Thermodynamics

Discussion For those who are wondering what happened to the rest of theenergy, here is a brief account:KE # out # V 2 g 8502ft>s42m 1100 lbm>s2e313288 fa 1 Btu>lbm2 225,037 ft 2 >s b 2 11,867 Btu>s132.2% 2·Q # out m # 1h 6 h 1 2 m # Q inc p 1T 6 T 1 2 1100 lbm>s2 10.24 Btu>lbm # R2311114 4202 R4AIRCRAFT 16,651 Btu>s145.3% 2Thus, 32.2 percent of the energy shows up as excess kinetic energy (kineticenergy of the gases relative to a fixed point on the ground). Notice that forthe highest propulsion efficiency, the velocity of the exhaust gases relative tothe ground V g should be zero. That is, the exhaust gases should leave thenozzle at the velocity of the aircraft. The remaining 45.3 percent of theenergy shows up as an increase in enthalpy of the gases leaving the engine.These last two forms of energy eventually become part of the internal energyof the atmospheric air (Fig. 9–51).Modifications to Turbojet EnginesThe first airplanes built were all propeller-driven, with propellers poweredby engines essentially identical to automobile engines. The major breakthroughin commercial aviation occurred with the introduction of the turbojetengine in 1952. Both propeller-driven engines and jet-propulsion-drivenengines have their own strengths and limitations, and several attempts havebeen made to combine the desirable characteristics of both in one engine.Two such modifications are the propjet engine and the turbofan engine.The most widely used engine in aircraft propulsion is the turbofan (orfanjet) engine wherein a large fan driven by the turbine forces a considerableamount of air through a duct (cowl) surrounding the engine, as shownin Figs. 9–52 and 9–53. The fan exhaust leaves the duct at a higher velocity,enhancing the total thrust of the engine significantly. A turbofan engineis based on the principle that for the same power, a large volume of slowermovingair produces more thrust than a small volume of fast-moving air.The first commercial turbofan engine was successfully tested in 1955.Chapter 9 | 525(propulsive power)W·P·KE out(excess kinetic energy)Q· out(excess thermal energy)FIGURE 9–51Energy supplied to an aircraft (fromthe burning of a fuel) manifests itselfin various forms.Low-pressurecompressorFan Duct BurnersLow-pressure turbineFan exhaustTurbine exhaustFIGURE 9–52A turbofan engine.FanHigh-pressurecompressorHigh-pressure turbineSource: The Aircraft Gas Turbine and ItsOperation. © United Aircraft Corporation (nowUnited Technologies Corp.), 1951, 1974.