|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.