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Dear Professor and classmates.r /
My presentation today treats the Turbofan Engine. In the course of my
presentation, I\'ll use some abbreviations, which you can find on the
blackboard. My presentation is structured in six parts:
1. General design
2. The fan
3. The compressor
4. The burner
5. The Turbine
6. Increase in performance
1. General Design:
Generally, there are two types of Turbofan Engines:
* Dual-spool turbofans: Here, the LPC and LPT are situated on the one spool
and the HPC, the HPT and the fan with an reduction gear on the other spool
* Triple-spool turbofans: Here the fan\'s got an own spool with a LPC.
A typical Turbofan Engine is the Trent 700 from Rolls-Royce. I will
describe this topic by means of this Engine.
A Turbofan Engine consists of the following main parts:
2. The fan:
The fan with a diameter of 120\" is actually half compressor - half
propeller. It is responsible for the high efficiency of this engine type at
high subsonic speeds, because it pushes most of the intaken air around the
other components. The relation of the air that goes around the engine and
the air that goes through the engine is called bypass ratio.
In this case it\'s 5,92 to 1.
3. The compressor:
Most Turbofan Engines use axial flow compressors, because of their axial
direction of airflow, which causes only small losses. But there is also a
disadvantage: the high complexity of the compressor construction, which you
can see on the foil.
A compressor stage consists of a single row of fixed compressor blades, the
stator, and rotating compressor blades, the rotor. The stator case is made
of stainless steel, Titan or cast magnesium and is divided into two halves.
The rotor is the most complicated part of the compressor. It has to
transmit a power of several thousand hp. The rotor consists of a number of
disks, which are attached to a cylinder. These disks hold the compressor
blades made of forged Titan.
The design of the blades is close to the design of an aircraft wing. The
only difference is the blade twist. The twist enables a constant
axial-velocity over bladeheight.
The Trent 700 has a eight stage low pressure compressor with variable
geometry. That means the angle of attack of the stator blades can be
adjusted to allow an optimal airflow. The high-pressure compressor has six
stages and a maximal rotation speed of 15183 rpm.
4. The burner:
In the burner fine atomized propellant is mixed to the highly-compressed
air and then burnt. The problem is that the air comes with about 150 m/s
from the HPC. But the combustion process requires a speed of only a few
meters per second, because when the air velocity is too high the flame will
be blown out. So the first part of the burner is a swirl vane combined with
a diffuser, the so-called primary zone. In the primary zone 20 - 30% of the
incoming air is mixed with the fuel. The rest of the air is led into the
air casing to cool the chamber, otherwise the material would melt, because
of temperatures up to 2000 ?C.
The burners can be divided into three groups:
Can-Type Burner:
This one is the oldest type of burner. The combustion chambers are placed
around the engine axis. The disadvantage of this design is the bad airflow
characteristic.
Annular-Type-Burner:
A very effective space-utilisation enable annular-type-burners, because
there is only one combustion chamber, which is located concentrically
around the engine axis.
The advantage of this kind of construction is the lower fuel consumption up
to -25%.
Can-Annular-Type-Burner:
This most common burner is a combination of the two previous burner-types.
The secondary air supply occurs from a common air casing, while the primary
air flows in through mouthpieces.
5. The Turbine
The task of the turbine is to provide the power to drive the compressors,
the fan and the accessories, like the oil pump or the lubrication system.
The turbine is actually a turned-around compressor, so it withdraws the hot
gas-stream energy.
As in the case of the compressor, there are two types of turbine designs,
the high-pressure turbine and the low-pressure turbine. Because of the
enormous heat stress, the turbine and stator blades have to be cooled. The
cooling air comes from the cold by-pass-stream. The cooling-air main leads
through the blades and enters it at the hub and exits at the tip. The
material the blade is made of in the most cases is the alloy NIMONIC 115
with the following components: Ni 57,29%, Co 15%, Cr 15%, Al 5%, Ti 4%, Mo
3.5%, C 0,16%, Zr 0,04%, B 0,014%.
But there are also other materials such as Udimet 700, B1900, Iconel 713 or
Waspaloy.
6. Increase in performance
There are two techniques to increase the performance of an engine. The
first, the afterburner, is only used at military turbojet engines, because
of the enormous fuel consumption.
The most suitable solution for civil applications is the water injection. A
water-methanol mixture is injected into the airflow to reduce its
temperature by vaporizing, because the lower the temperature, the higher
the density and the mass-flow. And high mass-flow means high thrust.
I hope you\'ve enjoyed my report and thank you for paying attention.
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