Understanding the Annular Combustion Chamber
Annular means pertaining to an annulus or ring. Ring shaped. An annular
combustion chamber is in the shape of a ring or a cylinder and the whole of the
annulus between the compressor and the turbine is used for combustion. Like
other layouts (multiple chamber, cannuluar chamber, etc.), there is a space
between the flame tube and the outer casing for secondary and tertiary airflow
prior to entry into the flame tube. This type of combustion chamber is lighter,
but control of airflow is difficult, as are repairs.
COMBUSTERS
All turbine engines have a
combustor, or burner, in which the fuel is combined with high pressure air and
burned. The resulting high temperature exhaust gas is used to turn the power
turbine and produce thrust when passed through a nozzle.
There are three main types of combustors, and all three designs are found
in modern gas turbines:
The burner at the left is an annular combustor with the liner sitting
inside the outer casing which has been peeled open in the drawing. Many modern
burners have an annular design.
The burner in the middle is an older can or tubular design. The photo at
the top left shows some actual burner cans. Each can has both a liner and a
casing, and the cans are arranged around the central shaft.
A compromise design is shown at the right. This is a can-annular design, in
which the casing is annular and the liner is can-shaped. The advantage to the
can-annular design is that the individual cans are more easily designed,
tested, and serviced.
The burner's life is not an easy one. In order to assure ignition, the mixture
of fuel and air should be stoichiometric, and not moving very fast. However,
far more air passes through the engine than is required for complete combustion
of the fuel. Hence, the burner starts by separating out a small portion of the
air and decelerates it for combustion in the ``primary zone''. The gases
leaving this region are far too hot to be tolerated by current technology
turbines. For this reason, the remaining air (sometimes called dilution air) is
then mixed with the hot gases from the primary zone to produce a nearly uniform
temperature stream entering the turbine.
There are several varieties of burner. The
earliest was the multiple can burner. This operated by totally separating the
flow into eight or so cans, each of which contained a flame tube and also dealt
with the flow around it.
The first major Improvement on this design led
to the can-annular burner, which retained the multiple flame tubes of the
previous type, but allowed the outer air to flow free around all the flame
tubes. Can-annular designs have less efficient combustion, but because of their
modular design they are easier to repair or replace.
Modern combustion chambers are totally
annular. The flame tube makes a ring around the entire engine. This results in
much higher combustion efficiency- nearly all of the fuel is burned completely.
This reduces noxious emissions. Such burners are, however, more difficult to
service.
The next step in burners may be a double-annular chamber, in which there
are two concentric flame tubes. Dual annular designs are being pursued in order
to lower emissions. By using two rows of burners, the dual-annular combustor
can use just a single row during low power (idle, descent) conditions, while
both sets are lit under high power (takeoff, climbout) condition
SUMMARY
First, a can combustor. It's round. It's small. Jet fuel is sprayed in.
Fire is started. BANG. These are typically arranged in a circle, about six of
them in a jet engine. The exhaust coming out of them powers the turbine, which
in turn runs the compressor section at the front of the engine.
Now, an ANNULAR combustor is a little different. Instead of six equally
spaced cans, an ANNULAR combustor basically forms a ring... a circle around the
shaft of the engine. It's one LARGER combustion chamber. It's lighter. Cheaper.
Lasts longer. But is harder to work on.
*This is a patent
*An online presentation is also available on this subject.
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