29-04-2014, 02:32 PM
FUNDAMENTALS OF GAS TURBINE ENGINES
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INTRODUCTION
The gas turbine is an internal combustion engine that uses air as the working fluid.
The engine extracts chemical energy from fuel and converts it to mechanical energy
using the gaseous energy of the working fluid (air) to drive the engine and propeller,
which, in turn, propel the airplane.
THE GAS TURBINE CYCLE
The basic principle of the airplane turbine engine is identical to any and all engines
that extract energy from chemical fuel. The basic 4 steps for any internal combustion
engine are:
1. Intake of air (and possibly fuel).
2. Compression of the air (and possibly fuel).
3. Combustion, where fuel is injected (if it was not drawn in with the intake air)
and burned to convert the stored energy.
4. Expansion and exhaust, where the converted energy is put to use.
In the case of a piston engine, such as the engine in a car or reciprocating airplane
engine, the intake, compression, combustion, and exhaust steps occur in the same
place (cylinder head) at different times as the piston goes up and down.
In the turbine engine, however, these same four steps occur at the same time but in
different places. As a result of this fundamental difference, the turbine has engine
sections called:
The inlet section
The compressor section
The combustion section (the combustor)
The turbine (and exhaust) section.
The turbine section of the gas turbine engine has the task of producing usable output
shaft power to drive the propeller. In addition, it must also provide power to drive the
compressor and all engine accessories. It does this by expanding the high
temperature.
SOME BASIC PRINCIPLES
As air passes through a gas turbine engine, aerodynamic and energy requirements
demand changes in the air’s velocity and pressure. During compression, a rise in the
air pressure is required, but not an increase in its velocity. After compression and
combustion have heated the air, an increase in the velocity of gases is necessary in
order for the turbine rotors to develop power. The size and shape of the ducts through
which the air flows affect these various changes. Where a conversion from velocity to
pressure is required, the passages are divergent. Conversely, if a conversion from
pressure to velocity is needed, a convergent duct is used.
Before further discussion, an explanation of convergent ducts, divergent ducts, and the
behavior of air within these ducts should be made. An understanding of the difference
between static pressure (Ps), impact pressure, (Pi), and total pressure (Pt) is also
needed.
The difference between static, impact, and total pressures is as follows. Static
pressure is the force per unit area exerted on the walls of a container by a stationary
fluid. An example is the air pressure within a car tire. Impact pressure, on the other
hand, is the force per unit area exerted by fluids in motion. Impact pressure is a
function of the velocity of the fluid. An example of impact pressure is the pressure
exerted on one's hand held outside a moving car’s window. Total pressure is the sum
of static and impact pressures.
PERFORMANCE AND EFFICIENCY
The type of operation for which the engine is designed dictates the performance
requirement of a gas turbine engine. The performance requirement is mainly
determined by the amount of shaft horsepower (s.h.p.) the engine develops for a given
set of conditions. The majority of aircraft gas turbine engines are rated at standard
day conditions of 59F and 29.92 inches Hg. This provides a baseline to which gas
turbine engines of all types can be compared.
Combustor
Once the air flows through the diffuser, it enters the combustion section, also called
the combustor. The combustion section has the difficult task of controlling the burning
of large amounts of fuel and air. It must release the heat in a manner that the air is
expanded and accelerated to give a smooth and stable stream of uniformly-heated gas
at all starting and operating conditions. This task must be accomplished with minimum
pressure loss and maximum heat release. In addition, the combustion liners must
position and control the fire to prevent flame contact with any metal parts.
The engine in this example uses a can-annular combustion section. Six combustion
liners (cans) are positioned within an annulus created by inner and outer combustion
cases. Combustion takes place in the forward end or primary zone of the cans.
Primary air (amounting to about one fourth of the total engine’s total airflow) is used to
support the combustion process. The remaining air, referred to as secondary or
dilution air, is admitted into the liners in a controlled manner. The secondary air
controls the flame pattern, cools the liner walls, dilutes the temperature of the core
gasses, and provides mass. This cooling air is critical, as the flame temperature is
above 1930C (3500'F), which is higher than the metals in the engine can endure. It is
important that the fuel nozzles and combustion liners control the burning and mixing of
fuel and air under all conditions to avoid excess temperatures reaching the turbine or
combustion cases. Maximum combustion section outlet temperature (turbine inlet
temperature) in this engine is about 1070C (>1950F).