unit-v centrifugal and axial flow compressors

8/13/2019 UNIT-V Centrifugal and Axial Flow Compressors

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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.

unit-v centrifugal and axial flow compressors

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