Buildings.Fluid.FixedResistances

Package with models for fixed flow resistances (pipes, diffusers etc.)

Information

k = m ⁄ √ΔP.

For models of valves and air dampers, see Buildings.Fluid.Actuators. For models of flow resistances as part of the building constructions, see Buildings.Airflow.Multizone.

The model Buildings.Fluid.FixedResistances.FixedResistanceDpM is a fixed flow resistance that takes as parameter a nominal flow rate and a nominal pressure drop. The actual resistance is scaled using the above equation.

The model Buildings.Fluid.FixedResistances.LosslessPipe is an ideal pipe segment with no pressure drop. It is primarily used in models in which the above pressure drop model need to be replaced by a model with no pressure drop.

The model Buildings.Fluid.FixedResistances.SplitterFixedResistanceDpM can be used to model flow splitters or flow merges.

Extends from Modelica.Icons.VariantsPackage (Icon for package containing variants).

Package Content

Name	Description
FixedResistanceDpM	Fixed flow resistance with dp and m_flow as parameter
LosslessPipe	Pipe with no flow friction and no heat transfer
Pipe	Pipe with finite volume discretization along flow path
SplitterFixedResistanceDpM	Flow splitter with fixed resistance at each port
Examples	Collection of models that illustrate model use and test models
BaseClasses

Buildings.Fluid.FixedResistances.FixedResistanceDpM

Fixed flow resistance with dp and m_flow as parameter

Information

This is a model of a 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 and dp_nominal are parameters. 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 follows:

• If the parameter use_dh is false (the default setting), the equation m_flow_turbulent = deltaM * abs(m_flow_nominal), where deltaM=0.3 and m_flow_nominal are parameters that can be set by the user.
• Otherwise, the equation m_flow_turbulent = eta_nominal*dh/4*π*ReC is used, 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.

The figure below shows the pressure drop for the parameters m_flow_nominal=5 kg/s, dp_nominal=10 Pa and deltaM=0.3.

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.

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.

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.

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.

Extends from Buildings.Fluid.BaseClasses.PartialResistance (Partial model for a hydraulic resistance).

Parameters

Connectors

Modelica definition

Buildings.Fluid.FixedResistances.LosslessPipe

Pipe with no flow friction and no heat transfer

Information

Model of a pipe with no flow resistance, no heat loss and no transport delay. This model can be used to replace a replaceable pipe model in flow legs in which no friction should be modeled. This is for example done in the outlet port of the base class for three way valves, Buildings.Fluid.Actuators.BaseClasses.PartialThreeWayValve.

Extends from Buildings.Fluid.Interfaces.PartialTwoPortInterface (Partial model transporting fluid between two ports without storing mass or energy).

Parameters

Connectors

Modelica definition

Buildings.Fluid.FixedResistances.Pipe

Pipe with finite volume discretization along flow path

Information

Model of a pipe with flow resistance and optional heat exchange with environment.

Heat loss calculation

There are two possible configurations:

1. If useMultipleHeatPorts=false (default option), the pipe uses a single heat port for the heat exchange with the environment. Note that if the heat port is unconnected, then all volumes are still connected through the heat conduction elements conPipWal. Therefore, they exchange a small amount of heat, which is not physical. To avoid this, set useMultipleHeatPorts=true.
2. If useMultipleHeatPorts=true, then one heat port for each segment of the pipe is used for the heat exchange with the environment. If the heat port is unconnected, then the pipe has no heat loss.

Pressure drop calculation

The default value for the parameter diameter is computed such that the flow velocity is equal to v_nominal=0.15 for a mass flow rate of m_flow_nominal. Both parameters, diameter and v_nominal, can be overwritten by the user. The default value for dp_nominal is two times the pressure drop that the pipe would have if it were straight with no fittings. The factor of two that takes into account the pressure loss of fittings can be overwritten. These fittings could also be explicitly modeled outside of this component using models from the package Modelica.Fluid.Fittings. For mass flow rates other than m_flow_nominal, the model Buildings.Fluid.FixedResistances.FixedResistanceDpM is used to compute the pressure drop.

For a steady-state model of a flow resistance, use Buildings.Fluid.FixedResistances.FixedResistanceDpM instead of this model.

Extends from Buildings.Fluid.FixedResistances.BaseClasses.Pipe (Model of a pipe with finite volume discretization along the flow path).

Parameters

Connectors

Modelica definition

Buildings.Fluid.FixedResistances.SplitterFixedResistanceDpM

Flow splitter with fixed resistance at each port

Information

Model of a flow splitter or mixer with a fixed resistance in each flow leg. In each flow lag, a pressure drop can be modeled, and at the fluid junction, a mixing volume can be modeled.

The pressure drop is implemented using the model Buildings.Fluid.FixedResistances.FixedResistanceDpM. If its nominal pressure drop is set to zero, then the pressure drop model will be removed. For example, the pressure drop declaration

  m_flow_nominal={ 0.1, 0.1,  -0.2},
  dp_nominal =   {500,    0, -6000}

would model a mixer that has the nominal flow rates and associated pressure drops as shown in the figure below. Note that port_3 is set to negative values. The negative values indicate that at the nominal conditions, fluid is leaving the component.

Optionally, at the fluid junction, a control volume can be modeled. This is implemented using the model Buildings.Fluid.Delays.DelayFirstOrder. The fluid volume is modeled if energyDynamics <> Modelica.Fluid.Types.Dynamics.SteadyState. The control volume has the size

  V = sum(abs(m_flow_nominal[:])/3)*tau/rho_nominal

where tau is a parameter and rho_nominal is the density of the medium in the volume at nominal condition. Setting energyDynamics=Modelica.Fluid.Types.Dynamics.FixedInitial can help reducing the size of the nonlinear system of equations.

Extends from Buildings.BaseClasses.BaseIcon (Base icon), Buildings.Fluid.BaseClasses.PartialThreeWayResistance (Flow splitter with partial resistance model at each port).

Parameters

Connectors

Modelica definition