Combustion ChamberIntroduction:-

When we talk about the combustion chamber that means we talk about an very important component in the gas turbine engine.

It is easy to say that "it is one of the main sources that gives the thrust of the aircraft".

The combustion chamber design was a complex process for many years due to involving fluid dynamics , combustion & mechanical design and these designations were much less amenable to theoretical treatment than other components. But in the recent years this designations became easier by using the methods of computational fluid dynamics (CFD).

It is known that the combustor is the third module of the engine and it is location is directly between the compressor & turbine module.

Combustor types:- There is a three types of the combustion chamber used in the aircraft applications listed as follows :

1- Annular. 2- Can ( Tublar ). 3- Can-Annular (Tubo-Annular ). The first two types are the basic types of the combustion chamber. 1st we will talk about the Annular combustion chamber.

Annular combustion chamber :-

This type of combustion chamber consists of a single flame tube completely annular in form which is contained in an inner & outer casing and it being open at the front of the compressor and at the rear to the turbine nozzle.

Operation -:

The compressed air is introduced into an annular space formed by the combustor liner around the turbine shaft.

Fuel is introduced through nozzles or injectors connected to a fuel manifold.

Tublar combustion chamber-:

This type of combustors used on the centrifugal compressor engines.

It is so arranged that air from the compressor enters each individual chamber through the adapter. Each individual chamber is composed of two cylindrical tubes, the combustion chamber liner and the outer combustion chamber.

Combustion takes place within the liner. Airflow into the combustion area is controlled by small louvers located in the inner dome, and by round holes and elongated louvers along the length of the liner.

Operation -:

By spark igniter plugs in two of the lower chamber the flame is started and it passes through the tubes and ignites the combustible mixture in the adjacent

chamber.

Tuboannular combustion chamber -:

This combustion chamber uses characteristics of both annular and can-type combustion chambers .

The can-annular combustion chamber consists of

an outer shell, with a number of individual cylindrical liners mounted about the engine axis.

After this looking about the

definition and operation of the three types we have to take a look abut the advantages & disadvantages of the three types.

Disadvantages Advantages Type1-Series buckling Problem on outer liner.

2-Rig testing necessitates full engine air mass flow .

3-Difficult to match fuel-flow & air flow patterns .

4-Difficult to maintain stable outlet temp. traverse.

1-Minimum weight & length.

2-Minimum engine frontal area.

3-Minimum pressure loss.

4-Easy light –round.

5-Strong.

6-Most efficient.

Annular combustor

5-Difficult in testing than can type combustor.

1-Bulky and heavy.

2-High pressure Loss .

3-Requires interconnectors .

4-incures problem of light –round .

5-Inefficient .

6-weak structure

.

1-Mechanically robust .

2-Fuel-flow & airflow Patterns are easily matched .

3-Rig testing necessitates only small .

4-Easy replacement .

Can combustor

1-Less compact than annular .

2-Requires Connectors .

3- incures problem of light –round .

4-Less efficient than annular one .

1- Mechanically robust .

2- Fuel-flow & airflow Patterns are easily matched .

3-Rig testing necessitates only small .

4-Low pressure

Can-Annular combustor

Loss .

5-Shorter & lighter than tublar combustor .

6-Strong .

7- Easy in replacement .

Combustor requirements-:

For our design of a gas turbine combustor must satisfy a wide range of requirements whose relative

importance varies among engine types.

The basic requirements of all combustors may be listed as follows:

1- High Combustion efficiency, the fuel should be completely burned so that all its chemical energy is liberated as heat.

2- Reliable and smooth ignition, both on the ground and on the case of aircraft engines, after a flameout at high altitude.

3- Wide stability limits, the flame should stay alight over wide range of pressure, velocity and air/fuel ratio.

4- Freedom from pressure pulsation and other manifestations of Combustion induced instability .

5- Low-pressure loss.

6- An outlet temperature distribution that is tailored to maximize the life of the blades and nozzle guide vanes.

7- Low emissions of smoke, unburned fuel and gaseous pollutant species.

8- Design for minimum cost and ease of maintenance.

9-Size and shape compatible with engine envelope.

10- Durability.

11- Multifuel capability.

12- Design for minimum weight .

13- Avoid flame out & cut off flame .

Combustion chamber components:-

There are 4 basic components in the combustor:

1- Diffuser.2- Dome.3- Cowl (Snout).4- The liner.

There are another components:

1- Igniter plugs.2- Fuel injectors.3- Swirl vanes.4- Air Casing.5- Fuel spray nozzle.

Air distribution along the combustion chamber:

This figure illustrate the different zones of air inside the combustor which have a great effect on the performance of the combustor like achievement of complete combustion , increasing life of the turbine blades , flame stabilization and another reasons.

Primary air zone:-

The function of the primary zone is to provide sufficient time, temperature, and turbulence to achieve essentially complete combustion of the fuel.

Primary zones may be roughly divided into can types, which employ large-scale recirculation of air, and pepperpot types, in which the eddies produced are essentially of small scale.

The primary zone volume could be reduced to the minimum if combustion were limited by chemical reaction rates.

It is percentage is between ( 15 : 20 % ) of the total air.

And it is function is: 1- Start combustion. 2- Good mixing by swirler.

Intermediate air zone :-

The intermediate zone must be allocated the proper length for the two main functions that it performs.

At low altitudes it severs as a region in which dissociation in the gas flow can be recovered, and the burning of any imperfectly mixed fuel-rich pockets of gas completed.

At high altitudes the rate of reaction in the primary zone is slower, owing to the lower concentration of fuel and air, and combustion is far from complete at the exit from the primary zone. Under these conditions the intermediate zone severs as an extension of the primary zone, providing an increased residence time at high temperature prior to cooling and freezing of reaction in the dilution zone.

And it is function is:1- For complete combustion.2- For flame stabilization.

Dillution air zone ( Tritory ):-

The role of the dilution zone is to admit the air remaining after the combustion, and to provide an outlet

steam with a mean temperature and a temperature distribution that are acceptable to the turbine. And it is function is:

1- Ensure flame cutout.2- Cooling for the air flow.

Combustion types :-

Before talking about combustion process we have to define the different types of combustion .

Complete combustion :-

In complete combustion, the reactant will burn in oxygen, producing a limited number of products. When a

hydrocarbon burns in oxygen, the reaction will only yield carbon dioxide and water.

In complete combustion :-

Incomplete combustion occurs when there isn't enough oxygen to allow the fuel (usually a hydrocarbon) to react completely with the oxygen to produce carbon dioxide and water. The reactant will burn in oxygen, but will produce numerous products .

Rapid combustion :-

Rapid combustion is a form of combustion in which large amounts of heat and light energy are released. This often occurs as a fire.

Slow combustion :-

Slow combustion is a form of combustion which takes place at low temperatures. Respiration is an example of slow combustion .

Turbulent combustion :- Turbulent combustion is a combustion characterized by turbulent flows. It is the most used for industrial application (e.g. gas turbines, diesel engines, etc.) because the turbulence helps the mixing process between the fuel and oxidizer.

Smouldering combustion :-

Smouldering combustion is a flameless form of combustion, deriving its heat from heterogeneous

reactions occurring on the surface of a solid fuel when heated in an oxidizing environment.

The fundamental difference between smouldering and flaming combustion is that in smouldering, the oxidation of the reactant species occurs on the surface of the solid rather than in the gas phase. The characteristic temperature and heat released during smouldering are low compared to those in the flaming combustion of a solid.

Diffuser :-

The diffuser has an expanding internal diameter to decrease the velocity and increase the static pressure of air.The air leaving compressor then through a diffuser section. The diffuser prepares air for entry the combustor at low velocity to permit proper mixing with fuel.

Swirl :-

Swirl vanes have two important functions:

1- High flame speed provides better mixing of air fuel And ensures spontaneous burning. 2- Low air velocity axially swirling prevents the flame Moving axially too rapidly .

.

Swirl vanes is used also for making a good recirculation during combustion process by aiding flame propagation because a high degree of turbulance in the early combustion and cooling stage is desirable .

Igniter plugs :-

Igniter plugs function only during starting being cut out of the circuit as soon as combustion is self supporting .

On engine shutdown or if the engine fails to start the combustor drain valve , a pressure actuated valve , automatically drain any remaining unburned fuel from the combustor .

The right figure illustrate the components inside the spark plug :

A- Terminal .B- Ceramic insulation .C- Center electrode .D- Gab .E- Ground electrode .

combustion process :-

It is mean mixing of liquid fuel droplets and vaporization of these mixing and breakdown hydrocarbons into lighter fractions then an intimate mixing between molecules of hydrocarbons & air molecules takes place then chemical reaction them selves .