Oh come on guys, he seeks this forum to learn something so lets tell the answers if we can. Isn't a forum there to learn something and spread some knowledge? After all I can't recall this question having been asked in this forum before. Although there are enough helpfull sites around he could check on before coming here, such as:Wikipedia
- notice the growing ammount of blue balls symbolizing the air molecules in the compressor stage. Play around with the animation.
Anyhow I'll give it a shot. The words Low Pressure Turbine and High pressure Turbine actually says it all. To explain we'll have to start of with Bernoulli's law for fluids and gases. His law states, that the static pressure decreases if the diameter of the tube, in which the fluid is flowing, decreases whereas the speed and dynamic pressure increases. Simply imagine a river bed that changes its width (A
volume)from large to small. What will happen with the water (Q
uantity)? It will accelerate (v
elocity) since the flow rate is constant. Q = A * v
The first stage of a turbine is its compressor (driven by the turbine) which has the task to add kinetic energy and transfer it into static pressure. Bernoulli's law is counteracted by adding kinetic energy with help of the rotor- and statorstage.
The first section of an axial compressor is its rotorstage (Low pressure compressor). By pushing the air into a decreasing diameter it gains in static pressure and accelerates the air at the same time. The second section is the statorstage (high pressure compressor) which slows the air down a bit but therefore gains in static pressure and temperature again. The air is pushed threw each stage and by "maintaining" its speed but limiting its space the pressure rises. Low pressure and high pressure configurations have different rpm and are fitted on two different drive shafts.
Modern jet engines have compressions of around 40:1 (compression ratio). The higher the compression the better the expansion of the mixture - same as in a turbo charged car. This immense pressure and high temperature that is needed for the combustion chamber. The ignition of the gasoline-air mixture again results in a higher temperature and pressure expansion which drives the High pressure turbine. Actually the ensemble of HPT and LPT is almost the opposite of the compressor with the only difference of extracting the dynamic pressure into kinetic energy needed for the compressor and dynamic pressure for the actual thrust of the engine.
High bypass or low bypass engine doesn't play a role here, the compressor and turbine settings stay about the same. The difference of a high bypass engine is in a better cooling of the actual turbine and less noise emission due to lesser ripping pressure between the hot and cold air leaving the engine.