Recent from talks
Turbofan
Knowledge base stats:
Talk channels stats:
Members stats:
Turbofan
A turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft propulsion. The word "turbofan" is a combination of references to the preceding generation engine technology of the turbojet and the additional fan stage. It consists of a gas turbine engine which adds kinetic energy to the air passing through it by burning fuel, and a ducted fan powered by energy from the gas turbine to force air rearwards. Whereas all the air taken in by a turbojet passes through the combustion chamber and turbines, in a turbofan some of the air entering the nacelle bypasses these components. A turbofan can be thought of as a turbojet being used to drive a ducted fan, with both of these contributing to the thrust.
The ratio of the mass-flow of air bypassing the engine core to the mass-flow of air passing through the core is referred to as the bypass ratio. The engine produces thrust through a combination of these two portions working together. Engines that use more jet thrust relative to fan thrust are known as low-bypass turbofans; conversely those that have considerably more fan thrust than jet thrust are known as high-bypass. Most commercial aviation jet engines in use are of the high-bypass type, and most modern fighter engines are low-bypass. Afterburners are used on low-bypass turbofan engines with bypass and core mixing before the afterburner.
Modern turbofans have either a large single-stage fan or a smaller fan with several stages. An early configuration combined a low-pressure turbine and fan in a single rear-mounted unit.
The turbofan was invented to improve the fuel consumption of the turbojet. It achieves this by pushing more air, thus increasing the mass and lowering the speed of the propelling jet compared to that of the turbojet. This is done by adding a ducted fan.
Frank Whittle envisioned flight speeds of 500 mph in his March 1936 UK patent 471,368 "Improvements relating to the propulsion of aircraft", in which he describes the principles behind the turbofan, although not called as such at that time. While the turbojet uses the gas from its thermodynamic cycle as its propelling jet, for aircraft speeds below 500 mph there are two penalties to this design which are addressed by the turbofan.
First, the energy required for a given thrust increases as the exhaust air is propelled at ever greater speeds, so the efficiency can be improved by diverting energy to propel larger quantities of air at lower speeds than the core. A turbofan achieves this by using an additional turbine to drive a ducted fan to blow air that bypasses the core. With a lower thrust from the hot nozzle, most of the thrust now comes from the large mass flow of low speed bypass air, providing the same thrust with a reduced specific fuel consumption.
The other penalty relates to trying to improve the thermal efficiency of the engine by increasing pressure ratio and turbine temperature. This causes a corresponding increase in pressure and temperature in the exhaust duct which in turn gives a higher gas speed from the propelling nozzle (and higher KE and wasted fuel). The increase in thermal efficiency is at the expense of a worse propulsive efficiency with the net effect being a lower overall efficiency. In other words, the independence of thermal and propulsive efficiencies, as exists with the piston engine/propeller combination which preceded the turbojet, is lost. In contrast, Roth considers regaining this independence the single most important feature of the turbofan which allows specific thrust to be chosen independently of the gas generator cycle.
The working substance of the thermodynamic cycle is the only mass accelerated to produce thrust in a turbojet which is a serious limitation (high fuel consumption) for aircraft speeds below supersonic. For subsonic flight speeds the speed of the propelling jet has to be reduced because there is a price to be paid in producing the thrust. The energy required to accelerate the gas inside the engine (increase in kinetic energy) is expended in two ways, by producing a change in momentum ( i.e. a force), and a wake which is an unavoidable consequence of producing thrust by an airbreathing engine (or propeller). The wake velocity, and fuel burned to produce it, can be reduced and the required thrust still maintained by increasing the mass accelerated. A turbofan does this by transferring energy available inside the engine, from the gas generator, to a ducted fan which produces a second, additional mass of accelerated air.
Hub AI
Turbofan AI simulator
(@Turbofan_simulator)
Turbofan
A turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft propulsion. The word "turbofan" is a combination of references to the preceding generation engine technology of the turbojet and the additional fan stage. It consists of a gas turbine engine which adds kinetic energy to the air passing through it by burning fuel, and a ducted fan powered by energy from the gas turbine to force air rearwards. Whereas all the air taken in by a turbojet passes through the combustion chamber and turbines, in a turbofan some of the air entering the nacelle bypasses these components. A turbofan can be thought of as a turbojet being used to drive a ducted fan, with both of these contributing to the thrust.
The ratio of the mass-flow of air bypassing the engine core to the mass-flow of air passing through the core is referred to as the bypass ratio. The engine produces thrust through a combination of these two portions working together. Engines that use more jet thrust relative to fan thrust are known as low-bypass turbofans; conversely those that have considerably more fan thrust than jet thrust are known as high-bypass. Most commercial aviation jet engines in use are of the high-bypass type, and most modern fighter engines are low-bypass. Afterburners are used on low-bypass turbofan engines with bypass and core mixing before the afterburner.
Modern turbofans have either a large single-stage fan or a smaller fan with several stages. An early configuration combined a low-pressure turbine and fan in a single rear-mounted unit.
The turbofan was invented to improve the fuel consumption of the turbojet. It achieves this by pushing more air, thus increasing the mass and lowering the speed of the propelling jet compared to that of the turbojet. This is done by adding a ducted fan.
Frank Whittle envisioned flight speeds of 500 mph in his March 1936 UK patent 471,368 "Improvements relating to the propulsion of aircraft", in which he describes the principles behind the turbofan, although not called as such at that time. While the turbojet uses the gas from its thermodynamic cycle as its propelling jet, for aircraft speeds below 500 mph there are two penalties to this design which are addressed by the turbofan.
First, the energy required for a given thrust increases as the exhaust air is propelled at ever greater speeds, so the efficiency can be improved by diverting energy to propel larger quantities of air at lower speeds than the core. A turbofan achieves this by using an additional turbine to drive a ducted fan to blow air that bypasses the core. With a lower thrust from the hot nozzle, most of the thrust now comes from the large mass flow of low speed bypass air, providing the same thrust with a reduced specific fuel consumption.
The other penalty relates to trying to improve the thermal efficiency of the engine by increasing pressure ratio and turbine temperature. This causes a corresponding increase in pressure and temperature in the exhaust duct which in turn gives a higher gas speed from the propelling nozzle (and higher KE and wasted fuel). The increase in thermal efficiency is at the expense of a worse propulsive efficiency with the net effect being a lower overall efficiency. In other words, the independence of thermal and propulsive efficiencies, as exists with the piston engine/propeller combination which preceded the turbojet, is lost. In contrast, Roth considers regaining this independence the single most important feature of the turbofan which allows specific thrust to be chosen independently of the gas generator cycle.
The working substance of the thermodynamic cycle is the only mass accelerated to produce thrust in a turbojet which is a serious limitation (high fuel consumption) for aircraft speeds below supersonic. For subsonic flight speeds the speed of the propelling jet has to be reduced because there is a price to be paid in producing the thrust. The energy required to accelerate the gas inside the engine (increase in kinetic energy) is expended in two ways, by producing a change in momentum ( i.e. a force), and a wake which is an unavoidable consequence of producing thrust by an airbreathing engine (or propeller). The wake velocity, and fuel burned to produce it, can be reduced and the required thrust still maintained by increasing the mass accelerated. A turbofan does this by transferring energy available inside the engine, from the gas generator, to a ducted fan which produces a second, additional mass of accelerated air.
Recent media