Thermodynamics

526 | ThermodynamicsFanLow pressurecompressorFan air bypassingthe jet engine2-stage high pressureturbine to turn outer shaftCombustorsLow pressure turbineto turn inner shaftHigh pressurecompressorThrustFIGURE 9–53A modern jet engine used to powerBoeing 777 aircraft. This is a Pratt &Whitney PW4084 turbofan capable ofproducing 84,000 pounds of thrust. Itis 4.87 m (192 in.) long, has a 2.84 m(112 in.) diameter fan, and it weighs6800 kg (15,000 lbm).Courtesy of Pratt & Whitney Corp.AirinletTwin spoolshaft to turn the fanand the compressorsThrustPropellerCompressor Burners TurbineFIGURE 9–54A turboprop engine.Source: The Aircraft Gas Turbine Engine and ItsOperation. © United Aircraft Corporation (nowUnited Technologies Corp.), 1951, 1974.Gear reductionThe turbofan engine on an airplane can be distinguished from the lessefficientturbojet engine by its fat cowling covering the large fan. All thethrust of a turbojet engine is due to the exhaust gases leaving the engine atabout twice the speed of sound. In a turbofan engine, the high-speed exhaustgases are mixed with the lower-speed air, which results in a considerablereduction in noise.New cooling techniques have resulted in considerable increases in efficienciesby allowing gas temperatures at the burner exit to reach over1500°C, which is more than 100°C above the melting point of the turbineblade materials. Turbofan engines deserve most of the credit for the successof jumbo jets that weigh almost 400,000 kg and are capable of carrying over400 passengers for up to a distance of 10,000 km at speeds over 950 km/hwith less fuel per passenger mile.The ratio of the mass flow rate of air bypassing the combustion chamber tothat of air flowing through it is called the bypass ratio. The first commercialhigh-bypass-ratio engines had a bypass ratio of 5. Increasing the bypass ratioof a turbofan engine increases thrust. Thus, it makes sense to remove thecowl from the fan. The result is a propjet engine, as shown in Fig. 9–54.Turbofan and propjet engines differ primarily in their bypass ratios: 5 or 6for turbofans and as high as 100 for propjets. As a general rule, propellers