Construction (combustion System)

Question 1

What is the primary requirement for a combustion system in terms of operation?

Answer 1

The system should give efficient, stable operation over a wide range of altitudes and flight speeds, and reduce the peak temperature to an acceptable level at the turbine.

Question 2

What materials can be used to build a GTE combustion chamber?

Answer 2

It can be built from light materials as there is no peak pressure in a GTE combustion chamber.

Question 3

What should the gases at the outlet from the combustion chamber system exhibit?

Answer 3

The gases should have a reasonably uniform velocity and temperature distribution.

Question 4

What is a key feature of the combustion system regarding engine shutdown?

Answer 4

It must be possible to re-light in flight after shutting down an engine.

Question 5

What is the expected lifespan of the combustion system compared to the engine?

Answer 5

It must be reliable and have a life at least as long as the overhaul life of the complete engine.

Question 6

How can the combustion chamber be maintained throughout its operational life?

Answer 6

With the introduction of ‘on condition maintenance’ and modular engine design, the combustion chamber can be monitored and replaced as necessary.

Question 7

What is the function of the Flame Tube in a combustion chamber?

Answer 7

The Flame Tube contains the flame during combustion and needs to withstand very high temperatures (1800 to 2000 deg. C).

Question 8

What material is typically used to manufacture the Flame Tube?

Answer 8

It is usually manufactured from thin heat resistant materials, usually Nimonic Alloy.

Question 9

Why can the Flame Tube be made from thin materials?

Answer 9

There are no peak pressures during the combustion process.

Question 10

What is the purpose of the holes around the circumference of the Flame Tube?

Answer 10

They allow air to be directed into the chamber for combustion and cooling purposes.

Question 11

What material is the Outer Air Casing made from?

Answer 11

It is manufactured from alloy steels.

Question 12

What characteristic helps identify overheating in the Outer Air Casing?

Answer 12

It is normally light in colour, so any overheating will show up as discolouration.

Question 13

What is the Primary Zone in a combustion chamber?

Answer 13

The Primary Zone is a turbulent, slow-moving zone of air that stabilises the flame by mixing with atomised fuel from the burner nozzle.

Question 14

What is the function of the Dilution Zone?

Answer 14

The Dilution Zone aids in cooling hot gases to a temperature acceptable to the turbine blades and shapes the flame to prevent it from contacting the sides of the flame tube.

Question 15

What is the typical air percentage that enters the combustion chamber?

Answer 15

A small amount of air, typically 18%-25% depending on engine type, enters the front of the combustion chamber.

Question 16

What is the typical overall air/fuel ratio in a combustion chamber?

Answer 16

The overall air/fuel ratio is approximately 60:1.

Question 17

What are the two main types of combustion chambers in common usage?

Answer 17

The two main types of combustion chambers are Annular and Reverse Flow.

Question 18

What is an annular combustion chamber?

Answer 18

An annular combustion chamber uses the whole of the annulus between the compressor and turbine for combustion.

Question 19

What is the structure of an annular combustion chamber?

Answer 19

It is a circular flame tube that allows the compressor shaft to go through the center, surrounded by an outer casing.

Question 20

What are the benefits of an annular combustion system?

Answer 20

1. Good utilisation of area
2. Structural part of the engine
3. Annular gas flow
4. Highly efficient
5. Large total combustion area

Question 21

What is a Reverse Flow combustion chamber?

Answer 21

A Reverse Flow combustion chamber is a circular flame tube with space for the compressor shaft to go through the center, featuring a single outer casing. It is shorter in overall length compared to other systems.

Used extensively in turbo-shaft helicopter engines and small gas turbines where length reduction is prioritized over cross-sectional area reduction.

Question 22

What is the function of Combustion Chamber Drains?

Answer 22

Combustion Chamber Drains allow residual fuel in the chamber to be drained overboard using an Automatic Drain Valve (ADV).

The ADV consists of a spring and plate valve that opens when combustion chamber pressure falls.

Question 23

How does the Automatic Drain Valve (ADV) operate?

Answer 23

The ADV opens to drain residual fuel when pressure falls and closes when pressure builds up during start.

The spring mechanism is key to its operation.

Question 24

What is the purpose of fuel nozzles in engines?

Answer 24

The purpose of any nozzle is to introduce fuel into the combustion chamber in a state in which it will burn efficiently.

Question 25

What are the two types of fuel nozzles?

Answer 25

The two types of fuel nozzles are Atomisers and Vaporisers.

Question 26

What is the function of atomisers?

Answer 26

Atomisers are designed to produce a controlled and efficient fuel burn.

Question 27

What are the three basic types of atomisers?

Answer 27

The three basic types of atomisers are Duplex, Single, and Multi-orifice.

Question 28

What is a Duplex atomiser?

Answer 28

A Duplex atomiser provides two supplies of fuel depending on the quantity of fuel required.

Question 29

How does the Duplex atomiser operate?

Answer 29

It includes a primary and main fuel manifold with two independent orifices, one smaller for lower flows and one larger for higher flows.

Question 30

What happens as fuel flow and pressure increase in a Duplex atomiser?

Answer 30

The pressurising valve moves to admit fuel to the main manifold and thus to the main orifice, allowing flow through both orifices.

Question 31

What advantages does the Duplex atomiser provide?

Answer 31

It provides effective atomisation over a wide flow range and efficient atomisation at lower flows required at high altitude.

Question 32

What is the significance of the primary and main orifice in a Duplex atomiser?

Answer 32

The primary and main orifice allow for effective atomisation at varying fuel flows.

Question 33

What is a key feature of the Duplex atomiser at high altitudes?

Answer 33

It is effective at high altitudes.

Question 34

What is a Rotary Atomiser?

Answer 34

A device where fuel is fed through a hollow compressor shaft, and centrifugal motion ejects the fuel in an atomised spray into the combustion chamber.

Question 35

What are the components of the Rotary Atomiser?

Answer 35

Key components include the rotor shaft, stubshaft, labyrinth, packing cone, fuel transfer tube recess, fuel ejection holes, air holes, rotating guide vanes, impeller, first stage turbine wheel, and second stage turbine wheel.

Question 36

What is the function of the airspray nozzle?

Answer 36

It mixes a proportion of the primary combustion air with the injected fuel to aerate the spray, reducing local fuel-rich concentrations, carbon formation, and exhaust smoke.

Question 37

What is an advantage of the airspray nozzle?

Answer 37

The low pressures required to atomise the fuel allow the use of a simpler high-pressure pump.

Question 38

What are the three types of atomisers?

Answer 38

1) Duplex Atomiser 2) Rotary Atomiser 3) Airspray Nozzle

Question 39

What is the function of the vaporiser in an engine?

Answer 39

The vaporiser sprays fuel into vaporising tubes, which are heated by combustion, vaporising the fuel before it enters the flame tube.

Question 40

How is primary air introduced in the vaporiser?

Answer 40

Primary air is fed into the vaporiser and mixes with the fuel as it passes down the tube.

Question 41

What role do the holes in the flame tube entry section serve?

Answer 41

The holes provide 'fans' of air to sweep the flame rearwards.

Question 42

What is necessary to start an engine using a vaporising system?

Answer 42

A set of spray nozzles must be incorporated with the igniter plugs to initiate the combustion process.

Question 43

What are the advantages of vaporisers over atomisers?

Answer 43

Vaporisers are effective over a wider operating range, allow easier combustion control, and produce less smoke.

Question 44

How does the placement of vaporisers affect flame stabilization?

Answer 44

Vaporisers are located behind the flame, stabilizing it at the front of the combustion chamber and allowing for a shorter combustion section.

Question 45

What is used to protect the burner in a vaporiser system?

Answer 45

Fine filters are used to protect the burner from debris.