1

Prepared byHEMANTHKRISHNAN R

ROLL NO 44

S5MA

NSSCE

MY TOPIC

COMBUSTION CHAMBER ARRANGEMENTS

3

A large single chamberUsually employed in in the heavy industrial power plants

Multiple ChambersAircraft applications

Annular ChamberBest suited to compressors of axial flow type

4

Why COMBUSTION CHAMBERS ?•Blends air and fuel proficiently

•Controls the burning of large amounts of fuel and air efficiently

•Dampens the hot combustion gases

•Ensures that the air is expanded and accelerated

** must be accomplished with minimum pressure loss and maximum

heat release

REQUIREMENTS OF AN INDUSTRIAL TYPE COMBUSTION CHAMBER

Required to operate economically & reliably over long periods without attention.

Compactness & weight restrictions are no longer important & is considered only if the engine has to be constrained to fit into an existing building or if delivery is made difficult.

Fuel economy, Low gas velocities & High combustion efficiency

Low pressure loss.

Low pollutant emission, Long life.

Minimum cost ,Accessibility for maintenance.

Minimal Shut down time.

5

Hence… To meet these objectives, industrial engine combustors

are normally larger than the ones in aeronautical engines. Thus, the residence time inside the combustors is longer.

This is an advantage when the fuel quality is poor.

Also, the pressure drop across the combustors is

smaller due to a lower velocity of the flow.

6

INDUSTRIAL COMBUSTION CHAMBERS:TYPES

7

The GE MS-7001, 80-MW gas turbines are one of the most successful industrial engines. There are 10 sets of combustion hardware in each machine. Each set includes a casing, an end cover, a set of fuel nozzles, a flow sleeve, a combustion liner, and a transition piece, as shown in Fig. 12.17. The flow sleeve has a cylindrical shape. It surrounds the liner and aids in distributing the air uniformly to all liners. Each combustor has one fuel nozzle in the conventional MS-7001. Multiple fuel nozzles are used for each combustor in the more advanced DLE versions. Some industrial engines have a single large combustor. It is installed outside the engine, as illustrated in Fig. 12.18. This design allows the combustor to meet the requirements of good combustion performance. The outer casing of the unit can be designed to withstand the high pressure. This arrangement has another advantage. It is the ease of inspection, maintenance, and repair. They can all be performed without removing the large components in the casing

It is preferable to use multiple fuel injectors (burners) for these combustors for the following two reasons: 1. They provide a shorter flame 2. The gases flowing into the dilution zone will have a more uniform temperature distribution

8

9

A number of “hybrid” burners are installed on the Siemens Silo combustors. They burn natural gas in either diffusion or premix modes. They emit a low level of pollutants over a wide range of loads. At low loads, the system operates as a diffusion burner. At high loads, it operates as a premix burner. Siemens used the same fuel burner in their silo-type combustors for engines having different power ratings. They only changed the number of burners to accommodate the changes in the size of the engine. However, the number of burners in their new annular combustors was fixed at 24. This was done to provide good pattern factor.

The main disadvantage of this design is that the size of the burners must vary with the rating of the machine. However, the basic design remained the same. The Siemens hybrid burner has been proven to provide low emissions for engines in the 150-MW rating. This design has also been used by MAN GHH to its THM-1304 engine, which is a 9-MW gas turbine. It has two tubular combustion chambers. They are mounted on top of the casing.

10

Silo type combustors

11

Silo type combustors and side combustors are found on large industrial turbines. They offer the advantage s of simplicity of design, ease of maintenance and long life due to low heat release rates. These combustors may be of the straight through or reverse flow design. In the reverse flow design, air enters the annulus between the combustor can and its housing ,usually via a hot gas pipe, to the turbine. Reverse flow designs have minimal length

12

1. (a) AIR FROM THE ENGINE COMPRESSOR ENTERS THE COMBUSTION CHAMBER AT A VELOCITY UP TO 500 ft/sec, BUT BECAUSE AT THIS VELOCITY THE AIR SPEED IS FAR TOO HIGH FOR 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 NORMAL MIXTURE RATIOS IS VERY LESS. ANY FUEL LIT EVEN IN THE DIFFUSED AIR-STREAM, WHICH NOW HAS 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 OPERATING CONDITIONS.

13

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

COMBUSTION CHAMBER CAN 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, IN WHAT IS CALLED A PRIMARY COMBUSTION ZONE.

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

a

14

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 AIR INDUCES A FLOW UPSTREAM OF THE CENTRE OF THE FLAME TUBE AND

PROMOTES THE DESIRED RECIRCULATION. THE AIR NOT PICKED UP BY THE SNOUT FLOWS INTO THE ANNULAR SPACE BETWEEN THE FLAME TUBE AND THE AIR CASING.

15

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

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

TURBULENCE IN THE PRIMARY ZONE, GREATLY ASSISTS IN BREAKING UP THE FUEL AND MIXING

IT WITH THE INCOMING AIR (i.e. ATOMISATION).

16

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 THE MAIN FLOW OF AIR PASSES INTO THE

PRIMARY ZONE.

THE AIR FROM THE SWIRL VANES AND THAT FROM THE

SECONDARY AIR HOLES INTERACTS AND CREATES A

REGION OF LOW VELOCITY RECIRCULATION. THIS TAKES THE FORM OF A TOROIDAL VORTEX, SIMILAR TO A SMOKE RING, WHICH HAS THE EFFECT OF STABILIZING AND ANCHORING THE FLAME.

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

THEM TO IGNITION TEMPERATURE.

17

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

SIMPLY BLOW OUT.

18

5. THE TEMPERATURE OF THE GASES RELEASED BY COMBUSTION

IS ABOUT 1,800 to 2,000 deg. C., WHICH IS FAR TOO HOTFOR ENTRY TO THE NOZZLE GUIDE VANES OF THE TURBINE.THE AIR NOT USED FOR COMBUSTION, WHICH AMOUNTS TOABOUT 60 % OF THE TOTAL AIR-FLOW, IS THEREFOREINTRODUCED PROGRESSIVELY INTO THE FLAME TUBE.

Approx. 1/3 PART OF THIS IS USED TO LOWER THE GAS

TEMPERATURE IN THE DILUTION ZONE BEFORE IT ENTERS

THE TURBINE AND THE REMAINDER IS USED FOR COOLINGTHE WALLS OF THE FLAME TUBE.

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

19

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

20

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, THIS CAN REDUCE THE REQUIRED WALL COOLING AIRFLOW BY UP TO 50% .

CAUTION:

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

And let’s conclude…..

21

22