UNIT1 FUNDAMENTALS OF GAS TURBINE ENGINES INTRODUCTION Comprehend

UNIT-1 FUNDAMENTALS OF GAS TURBINE ENGINES

INTRODUCTION • Comprehend thermodynamic processes occurring in a gas turbine • Comprehend the basic components of gas turbine engines and their basic operation • Comprehend the support systems associated with gas turbine engines

ADVANTAGES OF GTE’s – Weight reduction of 70% – Simplicity – Reduced manning requirements – Quicker response time – Faster Acceleration/deceleration – Modular replacement – Less vibrations – More economical

DISADVANTAGES OF GTE’s • • • Many parts under high stress High pitched noise Needs large quantities of air Large quantities of hot exhaust (target) Cannot be repaired in place

BRAYTON CYCLE Unlike diesels, operate on STEADY-FLOW cycle Open cycle, unheated engine 1 -2: Compression 2 -3: Combustion 3 -4: Expansion through Turbine and Exhaust Nozzle (4 -1: Atmospheric Pressure)

BASIC COMPONENTS

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

COMPRESSOR • Supplies high pressure air for combustion process • Compressor types – Radial/centrifugal flow compressor – Axial flow compressor

COMPRESSOR • Radial/centrifugal flow – Adv: 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 THRUST • • • PRESSURE TEMPERATURE DENSITY HUMIDITY ALTITUDE FORWARD VELOCITY

METHODS OF THRUST AUGMENTATION • AFTER BURNING • INJECTION OF WATER & ALCOHOL MIXTURE • BLEED BURN CYCLE

UNIT-II SUBSONIC & SUPERSONIC INLETS FOR JET ENGINES

INTRODUCTION Inlets 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 shouldn’t 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 EFFICIENCY The 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 POINT The 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-III COMBUSTION CHAMBERS

COMBUSTION CHAMBER • Where air & fuel are mixed, ignited, and burned • Spark plugs used to ignite fuel • Types – Can: for small, centrifugal compressors – Annular: for larger, axial compressors (LM 2500) – Can-annular: for really large turbines

UNIT-IV NOZZLES

INTRODUCTION • The 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 crosssection.

• 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 convergentdivergent 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-V COMPRESSORS

• Compressor – Draws in air & compresses it • Combustion Chamber – Fuel pumped in and ignited to burn with compressed air • Turbine – Hot gases converted to work – Can drive compressor & external load

COMPRESSOR • Controlling Load on Compressor – To ensure maximum efficiency and allow for flexibility, compressor can be split into HP & LP sections – Vane control: inlet vanes/nozzle angles can be varied to control air flow • Compressor Stall – Interruption of air flow due to turbulence

USE OF COMPRESSED AIR • Primary Air (30%) – Passes directly to combustor for combustion process • Secondary Air (65%) – Passes through holes in perforated inner shell & mixes with combustion gases • Film Cooling Air (5%) – Insulates/cools turbine blades

VELOCITY TRIANGLE

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