Bend
The Bend junction type must always have two connecting pipes. This junction type allows you to model the irrecoverable loss that occurs when the flow direction changes through a bend or elbow.
The Bend Properties window follows the first of the two basic Properties window formats, displaying the connecting pipes in a fixed format. A flow direction through the junction is adopted from the defined directions of the two connecting pipes.
The Bend Properties Window offers three standard types of bend losses: the Smooth Bend, the Standard Elbow (the standard short radius threaded elbow), and the Mitre Bend. The window shows a schematic of the selected geometry. Until you have input the information for the upstream pipe that allows flow area determination, the standard Bend junction loss factor is not calculated. For smooth radius bends, you also need to specify r/D, which is the radius of curvature over the pipe diameter. At r/D less than one the Reynolds number must be known to calculate the loss factor, and therefore cannot be displayed until output is available.
For bends that are not at a 90-degree angle, you can enter the angle and the standard loss factor will be calculated.
If the standard loss factors provided are inadequate for your application, you may specify a custom loss factor referenced to the upstream pipe flow area.
Note: In principle, the upstream and downstream pipe diameters should be the same. However, the Bend junction only uses the reference area specified under Basis Area for Loss Model, even if the physical flow is in the opposite direction. If you are working with a user-specified loss, your geometry may have different diameter pipes connecting to the elbow. In this case, you would purposely define different upstream and downstream pipe areas. However, you would still want to be careful that the custom loss factor you assign is referenced to the correct area that is specified in Basis Area for Loss Model.
CdA for Sonic Choking
As the fluid goes around the bend, it can separate from the wall. This results in a reduced physical flow area. When assessing sonic choking, it is this reduced area that is important. That is the purpose of the optional CdA input parameter. The minimum possible flow area is entered as the CdA and if sonic conditions are reached AFT Arrow will solve the system on a choked flow basis.
