Thermal CFD Analysis of Tubular Light Guides
2. CFD Model
- Simulation of thermal profile and determination of heat loss through tubular light guides;
- Specification of potential condensation risks on internal surface of the light guide tube at the interface between the tube and the roof construction;
- Simulation of air flow within tubular light guides under specified boundary conditions.
- Outdoor temperature interval is set to −15 °C and +15 °C to correspond with the winter and spring/autumn seasons of the temperate climate of the Central Europe region;
- Indoor temperature +20 °C and relative humidity of indoor air 50%.
2.1. Physical Models
2.2. Geometric Models
- The mesh adaptation on the internal surfaces of the light guide serves for the fine discretisation of boundary layers. It is essential for the simulation model accuracy;
- Widening of the horizontal mesh distances in the detail of the roof construction connected with the light guide tube is useful for reduction of the simulation time;
- 3D model discretisation of very thin metal tube (thickness 1 mm) needs the discretisation of millimetre fractions in the tube and also in the part of the roof construction that is in contact with the tube. The very fine discretisation extends the simulation time;
- The simplified model was created on the “shell conduction” method. In this model the light guide tube is substituted with a virtual layer of cells for simulation of heat transfer along the model its surface;
- The 2D rotation symmetrical model with unstructured mesh gives results that are comparable to the 3D models. The simple model offers possibility of more calculation results and geometric variations at a given simulation time;
- Performed mesh variants serve to show that the mesh independence of the geometric model is achieved.
2.3. The Final 2D Simplified Model
3. Results and Discussion
- Average temperature in the tubular light guide—difference about 7% (for temperatures in °C) and max difference 0.35% (compared temperatures in K);
- Minimal internal surface temperatures—difference max 6% (compared temperatures in °C) and max difference 0.30% (compared temperatures in K);
- Total heat flux and heat loss—difference to 7%;
- Maximal accessible velocity of air flow in the whole domain—to 50% (max. velocity is lower than 0.15 m·s−1);
- L3D is thermal coupling coefficient (W·K−1).
- Uj is thermal transmittance of the j element in the studied detail (W·m−2·K−1).
- Aj is area of the j element in the studied detail (m2).
- Ψ is linear thermal transmittance (W·m−1·K−1).
- b is length of the linear thermal bridge (m).
Example of the Real Evaluation of Tubular Light Guides
|Length||Tubular light guide diameter|
|d = 0.30 m||d = 0.60 m||d = 0.90 m|
|L (m)||TTE (-)||TT3D (W/K)||Q (W)||TTE (-)||TT3D (W/K)||Q (W)||TTE (-)||TT3D (W/K)||Q (W)|
- Variation I: 1 × TLG, d = 0.60 m, length 1.0 m: Total heat loss by TLG = 70 W—the tubular light guide increases the heat transmission loss by 74%.
- Variation II: 2 × TLG, d = 0.30 m, length 1.0 m: Total heat loss by TLG = 3.8 W—the tubular light guide increases heat transmission loss by 4%.
Conflicts of Interest
thermal coupling coefficient (W·K−1)
length of the linear thermal bridge(m)
linear thermal transmittance (W·m−1·K−1)
heat loss (W)
thermal conductivity (W·m−1·K−1)
heat loss coefficient (W·m−2·K−1)
local temperature (K)
thermal transmittance (W·m−2·K−1)
point thermal transmittance (W·K−1)
Tube transmission efficiency
an exponent in TTE (-)
specular reflectance (-)
portion of the zenithal sky (°)
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Šikula, O.; Mohelníková, J.; Plášek, J. Thermal CFD Analysis of Tubular Light Guides. Energies 2013, 6, 6304-6321. https://doi.org/10.3390/en6126304
Šikula O, Mohelníková J, Plášek J. Thermal CFD Analysis of Tubular Light Guides. Energies. 2013; 6(12):6304-6321. https://doi.org/10.3390/en6126304Chicago/Turabian Style
Šikula, Ondřej, Jitka Mohelníková, and Josef Plášek. 2013. "Thermal CFD Analysis of Tubular Light Guides" Energies 6, no. 12: 6304-6321. https://doi.org/10.3390/en6126304