AERO ENGINES

SYED HASSAN WAQAR GILANI LECTURE # 12

 Turbines
All gas turbine engines have a power turbine located downstream of the burner to extract energy from the
hot flow and turn the compressor. Work is done on the power turbine by the hot exhaust flow from the burner.
 Turbines
The upper right of the figure shows an actual power
turbine. The turbine, like the compressor, is
composed of several rows of airfoil cascades. Some
of the rows, called rotors, are connected to the
central shaft and rotate at high speed. Other rows,
called stators, are fixed and do not rotate. The job
of the stators is to keep the flow from spiraling
around the axis by bringing the flow back parallel to
the axis.
 Turbines
There are several interesting turbine
design details present on this slide.
Since the turbine extracts energy
from the flow, the
pressure decreases across the
turbine. The pressure gradient helps
keep the boundary layer flow
attached to the surface of the turbine
blades. Since the boundary layer is
less likely to separate on a turbine
blade than on a compressor blade,
the pressure drop across a single
turbine stage can be much greater
than the pressure increase across a
corresponding compressor stage. A
single turbine stage can be used to
drive multiple compressor stages.
 Multi Spool Turbines
Depending on the engine
type, there may be multiple
turbine stages present in the
engine. Turbofan and turboprop
engines usually employ a
separate turbine and shaft to
power the fan and gear box
respectively. Such an
arrangement is termed a two
spool engine. For some high
performance engines, an
additional turbine and shaft is
present to power separate parts
of the compressor. This
arrangement produces a three
spool engine. The power
turbine shown on the upper left
of the figure is for a two spool,
turbofan engine.
 Turbines
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 be made of
special materials that can withstand the heat, or they must be actively cooled. At the lower right of the figure, we show a
picture of a single, actively cooled turbine blade. The blade is hollow and cool air, which is bled off the compressor, is
pumped through the blade and out through the small holes on the surface to keep the surface cool. Because of the high
pressure change across the turbine, the flow tends to leak around the tips of the blades. The tips of turbine blades are
often connected by a thin metal band to keep the flow from leaking, as shown in the picture at the lower left.
 Exhaust and Nozzles
All gas turbine engines have a nozzle to produce thrust, to conduct the exhaust gases back to the free
stream, and to set the mass flow rate through the engine. The nozzle sits downstream of the power turbine.
Exhaust and Nozzles
 Exhaust and Nozzles
A nozzle is a relatively simple
device, just a specially shaped tube
through which hot gases flow.
However, the mathematics which
describe the operation of the nozzle
takes some careful thought. As
shown above, nozzles come in a
variety of shapes and sizes
depending on the mission of the
aircraft.
Simple turbojets, and turboprops, of
ten have a fixed
geometry convergent nozzle as
shown on the right of the figure.
 Exhaust and Nozzles
Turbofan engines often employ a co-annular nozzle as shown at the top left. The core flow exits the center
nozzle while the fan flow exits the annular nozzle. Mixing of the two flows provides some thrust
enhancement and these nozzles also tend to be quieter than convergent nozzles.
 Exhaust and Nozzles
Afterburning turbojets and
turbofans require a variable
geometry convergent-divergent -
CD nozzle as shown on the left. In
this nozzle, the flow first
converges down to the minimum
area or throat, then is expanded
through the divergent section to
the exit at the right. The flow
is subsonic upstream of the
throat, but supersonic downstream
of the throat. The variable
geometry causes these nozzles to
be heavier than a fixed geometry
nozzle, but variable geometry
provides efficient engine operation
over a wider airflow range than a
simple fixed nozzle.
 Thrust Vectoring Nozzles
All of the nozzles we have discussed thus far are
round tubes. Recently, however, engineers have
been experimenting with nozzles with
rectangular exits. This allows the exhaust flow to
be easily deflected, or vectored, as shown in the
figure. Changing the direction of the thrust with
the nozzle makes the aircraft much more
maneuverable.
Thrust Vectoring Nozzles
 Nozzles Afterbody
Because the nozzle conducts the hot
exhaust back to the free stream, there
can be serious interactions between
the engine exhaust flow and the
airflow around the aircraft. On fighter
aircraft, in particular, large drag
penalties can occur near the nozzle
exits. A typical nozzle-
afterbody configuration is shown in
figure for an F-15 with experimental
maneuvering nozzles.
 Advanced Turboprops
Conventional propellers lose their thrust production
capability when their tip operates in supersonic flow
and stalls. In the United States, Pratt &
Whitney/Allison Gas Turbine, GE Aviation and NASA
collaborated in developing the technology of advanced
turboprop engines in the 1970s and 1980s. These
engines are generally called Propfan, while GE’s
gearless, direct-drive ATP is called the Unducted Fan
(UDF). The advanced propellers operate with relative
supersonic tip Mach number (MT∼1.1–1.15) without
stalling! With increasing capability in relative tip Mach
number of the propeller, the cruise flight Mach number
is increased to M0∼0.8–0.82. Several configurations in
co- and counterrotating propeller sets and pusher
versus tractor configurations were developed and
tested. The advanced propellers are highly swept at the
tip (between 30–40◦) to improve tip efficiency at high
relative Mach numbers. Figure 1.18 shows an ATP.
Courtesy of GE Aviation and NASA.
 UHB Geared Turbofans
The technology of the ultra-high bypasss
(UHB) turbofan engine developed at Pratt &
Whitney utilizes an advanced gear system that
improves low-pressure spool operating
efficiency. The fan pressure ratio in UHB
engines is reduced to accommodate bypass
ratios of 12+, which improves propulsive
efficiency, cuts down on fuel consumption, and
reduces jet noise and engine emissions. The
first single-aisle transport aircraft equipped
with GTF entered service in 2013. The engine
architecture is readily scalable to include
widebody aircraft thrust levels as well. Figure
1.19 shows the cutaway of the P&W GTF
engine, that is, the PW1000G geared turbofan
engine family. The advanced fan gear system
on the low-pressure spool is visible in Figure
1.19 (trimetric view).