MOD 15 FUNDAMENTALS PDF

British Airways Global Learning Academy – Gas Turbine Engine Newton’s Laws of Motion to that is that the thing producing the force – the engine – must react in the opposite direction. For an understanding of piston (and gas turbine) engines, Newton’s laws as stated below. Thrust goes rearwards, e...

British Airways Global Learning Academy – Gas Turbine Engine Newton’s Laws of Motion to that is that the thing producing the force – the engine – must react in the opposite direction. For an understanding of piston (and gas turbine) engines, Newton’s laws as stated below. Thrust goes rearwards, engine reacts forwards. Simple! First Law A body remains in a state of rest, or of uniform motion in a straight line, unless it is acted upon by external forces. It is also known as the “Law of Inertia” It explains, amongst other things, various energy losses throughout the gas turbine engine when airflows are being made to change direction, for example. The gas wants to continue in a straight line, at a constant speed. To change its direction or speed requires an external force or mechanism. Second Law The rate of change of momentum is proportional to the force producing the change, this change taking place along the straight line in which the force acts. This gives the classic formula 𝐹 = 𝑚𝑎 A simple example of this is that the thrust from an engine will be the product of the mass of air used and the acceleration it is given. The method used to do this can have a great effect of fuel efficiency, for example. Third Law To every action there is an equal and opposite reaction. The obvious example is how a gas turbine engine moves an aircraft. It accelerates a mass of air rearwards, developing a force. The reaction Module 15B ETBN 0502 August 2022 Edition 5 15.1 Fundamentals British Airways Global Learning Academy – Gas Turbine Engine This provides a convenient way of determining the new value of pressure or volume after a change to the original conditions. The suffix 1 is given to the original conditions, and suffix 2 to the new values; thus the formula becomes : Gas Laws Gases can vary their volume, pressure and temperature and over the years physicists have endeavoured to establish the relationship between these three variables. Boyle's Law P1V1 = P 2V2 Boyle did his experiments at constant temperature. This is known as an adiabatic process where no heat is gained or lost in the system. He showed that for a fixed mass of gas, the volume is inversely proportional to pressure as shown in the diagram. 𝑃= Module 15B ETBN 0502 August 2022 Edition 1 𝑜𝑟 𝑃𝑉 = 𝐶𝑜𝑛𝑠𝑡𝑎𝑛𝑡 𝑉 6 15.1 Fundamentals British Airways Global Learning Academy – Gas Turbine Engine Charles’ Law Charles did his experiments at constant pressure. This is known as an isobaric process. He showed that for a fixed mass of gas, the volume was proportional to the temperature as shown in the diagram. When heat is added to the system, to allow the pressure to remain constant, the volume must be allowed to increase. This can be seen is the formula below. 𝑉 𝑉1 𝑉2 = 𝐶𝑜𝑛𝑠𝑡𝑎𝑛𝑡 𝑜𝑟 = 𝑇 𝑇1 𝑇2 He further showed that by projecting the graph back to zero pressure, the temperature for all gases was -273.16°C, now known as absolute zero. This was used to become 0K on the Kelvin scale which is the scale used by the Gas laws for all calculations containing temperature measurements . Module 15B ETBN 0502 August 2022 Edition 7 15.1 Fundamentals British Airways Global Learning Academy – Gas Turbine Engine Combined Gas Law The Combined Gas Law does as the name suggests and combines Boyle’s and Charles’ laws to cover the pressure, volume and temperature conditions in a lossless ideal gas. It can be expressed as: 𝑃1 𝑉1 𝑃2 𝑉2 = 𝑇1 𝑇2 This better explains the conditions found during the journey a gas makes through a gas turbine engine. Module 15B ETBN 0502 August 2022 Edition 8 15.1 Fundamentals British Airways Global Learning Academy – Gas Turbine Engine Bernoulli showed that for an ideal fluid Bernoulli’s Principle There are three kinds of energy in a gas flow: Potential Energy (represented by pressure). Kinetic Energy (represented by velocity). Thermal Energy (represented by temperature). TOTAL ENERGY IN A STEADY STREAMLINE FLOW REMAINS CONSTANT. This can be stated as: Explaining these a little further: Total Energy = Static pressure + Dynamic pressure. Potential energy has the ability to do work. A gas under increased static pressure has energy stored that can be used by expanding the gas. As one reduces, the other increases, they are inversely proportional This is the basic explanation of how compressors and turbines work. They are based on convergent and divergent ducts which make use of Bernoulli’s Principle. Kinetic energy is the energy of motion and it can also be used and have its state changed. It is worth noting that the temperature of a gas will follow the static pressure. An increase in pressure will increase the temperature, in accordance with Charles’ Law, or an increase in temperature will increase the static pressure. =;l This also leads to Bernoulli’s equation: 1 𝑃 + 𝜌𝑣 2 = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 2 where: 𝑃 is static pressure and 1 2 Module 15B ETBN 0502 August 2022 Edition 9 𝜌𝑣 2 is dynamic pressure 15.1 Fundamentals British Airways Global Learning Academy – Gas Turbine Engine The flow follows Bernoulli’s Principle where the total energy remains constant with the static and dynamic pressures altering as varying duct cross section affects the mass air flow speed. Convergent and Divergent Ducts There are many examples of convergent and divergent ducts within a gas turbine engine – between compressor or turbine blades, the shape of the combustion chamber, the intake and exhaust, for example. Although supersonic flow isn’t mentioned much in this module, the effects of convergent and divergent ducts are completely reversed for supersonic flow. A convergent duct will reduce in cross-sectional area whilst a divergent duct will increase. Assuming a given mass of air is flowing through the duct in a given time – a constant mass flow of air – then a change in the area must result in a change in the speed of the flow. If the cross section decreases, the speed increases and if the cross section increases, the speed decreases. These basic rules apply for air before compressibility starts to take effect at about 130 ms -1. For simplicity, and acceptable for this module, we say these rules apply to subsonic airflow. Divergent Duct Convergent Duct Subsonic Flow Module 15B ETBN 0502 August 2022 Edition 10 15.1 Fundamentals British Airways Global Learning Academy – Gas Turbine Engine The Brayton Cycle The working cycle of an engine describes the effects that engine has on the pressure and volume of the gas passing through it. The Brayton Cycle describes those effects for a gas turbine engine. The cycle is a theoretical constant pressure cycle where combustion is considered to happen at constant pressure. The reality is that there are some losses due to turbulence in the combustion chamber. The diagram shows the effects on pressure and volume as the air passes through the engine. • The gas compressor where ambient air is drawn into the compressor and pressurised (A to B). The volume decreases and the pressure and temperature increase in line with Combined Gas Law. • The combustion chamber where fuel is added and ignited and the surrounding air is heated during a constant-pressure process. As the temperature increase, so must the volume (Charles’ Law). Since the combustion chamber is open at both ends for inflow and outflow, the gas will expand and flow out of the rear of the combustion chamber. This is theoretically at constant pressure, but the diagram shows the practical losses (B to C) • The expansion turbine where the heated, pressurised gases from the combustion chamber gives up their energy, expanding through a turbine/turbines to drive the compressor/compressors at the front of the engine (C to D). This follows the Combined Gas Law, whereby the increase in volume results in a drop of temperature and static pressure. • At point D the gas exits the engine at having been expanded to ambient pressure but now at a greater volume. Module 15B ETBN 0502 August 2022 Edition 11 15.1 Fundamentals British Airways Global Learning Academy – Gas Turbine Engine Energy – is the ability to do work. Energy cannot be created or destroyed, only change its state. Converting energy from one state to another is what gas turbine engines do Force, Work, Power, Energy, Velocity and Acceleration Force – This is the energy required to change the motion of a body or, with gas turbines, a stream of gas. The motion to change can be speed and/or direction. (Newton’s second law). A speed in a given direction is referred to as velocity and a change in velocity is acceleration. Therefore, the force required to give a given mass an acceleration is calculated by These are some examples of energy conversions happening as the engine moves through the air: • The intake converts air’s kinetic energy (its movement) into static pressure and temperature energy 𝐹 = 𝑚𝑎 • The compressor converts the mechanical energy of rotation from the turbine in static pressure and heat and the units are Newtons (N). Any change in direction or speed needs energy to make it happen. • The combustion section converts potential chemical energy (fuel) into heat, pressure and kinetic energy in the gases Work – if the force moves the body over a certain distance, work has been performed. The amount of work can be calculated by: • The turbine converts heat, pressure and kinetic energy into mechanical to drive the compressor 𝑊𝑜𝑟𝑘 𝑑𝑜𝑛𝑒 = 𝑓𝑜𝑟𝑐𝑒 × 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 and the units of work are Joules (J). • And the propelling nozzle converts heat and static pressure into kinetic energy for thrust Power – the amount of work done is not time dependent. The rate at which that work is done is referred to as power. To accelerate an aircraft at a greater rate requires more power from the engines. As a formula: 𝑃𝑜𝑤𝑒𝑟 = Velocity – is the rate of change of distance with time. Simply, how fast something is moving, its speed - BUT, velocity also has a direction of travel. Change the direction, even if the speed is the same, and the velocity has changed. 𝑊𝑜𝑟𝑘 𝑑𝑜𝑛𝑒 𝑇𝑖𝑚𝑒 𝑡𝑎𝑘𝑒𝑛 Acceleration – is the rate of change of velocity. If the speed has increased, there has been an acceleration. If the speed stays the same but the direction changes, there has been an acceleration. If the speed and direction have changed – there’s been an acceleration. And that relates back to the Force needed to make the accelerations happen. and the units of work are Watts (W). Module 15B ETBN 0502 August 2022 Edition 12 15.1 Fundamentals British Airways Global Learning Academy – Gas Turbine Engine Gas Turbine Construction The Gas Generator The gas generator is the name given to the combination of the compressor, combustion chamber and turbine, its purpose being to produce a gas stream for energy to be extracted from as either mechanical energy or thrust. A modern gas turbine engine is modular in design. Rather than building the engine as one unit, there are 5 main modules involved. They are Intake – considered part of the airframe, it is crucial to efficient The mechanical energy will be extracted by a turbine that might be used to drive a fan, propeller or gearbox. operation of the engine. It has to slow the incoming air (during most flight operations) to the correct speed for the compressor and in so doing boost its static pressure. It will also present the air to the compressor correctly with minimal losses. The thrust will be produced by the gas exiting the engine rearwards, causing the engine to react forwards. Compressor – its purpose is to raise the static pressure and The Spool temperature prior to the combustion chamber. It will either be axial or centrifugal in design. The term ‘spool’ is given to a compressor and turbine being connected by a common shaft. It is possible for an engine to have more than one spool, each running independently of the other(s). Two spools is typical on many engines; three is available on a few, typically from one particular manufacturer. Combustion chamber – this is where the fuel is mixed with the air from the compressor and combusted. This is done at (theoretically) constant pressure, adding heat and volume to the gases. There are several designs of combustion chamber. Turbine – used to extract energy from the gases after the combustion chamber, to drive the compressor and other services. Usually in the form of an axial flow turbine. Exhaust and propelling nozzle – used to exit the gases from the engine and away from airframe components. For thrust engines, there will be a form of propelling nozzle to convert as much energy remaining in the exhaust gases into thrust. Each module will have its own maintenance records and can be swapped with different modules to make an engine, although this is very much an overhaul job! These modules will be explained in greater depth in the relevant chapters. Module 15B ETBN 0502 August 2022 Edition 13 15.1 Fundamentals British Airways Global Learning Academy – Gas Turbine Engine Gas Turbine Variants Turbojet The variants of gas turbine engines are grouped as follows:- A turbojet is an earlier type of gas turbine although still in service around the world today. The diagram shows the basic layout where the air is drawn into the intake and then presented to the compressor (a centrifugal one in this diagram, but that’s not really important). • Turbojet • Turbofan From the compressor, all the air passes via the combustion section – the main definition of a turbojet - , through the turbine and into the exhaust. Here, the propelling nozzle achieves the maximum acceleration to the air as the exhaust air is the only source of thrust. • Turboshaft • Turboprop Due to the high exhaust speeds, these engines are typically much noisier than other variants with similar thrust. Module 15B ETBN 0502 August 2022 Edition 14 15.1 Fundamentals British Airways Global Learning Academy – Gas Turbine Engine Turbofan The turbofan engine differs from the turbojet in that not all the air that enters the intake will go through the combustion section. Some air will by-pass the combustion section, the amount determining whether the engine is deemed a low- or high-bypass engine. As a result, this class of engine is usually referred to as bypass engine. The bypass air tends to be a higher mass of air which receives a lower acceleration from the compressor/fan. This tends to make the engine more fuel efficient and quieter. The explanation from this will be in following chapters. The bypass air is sometimes referred to as cold stream air. The air that passes via the combustion chamber, known as core air, or hot stream air, follows much the same journey as it would in a turbojet. In this diagram there is a triple spool axial flow compressor. The working of this will be explained in later chapters. . Module 15B ETBN 0502 August 2022 Edition 15 15.1 Fundamentals British Airways Global Learning Academy – Gas Turbine Engine turbofan does except that the turbine will extract most of the energy from the exhaust gases to drive the gearbox. (Here, the Gearbox is powering a generator and there is a Load Compressor for pneumatics, not found on thrust engines). Turboshaft Turboshaft engines are not thrust engines – instead, they are used to drive a gearbox which is then used to power the required services. This could be plane (helicopters, typically), trains or automobiles (some main battle tanks have such engines). The exhaust of the turboshaft produces little or no thrust and just directs the gases safely away from the installation. The most common location for a turboshaft to be seen is the Auxiliary Power Unit on an aircraft (explained later). The diagram shows a typical layout with the gas turbine (Power) section working as a gas generator much as the turbojet or core of the Module 15B ETBN 0502 August 2022 Edition 16 15.1 Fundamentals British Airways Global Learning Academy – Gas Turbine Engine Turboprop A turboprop develops its thrust via a propeller driven via a gearbox. In that way, it is similar to the turboshaft With a turboprop engine the objective is to convert the power (known as shaft horsepower) into rotation to drive a propeller through a reduction gearbox. The turbine stages of the turboprop engine absorb most of the gas energy to drive the propeller, leaving only a small amount (approx. 10%) of thrust at the propelling nozzle. The method of generating the required gas is the same as other gas turbine engines, with air passing through a compressor and combustion chamber before reaching the turbine. Because the propeller wastes less kinetic energy (see the Performance chapter) in its slipstream than a turbojet through its exhaust, it is subsequently more efficient at low and medium altitudes with speeds up to 350 kt There are different methods of driving the gearbox which will be covered in later chapters. Module 15B ETBN 0502 August 2022 Edition 17 15.1 Fundamentals