UCLA

Current trends in consumer electronics thermal management have prompted a strong push toward low-power and low-noise air moving devices due to the inability of conventional rotary fans to provide satisfactory performance for small-scale applications. Piezoelectric fans, on the other hand, offer an intriguing alternative owing to their simpler structure, less noise, and lower power consumption. Piezoelectric fans typically consist of a flexible plate attached to a piezoelectric actuation beam that oscillates the plate near its resonance frequency, causing the plate to undergo large-amplitude vibrations, which in turn induces net airflows in the streamwise direction and enhances convective heat transfer from heated surfaces. Despite being available for more than 40 years, a comprehensive understanding of the power consumption mechanisms, complex fluid dynamics, and their correlation with heat transfer performance of the piezoelectric fans is missing from the relevant literature. In the present study, we aim to fill this knowledge gap and provide practical guidelines to facilitate the optimized design of these fans for a variety of thermal applications. We first report a combined experimental and modeling study to help elucidate power consumption mechanisms in piezoelectric fans. We identify three main sources of power consumption, namely dielectric loss, hysteresis loss, and aerodynamic loss. We then correlate the aerodynamic loss, as the portion of the total power most related to airflow generation, with the thermal performance of the fan. Based on the models derived in this study, practical recommendations are provided in order to increase the power efficiency of the fans. Next, we perform a combined experimental and numerical study on the vortex regimes present in the wake of piezoelectric fans of systematically varied geometries, resonance frequencies and amplitudes. We focus on the two-dimensional wake vortices on the mid-span plane. Three distinct vortex regimes are identified in the wake based on the fan’s oscillatory Reynold number. A regime map is proposed next, denoting the incident of each vortex regime as a function of relevant dimensionless parameters. Finally, the effects of these vortex regimes on the flow generation capability, and hence power efficiency of the fans, are examined. We then extend the previous study to the three-dimensional characteristics of the time-averaged induced jet in the wake of piezoelectric fans and correlate these characteristics with the transient vortex structures in the wake and their temporal evolution. Our results reveal, for the first time, the unconventional dual region of the induced jet in the wake of pitching plates; a shrinking region immediately downstream of the trailing edge followed by an abrupt expansion region. Scaling laws are also provided to facilitate predicting the jet boundary at various operating conditions and plate geometries. Finally, we investigate the effect of the shape of the oscillating plate on the thermo-power efficiency of the fans. Plates with concave and convex trailing edges and varying upstream surface areas are compared in their flow generation capability, power consumption, and thermal performance. Based on our experimental results, recommendations are provided for choosing the most efficient plate shape based on application and space constraints. The effect of heated surface roughness is also examined as another means to enhance the thermo-power efficiency of the piezoelectric fans.