Combustor Section.pdf

PROGRAM STUDI TEKNIK PENERBANGAN & TEKNIK AERONAUTIKA

FAKULTAS TEKNOLOGI KEDIRGANTARAAN

UNIVERSITAS SURYADARMA

COMBUSTOR SECTION

Created by : Ir. Tri Susilo, MT


Introduction

The combustion chamber has the difficult task of burning large quantities of

fuel, 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

air is expanded and accelerated to give a smooth stream of uniformly heated

gas at all conditions required by the turbine.

This task must be accomplished with the minimum loss in pressure and with

the maximum heat release for the limited space available.

Efficient combustion has become increasingly important because of the rapid

rise in commercial aircraft traffic and the consequent increase in atmospheric

pollution, which is seen by the general public as exhaust smoke.


To function properly, the combustor or burner must mixture the air and the

fuel for efficient combustion. Then it must lower the temperature of the hot

combustion products enough that they will not overheat the turbine

components.

Combustor Function


The combustors, or burners, in a gas turbine engine have an outer casing, an

inner perforated liner, usually made of stamped sheet metal, a fuel injection

system, and an ignition system for starting.

Combustor Construction


Combustor Construction


Multiple Can Combustor Type

This older type of combustion chamber

(not commonly used today) consists of a

series of outer housings, each with its

own perforated inner liner. Each of the

multiple combustor cans is actually a

separate burner unit, with all of them

discharging into the open area at the

turbine nozzle inlet.

The individual combustors are

interconnected (interconnector) with

small flame propagation tubes so that

when combustion starts in the two cans

having igniter plugs, the flame will

travel through the tubes and ignite the

fuel-air mixture in the other cans.


Annular Combustor Type

The annular combustor consist of an

outer housing and a perforated inner liner

called a basket, with both parts encircling

the engine. Multiple fuel spray nozzles

stick out into the basket, and both

primary and secondary air for combustion

and cooling flow through it in the same

way as in the other combustor designs.

Annular combustors are in common use

today in both small and large engines.

They are the most efficient type from the

standpoint of both thermal efficiency and

weight, and they are also shorter than the

other types.


Can-Annular Combustor Type

The can-annular combustor (turbo-

annular combustor) is used for

commercial aircraft powered by Pratt and

Whitney engines. This type of combustor

consists of an outer case with multiple

inner liners located radialy around the

axis of the engine. Flame propagation

tubes connect the individual liners and

two igniter plugs are used for starting.

An advantage of this type of combustor is

its ease of on the wing maintenance

because the forward half of the outer

combustor casing may be unfastened to

slide rearward exposing the cans for

inspection.


Combustor Flow Configuration

Most combustors are of the through-flow configuration which is

sometimes called a through-flow combustor. Gas entering from diffuser

are ignited and then pass directly through the combustor into the turbine

section.


Reverse Flow Annular Combustor Type

The reverse-flow combustor serves the same function as the through-flow combustor,

but it differs by the air flowing around the chamber and entering from the rear, causing

the combustion gas flow to be in the opposite direction as the normal airflow through

the engine.

This allows for a shorter and lighter engine, and it also uses the hot gases to preheat

the compressor discharge air. These factors help make up for the loss of efficiency

cause by the gases having to reverse their direction as they pass through the

combustor.


Combustor Flow Configuration

Another configuration is the reverser-flow annular type where gases

leaving the diffuser flow to the rear of the combustor where fuel is sprayed

in and ignited. Then the burning gases follow a reverse-S path into the

turbine section.


Combustion Process

Air from the engine compressor enters the combustion chamber at a

velocity up to 500 to 800 ft/s, 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.

Heat energy is added to the flowing gases in the burners, and this energy

expands the gases (thermal energy) and accelerates (kinetic energy) them

as they leave the engine.

When heat energy from the fuel is added, the gases expand, but since the

area through which the gas must flow remains the same, the flowing

gases speed up.


Combustion Process

In normal operation, the overall air/fuel ratio of a combustion chamber can

vary between 45:1 and 130:1. However, kerosene will only burn efficiently

at, or close to, a ratio of 15:1, so the fuel must be burned with only part of

the air entering the chamber, in what is called a primary combustion zone.

The airflow through the combustor is divided into primary and secondary

air paths. Approximately 25 to 35 % of the air is routed to the area around

the fuel nozzle for combustion, this is the primary air. The secondary air, or

the remaining 65 to 75 %, forms a cooling air blanket on either side of the

liner and centers the flames so they do not contact the metal.

The secondary air also dilutes and cools the hot primary air to a

temperature that will not shorten the service life of the turbine

components.


Combustion Process

The secondary air in the combustors may flow at a velocity of up to several

hundred feet-per-second, but the primary airflow is slowed down by swirl

vanes, which gives the air a radial motion and retards its axial velocity to

about five or six feet-per-second before it is mixed with the fuel and burned.

The vortex created in the flame area provides the required turbulence to

properly mix the fuel and the air. This reduction in the airflow velocity is

important because of the slow flame propagation rate of kerosene-type

fuels.

If the primary airflow velocity was too high, it would literally blow the flame

out of the engine. As it is, the combustion process is complete in the first

third of the combustor length, and the burned and unburned gases then

mix to provide an even distribution of heat at the turbine nozzle.


Combustion Process


Combustion Stability

Combustion stability means smooth burning and the ability of the flame to

remain alight over a wide operating range.



There are two types of flameouts ; a lean flameout usually occurs at

low engine speed and low fuel pressure, at high altitude where the

flame from a weak mixture can be blown out by the normal airflow.

A rich flameout occurs during rapid engine acceleration where an

overly-rich mixture cause the combustion pressure to increase so much

that the compressor airflow stagnates and slow down, or even stops.

Turbulent weather, high altitude, slow acceleration during

maneuvers, and high speed maneuvers, turbulent inlet conditions are

some of the typical conditions which induce combustor instability which

could lead to flameout.



Combustor instability sometimes causes small gas pressure

fluctuations. These low pressure cycles cause high fuel flow pulsations,

which increase the combustor instability until the pilot makes the

necessary adjustments to the flight conditions or to the engine controls.

The combustor of gas turbine engine operates on a constant pressure

cycle, therefore any loss of pressure during the process of combustion

must be kept to a minimum. In providing adequate turbulence and

mixing, a total pressure loss varying from about 3 to 8 percent of the air

pressure at entry to the chamber is incurred.

The combustor efficiency is high, but only about one third of the mass

airflow is used for combustion, with the remainder of it used for cooling,

to keep the temperatures within acceptable limits for the combustor and

the turbine.


Combustion Efficiency

Combustor efficiency ranges between 95 and 99 percent, which means

that 95 and 99 percent of the heat energy in the fuel is release.


Combustion Emission

The unwanted pollutants which are found in the exhaust gases are

created within the combustion chamber. There are four main pollutants

which are legislatively controlled.

Unburnt hydrocarbons (unburnt fuel), smoke (carbon particles), carbon

monoxide and oxides of nitrogen. The principal conditions which affect

the formation of pollutants are pressure, temperature and time.

In the fuel rich regions of the primary zone, the hydrocarbons are

converted into carbon monoxide and smoke. Fresh dilution air can be

used to oxidize the carbon monoxide and smoke into non-toxic carbon

dioxide within the dilution zone. Unburnt hydrocarbons can also be

reduced in this zone by continuing the combustion process to ensure

complete combustion.


Combustion Emission

Oxides of nitrogen are formed under the same conditions as those

required for the suppression of the other pollutants. Therefore it is

desirable to cool the flame as quickly as possible and to reduce the time

available for combustion.

This conflict of conditions requires a compromise to be made, but

continuing improvements in combustor design and performance has led

to a substantially cleaner combustion process.


Combustor Material

The containing walls and internal parts of the combustion chamber

must be capable of resisting the very high gas temperature in the primary

zone. The temperature of the gas released by combustion is about 1.800

to 2.000 oC.

In practice, this is achieved by using the best heat-resisting materials

available, the use of high heat resistant coatings and by cooling the inner

wall of the flame tube as an insulation from the flame.

The combustion chamber must also withstand corrosion due to the

products of the combustion, creep failure due to temperature gradients

and fatigue due to vibration stresses.


Combustion Emission

With our innovative single-annular TAPS combustor, the GEnx is designed to be the

cleanest-burning engine in its class. This combustor will far and away comply with

all existing and expected regulations for NOx emissions -- positioning GEnx

operators for clean compliance for many years to come.

Cleaner combustion requires technology that delivers high efficiency and lower,

more uniform flame temperatures. This is achieved with our innovative pre-mixing

concept. By directing nearly all of the airflow through unique swirlers and around

nested fuel nozzles, we create ideal pre-mixed fuel/air environment. And because

NOx production is strongly driven by combustion temperature, these emissions will

be drastically reduced.

Additionally, because all of the combustion air enters through the dome and mixers,

no dilution holes are required on the new liner. This in turn reduces distress, leading

to longer liner life and reduced maintenance costs.

The lower and more uniform temperatures produced by this combustor have

another benefit, as well. They significantly improve the lives of all downstream

components.

This clean, easy-to-maintain combustor is one of the many innovations that will give

GEnx customers advantages unavailable to anyone else. Just image how we've

improved the other parts of this amazing new engine..


Combustion Emission


Combustor Design Requirements

a) Complete combustion.

b) Low total pressure loss.

c) Stability of combustion process.

d) Proper temperature distribution at exit with no hot spots.

e) Short length and small cross section

f) Freedom from flameout.

g) Relight ability

h) Operation over a wide range of mass flow rate, pressure and

temperatures.

THANK YOU

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