mayse@cs.uiuc.edu (Chip Mayse) (02/25/90)
From: Chip Mayse <mayse@cs.uiuc.edu> For: Ted Kim and others interested: The main difference between a turbojet and turbofan engine is, as you suspected, that a turbofan has a fan (one or more stages) which is usually in front of the compressor, and which induces a flow some (often most) of which does not pass through the "gas generator" section (compressors, combustors, and turbines). The bypass flow may either be mixed with the flow from the gas-generator section and expelled through a common nozzle, or may exit through an annular nozzle of its own. The former approach has a theoretical thermodynamic advantage, but requires more bypass-duct structure (hence engine weight), and the separate-exit approach is generally used for turbofans of "high" (e. g. 5:1) bypass ratios (like most airliner engines). The real reason to do this is that the propulsive efficiency of a jet engine--i. e., the efficiency of the process by which the momentum imparted to the gases passing through the engine is converted to forward thrust--is theoretically maximized when the exhaust exit velocity (relative to the engine) is twice the forward flight speed. The bypass flow in a turbofan doesn't have much energy added to it--just the work put in by the fan, plus perhaps a bit of heat from the gg section-- and doesn't accelerate to a very high velocity going through its exit nozzle. Thus if you want fuel-efficient cruising at a steady speed, you can design a fan engine with a combination of bypass ratio and bypass-flow and primary-flow energy contents which gives the best propulsive efficiency at that speed. A turbofan generally has less average velocity than a turbojet, and provides better fuel-efficiency at lower speeds (and, usually, much better takeoff performance). Since the slow-moving bypass flow doesn't interact as turbulently with the atmosphere as it exits as would a (faster) turbojet exhaust, it doesn't make as much noise. For both these reasons, turbofans have virtually taken over the airliner-engine business. At higher flight speeds, turbojets (and afterburning turbojets, with even higher exhaust velocities) make more sense. Most modern fighter engines are low-bypass-ratio afterburning turbofans, a combination intended to provide reasonable efficiency across a range of flight speeds. The use of a low bypass ratio also keeps the fan diameter (and rotational inertia) down, so the engine doesn't have (excessively) poor throttle response (unacceptable in a fighter engine). If interested, consult "Aircraft and Missile Propulsion, vol. I and II" by M. J. Zucrow, or "Jet Propulsion for Aerospace Applications," by W. J. Hesse and N. V. S. Mumford, or any similar textbook. Hesse and Mumford have a clear derivation of the propulsive-efficiency equation, and their book is more recent, so is probably preferred (I think Hesse was one of Zucrow's students at Purdue). Hope this is at least related to what you were wondering . . . Chip Mayse cmayse@ncsa.uiuc.edu