How jet engines produce thurst

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A description of how thrust and propulsion is achieved in jet engines

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How jet engines produce thurst

  1. 1. How Jet Engines Work and Produce Thrust Table of Contents2.0 Basic Components of a Jet Engine.......................................................................................3 2.2 Compressor.....................................................................................................................3 2.3 Combustor......................................................................................................................4 2.4 Turbine............................................................................................................................4 2.5 Exhaust Duct/Nozzle.......................................................................................................44.0 Creation of Thrust in a Turbojet Engine..............................................................................75.0 Conclusion...........................................................................................................................9 6.0 References......................................................................................................................9
  2. 2. Jet Engine Propulsion and Thrust 21.0 Introduction According to Hunecke (1997), jet engines, also known as gas turbine engines, arethe most widespread and most efficient method used for airplane propulsion currently.The Jet engine uses basic principles and concepts of motion but applying it using acombination of complex mechanical systems to achieve thrust. There are many typesof jet engines; however, this paper will concentrate on the Turbojet Engine to explainthe workings of the jet engine to achieve thrust and propulsion.1.1 How the turbojet Engine Works Turbojet Engines apply Newton’s Third Law of Motion that states, “For everymotion there is an equal and opposite reaction” (Hünecke, 1997, p. 4). Simply, when aburnt mixture is ejected backwards from an engine, a forward force is generated onthe engine and thus on the aircraft. The bigger the backward force the bigger theforward force (reaction force). Thrust is created when the burnt mixture pushed outthe back is ejected at higher velocity than that of the air being sucked in. (Hünecke,1997, p. 4)The engine’s fans suck air in at the front. A compressor, made up of fans with manyblades and attached to the shaft, elevates the pressure of the air. The compressed air isthen sprayed with fuel and an electric spark ignites the mixture. The burning gasesexpand and blast out through the nozzle, at the back of the engine. As the jets of gasshoot backward, the engine and the aircraft are thrust forward.
  3. 3. Jet Engine Propulsion and Thrust 32.0 Basic Components of a Jet Engine2.1 Air Intake/Inlet The air intake acts as a fluid flow duct, which directs the airflow to ensure thatthe engine functions correctly to generate thrust (Hunecke, n.d., p. 44). The intake hasto be designed to deliver the needed quantity of airflow to the engine and ensure thatairflow entering the compressor is stable and uniform. All these conditions have to bemet when the aircraft is on the ground and during flight. A good intake design isrequired to ensure that engine performance is close to figures obtained duringstandard testing. The Stators (stationary blades) guide the airflow of the compressedgases.2.2 Compressor The function of the compressor is to increase the pressure of the airflow thatcomes from the air intake. Mechanical energy is supplied to the compressor viarotating blades that exert aerodynamic forces on the airflow (Hunecke, n.d., p. 86).This not only adds energy to the airflow but at the same time squeezes (compresses) itinto a smaller space. The compressor is driven by the turbine. Important compressorperformance parameters include compressor efficiency, compressor total pressureratio and the airflow rate. These parameters influence the amount of energy requiredand the quality of energy conversion that is achieved (Hunecke, n.d., p.86).Compressor types used can be axial-flow, centrifugal-flow, axial-centrifugal-flow,double-centrifugal-flow compressors.
  4. 4. Jet Engine Propulsion and Thrust 42.3 Combustor The combustor provides a stream of hot gas that provides energy to the turbineand nozzle components of the engine. Heat is added by burning a mixture ofcompressed air and vaporized fuel (Hunecke, n.d., p. 125) Minimal loss of pressure isrequired in the combustion chamber during combustion. Types of combustors usedinclude can-type burners, annual-type burners and can-annular type burners.2.4 Turbine The turbine is used to drive the compressor by providing aerodynamic forces.A high turbine power is obtained by extracting all the energy present in the hot gas.On its own, a distinct turbine blade contributes approximately 250 hp (Hunecke, n.d.,p. 6)2.5 Exhaust Duct/Nozzle The function of the exhaust nozzle is to transfer gas potential energy intokinetic energy required to generate thrust (Hunecke, n.d., p. 155)
  5. 5. Jet Engine Propulsion and Thrust 5Figure 1: Parts of a Turbojet EngineHünecke, K. (1997). Jet engines: fundamentals of theory, design, and operation. WI, USA:Motorbooks International, p.4.3.0 The Process of Thrust and Propulsion At the intake, air is sucked from the compressor, where there a series of blades orairfoils. The Rotors (rotating blades), draw in the air and compress it at the same time,while the stators (stationary blades), guide the air through the compression chamber.As the air moves through the rows of rotors and rotors, pressure rises to as much as 40times, consequently the temperature rises. Most modern turbojet compressors haveoverall pressure ratios of 44:1. (Mattingly, 2002, p. 20) The compressed air is then pressed toward the back into the combustion chamber,where fuel injectors spray fuel to create a mixture. This mixture of pressurized air andfuel is then ignited. To control burn and flow, the fuel-air mixture must be brought
  6. 6. Jet Engine Propulsion and Thrust 6almost to a stop to ensure a steady, continuous and stable flame. This occurs at thevery beginning of the combustion chamber. The aft part of this flame front is allowedto advance toward the back. This guarantees complete combustion of the fuel as theflame becomes hotter when it leans out, and because of the shape of the combustionchamber the flow is accelerated rearwards. Some pressure slump is necessary, as it isthe reason why the expanding gases move out the rear of the engine rather than outthe front. Less than 25% of the air is involved in combustion, in some engines as littleas 12%, the rest acting as a reservoir to absorb the heating effects of the burning fuel.(Hünecke, K., 1997, p.4) The high-energy airflow coming exiting the combustion chamber goes into theturbine, causing the turbine blades to rotate. The turbines are linked by shafts (whichhave several ball-bearings in between) to turn the blades in the compressor and to spinthe intake fan at the front. This rotation reduces the energy. From the combustionchamber, the gases produced move through the turbine and spin its blades. After the turbine, the gases expand through the exhaust nozzle to atmosphericpressure, generating a high velocity jet in the exhaust plume. The nozzle pressureratio on a turbojet is generally high enough for the expanding gases to reach Mach 1.0and choke the throat. Usually, the flow will go supersonic in the exhaust plumeoutside the engine. (Mattingly, 2002, p. 12)
  7. 7. Jet Engine Propulsion and Thrust 74.0 Creation of Thrust in a Turbojet Engine The creation of thrust in a jet engine is based on Newton’s third law of motionapplied to a steady flow of air (Hünecke, K., 1997, p.32)The momentum of air leavingthe engine is supposed to be higher than the momentum of the air inflowing theengine. The outcome is a high kinetic energy for the jet. The high-energy input of thejet comes from burning of the fuel. According to Hunecke (1997), propulsion in a jetaircraft occurs based on the principle of reaction. This principle states that a gas jetexhausting at elevated velocity from a nozzle generates a force in the reverse directionthat is referred to as thrust (Hunecke, 1997, p.32). The thrust force will depend on theairflow passing through the jet engine and the exhaust velocity. The jet engineincreases the momentum of the airstream flowing through it.4.1 Momentum change Considering an engine on a pylon under a wing as shown below, the onlyforce applied is through the pylon. (See diagram 2 below)Figure 2: A High Bypass ratio engine installed under a wing (Cumpsty, 2003, p. 25).
  8. 8. Jet Engine Propulsion and Thrust 8 Assumptions made include the wing lift and drag being unaffected by theengine and the engine being unaffected by the wing. Additionally, it is assumed thatthere is uniform static pressure around the control surface (Cumpsty, 2003, p. 26). Aflow of fuel mf flows through the pylon, but with a low velocity, thus moment isinsignificant. Consequently, a mass flow of air mair enters the engine. For a bypassengine, the velocity differences between the inner engine and the bypass streams haveto mix out. Thrust will be calculated by taking into account the flux of momentumacross the control surface around the engine. The air enters the control surface with avelocity V. Air entering the control surface passes along the engine with only a smallportion, mair passing through the engine. According to Crumpsty (2003), the air thatpasses around the engine exits the control surface with the same velocity V as theflight speed; thus it does not contribute to thrust.Therefore, the flux of the momentum entering the engine is given by the equationbelowIn addition, the flux of momentum leaving the engine is given by the equation belowThe net thrust FN is given by the difference between the two fluxes Increase in kinetic energy causes an increase in velocity between the flow ofair entering the engine and that of the air leaving the engine (Cumpsty, 2003, p. 25).
  9. 9. Jet Engine Propulsion and Thrust 9The kinetic energy results from the effect of the work supplied by the engine to the airvia combustion.5.0 Conclusion This report provides an explanation of the basic components of a jet engineand the workings of each component to produce thrust that is used for propulsion.Airflow is directed to the compressor via the air intake. The compressor increases thepressure of the airflow before releasing it to the combustor. The combustor increasesthe energy of the airflow through burning using fuel. The turbine provides energy tothe compressor, which is used to increase pressure of the gas. The exhaust nozzleconverts the hot gas energy to kinetic energy, which generates thrust.6.0 ReferencesCumpsty, N. A., 2003. Jet Propulsion: A Simple Guide to the Aerodynamics and Thermodynamic Design and Performance of Jet Engines. Cambridge: Cambridge University Press. p. 25-26.Flack, R. D., 2005. Fundamentals of Jet Propulsion with Applications. New York: Cambridge University Press.
  10. 10. Jet Engine Propulsion and Thrust 10Hünecke, K., 1997. Jet engines: fundamentals of theory, design, and operation. WI: Motorbooks Internation. p 4-155.Jay, S., 1972. The Jet Engine. London: Coller Nacmillan.Mattingly, J. D., 2002. Aircraft Engine Design. New York: VA: American Institute of Aeronautics and Astronautics. p. 12-20.

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