Combustion chambers — Presentation Transcript COMBUSTION CHAMBER:JET ENGINE INTRODUCTION THE AIR/ FUEL MIXTURE BURNS INSIDE THE COMBUSTION CHAMBER. PROPER BURNING MUST TAKE PLACE DURING ENGINE OPERATION. PROPER STABILIZATION AND PROPAGATION OF FLAME IN THE COMBUSTION CHAMBERARE ESSENTIAL FOR OPTIMUM ENGINE POWER. THE AMOUNT OF FUEL ADDED TO THE AIR WILL DEPEND UPON THE TEMPERATURE RISEREQUIRED. THE MAXIMUM TEMPERATURE IS LIMITED TO 850 to 1700 deg. C BY THE MATERIALSFROM WHICH THE TURBINE BLADES AND EXHAUST NOZZLE GUIDE VANES ARE MADE. INTRODUCTION COMBUSTION CHAMBERS JET ENGINES EMPLOY VARIOUS TYPES OF COMBUSTIONCHAMBERS FOR BURNING AIR/FUEL MIXTURE AND GENERATING LARGE AMOUNT OF HEATDEPENDING UPON DESIGN / OPERATIONAL REQUIREMENTS. COMBUSTION CHAMBERS TO ENSURE EFFECTIVE AND EFFICIENT COMBUSTION OF AIR/FUELMIXTURE AND MAINTAIN OPTIMUM TEMPERATURES OF THE COMBUSTION PRODUCTS DURINGTHE ENTIRE RANGE OF ENGINE OPERATION. COMBUSTION CHAMBERS IN ORDER TO MEET VARIOUS OPERATIONAL REQUIREMENTS OF THE ENGINE, VARIOUSDESIGN FEATURES ARE INCORPORATED. HENCE, THEY VARY IN TERMS OF: -SHAPE -SIZE -MATERIAL -HEAT TREATMENT. THE COMBUSTION CHAMBER HAS THE DIFFICULT TASK OF BURNING LARGE QUANTITIES OFFUEL, SUPPLIED THROUGH THE FUEL SPRAY NOZZLES, WITH EXTENSIVE VOLUMES OF AIR,SUPPLIED BY THE COMPRESSOR, AND RELEASING THE HEAT IN SUCH A MANNER THAT THE AIRIS EXPANDED AND ACCELERATED TO GIVE A SMOOTH STREAM OF UNIFORMLY HEATED GAS ATALL CONDITIONS REQUIRED BY THE TURBINE. THIS TASK MUST BE ACCOMPLISHED WITH THEMINIMUM LOSS IN PRESSURE AND WITH THE MAXIMUM HEAT RELEASE FOR THE LIMITED
SPACE AVAILABLE. HENCE, THE COMBUSTION CHAMBER HAVE CERTAIN DESIGN / OPERATIONAL REQUIREMENTS . REQUIREMENTS OF COMBUSTION CHAMBER 1. The gas temperature required at the turbine varies with engine thrust, and in the case ofthe turbo-propeller engine upon the power required. Hence, combustion chamber must maintain a stable and efficient combustion over a widerange of engine operating conditions. 2. High combustion efficiency has become increasingly important because of the rapid risein commercial aircraft traffic and the consequent increase in atmospheric pollution. 3. Ability to operate efficiently over a wide range of conditions ( e.g. inlet pressure &temperature of air and air/fuel ratios ) . 4. The flame tube and spray nozzle atomizer components must be mechanically reliable . 5. The gas turbine engine operates on a constant pressure cycle. Therefore, during the process of combustion pressure loss should be minimum . In providing adequate turbulence and mixing, a total pressure loss varying from about 3 - 8% of the air pressure at entry to the chamber is incurred. REQUIREMENTS OF COMBUSTION CHAMBER 6. Combustion stability / satisfactory rich and weak mixture extinction limits). Combustion stability means smooth burning and the ability of the flame to remain alightover a wide operating range. For any particular type of combustion chamber there is both a richand weak limit to the air/fuel ratio, beyond which the flame is extinguished. 7. Uniform temperature and velocity distribution at the entrance to the turbine andsimplicity of control . 8. Heat & corrosion resistant . The containing walls and internal parts of the combustion chamber are subjected to theproducts of the combustion, creep failure and fatigue due to thermal and vibrational stresses. 9. Ease and cheapness of manufacture . Although there are several types of combustion chamber designs, they all have an inner andan outer section. The inner section or liner (as it is known) will be perforated with many holeswhich allow cooling and stabilising air to enter the combustion chamber throughout its length.
The outer section is not perforated and acts as a sealed vessel for the liner. The liner is usuallyconcentric within the outer casing and the passage formed between the two is used to form anarea of cool air to prevent the materials of the combustion chambers from melting intemperatures in the region of 2000 deg C. CONSTRUCTIONAL DETAILS TYPES OF COMBUSTION CHAMBER MULTIPLE OR CAN TYPE TYPES OF COMBUSTION CHAMBERS ANNULAR TYPE TYPES OF COMBUSTION CHAMBER CAN-ANNULAR TYPE 1. (a) AIR FROM THE ENGINE COMPRESSOR ENTERS THE COMBUSTION CHAMBER AT AVELOCITY UP TO 500 ft/sec , BUT BECAUSE AT THIS VELOCITY THE AIR SPEED IS FAR TOO HIGHFOR COMBUSTION, THE FIRST THING THAT THE CHAMBER MUST DO IS TO DIFFUSE IT, I.E.DECELERATE IT AND RAISE ITS STATIC PRESSURE. THE BURNING SPEED OF ATF AT NORMALMIXTURE RATIOS IS VERY LESS. ANY FUEL LIT EVEN IN THE DIFFUSED AIR-STREAM, WHICH NOWHAS A VELOCITY OF ABOUT 80 ft/sec , WOULD BE BLOWN AWAY. (b) THEREFORE, A REGION OF LOW AXIAL-VELOCITY HAS TO BE CREATED IN THE CHAMBER,SO THAT THE FLAME WILL REMAIN ALIGHT THROUGHOUT THE RANGE OF ENGINE OPERATINGCONDITIONS. COMBUSTION PROCESS AIR ENTRY 2. (a) IN NORMAL OPERATION, THE OVERALL AIR/FUEL RATIO OF A COMBUSTION CHAMBERCAN VARY BETWEEN 45:1 AND 130:1 . (b) ATF BURNS EFFICIENTLY AT A RATIO OF 15:1 approx . (c) THE FUEL MUST BE BURNED WITH ONLY PART OF THE AIR ENTERING THE CHAMBER, INWHAT IS CALLED A PRIMARY COMBUSTION ZONE . (d) THIS IS ACHIEVED BY MEANS OF A FLAME TUBE (COMBUSTION LINER) THAT HASVARIOUS DEVICES FOR METERING THE AIR-FLOW DISTRIBUTION ALONG THE CHAMBER. a PRIMARY ZONE 3. (a) Approx. 20% OF THE AIR MASS FLOW IS TAKEN IN BY THE SNOUT OR ENTRY SECTION.IMMEDIATELY DOWNSTREAM OF THE SNOUT ARE SWIRL VANES AND A PERFORATED FLARE,THROUGH WHICH AIR PASSES INTO THE PRIMARY COMBUSTION ZONE . THE SWIRLING AIRINDUCES A FLOW UPSTREAM OF THE CENTRE OF THE FLAME TUBE AND PROMOTES THEDESIRED RECIRCULATION . THE AIR NOT PICKED UP BY THE SNOUT FLOWS INTO THE ANNULARSPACE BETWEEN THE FLAME TUBE AND THE AIR CASING.
PRIMARY ZONE 3. (b) IT IS ARRANGED THAT THE CONICAL FUEL SPRAY FROM THE NOZZLE INTERSECTS THERECIRCULATION VORTEX AT ITS CENTRE. THIS ACTION, TOGETHER WITH THE GENERALTURBULENCE IN THE PRIMARY ZONE , GREATLY ASSISTS IN BREAKING UP THE FUEL AND MIXINGIT WITH THE INCOMING AIR (i.e. ATOMISATION ). PRIMARY ZONE 4. THROUGH THE WALL OF THE FLAME TUBE BODY, ADJACENT TO THE COMBUSTION ZONE,ARE A SELECTED NUMBER OF SECONDARY HOLES THROUGH WHICH A FURTHER 20 % OF THEMAIN FLOW OF AIR PASSES INTO THE PRIMARY ZONE . THE AIR FROM THE SWIRL VANES AND THAT FROM THE SECONDARY AIR HOLES INTERACTSAND CREATES A REGION OF LOW VELOCITY RECIRCULATION. THIS TAKES THE FORM OF ATOROIDAL VORTEX, SIMILAR TO A SMOKE RING, WHICH HAS THE EFFECT OF STABILIZING ANDANCHORING THE FLAME. THE RECIRCULATING GASES HASTEN THE BURNING OF FRESHLY INJECTED FUEL DROPLETSBY RAPIDLY BRINGING THEM TO IGNITION TEMPERATURE. COMBUSTION PROCESS SECONDARY ZONE UP TO 80% OF THE AIR ENTERING THE COMBUSTION CHAMBER IS USED TO COOL THE SIDESOF THE COMBUSTION CHAMBER AND TO STABILIZE THE FLAME. THIS FLAME STABILISATION ISIMPORTANT BECAUSE WITHOUT THIS, THE FLAME WOULD SIMPLY BLOW OUT. FLAME STABILISATION 5. THE TEMPERATURE OF THE GASES RELEASED BY COMBUSTION IS ABOUT 1,800 to 2,000deg. C ., WHICH IS FAR TOO HOT FOR ENTRY TO THE NOZZLE GUIDE VANES OF THE TURBINE.THE AIR NOT USED FOR COMBUSTION, WHICH AMOUNTS TO ABOUT 60 % OF THE TOTAL AIR-FLOW, IS THEREFORE INTRODUCED PROGRESSIVELY INTO THE FLAME TUBE. Approx. 1/3 PART OF THIS IS USED TO LOWER THE GAS TEMPERATURE IN THE DILUTIONZONE BEFORE IT ENTERS THE TURBINE AND THE REMAINDER IS USED FOR COOLING THE WALLSOF THE FLAME TUBE. THIS IS ACHIEVED BY A FILM OF COOLING AIR FLOWING ALONG THE INSIDE SURFACE OFTHE FLAME TUBE WALL, INSULATING IT FROM THE HOT COMBUSTION GASES. DILUTION IN TERTIARY ZONE 6. An electric spark from an igniter plug initiates combustion and the flame is then self-sustained. A RECENT DEVELOPMENT ALLOWS COOLING AIR TO ENTER A NETWORK OF PASSAGES
WITHIN THE FLAME TUBE WALL BEFORE EXITING TO FORM AN INSULATING FILM OF AIR, THISCAN REDUCE THE REQUIRED WALL COOLING AIRFLOW BY UP TO 50% . COMBUSTION SHOULD BE COMPLETED BEFORE THE DILUTION AIR ENTERS THE FLAMETUBE, OTHERWISE THE INCOMING AIR WILL COOL THE FLAME AND INCOMPLETE COMBUSTIONWILL RESULT.