Investigations On The Airflow Characteristics And Thermal Comfort In Buoyancy-driven Natural Ventilation Rooms

Natural ventilation can improve the air environment and thermal comfort indoors to some extent. The buoyancy-driven natural ventilation is one of the major types of natural ventilation. Since the timescale of the transient buoyancy-driven natural ventilation to its steady state may be comparable to the time of occupancy, it is worth paying more attention to indoor airflow characteristics and thermal comfort during the transient ventilation.Numerical modeling of the transient buoyancy-driven natural ventilation is carried out in this paper. The comparisons between the numerical predictions and the predictions of theoretical models, numerical simulations and experimental data reported in the literature reveal that the numerical predictions of this study agree well with the experimental results.The numerical model is then employed to predict the airflow behavior of 33 cases of transient natural ventilation in total. Based on the numerical simulations, the effects of vent characteristic and heat source characteristic on indoor air temperature and velocity distributions, flow rate and vertical temperature difference under different initial temperature differences between interior and exterior are investigated systematically. The results indicate that the air temperature is higher at higher positions and the temperature stratification becomes more and more obvious with time for the cases of ΔT0=0 and ΔT0>0. The transient natural ventilation for the case of ΔT0<0 is relatively complex because the air flows into and out of the room through the upper and lower vents respectively at the early stage of ventilation and the airflow direction would then turn reversely. Whether the two vents locate on the same wall or two opposite walls have little influence on the transient ventilation process, such as the air temperature distribution, thermal stratification interface height and flow rate. For the case of larger vent area, the flow rate is larger and the temperature differences between the head and foot when sitting and standing are both smaller. For the cases of ΔT0=0 and ΔT0>0, the higher the heat source power is, the larger the flow rate is. For the case of ΔT0<0, if the heat source strength is increased, the time taken for the transient ventilation to transform from downward flow to upward flow is longer and the flow rate increases more rapidly after that. Moreover, for the case of higher power of heat source, the temperature differences between the head and foot when sitting and standing are both larger. Putting the heat source higher above the floor would lead to smaller flow rate, smaller and larger temperature differences between the head and foot when sitting and standing, respectively. The vent shape (width-height ratio) and the horizontal position of heat source on the floor have negligible effects on the transient ventilation.Based on the numerical simulations of flow field and temperature distribution, the indoor comfortable temperature, Tc, is calculated by the adaptive model, which is used to judge whether the temperatures of all points in the occupied zone are within the comfortable temperature range. The ratio of the comfortable points to all points in the occupied zone is denoted as a and is used to evaluate indoor thermal comfort. The effects of vent characteristic and heat source characteristic on indoor thermal comfort during the transient ventilation are then analyzed in detail. The results show that indoor thermal comfort could be improved by increasing the vent area, heat source power or by locating the heat source in higher position. However, the vent shape (width-height ratio) and the horizontal position of heat source on the floor appear to have little effect on the thermal comfort level indoors during the transient ventilation.