• THE AIR/ FUEL MIXTURE BURNS INSIDE THE COMBUSTION THE AIR/ FUEL MIXTURE BURNS INSIDE THE COMBUSTION

CHAMBER.CHAMBER.• PROPER BURNING MUST TAKE PLACE DURING ENGINE PROPER BURNING MUST TAKE PLACE DURING ENGINE

OPERATION. OPERATION. • PROPER STABILIZATION AND PROPAGATION OF FLAME IN PROPER STABILIZATION AND PROPAGATION OF FLAME IN

THE COMBUSTION CHAMBER ARE ESSENTIAL FOR OPTIMUM THE COMBUSTION CHAMBER ARE ESSENTIAL FOR OPTIMUM ENGINE POWER.ENGINE POWER.

• THE AMOUNT OF FUEL ADDED TO THE AIR WILL DEPEND THE AMOUNT OF FUEL ADDED TO THE AIR WILL DEPEND UPON THE TEMPERATURE RISE REQUIRED. THE MAXIMUM UPON THE TEMPERATURE RISE REQUIRED. THE MAXIMUM TEMPERATURE IS LIMITED TOTEMPERATURE IS LIMITED TO 850 to 1700 deg. C850 to 1700 deg. C BY THE BY THE MATERIALS FROM WHICH THE TURBINE BLADES AND MATERIALS FROM WHICH THE TURBINE BLADES AND EXHAUST NOZZLE GUIDE VANES ARE MADE.EXHAUST NOZZLE GUIDE VANES ARE MADE.

COMBUSTION COMBUSTION CHAMBERSCHAMBERS

JET ENGINES EMPLOY VARIOUS JET ENGINES EMPLOY VARIOUS TYPES OF COMBUSTION CHAMBERS TYPES OF COMBUSTION CHAMBERS FOR BURNING AIR/FUEL MIXTURE AND FOR BURNING AIR/FUEL MIXTURE AND GENERATING LARGE AMOUNT OF GENERATING LARGE AMOUNT OF HEAT DEPENDING UPON DESIGN / HEAT DEPENDING UPON DESIGN / OPERATIONAL REQUIREMENTS.OPERATIONAL REQUIREMENTS.

COMBUSTION COMBUSTION CHAMBERSCHAMBERS

TO ENSURE EFFECTIVE AND TO ENSURE EFFECTIVE AND EFFICIENT COMBUSTION OF AIR/FUEL EFFICIENT COMBUSTION OF AIR/FUEL MIXTURE AND MAINTAIN OPTIMUM MIXTURE AND MAINTAIN OPTIMUM TEMPERATURES OF THE COMBUSTION TEMPERATURES OF THE COMBUSTION PRODUCTS DURING THE ENTIRE PRODUCTS DURING THE ENTIRE RANGE OF ENGINE OPERATION.RANGE OF ENGINE OPERATION.

COMBUSTION COMBUSTION CHAMBERSCHAMBERS

IN ORDER TO MEET VARIOUS IN ORDER TO MEET VARIOUS OPERATIONAL REQUIREMENTS OF THE OPERATIONAL REQUIREMENTS OF THE ENGINE, VARIOUS DESIGN FEATURES ARE ENGINE, VARIOUS DESIGN FEATURES ARE INCORPORATED. HENCE, THEY VARY IN INCORPORATED. HENCE, THEY VARY IN TERMS OF:TERMS OF:

-SHAPE-SHAPE-SIZE-SIZE-MATERIAL -MATERIAL -HEAT TREATMENT.-HEAT TREATMENT.

THE COMBUSTION CHAMBER HAS THE DIFFICULT TASK THE COMBUSTION CHAMBER HAS THE DIFFICULT TASK OF BURNING LARGE QUANTITIES OF FUEL, SUPPLIED OF BURNING LARGE QUANTITIES OF FUEL, SUPPLIED THROUGH THE FUEL SPRAY NOZZLES, WITH EXTENSIVE THROUGH THE FUEL SPRAY NOZZLES, WITH EXTENSIVE VOLUMES OF AIR, SUPPLIED BY THE COMPRESSOR, VOLUMES OF AIR, SUPPLIED BY THE COMPRESSOR, AND RELEASING THE HEAT IN SUCH A MANNER THAT AND RELEASING THE HEAT IN SUCH A MANNER THAT THE AIR IS EXPANDED AND ACCELERATED TO GIVE A THE AIR IS EXPANDED AND ACCELERATED TO GIVE A SMOOTH STREAM OF UNIFORMLY HEATED GAS AT ALL SMOOTH STREAM OF UNIFORMLY HEATED GAS AT ALL CONDITIONS REQUIRED BY THE TURBINE. THIS TASK CONDITIONS REQUIRED BY THE TURBINE. THIS TASK MUST BE ACCOMPLISHED WITH THEMUST BE ACCOMPLISHED WITH THE MINIMUM LOSS IN MINIMUM LOSS IN PRESSURE AND WITH THE MAXIMUM HEAT RELEASE PRESSURE AND WITH THE MAXIMUM HEAT RELEASE FOR THE LIMITED SPACE AVAILABLE. HENCE, THE FOR THE LIMITED SPACE AVAILABLE. HENCE, THE COMBUSTION CHAMBER HAVE CERTAIN COMBUSTION CHAMBER HAVE CERTAIN

DESIGN / OPERATIONAL REQUIREMENTSDESIGN / OPERATIONAL REQUIREMENTS..

1.1. The gas temperature required at the The gas temperature required at the turbine varies with engine thrust, and in turbine varies with engine thrust, and in the case of the turbo-propeller engine the case of the turbo-propeller engine upon the power required.upon the power required.

Hence, combustion chamber must Hence, combustion chamber must maintain amaintain a stable and efficient stable and efficient combustioncombustion over a wide range of over a wide range of engine operating conditions.engine operating conditions.

2. 2. High combustion efficiencyHigh combustion efficiency has become increasingly important has become increasingly important

because of the rapid rise in commercial because of the rapid rise in commercial aircraft traffic and the consequent aircraft traffic and the consequent increase in atmospheric pollution. increase in atmospheric pollution.

3.3. Ability to operate efficiently over a Ability to operate efficiently over a wide range of conditionswide range of conditions (( e.g. inlet e.g. inlet pressure & temperature of air and pressure & temperature of air and

air/fuel air/fuel ratiosratios))..

4.4. The flame tube and spray nozzle The flame tube and spray nozzle atomizer atomizer components must components must bebe

mechanically mechanically reliablereliable..

5.5. The gas turbine engine operates on a The gas turbine engine operates on a constant constant pressure cycle.pressure cycle.

Therefore, during the process of combustionTherefore, during the process of combustion pressure loss should be pressure loss should be minimumminimum..

In providing adequate turbulence and mixing, In providing adequate turbulence and mixing, a a total pressure loss varying from about total pressure loss varying from about 3 - 8 %3 - 8 %

of the air pressure at entry to the chamber is of the air pressure at entry to the chamber is incurred. incurred.

6.6. Combustion stability / Combustion stability / satisfactorysatisfactory rich and weak mixture extinctionrich and weak mixture extinction

limits).limits).

Combustion stabilityCombustion stability means smooth burning means smooth burning and the ability of the flame to remain alight and the ability of the flame to remain alight over a wide operating range. For any over a wide operating range. For any

particular particular type of combustion chamber there is type of combustion chamber there is both a both a rich and weak limit to the air/fuel ratio, rich and weak limit to the air/fuel ratio, beyond beyond which the flame is extinguished.which the flame is extinguished.

7.7. Uniform temperature and velocity Uniform temperature and velocity distributiondistribution at the entrance to the turbine and at the entrance to the turbine and simplicity of simplicity of controlcontrol..

8.8. Heat & corrosion resistantHeat & corrosion resistant..

The containing walls and internal parts of the The containing walls and internal parts of the combustion chamber are subjected to the products of combustion chamber are subjected to the products of the combustion, creep failure and fatigue due to the combustion, creep failure and fatigue due to thermal thermal and vibrational stresses.and vibrational stresses.

9.9. Ease and cheapness of manufactureEase and cheapness of manufacture..

Although there are several types of combustion Although there are several types of combustion chamber designs, they all have an inner and an chamber designs, they all have an inner and an outer section. The inner section or liner (as it is outer section. The inner section or liner (as it is known) will be perforated with many holes which known) will be perforated with many holes which allow cooling and stabilising air to enter the allow cooling and stabilising air to enter the combustion chamber throughout its length. The combustion chamber throughout its length. The outer section is not perforated and acts as a outer section is not perforated and acts as a sealed vessel for the liner. The liner is usually sealed vessel for the liner. The liner is usually concentric within the outer casing and the concentric within the outer casing and the passage formed between the two is used to form passage formed between the two is used to form an area of cool air to prevent the materials of the an area of cool air to prevent the materials of the combustion chambers from melting in combustion chambers from melting in temperatures in the region of 2000 deg C.temperatures in the region of 2000 deg C.

TYPES OF COMBUSTION CHAMBERTYPES OF COMBUSTION CHAMBER

TYPES OF COMBUSTION CHAMBERSTYPES OF COMBUSTION CHAMBERS

TYPES OF COMBUSTION CHAMBERTYPES OF COMBUSTION CHAMBER

1. (a)1. (a) AIR FROM THE ENGINE COMPRESSOR ENTERS THE AIR FROM THE ENGINE COMPRESSOR ENTERS THE COMBUSTION CHAMBER AT A VELOCITY UP TO COMBUSTION CHAMBER AT A VELOCITY UP TO 500 ft/sec500 ft/sec, BUT , BUT BECAUSE AT THIS VELOCITY THE AIR SPEED IS FAR TOO HIGH FOR BECAUSE AT THIS VELOCITY THE AIR SPEED IS FAR TOO HIGH FOR COMBUSTION, THE FIRST THING THAT THE CHAMBER MUST DO IS COMBUSTION, THE FIRST THING THAT THE CHAMBER MUST DO IS

TO TO DIFFUSEDIFFUSE IT, I.E. DECELERATE IT AND RAISE ITS STATIC IT, I.E. DECELERATE IT AND RAISE ITS STATIC PRESSURE. THE BURNING SPEED OF ATF AT NORMAL MIXTURE PRESSURE. THE BURNING SPEED OF ATF AT NORMAL MIXTURE RATIOS IS VERY LESS. ANY FUEL LIT EVEN IN THE DIFFUSED AIR-RATIOS IS VERY LESS. ANY FUEL LIT EVEN IN THE DIFFUSED AIR-STREAM, WHICH NOW HAS A VELOCITY OF ABOUT STREAM, WHICH NOW HAS A VELOCITY OF ABOUT 80 ft/sec80 ft/sec, , WOULD BE BLOWN AWAY. WOULD BE BLOWN AWAY.

(b)(b) THEREFORE, A REGION OF LOW AXIAL-VELOCITY HAS TO THEREFORE, A REGION OF LOW AXIAL-VELOCITY HAS TO BE CREATED IN THE CHAMBER, SO THAT THE FLAME WILL REMAIN BE CREATED IN THE CHAMBER, SO THAT THE FLAME WILL REMAIN ALIGHT THROUGHOUT THE RANGE OF ENGINE OPERATING ALIGHT THROUGHOUT THE RANGE OF ENGINE OPERATING CONDITIONS.CONDITIONS.

2. (a)2. (a) IN NORMAL OPERATION, THE OVERALL AIR/FUEL RATIO OF IN NORMAL OPERATION, THE OVERALL AIR/FUEL RATIO OF

A COMBUSTION CHAMBER CAN VARY BETWEEN A COMBUSTION CHAMBER CAN VARY BETWEEN 45:1 45:1 ANDAND 130:1 130:1. .

(b)(b) ATF BURNS EFFICIENTLY AT A RATIO OF ATF BURNS EFFICIENTLY AT A RATIO OF 15:1 15:1 approxapprox..

(c)(c) THE FUEL MUST BE BURNED WITH ONLY PART OF THE AIR THE FUEL MUST BE BURNED WITH ONLY PART OF THE AIR

ENTERING THE CHAMBER, IN WHAT IS CALLED A ENTERING THE CHAMBER, IN WHAT IS CALLED A PRIMARY PRIMARY COMBUSTION ZONECOMBUSTION ZONE. .

(d) THIS IS ACHIEVED BY MEANS OF A FLAME TUBE (COMBUSTION (d) THIS IS ACHIEVED BY MEANS OF A FLAME TUBE (COMBUSTION LINER) THAT HAS VARIOUS DEVICES FOR METERING THE AIR-LINER) THAT HAS VARIOUS DEVICES FOR METERING THE AIR-FLOW DISTRIBUTION ALONG THE CHAMBER.FLOW DISTRIBUTION ALONG THE CHAMBER.

a

3. (a) Approx. 3. (a) Approx. 20% OF THE AIR MASS FLOW IS TAKEN IN BY THE 20% OF THE AIR MASS FLOW IS TAKEN IN BY THE SNOUT OR ENTRY SECTION. IMMEDIATELY DOWNSTREAM OF SNOUT OR ENTRY SECTION. IMMEDIATELY DOWNSTREAM OF THE SNOUT ARE SWIRL VANES AND A PERFORATED FLARE, THE SNOUT ARE SWIRL VANES AND A PERFORATED FLARE, THROUGH WHICH AIR PASSES INTO THE PRIMARY THROUGH WHICH AIR PASSES INTO THE PRIMARY COMBUSTION COMBUSTION ZONEZONE. THE SWIRLING AIR INDUCES A FLOW . THE SWIRLING AIR INDUCES A FLOW UPSTREAM OF UPSTREAM OF THE CENTRE OF THE FLAME TUBE AND THE CENTRE OF THE FLAME TUBE AND PROMOTES THE PROMOTES THE DESIRED DESIRED RECIRCULATIONRECIRCULATION. THE AIR NOT . THE AIR NOT PICKED UP BY PICKED UP BY THE SNOUT FLOWS INTO THE ANNULAR SPACE THE SNOUT FLOWS INTO THE ANNULAR SPACE BETWEEN THE BETWEEN THE FLAME TUBE AND THE AIR CASING.FLAME TUBE AND THE AIR CASING.

3. (b)3. (b) IT IS ARRANGED THAT THE CONICAL FUEL SPRAY IT IS ARRANGED THAT THE CONICAL FUEL SPRAY FROM THE NOZZLE INTERSECTS THE FROM THE NOZZLE INTERSECTS THE

RECIRCULATION VORTEXRECIRCULATION VORTEX AT ITS CENTRE. THIS AT ITS CENTRE. THIS ACTION, TOGETHER WITH THE GENERAL ACTION, TOGETHER WITH THE GENERAL TURBULENCE TURBULENCE

IN THE IN THE PRIMARY ZONEPRIMARY ZONE, GREATLY , GREATLY ASSISTS IN ASSISTS IN BREAKING UP THE FUEL AND MIXING IT BREAKING UP THE FUEL AND MIXING IT WITH THE WITH THE

INCOMING AIR (i.e. INCOMING AIR (i.e. ATOMISATIONATOMISATION).).

4.4. THROUGH THE WALL OF THE FLAME TUBE BODY, THROUGH THE WALL OF THE FLAME TUBE BODY,

ADJACENT ADJACENT TO THE COMBUSTION ZONE, ARE A SELECTED TO THE COMBUSTION ZONE, ARE A SELECTED NUMBER OF NUMBER OF SECONDARY HOLES THROUGH WHICH A FURTHER 20 SECONDARY HOLES THROUGH WHICH A FURTHER 20

% OF % OF THE MAIN FLOW OF AIR PASSES INTO THE THE MAIN FLOW OF AIR PASSES INTO THE PRIMARY PRIMARY ZONEZONE..

THE AIR FROM THE THE AIR FROM THE SWIRL VANESSWIRL VANES AND THAT FROM THE AND THAT FROM THE

SECONDARY AIR HOLESSECONDARY AIR HOLES INTERACTS AND CREATES A INTERACTS AND CREATES A REGION OF LOW VELOCITY RECIRCULATION. THIS TAKES REGION OF LOW VELOCITY RECIRCULATION. THIS TAKES

THE THE FORM OF A TOROIDAL VORTEX, SIMILAR TO A SMOKE RING, FORM OF A TOROIDAL VORTEX, SIMILAR TO A SMOKE RING, WHICH HAS THE EFFECT OF STABILIZING AND ANCHORING WHICH HAS THE EFFECT OF STABILIZING AND ANCHORING

THE THE FLAME. FLAME.

THE RECIRCULATING GASES HASTEN THE BURNING OF THE RECIRCULATING GASES HASTEN THE BURNING OF FRESHLY INJECTED FUEL DROPLETS BY RAPIDLY BRINGING FRESHLY INJECTED FUEL DROPLETS BY RAPIDLY BRINGING

THEM TO IGNITION TEMPERATURE.THEM TO IGNITION TEMPERATURE.

UP TO 80%UP TO 80% OF THE AIR ENTERING THE COMBUSTION OF THE AIR ENTERING THE COMBUSTION CHAMBER CHAMBER IS USED TO COOL THE SIDES OF THE COMBUSTION IS USED TO COOL THE SIDES OF THE COMBUSTION CHAMBER CHAMBER AND TO STABILIZE THE FLAME. THIS FLAME AND TO STABILIZE THE FLAME. THIS FLAME STABILISATION IS STABILISATION IS IMPORTANT BECAUSE WITHOUT THIS, THE IMPORTANT BECAUSE WITHOUT THIS, THE FLAME WOULD FLAME WOULD SIMPLY BLOW OUT.SIMPLY BLOW OUT.

5.5. THE TEMPERATURE OF THE GASES RELEASED BY THE TEMPERATURE OF THE GASES RELEASED BY COMBUSTION IS ABOUT COMBUSTION IS ABOUT 1,800 to 2,000 deg. C1,800 to 2,000 deg. C.,., WHICH IS FAR WHICH IS FAR TOO HOT FOR ENTRY TO THE NOZZLE TOO HOT FOR ENTRY TO THE NOZZLE GUIDE VANES OF GUIDE VANES OF THE TURBINE. THE AIR NOT USED FOR COMBUSTION, WHICH THE TURBINE. THE AIR NOT USED FOR COMBUSTION, WHICH AMOUNTS TO ABOUT 60 % OF THE TOTAL AIR-FLOW, IS AMOUNTS TO ABOUT 60 % OF THE TOTAL AIR-FLOW, IS THEREFORE INTRODUCED PROGRESSIVELY INTO THE FLAME THEREFORE INTRODUCED PROGRESSIVELY INTO THE FLAME TUBE. TUBE.

Approx. 1/3 PART OF THIS IS USED TO LOWER THE GAS Approx. 1/3 PART OF THIS IS USED TO LOWER THE GAS TEMPERATURE IN THE TEMPERATURE IN THE DILUTION ZONEDILUTION ZONE BEFORE IT ENTERS BEFORE IT ENTERS THE TURBINE AND THE REMAINDER IS USED FOR THE TURBINE AND THE REMAINDER IS USED FOR COOLINGCOOLING THE WALLS OF THE FLAME TUBE. THE WALLS OF THE FLAME TUBE.

THIS IS ACHIEVED BY A FILM OF THIS IS ACHIEVED BY A FILM OF COOLING AIR FLOWING COOLING AIR FLOWING ALONG THE INSIDE SURFACE OF THE FLAME TUBE WALL, ALONG THE INSIDE SURFACE OF THE FLAME TUBE WALL, INSULATING IT FROM THE HOT COMBUSTION GASES. INSULATING IT FROM THE HOT COMBUSTION GASES.

6. 6. An electric spark from an igniter plug initiates An electric spark from an igniter plug initiates combustion and the flame is then self-sustained.combustion and the flame is then self-sustained.

A RECENT DEVELOPMENT ALLOWS COOLING A RECENT DEVELOPMENT ALLOWS COOLING AIR TO ENTER A NETWORK OF PASSAGES AIR TO ENTER A NETWORK OF PASSAGES WITHIN THE FLAME TUBE WALL BEFORE WITHIN THE FLAME TUBE WALL BEFORE EXITING TO FORM AN INSULATING FILM OF AIR, EXITING TO FORM AN INSULATING FILM OF AIR, THIS CAN REDUCE THE REQUIRED WALL THIS CAN REDUCE THE REQUIRED WALL COOLING AIRFLOW BY UP TO 50% . COOLING AIRFLOW BY UP TO 50% .

COMBUSTION SHOULD BE COMPLETED BEFORE COMBUSTION SHOULD BE COMPLETED BEFORE THE DILUTION AIR ENTERS THE FLAME TUBE, THE DILUTION AIR ENTERS THE FLAME TUBE, OTHERWISE THE INCOMING AIR WILL COOL THE OTHERWISE THE INCOMING AIR WILL COOL THE FLAME AND INCOMPLETE COMBUSTION WILL FLAME AND INCOMPLETE COMBUSTION WILL RESULT. RESULT.