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 an exit 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, then specify the exit pressure and, if a heat transfer fluid model is used, the exit temperature.

Orifice Types

AFT Fathom 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 Fathom provides, you may 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 specify polynomial constants, fit a curve to available data, or use interpolated x-y data. When a variable loss is specified, AFT Fathom modifies the loss factor in the Solver (the part of AFT Fathom that applies the governing incompressible flow equations to obtain a solution to the pipe system) to agree with the solution. Here you can specify any of the available optional flow and pressure parameters using your preferred units.

An optional input in the Orifice Properties window is the Restricted Flow Area. This parameter describes the effective area restriction in the orifice. This is done for the purpose of assessing cavitation. In most cases, information on the size of the restricted area must be obtained from handbook or test data.