Gas Turbine Operation - Coursework Example

?Contents Gas Turbine Design 1. The Turbofan 1 Gas Turbine operation …………………………………………….... 1 2. Design issues of a turbofan ……………………………………………… 2 2.1 Turbofan Aerodynamics ………………………………………………… 3 2.1.1 Intake ……………………………………………………………... 3 2.1.2 Compressor ………………………………………………………. 3 2.1.3 Combustion chamber ……………………………………………. 4 2.1.4 Turbine …………………………………………………………… 4 2.1.5 Exhaust nozzle …………………………………………………… 4 2.2 Turbofan Theory ………………………………………………………… 5 2.2.1 Fluid mechanics ………………………………………………….. 5 2.2.2 Thermodynamics ………………………………………………… 6 Works Cited ……………………………………………………………………… 7 Gas Turbine Design One of the technological leaps in the history of science that led us to a wide spectrum of newer prospects of energy production and transportation was that of the gas turbines engines or what we simply call gas turbines. Today, they have developed into more efficient and dependable machines and have surpassed other types of mechanical prime movers, such as internal combustion engines, both in efficiency and mechanical robustness. The aim of this report is to explore the design ideas and scientific and thermodynamic principles that have helped us develop these valuable machines. Our study will revolve around the design of a turbofan that is a type of gas turbine engine commonly used in airplanes and energy production. 1. The Turbofan Turbofan is a type of gas turbine engine that itself is a common type of air-breathing engine. In order to understand its design, its construction and the function of its important components should be understood. The important parts of a turbofan are as follows: 1. Inlet 2. Compressor 3. Combustor 4. Turbine 5. Exhaust nozzle Figure 1 shows the location of each of these components in a turbofan. To understand the importance and function of every component mentioned above, we will briefly describe the working principle of a turbofan. 1.1 Gas Turbine Operation The operation of an air breathing engine can be understood by looking at the path of the air through the engine and the temperature and pressure variations that it undergoes. Figure 1: Turbofan (2-spool) schematic. (Turbofan) Air from the atmosphere is received by the engine through its inlet and is divided into bypass stream and the engine core stream. The fan along with the compressor increases the pressure of the air in order to enable a greater air mass flow rate through the engine core, which in turn in required for efficient fuel combustion and the production of sufficient thrust. The compressed air from the compressor is brought into the combustor – or the combustion chamber – where a fine stream of fuel is added into the compressed air and ignited using a flame torch setup. The temperature of the air rises substantially during combustion and as it tends to expand, it is directed towards the turbine which uses the energy of this very hot air to drive the compressor. The rest of the energy gained by the air makes it expel the exhaust nozzle at a high speed and provides the thrust required to accelerate the plane. 2. Design issues of the turbofan Having looked into the operation and role of every component in it, we can classify the design problems of a turbofan into two categories: 1. Aerodynamics 2. Theory or Science Now we will delve deeper into these modes of design separately and identify the design complexities and the solutions that have been proposed to them. 2.1 Turbofan Aerodynamics To have a sketch of the overall aerodynamic design of the turbofan, we will explain the aerodynamic role and design preferences of the individual components first. 2.1.1 Intake Intake is primarily responsible for channeling the air from the atmosphere to the fan of the engine at a high pressure (Gordon). In addition to having an aerodynamically efficient front profile, its shape has to be appropriate for required air mass flow required by the engine as well as for required fluid velocity translation to a lesser magnitude. Air enters the inlet at a high speed and is slightly slowed down to assist the fan in increasing the pressure. This also increases the pressure and is achieved by using a divergent intake duct (Jet Propulsion). Depending on the bypass ratio, the air flow capacity of the intake is decided. In abnormal flying conditions, for example when an engine fails, the air mass flow rate required is greater. Intake should be able to handle high speeds of air in such situations to fulfill the thrust needs. 2.1.2 Compressor The compressor of the turbofan is, aerodynamically, the most important and complex component. It consists of alternate rotating airfoils and the stator blades. The air is accelerated in the moving airfoils as it is pushed ahead and again slowed down in the stator blades (Figure 2). This results in an increase in the pressure of the air when it reaches the combustor. Usually the compressor has more than one stage to accommodate different ranges of speeds and pressures, e.g. low pressure and high pressure sections as shown in Figure 1. Moreover, the cross-section of the passage is made to gradually decrease, so as to compensate for the velocity decrease that occurs due to pressure increase. This is required to maintain maximum thrust, because air flow rate is both dependent on the velocity as well as the pressure of the air. Major design constraints for an axial compressor, which is the most common type used in a turbofan, are stall condition and compressor surge. These refer to instable aerodynamic conditions when the air flow is disturbed in a way that leads to very less compression. This may result in severe engine power drop. Figure 2: Axial compressor. 2.1.3 Combustion chamber Aerodynamics of the combustion chamber is a very interesting and important part of the overall engine design. Generally the aerodynamic role of the combustor starts with the diffuser that slows down the air and translates the kinetic energy of the air to an optimum pressure. Air then mixes up with the fuel injected into the combustor and ignites it in the presence of a flame. This is where it tends to expand after getting very hot and the reverse aerodynamic cycle of the engine starts. 2.1.4 Turbine Aerodynamically, turbine is the opposite of the compressor and is usually constructed as multi-stage to drive a similar compressor. The air that expands after ignition in the combustor turns the turbine and accelerates as its pressure drops. As the air enters the turbine, it is very hot and at a very high pressure. The pressure drops as the turbine blades turn this potential energy to kinetic energy and the air accelerates to accommodate the air mass flow rate being maintained by the compressor. 2.1.5 Exhaust nozzle After passing through the turbine, the hot air enters the exhaust nozzle of the engine. The exhaust is not only an end to the engine cycle but also a power delivery point for the flight vehicle. Air is further accelerated using a converging duct at the exhaust and provides a part of the thrust that powers the plane. As we that the momentum is the product of mass and velocity, the more the mass and the velocity of the air that flows through the engine, the more momentum it has. This results in a greater thrust as dictated by the law of conservation of momentum. To summarize, in the aerodynamic cycle, the air is compressed by the inlet and the compressor section and as a result its temperature increases. In the combustor it further heats up and achieves even greater pressure. It then expands in the turbine and cools down as well as accelerates as it passes through the exhaust nozzle to produce thrust and drive the compressor. 2.2 Turbofan Science The gas turbine engine, like every mechanical device, obeys a set of scientific laws and several principles of thermodynamics. To properly understand the design issues of turbofan, the relevant gas laws and theory of fluid mechanics should be known. In this regard, we can divide our discussion into two main parts: 1. Fluid mechanics 2. Thermodynamics Fluid mechanics encompasses the theory of fluid flow through the engine from the point of intake to exhaust and the related thermodynamics explains what gas laws are relevant to the processes that take place in the different components of the engine. 2.2.1 Fluid mechanics Working of a turbofan is closely related to continuity equation and the Bernoulli’s principle. The assumption that fluid flow remains in the subsonic domain invokes these two concepts. The equation of continuity is an instance of the law of conservation of mass for flowing fluids. In simple terms, it can be written as (Halliday): A1v1 = A2v2, (1) where A1 and A2 are the cross-sections at two different locations of the duct and v1 and v2 are the corresponding speeds of fluid flow. This equation means that when the cross section decreases, the speed of flow increases so as to keep the rate of flow constant. This principle is harnessed in inlet and exhaust nozzle of the turbofan, i.e. to increase the speed of the gas, all we need is to decrease the cross-section (or in other words, use a converging duct). Bernoulli’s principle is in fact the law of conservation of energy for fluids. It relates the pressure of the fluid to its velocity and height; generally, the faster the gas flows, the lesser the pressure it exerts. This can be understood as the conversion of kinetic to potential energy and vice versa. Mathematically (Halliday): P + ? (?v2) + ?gh = constant, (2) where P is the pressure, v is the speed and h is the height above the ground. When height is not the point of concern, the equation for two locations of different speed of flow becomes: P1 + ? (??v12) = P2 + ? (??v22) (3) Equation (3) indicates that where the speed will increase, the pressure will decrease. This principle governs the lift of the plane as well as the compression process. In fact, compression in an axial compressor is done by decreasing the speed of flow of the fluid. 2.2.2 Thermodynamics The fundamental gas laws, i.e. Boyle’s law (P V = constant), Charles’s law (V = k T) and Gay-Lussac’s law (P = k’ T) form the basics of turbofan thermodynamics. When the air is compressed in the compressor, its temperature rises. The reverse happens in the turbine section after the gas expands due to its very high temperature. Moreover the ideal behavior of an engine is described under certain conditions that define thermodynamic processes. In the context of turbofan, we treat the whole process – from intake to exhaust – as an isentropic process: the entropy of the system remains constant. This is because of its reversible adiabatic nature, and such a process is isentropic according to the second law of thermodynamics. Works Cited Gordon, C. Oates. Aerothermodynamics of Gas Turbine and Rocket Propulsion, 3rd ed. Virginia: American Institute of Aeronautics and Astronautics Inc. 1997. Print. Halliday, David, Robert Resnick, and Kenneth S. Krane. Physics v. 1. New York: John Wiley & Sons. 2001. Print. "Jet Propulsion." Wikipedia, The Free Encyclopedia. Wikimedia Foundation, Inc. 22 July 2004. Web. 19 May 2011. "Turbofan." Wikipedia, The Free Encyclopedia. Wikimedia Foundation, Inc. 22 July 2004. Web. 19 May 2011. Yin, J. and P. Pilidis. “Influence of inlet profile on high-BPR turbofan performance using a radial profile map”. 23rd Congress of International Council of Aeronautical Sciences, Toronto. 2002. Read More