Simscape Reference

Constant Area Pneumatic Orifice (ISO 6358)
b = 0.41 + 0.272 d / D
This equation is not used by the block and you must specify the critical pressure ratio
directly.
If the orifice is parameterized in terms of the C v [2] coefficient, then the C v coefficient is
turned into an equivalent effective orifice area for use in the Gidlund formula:
A = 1.6986e – 5 C v
By definition, an opening or restriction has a C v coefficient of 1 if it passes 1 gpm (gallon
per minute) of water at pressure drop of 1 psi.
If the orifice is parameterized in terms of the K v [2] coefficient, then the K v coefficient is
turned into an equivalent effective orifice area for use in the Gidlund formula:
A = 1.1785e – 6 C v
K v is the SI counterpart of C v . An opening or restriction has a K v coefficient of 1 if it
passes 1 lpm (liter per minute) of water at pressure drop of 1 bar.
The heat flow out of the orifice is assumed equal to the heat flow into the orifice, based on
the following considerations:
• The orifice is square-edged or sharp-edged, and as such is characterized by an abrupt
change of the downstream area. This means that practically all the dynamic pressure
is lost in the expansion.
• The lost energy appears in the form of internal energy that rises the output
temperature and makes it very close to the inlet temperature.
Therefore, q i = q o , where q i and q o are the input and output heat flows, respectively.
The block positive direction is from port A to port B. This means that the flow rate is
positive if it flows from A to B.
Basic Assumptions and Limitations
• The gas is ideal.
• Specific heats at constant pressure and constant volume, c p and c v , are constant.
• The process is adiabatic, that is, there is no heat transfer with the environment.
• Gravitational effects can be neglected.
• The orifice adds no net heat to the flow.
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