Most modern passenger and military aircraft are powered by
gas turbine engines, which are also called
jet engines. The first and most simple type
of gas turbine is the
turbojet.
On this slide we show how the flow temperature varies through a
typical turbojet engine. The temperature is color-coded, with blue
indicating the lowest temperature and white the highest temperature.
Air is brought into the turbojet through the inlet
at the left of the computer drawing. At the rear of the inlet, the
air enters the compressor. The compressor
acts like many rows of airfoils, with each
row producing a small increase in pressure. The increase in pressure
is accompanied by an increase in temperature since the compressor is
doing work on the flow. In the burner a
small amount of fuel is combined with the air and ignited at near
constant pressure. The temperature of the flow reaches a maximum in
the burner. Leaving the burner, the hot exhaust is passed through the
turbine. Energy is extracted from the flow
by the turbine to turn the compressor, which is linked
to the turbine by a central shaft. The temperature decreases across
the turbine during this process, but the temperature entering the
nozzle is still greater than free stream. The nozzle
then converts the high pressure and temperature into high velocity.
Because the exit velocity is greater than the free stream velocity,
thrust is created as described by the thrust
equation.
The engine temperature ratio (ETR) is defined to be the
total temperature ratio across the engine. Using our station
numbering system, ETR is the ratio of nozzle total temperature
Tt8 to compressor face total temperature Tt2. ETR can be easily
measured on an operating engine and displayed to the pilot on a
cockpit dial. If we know ETR, and the corresponding engine
pressure ratio, EPR, we can easily determine the thrust
of an engine using the nozzle performance
information and the thrust equation. The
ETR is simply the product of the temperature ratio across all of the
engine components.
ETR = Tt8 / Tt2 = (Tt3 / Tt2) * (Tt4 / Tt3) * (Tt5 / Tt4) * (Tt8 / Tt5)
ETR = compressor temperature ratio * burner temperature ratio * turbine temperature ratio * nozzle temperature ratio
For a given engine design, we can determine the
temperature ratio of each component as given on each of the component
thermodynamic slides.
Here is an animated version of the graphic:
You can investigate the variation of temperature through an engine
by using the
EngineSim
interactive Java applet.
You can vary the performance of any of the engine parts and investigate
the effects on thrust and fuel flow. EngineSim can also plot the variation
of temperature through the engine.
Activities:
Guided Tours
-
EngineSim - Engine Simulator:
-
Thrust Equation:
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