Gas Turbine Fundamentals.pdf

Physics (Recap) Physics Fundamentals For a clear understanding of jet propulsion principles, it is necessary to understand the applicable principles of physics which govern the action of mass or matter. The physics described here are intended to present the basic ideas necessary for...

Physics (Recap) Physics Fundamentals For a clear understanding of jet propulsion principles, it is necessary to understand the applicable principles of physics which govern the action of mass or matter. The physics described here are intended to present the basic ideas necessary for understanding the physical relationships between gases and the turbo-machinery within a gas turbine engine. The mass-flow of gases referred to is atmospheric air, which is compressed and accelerated in the gas turbine engine to create useful work at the turbine wheel and, ultimately, thrust. The thrust is created by either a pure reaction to the flowing gases or by a propeller or fan driven by a turbine. Power in Gas Turbine Engines Reciprocating engines measure power in horsepower or kilowatts. Gas turbine engines are more concerned with thrust, and as such the engine is rated on thrust produced or a combination of thrust produced and shaft horsepower in turboprop applications. Thrust is called force (F) when calculating the output of thrust-producing gas turbine engines. 2022-08-24 B1-15a Gas Turbine Engine Page 11 of 244 CASA Part 66 - Training Materials Only  Force Force is defined as the capacity to do work or the tendency to produce work. It is also a vector quantity that tends to produce acceleration of a body in the direction of its application. It can be measured in pounds. Turbojet and turbofan engines are rated in pounds of thrust. The formula for force is: F orce = P ressure × Area ⟹ F = P × A Where: F = Force in pounds P = Pressure in pounds per square inch A = Area in square inches Worked Example The pressure across the opening of a jet tailpipe (exhaust nozzle) is 6 psi above ambient and the opening is 300 in2. What is the force present in pounds? F = P × A 2 F = 6 psi × 300 in F = 1800 lb The force mentioned here is present in addition to reactive thrust in most gas turbine engine designs. 2022-08-24 B1-15a Gas Turbine Engine Page 12 of 244 CASA Part 66 - Training Materials Only  Work Mechanical work is present when a force acting on a body causes it to move through a distance. The formula for work is: W ork = F orce × Distance W = F × d Where: W = Work in foot pounds F = Force in pounds d = Distance in feet A force can act on an object: 1. Vertically (opposite the effect of gravity), for example, lifting an engine. 2. Horizontally (90° to the effect of gravity), 3. or anywhere in between the two. Power Power depends on three factors: Force used Distance the force moves Time required to move the force. The definition of work makes no mention of time. Whether it takes 5 s or 5 hr to move an object, the same amount of work is accomplished. Power, by comparison, does take time into account. Lifting a 10-lb object 15 ft off the floor in 5 s requires significantly more power than lifting it in 5 hr. Work performed per unit of time is power. Power is measured in foot-pounds per second, foot-pounds per minute, or mile-pounds per hour. Expressed simply, power is the rate at which work is performed. The formulae for power and work respectively are: W ork P ower = T ime 2022-08-24 B1-15a Gas Turbine Engine Page 13 of 244 CASA Part 66 - Training Materials Only  therefore (substituting the work formula into the power formula), the power formula becomes: F orce × Distance P ower = T ime D ⟹ P = F × t Where: P = Power in foot-pounds per minute D = Distance/displacement in feet t = Time in minutes F orce × Displacement P ower = T ime ∴ P ower = F orce × V elocity Worked Example A 2500-lb engine is to be hoisted a height of 9 ft in 2 min. How much power is required? D P = F× t 9 ft P = 2500 lb× 2 min lb − f t P = 11250 min Hoisted turbine engine 2022-08-24 B1-15a Gas Turbine Engine Page 14 of 244 CASA Part 66 - Training Materials Only  Translating this knowledge to flight physics, it can be seen that accelerating a 1500-kg vehicle over 1/4 mi in 7 s would require significantly more power than doing so in 5 min. Vectors and Scalars Displacement vs Distance Displacement and distance are commonly used interchangeably, as in many cases, they are the same value. However, these two terms have distinct definitions. Thus far, distance has been used in work and power equations. Distance is a scalar quantity (i.e. how far along the ground an object has travelled). Displacement is a vector quantity (i.e. how far out of place an object is from its origin). If an object travels in a straight line, distance and displacement are the same. Displacement vs distance 2022-08-24 B1-15a Gas Turbine Engine Page 15 of 244 CASA Part 66 - Training Materials Only  Velocity vs Speed Velocity is a vector quantity; it has both speed and direction. Speed is a scalar quantity and does NOT have direction associated with it. Distance Speed = T ime Displacement V elocity = T ime In a straight line, these two equations are the same. So, power can also be related to velocity (as demonstrated in the power equations). Velocity The velocity of the gas flow throughout the gas turbine engine is particularly important as it is responsible for the resultant thrust, in line with Newton’s third law, and is critical to controlling engine operating temperatures. The formula for velocity is: Displacement V elocity = T ime In the following diagram, both aircraft are flying at the same speed, 200 kt. The velocity of the aircraft on the left is 200 kt east. The velocity of the aircraft on the right is 200 kt west. Velocity is a vector quantity 2022-08-24 B1-15a Gas Turbine Engine Page 16 of 244 CASA Part 66 - Training Materials Only  Acceleration In physics, acceleration is defined as a change in velocity with respect to time. Observe that distance travelled is not considered, only loss or gain of velocity with time. The typical (imperial) units for acceleration are feet per second per second (fps/s) and miles per hour per second (mph/s). Feet per second per second are sometimes referred to as feet per second squared (fps2). The SI unit is metres per second squared. The formula for calculating acceleration is: Vf inal − Vinitial V2 − V1 Acceleration = ⟹ T ime t 2022-08-24 B1-15a Gas Turbine Engine Page 17 of 244 CASA Part 66 - Training Materials Only  Jet Propulsion Principles Newton's Laws of Motion Jet engines and propellers develop thrust in accordance with Sir Isaac Newton’s laws of motion. Thrust is defined as a forward force that imparts momentum to a mass of air behind it. In other words, thrust is a reaction to the rearward momentum of a gas. Jet engines work on the principle of reaction. Therefore, the reaction to the acceleration of the air through the engine is felt as thrust. Knowledge of these laws will help you understand thrust. The inertia of the mass airflow The momentum of the mass airflow The reaction to the mass airflow. Newton’s First Law Newton’s First Law of motion states: "A body at rest tends to remain at rest, and a body in motion tends to remain in motion in a straight line, unless acted upon by an external force." This law is often termed the Law of Inertia. Inertia is the quantity which depends solely on mass. The more mass, the greater the force required to change an object’s motion, and the more inertia it will have when in motion. The mass airflow through a jet engine or propeller remains motionless or constant until it is moved by an external force, such as when the engine rpm increases or decreases. The inertia of the propeller and/or the engine’s rotating assembly resists any change of motion. More force is required to overcome this inertia – more power to increase the mass airflow, or more drag to reduce mass airflow. 2022-08-24 B1-15a Gas Turbine Engine Page 18 of 244 CASA Part 66 - Training Materials Only  Newton’s Second Law Newton’s Second Law states: "The acceleration of a body is directly proportional to the force causing it and inversely proportional to the mass of the body." In simple terms, this means a body accelerates in proportion to the force applied to it. Vf inal − Vinitial Acceleration = T ime This law deals with acceleration and is the one that explains, to a great extent, the thrust produced by a turbine engine. The acceleration of a body (air) is directly proportional to the force causing it (engine or components) and inversely proportional to the mass of the body. In other words, a change in motion is proportional to the force applied. This may be expressed by the equation: F orce = M ass × Acceleration F = m × a The second law relates to aircraft engines and propellers accelerating their mass airflow to produce thrust. The above equation is the basic thrust formula. The propeller moves a large mass of air rearwards with a relatively small change in velocity, while the exhaust gas stream from a turbojet engine has a relatively small mass, but the acceleration that has taken place within the engine is large. Both types of acceleration produce thrust. Large mass and small velocity 2022-08-24 B1-15a Gas Turbine Engine Page 19 of 244 CASA Part 66 - Training Materials Only  Small mass and large velocity Newton’s Third Law Newton’s Third Law of motion states: "For every action, there is an equal and opposite reaction." When a jet engine, or an engine turning a propeller, accelerates a mass of air backwards, an equal amount of force is produced that moves the aircraft forward. This law is best understood by observing a deflating balloon as shown in the illustration. Newton’s Third Law - balloon example Due to an imbalance in the balloon’s internal pressure, the air is accelerated out the neck. An equal and opposite force reacts to the force accelerating the air and causes the balloon to move. As with all forms of jet propulsion, the balloon is not propelled forward by the escaping air pushing on anything outside, but by the reaction force inside the balloon. Both action and reaction forces occur inside all engines; this concept will be covered in detail during 15.2 Engine Performance. 2022-08-24 B1-15a Gas Turbine Engine Page 20 of 244 CASA Part 66 - Training Materials Only  Remember that all three of Newton’s laws take place simultaneously and are inseparable. Bernoulli's Theorem Bernoulli’s principle finds that if air is passed through a venturi, as the air velocity increases, the pressure decreases, and as the velocity decreases, the pressure increases. A venturi is simply a narrowing in a tube, as can be seen in the diagram below. Venturi (narrowing in a tube) Bernoulli’s theorem states: "The total energy of a particle in motion is constant at all points on its path at a steady flow." In its simplest form, the theorem means that in a venturi, pressure is inversely proportional to velocity. Another way of stating this is that if pressure increases, velocity decreases proportionally, or if pressure decreases, velocity increases proportionally. The significance of this discovery is that it is one of the basic principles of operation of a jet engine and will become evident during later topics. The diagram below shows how varying the size of a tube affects the velocity and pressure of a fluid, but the total energy always remains constant. In the diagram, the term static is used to describe pressure of a fluid, and static plus dynamic pressure make up total pressure. Static P ressure + Dynamic P ressure = T otal P ressure 2022-08-24 B1-15a Gas Turbine Engine Page 21 of 244 CASA Part 66 - Training Materials Only  Venturi showing variations in airspeed are pressure Venturi showing variations in airspeed and pressure Convergent and divergent ducts 2022-08-24 B1-15a Gas Turbine Engine Page 22 of 244 CASA Part 66 - Training Materials Only  Bernoulli's Principle inside an engine 2022-08-24 B1-15a Gas Turbine Engine Page 23 of 244 CASA Part 66 - Training Materials Only  Potential and Kinetic Energy Energy is used to perform useful work. In the gas turbine engine, this means producing motion and heat. The two forms of energy which best describe the propulsive power of the jet engine are potential and kinetic. Potential Energy Potential energy is stored energy because of position, such as water behind a dam or a fully charged battery. An aircraft in flight has potential energy due to its mass, height and velocity. Fuel has potential energy which is released during combustion. Potential energy in fuel 2022-08-24 B1-15a Gas Turbine Engine Page 24 of 244 CASA Part 66 - Training Materials Only  Kinetic Energy Kinetic energy is the energy possessed by a body because of its motion. The kinetic energy of an object depends on its mass and velocity. Potential energy is released as kinetic energy. When the aircraft lands, that is, when its potential energy is converted into kinetic energy, its kinetic energy is converted into heat by the brakes. Kinetic energy converted to heat by braking Kinetic energy released by combustion Another example of kinetic energy is the energy of motion, such as in releasing water from a dam. 2022-08-24 B1-15a Gas Turbine Engine Page 25 of 244 CASA Part 66 - Training Materials Only  The formula for kinetic energy is: 1 1 2 2 Kinetic Energy = mv = × M ass × V elocity 2 2 The thrust of the engine is kinetic energy. 2022-08-24 B1-15a Gas Turbine Engine Page 26 of 244 CASA Part 66 - Training Materials Only  Conservation of Energy Recall the law of conservation of energy: "Energy can neither be created nor destroyed. It can only be changed from one form to another." The gas turbine engine relies on the first law of thermodynamics in that a cycle of energy conversion is constantly taking place: Air Inlet In the inlet (intake) - Kinetic energy in the form of airflow velocity is being converted to potential energy in the form of pressure by the divergent design of the inlet. Compressor Across the compressor - Pressure or potential energy is continually being converted to velocity or kinetic energy across the compressor rotor(s). Kinetic energy is continually being converted back to potential energy across the compressor stators. Combustor In the combustion area - Potential energy developed in the compressor is increased by the addition of heat energy. Kinetic energy is developed due to the expansion caused by the application of heat energy. Turbine Across the turbine area - Due to the velocity of expanding gas, kinetic energy is formed and converted into mechanical energy by the turbine to drive the compressor. Mechanical energy is the source of the air pressure increase within the compressor. The loss of kinetic energy across each stage of turbine is compensated for by utilising velocity stabilisation methods, preparing the airflow for the next stage of the turbine: Velocity is stabilised by blade and/or stator design. To maintain velocity, a subsequent drop in potential energy (pressure is affected) is implemented across the turbine pack. 2022-08-24 B1-15a Gas Turbine Engine Page 27 of 244 CASA Part 66 - Training Materials Only  Laws of Thermodynamics The effects of heat in a jet engine, or in any engine, are explained by the laws of thermodynamics. It is necessary for you to understand what is involved in the process converting fuel into mechanical work, as it will give you a better understanding of the internal operation of gas turbine engines. The two laws of thermodynamics are as follows: First Law "Energy can neither be created nor destroyed, but can be changed in form." This is also known as the law of conservation of energy. In a gas turbine engine, heat energy is imparted to the air by the compressor, while additional heat is added when the fuel is burned. The heat energy is changed to thrust and the gases are cooled as they pass through the turbine section and out the jet nozzle. With this law in mind, it is reasonable to assume that the total quantity of energy in a cycle is equal to the amount of energy that can be accounted for in any of the forms in which it can occur throughout the cycle, i.e. mechanical energy, heat energy, pressure energy, etc. Obtaining 100% efficiency from a heat engine is a practical impossibility. An engine, as it converts heat into work, has to lose some heat. This phenomenon is the basis of the second law of thermodynamics. Second Law "Temperature differences between systems in contact with each other tend to even out, and work can be obtained from these differences, but a loss of heat occurs when work is done." In other words, no cyclic process is possible in which heat is absorbed from a reservoir at a single temperature and converted completely into mechanical work. Another definition of the same law states that heat cannot flow from a cooler body to a hotter body but must flow from hotter towards cooler. The cooling of an engine involves this principle in that heat is transferred from hotter bodies or substances to cooler bodies or substances. If cooling is not introduced, the components will continue to get hotter until they fail. Another variant explains that mechanical energy can be converted entirely into heat energy, but heat energy cannot be converted entirely to mechanical energy. 2022-08-24 B1-15a Gas Turbine Engine Page 28 of 244 CASA Part 66 - Training Materials Only  Gas Turbine Engine Construction Principles The mechanical layout of the gas turbine engine is simple in that it consists of four basic sections: compressor combustors turbines exhaust. Only two of these parts rotate: the compressor and turbine. One or more combustors direct the mass airflow through the turbine into the dynamically designed exhaust system. The illustration shows the layout of a basic gas turbine engine. Sections of a gas turbine engine In a gas turbine engine, ambient air enters the inlet, where it is subjected to changes in pressure, velocity and temperature. The air is directed at the optimal angle into the compressor, where pressure and temperature are increased mechanically. The air continues to the diffuser, where, by the divergent nature of the duct, the pressure is further increased and velocity is decreased. As the air enters the combustion section, it is mixed with fuel and ignited. The ignition of the air/fuel mixture increases the temperature and volume, which converts the potential energy of the fuel to kinetic energy. The hot gases expand through a convergent exhaust nozzle. The expansion of the gases, caused by the addition of heat energy, creates the necessary action to give the reacting thrust. 2022-08-24 B1-15a Gas Turbine Engine Page 29 of 244 CASA Part 66 - Training Materials Only  The Brayton Cycle The working cycle of the gas turbine engine is similar to that of the four-stroke reciprocating engine. In the gas turbine engine, combustion in the combustion chamber takes place at a constant pressure and is referred to as the Brayton cycle, whereas in the reciprocating engine, or Otto cycle, it occurs at a constant volume. The Four Stroke or Otto Cycle The Otto cycle (Piston Engines) is combustion at constant volume. The Brayton cycle (Gas Turbine Engines) is combustion at constant pressure. Gas Turbine - The Brayton Cycle The Brayton cycle is also widely known as a constant pressure cycle because in the gas turbine engine, pressure is fairly constant across the combustion chamber section as volume increases and gas velocities increase. Both engine cycles show that in each instance there is induction, compression, combustion and exhaust. In the reciprocating engine, these processes are intermittent and occur in the same place. In the gas turbine engine, however, the cycle is continuous and occurs in different places. 2022-08-24 B1-15a Gas Turbine Engine Page 30 of 244 CASA Part 66 - Training Materials Only  Due to the continuous action of the turbine engine and the fact that the combustion chamber is not a fully enclosed space, the pressure of the air does not rise like that of a piston engine, but its volume does increase. This process is known as heating at a constant pressure. Bernoulli’s Theorem Applied to Engine Theory This is the application of Bernoulli’s Theorem in a typical single-spool axial flow turbojet engine. The diagram below shows the changes in pressure, velocity and temperature (turbojet) during ground run-up. © RR The Jet Engine Bernoulli’s theorem - pressure, velocity, temperature graph 2022-08-24 B1-15a Gas Turbine Engine Page 31 of 244 CASA Part 66 - Training Materials Only  Gas Turbine Engines The History of Propulsion In 1930, Frank Whittle, while a cadet in the British Royal Air College, wrote a thesis advocating use of the gas turbine engine. If the engine could be made light enough in weight, the ram effect of the incoming air in flight would provide sufficient power to make it an effective aircraft power plant. He patented the first turbojet aircraft engine which used a compressor impeller, driven by a turbine. The engine developed was a pure reaction turbojet. That is, its total thrust came from reaction to the hot gas stream emitted from a propelling nozzle. The engine featured an impeller type compressor, a multiple-can combustion chamber and a single- stage turbine wheel. Today, the gas turbine engine receives its name from this design, wherein flowing gas drives the turbine wheel which is attached to, and drives, the compressor impeller. Frank Whittle and his pure reaction turbojet 2022-08-24 B1-15a Gas Turbine Engine Page 32 of 244 CASA Part 66 - Training Materials Only  Pure reaction turbojet The Gas Turbine Engine The aircraft gas turbine is a heat engine using air as a working fluid. In its most basic form, it consists of a compressor for compressing the air, a combustion chamber for burning the air/fuel mixture and a turbine for extracting energy from the high-velocity exhaust gases. Some of the energy in the highly heated gases is required to drive the compressor and accessories, the remainder being available to produce power or thrust. The turbine-type jet and, more specifically, the gas turbine engine is a name given to a family of engines based on the Whittle design, which include the turbojet, turboprop, turboshaft and turbofan. These four gas turbines are discussed in detail throughout this chapter. Supplementary Media Relevant Youtube link: Gas Turbine Engines - Lecture 01 Relevant Youtube link: Gas Turbine Engines Video - Lecture 02 Relevant Youtube link: Gas Turbine Engines Video - Lecture 03 2022-08-24 B1-15a Gas Turbine Engine Page 33 of 244 CASA Part 66 - Training Materials Only  Engine Constructional Configurations All gas turbine engines consist of the same basic components. However, the nomenclature used to describe each component varies among manufacturers. Nomenclature and overhaul differences are reflected in applicable maintenance manuals. The following discussion uses the terminology that is most commonly used in industry. There are seven basic sections within every gas turbine engine: Air inlet Compressor section Combustion section Turbine section Exhaust section Accessory section Ancillary systems Ancillary systems are required for: Starting Lubrication Fuel supply Anti-icing Cooling Pressurisation Additional terms you often hear include hot section and cold section. A turbine engine’s hot section includes the combustion, turbine and exhaust sections. The cold section, on the other hand, includes the air inlet duct and the compressor section. The gas turbine-powered jet is further broken down into four types: turboshaft turbo-propeller turbojet turbofan. These four types of engines are the ones most common in today’s aircraft. 2022-08-24 B1-15a Gas Turbine Engine Page 34 of 244 CASA Part 66 - Training Materials Only  Turboshaft The gas turbine engine that delivers power through a shaft to operate something other than a propeller is referred to as a turboshaft engine. The turboshaft power take-off may be coupled to and driven by the turbine that drives the compressor, but is more likely to be driven by a turbine of its own. They are known as a fixed turbine or a free turbine turboshaft engine. A free turbine turboshaft engine has two major sections: the gas generator and free turbine sections. The diagram shows an example of these two sections of turboshaft engines. Turboshaft engine The function of the gas generator is to produce the required energy to drive the free turbine system. The gas generator extracts about two thirds of the energy available from the combustion process, leaving the other third to drive the free power turbine. These engines are widely used in industrial applications, such as in electrical power-generating plants and surface transportation systems (mainly high-speed naval vessels), while in aviation, turboshaft engines are used to drive the rotors of many modern helicopters. Aircraft auxiliary power units (APUs) are also often turboshaft engines which are used in aircraft to drive generators and hydraulic pumps. 2022-08-24 B1-15a Gas Turbine Engine Page 35 of 244 CASA Part 66 - Training Materials Only  Turbo-propeller (Turboprop) Commonly called the ‘turboprop’ engine, this engine is similar in design to the turbojet, with the exception that it delivers the power produced in the engine to a shaft which feeds into a reduction gearbox and onwards to the propeller. The reduction gearbox is used to slow the propeller’s rotational speed and to increase torque capability. Most of the power produced in the engine is used to drive the propeller, and therefore little thrust is produced from the engine exhaust. Shown below are two different examples of turboprop engines. Turboprop engine - single spool type 2022-08-24 B1-15a Gas Turbine Engine Page 36 of 244 CASA Part 66 - Training Materials Only  Turboprop engine - free power turbine type Turboprop engines are best suited to speeds below approximately 450 mph. Up to this speed they are more efficient than turbojet engines, but propeller efficiency falls away rather rapidly at speeds above 450 mph due to disturbance of the airflow at the propeller blade tips. The advantages of the turboprop have been largely offset by advances in turbofan technology. © Aviation Australia Turboprop aircraft 2022-08-24 B1-15a Gas Turbine Engine Page 37 of 244 CASA Part 66 - Training Materials Only  Turbojets Modern turbojets use many variations on this theme, but the components are still basically unchanged. The illustration provides a typical example of a modern turbojet. Turbojet engine The turbojet engine uses the acceleration of airflow throughout the engine to produce thrust. The turbojet is well suited to high-speed, high-altitude operations due to enhanced efficiencies under these conditions. The basic operating principles of a turbojet engine are relatively straightforward: air enters through an inlet duct and proceeds to the compressor, where it is compressed. Once compressed, the air flows to the combustor section, where fuel is added and ignited. The heat generated by the burning fuel causes the compressed air to expand and flow towards the rear of the engine. As the air moves rearwards, it passes through a set of turbine wheels that are attached to the same shaft as the compressor blades. The expanding air spins the turbines, which in turn drives the compressor. Once past the turbines, the air exits the engine at a much higher velocity than the incoming air. It is this difference in velocity between the entering and exiting air that produces thrust. From 450 mph up, the turbofan or turbojet is most widely used. The turbofan is newer and has become the most popular power plant for commercial and business jets because its design affords the greatest propulsive power at higher subsonic cruising speeds. The turbofan engine was developed to permit the use of higher turbine temperatures without a corresponding increase in jet velocity, because a high jet velocity is not efficient for subsonic flight. The turbojet engine is less efficient and has, for all practical purposes, been replaced by the turbofan. 2022-08-24 B1-15a Gas Turbine Engine Page 38 of 244 CASA Part 66 - Training Materials Only  Turbofan/Bypass Engines The turbofan engine, or bypass engine, consists basically of a multi-bladed ducted propeller driven by a gas turbine. There are several different configurations of turbofan engines. Some new designs have the fan driven through a reduction gearbox from the compressor, while others are connected directly to the compressor. Turbofan geared engine Fan driven directly by turbine 2022-08-24 B1-15a Gas Turbine Engine Page 39 of 244 CASA Part 66 - Training Materials Only  In a turbofan engine, the fan makes a substantial contribution to the total thrust. The core engine, which is the portion of the turbofan that resembles a typical turbojet, continues to contribute to thrust only at a lower ratio. The fans of a turbofan engine produce 30%–90% of the total thrust, the actual amount depending principally on the bypass ratio. Turbofan engines have turbojet-type cruise speed capability, yet retain some of the short-field take- off capability of a turboprop. Nearly all present-day airliners are powered by turbofan engines for the reasons just mentioned as well as because turbofans are very fuel efficient. A turbofan engine may have the fan mounted to either the front or back of the engine. Engines that have the fan mounted in front of the compressor are called forward-fan engines, while turbofan engines that have the fan mounted to the turbine section are called aft-fan engines. The inlet air that passes through a turbofan engine is divided into two separate streams of air. 2022-08-24 B1-15a Gas Turbine Engine Page 40 of 244 CASA Part 66 - Training Materials Only  Bypass Ratios Low bypass (1:1) Medium bypass (2:1 or 3:1) High bypass (4:1 to < 9:1) Ultra-high bypass (> 9:1) Low Bypass In the low-bypass engine, the airflow is divided approximately into two halves between the fan and the compressor. Air that is being discharged by the fan may be ducted overboard from a short duct, or it may pass down a duct that extends the full length of the core engine and is known as the ‘cold gas stream’. The core engine air is compressed, combusted and discharged in the normal manner out the hot exhaust nozzle. The air that passes through the core engine is known as the ‘hot gas stream’. The turbofan illustrated below has a non-mixed exhaust. This means the air being discharged from the fan is not mixed with that from the core engine before reaching the outside air. Non-mixed exhaust The diagram below also shows a fully ducted fan engine; however, the hot gas stream (from the core engine) is mixed with the cold gas stream (from the fan) in the exhaust before they enter the atmosphere. This design offers the advantage of diluting the hot gases in the common exhaust, which aids in noise suppression, helping to lessen noise pollution. 2022-08-24 B1-15a Gas Turbine Engine Page 41 of 244 CASA Part 66 - Training Materials Only  Mixed exhaust High Bypass The high-bypass engine has a fan ratio of 4:1 to < 9:1 (this means that four to nine parts of air go through the fan for every part that goes to the core engine). To accomplish this ratio, a large-diameter fan is required. These engines are of the type fitted to large aircraft commercial jets and produce the greater percentage of their thrust from the fan (up to 80%). High bypass fan 2022-08-24 B1-15a Gas Turbine Engine Page 42 of 244 CASA Part 66 - Training Materials Only  Ultra-High Bypass The ultra-high bypass engine has a fan ratio of 9:1 or greater (this means nine or more parts of air go through the fan for every part that goes to the core). To accomplish these ratios, a larger fan diameter is required, and some engine manufactures are also using a geared fan. These engines are of a type fitted to commercial aircraft jets and produce the greater percentage of their thrust from the fan (up to 90%). Current engine examples are the CFM LEAP (Leading Edge Aviation Propulsion) with a bypass ratio of 11:1, the RR Trent 1000 with a bypass ratio of 10:1, and the P&W 1000G geared fan with a bypass ratio of 12.5:1. Ultra-high bypass fan (courtesy of Rolls Royce) The turbofan engine is now the most widely used gas turbine engine in the aircraft industry, both military and commercial. It offers higher performance in comparison to the turbojet, low speed efficiencies on the order of the turbo-propeller and the best fuel economy of them all. 2022-08-24 B1-15a Gas Turbine Engine Page 43 of 244 CASA Part 66 - Training Materials Only  Ducted Fan Operation The ducted fan engine may be regarded as a development of the bypass principle. The requirement for high-bypass ratios of up to 5:1 is largely met by using the front fan in a twin- or triple-spool configuration. The front compressor stage (fan) is housed in an aerodynamic duct or shroud. Up to 90% of the airflow accelerated by the fan rotor blades is ducted past the core engine, while the air from the lower portion of the blades flows into the engine itself. Ducted fan engine On some front-fan engines, the bypass airstream is ducted overboard, either directly behind the fan through short ducts or at the rear of the engine through longer ducts as illustrated below; hence the name ducted fan. Ducted fan engine 2022-08-24 B1-15a Gas Turbine Engine Page 44 of 244 CASA Part 66 - Training Materials Only  Fan tip speed may be allowed to exceed Mach 1 so the compressor can deliver the correct amount of air. Pressure within the fan duct helps retard airflow separation from the blades at speeds over Mach 1 so there is an effective transfer of energy to the air at the required compression ratio. High-bypass engines and ducted fan engines produce more fan thrust than low-bypass engines because they suffer less loss through skin friction with their short ducts as well as being designed to carry much larger airflow mass. Another seldom used variation is the aft fan, where the fan is arranged either behind the turbine and powered by shaft from the turbine or is an extension of the turbine blades. Engine Stations Although the terms hot and cold sections are useful indicators of engine position, they are not specific enough when referring to maintenance manuals/tasks. To make it easier to identify the position (or station) of a component or fitting, a reference system using engine stations has been developed. An engine station is a numbered location along the axis line of the engine which follows the gas path and refers to basic locations such as the compressor inlet, compressor outlet, and turbine inlet and outlet. Engine stations example The number of designated stations varies according to engine complexity. It is therefore possible for the same station number to refer to different positions on different engine types or by different manufacturers. Engine symbols such as Pt and Tt are often used in conjunction with station numbers. Station numbers, in addition, locate the position of temperature and pressure sensing. For example, total temperature (Tt) is used for indicating instruments and engine monitoring and control via electronic computers, and pressure ratio, or EPR, compares air pressure at the exhaust Pt7 with air pressure at the inlet Pt2 to indicate thrust to the pilots. 2022-08-24 B1-15a Gas Turbine Engine Page 45 of 244 CASA Part 66 - Training Materials Only  Engine Performance Stations - single spool engine Engine performance stations of a dual spool engine 2022-08-24 B1-15a Gas Turbine Engine Page 46 of 244 CASA Part 66 - Training Materials Only  Engine Performance Stations - turboprop From this discussion, it is evident that station numbers, when used as a subscript to an uppercase prefix, greatly abbreviate cumbersome terminology for describing locations and functional data of the engine. In addition to the station numbers, prefixes are used to show various parameters occurring at these stations within the engine. For example, temperature has the prefix T. The temperature occurring at Station 5 is called T5. Pressure has a prefix P and can be further divided into: Pt = total pressure Ps = static pressure. The static pressure at Station 3 is known as Ps3. 2022-08-24 B1-15a Gas Turbine Engine Page 47 of 244 CASA Part 66 - Training Materials Only  Engine Terminology Many different terms are used to describe parts of or positions (stations) on the engine. Generally speaking, the terms referred to here are universally acceptable. The engine’s cold and hot sections are the sections exposed to cold and hot gases respectively. Cold Section Basically, the front part of the engine, which handles the colder airflow, is termed the cold section. The illustration shows the cold section, which consists of the: Engine inlet Compressor Diffuser. Engine Cold Section 2022-08-24 B1-15a Gas Turbine Engine Page 48 of 244 CASA Part 66 - Training Materials Only  Hot Section The hot section of the engine is exposed to hot air and includes the: Combustion chamber Turbine assembly Exhaust. The air that flows through the hot section is known as the ‘hot gas stream’. The diagram below illustrates the hot section and identifies its component parts. Engine Hot Section Ambient Air Ambient air is the natural air surrounding the engine. The pressure and temperature of ambient air are usually required when an aircraft requires a ground run. They are usually obtained by contacting the base meteorology section. 2022-08-24 B1-15a Gas Turbine Engine Page 49 of 244 CASA Part 66 - Training Materials Only  Gas Generator The gas generator is the part of the gas turbine engine that produces the basic gas. Basic gas is the gas that travels through the compressor(s) to the combustion area, and onwards to the turbine. The gas generator section of a jet engine excludes the engine inlet and exhaust nozzle. The image below illustrates the gas generator portion of a turbojet engine. For a dual-spool engine, the high-speed compressor (N2), combustor and high-speed turbine make up the gas generator. Gas Generator Section 2022-08-24 B1-15a Gas Turbine Engine Page 50 of 244 CASA Part 66 - Training Materials Only