Turbofan engine the turbofan engine is the dominant

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TurboFan Engine The turbofan engine is the dominant gas turbine engine in commercial aircraft and is extensively employed in military aircraft is well. Its primary feature is a large fan that accelerates a large mass of unheated air in an annular duct surrounding the central core engine, as in Figures 1 which shows a photo-graph of the General Electric CF6-80C2 high-bypass-ratio engine. The large fan diameter produces a large jet exhaust consisting of a cylindrical wake of hot combustion gas surrounded by an annular flow of slower- moving warm air. Figure 1 (B747 engine) The bypass rati o, B , is the ratio of the mass flow rate through the outer cooler duct, c m , to the flow rate of the hot core engine, h m : h c m m B = The bypass ratio is a design parameter that is primarily determined by the mission of the aircraft. High-bypass-ratio engines are desirable for long-range commercial aircraft because of their excellent fuel economy. The CF6-80C2 engine has a bypass ratio of 5.05 and a total airflow of 802 kg/s.
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The bypass air may have its own nozzle, separate from the core engine as in the CF6 engine, or the core and bypass flows may be mixed in a specially designed nozzle. The mixing nozzle helps to reduce jet noise by transferring momentum from the fast- moving core gas to the slower-moving bypass air, thereby reducing the wake shear noise source. The mixing process, however, involves a thrust-loss penalty. Figure 2 presents some of the nomenclature and notation that will be used in discussing the turbofan engine. For the configuration shown in the figure, the fan pressurizes the compressor inlet air as well as delivering the power needed to accelerate the bypass air through its nozzle downstream. While other fan configurations are possible, this frequently used arrangement is the only one analyzed here. Figure 2 In addition to the bypass ratio, a second important design parameter is the fan pressure rati o, the ratio of the stagnation pressure downstream to that upstream of the fan: 1 2 p p FPR = The fan pressure ratio, together with the bypass ratio, determines the power transferred between the hot core engine and the bypass flow. For a given core engine configuration, higher bypass ratios and fan pressure ratios cause more power to be extracted from the turbine and passed to the bypass air by the fan. This produces higher bypass duct thrust. However, as more power is extracted from the core flow, the core nozzle velocity and core engine thrust are reduced. The determination of the design
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values of these parameters therefore involves a complex tradeoff with numerous other design factors. Let us now consider the analysis of the turbofan configuration shown in Figure 2. The following parameters are assumed to be specified: Figure 3: T-s diagram for turbofan engine
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Figure 4: T-s diagram for Fan part Fan analysis Fan efficiency; 1 2 1 ' 2 T T T T fan - - = η Fan nozzle efficiency; 2 ' 9 2 9 T T T T Nozzlefan - - = η Fan work, ) ( 1 2 T T C w p fan - = Fan jet energy, ) ( 2 9 2 2 T T C V p jetFan - = Thrust net, a a NozleFan
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