humidity diffusion

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HumidityDiffusion

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  • Semi Permeable Humidity Diffusion

    Introduction

    This document describes the implementation of a feature to model the diffusion of water vapor through semi-permeable membranes such as Gore-Tex in ESC.

    Required ESC Versions

    In order to use this feature, you must run with one of the TMG/ESC patches beginning with the following version numbers:

    NX Ideas 5.1072 NX 5.1072 NX Ideas 6.822 NX 6.822 NX 7.239

    The NX Ideas 5.1072 patch can be obtained from the MAYA download site,

    ftp://ftp.mayahtt.com/downloads/patches/ideas/nxideas5/

    Installation instructions can be found here:

    ftp://ftp.mayahtt.com/downloads/patches/Installation_Instructions.html

    Usage

    Because we are not able to deliver user interface (I-DEAS interface) changes in a software patch, the feature is triggered via a special keyword in the name of the flow surface.

    The semi-permeable diffusive flow surface should be named in the form _#MEMBRANE_R#1.2#FLOWSURFACE where, in this example, 1.2 is the value of the diffusive resistance. The units of the diffusion resistance of the humidity should be in units consistent with the solve units of the model. For example, in S.I. units it is sec/m. When a flow surface is named this way, a diffusive boundary condition to ambient air is applied on the wall. If humidity is defined at ambient, it diffuses to the ambient air humidity. Otherwise the ambient humidity is treated to be zero.

    ( )R

    Amb =

    FA M A = area of the fluid face

    F = density of water vapor in the fluid

  • Amb = density of water vapor at the ambient condition R = diffusion resistance of Gore-Tex M=mass flow

    M, the mass flow, is added as a source term to the mass conservation and humidity conservation equations to ensure the overall balance of the humidity.

    Example

    Following is a simple model with an inlet, opening and specified ambient humidity of zero, and a flow surface with diffusive resistance of 1.2 sec/mm specified as _#MEMBRANE_R#1.2#FLOWSURFACE.

    Figure 1 shows the test model: In blue is the flow surface where humidity diffusion to ambient takes place. The

    resistance coefficient is 1.2 in the model units, which are mm. Figure 2 shows the flow surface details, in particular the flow surface name which contains the resistance.

    The face on the -X plane is an inlet with 0.1 m/s velocity and 20% relative humidity.

    The face on the +X plane is a vent to ambient.

    Figures 3 and 4 show the velocity and relative humidity results on a cutting plane situated midway between the +/-Z surfaces of the model. We have a gradient of velocity and humidity in the x-y plane indicating the flow of humidity from the top. Humidity is lower near the surface and increases towards the center because of high convection. Clearly, the humidity is leaving from the top near the flow surface.

  • Figure 1: General overview of the test model.

    Figure 2: Flow Surface specification in IDEAS.

  • Figure 3: Velocity on cutting plane.

    Figure 4: Relative humidity on cutting plane.

  • The overall mass balance reported in the message file is Overall Mass balance is

    +--------------------------------------------------------------------+

    | Boundary Mass Flow and Total Source Term Summary | +--------------------------------------------------------------------+

    ADIABATIC FACES -2.6372E-08 INLET _FAN 1_FAN 1 1.2025E-03 VENT 1_VENT 1 -1.2025E-03 OPEN_HEXAHEDRAL_ELEMENTS -1.8282E-11 -----------

    Global Mass Balance: 5.8025E-09

    Global Imbalance, in %: 0.0005 %

    Adiabatic faces are flow surfaces that can be slip or no-slip wall faces, but do not convect heat to the surroundings. Convecting faces are also flow surfaces that can be slip or no-slip wall faces, but do convect heat to the surroundings. We normally do not allow mass flow through the flow surfaces (adiabatic or convecting). But here we can see that mass is flowing through the adiabatic faces.

    Humidity balance is

    +--------------------------------------------------------------------+

    | Boundary Humidity Flow and Total Source Term Summary | +--------------------------------------------------------------------+

    ADIABATIC FACES -2.6381E-08 INLET _FAN 1_FAN 1 3.4581E-06 VENT 1_VENT 1 -3.4310E-06 OPEN_HEXAHEDRAL_ELEMENTS 0.0000E+00 -----------

    Global Humidity Balance: 7.7739E-10

    Global Imbalance, in %: 0.0225 %

    The report clearly shows that we have humidity flow of 2.6381E-08 leaving through the flow surface (adiabatic faces).

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