WARNING: I AM A COMPLETE N00B AT THERMODYNAMICS.
I have been reading about gas turbines lately as they are fascinating. The tolerances, materials and fluid dynamics that make these things work as well as they do work is fascinating and most impressive.
One thing I do not understand is how increasing the pressure ratio; that is, the ratio of pressure between the last stage of the compressor and atomospheric air helps to improve the efficiency of the engine. I am certain that increasing the pressure ratio does improve the efficiency of the engine, since every resource written by anyone who's actually worked on the things says so, and also because increasing pressure ratio makes the little box on the P/V chart get bigger. However, this is clearly not a scientifically rigorous explanation.
The problem, as I see it, is that if the compressor is compressing the air more then it is performing more work on the air. To perform more work on the air it needs to be provided with more power (assuming that compressor efficiency is constant). Power for the compressor in a jet engine comes from the turbine stage. The turbine works by converting some of the heat of the air flowing through the engine after it has been combusted with fuel into mechanical power. So, for the compressor to perform more work, more work needs to be taken out of the system by the turbine. Clearly, the turbine gets that work back minus leaks because the air that the compressor acts on is hotter. But at that point you're just recovering (most of) the work that you already took out of the gasses.
But how on earth does this improve efficiency?
I have been reading about gas turbines lately as they are fascinating. The tolerances, materials and fluid dynamics that make these things work as well as they do work is fascinating and most impressive.
One thing I do not understand is how increasing the pressure ratio; that is, the ratio of pressure between the last stage of the compressor and atomospheric air helps to improve the efficiency of the engine. I am certain that increasing the pressure ratio does improve the efficiency of the engine, since every resource written by anyone who's actually worked on the things says so, and also because increasing pressure ratio makes the little box on the P/V chart get bigger. However, this is clearly not a scientifically rigorous explanation.
The problem, as I see it, is that if the compressor is compressing the air more then it is performing more work on the air. To perform more work on the air it needs to be provided with more power (assuming that compressor efficiency is constant). Power for the compressor in a jet engine comes from the turbine stage. The turbine works by converting some of the heat of the air flowing through the engine after it has been combusted with fuel into mechanical power. So, for the compressor to perform more work, more work needs to be taken out of the system by the turbine. Clearly, the turbine gets that work back minus leaks because the air that the compressor acts on is hotter. But at that point you're just recovering (most of) the work that you already took out of the gasses.
But how on earth does this improve efficiency?
via JREF Forum http://ift.tt/1ltpm08
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