Flow Resistance (MA)
Generalised resistance in a wet air network.
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Description
The Flow Resistance (MA) unit modulates the total pressure drop in the humid air network. The pressure drop is proportional to the square of the mass flow rate and inversely proportional to the wet air density. The proportionality factor is determined based on the nominal operating condition specified in the unit parameters.
This block is used when only the pressure drop as a function of mass flow rate is known about the component and detailed geometry information is not available.
Mass Conservation:
,
,
,
Where:
-
- mass flow rate.
-
Lower indices and are ports A and B respectively.
-
Lower indices and indicate water vapour and gas impurity properties respectively.
Energy balance:
,
where and are the energy flow through ports A and B respectively.
If the value of Nominal mixture density, , is greater than zero, the unit calculates the pressure drop as:
,
where:
-
and are pressures at ports A and B respectively.
-
- is the mass flow rate fraction for the laminar-turbulent transition.
-
- inlet pressure ( and depending on the direction of flow).
-
- inlet temperature ( and depending on the direction of flow).
-
- Proportionality factor calculated on the basis of nominal values:
.
If the value of Nominal mixture density, , is zero, the unit calculates the pressure drop as:
,
where is the proportionality factor:
.
The flow resistance is considered adiabatic, so the specific total enthalpies of the mixture are equal to
Ports
Non-directional
A - inlet or outlet
`moist air
Wet air port, corresponds to the input or output of the generalised resistance. This block has no defined directionality.
B - input or output
`humid air
Wet air port, corresponds to the input or output of the generalised resistance. This block has no defined directionality.
Parameters
Nominal pressure drop - pressure drop at known operating mode
`1000 Pa (by default)
Pressure drop from inlet to outlet at known operating mode. The unit uses the nominal parameters to calculate the coefficient of proportionality between differential pressure and mass flow.
Nominal mass flow rate - mass flow rate at known operating conditions
0.1 kg/s (By default).
Mass flow rate from inlet to outlet at known operating mode, kg/s. The unit uses the nominal parameters to calculate the coefficient of proportionality between pressure drop and mass flow rate.
Nominal density - density at known operating mode
`0 (By default)
Mass density inside the generalised resistance at known operating mode, kg/m³. The unit uses nominal parameters to calculate the coefficient of proportionality between pressure drop and mass flow rate. Set this parameter to zero to ignore the dependence of pressure drop on wet air density.
Cross-sectional area at ports A and B - flow area at ports of generalised resistance
0.01 m² (by default).
Flow area at ports of generalised resistance, m². It is assumed that the ports are identical in size.
Fraction of nominal mass flow rate for laminar flow - ratio of threshold flow rate to nominal mass flow rate
1e-3 (by default).
Ratio of threshold mass flow rate to nominal mass flow rate. The unit uses this parameter to calculate the threshold mass flow rate - and ultimately to set the linearisation limits for the differential pressure.