Mechanisms of power dissipation in piezoelectric fans and their correlation with convective heat transfer performance

Piezoelectric fans offer an intriguing alternative to conventional rotary fans for thermal management of portable and wearable electronics due to their scalability, low power consumption and simple mechanical construct. We report a combined experimental and modeling study to help elucidate power dissipation mechanisms in piezoelectric fans. To analyze contributions from these different mechanisms, mathematical models that account for mechanical hysteresis, dielectric loss and viscous damping from generated air flows are used in conjunction with vibration amplitudes and power consumption data obtained experimentally from piezoelectric fans of different blade lengths, thicknesses and mass distributions. In parallel, we perform experiments on convective heat transfer coefficients and aerodynamic forces acting on surfaces that are oriented perpendicular with respect to fan-induced air flows. These experiments establish that the portion of power dissipation ascribed to air flows correlates well with the heat transfer performance and aerodynamic force. A power ratio, defined as the fraction of the air flow power to the total power dissipation, is then proposed as a useful indicator of the power efficiency of the piezoelectric fans. We show that the power efficiency exhibits a peak at a particular bias voltage amplitude, and provide a guideline to determine this optimum voltage. Lastly, we relate the air flow power to the blade's geometrical parameters to facilitate systematic optimization of the blades for both cooling performance and power efficiency. (C) 2018 Elsevier B.V. All rights reserved.