M15.05 Combustion section

Question 1

What are the requirements of the combustion chamber?

Answer 1

For a safe and efficient operation of the engine the combustion chamber must
fulfil the following requirements:
* It must permit safe ignition of the fuel air mixture both on ground and in
flight.
* Give stable combustion in all operating conditions of the engine.
* Combustion must also be complete. This means that no unburned fuel
should leave the combustion chamber.
Other combustion chamber requirements are:
* To give equal temperature distribution at the combustion chamber outlet.
This is important for the first turbine stage.
* Combustion should happen with a minimum of pressure loss in the
combustion chamber to increase the engine efficiency.
The combustion chamber:
* Must be as small and light as possible to save weight .
* It must have a dependable cooling system for all materials which come in
contact with the hot gas flow.
* Should have a high operating lifetime as the replacement of these engine
components is very time consuming.

Question 2

What forms the outer shield of the combustor?

Answer 2

The combustor casing

Question 3

What does the combustor casing do?

Answer 3

It takes the air pressure loads and protects the internal and external engine
parts from the hot combustion gases.
The housing also gives support to the flame tube and other combustion
chamber components, such as fuel nozzles and igniter plugs

Question 4

What is the purpose of the flame tube?

Answer 4

The flame tube controls and guides the flame. It is made up of many segments
with a lot of holes and slots

Question 5

What are the two primary conditions for combustion to reliably occur?

Answer 5

Two primary conditions are necessary for correct and safe combustion:
1. The ratio of air and fuel must be correct.
2. The air velocity must be slow enough to make sure that the combustion is
completed inside the combustion chamber

Question 6

What is the ratio of fuel to air to sustain complete combustion?

Answer 6

15 parts air to one part fuel

Question 7

Air from the compressor is split into what to type of airflow?

Answer 7

The primary and secondary air

Question 8

What is the percetage of used in the primary aiflow?

Answer 8

about 20% to 30%

Question 9

what is the purpose of the secondary air?

Answer 9

In this area, some of the secondary air is used to keep the flame away from
the flame tube walls. This is necessary because the temperature in the centre
of the flame reaches approximately 2000°C.
The secondary air is mainly used for cooling, as combustion chamber
materials can melt at these high temperatures.
Most of the secondary airflow enters the dilution zone of the flame tube via
dilution air holes

Question 10

What is the airspeed of air from the compressor?

Answer 10

150 m/s

Question 11

Explain how the airflow from the compressor is slowed down

Answer 11

Air from the compressor is much to fast, to slow it down the outlet of the compressor is a diffuser. this slows down the air to about 25 m/s which is still to fast. to further slow the air down it must pass through the restrictor and swirl vanes.

Question 12

What is a secondary benefit of the swirl vanes?

Answer 12

It iduces a swirl unto the air, which helps to mix the fuel.

Question 13

What is the optimum velocity of air for a kerosene air mixture?

Answer 13

2 m/s to 15 m/s

Question 14

Name the main components of a can type combustor

Answer 14

Each combustion chamber has its own:
* Air supply duct
* Fuel nozzle
* Flame tube
* Casing

Question 15

How are multiple can type combustors joined together?

Answer 15

By interconnector tubes which also equalize the pressure between cans

Question 16

How many can combustors on an engine will have igniter plugs?

Answer 16

2

Question 17

What are the advantages and disadvantages of can type combustors?

Answer 17

The advantage of can-type combustion chambers is that they are simple in
design, they have very good structural strength and they can be removed and
replaced individually.
However, the disadvantage of these combustion chambers is, that they are
heavy and need a lot of space, they also need complicated air supply ducts
from the compressor. These ducts lead to very high aerodynamic losses.
Another disadvantage of multiple can-type combustion chambers is the
difficult ignition from one chamber to the other.

Question 18

What are the advantages and disadvantages of the can annular combustor?

Answer 18

The advantage of the can-annular combustion chamber is that it is smaller
and lighter than a similar multiple can-type combustion chamber. It does not
need complicated air supply ducts from the compressor and it still has good
structural strength.
The disadvantage of this type is that the aerodynamic losses are quite high
and the ignition from one flame tube to the other is very difficult.

Question 19

With the annular combustor, Explain its construction

Answer 19

The annular combustion chamber is most commonly used on modern gas
turbine engines.
This combustion chamber has only one annular flame tube. This flame tube is
supported by an inner and outer combustion chamber casing.
The combustion chamber casings are part of the load carrying engine
structure.
Together with the flame tube, they also make the air channel for the
secondary air.

Question 20

What are the main parts of an annular flame tube?

Answer 20

The dome
The inner liner
The outer liner

Question 21

What are the advantages and disadvantages of the annular combustor?

Answer 21

They are smaller and lighter at the same airflow.
The large combustion area means that they have better efficiency, the
combustion is complete and smokeless, and the flame can spread easily over
the circumference of the flame tube. They also have a better thermal load
distribution for the turbine
The disadvantages of annular combustion chambers are they are expensive to
make and removal is very difficult and time consuming.
These disadvantages, however, are compensated by a very high service life.

Question 22

What are the main pollutants in the exhaust gas?

Answer 22

Unburned fuel and hydrocarbons

Question 23

What is the effect of a rich mixture?

Answer 23

It is called a rich mixture if there is too much fuel for the primary air in the
combustion chamber. In this condition, some of the fuel cannot be burned
and is released into the exhaust.
The excessive fuel also supports the build-up of carbon monoxide

Question 24

What are the ratios that fuel can safely be burned at?

Answer 24

An air fuel mixture can burn safely if the ratio is between 7.5:1 and 24:1.

Question 25

How can the emmision of hydrocarbons and carbon monoxide be reduced?

Answer 25

Optimum fuel ratios being used

Question 26

How can nitrogen oxide emission be reduced?

Answer 26

by reducing the contact time of the gas in the hot zone, or by reducing the combustion temperatures

Question 27

What is the effect of low combustion temps?

Answer 27

Reduced output of nitrogen oxide, but an increase in carbon monoxide emission

Question 28

What is the effect of increasing the combustion temp?

Answer 28

Carbon monoxide and hydrocarbon emission will decrease but nitrogen oxide emission will increase

Question 29

What is a method of reducing the emissions from an engine?

Answer 29

By using a duel dome combustor

Question 30

What is a duel dome combustor?

Answer 30

The annular combustion chamber has an outer and inner set of combustion areas. One combustion area, called the pilot stage, always operates. The other stage, called the main stage, operates only in high power conditions. For each of these stages the air fuel ratio is controlled much better than on standard combustion chambers. This reduces carbon monoxide and hydrocarbon emissions. The dual dome combustion chamber is much shorter than standard combustion chambers. This reduces the time that the exhaust gas is in the hot area and therefore the build-up of nitrogen oxides

Question 31

What can the combustion section house and wat forces can it carry?

Answer 31

The combustion case carries the structural loads between the compressor case and the turbine case and houses the flame tube. On some engines it even houses parts of the high pressure compressor and high pressure turbine. The flame tube stabilizes the flame during combustion. The flame tube has cooling air holes necessary for the airflow to cool down the combustion gases. External components of the combustion section are the fuel nozzles and igniter plugs. They are installed on the outer wall of the combustion case. Also on the outer wall of the combustion case are mounting flanges for bleed air tubes and turbine cooling air tubes

Question 32

How is the combustion case assembled?

Answer 32

This combustion case has: * An outer wall, * An inner wall * A diffuser The rear portion of the inner wall is usually removable so that the flame tube can be installed. The rear flange of the inner wall is used to attach the nozzle guide vanes to the high pressure turbine and support the flame tube. In front of the inner wall flange is the seal surface of the Compressor Discharge Pressure (CDP) seal. Integral parts of the combustion case are the stator vanes of the last stage of the high pressure compressor. These stator vanes are often known as exit guide vanes because of their location. Another integral part of the combustion case is the diffuser

Question 33

What is the combustor frame known as?

Answer 33

On many engines the combustion case is a frame, often known as the compressor rear frame. It houses the bearings for the high pressure rotor. The bearing area is usually in the inner annulus of the combustion case.

Question 34

Explain flame tube construction

Answer 34

The main components of the flame tube are: * Outer and inner liner * Outer and inner cowl * Dome The dome section of the flame tube connects the inner liner with the outer liner. It also houses the swirlers with swirl vanes. Usually, each swirler has a bore to hold the nozzle tip in its centre. The flame tube is bolted to the combustion case structure and held in place by outer and inner liner supports. The attachment of flame tubes differs from engine to engine, but it is always fixed at one end and moveable at the opposite end. The outer liner support fits to the inner surface of the outer wall. It is fixed in the axial direction by a recess of the combustion case. The inner liner support is bolted to the inner wall of the combustion case. The flame tube is fixed in all three dimensions at the aft end. As heat from the combustion process creates thermal expansion of the flame tube, the tube expands in an axial and radial direction without build-up of stress. This mounting principle ensures a long service life for the flame tube. Sometimes you also find that the flame tube is fixed at the front end of the combustion case. This leaves it free to expand at the rear

Question 35

Explain film cooling

Answer 35

The primary airflow enters the flame tube through the swirl vanes and the secondary airflow passes the cowl and enters the flame tube through the liners. This secondary airflow forms a cooling film on the inner surface of the liners, which helps to protect the liner from the intense heat of the combustion gases. This cooling method is known as film cooling

Question 36

Explain the different types of film cooling

Answer 36

There are many different methods of designing flame tubes to get an optimum cooling film. Machined cooling rings can be found on modern flame tubes. Corrugated strips or splash cooling strips were used on older flame tubes.

Question 37

Explain transpiration cooling

Answer 37

A more sophisticated method of film cooling is known as the transpiration cooling method. This method requires less air for cooling the flame tube liners. For this method, the inner surfaces of the flame tube liners are covered with detachable tiles. These tiles are known as liner segments. The secondary airflow enters the flame tube through radial holes in the liner. This air then flows through the gap between the liner segments and the liner. Here, the air cools the outer surface of the liner segments and the liner. When the air leaves the gap, it creates a cooling film on the inner surface of the next liner segment.