Combustion Chambers Part 2 - Aircraft Gas Turbine Engines #09

Summary

This 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

00:00

🔥 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.

05:01

🌀 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.

10:02

💧 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.

Keywords

💡Fuel Drain System

The fuel drain system is a mechanism designed to remove excess fuel from an engine. It is crucial for maintaining engine efficiency and preventing fuel wastage. In the context of the video, after a 'wet start,' the fuel drain system helps the engine to find its own level by allowing excess fuel to flow from the top of the engine to the bottom chamber and then exit the engine. This process is essential for the proper functioning and safety of the engine, ensuring that it operates with the correct amount of fuel.

💡Evaporation

Evaporation, in the context of the video, refers to the process of converting liquid fuel into vapor to eliminate any remaining traces within the engine components such as combustion chambers, turbine, and jet pipe. This is necessary after a wet start to ensure that no fuel residue interferes with the engine's operation. The video mentions a 'blowout cycle,' during which the engine is motored over without high-pressure fuel and ignition, allowing air to flow through and assist in the evaporation process.

💡Combustion Chambers

Combustion chambers are the parts of an engine where fuel is burned to produce energy. The video discusses different types of combustion chambers, including the annular system, which is a more compact design compared to the multiple combustion chamber system. The efficiency and design of combustion chambers are critical to the engine's performance, as they directly affect the heat release and combustion efficiency.

💡Turbine

A turbine is a device that extracts energy from a flow of fluid, such as air or steam. In the video, the turbine is part of the gas turbine engine where the expanding gases from the combustion chamber drive the turbine blades, converting thermal energy into mechanical energy. The turbine's efficiency is influenced by the pressure distribution of the gases impinging on it, which is discussed in the context of the annular combustion chamber system.

💡Jet Pipe

The jet pipe, as mentioned in the video, is a component through which exhaust gases are expelled from the engine. It is part of the exhaust system and plays a role in the overall efficiency and performance of the engine by managing the flow of gases post-combustion. The design and efficiency of the jet pipe can impact the thrust produced by the engine.

💡Annular Combustion Chamber System

The annular combustion chamber system is a type of combustion chamber design that features a single, continuous chamber around the engine's axis. The video highlights the advantages of this system over multiple combustion chamber systems, such as a more compact design, no flame propagation problems, and improved combustion efficiency. This system is integral to the engine's performance, as it allows for a more uniform distribution of gases and a more even load on the turbine.

💡Air-Fuel Ratio

The air-fuel ratio is the ratio of the mass of air to the mass of fuel in an internal combustion engine. The video emphasizes the importance of using the chemically correct air-fuel ratio of 15 to 1 for maximum heat release in the combustion chambers. This ratio is critical for achieving efficient and stable combustion without causing detonation or dissociation, which would be problematic in a piston engine but is not an issue in a gas turbine engine.

💡Self-Sustaining Speed

Self-sustaining speed refers to the speed at which an engine can continue to operate without the assistance of a starter motor after it has been started. The video explains that once the engine reaches this speed, the ignition system is switched off, and the engine continues to run on its own. This is an important parameter for engine operation, as it indicates the engine's ability to maintain combustion without external assistance.

💡Flameout

A flameout is the extinction of the flame in a combustion chamber, which can occur due to various unusual circumstances, such as the ingestion of large quantities of water into the engine intakes. The video mentions that a flameout can require the use of the ignition system to reignite the engine, especially in situations where the engine has flamed out at high speed or high altitude, necessitating a slowdown or descent for a successful relight.

💡Combustion Efficiency

Combustion efficiency is a measure of how effectively the potential heat energy in the fuel is converted into useful work in a combustion process. The video discusses how modern gas turbine engines can achieve very high combustion efficiencies, nearing 99% at high power conditions and still maintaining around 95% at idle. This high efficiency is attributed to the design of the fuel spray nozzles, which atomize or vaporize the fuel to ensure complete combustion.

💡Atomization

Atomization is the process of breaking down a liquid into small droplets, which is crucial for fuel combustion in engines. The video explains the challenges of atomizing fuel at low pressures and high airstream velocities within the combustion chamber. It also discusses various methods, such as the use of air spray systems, duplex spray nozzles, and vaporizing tubes, to ensure effective atomization at different engine operating conditions.

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