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INTRODUCTION

The compressor section houses the compressor rotors and stators, and works to supplyair in sufficient quantity to satisfy the needs of the combustor. Compression results whenfuel energy of combustion and mechanical work of the compressor and turbine areconverted into potential energy. Compressors operate on the principle of acceleration ofa working fluid followed by diffusion to convert the required kinetic energy to a pressurerise. The primary purpose of the compressor is to increase the pressure of the mass ofair entering the inlet and discharge it to the diffuser, and then, to the combustor sectionat the correct velocity, temperature and pressure. There are 2 types of compressors:centrifugal flow compressor and axial-flow compressor.

CENTRIFUGAL FLOW COMPRESSORS

The centrifugal flow compressor, sometimes refer to as a radial out-flow compressor isthe oldest design and is still in use today. A Centrifugal flow compressor consistsbasically of an impeller rotor, a diffuser and a manifold as shown.

Fig. 3-1 Components of a Centrifugal Flow Compressor

BASIC CONSTRUCTION OF A CENTRIFUGAL FLOW COMPRESSOR

The impellers are usually made from aluminum or titanium alloy and can be either single

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or dual sided. The diffuser consists of a number of vanes formed tangential to theimpeller. The vane passages are divergent to convert the kinetic energy into pressureenergy. The compressor manifold then distributes the air in a turbulence free conditionto the combustion section.

The single sided impeller, Fig. 3-2 benefits from ram effect and less turbulent air entry. Itis for this reason that this type of impeller is well suited to many aircraft installations.

Fig. 3-2 Single Stage, Single-sided Impeller

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The single stage dual-sided impeller design, Fig. 3-3 allows for a narrower overall enginediameter and higher mass airflow. However, it does not benefit from the ram effectbecause the air has to turn radially inward from a plenum chamber into the center of theimpellers.

Fig. 3-3 Single Stage, Dial-sided Impeller

Compression ratios attainable are about the same for both previously mentioned singlestage types of impellers. However, as seen in Fig. 3-4, more than two stages of singleentry type are considered impractical. The energy loss to the airflow (slowing down)when making the turns from one impeller to the next, the added weight and the driveshaft power extraction all seem to offset the benefit of additional compression with morethan two stages.

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Fig. 3-4 Two-stage, Single-sided Impeller

The most commonly seen centrifugal compressor is the single sided type in either one ortwo stages and is commonly used in conjunction with the axial flow compressor in smallengines for rotorcraft or small turboprop aircraft. All larger engines are of the axial flowtype.

PRINCIPLE OF OPERATION OF A CENTRIFUGAL COMPRESSOR

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The impeller is rotated at a high speed by the turbine and air is induced into the eye(center) of the impeller. Centrifugal force caused it to flow radically outwards along thevanes to the impeller tip, thus accelerating the air and also raising the pressure.

The air, on leaving the impeller, passes into the diffuser section where the passages formdivergent ducts that convert most of the kinetic energy into pressure energy. In practice,it is usual to design the compressor so that half of the pressure rise occurs in theimpeller and half in the diffuser.

The air mass flow through the compressor and the pressure rise depend on therotational speed of the impeller; therefore, impeller is designed to operate at tip speed upto 1600 ft/sec. By operating at such high tip speeds, the air velocity from the impeller isincreased so that greater energy is available for conversion to pressure

Fig.3-5 Pressure & Velocity Changes through a Centrifugal Compressor

COMPRESSOR INLET CONDITION

To ensure that the impeller can accept the resultant airflow smoothly, the impeller vanesat the eyes are bent in the direction of rotation. These vanes are known as rotating guidevanes. It should be appreciated that the impeller’s rotational speed increases as theimpeller diameter increased. This means that the angle that the rotating guide vanes arebent must increase with the radius to maintain a constant angle of approach.

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Fig. 3-6 Rotating Guide Vanes

ADVANTAGES/DISADVANTAGES OF THE CENTRIFUGAL FLOWCOMPRESSOR

The advantages of the centrifugal compressor are as follows:

i. High pressure rise per stage – Up to 10:1 and 15:1 in a dual stage,

ii. Good efficiency (compression) over a wide rotational speed range,

iii. Simplicity of manufacture and low cost as compared to axial flow compressor,

iv. Low weight,

v. Lower starting power requirement.

The disadvantages are:

i. Large frontal area for a given airflow,

ii. More than two stages of compression is not practical because of the energy losses

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between stages.

BASIC CONSTRUCTION OF THE AXIAL FLOW COMPRESSOR

An axial flow compressor consists of one or more rotor assemblies that carry blades ofairfoil section. These assemblies are mounted between bearing in the casings whichincorporate the stator vanes. The compressor is a multi-stage unit as the amount ofpressure increase by each stage is small. A stage of compression consists of a row ofrotating blades (Rotors) followed by a row of stator vanes.

From the front to the rear of the compressor, ie. from the low to the high pressure end,there is a gradual reduction of the air annulus area between the rotor shaft and the statorcasing. This is necessary to maintain a near constant air axial velocity as the densityincreases through the length of the compressor. Where several stages of compressionoperate in series on one shaft, it becomes necessary to vary the stator vane angle toenable the compressor to operate effectively at speeds below the design condition.

As the pressure ratio is increased the incorporation of variable stator vanes ensures thatthe airflow is directed onto the succeeding stage of rotor blades at an acceptable angle.

Fig.3-7 Variable Inlet Guide Vanes and Stator Vanes

PRINCIPLES OF OPERATION OF AN AXIAL FLOW COMPRESSOR

During operation, the rotor is turned at high speed by the turbine, so that air iscontinuously induced into the compressor, where it is accelerated by the rotating bladesand swept rearwards onto the adjacent row of stator vanes. The pressure rise in theairflow results from the energy imparted to the air in the rotor, which increases the airvelocity. The air is then decelerated (diffused) in the following stator passage and thekinetic energy translated into pressure. Stator vanes also serve to correct the deflectiongiven to the air by the rotor blades and to present the air at the correct angle to the nextstage of rotor blades.

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The last row of stator blades usually act as ‘ air straighteners’ to remove the swirl fromthe air prior to entry into the combustion system at a reasonably uniform axial velocity.The changes in pressure and velocity that occur in the airflow through the compressorare shown diagrammatically in Fig. 3-8. The changes are accompanied by a progressiveincrease in air temperature as the pressure increases.

Fig. 3-8 Pressure and Velocity Changes through an Axial Flow Compressor

TYPES OF AXIAL FLOW COMPRESSOR

A single-spool compressor consists of one rotor assembly and stators with as manystages as necessary to achieve the desired pressure ratio and all the airflow from theintake passes through the compressor.

The multi-spool compressor consists of two or more rotor assembles, each driven bytheir own turbine at an optimum speed to achieve higher pressure ratios. Although atwin-spool compressor can be used for a pure jet engine, it is most suitable for the by-pass type of engine where the front or low pressure compressor is designed to handle alarger airflow than the high pressure compressor. Only a percentage of the air from thelow pressure compressor passes into the high pressure compressor. The remainder ofthe air is ducted around the high pressure compressor.

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Fig. 3-9 Typical Axial Flow Compressors

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ROTORS

In compressor designs shown in Fig. 3-10, the rotational speed is such that a disc isrequired to support the centrifugal blade load. Where a number of discs are fitted ontoone shaft they may be coupled and secured together by a mechanical fixing butgenerally the discs are assembled and welded together, close to their periphery, thusforming an integral drum.

Fig.3-10 Rotors of Drum and Disc Construction

METHODS OF SECURING BLADES TO DISC

Typical methods of securing rotor blades to the disc are shown in Fig. 3-11. Fixing maybe circumferential or axial. In general the aim is to design a securing feature that impartsthe lightest possible load on the supporting disc thus minimizing disc weight. Whilst mostcompressor designs have separate blades for manufacturing and maintainabilityrequirements, it becomes more difficult on the smallest engines to design a practicalfixing. However this may be overcome by producing blades integral with the disc; the so-

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called ‘blisk’.

Fig.3-11 Methods of securing Blades to Disc

ROTOR BLADES

The rotor blades are of airfoil section shown in Fig.3-12 and usually designed to give apressure gradient along their length to ensure that the air maintains a reasonablyuniform axial velocity. The higher pressure towards the tip balances out the centrifugalaction of the rotor on the airstream. To obtain these conditions, it is necessary to ‘twist’the blade from root to tip to give the correct angle of incidence at each point.

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Fig.3-12 Typical Rotor Blade showing Twisted Contour

REDUCE VIBRATION AND AIR LEAKAGE BY ‘KNIFE-EDGE’ TIP

The clearance between the blades and the compressor casing is very important becausethe compressor can only operate efficiently when the air leakage is held to an absoluteminimum. Compressor blades are therefore often manufactured with a knife-edge tip.

The assembled compressor rotor fits easily into the compressor case when the bladesare cold. But when the blade expands from the heat generated within the compressor,they rub against the case. As they rub, the knife-edge tips grind off and establishclearances, which are less than could be achieved in any other way.

STATOR VANES

The stator vanes are again of airfoil section and are secured into the compressor casingor into stator vane retaining rings, which are themselves secured to the casing as shownin Fig.3-13. The vanes are often assembled in segments in the front stages and may beshrouded at their inner ends to minimize the vibration effect of flow variations on thelonger vanes. The stator vanes are also locked in such a manner by using set-screws sothat they will not rotate around the casing.

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Fig.3-13 Methods Of Securing Blades To Compressor Casing

ADVANTAGES/DISADVANTAGES OF THE AXIAL FLOWCOMPRESSOR

The advantages of the axial flow compressor are as follows:

i. High peak efficiencies (compressor pressure ratio) created by its straight throughdesign,

ii. Higher peak efficiencies (pressure) attainable by addition of compression stages asdesired,

iii. Small frontal area and resulting low drag

The disadvantages are:

i. Difficulties and high cost of manufacture,

ii. Relatively high weight,

iii. Higher starting power required,

iv. Low pressure rise per stage (approximately 1.3:1 maximum).

PRESSURE RATIO AND COMPRESSOR RATIO

In the axial-flow compressor, a desired compression ratio is achieved by simply addingmore stages onto the compressor. The amount of pressure rise or compression ratiodepends on the mass of air discharged by the compressor, the restriction to flowimposed by the parts of the engine through which the air must pass, and the operatingcondition (pressure) inside the engine compared to the ambient air pressure at thecompressor intake. The final pressure is the result of multiplying the pressure rise ineach stage in each stage.

Example: A 13-stage compressor has a pressure ratio across each stage have 1.2 andan ambient inlet pressure of 14.7 psi. What is the final pressure? What is thecompression ratio?

Stage 1 Stage 2 Stage 3 Stage 4 Stage 5 Stage 6 Stage 7

14.7 X 1.2 17.64 X 1.2 21.17 X 1.2 25.4 X 1.2 30.48 X 1.2 36.58 X 1.2 43.89 X 1.2

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= = = = = = =

Stage 8 Stage 9 Stage 10 Stage 11 Stage 12 Stage 13

52.67 X 1.2=

63.21 X 1.2=

75.85 X 1.2=

91.02 X 1.2=

109.22 X 1.2=

131.07 X 1.2=

157.28

Final pressure = 157.28 psi

Initial pressure = 14.7 psi

Or

Compression ratio (CR) = 157.28 / 14.7 = 10.7:1

Notice that the pressure rise across the first stage is:

17.6 psi : at the back of 1st stage

-14.7 psi : at the front of 1st stage

2.9 psi : pressure rise across the first stage

and the pressure rise across the last stage is:

157.28 psi : at the back of 13th stage

-131.07 psi : at the front of 13th stage

psi : pressure rise across the 13th stage

(Optional method to find the final pressure)

Final pressure = 1.2 to the power of n * 14.7 (Formulae)

= 1.2 to the power of 13 * 14.7

= 157.28 psi

The pressure ratio is the same in both cases but the actual increase in pressure is muchgreater toward the rear of the compressor than the front. The compression ratio willincrease and decrease with engine speed and the compressor inlet temperature.

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