The Use of Ceiling Fans in Reverse-Flow Mode for Comfort Cooling: Physiological Aspects and Relationship with International Standards

摘要

In a warming planet, the energy demand for space cooling is constantly increasing. Air movement can be considered an energy efficient mean to provide thermal comfort in warm season. Ceilingfans are among the most adopted sources of air movement in buildings, and they are widely used in many countries. In warm environmental conditions, ceilingfans usually blow air from the ceiling to the occupants. While the heat generated from a human body does not vary, this forced convection accelerates the heat transfer between the parts of the body skin hit by the airflow and the surrounding thermal environment. The elevated air speed increases the convective heat transfer coefficients and the rate of evaporation from those parts of the skin. In cold seasons, ceilingfans are often used in reverse flow mode for destratification purposes. In this case, the air speed is usually much lower than the direct flow, but considerably more uniform in the room. Some manufacturing companies lately raised their interest in analysing the reverse-flow as a means to provide thermal comfort also in summer, providing a gentler airflow that might be less likely to cause draft for the users. Hence, the aim of this study is to investigate the extent to which the reverse-flow could be used also for cooling in warm conditions. CFD simulations were used to evaluate (ⅰ) the air speed field generated using direct and reverse flow (magnitude and spatial distribution), and (ⅱ) how the heat transfer coefficients on the different body parts vary in both cases. The former enables to provide thermo -physiological insights, while the latter a comparison with air speed values reported in standards such as ASHRAE55. The results indicate that, although more evenly distributed within the room, the generated air speed with the reverse flow is considerably low. Hence, from a physiological point of view, it would not be sufficient to provide comfort cooling. Further work with human participants will investigate whether this low speed might generate a subjective perception of comfort cooling.