Aerodynamics and Propulsion principles - Coursework Example

?Table of Contents The Turbofan 2. Turbofan Aerodynamics …………………………………………………. 3 2 Intake ………………………………………………………………... 3 2.2 Compressor …………………………………………………………. 3 2.3 Combustion chamber ………………………………………………. 4 2.4 Turbine ……………………………………………………………… 4 2.5 Exhaust nozzle ……………………………………………………… 4 2.6 Overall design issues of turbofan ………………………………….. 4 3. Engine Science ……………………………………………………………. 5 3.1 Continuity equation and mass flow of air ……….………………... 5 3.2 Compressor and Turbine theory ………………………………….. 6 3.3 Combustion chamber thermodynamics …………………………... 6 Works Cited ………………………………………………………………………. 7 Aerodynamics and Propulsion principles 1. The Turbofan Turbofan is the most ubiquitous type of air-breathing engine and refers to a gas turbine engine that is designed to work for subsonic speeds. It is a unique type of internal combustion engine which does not have any reciprocating parts, such as the car engine. To understand the operation of a turbofan, first we need to look into its construction and the function of individual components. Figure 1 describes the arrangement of the components of a turbofan and indicates the direction of normal air flow in the engine. All the 5 components: intake, compressor, combustion chamber, turbine and the exhaust nozzle have their unique role to play in the working of a turbofan and contribute in providing the necessary thermodynamic and aerodynamic requirements of the overall system. By understanding the individual function of these components, we can determine the performance measures and the complexities that have to be considered in the design of a turbo fan. Before we proceed to the operation of a gas turbine, we will describe the aerodynamic role of individual parts. Figure 1: Turbofan (2-spool) schematic. (Aircraft) 2. Aerodynamics of a Turbofan In this section, we will describe the aerodynamic behavior of the engine components that were mentioned in section 1 in the same order as the air encounters them in a turbofan. 3.1 Intake Intake guides the air from the atmosphere to the fan of the engine and assists the engine by increasing the pressure before the air is sucked in by the engine (Gordon). It has an aerodynamic design to minimize the drag and is basically a nozzle with increasing cross section (Jet Propulsion) that results in a higher pressure at the fan. Moreover, its front should not generate any turbulence in the flow of air as it can cause severe conditions inside the engine and lift may dangerously drop. Intake has to accommodate both the bypass and engine streams of air so that no considerable pressure gradients result at the face of the fan. For faulty conditions, intake may have to provide a larger mass of air than normal requirement of an engine and so has to have a reasonable choke limit. 3.2 Fan and Compressor Compressor is the first component of considerable aerodynamic complexity that the air meets in its way to the exhaust. A compressor is made up of several fan-like moving blades called airfoils alternately arranged with the stators are the stationary blades (Figure 2). Figure 2: Axial compressor. (Turbine Engines) The airfoils blow the air ahead and stators decelerate it, thus increasing the pressure with successive sections. There are usually two separate parts of a compressor: the low pressure and the high pressure compressor. This division is necessary because as the pressure increases, the speed of rotating airfoils has to increase. To maintain the air mass flow rate which depends both on the speed of flow and pressure, compressor section is enclosed in a nozzle with a decreasing cross section. Usually compressors are designed to operate at speeds considerably greater than the normal operating speeds to prevent any compressor stall and resulting surge conditions which are a result of turbulence that may occur at high speeds. 3.3 Combustion chamber Although the combustion chamber has a little aerodynamics role to play, it has some interesting fluid flow requirements. The diffuser that directs the air from the compressor to the combustor slows it down for efficient fuel combustion. Still it has to maintain a flow speed that keeps the flame sufficiently straight and prevents any damage to the combustor casing. A major change in the whole cycle of air occurs when it ignites the fuel and gets very hot and extremely compressed. 3.4 Turbine While the compressor converts the kinetic energy (velocity) of the gas into potential energy (pressure), turbine the does the opposite (Figure 3). The construction of the turbine is also similar to the compressor except that its blades are in the opposite orientation and are driven by the flow of air. Turbine also has a low and a high pressure section which are connected to the corresponding sections of the compressor. The air from the combustor loses its potential energy against the blades of the turbine and rotates it, in turn driving the compressor. 3.5 Jet nozzle The hot and highly compressed air from the combustion chamber is already at a relatively low pressure when it reaches the exhaust or the jet nozzle. Here the air is further accelerated to reduce its pressure close to the ambient pressure, as it provides better aerodynamic matching with the atmosphere. As the air leaves the exhaust at a high speed, it provides a forward thrust to the flight vehicle. According the law of conservation of momentum, the more the mass flow rate, the more will be the thrust. To cope with abnormal flying conditions, jet nozzle is also designed to operate below its choke mode. The previous section explained the aerodynamic properties of different components of a gas turbine. In the next section we will try to understand the scientific principles that govern the operation of a turbofan. 3. Overall Turbofan design issues In the overall performance of a jet engine, the most important requirement is the matching of the engine to the external air. Matching means that no loading conditions arise when the air is sucked in through the intake and exhausted. This is the subject of intake and exhaust aerodynamics, and depending on the capacity of the engine, they have to efficiently translate the ambient pressure to required pressure at the face of the fan and from the extremely high pressure after the combustor again to the atmospheric pressure to maintain a streamlined flow of the fluid. Another major design complication is about the high pressure turbine that has to face extremely high temperatures and pressure at the same time. Materials scientists are working on manufacturing high strength materials that could withstand extreme temperature and centrifugal stresses at such high speeds of rotation. 4. Engine Science Every machine is governed by a set of scientific laws or principles that decide the performance and design of the machine. Looking at the operation and construction of a gas turbine, we can divide its theory into three parts: 1. Continuity equation and mass flow of air 2. Compressor and Turbine theory 3. Combustion chamber thermodynamics We will briefly describe the relevance of these sections to a turbofan. 4.1 Continuity equation and the mass flow of air The equation of continuity states of the law of conservation of mass for fluids. Mathematically, for a duct with a cross section of A1 and A2 at two different positions, the respective speed of flow, v1 and v2, are related as follows: A1v1 = A2v2, (1) Physically this means that for smaller cross section the speed of flow increases. This is the principle behind the operation of nozzles and is the basic principle by which the intake decreases the speed of the air at the face of the fan. As described earlier, the intake has to slow down the air; this is achieved by increasing the cross section of the inlet nozzle, so that the total mass flow of the air remains constant. To increase the mass flow of air, the intake has to have a greater front cross-section. 4.2 Compressor and Turbine theory The compressor operation puts into practice both the thermodynamic and fluid flow principles. Gas laws, such as Boyle’s law (P V = constant), Charles’s law (V = k T) and Gay-Lussac’s law (P = k’ T) govern the fundamental thermodynamics whereas the Bernoulli’s principle dictates how and in what proportion the pressure of the air can be increased. The compression process is achieved in fact by slowing down and accelerating the air at alternate stages, which results in the conversion of its kinetic energy into potential energy. Also, as the pressure of the air increases, its temperature does so as well as dictated by the fundamental gas laws. At this point we need a reference to the Bernoulli’s equation to understand the qualitative and quantitative relationship between the speed of flow and the pressure exerted by the air. For fluids, Bernoulli’s equation defines a relationship between the pressure of the fluid and its velocity. Slowing down the fluid will cause its pressure to increase. Mathematically (Halliday): P + ? (?v2) + ?gh = constant, (2) where P is the pressure exerted by the fluid, v is its speed and h is its height above the ground. When the system is horizontal, such as a turbofan, Bernoulli’s equation for two points with different fluid velocity can be written as: P1 + ? (??v12) = P2 + ? (??v22) (3) This equation is directly related to the concept of lift as well. So the design of a compressor, turbine and a fan has to obey this relationship. A turbine is only a reversed compressor except the fact that it has to endure several thousand degrees of temperature and has to revolve at much greater speeds. 4.3 Combustion chamber thermodynamics Combustion chamber is the part of the engine that has a most prominent thermodynamic role to play in the overall function of a gas turbine engine. Due to combustion the temperature of the air rises substantially in this part of the engine and due to that, the pressure also increases greatly. This pressure in turn is used to do work against the blades of the turbine and rotate it to produce a kinetic energy output. This work done by the compressed gas can be understood as an instance of inter-conversion of potential and kinetic energy. As the temperature rises, due to confined volume only the potential energy or the pressure of the air can increase. Turbine is a device that converts this potential energy into rotational kinetic energy and drives the compressor as well as the bypass stream which is a major contributor to the thrust provided to the flight vehicle. Works Cited "Aircraft." Quest for Performance: The Evolution of Modern Aircraft. Ch. 10. Web. 19 May 2011. 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. “Turbine Engines”. Free Online Private Pilot Ground School. Web. 21 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