Thermodynamics

Chapter 9 | 527Fuel nozzles or spray barsAir inletFlame holdersJet nozzleFIGURE 9–55A ramjet engine.Source: The Aircraft Gas Turbine Engine and ItsOperation. © United Aircraft Corporation (nowUnited Technologies Corp.), 1951, 1974.are more efficient than jet engines, but they are limited to low-speed andlow-altitude operation since their efficiency decreases at high speeds and altitudes.The old propjet engines (turboprops) were limited to speeds of aboutMach 0.62 and to altitudes of around 9100 m. The new propjet engines( propfans) are expected to achieve speeds of about Mach 0.82 and altitudesof about 12,200 m. Commercial airplanes of medium size and range propelledby propfans are expected to fly as high and as fast as the planes propelledby turbofans, and to do so on less fuel.Another modification that is popular in military aircraft is the addition ofan afterburner section between the turbine and the nozzle. Whenever aneed for extra thrust arises, such as for short takeoffs or combat conditions,additional fuel is injected into the oxygen-rich combustion gases leaving theturbine. As a result of this added energy, the exhaust gases leave at a highervelocity, providing a greater thrust.A ramjet engine is a properly shaped duct with no compressor or turbine,as shown in Fig. 9–55, and is sometimes used for high-speed propulsion ofmissiles and aircraft. The pressure rise in the engine is provided by the rameffect of the incoming high-speed air being rammed against a barrier. Therefore,a ramjet engine needs to be brought to a sufficiently high speed by anexternal source before it can be fired.The ramjet performs best in aircraft flying above Mach 2 or 3 (two orthree times the speed of sound). In a ramjet, the air is slowed down to aboutMach 0.2, fuel is added to the air and burned at this low velocity, and thecombustion gases are expended and accelerated in a nozzle.A scramjet engine is essentially a ramjet in which air flows through atsupersonic speeds (above the speed of sound). Ramjets that convert toscramjet configurations at speeds above Mach 6 are successfully tested atspeeds of about Mach 8.Finally, a rocket is a device where a solid or liquid fuel and an oxidizerreact in the combustion chamber. The high-pressure combustion gases arethen expanded in a nozzle. The gases leave the rocket at very high velocities,producing the thrust to propel the rocket.9–12 ■ SECOND-LAW ANALYSISOF GAS POWER CYCLESThe ideal Carnot, Ericsson, and Stirling cycles are totally reversible; thus theydo not involve any irreversibilities. The ideal Otto, Diesel, and Brayton cycles,however, are only internally reversible, and they may involve irreversibilities