Aircraft Propulsion, 2nd Edition

4.3.3 The Turbofan Engine

4.3.3.1 Introduction

To create a turbofan engine, a basic gas generator is followed by an additional turbine stage(s), which tap into the exhaust stream thermal energy to provide shaft power to a fan. This arrangement of multiple loading demands on the turbine stages leads to a multiple shaft arrangement, referred to as spools. The fan stages and potentially several (low-pressure) compressor stages may be driven by a shaft, which is connected to the low-pressure turbine. The high-pressure compressor stages are driven by the high-pressure turbine stage(s). The rotational speeds of the two shafts are called N₁ and N₂, respectively, for the low- and high-pressure spools. An additional two new parameters enter our gas turbine vocabulary when we consider turbofan engines. The first is the bypass ratio and the second is the fan pressure ratio. The ratio of the flow rate in the fan bypass duct to that of the gas generator (i.e., the core) is called the bypass ratio α. The fan pressure ratio is the ratio of total pressure at the fan exit to that of the fan inlet. It is given a symbol π_f.

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The principle behind the turbofan concept comes from sharing the power with a larger mass flow rate of air at a smaller velocity increment pays dividend at low-speed ...