modelMassExchange
Extends from Buildings.BaseClasses.BaseIcon (Base icon).
Information
This model computes the mass transfer based on similarity laws between the convective sensible heat transfer coefficient and the mass transfer coefficient.
Using the Lewis number which is defined as the ratio between the heat and mass diffusion coefficients, one can obtain the ratio between convection heat transfer coefficient h in (W/(m^2*K)) and mass transfer coefficient hm in (m/s) as follows:
h ⁄ hm = ρ cp Le(1-n) ⁄ hm
where ρ is the mass density, cp is the specific heat capacity of the bulk medium and n is a coefficient from the boundary layer analysis, which is typically n=1/3. From this equation, we can compute the water vapor mass flow rate nA in (kg/s) as
nA = Gc ⁄ (cp Le(1-n)) (Xs - X∞),
where Gc is the sensible heat conductivity in (W/K) and Xs and X∞ are the water vapor mass per unit volume in the boundary layer and in the bulk of the medium. In this model, Xs is the saturation water vapor pressure corresponding to the temperature Tsur which is an input.
Parameters
| Type | Name | Default | Description |
|---|---|---|---|
| Real | Le | 1 | Lewis number (around 1 for water vapor in air) |
| Real | n | 1/3 | Exponent in bondary layer ratio, delta/delta_t = Pr^n |
Connectors
| Type | Name | Default | Description |
|---|---|---|---|
| Modelica.Blocks.Interfaces.RealInput | XInf | Water mass fraction of medium | |
| Modelica.Blocks.Interfaces.RealInput | TSur | Surface temperature | |
| Modelica.Blocks.Interfaces.RealOutput | mWat_flow | Water flow rate | |
| Modelica.Blocks.Interfaces.RealInput | Gc | Signal representing the convective (sensible) thermal conductance in [W/K] |
Components
| Type | Name | Default | Description |
|---|---|---|---|
| Buildings.Utilities.Psychrometrics.X_pW | humRatPre | Model to convert water vapor pressure into humidity ratio | |
| Buildings.Utilities.Psychrometrics.pW_TDewPoi | TDewPoi | Model to compute the water vapor pressure at the dew point | |
| Modelica.Blocks.Math.Gain | gain | Constant to convert from heat transfer to mass transfer | |
| Modelica.Blocks.Math.Product | mWat | Water flow rate | |
| Modelica.Blocks.Math.Min | min | ||
| Modelica.Blocks.Sources.Constant | zero | Constant for zero | |
| Modelica.Blocks.Math.Add | delX | Humidity difference |
Contents
| Name | Description |
|---|---|
 Medium | Fluid medium model |
Revisions
-
April 12, 2017, by Michael Wetter:
Removed temperature that is no longer needed.
This is for issue Buildings #704. -
December 14, 2012 by Michael Wetter:
Renamed protected parameters for consistency with the naming conventions and fixed error in the documentation. -
August 13, 2008 by Michael Wetter:
First implementation.