Electroelastic Modeling and Testing of Direct Contact Ultrasonic Clothes Drying Systems

Energy efficient appliances and devices are becoming increasingly necessary as emissions from electricity production continue to increase the severity of global warming. Many of such appliances have not been substantially redesigned since their creation in the early 1900s. One device in particular which has arguably changed the least and consumes the most energy during use is the electric clothes dryer. The common form of this technology in the United States relies on the generation of thermal energy by passing electrical current through a metal. The resulting heat causes liquid within the clothing to evaporate where humid air is ejected from the control volume. While the conversion of energy from electrical to thermal through a heating element is efficient, the drying characteristics of fabrics in a warm humid environment are not, and much of the heat inside of the dryer does not perform work efficiently.

In 2016, researchers at Oak Ridge National Laboratory in Knoxville, Tennessee, proposed an alternative mechanic for the drying of clothes which circumvents the need for thermal energy. This method is called direct-contact ultrasonic clothes drying, utilizing atomization through direct mechanical coupling between mesh piezoelectric transducers and wet fabric. During the atomization process, vertical oscillations of a contained liquid, called Faraday excitations, result in the formation of standing waves on the liquid surface. At increasing amplitudes and frequencies of oscillation, wave peaks become extended and form "necks" connecting small secondary droplets to the bulk liquid. When the oscillation reaches an acceleration threshold, the droplet momentum is sufficient to break the surface tension of the neck and enable the droplets to travel away from the liquid. For smaller drops where surface tension is high, a larger magnitude of acceleration is needed to reach the critical neck lengths necessary for droplet ejection. The various pore sizes within the many fabrics comprising our clothing results in many sizes of droplets retained by the fabric, affecting the rate of atomization due to the differences in surface tension.

In this study, we will investigate the physical processes related to the direct contact ultrasonic drying process. Beginning with the electrical actuation of the transducer used in the world's first prototype dryer, we will develop an electromechanical model for predicting the resulting deformation. Various considerations for the material properties and geometry of the transducer will be made for optimizing the output acceleration of the device. Next, the drying rates of fabrics in contact with the transducer will be modeled for identification of parameters which will facilitate timely and energy efficient drying. This task will identify the first ever mechanically coupled drying equation for fabrics in contact with ultrasonic vibrations. The ejection rate of the water atomized by the transducer and passed through microchannels to facilitate drying will then be physically investigated to determine characteristics which may improve mass transport. Finally, future considerations and recommendations for the development of ultrasonic drying will be made as a result of the insight gained by this investigation.