unit-v centrifugal and axial flow compressors
TRANSCRIPT
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CENTRIFUGALAND
AXIAL FLOW
COMPRESSORS
By
Mr. Manoj Kumar Elipey
Assistant professor
Vardhaman College Of Engineering
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Tip Speed & Materials
The precise stress limits of a given impeller
material will depend upon factors such as the requiredcyclic duty (number of startstop cycles per unit time).
Cast aluminium to be used up to a tip speed of
around 200
300 m/s,
forged machined aluminium up to where a
maximum of about 500 m/s.
titanium up to around 650
700 m/s.
Titanium aluminides and titanium metal matrix
composites are currently being researched for the highertip speeds.
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Efficiency of A Centrifugal Compressor
Broadly speaking, two approaches are used for todetermining stage efficiency at the preliminary design stage.
An approach that at first sight appears to be less
dependent upon empiricism is to formulate a general 1D
compressor model that includes some system of lossestimation for the principal flow elements of the stage.
The most comprehensive method includes, models for IGV,
impeller, vaneless space and vaned diffuser.
The loss models are tuned so that the method obtains
reasonable agreement with a representative range of test cases
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Centrifugal compressors sometimes
termed radial compressors, are a sub-class of
dynamic axisymmetric work-absorbing turbomachinery.
The idealized compressive dynamic turbo-machine achieves a pressure rise by adding kinetic
energy/velocity to a continuous flow of fluid through
the rotor or impeller. This kinetic energy is then
converted to an increase in potential energy/static
pressure by slowing the flow through a diffuser. The
pressure rise in impeller is in most cases almost equal
to the rise in the diffuser section.
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Centrifugal impeller
The key component that makes a compressor
centrifugal is the centrifugal impeller, It is the impeller's
rotating set of vanes (or blades) that gradually raises the
energy of the working gas. This is identical to an axial
compressor with the exception that the gases can reach
higher velocities and energy levels through the impeller'sincreasing radius. In many modern high-efficiency centrifugal
compressors the gas exiting the impeller is travelling near the
speed of sound.
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Diffuser
The next key component to the simple
centrifugal compressor is the diffuser. Downstream of
the impeller in the flow path, it is the diffuser'sresponsibility to convert the kinetic energy (high
velocity) of the gas into pressure by gradually slowing
(diffusing) the gas velocity. Diffusers can be vane less,vaned or an alternating combination. High efficiency
vaned diffusers are also designed over a wide range of
solidities from less than 1 to over 4.
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Collector
The collector of a centrifugal compressor cantake many shapes and forms. When the diffuser
discharges into a large empty chamber, the collector
may be termed a Plenum. When the diffuser
discharges into a device that looks somewhat like a
snail shell, bull's horn or a French horn, the collector
is likely to be termed a voluteor scroll. As the name
implies, a collectors purpose is to gather the flowfrom the diffuser discharge annulus and deliver this
flow to a downstream pipe.
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Surge
Surge is the point at which the compressor cannot add enough
energy to overcome the system resistance or backpressure
This causes a rapid flow reversal (i.e. surge). As a result, high
vibration, temperature increases, and rapid changes in axial thrust canoccur. These occurrences can damage the rotor seals, rotor bearings,
the compressor driver and cycle operation. Most turbo machines are
designed to easily withstand occasional surging. However, if the
machine is forced to surge repeatedly for a long period of time, or if it ispoorly designed, repeated surges can result in a catastrophic failure. Of
particular interest, is that while turbo machines may be very durable,
the cycles/processes that they are used within can be far less robust.
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Surge Line
The Surge-line is the curve that passes throughthe lowest flow points of each of the four speed lines.
As a test map, these points would be the lowest flow
points possible to record a stable reading within the
test facility/rig. In many industrial applications it maybe necessary to increase the stall line due to the system
exhibits a stalling characteristic or positive slope
within that range of flows. When placed in a different
system those lower flows might not be achievablebecause of interaction with that system.
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Choke
The speed lines of gas turbine centrifugalcompressors typically exhibit choke. This is a situation
where the pressure ratio of a speed line drops rapidly
(vertically) with little or no change in flow. In most
cases the reason for this is that close to Mach 1velocities have been reached somewhere within the
impeller and/or diffuser generating a rapid increase in
losses. Higher pressure ratio turbocharger centrifugal
compressors exhibit this same phenomenon. Real choke
phenomena is a function of compressibility as measured
by the local Mach number within an area restriction
within the centrifugal pressure stage.
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Axial flow compressors
Axial compressors are rotating, aerofoil basedcompressors in which the working fluid principally flows parallel
to the axis of rotation. This is in contrast with centrifugal,
axicentrifugal and mixedflow compressors where the air may
enter axially but will have a significant radial component on exit.
Axial flow compressor is capable of higher pressure ratio
on a single shaft. The energy transfer in a single stage is very
limited (stage pressure ratio of about 1.2).But ease of combining
axial flow stages leads to pressure ratios of up to 6/1 or even
higher. Thus axial flow compressor is considered as consisting ofmany stages. Single stage is considered as a fan For most
aircraft & industrial gas turbine, axial flow compressor is used in
preference to radial flow type.
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Axial flow compressor basic operation
Working fluid initially accelerated by rotor
blades, then decelerated in stator blades where
kinetic energy transferred in rotor is converted to
static pressure. Many stages necessary for
required overall pressure ratio. Flow alwayssubject to adverse pressure gradient Process
consists of series of diffusions in both rotor &
stator blade passages. Careful design ofcompressor balding necessary to prevent wasteful
losses and minimize stalling. Flow reversals may
occur at mass flow conditions different from blade
desi n conditions.
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Isentropic efficiency
Isentropic efficiency is ratio of ideal specificwork input, or total T rise, for given pressure
ratio to actual. Definition of isentropic is adiabatic
reversible. Total T rise & power input to sustaingiven P ratio is proportional to inlet total
temperature Polytropic efficiency is isentropic
efficiency of an infinitesimally small
compression step, such that its magnitude is
constant throughout Isentropic efficiency falls as
pressure ratio is increased for same polytropic
efficiency.