AFT Arrow is a highly accurate and powerful compressible flow analysis tool and rather than solving various compressible flow correlations, AFT Arrow is simultaneously solving the fundamental mass, momentum, energy, Equation of State, and Mach number equations.  Although AFT Arrow accurately calculates the fluid physical properties throughout the compressible pipe network, there is also a high level of complexity associated with these calculations.

We have seen several models that have various convergence issues and there is a common troubleshooting technique that solves the majority of these issues.  This technique is simply changing the specific heat ratio accuracy option from “High” to “Standard” in the System Properties window.

System Properties Window

By default, the High Accuracy specific heat ratio is used and is defined below as:

High Accuracy Specific Heat Ratio

When the Standard Accuracy option for the specific heat ratio is chosen, an approximation is made such that the specific heat ratio is defined as:

Standard Accuracy Specific Heat Ratio

For superheated gases, this is a very reasonable approximation to make without significant loss of accuracy and thus, is a small change that will help solve a majority of convergence issues.

In order to see if the standard accuracy option is reasonable, compare the fluid properties used in the model with the saturation conditions of the fluid.  If the model predicts pressures and temperatures that are reasonably departed from saturation conditions, then using Standard Accuracy is reasonable.  If model predictions are close to saturation conditions, then high accuracy should be used for the specific heat ratio and other model modifications should be made.