Buildings.Fluid.MixingVolumes

Package with mixing volumes

Information

This package contains models for completely mixed volumes.

For most situations, the model Buildings.Fluid.MixingVolumes.MixingVolume should be used. The other models are only of interest if water should be added to or subtracted from the fluid volume, such as in a coil with water vapor condensation.

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

Package Content

Name	Description
MixingVolume	Mixing volume with inlet and outlet ports (flow reversal is allowed)
MixingVolumeMoistAir	Mixing volume with heat port for latent heat exchange, to be used with media that contain water
Examples	Collection of models that illustrate model use and test models
Validation	Collection of validation models
BaseClasses	Package with base classes for Buildings.Fluid.MixingVolumes

Buildings.Fluid.MixingVolumes.MixingVolume

Mixing volume with inlet and outlet ports (flow reversal is allowed)

Information

This model represents an instantaneously mixed volume. Potential and kinetic energy at the port are neglected, and there is no pressure drop at the ports. The volume can exchange heat through its heatPort.

The volume can be parameterized as a steady-state model or as dynamic model.

To increase the numerical robustness of the model, the constant prescribedHeatFlowRate can be set by the user. This constant only has an effect if the model has exactly two fluid ports connected, and if it is used as a steady-state model. Use the following settings:

• Set prescribedHeatFlowRate=true if the only means of heat transfer at the heatPort is a prescribed heat flow rate that is not a function of the temperature difference between the medium and an ambient temperature. Examples include an ideal electrical heater, a pump that rejects heat into the fluid stream, or a chiller that removes heat based on a performance curve. If the heatPort is not connected, then set prescribedHeatFlowRate=true as in this case, heatPort.Q_flow=0.
• Set prescribedHeatFlowRate=false if there is heat flow at the heatPort computed as K * (T-heatPort.T), for some temperature T and some conductance K, which may itself be a function of temperature or mass flow rate.
  If there is a combination of K * (T-heatPort.T) and a prescribed heat flow rate, for example a solar collector that dissipates heat to the ambient and receives heat from the solar radiation, then set prescribedHeatFlowRate=false.

Options

The parameter mSenFac can be used to increase the thermal mass of this model without increasing its volume. This way, species concentrations are still calculated correctly even though the thermal mass increases. The additional thermal mass is calculated based on the density and the value of the function HeatCapacityCp of the medium state state_default.
This parameter can for instance be useful in a pipe model when the developer wants to lump the pipe thermal mass to the fluid volume. By default mSenFac = 1, hence the mass is unchanged. For higher values of mSenFac, the mass will be scaled proportionally.

Set the parameter use_C_flow = true to enable an input connector for the trace substance flow rate. This allows to directly add or subtract trace substances such as CO2 to the volume. See Buildings.Fluid.Sensors.Examples.PPM for an example.

Implementation

If the model is operated in steady-state and has two fluid ports connected, then the same energy and mass balance implementation is used as in steady-state component models, i.e., the use of actualStream is not used for the properties at the port.

The implementation of these balance equations is done in the instances dynBal for the dynamic balance and steBal for the steady-state balance. Both models use the same input variables:

• The variable Q_flow is used to add sensible and latent heat to the fluid. For example, Q_flow participates in the steady-state energy balance

      port_b.h_outflow = inStream(port_a.h_outflow) + Q_flow * m_flowInv;
  

  where m_flowInv approximates the expression 1/m_flow.
• The variable mXi_flow is used to add a species mass flow rate to the fluid.

For the rationale of selecting different energy and mass balances, and for the use of prescribedHeatFlowRate, see the documentation of Buildings.Fluid.MixingVolumes.BaseClasses.PartialMixingVolume.

For simple models that uses this model, see Buildings.Fluid.HeatExchangers.HeaterCooler_u and Buildings.Fluid.MassExchangers.Humidifier_u.

Extends from Buildings.Fluid.MixingVolumes.BaseClasses.PartialMixingVolume (Partial mixing volume with inlet and outlet ports (flow reversal is allowed)).

Parameters

Connectors

Modelica definition

Buildings.Fluid.MixingVolumes.MixingVolumeMoistAir

Mixing volume with heat port for latent heat exchange, to be used with media that contain water

Information

Model for an ideally mixed fluid volume and the ability to store mass and energy. The volume is fixed, and latent and sensible heat can be exchanged.

This model represents the same physics as Buildings.Fluid.MixingVolumes.MixingVolume, but in addition, it allows adding or subtracting water in liquid phase. The mass flow rate of the added or subtracted water is specified at the port mWat_flow. The water flow rate is assumed to be added or subtracted at the temperature of the input port TWat. Adding water causes a change in enthalpy and species concentration in the volume.

Note that this model can only be used with medium models that include water as a substance. In particular, the medium model needs to implement the function enthalpyOfLiquid(T) and the integer variable Water that contains the index to the water substance. For medium that do not provide this functionality, use Buildings.Fluid.MixingVolumes.MixingVolume.

To increase the numerical robustness of the model, the constant prescribedHeatFlowRate can be set by the user. This constant only has an effect if the model has exactly two fluid ports connected, and if it is used as a steady-state model. Use the following settings:

• Set prescribedHeatFlowRate=true if the only means of heat transfer at the heatPort is a prescribed heat flow rate that is not a function of the temperature difference between the medium and an ambient temperature. Examples include an ideal electrical heater, a pump that rejects heat into the fluid stream, or a chiller that removes heat based on a performance curve. If the heatPort is not connected, then set prescribedHeatFlowRate=true as in this case, heatPort.Q_flow=0.
• Set prescribedHeatFlowRate=false if there is heat flow at the heatPort computed as K * (T-heatPort.T), for some temperature T and some conductance K, which may itself be a function of temperature or mass flow rate.
  If there is a combination of K * (T-heatPort.T) and a prescribed heat flow rate, for example a solar collector that dissipates heat to the ambient and receives heat from the solar radiation, then set prescribedHeatFlowRate=false.

Options

The parameter mSenFac can be used to increase the thermal mass of this model without increasing its volume. This way, species concentrations are still calculated correctly even though the thermal mass increases. The additional thermal mass is calculated based on the density and the value of the function HeatCapacityCp of the medium state state_default.
This parameter can for instance be useful in a pipe model when the developer wants to lump the pipe thermal mass to the fluid volume. By default mSenFac = 1, hence the mass is unchanged. For higher values of mSenFac, the mass will be scaled proportionally.

Set the parameter use_C_flow = true to enable an input connector for the trace substance flow rate. This allows to directly add or subtract trace substances such as CO2 to the volume. See Buildings.Fluid.Sensors.Examples.PPM for an example.

Extends from BaseClasses.PartialMixingVolume (Partial mixing volume with inlet and outlet ports (flow reversal is allowed)).

Parameters

Connectors

Modelica definition