Spray Discharge
The Spray Discharge junction must have at least one connecting pipe, and it allows up to four. This junction offers a flexible way of modeling a flow exiting the system through a nozzle or spray device.
The Spray Discharge Properties window follows the second of the two basic Properties window formats. A table displays the connecting pipe information. The pipe table grows in size to accommodate up to four connecting pipes.
The exit pressure to which the Spray Discharge junction discharges must be entered, as well as the temperature in case of inflow.
Two geometries can be modeled. The first is a normal, one hole spray, where the discharge flow area of the hole is entered. The second is a sparger which, in principle, works in the same way as a spray. For the sparger, the flow area of a single hole and the number of holes are entered. This assumes that the holes are hydraulically close in proximity and are the same area.
For each connecting pipe, the pipe table on the Pipe Losses tab lists the reference flow direction and up to two loss factors. The first loss factor is for physical flow out of the pipe and into the junction, the second is for physical flow into the pipe and out of the junction. The loss factors can be specified independently or left as zero. To edit within the pipe table, click in the appropriate column and row.
The discharge coefficient for the junction is entered in the appropriate box. The physical area of the exit flow can also be entered. These two parameters are used to calculate the discharge flow rate, which depends on the difference between the internal pressure in the system and the specified exit pressure.
If you select a Resistance Curve, the Spray Discharge Properties window makes additional features available. Using these new features you can input the spray pressure drop that varies 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 Arrow modifies the loss factor in the solver to agree with the solution. You can choose any of the optional flow and pressure parameters provided, and you can specify the most convenient units.
CdA for Sonic Choking
An optional input in the Spray Discharge Properties window is the CdA. This parameter describes the effective area restriction in the spray for the purpose of calculating sonic choking.
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.
Special Conditions
Spray Discharge junctions have the following Special Conditions on the Optional tab:
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None - Flow is allowed to enter/exit the system at the junction as determined by the system hydraulics and external pressure.
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Closed - Flow is not allowed to enter/exit the system at the junction, but flow can still go in/out of the connected pipes as determined by the system hydraulics.
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No Inflow - Flow is not allowed to enter the system from the surroundings, but is allowed to exit the system as determined by the system hydraulics and external pressure.
Flow Net vs Through
The Output Control allows reporting of Mass/Volumetric "Flow Rate Net At Junction" or "Flow Rate Through Junction". Depending on the number of connected pipes and flow directions determined by the Solver, the Flow Rate Through Junction may report values of N/A. This is because there may not be a unique flow path "through" the junction.
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Net At Junction - Generally this parameter is measuring the flow entering/exiting the system. It is calculated by summing the flow into the junction and subtracting the sum of flow out of the junction. A positive value indicates flow exiting the system, and a negative value indicates flow entering the system.
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Through Junction - Generally this parameter is measuring the flow through a unique path in the junction if one is available. A unique flow path could include a scenario where the Special Condition is set to Closed, but flow enters from one upstream pipe, passes through the junction (lossless), and exits into one downstream pipe. Another example of a unique flow path is if the Spray has only one connected pipe.
Note: Pipes with inverted Reference Positive Flow Directions and negative flowrates can cause confusion when interpreting results with the Flow Rate Net At Junction and Flow Rate Through Junction parameters. Care should be taken when observing the signs of the parameters.
Transient
(XTS Module Only) On the Transient tab the Subsonic and Sonic CdA data can be varied with time to simulate the opening or closing of the junction. For more information on transient data, including event transients, see the Junction Transient Data topic.
