Several advanced settings are available to fine-tune the accuracy of the fluid properties for the xStream simulation. These options can be enabled from Tools -> User Options by going to the Show options under General (global).

Once enabled the Fluid Accuracy settings are located on the Fluid panel below the Fluids Available in Library area (Figure 1).

Specific Heat Ratio

There is a choice of standard or high accuracy for the specific heat ratio. The standard accuracy uses Equation 80 for gamma(γ). The high accuracy uses Equation 81. The standard accuracy will usually be suitable, but for completeness you may want to use the high accuracy in your final runs and see if the results change.

The high accuracy option should not have a significant affect on run time (at most 10%). However, it can result in convergence problems.

This option is applicable to both the AFT Standard as well as Chempak fluids.

State Properties

The State Properties option influences the number of elements used in curve fits for calculating property data. You will usually want to start with standard accuracy, and use high accuracy for final runs, depending on the importance and run time for the model. The high accuracy option will increase run time by about a factor of two.

When performing interpolations for properties, a 4 point or 16 point interpolation scheme can be used. The 4 point scheme implemented in AFT xStream is the Standard Accuracy option. The 16 point scheme is the high accuracy option. Use of the High accuracy option will roughly double the run times for models.

This option is applicable to only to Chempak fluids.

Sonic Velocity

The standard accuracy sonic velocity option will usually be acceptable. The standard option uses Equation 87 for all sonic velocity calculations. The high accuracy option uses Equation 84, calculating the derivative using a numerical difference (also called perturbation derivative) along the isentropes. This is time consuming, and can cause models to run ten times as long. You should not use this option unless you have reason to believe Equation 87 has unacceptable accuracy as evidenced from the output values of sonic velocity.

This option is applicable to only to Chempak fluids.