Analysis of convective heat transfer coefficient correlations for ventilative cooling based on reduced-scale measurements

Ventilative cooling can be used as a passive cooling measure to reduce the cooling energy demand of buildings. It can be used during the day, directly removing excessive heat gains, or during the night (i.e. night flush), in which cold outdoor air flows through the building and cools down the indoor air volume and subsequently the thermal mass of the building. Night flushing reduces the indoor air temperatures at the beginning of the next day and the cooling demand over the day. To assess the impact of ventilative cooling on the temperatures in a building and the resulting cooling energy demand, building energy simulations (BES) can be performed. An important parameter to set in BES is the convective heat transfer coefficient (CHTC) (or CHTC correlation) for the interior surfaces to calculate convective heat transfer from these surfaces to the indoor environment and vice versa. The majority of the available CHTC correlations for internal surfaces in BES are based on natural convection, with a temperature difference as the driving force for convection. However, in case of night flush the airflow rates through the building can be quite large and mixed convection can occur due to the possible presence of relatively high indoor air velocities. A solution to this problem could be the use of convective heat transfer correlations for forced convection, or correlations for external surfaces subjected to the atmospheric boundary layer (ABL) wind flow, which generally calculate the convective heat transfer based on a reference wind speed at a certain location. This paper presents reduced-scale experiments (ABL wind tunnel measurements) of velocities, turbulence levels, air temperatures, surface temperatures and convective heat fluxes in a generic cubical cross-ventilated enclosure. One of the walls of the enclosure is heated and has a higher temperature than the ambient air, resulting in convective heat exchange between this surface and the air inside the enclosure. The experimental results are used to calculate the values of CHTC for this mixed convection case, in which the contribution of forced convection dominates, as indicated by the calculated Richardson number. The measurement results are subsequently compared with both CHTC correlations for natural and forced convection. The results indicate that the average CHTC values from the experiments generally show a fair to good agreement with the values obtained using CHTC correlations for forced convection, while the agreement with the CHTC values from the CHTC correlations based on natural convection is, as expected, much worse (72-91% difference). This finding is in line with earlier publications in which the use of CHTC correlations for ventilative cooling assessment in BES is discussed. A proper definition of the CHTC correlations for ventilative cooling applications is required to correctly estimate the reduction in cooling energy demand and is part of a larger ongoing research effort.