supply coming from the primary fuel
line
the primary fuel line feeds the primary
orifice a relatively small hole which is
capable of providing a finely atomized
spray at lower fuel pressures
when the engine accelerates during start
fuel pressure builds until the
pressurizing valve starts to open this
allows fuel to flow through the main
orifice where it will supplement the
spray of fuel from the primary orifice
in the vaporizing tube method
Illustrated here the fuel is sprayed
from feed tubes
into vaporizing tubes
which are positioned inside the flame
tube
primary air is fed into the flame tube
through the fuel feed tube opening and
also through holes in the flame tube
entry section
the field is turned through 180 degrees
and as the vaporizing tubes are heated
by combustion the fuel is vaporized
before passing into the flame tube
this concludes the lesson on combustion
chambers
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