[ChrisA]

I'm doing some enthalpy comparisons for a simulation of a binary gas mixture in FLUENT 14.5 and I'm having some issues reconciling the reported values with what I'm expecting from the theory guide. I'm using the density based steady solver in 2D for a laminar, compressible, flat plate simulation with implicit solution formulation.

The theory guide for the energy equation (5.2.1.1) states that the mixture enthalpy is calculated from the summation of the mass fraction weighted individual species contributions with the individual species contributions being calculated from the integral of Cp*dT.

However, when I write enthalpy and look at it in tecplot I see values that are closer to Cp*T rather than the integral quantity. I should note that I'm using peicewise polynomials for the individual gas Cp and have mixing law selected for the mixture-template Cp. I have a summary of the reference values (on boundaries where know the exact temperature and composition) I looked at for one of the species (air) below:

T = 52.29K (air)

Difference between FLUENT reported enthalpy and integral enthalpy: 11.7%
Difference between FLUENT reported enthalpy and Cp(T)*T: 0.206%

T = 314K (air)

Difference between FLUENT reported enthalpy and integral enthalpy: 0.37%
Difference between FLUENT reported enthalpy and Cp(T)*T: 0.13%

Is FLUENT actually calculating and using enthalpy with Cp*T rather than integrating Cp*dT like the theory guide states? If so where is this stated/why is it done and is there any way to change my simulation to use the "proper" definition of enthalpy?

I think I included all the case set-up information relevant to the question, please let me know if I missed anything.