Local Restriction (TL)
Local flow constriction in a thermal liquid network.
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Description
The Local Restriction (TL) block simulates the pressure drop due to a localised flow restriction, such as a valve or orifice, in a thermal liquid network.
Ports A and B represent the input and output of the Local Restriction (TL) block. Depending on the Restriction type parameters, the constriction area can be fixed or controlled. In the case of controlled, the input signal on the AR port sets the cross-sectional area of the constriction.
The block icon changes depending on the value of the Restriction type parameters.
In a localised constriction there is no heat exchange with the environment, the process is diabatic.
Local Restriction (TL) consists of a sudden contraction followed by a sudden expansion. At the point of contraction, the fluid accelerates, causing a drop in pressure. In the expansion zone, if the Pressure recovery parameters are off, the momentum of the accelerated fluid is lost. If the Pressure recovery parameters are enabled, the sudden expansion recovers some of the momentum and allows the pressure to rise slightly after the constriction.
The figure shows a schematic representation of a localised constriction.
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Conservation of mass
The mass conservation equation for Local Restriction (TL) is as follows
Where:
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- mass flow rate through port A;
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- mass flow rate through port B.
Pulse balance
The pressure difference between ports A and B follows from the momentum balance:
where:
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- pressure drop;
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- liquid density;
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- dynamic viscosity of the liquid;
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- cross-sectional area of ports A and B;
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- cross-sectional area of the constriction;
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- liquid velocity in the constriction;
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- critical fluid velocity;
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- critical Reynolds number;
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- flow coefficient.
If the Pressure recovery parameters are off, then
where:
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- port pressure A.
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- port pressure B.
If the Pressure recovery parameters are enabled, then
Ports
Inlet
AR - value of the cross-sectional area of the constriction, m²
scalar
Input signal specifying the value of the cross-sectional area of the constriction. The value is limited by the minimum and maximum limits set by the block parameters.
Dependencies
This port is only used if the Restriction type parameter is set to Variable.
Non-directional
A - inlet or outlet port
thermal liquid
Thermal liquid port, corresponds to the inlet or outlet of the local limitation. This unit has no internal directionality.
B - inlet or outlet port
thermal liquid.
Thermal liquid port, corresponds to the inlet or outlet of the local limitation. This unit has no internal directionality.
Parameters
Restriction type - possibility to change the cross sectional area of the constriction
Variable (by default) | Fixed
Select whether the cross-sectional area of the constriction can be changed during the simulation:
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Variable- the input signal on the AR port defines the cross-sectional area that can be varied during the simulation. The parameters Minimum restriction area and Maximum restriction area set the lower and upper limits of the cross-sectional area. -
Fixed- the cross-sectional area of the constriction set by the Restriction area parameters remains constant during the simulation. In this case the AR port is hidden.
Minimum restriction area - lower limit of the value of the constriction cross-sectional area
1e-10 m² (by default).
Lower limit of the cross-sectional area of the constriction. You can use this parameter to set the leakage area. The AR input signal is limited to this value to prevent further reduction of the cross-sectional area.
Dependencies
To use this parameter, set the Restriction type parameter to Variable.
Maximum restriction area - upper limit of the value of the cross-sectional area of the constriction
0.005 m² (by default).
Upper limit of the cross-sectional area. The AR input signal reaches the upper limit at this value to prevent further increase of the cross-sectional area.
Dependencies
To use this parameter, set the Restriction type parameter to Variable.
Restriction area is the cross-sectional area of the constriction
1e-3 m² (by default).
Cross-sectional area of the constriction normal to the flow direction.
Dependencies
To use this parameter, set the Restriction type parameters to Fixed.
Cross-sectional area at ports A and B - cross-sectional area at ports A and B
0.01 m² (by default).
The cross-sectional area of the flow at ports A and B. This area is assumed to be the same for the two ports.
Discharge coefficient - ratio of actual mass flow rate to theoretical mass flow rate through the constriction
`0.64 (by default).
Discharge coefficient is a semi-empirical parameter defined as the ratio of the actual mass flow rate to the theoretical mass flow rate through the restriction.
Critical Reynolds number - critical Reynolds number
150 (By default).
Reynolds number at which the transition from laminar to turbulent regime occurs.
Pressure recovery - pressure recovery accounting
On (By default) | Off.
Determines whether pressure recovery at the outlet of the local constriction is taken into account.