High-bypass turbofan vs. ducted fan ??

[PeteCO]

Isn't the high-bypass section just a ducted fan? Or is there more to it?

I guess what I'm getting at is whether the difference in naming is mostly semantic, or is bypass ratio / pressure differential the determining factor? Number of stages in the bypass section maybe? (as far as I know, the fan section is single stage / one fan, right?)

It sounds to me like a HBPTF is a ducted fan with several qualifying factors - the engine is within the duct, it is driven by a turbojet, the fan contributes as a part of the compressor section, and ????

 

[VNugget]

I'm no expert, but I'd say that a high bypass turbofan still gets a significant portion of its thrust through the jet core, while a ducted fan would be designed to get as much thrust out of the fan as possible, with only enough from the core to be able to run, just like a turboprop. That's at least my guess.

 

[FlyingBuckI]

A ducted fan derives ALL of it's power from ducted air, whereas a turbofan is just a percentage of ducted air (bypass ratio). The cold air around the turbine helps to shroud engine noise, and the amount of contributing thrust depends on altitude. Second, the bypass fan is in front of, not part of, the compressor. Your logic is mostly correct, but be careful with your terminology. A turbojet is a turbine, but the reverse is not always correct (ie. a turbofan is driven by a turbine, not a turbojet -- though they are essentially the same, some people would argue otherwise )

-FB

 

[Berkut]

Well, since we are talking terminology...

Second, the bypass fan is in front of, not part of, the compressor.

The fan is considered to be the first stage (N1) of the compressor. It is part of the compressor.

A turbojet is a turbine

A turbojet is a turbine engine, not a turbine itself. A turbine is a device which converts the kinetic energy of a moving fluid to mechanical power. The turbine is only a part of the turbojet, not the whole engine.

 

[FearlessFreep]

High Bypass Turbo Fan Engines derive the majority of their thrust from the fan.

The ratio is about 70% to 75% thrust from the fan and 25% to 30% from the turbine.

I don't know what the latest efficiency ratings on turbine engines are, the last time I heard they were about 30% efficient, in that it takes 70% of the energy inputted just to operate it. Reciprocating internal combustion engines are somewhere around 17% efficient. Mind you I am not certain on these figures as there has been a myriad of improvements on these engines since I learned of these figures.

 

[skiddriver]

It might help to remember that ducted fans are used in some helicopters as anti-torque devices (i.e. tail rotors) and are shaft driven. Also, the VTOL version of the F-35 JSF uses a shaft driven ducted fan for vertical propulsion. So ducted fans are a larger set of propulsion devices which can be driven in a number of ways. HBTFEs use ducted fan technology to improve efficiency, reduce noise, etc.

 

[VNugget]

It might help to remember that ducted fans are used in some helicopters as anti-torque devices (i.e. tail rotors) and are shaft driven. Also, the VTOL version of the F-35 JSF uses a shaft driven ducted fan for vertical propulsion. So ducted fans are a larger set of propulsion devices which can be driven in a number of ways. HBTFEs use ducted fan technology to improve efficiency, reduce noise, etc.

It's also interesting to note that the Campini CC2 thermojet could be, and was, flown with the burner section removed, in effect turning it into a reciproocating-engine-driven ducted fan.

 

[FlyingBuckI]

Well, since we are talking terminology...
The fan is considered to be the first stage (N1) of the compressor. It is part of the compressor.

Not so. N1 is simply the compressor (pre-combustion), and N2 is the turbine (aft combustion). The fan is NOT considered part of N1. (see FAR 33...it specifically refers to the fan and compressor as two individual components)

A turbojet is a turbine engine, not a turbine itself. A turbine is a device which converts the kinetic energy of a moving fluid to mechanical power. The turbine is only a part of the turbojet, not the whole engine.

You're right on this one. I meant to imply that turbine=turbine engine, my mistake...that's what I get for replying just after waking up!

 

[VNugget]

Not so. N1 is simply the compressor (pre-combustion), and N2 is the turbine (aft combustion). The fan is NOT considered part of N1. (see FAR 33...it specifically refers to the fan and compressor as two individual components)

I'm not sure what you mean, but the way you have it written is ambiguous and makes no sense. For a single shaft, there's only one measure of RPM. N1 and N2 differentiate between the different spools on twin-spool engines where you have one turbine driving one compressor, and another turbine driving another compressor (and maybe a fan) through another shaft.

And yes, on most twin-spool turbofans the fan IS part of N1. See JT8D.

 

[Berkut]

Not so. N1 is simply the compressor (pre-combustion), and N2 is the turbine (aft combustion).

N1, N2, and N3 all refer to compressor sections. On a twin spool turbofan, N1 would be the fan and N2 would be the high pressure compressor. For a triple spool you add N3 and an intermediate compressor in there. Nx is not typically used to refer to turbine sections other than by associating them with compressor sections on the same shaft.

The fan is NOT considered part of N1. (see FAR 33...it specifically refers to the fan and compressor as two individual components)

I kinda browsed through there but didn't find anything. Can you be a little more specific? I'd like to check it out. That's a good-sized chunk of text to wade through.

Originally posted by General Electric
In a turbofan engine only a portion of the incoming air goes into the combustion chamber. The remainder passes through a fan, or low-pressure compressor, and is ejected directly as a "cold" jet or mixed with the gas-generator exhaust to produce a "hot" jet

Originally posted by The FAA
The turbofan engine

A turbofan engine is simply a turbine engine where the first stage compressor rotor is larger in diameter than the rest of the engine. This larger stage is called the fan.

 

[gern_blanston]

High Bypass Turbo Fan Engines derive the majority of their thrust from the fan.

At low levels this is true, but at high altitude, the core puts out more thrust than the fan, since there's not much air for the fan to push.

As for fan location, take a look at the General Electric CF-700. The fan is stuck behind the core at the back of the engine with its center in the exhaust stream.