ADIABATIC COMPRESSION
factors such as bleed air, pressure of fuel added,
and heat loss to turbine parts.
In an adiabatic stage change, no transfer of
As the high-temperature, high-pressure (HP)
heat to or from the system occurs during the
gases enter the turbine section, they expand
rapidly. Relatively little change in the temperature
process. Theoretically, in the ideal GTE,
the air enters the compressor and is compressed
of the gases occurs. The net power available from
adiabatically. In many real processes, adiabatic
the turbine is the difference between the turbine-
changes can occur when the process is performed
developed power and the power required to
rapidly. Since heat transfer is relatively slow, any
operate the compressor.
rapidly performed process can approach an
adiabatic state. Compression and expansion of
FACTORS AFFECTING
working fluids are often achieved almost
ENGINE PERFORMANCE
adiabatically. This is the case in todays's GTEs.
Many factors, such as aerodynamics and
Figure 1-9 is a schematic representation of a
thermodynamics, have a direct effect on efficient
stationary simple GTE. It uses two graphs to show
GTE performance. In this chapter we will discuss
the pressure-temperature-volume relationships at
only two common factors, the effect of ambient
various process states. The major components of
temperatures and the effect of compressor
the GTE are the compressor for the process from
cleanliness. As a gas turbine technician, you will
1 to 2, the combustor for the process from 2 to
be concerned with these in your daily operation
3, and the turbine for the process from 3 to 4.
of the GTE.
The heat rejection process from 4 to 1, which
completes the cycle, is carried out by exhausting
Effect of Ambient Temperature
the hot gas and allowing it to mix with the
atmosphere.
In discussions of temperature effects on GTEs,
In an initial simplified analysis, the com-
you will often hear the term Navy standard day.
pressor is taken to include the inlet nozzle and
This term refers to a theoretical condition seldom
ducting and any exit diffuser leading to the
duplicated except in some permanent test
combustor. Thus, for the compressor inlet
situations and is used only as a reference or
condition (point 1), the air being drawn from
standard. A standard day is indicated by the
the surroundings has zero velocity, ambient
following conditions at sea level: barometric
temperature, and ambient pressure. For the exit
pressure--29.92 Hg, humidity (water vapor
condition (point 4), the air has zero velocity at
pressure)--0.00 Hg, and temperature--59F.
some elevated pressure and temperature that are
Operation of engines above or below 50F will
measured. View A of figure 1-9 depicts a pressure-
proportionally affect engine power output by as
temperature graph for a simple GTE, while view
much as 15 or 20 percent.
B depicts a pressure-volume graph. The distance
The power and efficiency of a GTE are
between adjacent numbers on each of the
affected by both outside and inside variables. Air
diagrams represents an event of the combustion
has volume that is directly affected by its
cycle. A combustion cycle includes compression
temperature. As the temperature decreases, the
of air, burning of the compressed air and fuel
volume of air for a given mass decreases and its
mixture, expansion of gases, and removal of
density increases. Consequently, the mass weight
gases. By comparing the numerical points on both
of the air increases, causing the engine to operate
graphs (point 1 to 2 on view A with point 1 to
more efficiently. This happens because less energy
2 on view B), you can get a better understanding
is needed to achieve the same compression at the
of the pressure-temperature-volume relationship
combustion chambers. Also, cooler air causes
of a simple GTE.
lower burning temperatures. The resulting
During operation the work produced by the
temperatures extend turbine life. For example, a
propulsion GTE is operating at 100 percent GG
compressor turbine rotor is almost the same
amount as the work required by the compressor.
speed with 100 percent PT speed. The ambient
The mass flow available to the compressor
(external air) temperature is 70F. If the
temperature were increased to 120F, the volume
turbine is about the same as the mass flow handled
of air required would increase. The mass weight
by the compressor. This allows the heat of
would decrease. Since the amount of fuel added
compression to be about the same value as the
is limited by the inlet temperature the turbine will
heat of expansion. Allowances are made for
1-9
