Convective heat transfer coefficients over human body under non-uniform approaching airflow conditions

Abstract

Elevated air movement, achieved through natural ventilation or electrical fans, has been advocated in the latest indoor climate standards as a more energy-efficient and cost-effective alternative or enhancement to full air conditioning for improving occupant thermal comfort in warm environments. However, the flow generated by electrical fans is typically non-uniform, and the relationship between the airflow profile and convective heat transfer from the human body remains unclear. In this study a multi-fan wind tunnel is utilized to generate arch-shaped and bowl-shaped airflow profiles, and the convective heat transfer coefficient (hc) over a thermal manikin were measured. The findings reveal that hc in non-uniform airflow differs significantly from that in uniform airflow. For arch-shaped airflow profiles, the whole-body hc can be higher by up to 20%, with the most obvious increment in hc over the limbs. In contrast, a notable increase in the hc over the torso is observed at the highest profile curvature under bowl-shaped airflow profile, while the hc of the limbs and whole-body is less affected by the profile curvature. The results have implications to future fan designs that could generate airflow profile most effective to human body cooling. As demonstrated by a case study that simulates the cooling provided by elevated airflow to pedestrians after traversing in hot and humid summertime, a tailored arch-shaped airflow profile has the potential of reducing the required cooling time by half. In addition, a new set of regression formula is proposed in this study for predicting hc under non-uniform air flow conditions, which could be integrated into a thermo-physiological model to improve the accuracy of thermal comfort prediction.