CFD simulering av kallras : Undersökning av temperatur- och luftbeteende intill höga glasfasader och i vistelsezon med golvvärme som en värmekälla

• Main Author: Al Taweel, Maher
• Format: Others
• Language: Swedish
• Published: KTH, Tillämpad termodynamik och kylteknik 2013
• Subjects: cold downdraughts; radiation; underfloor heating; surface temperature; air velocities; glass facades; CFD Star-CCM+; kallras; strålningsdrag; golvvärme; yttemperatur; lufthastigeter; glasfasader
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-133483

Glass has sophisticated front properties and are used as facades in high buildings. During cold periods, these glass facades could cause thermal discomfort, due to cold downdraught. Cold downdraught can be countered by placing heaters under glass surfaces. Nowadays technology offers highly insulating windows, which is why there is an interest to investigate the indoor climate with only underfloor heating. The research in this area is limited, and few empirical methods are available. Theoretical analysis has begun but it still brand new. The aim of this investigation was to present the thermal indoor climate influenced by various parameters, such as outdoor temperature, U-value and the glass height. The results were also meant to be used as reference tools in future projects. A reference building was modeled in simulation software called CFD Star-CCM+. The assignment was initiated by Incoord, a leading consulting company in energy, indoor climate and installation planning. The results showed that the air velocity increases with decreasing outdoor temperature and decreases with increasing thermal insulation (lower U-value). At the edges of the glass the air velocity becomes twice as large compared to the velocity of the air in the middle of the atrium. The air velocity (maximum and average) at 0.1 m above the floor is always higher than at 2.0 m. The lowest air velocities start from about 0.25 m/s at 0 ℃ and reaches to 0.60 m/s at -20 ℃. That means these air velocities are too high for what is accepted as a good indoor climate, where the maximum allowable air velocity is 0.15 m/s. The outdoor temperatures and the glass facade’s U-value also have an effect on the surface temperature of the glass facade. This decreases the surface temperature with decreased outdoor temperature, and the surface temperature increases at lower U-value. The height of the glass facades proved to affect both the air velocity in the occupied zone and in the glass surface temperature. The air velocity increases with the glass’ height. The increase is higher at 0.1 m than at 2.0 m above the floor. The result shows also that the average air velocity is lower than 0,15 m/s at window height lower than 5 m. But, at the same height the maximum air velocity is higher than 0.3 m/s. The surface temperature of the glass facades increases with the glass’ height. This is because the indoor heat transfer coefficient increases with height. The outdoor heat transfer coefficient is a function of the wind speed and was assumed to be constant. The underfloor heating, which is represented in the simulations with a floor surface temperature of 27 ℃, is not enough to maintain a good indoor climate in any of simulations. The results of this thesis showed a strong relation between indoor climate, outdoor temperature, U-value and the glass height. This study also showed that the floor heating is not enough to counteract the cold draft during extreme cold periods, in high glass buildings. The presented results can be used as a reference tool for the assessment of air velocities and surface temperatures, in similar high buildings.