DEPARTMENT OF AERONAUTICAL ENGINEERING

AE-1304PROPULSION-1

UNIT-1FUNDAMENTALS OF GAS TURBINE ENGINES

INTRODUCTIONComprehend the thermodynamic processes occurring in a gas turbineComprehend the basic components of gas turbine engines and their basic operationComprehend the support systems associated with gas turbine engines

ADVANTAGES OF GTEsWeight reduction of 70%SimplicityReduced manning requirementsQuicker response timeFaster Acceleration/decelerationModular replacementLess vibrationsMore economical

DISADVANTAGES OF GTEsMany parts under high stressHigh pitched noiseNeeds large quantities of airLarge quantities of hot exhaust (target)Cannot be repaired in place

BRAYTON CYCLEUnlike diesels, operate on STEADY-FLOW cycleOpen cycle, unheated engine1-2: Compression

2-3: Combustion

3-4: Expansion through Turbine and Exhaust Nozzle

(4-1: Atmospheric Pressure)

BASIC COMPONENTS

NUMBERING OF TURBINE ENGINESintake compressor burner turbine afterburner (AB) nozzle (n)(diffuser)(e.g., turbofan)

COMPRESSORSupplies high pressure air for combustion processCompressor typesRadial/centrifugal flow compressorAxial flow compressor

COMPRESSORRadial/centrifugal flowAdv: simple design, good for low compression ratios (5:1)Disadv: Difficult to stage, less efficient

Axial flow Good for high compression ratios (20:1)Most commonly used

THE THRUST EQUATION

FACTORS AFFECTING THRUSTPRESSURETEMPERATUREDENSITYHUMIDITYALTITUDEFORWARD VELOCITY

METHODS OF THRUST AUGMENTATIONAFTER BURNINGINJECTION OF WATER & ALCOHOL MIXTUREBLEED BURN CYCLE

UNIT-IISUBSONIC & SUPERSONIC INLETS FOR JET ENGINES

INTRODUCTIONInlets are very important to the overall jet engine performance & will greatly influence jet engine thrust output.The faster the airplane goes the more critical the inlet duct design becomes.Engine thrust will be high only if the inlet duct supplies the engine with the required airflow at the highest possible pressure.

The nacelle/duct must allow the engine to operate with minimum stall/surge tendencies & permit wide variation in angle of attack & yaw of the aircraft.For subsonic aircraft, the nacelle shouldnt produce strong shock waves or flow separations & should be of minimum weight for both subsonic & supersonic designs.For certain military applications, the radar cross sectional control or radar reflectance is a crucial design requirements.

Inlet ducts add to parasite drag skin friction+ viscous drag) & interference drag.It must operate from static ground run up to high aircraft Mach number with high duct efficiency at all altitude, attitudes & flight speeds.It should be as straight & smooth as possible & designed such a way that Boundary layer to be minimum.It should deliver pressure distribution evenly to the compressor.

Spring loaded , blow-in or such-in-doors are sometimes placed around the side of the inlet to provide enough air to the engine at high engine rpm & low aircraft speed.It must be shaped such a way that ram velocity is slowly & smoothly decreases while the ram pressure is slowly & smoothly increases.

DUCT EFFICIENCYThe duct pressure efficiency ratio is defined as the ability of the duct to convert the kinetic or dynamic pressure energy at the inlet of the duct to the static pressure energy at the inlet of the compressor without a loss in total pressure . It is in order of 98% if there is less friction loss.

RAM RECOVERY POINTThe ram recovery point is that aircraft speed at which the ram pressure rise is equal to the friction pressure losses or that aircraft speed at which the compressor inlet total pressure is equal to the outside ambient air pressure.A good subsonic duct has 257.4 km/h.

SINGLE ENTRANCE DUCT

SUBSONIC DUCTS

VARIABLE GEOMETRY DUCT FOR SUPERSONIC A/C

NORMAL SHOCK RELATION

OBLIQUE SHOCK RELATIONS

BOUNDARY LAYER

UNIT-IIICOMBUSTION CHAMBERS

COMBUSTION CHAMBERWhere air & fuel are mixed, ignited, and burnedSpark plugs used to ignite fuelTypesCan: for small, centrifugal compressorsAnnular: for larger, axial compressors (LM 2500)Can-annular: for really large turbines

UNIT-IVNOZZLES

INTRODUCTIONThe primary objective of a nozzle is to expand the exhaust stream to atmospheric pressure, and form it into a high speed jet to propel the vehicle. For air breathing engines, if the fully expanded jet has a higher speed than the aircraft's airspeed, then there is a net rearward momentum gain to the air and there will be a forward thrust on the airframe.

Many military combat engines incorporate an afterburner (or reheat) in the engine exhaust system. When the system is lit, the nozzle throat area must be increased, to accommodate the extra exhaust volume flow, so that the turbo machinery is unaware that the afterburner is lit. A variable throat area is achieved by moving a series of overlapping petals, which approximate the circular nozzle cross-section.

At high nozzle pressure ratios, the exit pressure is often above ambient and much of the expansion will take place downstream of a convergent nozzle, which is inefficient. Consequently, some jet engines (notably rockets) incorporate a convergent-divergent nozzle, to allow most of the expansion to take place against the inside of a nozzle to maximise thrust. However, unlike the fixed con-di nozzle used on a conventional rocket motor, when such a device is used on a turbojet engine it has to be a complex variable geometry device, to cope with the wide variation in nozzle pressure ratio encountered in flight and engine throttling. This further increases the weight and cost of such an installation.

The simpler of the two is the ejector nozzle, which creates an effective nozzle through a secondary airflow and spring-loaded petals. At subsonic speeds, the airflow constricts the exhaust to a convergent shape. As the aircraft speeds up, the two nozzles dilate, which allows the exhaust to form a convergent-divergent shape, speeding the exhaust gasses past Mach 1. More complex engines can actually use a tertiary airflow to reduce exit area at very low speeds. Advantages of the ejector nozzle are relative simplicity and reliability. Disadvantages are average performance (compared to the other nozzle type) and relatively high drag due to the secondary airflow. Notable aircraft to have utilized this type of nozzle include the SR-71, Concorde, F-111, and Saab Viggen

NOZZLE

1-D ANALYSIS OF GAS

MASS FLOW RELATION

UNIT-VCOMPRESSORS

CompressorDraws in air & compresses itCombustion ChamberFuel pumped in and ignited to burn with compressed airTurbineHot gases converted to workCan drive compressor & external load

COMPRESSORControlling Load on CompressorTo ensure maximum efficiency and allow for flexibility, compressor can be split into HP & LP sectionsVane control: inlet vanes/nozzle angles can be varied to control air flowCompressor StallInterruption of air flow due to turbulence

USE OF COMPRESSED AIRPrimary Air (30%)Passes directly to combustor for combustion processSecondary Air (65%)Passes through holes in perforated inner shell & mixes with combustion gasesFilm Cooling Air (5%)Insulates/cools turbine blades

VELOCITY TRIANGLE

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