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Topic: Aircraft Engine
Username: surajit001
Posted 2012-12-20 03:37:16 and read 2056 times.

I am relatively new to the concept of aircraft engine. So please pardon me if my questions sounds foolish.
While going through the Turboprop engine, i read that most of the energy of the exhaust gases are used to drive the propeller blades. How is it done ? Because i find the design of turboprop engine and gas turbine engine pretty similar except in turboprop, there is propeller and a gearbox put forward. If i am not wrong there is turbine placed at the end of gas turbine engine and also turboprop. While in a gas-turbine engine, the exhaust gases produce the thrust, while in a turbo prop how does the same turbine work in different way to harness the same energy of the exhaust gases to run the propeller. Please kindly explain me in a simple way if possible as i am relatively a starter to this concept.

Thank you again.

Topic: RE: Aircraft Engine
Username: b78710
Posted 2012-12-20 06:01:23 and read 2006 times.

A turbo prop is essentially a regular gas turbine engine. It just uses the last stage of the turbine to drive a gearbox and propeller arrangement. They call this a free turbine. They are designed use up all the energy from combustion so there is no thrust produced.

So if you look at a regular GE90 say, the last turbine spool (N1) is used to drive the fan. And on a turboprop it is used to drive a gearbox and propeller

Topic: RE: Aircraft Engine
Username: tdscanuck
Posted 2012-12-20 08:30:08 and read 1960 times.

Quoting surajit001 (Thread starter):
Because i find the design of turboprop engine and gas turbine engine pretty similar except in turboprop, there is propeller and a gearbox put forward.

In concept, they're exactly the same. Turbines, regardless of ultimate engine design, extract shaft power from moving gas. Some of the turbines are extracting power to run the compressor. The energy that's left in the gas is available to do something useful.

In a turbojet, you just extract enough power to run the compressor and the rest of the gas goes screaming out the back through a nozzle to give you thrust. This is great for high speed but not very efficient.

In a turbofan, most of the energy is extracted by the low pressure turbine and that power is used to turn the fan. The fan provides the bulk of the thrust. A little bit of energy is left in the core flow coming out the back of the turbine and that provides some thrust but most of it is coming from the fan. The balance point is that each additional turbine stage gets bigger and heavier and recovers less energy from the flow, so you hit a balance point where it's not worth extracting any more power for the fan. This is much more efficient but limits your speed to just below supersonic.

In a turboprop, as much of the energy as practical is extracted by the low pressure turbine and that power is used to turn the propeller (through a gearbox). The propeller provides the bulk of the thrust. A little bit of energy is left in the core flow and that provides some thrust but it's usually very small (proportionally smaller than for a turbofan). This is even more efficient but limits your speed even more.

In a ground turbine (e.g. power generation or ship propulsion) it's basically a turboprop but, instead of a propeller, you connect the shaft to whatever you want to spin (generator, water propeller, etc.). These will often have more turbine stages than the aircraft equivalent because they don't care about weight and usually can't do anything useful with the exhaust gas.

Tom.

Topic: RE: Aircraft Engine
Username: prebennorholm
Posted 2012-12-20 18:52:05 and read 1808 times.

Quoting tdscanuck (Reply 2):
In a ground turbine (e.g. power generation or ship propulsion) it's basically a turboprop but, instead of a propeller, you connect the shaft to whatever you want to spin (generator, water propeller, etc.). These will often have more turbine stages than the aircraft equivalent because they don't care about weight...

  

Quoting tdscanuck (Reply 2):
....and usually can't do anything useful with the exhaust gas.

In many applications the heat in the exhaust gas is used in a boiler, which runs a steam turbine. That may increase overall efficiency quite significantly.

The same system is used on some modern, very large diesel engines on ships - ocean going container ships and such.

Some even have two stage heat recovery, first a high pressure boiler and its steam turbine, then a low pressure boiler and one more turbine.

On ships these steam turbines usually pull electric power generators which power an electric motor placed around the propeller shaft.

Weight and volume make such systems totally irrelevant for aircraft use.

Topic: RE: Aircraft Engine
Username: BMI727
Posted 2012-12-20 21:53:55 and read 1771 times.

Quoting prebennorholm (Reply 3):
In many applications the heat in the exhaust gas is used in a boiler, which runs a steam turbine. That may increase overall efficiency quite significantly.

You can also used regeneration to apply exhaust heat to the compressed fluid without using fuel.


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