Turbofan: Bypass Engine
The energy extraction through the additional turbine lowers the rejected energy at the exhaust, resulting in a lower exhaust velocity, pressure, and temperature. The additional turbine drives a fan in front of the compressor. The large amount of air – mass flowing through the fan provides thrust. Part of the intake airmass flow through the fan is diverted (i. e., bypassed as the cold secondary flow) around the engine core and does not burn. The primary flow flows through the combustion chamber and is known also as the core flow or hot flow. Figure 10.5 is a schematic diagram of a turbofan engine (the top of the figure is a bare PW 4000). The lower exhaust velocity reduces engine noise. At the design point (i. e., LRC), the lower exhaust pressure permits the nozzle exit area to be sized to make the exit pressure equal to the ambient pressure (i. e., in a perfectly expanded nozzle). This is unlike simple turbojets, which can have a higher exit pressure.
Readers should note that the component-station-numbering system follows the same pattern as for the simple straight-through turbojet. The combustion chamber in the middle maintains the same numbers (i. e., 2-3). The only difference is the fan exit, which has the subscript f. The intermediate stages of the compressor and the turbine are primed.
Typically, the BPR (see Equation 10.2) for commercial jet-aircraft turbofans (i. e., high-subsonic flight speeds of less than Mach 0.98) is around 4 to 7. Recently, turbofans for the newer Boeing787, Airbus350 and Bombardier Cseries have reached BPR of 8 to 12. For military aircraft applications (i. e., supersonic flight speeds of up to Mach 2.5), the BPR is around 1 to 3. A lower BPR keeps the fan diameter smaller and, hence, lowers the frontal drag. Multispool drive shafts offer better efficiency and response characteristics, mostly with two concentric shafts. The shaft driving the low-pressure (LP) section runs inside the hollow shaft of the high – pressure (HP) section (see Figure 10.5). Three shaft turbofans have been designed, but most of the current designs use a twin spool. The recent advent of a geared turbofan is indicative of better fuel efficiencies.
Shaded area is the HP module (compressor and turbine)
HP shaft goes through hollow LP shaft
Figure 10.5. Schematic diagram of a pod-mounted, long-duct, two-shaft turbofan engine
A lower fan diameter compared to the propeller permits higher rotational speed and provides the scope for a thinner aerofoil section to extract better aerodynamic benefits. The higher the BPR, the better is the fuel economy. A higher BPR demands a larger fan diameter when reduction gears may be required to keep the revolutions per minute (rpm) at a desired level. Ultra-high BPR (UHBPR) turbofans approach the class of a ducted-fan, ducted-propeller, or propfan engine. This type of engine has been built, but its cost versus performance has prevented it from breaking into the market.