Gas Turbine Handbook : Principles and Practices

Gas Turbine Systems Theory 65
As the (subsonic) velocity of the air decreases with the expanding
shape of the divergent duct, its static pressure increases, although
the total pressure remains the same. (Note that total pressure is the
sum of the static pressure and the velocity pressure; where velocity
pressure is the pressure created by the movement of the air.)
The fraction of the velocity head that is converted to static pressure
( diffuser efficiency) is a function of the area ratio and diffuser
angle. Diffuser efficiency is defined as
η D =
1– P o
P i
σ
1– T o
T i
(4-26)
where subscripts i and o refer to the pressure and temperature entering
and exiting the diffuser, respectively. 1 It must be noted that diffuser
effectiveness is the designers problem and for most applications
it can be considered part of the combustor efficiency.
The fuel nozzles function is to introduce the fuel into the burner
in a form suitable for rapid and complete mixing with air. There are
several types of fuel nozzles: gaseous, liquid, dual gas/liquid, and
dual liquid/liquid. The liquid fuel nozzle must atomize the fuel into
a fine, uniform spray. Furthermore, it must maintain this spray pattern
throughout the range of operation from starting to maximum
power output. To accomplish this, dual pressure features have been
designed into the fuel nozzles. These are commonly referred to as duplex,
dual orifice, multi-jet, and pintle fuel nozzles. The gaseous fuel
nozzles do not share this problem, as the gas is already in a vapor
form.
The combustor itself may be either a series of combustor cans
in an annular configuration, an annual combustor, or a single “stand
alone” design. Combustors typically have primary air holes at the
dome for mixing with the fuel and providing the critical air ratio for
ignition, and secondary air holes downstream for additional cooling
(Figure 3-12). Combustor design continues to be an art and, although
computer modeling of the air and fuel flow within the combustor is
progressing, designs are developed and improved by trial and error
testing.