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BuildingElement Boundary Condition

A comprehensive thermal boundary condition for building walls in OpenFOAM that accounts for:

  • Conduction through wall (U-value or multi-layer)
  • Internal and external convection
  • Solar radiation gains
  • Long-wave radiation to sky
  • Integration with OpenFOAM radiation models

Features

Simple Mode (U-value)

  • Specify overall thermal transmittance (U-value)
  • Quick setup for standard walls

Multi-layer Mode

  • Define individual layer properties (conductivity, density, specific heat)
  • Transient heat storage in wall layers
  • Automatic temperature distribution calculation

Radiation Support

  • Solar gains through g-value
  • Sky radiation with view factor
  • Integration with OpenFOAM radiation models (e.g., P1, viewFactor)

Debug Options

  • Temperature and heat flux monitoring
  • Face-specific or patch-wide statistics
  • Configurable output intervals

Usage

Simple Mode Example

wall
{
    type            buildingElement;
    
    // Thermal properties
    uValue          0.24;        // W/(m²K)
    gValue          0.6;         // Solar heat gain coefficient
    
    // External conditions
    tempExt         283;         // External air temperature [K]
    tempSky         273;         // Sky temperature [K]
    solarExt        500;         // Solar radiation [W/m²]
    hExt            25;          // External convection coefficient [W/(m²K)]
    
    // Radiation properties
    emissivity      0.9;         // Surface emissivity
    viewSky         0.5;         // View factor to sky
    radField        qr;          // Internal radiation field name (optional)
    
    // Initial value
    value           uniform 293;
}

Multi-layer Mode Example

wall
{
    type            buildingElement;
    
    // Layer definition
    layers
    (
        // Inner layer (e.g., plaster)
        {
            lambda      0.7;         // Thermal conductivity [W/(mK)]
            thickness   0.02;        // Layer thickness [m]
            rho         1200;        // Density [kg/m³]
            cp          1000;        // Specific heat [J/(kgK)]
            nNodes      5;           // Discretization nodes
        }
        // Insulation layer
        {
            lambda      0.04;
            thickness   0.2;
            rho         30;
            cp          1000;
            nNodes      10;
        }
        // Outer layer (e.g., concrete)
        {
            lambda      1.4;
            thickness   0.1;
            rho         2200;
            cp          900;
            nNodes      5;
        }
    );
    
    gValue          0.6;
    tempExt         283;
    tempSky         273;
    solarExt        500;
    hExt            25;
    emissivity      0.9;
    viewSky         0.5;
    radField        qr;
    
    value           uniform 293;
}

Debug Options

    // Debug options (optional)
    debugTemp       true;        // Output temperatures
    debugFlux       true;        // Output heat fluxes
    debugFace       0;           // Face index to monitor (-1 for none)
    debugInterval   60;          // Output interval [s]

Compilation

cd buildingElementBC
wmake

Integration with Solvers

Add to your solver's controlDict:

libs ("libbuildingElementBC.so");

Or for Windows:

libs ("libbuildingElementBC.dll");

Heat Flux Convention

  • Positive flux = heat entering the wall from inside
  • Negative flux = heat leaving the wall to inside

Output Variables

When debug is enabled:

  • q_conv_int: Interior convective heat flux
  • q_rad_int: Interior radiative heat flux (from OpenFOAM radiation model)
  • q_conduction: Conduction through wall
  • q_conv_ext: Exterior convective heat flux
  • q_rad_ext: Exterior radiative heat flux to sky/environment
  • q_solar: Solar heat gains
  • q_total: Net heat flux balance

Notes

  • The boundary condition automatically handles the interaction with OpenFOAM radiation models
  • For multi-layer mode, ensure time step satisfies stability criteria (Fourier number < 0.5)
  • External surface resistance (0.04 m²K/W) and internal surface resistance (0.13 m²K/W) are included in U-value calculation