Experimental Study of Vibration Effects on Heat Transfer during Solidification of Paraffin in a Spherical Shell

Abstract

Two effects that have been observed when metals and metal alloys are vibrated during solidification are a decrease in dendritic spacing, which directly affects density, and faster cooling rates and associated solidification times. Because these two effects happen simultaneously during solidification it is challenging to determine the one effect independently from the other. Most previous studies were on metals and metal alloys. In these studies, the one effect, i.e. the decrease in dendritic spacing, might influence the other, i.e. the faster cooling rates, and vice versa. The direct link between vibration and heat transfer has not yet been studied independently. The purpose of this study was to experimentally investigate the effect of vibration only on heat transfer and thus solidification rate. Experiments were conducted on paraffin wax, because it had a clearly defined macroscopic crystal structure consisting of mostly large straight-chain hydrocarbons. The advantage of the large straight-chain hydrocarbons was that the dendritic spacing was not affected by the cooling rate.Experiments were done with paraffin wax inside hollow plastic spheres of 40 mm 2 diameter with 1 mm wall thickness. The paraffin wax was initially in a liquid state at a uniform temperature of 60°C and then submerged into a thermal bath at a uniform constant temperature of 15°C, which was approximately 20°C below the mean solidification temperature of the wax. Experiments were conducted in approximately 300 samples, with and without vibration at frequencies varying from 10 -300 Hz.The first set of experiments were conducted to determine the solidification times. In the second set of experiments, the mass of wax solidified was determined at discrete time steps, with and without vibration. The results showed that paraffin wax had vibration independent of solid density contrary to other materials, eg. metals and metal alloys. Enhancement of heat transfer resulted in quicker solidification times and possible control over the heat transfer rate. The increase in heat transfer leading to faster solidifcation times was observed to first occur, as frequency increased and then to decrease.

Numerical study of dynamic melting enhancement in a latent heat thermal energy storage system

“…They can be categorized into four different groups. The first technique is the implementation of ultrasonic vibration in the LHTES system with the enhancements mainly due to the combination of cavitation, acoustic streaming, and thermally oscillating flow initiated by the ultrasonic vibration [8][9][10][11][12]. Oh et al [8] experimentally investigated it during the melting process.…”

“…They observed that they could accelerate the melting process up to 2.5 times if compared to the rate of natural melting. Other authors experimentally and numerically investigated the implementation of this technique during the solidification process [9][10][11][12]. They focused on preventing the adhesion of the PCM solid layers to the cooling wall and on the generation of solid PCM slurries in subcooled liquid PCM, obtaining better results because of the longer absence of these solid layers.…”