.Buildings.Fluid.FixedResistances.HydraulicDiameter

Information

This is a model of a flow resistance with a fixed flow coefficient. The mass flow rate is computed as

ṁ = k √ΔP,

where k is a constant and ΔP is the pressure drop. The constant k is equal to `k=m_flow_nominal/sqrt(dp_nominal)`, where `m_flow_nominal` is a parameter.

Assumptions

In the region `abs(m_flow) < m_flow_turbulent`, the square root is replaced by a differentiable function with finite slope. The value of `m_flow_turbulent` is computed as `m_flow_turbulent = eta_nominal*dh/4*π*ReC`, where `eta_nominal` is the dynamic viscosity, obtained from the medium model. The parameter `dh` is the hydraulic diameter and `ReC=4000` is the critical Reynolds number, which both can be set by the user.

Important parameters

By default, the pressure drop at nominal flow rate is computed as

```dp_nominal = fac * dpStraightPipe_nominal,
```

where `dpStraightPipe_nominal` is a parameter that is automatically computed based on the nominal mass flow rate, hydraulic diameter, pipe roughness and medium properties. The hydraulic diameter `dh` is by default computed based on the flow velocity `v_nominal` and the nominal mass flow rate `m_flow_nominal`. Hence, users should change the default values of `dh` or `v_nominal` if they are not applicable for their model.

The factor `fac` takes into account additional resistances such as for bends. The default value of `2` can be changed by the user.

The parameter `from_dp` is used to determine whether the mass flow rate is computed as a function of the pressure drop (if `from_dp=true`), or vice versa. This setting can affect the size of the nonlinear system of equations.

If the parameter `linearized` is set to `true`, then the pressure drop is computed as a linear function of the mass flow rate.

Setting `allowFlowReversal=false` can lead to simpler equations. However, this should only be set to `false` if one can guarantee that the flow never reverses its direction. This can be difficult to guarantee, as pressure imbalance after the initialization, or due to medium expansion and contraction, can lead to reverse flow.

If the parameter `show_T` is set to `true`, then the model will compute the temperature at its ports. Note that this can lead to state events when the mass flow rate approaches zero, which can increase computing time.

Notes

For more detailed models that compute the actual flow friction, models from the package Modelica.Fluid can be used and combined with models from the `Buildings` library.

For a model that uses `dp_nominal` as a parameter rather than geoemetric data, use Buildings.Fluid.FixedResistances.PressureDrop.

Implementation

The pressure drop is computed by calling a function in the package Buildings.Fluid.BaseClasses.FlowModels, This package contains regularized implementations of the equation

m = sign(Δp) k √ Δp

and its inverse function.

To decouple the energy equation from the mass equations, the pressure drop is a function of the mass flow rate, and not the volume flow rate. This leads to simpler equations.

Revisions

• December 1, 2016, by Michael Wetter:
First implementation for #480.

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