[Fat shady]

Hello, everyone. I have a question for you.I'm running a simulation of the turbo expander with CFX. Why choose to use a fixed composition mixture as the material, and after setting the isentropic efficiency formula, the solution will report an error?
Fixed composition mixture is as follows:
I used RGP Generator to generate a single substance RGP property file of methane and ethane。 The Fixed Composition Mixture in the CFX Pre sets the ratio of methane and ethane to 0.964 and 0.036, respectively.
The isentropic efficiency formula is as followsEFFICIENCY)
100*(massFlowAve(Total Enthalpy)@S1 Inlet-massFlowAve(Total Enthalpy)@R1 Outlet)/(massFlowAve(Total Enthalpy)@S1 Inlet-massFlowAve(Isentropic Total Enthalpy)@R1 Outlet)
CFX-Solver reports the following error:
Monitor Point: EFFICIENCY
+--------------------------------------------------------------------+
| ERROR #001100279 has occurred in subroutine ErrAction. |
| Message: |
| An error: Illegal argument for callback function massFlowAve(Isen- |
| tropic Total Enthalpy)@R1 Outlet |
| |
+--------------------------------------------------------------------+
+--------------------------------------------------------------------+
| ERROR #001100279 has occurred in subroutine ErrAction. |
| Message: |
| Stopped in routine GET_CBCK_VALUE

I kind of know that the reason it's wrong is that you can't calculate the total isentropic enthalpy of two or more mixtures. Individual substances such as methane or ethane can be counted, but mixtures cannot. So I want to ask for your help.Thank you!

[Martin_Sz]

You're absolutely right! The error you're encountering in CFX is due to the attempt to calculate the isentropic enthalpy of a mixture directly. Here's how to address this issue:

**The Problem:**

* Fixed composition mixtures don't have a single isentropic enthalpy value.
* The `massFlowAve(Isentropic Total Enthalpy)@R1 Outlet` function doesn't work for mixtures like yours.

**The Solution:**

There are two main approaches to calculate the isentropic efficiency for your methane-ethane mixture:

**1. Individual Component Isentropic Enthalpy:**

* Calculate the isentropic enthalpy for each component (methane and ethane) separately using their pure substance properties. You can likely find these properties within the RGP file generated by RGP Generator.
* Use a mass-weighted average of the individual component isentropic enthalpies to represent the mixture's isentropic enthalpy.

Here's a modified efficiency formula based on this approach:

```
EFFICIENCY = 100 * (
(massFlowAve(h_isentropic,methane) * mass_fraction_methane + massFlowAve(h_isentropic,ethane) * mass_fraction_ethane) @ S1 Inlet -
(massFlowAve(Total Enthalpy) @ R1 Outlet)
) / (
(massFlowAve(h_isentropic,methane) * mass_fraction_methane + massFlowAve(h_isentropic,ethane) * mass_fraction_ethane) @ S1 Inlet -
(massFlowAve(h_isentropic,methane) * mass_fraction_methane + massFlowAve(h_isentropic,ethane) * mass_fraction_ethane) @ R1 Outlet
)
```

**2. Use Material Property Function (MPF):**

* Define a User Defined Material Property Function (UDMPF) in CFX-Pre.
* This function should take the mixture composition (mass fractions of methane and ethane) and temperature/pressure as inputs.
* Within the UDMPF, calculate the mixture's isentropic enthalpy based on the individual component properties and their mass fractions.
* Use the UDMPF in your efficiency formula to access the isentropic enthalpy at the desired locations.

This approach requires more upfront work but offers greater flexibility for complex property calculations involving mixtures.

**Additional Tips:**

* Ensure the mass fractions of methane and ethane in your CFX model match the values used during RGP property generation.
* Double-check the syntax of your modified efficiency formula in CFX-Solver.

By implementing one of these solutions, you should be able to calculate the isentropic efficiency for your turbo expander simulation in CFX without encountering the error related to the mixture's isentropic enthalpy.