OpenMDAO
Why bleeds do require both frac_P and frac_work?
I have a seemingly simple question, but I haven't been able to easily find an answer.
Why do the bleeds defined within compressors require both the parameters frac_P and frac_work as input?
If I understand correctly, when defining a compressor component, it is possible to specify the presence of bleeds. To characterize these bleeds, three parameters must be provided: frac_W, frac_P, and frac_work.
Their definitions are fairly straightforward: they respectively indicate the fraction of mass flow rate, pressure, and enthalpy the bleed possesses relative to the entire compressor.
However, I cannot understand from a thermodynamic perspective why it is necessary to define both the pressure fraction and the work fraction. To be clear, I am fully aware that the two values are not equal — depending on the polytropic efficiency of the machine and the operating conditions, it is absolutely possible for a 50% increase in pressure not to correspond to a 50% increase in enthalpy. However, I don’t understand why the code requires both, since, according to my understanding of the code, it should be able to calculate one from the other.
It almost seems as though the model requires two inputs that are related to each other, and I can’t understand why. Is there anyone more familiar with the code or the physics it models who could help clarify this doubt?
Thank you.
This design follows a super generalized design that's been used in the base line elements for NPSS for a couple of decades.
You are correct that in practice, if you think of a compressor as a 1D flow from low pressure to high pressure, lets say going from 0 to 1 (generic parametric definition).
Then at at 0, you'd take the bleed air at P_in, and 0 work done. At 1 you'd take the bleed at P_out and 100% work done. Anything in between you could compute a nonlinear link between these two as well. Thats certainly one way to link these two and it would work fine.
But modern compressors are very complex and opperate at the ragged edge of their performance envelopes. So where and how you take the bleed matters a lot. For example, If you take it from the outer casing, or the inner one will change the character of the flow you get. Another thing that can affect is is the individual efficiencies of each compressor stage (as the blades get smaller, large amounts of losses will come from them). This can cause some fairly wonky behaviors (e.g. maybe you don't get much pressure, but still lots of enthalpy), and if you're trying to model weird things then you may benefit from this kind of complexity to match reality.
This design allows the flexibility to do both. If you'd like to make a component to link them up using 1D flow thermodynamics then you definitely could. If you want to try and handle some of the more subtle issues that come from modern high ratio compressors, then you have the flexibility to do that too.
Thank you very much for the clear and thorough response, everything definitely makes much more sense now.