MODULE SIX GAS TURBINE ENGINE PRINCIPLES 0F OPERATION
OBJECTIVE • WITHOUT REFERENCE TRAINEES WILL BE ABLE TO DESCRIBE, THE FUNCTION,CONSTRUCTION AND CLASSIFICATION OF GAS TURBINE ENGINES AS PER THE APPLICABLE TEXT BOOKS, MATERIALS AND MANUALS OF INSTRUCTIONS.
MODULE SIX OVERVIEW A. Module Six Contents 1.History and development of jet propulsion principles 2.Early jet engine development 3.Turbojet engine development 4.The world’s first turbojet powered aircraft flight 5.Gas turbine engine fundamentals:
Contd. a. Principal terms, definitions and abbreviations b. Symbols and their meanings c. Laws of physics as applied to jet engines d. Principles of jet engine aerodynamics e. Principles of jet engine thermodynamics f. Gas turbine engine theory g. Thrust compared with horsepower h. Factors affecting thrust i. Jet engine requirement and thrust distribution j. Thrust measurement k. Compressibility
Contd. B. Evaluation Measures 1.Feedback questions 2.Test items a. Multiple choice b. Essay type c. Fill in the blanks NB. The passing mark is 70%
Contd. C. Reference Materials 1.Aircraft Powerplants by Bent/McKinley fifth edition pages 429-437 2.Airframe & Powerplant Mechanic, Powerplant Handbook(AC6512A) pages 38-42 3.Aviation Technician Integrated Training Program (ITP) pages 131 4.Jet Aircraft Power Systems Third Edition pages 1-20 5.The Aircraft Gas Turbine Engine and its Operation pages 1-23 and 44-46
HISTORY AND DEVELOPMENT OF JET PROPULSION ENGINES
The Squid and Cuttlefish • •
Earliest form of jet propulsion Aquatic fish uses jet propulsion for locomotion
Aeolipile • Built by Heron of Alexandria, Egypt, about 250 BC. • Steam pressure through jets rotates sphere
Rockets • Rockets were constructed as early as 1232 by Mongols for use in war and fireworks displays
Chinese Rocket • Chinese scholar named Wan Hu built a sledge for transportation using rockets as a means of propulsion. • The rocket blast was excessive and he became the first martyr in humanity’s struggle to achieve flight
Da Vinci’s Chimney Jack • Leonardo Da Vinci described the chimney jack around 1500 AD • A chimney fitted with spit used to turn roasting meat
Branca’s Stamping Mill • Giovanni Branca built the stamping mill in 1629. • Boiler produces steam to rotate the first actual impulse turbines • Rotating turbine is used to do work.
Newton’s Steam (Horseless) Carriage • It is thought that Willem Jako Gravesande, a Dutchman, actually designed this horseless carriage in 1680 and that Isaac Newton may have only supplied the idea • Provides illustration of Newton’s third law of motion – For every action, there is an equal and opposite reaction.
First Gas Turbine • In 1791 John Barber, an Englishman, was the first to patent a design utilizing the thermodynamic cycle of the modern gas turbine • He suggested its use for jet propulsion
BRITISH • In 1926 Dr. A.A Griffith developed a theory • In 1930 Sir Frank Whittle added the theory into a better form. • In 1936 the first jet company was established and production started • In 1941 First flight by Whittle W-1 engine on Gloster E28/39 experimental airplane.
AMERICANS • In 1918 Dr. Sanford A. Moss supervised the production of the gas turbine driven turbo supercharger for reciprocating engines • Credit for the basic idea for the turbo supercharger is given to Rateau of France
Americans • The first GTE built in USA was in 1902 under the supervision of Dr Sanford A. Moss. • First successful flight by Bell XP-59 aircraft powered by two GE-1A engines in 1942
Germans • In 1937 Han Von Ohain built and ran his first demonstration jet engine. • On August 27, 1939 the first jet engine (Hes3B) flight was made on the Heinkel He 178 aircraft.
Italian • In 1940 Secundo Campini designed jet propelled monoplane at Caproni Company • It consisted of a ducted fan engine where the compressor of the engine was driven by a reciprocating engine. • Engine was inefficient due to its excessive weight
Russians • Since everything was secret in the former Soviet Union the jet engine production was known to have been built after the second world war. • The Russian technology has contributed for engine development in various aspects
Gas Turbine Engines after the Second World War • Pratt & Whitney, General Electric, Rolls Royce, Garrett, General Motors-Allison, Snecma, Textron-Lycoming, and etc. companies now produce the bulk of gas turbine engines for aviation industry . • These companies and others have all capitalized on earlier technological discoveries.
GAS TURBINE ENGINE FUNDAMENTALS I. II. III. IV.
Physics laws applicable to engines Propulsion & thrust calculations Jet engine aerodynamics Factors that affect GTE operation
Comparison of GTE & propellers
Applicable Physics Laws • Gas laws – Boyle’s law – Charles’s law – Gay-Lussac’ law – General gas law
• Newton’s laws of motion • Thermodynamics laws
BOYLE’S LAW • When the temperature of a confined gas was kept constant and the pressure doubled the volume was reduced to half the former value. PV = Constant
CHARLE’S LAW • All gases expand and contract in direct proportion to the change in the absolute temperature, provided the pressure is kept constant. V/T = Constant
GENERAL GAS LAW • It is the combination of Boyle’s and Charles’s laws. • Given initial state it can be used to define state of the gas after a certain process.
P1V1 / T1 = P2V2 / T2
Newton’s laws of motion • First law: law an object remains at rest unless disturbed by an external force • Second law: law the acceleration of a body is directly proportional to the fraction of the force and the mass of the body • Third law: law for every action there is an equal and opposite reaction
Description of Newton’s Law
Equal pressure in all direction (zero net force)
This pressure remains
This pressure is released
Unequal pressure causes moving force
Maintain Constant Pressure Net force
• Maintaining Balloon pressure assures the resultant net force to act continuously.
Placing The Burner • Placing a burner will increase the volume flow rate of the air stream. • It will increase the change in momentum • Some of these energy is used to drive the turbine thus the compressor.
Newton’s Third Law For every action, there is an equal and opposite reaction. Example: – – – –
Pistol Recoil Lawn sprinkler Motor boat Jet-engine thrust
Newton’s Third Law Example: – – – –
Pistol Recoil Fire hose Balloon Jet-engine thrust
THERMODYNAMICS • The branch of physics which seeks to derive, from a few basic postulates, relationships between properties of matter, especially those which are affected by changes in temperature. • It also describes of conversion of energy from one form to another. – First law – Second law
First Law of Thermodynamics • It is the law of conservation of energy. • When heat energy is added to (removed from) a system an equal amount of some other form of energy appears (disappears). • This law applies for all heat engines – Reciprocating engine – Turbine engine, etc.
Second Law of Thermodynamics • Heat flows spontaneously from a hotter to a colder object, but not vice versa. Because of this, it is impossible for a system to transfer heat from a lower temperature body to a higher temperature body in definitely unless external work is done on the system. • This law is used to explain temperature difference between combustion chamber and turbine or jet nozzle.
Thermodynamics application • GTEs follow the Brayton engine cycle • It is a constant pressure cycle Point
1 2 3 4
5
6
WHAT IS PROPULSION? • Latin words: pro meaning forward and pellere 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. • On airplanes, thrust is usually generated through some application of Newton's third law of motion (action and reaction).
What is thrust? LIFT THRUS T
DRA G
LOAD
Thrust Equation • 1. F=ma » Where F= Force » m= Mass » a= Acceleration
• 2.F=W/g x (V2-V1) » Where F= Force in pounds » W= Flow rate in lbs/sec of air, gas, or a liquid, . such as fuel » V1= Initial velocity of a mass of air, gas, or a . liquid in ft/sec » V2= Final velocity of a mass of air, gas, or a . liquid in ft/sec » g= Gravitational acceleration, 32.2 ft/sec/sec
• Note: m=W/g; Where m= Mass
Thrust Equation • Momentum is the impulse imparted to the air, fuel, and products of combustion that pass through the engine.
• 3.F=(W/g x V2) – (W/g x V1) • The force generated when a mass is accelerated through a jet engine becomes the difference between the final momentum of the mass and the initial momentum of the mass.
• 4.Fn=Outgoing exhaust gas momentum – (Incoming air momentum + Incoming fuel momentum) » Where Fn= Net thrust in lbs
Thrust Equation • Net thrust is the thrust that results from the change in momentum of the mass of air and fuel that passes through the engine.
• 5.Fn=[(Wa/g+Wf/g) x Vj] – [(Wa/g x Va) + (Wf/g x Vf)] – Where Wa = Airflow through the engine in lbs/sec – Wf = Fuel flow in lbs/sec – Vj = Exhaust gas velocity in ft/sec – Va= Incoming air velocity in ft/sec – Vf= Incoming fuel velocity in ft/sec
Thrust Equation • The incoming air velocity, Va, will be approximately equivalent to the speed of the aircraft. • The incoming fuel velocity, Vf, is considered zero because the fuel is carried aboard the aircraft, and therefore will have no initial velocity relative to the engine
• 6.Fn=[(Wa/g+Wf/g) x Vj] – (Wa/g x Va + 0) • Or, transposing
• 7.Fn=Wa/g(Vj-Va) + Wf/g(Vj)
Thrust Equation • When conventional subsonic jet nozzles are used, all of the pressure within the engine cannot be converted to velocity. • This unconverted pressure and the thrust it generates become more and more pronounced as the speed of the aircraft increases, particularly at supersonic air speeds.
• 8.Fn=Wa/g(Vj-Va) + Wf/g(Vj) + Aj(Pj-Pam) – Where Aj=Area of engine jet nozzle in sq.ft. – Pj=Static pressure at the jet nozzle discharge in lbs/sq.ft – Pam=Static pressure of the ambient (outside) air at the . jet nozzle in lbs/sq.ft
Thrust Equation • In actual practice, fuel flow is usually neglected when net thrust is computed, because the weight of air that leaks from various sections of the engine is assumed to be approximately equivalent to the weight of the fuel consumed. • Therefore, the final equation (not considering fuel flow) for computing the net thrust produced by a turbojet engine becomes:
• 9. Fn=Wa/g (Vj-Va) + Aj (Pj-Pam)
Thrust with Choked Nozzle
Thrust Equation • Gross thrust is the thrust developed at the engine exhaust nozzle. • This includes both the thrust generated by the outgoing momentum of the exhaust gases and the additional thrust resulting from the difference between the static pressure at the nozzle and the static pressure of the ambient air. • Gross thrust does not take the incoming momentum of the air and fuel into consideration. • Zero incoming momentum is assumed, which is true only when the engine is static.
Thrust Equation • Without considering fuel flow, the equation for gross thrust is:
• Fg=Wa/g(Vj) + Aj(Pj-Pam) » Where Fg=Gross thrust
• When an aircraft and engine are static, as when the aircraft is parked, or when an engine is being run up prior to takeoff at the end of a runway, net thrust and gross thrust are equal. • The same is true when an engine is being operated in a ground test stand.
Thrust Measurement • For small shaft driving engines – Torque – RPM
• For other engines a relative scale is used – Floating stand with a calibrated scale – Dynamometer – Engine pressure ratio
Jet Engine Aerodynamics • Unlike reciprocating engines air flows continuously from the inlet to the exhaust. • This flow involves many changes in velocity (but no stops or starts)
Jet Engine Aerodynamics • 1.Airbreathing • A jet engine is often called an airbreathing engine. It breathes continuously as opposed to intermittent type of breathing of the lung. It breathes air continuously from the entrance to the jet nozzle. The jet engine is a tube engine.
• 2.Primary Flow • It is the flow of air used for combustion.
• 3.Secondary Flow • It is the flow of air used for cooling.
Jet Engine Aerodynamics • 4.Axial Flow • Flow of air parallel to the axis or lengthwise center line of the engine.
• 5.Centrifugal Flow • Flow of air compressed by centrifugal force. Flow of air from center to outward.
• 6.Compressor Aerodynamics • • • •
Compressor blades are small airfoils. They have camber difference. Camber difference creates pressure difference. Therefore, the pressure in the compressor varies from stage to stage.
Jet Engine Aerodynamics • Various parts of a GTE contribute to the total thrust
Jet Engine Aerodynamics • A bypass engine produces thrust outside the core engine.
GTE Stationing • For ease of discussion GTE is divided into two sections, namely HOT & COLD sections
GTE Stationing • Further the engine is sub-divided into functional blocks – Inlet – Compressor – Burner – Turbine – Exhaust
GTE Stationing • Station numbers denote specific locations on engine
GTE Stationing • Station numbers denote specific locations on engine
GTE Stationing • Station numbers denote specific locations on engine
Review Questions 1. Considering the formulas for work, force, power, horsepower, velocity, and acceleration, which three formulas are expressed in units with respect to time? 2. Which of Newton’s laws states the principle of action-reaction? 3. What type of thermodynamic cycle of events is known as the Brayton cycle? 4. Bernoulli’s principle describes the relationship existing between velocity and pressure of a fluid moving through a duct. Is this relationship direct or inverse?
Review Questions 5.Is thrust, calculated when an aircraft is in flight, referred to as gross thrust or net thrust? 6.Which one is greater among gross thrust and net thrust? 7.Does a choked nozzle add additional supersonic velocity or additional thrust? 8.What are the three most important factors that will affect the thrust of a gas turbine engine during operation?
Factors affecting GTE operation • • • • •
Atmospheric air density Atmospheric air temperature Engine air speed Engine RPM Type of fuel being used
Factors affecting GTE operation
Factors affecting GTE operation
Factors affecting GTE operation
Factors affecting GTE operation
Factors affecting GTE operation
Factors affecting GTE operation
SUMMARY OF MODULE-6 • History of jet engine development • Jet engine fundamentals – Physics laws – Propulsion & thrust calculations – Jet engine aerodynamics – Factors affecting engine thrust