Introduction to gas turbine engine
- 1. Ankit Shukla 04/03/2018 Introduction to Gas Turbine Engines -Ankit
- 2. Working Principle • Jet Propulsion is a practical application of Newton’s III Law. • For every force acting on a body there is an equal and opposite reaction. • In the case of aircraft propulsion “ The body” is atmospheric air that is caused to accelerate as it passes through the engine. • A Propulsion system is a machine that produces thrust or power to push an object forward. • The gas or the working fluid is accelerated by the engine and reaction to this acceleration produces a force on the engine.
- 3. The Propulsion System Propulsion = pro + peller pro: Before or forwards peller: Meaning to drive. Propulsion means to push forward or drive an object forward. A propulsion system is a machine that produces thrust to push an object forward. A gas, or working fluid, is accelerated by the engine, and the reaction to this acceleration produces a force on the engine.
- 4. Purpose of Propulsion System The Airplane propulsion system must serve two purposes. 1. Thrust from the propulsion system must balance the drag of airplane when the airplane is cruising. 2. The thrust from the propulsion system must exceed the drag of the airplane for the airplane to accelerate, climb and maneuver This is called as excess thrust over drag known as Thrust – Drag i.e. (T – D)
- 5. Purpose of Jet Propulsion Propulsion system Performance • During straight and level flight called cruise, the engine must produce sufficient thrust to balance the aircraft drag. • For civil or commercial engines fuel economy or specific fuel consumption is of prime importance to get maximum range. • Specific fuel consumption is defined as: fuel flow / thrust of the engine. • For fighter aircraft application short takeoff, fast acceleration, fast rate of climb and good maneuver performance are of prime importance for which additional thrust is required. • Excess thrust over drag i.e. ( Thrust - Drag ) is used for climbing to higher altitudes or for accelerating from One Mach number to another Mach number and also for maneuver for military aircraft. • Mach Number is defined as the ratio of the velocity of the object to the velocity of the sound. It is non- dimensional quantity. • Thrust to weight ration ( T/W) is one of the important figures of merit especially for fighter aircraft. • Similarly for the important figure of merit is Lift / Drag ratio i.e. L/D ratio
- 6. Aero gas turbine engine Technology growth For the past about 4.5 decades the aero gas turbine engine technology has grown tremendously in terms of engine Over all pressure, Turbine entry temperature and Thrust / Weight ratio. The over all pressure ratio has gone up almost by about 9 to 10 times, Turbine entry temperature has doubled and Thrust to Weight ratio has increased by about 2.5 times. This has resulted in lesser engine Assemblies and Sub assemblies, engine part counts, major reduction in engine length and diameter. All these Technology improvements have been made possible by improved Aerothermodynamics, Computational Fluid Dynamics techniques, Advancement in manufacturing and Fabrication technologies, Advanced Control Systems and Advanced materials.
- 7. The classification can be made in two ways namely: Based on the fluid under which the Gas turbine engine operates Application based classification Classification of Gas turbine engine
- 8. Propulsion Engines Air breathing engines Uses atmospheric air as working fluid Reciprocating or Piston engines Gas turbine Engine ( Need not be always propulsive) Ramjet Gas turbine Turbo Ramjet Turbo Rocket` •Does not use Atmospheric air as working fluid •Uses Gas produced by chemical reaction of fuel and oxidizer •The fuel + Oxidant Mixture is called propellant Propulsion System Classification ( Working fluid based)
- 9. Propulsion System Classification (Application based) Aircraft Marine Industrial UAV Launch vehicle
- 10. • Gas Turbine Engine operates on a thermodynamic cycle known as the Brayton cycle. • Air is drawn from atmosphere. • Pressure rise ( Compression ) takes place in the compressor. • High pressure air is mixed with fine atomized fuel spray and ignited with high energy spark. Combustion takes place at constant pressure. • Hot gases arising out of combustion impinge on the turbine and rotate it and hence called gas turbine. • The Turbine drives the compressor and Turbine-compressor combination becomes self sustaining after start. • Self sustaining RPM is that RPM at which the Turbine produces sufficient power to drive the compressor. • Balance pressure energy is converted into velocity in the exhaust nozzle and the rate of change of momentum produces the thrust which is equal to the Mass flow rate times the change in velocity from front to the rear of the engine. • For taking the engine up to self sustaining speed an external starting system is required. Gas Turbine Engines
- 11. • Similar to that of a 4-stroke piston engine. • In Gas turbine engine combustion occurs at constant pressure whereas in piston engine it occurs at constant volume. • In Both cases the cycle comprises of Induction, Compression, Combustion and exhaust. • In Piston engine the cycle is intermittent, piston being concerned in all 4 strokes. • In Gas turbine engine the cycle is continuous with a separate compressor, combustor, Turbine and the exhaust system. Working Cycle of Gas Turbine Engine
- 12. • The Continuous Cycle and absence of reciprocating parts give a smoother engine and enable more energy to be released for a given size. Peak pressures that occur in piston engine are avoided. • During compression the work is done on the air which increases the pressure and temperature and decreases the volume of the air. • During combustion fuel is added to the compressed air and burnt. This increases the temperature and the volume of air while the pressure remains almost constant since the engine operates on a constant pressure cycle. •During expansion when the work is taken from the gas stream by the turbine to drive the compressor, pressure and temperature decrease while the volume increases. • Turbo-propeller engines does the conversion of gas energy into mechanical power to drive the propeller. Only small amount of jet thrust is available from the exhaust system. Working Cycle of Gas Turbine Engine (Cont.)
- 13. Pure Turbo jet (Zero by pass ratio, i.e. Straight jet) Turbo fan (High and Low by pass engines) Turbo Prop Turbo Shaft Industrial Gas Turbine engines Different types of aero gas turbine engines
- 14. • Jet Engine with high bypass ratio. Bypass ratio is defined as the ratio of the bypass air ( cold air ) to the core air ( Gas generator air ). • This Bypass ratio is quite high of the order of 5 to 6 in Turbofan engines resulting in good fuel efficiency namely Good Specific fuel consumption ( SFC). • Two types of thrust namely Cold thrust and Hot thrust are produced and sum of the two is the total thrust Turbo Fan Engine
- 15. • This can be considered as a turbofan engine with low (small) bypass ratio in the range of 0.2 to 1 • This is quite suitable for military engines where both high thrust and moderate fuel efficiency ( SFC) are of prime importance. • Small bypass ratio results in smaller size, high specific thrust and moderately low SFC. • These are of mixed type i.e. both the cold and the hot streams are mixed. Bypass Engine
- 16. • No Rotating parts ( i.e. no compressor and turbine ) and consists of a duct with a divergent entry, combustion chamber and convergent- divergent nozzle exit. • It can not be started under static condition and air has to be forced into the air intake. • In other words it is not self propelling at zero velocity. • To Initiate the operation the Ramjet must be either launched from airplane in flight or be given an initial velocity by some auxiliary means. Ram Jet Engine
- 17. Turbo-Ramjet: • Turbo Ramjet combines the Turbojet engines (up to M = 3) with the ramjet engine which has good performance at high MACH number. Turbo-Ramjet & Turbo-Ram-Rocket Turbo- Ram Rocket: •A combination of Turbojet, Ram jet and Rocket engine which can be used for hypersonic propulsion (M = 5 to 15).
- 18. • Does not use Atmospheric air as working fluid. •Produces its own propelling fluid by the combustion of liquid or chemically decomposed fuel with oxygen which it carries, thus enabling it to operate out side the earth’s atmosphere. •Hence it is suitable only for operation over short periods. Rocket Engines
- 19. Gas Turbine engine components and Subsystems The gas turbine engine can be divided into various components and sub systems. These components and sub systems are also called as engine modules when the engine is built in a modular fashion. Generally those items which perform some thermodynamic process are called as Components and those items which aid these components to perform the thermodynamic are called as Sub Systems. But the above is only a generic classification and they can be interchanged i.e. the sub systems can be called as components and vice versa.
- 20. Aero Gas Turbine Engines High and low pressure spools Rotor support system Engine Lubrication System Combustion system Engine Control system Engine accessories gearbox Secondary air & Anti-icing system Exhaust system Gas Turbine Engine Components & Subsystems Compression system - Low Pressure - High Pressure - Combustor - After burner Expansion system - High Pressure - Low Pressure - Nozzle
- 21. Functional concept of single and twin spool engines The older engines were single spool engines having a large number of compressor stages resulting in large engine length, operational complexity and increased cost. Since the compression is an adverse pressure gradient process it was felt that the engine compressor can be split into low and high pressure compressors resulting in two spool engines. There are a few three spool engines also mainly from Rolls Royce (RR Trent series of engines, RB 199, RB 211 engines). The advantage of multi spool ( two and three spool) engines is that it increases the operational flexibility of the aero gas turbine engines. • In a twin spool engine, typically: High pressure turbine generates power to drive high pressure compressor Low pressure turbine generates power to drive low pressure compressor Thrust is obtained by expanding the gases through the exhaust nozzle The Low pressure compressor, LP shaft and Low pressure turbine forms the LP spool The high pressure compressor, HP shaft and High Pressure turbine forms the HP spool
- 22. LPC HPC LPTHPT • There is no mechanical coupling between the HP & LP spools, only thermodynamic coupling is there i.e. the two spools rotate at different mechanical RPMs Set of compressor & Turbine coupled thermodynamically is called spool Multi spool design
- 23. Although 2 spools are mechanically independent, their speed are related aerodynamically. There is a unique RPM relationship between the HP and LP spools. It may also be necessary to use bleed valve at intermediate location in the compressor to handle the serious flow mismatch occurring during start-up. Multi spool design
- 24. Aircraft engine requirements Fighters: Supersonic Persistence Reduced Observables Low radar cross section Multi mission capabilities Good maneuverability Civil engines: Minimum specific fuel consumption Maximum range and endurance Long operational life Least noise and pollution levels