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Need some help with Ideal Gas Law please

  1. Dec 13, 2008 #1
    What is the mole/ Mass ratio for ambient air at a given Temp/ Humidity?

    I am trying to get a very rough calculation of a pressure increase in a turbine engine with a given increase in temperature.

    In the ideal gas law alternative of PV=m/M(RT), The only part of the equation I am having problems with is the (m/M) portion for ambient air and fuel in the ratio stated below. I would like to know a mole/Mass equivelency of the Air and Fuel...

    As constants I will establish the following: Air mass= .5kg. Fuel / Air ratio = .10235 (.051175kg) (E85), Ambient air temp =60 F/ 15.5 C, Relative Humidity =30%, Ambient air pressure 1.0332275548 kg/cm2

    I would like to graph pressure increases from 600-2000 deg F.

    Also, help with the density calculation for the above variables?
  2. jcsd
  3. Dec 15, 2008 #2
    I know there is a LOT of people on these forums more than capable of helping me with this problem... Did I not ask the right question? Did I not pose it in the proper manner?

    I am not physics major, however, I want to learn. I have a desire for knowledge that, unfortunately, came later in life.

    If someone would please spend some time to help me gain some knowledge I would greatly appreciate it.
  4. Dec 15, 2008 #3


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    I don't know about designing turbines, but if you're looking for a rough calculation of pressure, could you just go with the ideal gas law,

    P = nRT/V ?

    And for a rough estimate of air's molar mass, use 20% O2 and 80% N2:

    0.2×32.0 + 0.8×28.0 grams/mole
    = 28.8 grams/mole​

    where 32.0 and 28.8 are the molar masses of O2 and O2, respectively?

    EDIT: that should be, "where 32.0 and 28.0 are the molar masses of O2 and N2, respectively" (no "?")

    For turbine questions, you might try one of the engineering subforums here:
    https://www.physicsforums.com/forumdisplay.php?f=98 [Broken]
    Last edited by a moderator: May 3, 2017
  5. Dec 15, 2008 #4

    Thank you so much, that is exactly what I was looking for! Now I think I can finish the calcs on my own (maybe). If I can not I will certainly ask.

    If a admin thinks I will get farther in that subforum, I ask that the thread be switched.

    One thought on the atomized fuel and vapor. I would imagine liquid droplets of fuel in suspension can not be calculated into the equation since they can not be compressed or should it? The fuel is going to have an endothermic presence and is one aspect if it's usage predetermined as a benefit for sustaining controlable temeratures.

    I remember back in the day someone at Garret giving me a base (hypothetical of pressure vs temp on a 70-80% adiabetic compression efficiencey) of an increase of (x) amount of pressure results in a (Y) increase in medium temperature. For the life of me I can't remember what it was.
    Last edited by a moderator: May 3, 2017
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