超音波 振動이 相變化 熱傳達 促進에 미치는 影響

Abstract

This study presents experimental and numerical works on the effect of ultrasonic vibrations on enhancement of Phase Change Heat Transfer. In order to investigate the melting process of a phase change material (PCM), the present study was applied experimental condition as following : one without ultrasonic vibrations (i.e., natural melting) and the other with ultrasonic vibrations. And a constant heat flux condition was considered and modified dimensionless number such as Ste* and Ra* were adopted. Also, heat flow phenomena were visually measured by a particle image velocimetry (PIV) and a thermal infrared camera with and without ultrasonic vibrations. Numerical work was applied to calculate the acoustic pressure in the liquid paraffin, instead of experimental measurement. For the numerical work, a coupled finite element-boundary element method (coupled FE-BEM) was applied. The experimental results revealed the acoustic streaming could accelerate the melting process as much as 2.5 times, compared to the rate of the melting without ultrasonic vibration and was saved the total consumed electricity about 2.3 ~ 2.9 Wh. In addition, temperature and Nusselt number over time provided conclusive evidences which are the important roles of acoustic streaming on the melting phenomena of the phase change material. The numerical results of the profiles of acoustic pressure variation are consistent with those of augmentation of heat transfer. That is, the higher acoustic pressure distributions are, the higher augmentation ratio of heat transfer is near to ultrasonic transducers. In the end, it is concluded that acoustic pressure variations are closely related to the augmentation of heat transfer. This mechanism is believed to increase a coefficient of heat transfer