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The high-bypass turbofan engines used on many modern airliners is an example of a very successful and popular use of ducted fan design. The duct increases thrust efficiency by up to 90% in most cases [citation needed], in comparison to a similar-sized propeller in free air. Ducted fans are quieter, and offer good opportunities for thrust vectoring.
About 70% of the air passing through the fan goes down the bypass duct. The JT15D-4 and later variants use a "booster" axial stage behind the fan which runs at the same speed as the fan and directs the remaining 30% of the air into the high-pressure compressor, after which it passes into a reverse-flow annular combustor. The hot gases flow ...
Figure 1: Components of a centrifugal fan An external motor belt driven inline centrifugal fan discharging inline to the exterior of a building through a duct. Unlike non-inline/non-concentric impeller casing design with a cutoff blade above, the concentrically symetric cylinder casing and impeller geometry of inline type redirects the outflow around so that it is parallel to the inflow of gases.
Schematic diagram illustrating a 2-spool, high-bypass turbofan engine with an unmixed exhaust. The low-pressure spool is coloured green and the high-pressure one purple. Again, the fan (and booster stages) are driven by the low-pressure turbine, but more stages are required. A mixed exhaust is often employed.
The combustor is an impingement cooled annular design. Fuel is delivered to the combustor through an unusual rotating fuel nozzle system, rather than the standard fuel-air mixers or vapourisers. The bypass duct runs the full length of the engine. The FJ44-2A has two additional booster compressor stages.
A "cropped fan" derivative of the -2, the -3 engine has a smaller fan diameter at 60 in (1.5 m) but retains the original basic engine layout. The new fan was primarily derived from GE's CF6-80 turbofan rather than the CFM56-2, and the booster was redesigned to match the new fan. [26]