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Just putting it out there (Thermo help)

Discussion in 'The Pub' started by Ljiljan, Jun 14, 2010.

  1. Come across a problem I can't get the answer to. I had one method but that gave me the completely wrong answer.

    The question I have is this: is dh = Cp.dT for a constant pressure system only?

  2. WTF????

  3. :WStupid:
  4. ??? Wow. Complicated thermal dynamics in a motorcycle forum.
  5. There are a few mechanical engineers on here and they may be still fresh enough to remember.

    I know that the formula is derived based on constant pressure closed systems, but I'm wondering if can also be applied to open systems
  6. 42 - always 42
  7. thats pretty close to what I got and unfortunately its wrong. It should be closer to 142.
  8. Ah that just means we don't understand the question. (well I sure as hell don't!!!)
  9. yes I saw that. Problem is it doesn't really help me.

    Oh well, I'll try and find answers elsewhere.
  10. What about this:

    Not that I know anything about thermodynamics ...
  11. yep, I got that. Already had that.

    My problem is that in my lecture notes, the lecturer used that formula in a turbine compressor analysis which is an open system application and almost by definition not constant pressure. After trying to do something similar, I've not found the answer I should have. To make things more generally annoying, the same formula gave me the correct answer in a diffuser analysis which is also open system :-s
  12. Maybe the person who wrote the question / answer made an assumption of constant pressure?

    On second thoughts, I think the_blacke was onto something ...
  13. Never mind, crisis avoided. After telling plenty of people that the textbook is usually right and they are usually wrong, I should have listened to my own advice. I had a mistake in the units. At least my initial question got answered in solving the problem.
  14. I still haven't got a clue.......:?
  15. naaah its all bullshit Lilley, after you graduate if you want information like that you will simply ring up a Compressor supplier and ask em to size a compressor for your application....... JOB DONE

    Engineering....... the art of making educated guesses

  16. that wont go down to well in an exam
  17. Well, I'm a mech and I can't bloody remember, if I ever knew.

    I was always a bit shy of Thermodynamics. Anything with that many assumptions, approximations and partial differentials has no business considering itself a proper engineering subject :grin:.

    The main thing to remember is that, out in real engineering, there are very few problems that cannot be approximated as either a simply supported beam or one dimensional flow :grin:.
  18. I thought that was what engineering was all about. ;)

    Thats good to know because I havn't gone past them yet. Thermofluids starts next semester, further solids and mechanics - next year.
  19. dh = Cp.dT = left glove first