Engee documentation

Flow Resistance (MA)

Generalised resistance in a wet air network.

blockType: AcausalFoundation.MoistAir.Elements.FlowResistance

flow resistance (ma)

Path in the library:

/Physical Modeling/Fundamental/Moist Air/Elements/Flow Resistance (MA)

Description

The Flow Resistance (MA) unit simulates 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 density of the moist air. The proportionality factor is determined based on the nominal operating condition specified in the parameters of the unit.

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 parameters Nominal mixture density, ρ , 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:

    ρ .

If the value of the parameters Nominal mixture density, ρ , is zero, the unit calculates the pressure drop as:

,

where is the proportionality factor:

.

The flow resistance is assumed to be adiabatic, so the specific total enthalpies of the mixture are equal:

.

Assumptions and limitations

  • The resistance is adiabatic. The unit does not exchange heat with the environment.

  • The pressure drop is assumed to be proportional to the square of the mass flow rate of the mixture and inversely proportional to the density of the mixture.

Ports

Conserving

# A — input or output
moist air

Details

Wet air port, corresponds to the input or output of the generalised resistance. This block has no defined directionality.

Program usage name

port_a

# B — input or output
moist air

Details

Wet air port, corresponds to the input or output of the generalised resistance. This block has no defined directionality.

Program usage name

port_b

Parameters

Parameters

# Nominal pressure drop — pressure drop at known operating conditions
Pa | GPa | MPa | atm | bar | kPa | ksi | psi | uPa | kbar

Details

Pressure drop from inlet to outlet at known operating mode. The unit uses nominal parameters to calculate the proportionality factor between pressure drop and mass flow rate.

Units

Pa | GPa | MPa | atm | bar | kPa | ksi | psi | uPa | kbar
Default value

0.001 MPa
Program usage name

delta_p_nominal
Evaluatable

Yes

# Nominal mixture mass flow rate — mass flow rate at known operating mode
kg/s | N*s/m | N/(m/s) | lbf/(ft/s) | lbf/(in/s)

Details

Mass flow rate from inlet to outlet at known operating mode. The unit uses nominal parameters to calculate the proportionality factor between pressure drop and mass flow rate.

Units

kg/s | N*s/m | N/(m/s) | lbf/(ft/s) | lbf/(in/s)
Default value

0.1 kg/s
Program usage name

mdot_nominal
Evaluatable

Yes

# Nominal mixture density — density at known operating conditions
g/cm^3 | kg/m^3 | lbm/gal

Details

Mass density within the generalised resistance at known operating mode. 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.

Units

g/cm^3 | kg/m^3 | lbm/gal
Default value

0.0 kg/m^3
Program usage name

rho_nominal
Evaluatable

Yes

# Cross-sectional area at ports A and B — flow area at the ports of generalised resistance
m^2 | cm^2 | ft^2 | in^2 | km^2 | mi^2 | mm^2 | um^2 | yd^2

Details

Flow area at the ports of generalised resistance. It is assumed that the ports are identical in size.

Units

m^2 | cm^2 | ft^2 | in^2 | km^2 | mi^2 | mm^2 | um^2 | yd^2
Default value

0.01 m^2
Program usage name

port_area
Evaluatable

Yes

# Fraction of nominal mixture mass flow rate for laminar flow transition — the ratio of the threshold flow rate to the nominal mass flow rate

Details

The ratio of the threshold mass flow rate to the nominal mass flow rate. The unit uses this parameter to calculate the mass flow threshold and to set the linearisation limits for the differential pressure.

Default value

1.e-3
Program usage name

laminar_fraction
Evaluatable

Yes