modelPartialSaturatedControlVolume

Information

This model represents a partial control volume for either condensation or evaporation processes of water with liquid and vapor phases in equilibrium and at a saturated state. Models that extend this base class need to assign the mass flow rate at each port and the enthlapy at each port, as exemplifed in the evaporation and condensation models listed below. The volume can exchange heat through its heatPort when configured with dynamic mass and energy balances. In steady state, the heat port is conditionally removed in order to maintain a consistent set of equations.

This model is similar to Modelica.Fluid.Examples.DrumBoiler.BaseClasses.EquilibriumDrumBoiler with the following exceptions:

• Rather than a two-phase medium, fluid mediums are modeled as two single-state fluids, with liquid water at the up-stream port(port_a), and steam vapor at the downstream port (port_b) for instances of this base class that model evaporation (the opposite for condensation);
• The metal drum is excluded from the mass and energy balances;

Implementation

This model is configured to allow both steady state and dynamic mass and energy balances. The heat transfer through the heatPort is disabled in steady state balance. This is required because the fluid is restricted to a saturated state; thus, the heat transfer rate is a function of mass flow rate only if the volume is steady. The fluid mass m in the volume is calculated as

m = ρ_sV_s + ρ_wV_w

where ρ is density,V is volume, and subscripts represent the steam and liquid water components, respectively. The total internal energy U is

U = ρ_sV_sh_s + ρ_wV_w − pV

where h is specific enthalpy, p is pressure, and the total volume of fluid V=V_s+V_w.

The steady state mass balance is given as

ṁ_s + ṁ_w = 0,

while no additional equation is given for the steady state energy balance, since the heat flow rate into the water must be removed from the system in which the control volume is used.

The dynamic mass and energy balances are given as

dm/dt = ṁ_s + ṁ_w
dU/dt = Q̇ + ṁ_s h_s + ṁ _w h_w

where ̇ṁ_s and ṁ_w are the mass flow rates of steam and liquid water respectively; Q̇ is the heat flow rate into the control volume; h_s and h_w are the specific enthalpies of steam and liquid water, respectively. Note that with an evaporation process, the liquid phase (water) is always assigned at the port_a (inlet), while the vapor phase (steam) is always at the port_b (outlet). The opposite holds for a condensation process.

Assumptions

Three principal assumptions are made with this model:

• The fluid within the volume is wet steam.
• Liquid and vapor subcomponents are at equilibrium; and
• Fluid is discharged from the volume as ei ther saturated liquid or saturated vapor.

Models that extend this base class include Buildings.DHC.Plants.Steam.BaseClasses.ControlVolumeEvaporation and Buildings.DHC.Loads.Steam.BaseClasses.ControlVolumeCondensation.

Reference

Hinkelman, Kathryn, Saranya Anbarasu, Michael Wetter, Antoine Gautier, and Wangda Zuo. 2022. “A Fast and Accurate Modeling Approach for Water and Steam Thermodynamics with Practical Applications in District Heating System Simulation.” Preprint. February 24. doi:10.13140/RG.2.2.20710.29762.

Parameters

Connectors

Components

Contents

Name	Description
MediumWat	Liquid water medium
MediumSte	Steam medium

Revisions

• May 4, 2022 by David Blum:
  Update stateSte to use MediumSte instead of MediumWat.
• February 26, 2022 by Kathryn Hinkelman:
  First implementation.