The objective of this guide is to show how to set boundary conditions to Walls and Windows in Thermal Comfort (Convective Heat Transfer) Analysis.
Walls in reality are composed of multiple layers of different materials, such as concrete, brick, insulation, paint, etc. In thermal comfort simulations, we are only interested in the temperature of the air domain and use some simplifications to imitate heat transfer between the walls of the domain with external conditions. Turbulent flowWalls in reality are composed of multiple layers of different materials, such as concrete, brick, insulation, paint, etc. In thermal comfort simulations, we are only interested in the temperature of the air domain and use some simplifications to imitate heat transfer between the walls of the domain with external conditions.
Solid surfaces are treated with “Wall” boundary condition. As a default, every surface, which has no boundary condition is treated as Adiabatic Wall. In the following section, you will see the settings for some common wall conditions.
Adiabatic wall means that there should not be any heat transfer between the surface (surface of the fluid domain) and the surroundings. Usually, internal walls are assumed to be adiabatic, since same temperature conditions are expected in the neighboring rooms.
As a rule of thumb, one can use Laminar flow for natural convection, and k-omega SST for forced convection simulations. Adiabatic applies a zero gradient condition meaning, cell values on both sides of the surface are assumed to be same (therefore, the value won’t change).
Adiabatic Wall – Using Radiative Heat Transfer
If Radiation is enabled, we need to define radiative behavior of the surface.
- Grey-Body: This will ensure that the wall will receive and/or radiate heat.
- Emissivity: Emissivity is a constant, which defines the ratio of thermal radiation from a surface to the radiation form an ideal black surface with respect to the Stefan-Boltzmann law. The ratio varies from 0 to 1 (1 represents the ideal black body). Emissivity depends on many factors such as material, surface finish, temperature of the surface, wavelength and angle. For simplicity, one can use commonly published emissivity values for common materials :
|Wood||0.82 – 0.92|
|Soil||0.93 – 0.96|
|Vegetation||0.92 – 0.96|
|Concrete||0.92 – 0.96|
A constant temperature, heat flux or power source can be added to represent the behavior of heating or cooling walls. If initial boundary temperature predicted by the user, the solver will achieve the convergence faster.
External walls are the walls, assumed to be interacting often with ambient conditions. Often a considerable temperature difference is existed, therefore a high heat transfer is expected.
- Heat transfer coefficient: Convection coefficient between the fluid surface and exterior surrounding.
- (T) Ambient temperature: Temperature of the exterior surrounding
- Contact conductance: U-value of the wall
- (K) Thermal conductivity: Thermal conductivity of the wall material
- Layer thickness: Thickness of the wall material
First model: Wall is assumed to be too thin and/or conductivity is too high, so wall resistance is expected
Second model: Building walls have multiple layers. Overall U-value of the wall is assigned to define multilayered wall.
Third model: A single wall, with single material. Thermal conductivity and layer thickness is assigned. Solver calculates resistance.
Windows can be defined similarly to walls, except the radiative behavior is different. SimScale does not differentiate emissivity of different spectrums. Currently, we recommend to assign radiative behaviour as Transparent. This will ensure that the window surface is not going to be included in radiative heat transfer. Thus, assigning an additional radiative source will release energy from the window surface, without changing the window temperature.
The following picture represents an example case to clarify the conditions explained above. Boundary conditions are defined as follows:
- Internal walls: Adiabatic wall, grey body
- Cylindrical obstacle: Adiabatic wall, grey body (this represents a furniture, blocking the sunlight)
- External wall: External convection, a hot climate, a constant u-value, grey body
- Ceiling: External convection, a hot climate, a constant u-value, grey body
- Cooling floor: Constant temperature, grey body (this represents floor cooling system)
- Window: External convection, a hot climate, a constant u-value, transparent, additional radiative heat source
The temperature results on the domain clearly shows the temperature gradient between the cold surface and poorly insulated ceiling, which is exposed to hot ambient conditions. In front of the window, a high temperature region is seen. This shows that there is a hot object below the ceiling in this region.
To be able to see the temperature on the walls more clearly fluid domain, some walls, and ceiling are removed. Now the floor, adiabatic walls, and the window is visible. One can notice that the frontal surfaces of the adiabatic obstacle looks warmer than the other surfaces. The possible reason might be that the frontal surface receives radiation energy from the window.
Net Radiative Heat Flux contour shows that the frontal surface of the cylinder receives most of the radiation energy. In addition, one can see that warmer objects (adiabatic wall and window) has negative net radiative heat flux value, while colder objects (floor and cylinder) has a positive one. Finally, lower net radiative heat flux on the floor, behind the obstacle shows that furniture blocks the radiative energy dissipation coming through the window.