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.

Principles

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. 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 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 is expended in two ways, by producing a change in momentum, and a wake which is an unavoidable consequence of producing thrust by an airbreathing engine. 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.
The transfer of energy from the core to bypass air results in lower pressure and temperature gas entering the core nozzle, and fan-produced higher pressure and temperature bypass-air entering the fan nozzle. The amount of energy transferred depends on how much pressure rise the fan is designed to produce. The best energy exchange between the two flows, and how the jet velocities compare, depends on how efficiently the transfer takes place which depends on the losses in the fan-turbine and fan.
The fan flow has lower exhaust velocity, giving much more thrust per unit energy. Both airstreams contribute to the gross thrust of the engine. The additional air for the bypass stream increases the ram drag in the air intake stream-tube, but there is still a significant increase in net thrust. The overall effective exhaust velocity of the two exhaust jets can be made closer to a normal subsonic aircraft's flight speed and gets closer to the ideal Froude efficiency. A turbofan accelerates a larger mass of air more slowly, compared to a turbojet which accelerates a smaller amount more quickly, which is a less efficient way to generate the same thrust.
The ratio of the mass-flow of air bypassing the engine core compared to the mass-flow of air passing through the core is referred to as the bypass ratio. Engines with more jet thrust relative to fan thrust are known as low-bypass turbofans, those that have considerably more fan thrust than jet thrust are known as high-bypass. Most commercial aviation jet engines in use are high-bypass, and most modern fighter engines are low-bypass. Afterburners are used on low-bypass turbofans on combat aircraft.

Bypass ratio

The bypass ratio of a turbofan engine is the ratio between the mass flow rate of the bypass stream to the mass flow rate entering the core. A bypass ratio of 6, for example, means that 6 times more air passes through the bypass duct than the amount that passes through the combustion chamber.
Turbofan engines are usually described in terms of BPR, which together with overall pressure ratio, turbine inlet temperature and fan pressure ratio are important design parameters. In addition BPR is quoted for turboprop and unducted fan installations because their high propulsive efficiency gives them the overall efficiency characteristics of very high bypass turbofans. This allows them to be shown together with turbofans on plots which show trends of reducing specific fuel consumption with increasing BPR. BPR can also be quoted for lift fan installations where the fan airflow is remote from the engine and doesn't flow past the engine core.
Considering a constant core, core and bypass jet velocities equal and a particular flight condition the fuel consumption per lb of thrust decreases with increase in BPR. At the same time gross and net thrusts increase, but by different amounts. There is considerable potential for reducing fuel consumption for the same core cycle by increasing BPR.This is achieved because of the reduction in pounds of thrust per lb/sec of airflow and the resultant reduction in lost kinetic energy in the jets.
If all the gas power from a gas turbine is converted to kinetic energy in a propelling nozzle, the aircraft is best suited to high supersonic speeds. If it is all transferred to a separate big mass of air with low kinetic energy, the aircraft is best suited to zero speed. For speeds in between, the gas power is shared between a separate airstream and the gas turbine's own nozzle flow in a proportion which gives the aircraft performance required. The trade off between mass flow and velocity is also seen with propellers and helicopter rotors by comparing disc loading and power loading. For example, the same helicopter weight can be supported by a high power engine and small diameter rotor or, for less fuel, a lower power engine and bigger rotor with lower velocity through the rotor.
Bypass usually refers to transferring gas power from a gas turbine to a bypass stream of air to reduce fuel consumption and jet noise. Alternatively, there may be a requirement for an afterburning engine where the sole requirement for bypass is to provide cooling air. This sets the lower limit for BPR and these engines have been called "leaky" or continuous bleed turbojets and low BPR turbojets. Low BPR has also been used to provide surge margin as well as afterburner cooling for the Pratt & Whitney J58.

Efficiency

engines are most efficient for low speeds, turbojet engines for high speeds, and turbofan engines between the two. Turbofans are the most efficient engines in the range of speeds from about, the speed at which most commercial aircraft operate.
In a turbojet engine, the high temperature and high pressure exhaust gas is accelerated when it undergoes expansion through a propelling nozzle and produces all the thrust. The compressor absorbs the mechanical power produced by the turbine. In a bypass design, extra turbines drive a ducted fan that accelerates air rearward from the front of the engine. In a high-bypass design, the ducted fan and nozzle produce most of the thrust. Turbofans are closely related to turboprops in principle because both transfer some of the gas turbine's gas power, using extra machinery, to a bypass stream leaving less for the hot nozzle to convert to kinetic energy. Turbofans represent an intermediate stage between turbojets, which derive all their thrust from exhaust gases, and turbo-props which derive minimal thrust from exhaust gases. Extracting shaft power and transferring it to a bypass stream introduces extra losses which are more than made up by the improved propulsive efficiency. The turboprop at its best flight speed gives significant fuel savings over a turbojet even though extra machinery, with its own losses, is additional to the turbojet's simple low-loss nozzle.
Froude, or propulsive, efficiency can be defined as
where