Most modern passenger and military aircraft are powered by
gas turbine engines, which are also called
jet engines. There are several different types
of jet engines. But all jet engines have some partsin common.
All jet engines have a turbine to drive
the compressor. The job of the turbine is
to extract energy from the heated flow exiting the
burner.
The
turbine is connected to the shaft, which
is also connected to the compressor. As the flow passes through the
turbine, the total pressure pt and temperature Tt decrease. We measure the
decrease in pressure by the turbine pressure ration (TPR), which is the
ratio of the air pressure exiting the turbine to the air pressure
entering the turbine. This number is always less than 1.0.
Referring to our station
numbering, the turbine entrance is station 4 and the turbine exit
is station 5.
The TPR is equal to
pt5 divided by pt4
TPR = pt5 / pt4 <= 1.0
In the axial turbine, cascades of small airfoils are mounted on a
shaft that turns at a high rate of speed.
Since no external heat is being added to or extracted from the turbine
during this process, the process is
isentropic. The temperature
ratio across the turbine is related to the pressure ratio by the
isentropic flow equations.
Tt5 / Tt4 = (pt5 / pt4) ^((gam -1) / gam)
where gam is the ratio of
specific heats.
Work
is done by the flow to turn the turbine and the shaft. From the
conservation of energy, the turbine
work per mass of airflow (TW) is equal to the change in the
specific enthalpy ht of the flow from the
entrance to the exit of the turbine.
TW = ht4 - ht5
The term "specific"
means per mass of airflow. The enthalpy at the entrance and exit is
related to the total temperature Tt at those station:
TW = cp * (Tt4 - Tt5)
Using algebra, we arrive at the equation:
TW = (nt * cp * Tt)4 * [1 - TPR ^((gam -1) / gam)]
that relates the work done by the
turbine to the turbine pressure ratio, the incoming total
temperature, some properties of the gas,
and an efficiency factor nt.
The efficiency factor is included
to account for the actual performance of the turbine as opposed to
the ideal, isentropic performance. In an ideal world, the value of
the efficiency would be 1.0. In reality, it is always less than 1.0.
Because of mechanical inefficiencies, you cannot get 100% of the
available work from the turbine.
The turbine blades exist in a much more hostile environment than
compressor blades. Sitting just downstream of the burner, the blades
experience flow temperatures of more than a thousand degrees
Fahrenheit. Turbine blades must, therefore, be made of
special materials
that can withstand the heat, or they must be actively cooled.
You can now use
EngineSim
to study the effects
of different materials on engine operation.
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