COMBUSTION
SECTION
AAMT 1-I GROUP 2
COMBUSTION SECTION
A combustion section is typically located directly
between the compressor diffuser and turbine section.
All combustion sections contain the same basic
elements:
one or more combustion chambers (combustors)
a fuel injection system
an ignition source
and a fuel drainage system.
COMBUSTION CHAMBER
(COMBUSTOR)
The combustion chamber or combustor in a turbine
engine is where fuel and air are mixed and burned. A
typical combustor consists of an outer casing with a
perforated inner liner. The perforations are various
sizes and shapes, all having a specific effect on the
propagation of the flame within the liner.
FUEL INJECTION SYSTEM
The fuel injection system meters the appropriate
amount of fuel through the fuel nozzles into the
combustors. Fuel nozzles are located in the
combustion chamber case or in the compressor outlet
elbows. Fuel is delivered through the nozzles into the
liners in a finely atomized spray. This ensures
thorough mixing with the incoming air. The finer the
spray, the more rapid and efficient the combustion
process is.
IGNITION SOURCE
A typical ignition source for gas turbine engines is the
high-energy capacitor discharge system, consisting of
an exciter unit, two high-tension cables, and two
spark igniters. This ignition system produces 60 to
100 sparks per minute, resulting in a ball of fire at the
igniter electrodes. Some of these systems produce
enough energy to shoot the sparks several inches, so
take care to avoid a lethal shock during maintenance
tests.
FUEL DRAINAGE SYSTEM
A fuel drainage system accomplishes the important
task of draining unburned fuel after engine shutdown.
Draining accumulated fuel reduces the possibility of
exceeding tailpipe or turbine inlet temperature limits
(caused by an engine fire after shutdown). In
addition, draining unburned fuel helps prevent gum
deposits caused by fuel residue in the fuel manifold,
nozzles, and combustion chambers.
COMBUSTION SECTION
To efficiently burn the fuel/air mixture, a
combustion chamber must:
1. Mix fuel and air effectively in the best ratio for good
combustion.
2. Burn the mixture as efficiently as possible.
3. Cool the hot combustion gases to a temperature
that the turbine blades can tolerate.
4. Distribute hot gases evenly to the turbine section.
COMBUSTION SECTION
To enable the combustion section to mix the incoming
fuel and air, ignite the mixture, and then sufficiently
cool the combustion gases, airflow through a
combustor is divided into primary and secondary
paths. Approximately 25 to 35 percent of the
incoming air is designated as primary, and the
remaining air is designated as secondary.
PRIMARY (COMBUSTION AIR)
Primary, or combustion air, is directed inside the liner in the front end of a
combustor. As this air enters the combustor, it passes through a set of swirl
vanes, which slow the air down to five or six feet per second and impart a
vortex motion.
The reduction in airflow velocity is important because kerosene-based fuels
have a slow flame propagation rate. An excessively high-velocity airflow
could literally blow the flame out of the engine. Such a condition is known
as a flameout. The vortex created in the flame area provides the
turbulence required to mix the fuel and air properly, after which the
combustion process is complete in the first third of a combustor.
SECONDARY AIRFLOW
The secondary airflow in the combustion section flows at a velocity of
several hundred feet per second around the combustor's periphery. This
flow of air forms a cooling air blanket on both sides of the liner and centers
the combustion flames to prevent contact with the liner. Some secondary
air slows and enters the combustor through the perforations in the liner.
This air supports the combustion of any remaining unburned fuel.
Secondary air also mixes with the burned gases and cool air to provide an
even distribution of energy to the turbine nozzle at a temperature that the
turbine section can withstand.
3 BASIC TYPES
OF COMBUSTION
CHAMBERS
MULTIPLE-CAN
COMBUSTORS
The multiple-can combustion chamber consists of a series of individual combustor cans that
function as individual burner units. This type of combustion chamber is well suited to
centrifugal compressor engines because of the way compressor discharge air is equally
divided at the diffuser. Each can is constructed from a perforated stainless steel liner inside
an outer case. The inner liner is highly heat resistant and easily removed for inspection. Each
combustion can has a large degree of curvature that provides high resistance to warpage.
However, this shape is inefficient because of the volume of space it requires and the
additional weight.
The individual combustors in a typical multiple-can combustion chamber are interconnected
with small flame propagation tubes. The combustion starts in the two cans equipped with
igniter plugs; the flame travels through the tubes to ignite the fuel/air mixture in the other
cans. Each flame propagation tube consists of a small tube surrounded by a larger tube (or
jacket). The small inner tube contains the flame and the outer tube contains cooling and
insulating airflow between the cans. A typical multiple-ca11 combustion section includes 8 or
10 cans. On most American-built engines, cans are numbered clockwise with the number one
can on the top when facing the rear of the engine. All of the combustor cans discharge
exhaust gases into an open area at the turbine nozzle inlet.
ANNULAR COMBUSTORS
Today, annular combustors are commonly used in both small and large
engines. From a standpoint of thermal efficiency, weight, and physical size,
the annular combustor is the most efficient. An annular combustion chamber
consists of a housing and perforated inner liner, or basket. The liner is a
single unit encircling the outside of the turbine shaft housing. The shroud is
shaped to contain one or more concentric baskets. An annular combustor
with two baskets is known as a double-annular combustion chamber.
Normally, the ignition source consists of two spark igniters similar to the type
found in multiple-can combustors.
In a conventional annular combustor, airflow enters at the front and is
discharged at the rear and primary and secondary airflow are much the same
as the multiple-can design. However, unlike can combustors, an annular
combustor must be removed as a single unit for repair or replacement. This
usually involves complete separation of the engine at a major flange.
ANNULAR COMBUSTORS
Some annular combustors are designed with a reverse direction airflow.
Reverse-flow combustors work the same as conventional flow combustors,
except that the air flows around the chamber and enters from the rear. This
causes the combustion gases to flow through the engine opposite the
normal airflow. This idea was first employed by Whittle in his early designs.
In a typical reverse-flow annular combustor, the turbine wheels are inside
the combustor instead of downstream, as with the conventional flow
designs. This design allows for a shorter and lighter engine that uses the
hot gases to preheat the compressor discharge air. These factors help
make up for the loss of efficiency caused by the gases passing through the
combustor in the reverse direction.
CAN-ANNULAR
COMBUSTORS
Can-annular combustion sections are a combination of the multiple-can
and the annular type combustors. The can-annular combustor was
invented by Pratt & Whitney and consists of a removable steel shroud that
encircles the entire combustion section. Inside the shroud, or casing,
multiple burner cans are assembled radially about the engine axis with
bullet-shaped perforated liners. A fuel nozzle cluster is attached to the
forward end of each burner can, and pre-swirl vanes are placed around
each fuel nozzle. The pre-swirl vanes enhance combustion by promoting a
thorough mixing of fuel and air and slowing the velocity of axial air in the
burner can. Flame propagation tubes connect the individual liners, and two
igniter plugs initiate combustion. Each can and liner is removed and
installed as a single unit for maintenance. This design combines the ease of
overhaul and testing of the multiple-can arrangement with the compact
design of the annular combustor.
FLAMEOUT
As mentioned earlier, the combustion flame can be extinguished by high
airflow rates. Additionally, excessively slow airflow rates can cause this
problem. Although flameout is uncommon in modern
engines, combustion instability still occurs and occasionally causes a
complete flameout. The correct set of circumstances (turbulent weather,
high altitude, slow acceleration, and high-speed maneuvers) can induce
combustion instability that results in a flameout. The two types of
flameouts are the lean die-out and the rich blowout. A lean die-out usually
occurs at high altitude where low engine speeds and low fuel pressure form
a weak flame that is extinguished in a normal airflow. The conditions for a
rich blowout typically occur during rapid engine acceleration with an overly
rich mixture, in which either the fuel temperature drops below the
temperature necessary for combustion or there is insufficient airflow to
support combustion.
