Feasibility of using ultrasonic flexural waves as a cooling mechanism

The potential convective heat transfer capability of an ultrasonic flexural wave (UFW) is experimentally investigated. The UFW includes an ultrasonic flexural standing wave (USW) and an ultrasonic flexural traveling wave (UTW). The factors that might affect the cooling performance of the UFW are investigated. Those include the vibration amplitude of the UFW, the gap between the cooling source and the object above it, and the temperature of the object being cooled. It was observed that the temperature drop increased with the vibration amplitude. At gaps below 100 μm, a temperature drop was not observed. As the gap was increased to more than 100 μm, the temperature drop increased until it reached an optimum gap producing maximum temperature drop. Beyond the optimum gap, the temperature drop began to decrease. Also, it was observed that the temperature drop increased as the temperature difference between the object and ambient air increased. The cooling performance of the USW and UTW was investigated and compared. The differences in cooling performance were found to be insignificant. This indicates that acoustic streaming is the dominant factor in the convective heat transfer using the UFW. However, using resonance, the UTW creates a temperature drop six times greater than the UTW for a given power supply. With the USW having a vibration amplitude of 25 μm, an object at 98°C was cooled down to 58°C in 5 min. The temperature drop obtained by using the USW was approximately 80% of a conventional fan oriented with respect to the heated object such that the maximum heat transfer occurs. The UFW-based fan offers advantages over the conventional fan, such as silent operation, minimal heat dissipation, lack of wearing parts, and slim profile. These benefits make the fan an ideal candidate for cooling miniature parts in an enclosed workspace. Finally, a possible design option for minimizing the fan using thin-film PZT is presented