Coupled heat and mass transfer analysis for indoor air quality and thermal comfort in naturally ventilated offices

Abstract

Multiple discrete heat and pollution sources have a significant impact on the thermal comfort and indoor environment of naturally ventilated offices. Based on the heat-mass coupling mechanism, Computational Fluid Dynamics (CFD) numerical simulation combined with measurements were used to evaluate the influencing factors such as equipment load, inlet air temperature and ventilation strategy. Furthermore, key indicators such as air diffusion performance index (ADPI), air exchange efficiency (AEE) and pollutant removal rate (PRR) were quantified to analyze the thermodynamic coupling relationship between indoor airflow, temperature field and pollutant concentration field. The results show that from the perspective of heat transfer, when the indoor temperature is significantly higher than the outdoor temperature (ΔT = 4 °C), buoyancy-driven natural convection dominates the flow, enhancing air exchange while causing uneven temperature distribution that can reduce thermal comfort. Under strong buoyancy, pollutants such as CO₂ rise with the warm air, leading to clear stratification in the upper part of the room. In contrast, a slight negative temperature difference (ΔT = –1.6 °C) causes cold air to sink, which suppresses natural convection. This results in localized heat accumulation, air stagnation, and the buildup of pollutants near the floor. From a mass transfer perspective, the heat output from equipment primarily affects the temperature field and has minimal impact on airflow velocity. Global sensitivity analysis (GSA) identifies the ΔT as the primary factor influencing thermal comfort (PMV), followed by CO₂ concentration. Moreover, the combined effect of door and window openings (θ_w, θ_d) contributes up to 68 % of the PRR, emphasizing the importance of balanced ventilation to maintain effective diffusion. The research results provide a scientific basis for optimizing thermal comfort and pollutant control in natural ventilation environments.