High bypass turbofan question

[Patco Lch 1,469]

Prehaps a young Eienstien can help me understand something that perplexes my unlearned mind. Where does the increase in thrust from the bypass turbine come from? I understand on a turbojet the intake turbines drive air into the combustion chamber increasing power phrehaps similar to a turbocharger. But the bypass blades blow air past the combustion chamber adding thrust in addition to the ignited gasses. It would seem the energy lost by the exhaust gasses having to turn these huge blades up front would slow down the exhaust there by negating any advantage. Kind of like breaking a dollor into two fifty cent pieces to have more money. This has baffled me for years so I now approach the oracles of wisdom. Thanks folks for suffering my ignorance!

[WarpD 824]

At a glance, your conclusion would be correct. However, turbofans are more efficient than turbojets in the speed ranges of commercial aircraft. So while the turbofan is indeed using bypassed air flow as a part of thrust by driving a ducted fan with exhaust from the gas turbine... it's performance range is superior for commercial aircraft. It results in lower volume of noise as well because of the reduced exhaust velocity.

 

In short, the performance loss because of driving the ducted fan with the gas turbine's exhaust would probably only start to become apparent above speeds of 500kts or so.

[PhantomTweak 3]

The turbo-fans are inside a ducted-fan arrangement, which increases their thrust dramatically, and the energy created by the hi-temp tubines is more than enough to turn them. Their bearings are amazingly free to turn, and very well lubricated. Also, the fan arrangements are as well or more well balanced than the compressor and turbine sections. They also provide more than enough air to the actual engine, providing air to the compressor section, and then to the burner section under the proper copression ratio. In fact, most turbine engines will have more compressor sections past the turbofans. Also, to make the pressure correct in the burner section, there are usually bleed air pick-offs in certain compressor sections, lets say the 10 and 14 sections, just for an example. This air can be very hot, and the tempurature of the areas surrounding the bleed-air ducts is carefully monitored, in case of  bleed-air leak, which can cause fuselage fires.

These are used to provide temperature-controlled air to the cabins via heat-exchangers. They ALSO provide pressurized air to the fuel tanks, and is available to the other engines for crossbleed startups, if the APU is shut down. Additionally, they provide cooling to the avionics and battery areas, again, via heat exchangers.

I hope all this helps a little bit

Pat☺

[G550flyer 1,015]

Lets try a easy simple way to explain. In the old turbo jet, about 70% of the thrust was caused by the air moving through the core of the engine. You burn a lot of fuel, definitely at the lower altitudes. With the Turbofan, the core is a lot smaller by comparison and it puts out about 30% of the air. The core is specially tuned and machined to give optimum efficient performance. The small efficient core uses its efficient power to turn the larger light weight efficient fan. Older turbojets had more and heavier fan blades and less fan turbine blades. The turbofan has one fan and more fan turbine blades to extract maximum energy from the core's exhaust. The efficient computer controlled variable vanes in the core also leads to performance. In a sense, the smaller efficient core is heavily extracted giving the fan it's power and efficiency. This also makes the engine quieter. The engine produces more power with less fuel flow. 

 

You will notice the exhaust gas temperature difference also. The old JT3D engines that were on the old jets I flew maxed at about 590C. The CF-6s on the DC10s I flew maxed out at 945C and the BR710s on the G550s I fly max at 905C. You can see that the two turbofans burn very hot producing more exhaust force than the old turbojet I mentioned.

[KingGhidorah 209]

Why don't we ever see ducted fans geared to piston engines?

[brucek 38]

Moving the same mass of air (for the purpose of generating thrust)  can be done either:

1. High volume at low speeds

2. Lower volume at higher speeds.

 

The losses tend to increase at a rate of the square of the speed, as does the noise pollution.  Therefore, option 1. above is more efficient-  not to mention the ability to generate a lot more thrust as volume is more easily increased than speed.

 

I may have some fo this incorrect,  I'm trying to recall an article I read a long time ago 

 

Thanks,  Bruce.

 

 

 

 

[Patco Lch 1,469]

Thanks for taking the time to answer my question.  I'll have to study these a awhile to get my head around the concepts but it is very interesting. I see a high bypass engine would not go supersonic.

[Bjoern 1,157]

Why don't we ever see ducted fans geared to piston engines?

 

Despite the fuel consumption at low speeds and alitiudes, gas turbines are smaller, lighter and not as complex as piston engines of comparable power.

Maybe on a private plane, or so, but not on an airliner.

 

Or something like this:

http://en.wikipedia.org/wiki/Motorjet

 

 

 

 

 

Moving the same mass of air (for the purpose of generating thrust)  can be done either:

1. High volume at low speeds

2. Lower volume at higher speeds.

 

The losses tend to increase at a rate of the square of the speed, as does the noise pollution.  Therefore, option 1. above is more efficient-  not to mention the ability to generate a lot more thrust as volume is more easily increased than speed.

 

Yep.

[PhantomTweak 3]

Despite the fuel consumption at low speeds and alitiudes, gas turbines are smaller, lighter and not as complex as piston engines of comparable power.

Maybe on a private plane, or so, but not on an airliner.

 

 

Absolutely, completely, 100% true! For the life of me I've never been able to understand why the heck we insist on converting linear motion (the pistons go up and down...)  into rotary motion (the transmission and wheels on the bus go round and round...). It is SO much more efficient to convert the rotary motion (the central shaft of the turbine engine) into rotary motion (see above). Always has bothered me...

Besides, as Bjoern mentioned, they are SO much simpler. ONE moving part! How many in a piston engine?? more than many, that's for sure!

I suspect both shenanigans and politics at work here...

Pat☺

[vololiberista 205]

Why don't we ever see ducted fans geared to piston engines?

Piston engines can't really drive the fan at a fast enough speed. Fuel consumption would be through the roof. It's a technical possibility, but prohibitively expensive and inefficient.

[Bjoern 1,157]

Absolutely, completely, 100% true! For the life of me I've never been able to understand why the heck we insist on converting linear motion (the pistons go up and down...)  into rotary motion (the transmission and wheels on the bus go round and round...). It is SO much more efficient to convert the rotary motion (the central shaft of the turbine engine) into rotary motion (see above). Always has bothered me...

Besides, as Bjoern mentioned, they are SO much simpler. ONE moving part! How many in a piston engine?? more than many, that's for sure!

I suspect both shenanigans and politics at work here...

 

What context are we talking about?

 

In aviation, turbines are, from a certain airplane size onward, the best option for propulsion. No "if"s, no "but"s.

Piston engines offer much better fuel economy and heat and noise levels in automotive applications.

In the same vein, fuel economy is the biggest "pro" for piston engines in maritime applications, although gas turbines are also used, mostly for supplemental electrical current generation or for propelling fast (mostly military) ships.

Rail transport is also a piston-only domain if electrification of the track is not feasible. Gas turbines are also suffering from pure fuel economy in this domain.

For generation of electrical power in power plants, however, the - steam or gas - turbine is uncontested for their reliability and simplicity. Gas turbines even have the added bonus of being capable of much better response times to increased electrical load demands.

 

One general drawback of turbines is that they react to huge mechanical torque demands by a very high increase in fuel consumption. While fighting air friction or electrical field forces is quite easy, fighting ground or water friction is an entirely different thing.