CFX Turbo Expander: Isentropic Efficiency Error with Fixed Composition Mixture (Solved)

 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.