Internal Compressible Flow

Internal compressible flow is often found in industrial valves and nozzles. Such flow is typically fed by either a blow-down tank or a known flow rate. The physical mechanism used to feed internal compressible flow should be understood as it determines the analysis strategy.

The first question to answer is if the flow is truly compressible. A good test is to set up the analysis as an incompressible flow (following standard best practices for incompressible flows), and run it. If the Mach number in a region of the device approaches or exceeds 0.8, then the flow is likely compressible. Follow the appropriate procedure to run the analysis as compressible.

If the device is fed from a pressurized holding tank, and the outlet pressure is known

• Inlet: Specify total pressure

  Note: The total pressure will be the static pressure of the non-moving air within the tank.

• Inlet: If running heat transfer, specify total temperature.

• Outlet: Specify static pressure (P = 0)

If the device is fed from a pressurized holding tank, and the outlet pressure is not known

• Inlet: Specify total pressure.

  Note: The total pressure will be the static pressure of the non-moving air within the tank.

• Inlet: If running heat transfer, specify total temperature.

• Outlet: Specify static pressure (P = 0)

• Add an outlet extension to the device so that the specified static pressure does not influence or interfere with the flow as it leaves the physical device. This will allow for the calculation of the true outlet pressure, and for any shocks or expansion fans to form.

If the model cannot be extended at the outlet, and the inlet velocity (or volume flow rate) is known

• Inlet: Specify velocity (or volume flow rate) and total pressure.

  Note: The total pressure will be the static pressure of the non-moving air within the tank.

• Outlet: Specify Unknown condition

• This set up will not allow for shocks or expansion fans at the device outlet.

If the model cannot be extended at the outlet, and the inlet mass flow rate is known

• Inlet: Specify mass flow rate
• Outlet: Specify Unknown condition
• This set up will not allow for shocks or expansion fans at the device outlet.

Material Environment

For all cases, select Variable on the Material Environment dialog:

1. On the Material quick edit dialog, click Edit on the Environment line. (Alternatively, right click on the material name branch in the Design Study bar.)
2. Select Variable on the Material Environment dialog.

Specify the Environment Pressure to be the ambient pressure in the analysis:

1. On the Design Study bar, right click on the Material branch, and select Edit environment reference...
2. Specify the Pressure on the Material Environment dialog.

Related Topics

External Internal Compressible Flow Analysis

External Compressible Flow