Numerical study of convection during night cooling and the implications for convection modeling in Building Energy Simulation models

When predicting the performance of night ventilation with Building Energy Simulation models, the results are highly sensitive to the selection of the convective heat transfer coefficient (CHTC). Therefore, a numerical study was conducted, simulating rooms with Fluent 12 for 8 h of flow time. The study was limited to 2D and results must be considered as qualitative. 14 initial conditions were applied to 4 geometries, including the thermal mass in floor and ceiling. The ACH was varied from 4 to 11 h^-1 and initial ΔK between room air and supply air from 1.25 to 10 K. The flow development showed three flow phases, influencing the CHTC at the ceiling. The implications for the selection of CHTC in BES are discussed and applied in a sensitivity study in TRNSYS 17. The sensitivity on the thermal comfort was investigated through a parameter variation, using convection correlations from literature. When comparing with the internal CHTC calculation in TRNSYS, the usage of natural convection correlations increased the predicted weighted overheating hours at 4 and 12 ACH respectively with 18 K h (19%) and 105 K h (12%), whereas forced convection correlations reduced this at 4 and 12 ACH respectively with 30 K h (11%) and 210 K h (29%).