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:
1. The inlet section
2. The compressor section
3. The combustion section (the combustor)
4. 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, pressure, and velocity gas and converting the gaseous energy to
mechanical energy in the form of shaft power.
A large mass of air must be supplied to the turbine in order to produce the necessary
power. This mass of air is supplied by the compressor, which draws the air into the
engine and squeezes it to provide high-pressure air to the turbine. The compressor does this by converting mechanical energy from the turbine to gaseous energy in the
form of pressure and temperature.
If the compressor and the turbine were 100% efficient, the compressor would supply
all the air needed by the turbine. At the same time, the turbine would supply the
necessary power to drive the compressor. In this case, a perpetual motion machine
would exist. However, frictional losses and mechanical system inefficiencies do not
allow a perpetual motion machine to operate.Additional energy must be added to the
air to accommodate for these losses. Power output is also desired from the engine
(beyond simply driving the compressor); thus, even more energy must be added to the
air to produce this excess power. Energy addition to the system is accomplished in
the combustor. Chemical energy from fuels it is burned is converted to gaseous
energy in the form of high temperatures and high velocity as the air passes through
the combustor. The gaseous energy is converted back to mechanical energy in the
turbine, providing power to drive the compressor and the output shaft.
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.
Figure 2-1 illustrates the methods used to measure pressures.Part (a) illustrates the
measurement of static pressure. Static pressure will not take into account the velocity
of the air. Part (b) illustrates the measurement of total pressure, which accounts for
both static pressure and the pressure due to the moving fluid (impact pressure). In
order to obtain impact pressure, the value of the static pressure is subtracted from the
value of total pressure.
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