Orifice
The Orifice junction type requires two connecting pipes, unless you specify it as an exit orifice, in which case only one connecting pipe is allowed. This junction type allows you to model the irrecoverable loss that occurs through a plate-type flow restriction. It also allows you to specify loss factors as a function of a flow parameter and an orifice loss for pipes with an area change.
The Orifice Properties window follows the first of the two basic Properties window formats, displaying the connecting pipes in a fixed format. The Orifice junction does not have an explicit flow direction, but adopts a flow direction from the connecting pipes.
Orifices that are located at exit points are specified as exit orifices. These orifices require an associated back pressure definition. Exit orifices can only be connected to a single upstream pipe. To specify an exit orifice, select the Exit Orifice checkbox on the Loss Model tab. Then specify the exit pressure and temperature.
Orifice Types
AFT Arrow provides two standard orifice types: sharp-edged and rounded. These loss factors are calculated based on correlations. In addition to the standard orifices that AFT Arrow provides, you can enter your own custom orifice loss factor that is consistent with the application.
For convenience, you can specify constant loss characteristics of an orifice as a discharge coefficient (Cd) or as a loss factor (K). The standard orifice geometries include a functional dependence on the upstream and downstream pipe areas, which may be different.
When you select Resistance Curve, the Orifice Properties window makes additional features available. Using these new features you can input Orifice loss factors or pressure drops that vary with flow. To enter these factors, you can fit a curve to available data, or use interpolated x-y data. You can choose any of the optional flow and pressure parameters provided; you can also specify the most convenient units.
CdA for Sonic Choking
An optional input in the Orifice Properties window is the CdA for Sonic Choking. This parameter describes the effective area restriction in the orifice for the purpose of calculating sonic choking. In most cases, information on the CdA must be obtained from handbook or test data.
Note: The CdA for sonic choking may be different from the subsonic CdA loss model option in Arrow. The discharge coefficient can vary at different pressure ratios due to the vena contracta moving closer to or farther from the orifice restriction. For the highest accuracy the CdA used for subsonic and sonic losses should be tested and entered separately. See the "Modeling Choked Flow Through an Orifice" white paper on AFT's website for more information.
