1.19.
The compressor is the section of the engine that produces an
increase in air pressure. It is made up of rotating and stationary
vane assemblies. The first stage compressor rotor blades accelerate
the air rearward into the first stage vane assemblies. The first
stage vane assemblies slow the air down and direct it into the second
stage compressor rotor blades. The second stage compressor rotor
blades accelerate the air rearward into the second stage vane
assemblies, and so on through the compressor rotor blades and vanes
until air enters the diffuser section. The highest total air
velocity is at the inlet of the diffuser. As the air passes rearward
through the diffuser, the velocity of the air decreases and the
static pressure increases. The highest static pressure is at the
diffuser outlet.
The compressor rotor may be thought of as an air pump. The
volume of air pumped by the compressor rotor is basically
proportional to the rotor rpm. However, air density, the weight of a
given volume of air, also varies this proportional relationship. The
weight per unit volume of air is affected by temperature, compressor
air inlet pressure, humidity, and ram air pressure*. If compressor
air inlet temperature is increased, air density is reduced. If
compressor air inlet pressure is increased, air density is increased.
If humidity increases, air density is decreased. Humidity, by
comparison with temperature, and pressure changes, has a very small
effect on density. With increased forward speed, ram air pressure
increases and air temperature and pressure increase.
The following is an example of how air density affects
compressor efficiency of the T62 gas turbine. At 100 percent N1 rpm,
the compressor rotor pumps approximately 40.9 cubic feet of air per
second. At standard day static sea level conditions, 59 F outside
air temperature and 29.92" Hg barometric pressure, with 0 percent
relative humidity and 0 ram air, air density is .07651 pound per
cubic foot. Under these conditions, 40.9 cubic feet per second times
.07651 pound per cubic feet equals approximately 3.13 pounds per
second air flow through the engine. If the air density at the
compressor inlet is less than on a standard day, the weight of air
flow per second through the engine is less than 3.13 pounds per
second. If N1 is less than 100 percent rpm on a standard day, the
weight of air flow per second through the engine will be less than 3.
13 due to decreased volume flow at lower rpm. Because of this, N1 rpm
varies
________________
*ram air pressure free stream air pressure provided by the forward
motion of the engine.
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