Basic model for an ideally mixed fluid volume with the ability to store mass and energy. It implements a dynamic or a steady-state conservation equation for energy and mass fractions. The model has zero pressure drop between its ports.

If the constant simplify_mWat_flow = true then adding moisture does not increase the mass of the volume or the leaving mass flow rate. It does however change the mass fraction medium.Xi. This allows to decouple the moisture balance from the pressure drop equations. If simplify_mWat_flow = false, then the outlet mass flow rate is mout = min (1 + Δ Xw), where Δ Xw is the change in water vapor mass fraction across the component. In this case, this component couples the energy calculation to the pressure drop versus mass flow rate calculations. However, in typical building HVAC systems, Δ Xw < 0.005 kg/kg. Hence, by tolerating a relative error of 0.005 in the mass balance, one can decouple these equations. Decoupling these equations avoids having to compute the energy balance of the humidifier and its upstream components when solving for the pressure drop of downstream components. Therefore, the default value is simplify_mWat_flow = true.

Typical use and important parameters

Set the parameter use_mWat_flow_in=true to enable an input connector for mWat_flow. Otherwise, the model uses mWat_flow = 0.

If the constant simplify_mWat_flow = true, which is its default value, then the equation

  port_a.m_flow + port_b.m_flow = - mWat_flow;

is simplified as

  port_a.m_flow + port_b.m_flow = 0;

This causes an error in the mass balance of about 0.5%, but generally leads to simpler equations because the pressure drop equations are then decoupled from the mass exchange in this component. The model Buildings.Fluid.MixingVolumes.Validation.MixingVolumeAdiabaticCooling shows that the relative error on the temperature difference between these two options of simplify_mWat_flow is less than 0.1%.


When extending or instantiating this model, the input fluidVolume, which is the actual volume occupied by the fluid, needs to be assigned. For most components, this can be set to a parameter.

Input connectors of the model are

The model can be used as a dynamic model or as a steady-state model. However, for a steady-state model with exactly two fluid ports connected, the model Buildings.Fluid.Interfaces.StaticTwoPortConservationEquation provides a more efficient implementation.

For a model that instantiates this model, see Buildings.Fluid.MixingVolumes.MixingVolume.


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