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I am writing an essay on how a jet engine works, so I need specific answers on this.

What percent of the air that is sucked into the engine is bypass air? I have heard that bypass air provides about 60% of the thrust. How does cold air provide thrust? I was under the impression that hot air makes much more thrust than cold.

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    $\begingroup$ "How does cold air provide thrust", the big fan at the front of a turbofan is mostly a propeller, the rest of the engine is here to spin the fan. $\endgroup$ – mins May 20 '17 at 18:11
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Cold air accelerated backwards provides forward thrust by reaction. What really matters is the amount of mass accelerated backwards. Its temperature is not important...

Concerning the 'percent of air sucked', it depends on the design. A pure turbojet doesn't produce 'bypass air', and a turbofan may reach a ratio of 10:1, that ratio being the relation between the mass of 'cold' air pushed backwards by the fan in a given time and the mass of air that passes through the combustion chamber...

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It all depends on the bypass ratio of a turbofan engine, with is the ratio of the air flowing through the fan and cold section diffuser vs the airflow through the gas core of the engine. High bypass ratios can have at or greater than 90% of the total airflow moving through the cold diffuser of the nacell with only 10% or less of the air moving through the gas core. The bypass air also produces nearly 90% of the reactive thrust from the engine as well. This is reversed in low bypass or turbojet engines where very little or no reactive force is created from bypass air through the engine core.

What produces thrust is a change in momentum of a mass of air over time. This can be accomplished by the fan and cold diffuser, in fact a turbofan engine is just that - a ducted fan powered by a gas turbine engine, functioning much like a turboprop. The major advantage - and the reason we use high bypass turbofan engines to power commercial airliners - is the fuel savings in the design for lower speeds. Engineers found that the more air which was fed through the gas core, the more fuel would need to be burned, which decreased efficiency of the engine. For lower speed flight the change in momentum of the air through the engine could be lower and save fuel by simply accelerating the air through a fan or first stages of the LPC, then bypass it around the core through a diffuser and out the back of the engine. The design provides acceptable amounts of thrust in the Mach 0.85 - 0.90 range.

Supersonic flight requires a higher change in momentum for the required impulse and this is only achievable by means of higher gas enthalpies prior to exiting the diffuser, making the turbojet the most realistic option for cruise in the Mach 1 - Mach 4 range. Ramjets as well work good in this flight regime but are not capable of effective operation in low speed flight, requiring additional power plants for operation here.

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