Junction X is not sonically choked, but has a pressure ratio at or below the critical pressure ratio calculated by the Perry's discharge coefficient loss model
A Caution message in xStream
Full Message Text
Junction X is not sonically choked, but has a pressure ratio at or below the critical pressure ratio calculated by the Perry's discharge coefficient loss model. See product help for more information.
Note: This message is based on the steady-state pressure ratio and will not appear when the steady-state pressure ratio is above the critical pressure ratio. It is still possible that the transient flow will be limited by the critical pressure ratio when this message is not present.
Explanation
This message may appear when using the Cd (Inline) subsonic loss model for orifices or the Cd (Perry's) subsonic loss model for venturis. These subsonic loss models use the discharge coefficient method for orifices and nozzles/venturis defined in Perry, 10-14 (1997)Perry, R. H., Green, D. W., Perry's Chemical Engineers' Handbook, 7th Edition, McGraw-Hill, New York, NY, 1997..
The subsonic loss model is used when the junction is not considered choked by the restriction choking criteria discussed in Sonic Choking Detailed Description. However, due to differences in sonic choking calculations between methods, it is possible for Perry's method to predict sonic choking when xStream's method does not. For steady-state flow, this may lead to flow rates in excess of Perry's critical flow value. For transient flow, the choked flow state may not be reflected in the Event Messages when flow is increasing gradually because the maximum subsonic flow is Perry's critical flow rate.
When the diameter ratio is less than 0.2, the ideal gas equation of state is used, and subsonic and sonic CdA values are equal, both sonic flow models will be identical.
Required Action
No changes to the model are typically required, the message is informational for users verifying results with hand calculations.
However, this situation may contribute to expansibility factor convergence issues.
