.AixLib.Fluid.MixingVolumes.MixingVolume

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 AixLib.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 AixLib.Fluid.MixingVolumes.BaseClasses.PartialMixingVolume.

For simple models that uses this model, see AixLib.Fluid.HeatExchangers.HeaterCooler_u and AixLib.Fluid.Humidifiers.Humidifier_u.

Revisions

• October 19, 2017, by Michael Wetter:
  Set initialize_p to final so that it does not appear as a user-selectable parameter. This is done because initialize_p has been changed from a constant to a parameter for Buildings, issue 1013.
• April 11, 2017, by Michael Wetter:
  Changed comment of heat port, as this needs to be the total heat flow rate in order to be able to use this model for modeling steam humidifiers and adiabatic humidifiers.
  Removed blocks QSen_flow and QLat_flow.
  This is for issue Buildings #704.
• April 11, 2016 by Michael Wetter:
  Corrected wrong hyperlink in documentation for issue 450.
• January 19, 2016, by Michael Wetter:
  Updated documentation due to the addition of an input for trace substance in the mixing volume. This is for issue 372.
• January 17, 2016, by Michael Wetter:
  Removed protected block masExc as this revision introduces a conditional connector for the moisture flow rate in the energy and mass balance models. This change was done to use the same modeling concept for the moisture input as is used for the trace substance input.
• December 2, 2015, by Filip Jorissen:
  Changed code for handling trace substance insertions using input C_flow.
• May 1, 2015 by Michael Wetter
  Set final keyword for masExc(final k=0). This addresses issue 230.
• February 11, 2014 by Michael Wetter:
  Redesigned implementation of latent and sensible heat flow rates as port of the correction of issue #197.
• February 7, 2012 by Michael Wetter:
  Revised base classes for conservation equations in AixLib.Fluid.Interfaces.
• September 17, 2011 by Michael Wetter:
  Removed instance medium as this is already used in dynBal. Removing the base properties led to 30% faster computing time for a solar thermal system that contains many fluid volumes.
• September 13, 2011 by Michael Wetter:
  Changed in declaration of medium the parameter assignment preferredMediumStates=true to preferredMediumStates= not (energyDynamics == Modelica.Fluid.Types.Dynamics.SteadyState). Otherwise, for a steady-state model, Dymola 2012 may differentiate the model to obtain T as a state. See ticket Dynasim #13596.
• July 26, 2011 by Michael Wetter:
  Revised model to use new declarations from AixLib.Fluid.Interfaces.LumpedVolumeDeclarations.
• July 14, 2011 by Michael Wetter:
  Added start values for mass and internal energy of dynamic balance model.
• May 25, 2011 by Michael Wetter:
  
  • Changed implementation of balance equation. The new implementation uses a different model if exactly two fluid ports are connected, and in addition, the model is used as a steady-state component. For this model configuration, the same balance equations are used as were used for steady-state component models, i.e., instead of actualStream(...), the inStream(...) formulation is used. This changed required the introduction of a new parameter m_flow_nominal which is used for smoothing in the steady-state balance equations of the model with two fluid ports.
  • Another revision was the removal of the parameter use_HeatTransfer as there is no noticeable overhead in always having the heatPort connector present.
• July 30, 2010 by Michael Wetter:
  Added nominal value for mC to avoid wrong trajectory when concentration is around 1E-7. See also https://trac.modelica.org/Modelica/ticket/393.
• February 7, 2010 by Michael Wetter:
  Simplified model and its base classes by removing the port data and the vessel area. Eliminated the base class PartialLumpedVessel.
• October 12, 2009 by Michael Wetter:
  Changed base class to AixLib.Fluid.MixingVolumes.BaseClasses.ClosedVolume.