Calculating Energy of an inlet gas stream; 2 conflicting methods

[parislad]

Homework Statement

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.

Homework Equations

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

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.

 

[mark726]

Hello,

Thank you for your post. It sounds like you are on the right track with using the equations for heat transfer (Q = m_dot * Cp * T and Q = m_dot * H). However, I would like to clarify a few things to help you with your calculations.

First, it is important to note that the energy of a gas stream is not a state variable, as it is dependent on the flow rate and temperature of the gas. A state variable, on the other hand, is a property of a system that is independent of its history or how it was created. Examples of state variables include pressure, temperature, and volume.

Secondly, when calculating the energy of a gas stream, it is important to consider the specific heat capacity (Cp) of the gas mixture, rather than just the individual gases. This is because the specific heat capacity of a gas mixture is not simply the sum of the individual gas specific heat capacities, but is affected by the composition of the mixture. You can use lookup tables or equations to calculate the specific heat capacity of air at different temperatures and compositions.

Finally, to answer your question about whether you are calculating the internal energy of the gas, the answer is yes. The internal energy of a gas is the sum of the kinetic and potential energies of its molecules, and is a measure of the internal energy of the gas stream.

I hope this helps clarify some things for you. Let me know if you have any further questions or need any additional assistance with your calculations. Best of luck!