Turbofan Engines
The objective of modern turbofans, see Fig. 1.10a, is to achieve acceptable
thrusts and efficiencies at higher flight speeds than a turboprop can. As
with a turboprop, the main engine can operate in any regime of Fig. 1.2
without disturbing the effectiveness of the thrust-producing components.
Similar to a propeller, a fan imparts energy to a large quantity of air: the
quantity is less than that of a propeller, however, being limited by factors
affecting the installation weight and drag, as well as fan efficiency.
Figure 1.10a differs from Fig. 1.9 only by the fact that the turbine located
between stations 5 and 7 drives an axial flow fan or compressor. The fan
raises the pressure of air passing through it so that a portion of this flow,
~VrF, can be discharged with the velocity VFj, which is lower than that
prevalent in turbojets, but higher than that behind propellers. The rest, of
the flow into the fan blades, I~E, passes through the engine. We call the
ratio WF/W ~ the bypass ratio.
The tolerable Mach numbers of the tips of the fan rotors are much
higher than those of conventional propeller blades. Available compressor
technology can be applied to produce the desired fan pressure ratios with
high efficiencies at elevated Mach numbers (the relative close spacing of the
fan blades and the presence of a casing surrounding the fan are the
principal reasons for this difference).
The designer of fan engines has the responsibility, however, of selecting
the energy delivered to the fan and the mass flow through it. The ensuing
discussion calls attention to the principal elements governing that decision.
The objective of modern turbofans, see Fig. 1.10a, is to achieve acceptable
thrusts and efficiencies at higher flight speeds than a turboprop can. As
with a turboprop, the main engine can operate in any regime of Fig. 1.2
without disturbing the effectiveness of the thrust-producing components.
Similar to a propeller, a fan imparts energy to a large quantity of air: the
quantity is less than that of a propeller, however, being limited by factors
affecting the installation weight and drag, as well as fan efficiency.
Figure 1.10a differs from Fig. 1.9 only by the fact that the turbine located
between stations 5 and 7 drives an axial flow fan or compressor. The fan
raises the pressure of air passing through it so that a portion of this flow,
~VrF, can be discharged with the velocity VFj, which is lower than that
prevalent in turbojets, but higher than that behind propellers. The rest, of
the flow into the fan blades, I~E, passes through the engine. We call the
ratio WF/W ~ the bypass ratio.
The tolerable Mach numbers of the tips of the fan rotors are much
higher than those of conventional propeller blades. Available compressor
technology can be applied to produce the desired fan pressure ratios with
high efficiencies at elevated Mach numbers (the relative close spacing of the
fan blades and the presence of a casing surrounding the fan are the
principal reasons for this difference).
The designer of fan engines has the responsibility, however, of selecting
the energy delivered to the fan and the mass flow through it. The ensuing
discussion calls attention to the principal elements governing that decision.
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