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Homework Help: Calculating Energy of an inlet gas stream; 2 conflicting methods

  1. Dec 4, 2012 #1
    1. The problem statement, all variables and given/known data

    I want to calculate the energy of an inlet gas stream (I will add heat later but need to know the rate at which energy is coming into the system [J/s]).

    The fluid is air and I can decompose it into molar flow rates of the constituent gases. I now need to use lookups to find the energy of each constituent gas.

    I believe what I'm after here is a state variable because its based on the ambient condition and not any process that happened beforehand.

    2. Relevant equations

    I can think of 2 equations here:
    Q = m_dot * Cp * T
    Q = m_dot * H
    (where H = enthalpy)

    3. The attempt at a solution

    I have found some lookup data for values of H and Cp of oxygen and nitrogen. However the 2 equations give different plots; when I plot T vs H and T vs (Cp * T) the plots are similar but significantly different.

    I'm not sure why they are different or which one I should use. Also, is what I'm trying to calculate defined as the internal energy of the gas?

    Thanks for your help.
  2. jcsd
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