Flow in microchannels differs substantially from the flow in the macroscopic scale. Despite numerous works on two-phase flow and oscillatory single-phase flow in microchannels, oscillatory two-phase flow has not been thoroughly investigated. One of the situations where this type of flow occurs is in the ultrasonic drying device recently pioneered by Oak Ridge National Laboratory. An ultrasonic oscillatory piezoelectric transducer with microchannels is designed to dry the fabric by atomization and draining the water through the microchannel outlet. In this work, computational fluid dynamics is utilized to investigate the air-water two-phase flow driven by the ultrasonic vibrating microchannel. Our results indicate the importance of microchannel geometry and vibration conditions on drying efficiency.
An ultrasonic clothes dryer was developed by researchers at Oak Ridge National Laboratory based on a novel approach of using high-frequency mechanical vibration instead of heat to extract moisture as a cold mist. This technology is based on direct mechanical coupling between mesh piezoelectric (PZT) transducers and wet fabric. The vibration introduces sufficient momentum to the droplets trapped in the fabric pores to atomize them and leave the garment in a cold state. In the vibrating transducer, deformation followed by the effects of boundary layer acoustic streaming results in ejection of the atomized droplets. The research presented bridges the vibration of a PZT mesh transducer to the induced acoustic field and to capillary-wave theory. Mathematical modeling studies free and forced vibrations of a mesh-like PZT structure, using the structural parameters identified by actuation testing in several case studies. Computational fluid–structure interaction modeling is performed to couple the vibrations of a PZT transducer with an in-contact droplet. The results obtained are used to investigate (1) the transverse deformation of the vibrating mesh transducer in contact with a droplet, (2) the resultant boundary layer acoustic streaming in the fluid surrounding the vibrating surface, and (3) the droplet deformation and fluid ejection. The physics of atomization are linked to the level of the near-wall droplet vibrations induced by the surface deformation of the transducer. Then the surface deformation is linked to the properties of the PZT mesh transducer and input actuation frequency and power.