Characteristics and comfort evaluation of sinusoidal airflows by regulating motor rotating frequency of a floor fan

Increasing air movement by utilizing electric fans is among the common approaches for comfort and energy savings in buildings in summer; however, the use of electric fans is usually the forced constant airflow. This study reformed the one-chip computer program of a floor fan motor and simulated dynamic airflow through controlling the rotating frequency only. The flow field characteristics of constant, oscillated, sinusoidal airflows with periods of 10 s, 30 s, 60 s, 100 s were measured. The comfort performance was evaluated by chamber experiments, with 20 subjects exposed to six airflow patterns under 30 $dgC, 70%RH. The results showed that the simulated sinusoidal airflows had relatively higher turbulence intensity (32%-37%) and beta values (> 0.4). While subjects' thermal sensations were not statistically significant among six airflows, their reported discomfort symptoms during 60 min exposure were reduced under sinusoidal airflows. The calculated convective heat transfer shared similar variations to instantaneous air velocity and skin temperature. A large fluctuation of 10-50 W/m(2) and higher total convective heat loss (3000-3500 W/m(2)) were found for sinusoidal period 30 s. This study develops a new method to simulate varying air velocities through conveniently controlling the fan motor amplitude and frequency, and verifies the comfortable feelings to dynamic sinusoidal airflows. The work benefits to improve the performance of the current electric fans with lower costs and promote the applications of personal ventilation devices in buildings, thus optimizing human thermal comfort, reducing dependences on air conditionings and achieving building energy efficiency.