Mechanical Vibration Effects on Enhanced Melting of Phase Change Materials within Latent Heat Thermal Energy Storage Devices Under Different Container Orientations

Abstract

Phase change materials (PCM) are playing a bigger role in our daily life, from cooling of electric vehicle batteries to spacecrafts in the outer space. Research on PCM melting behavior and system performance has generally focused on cases without mechanical vibrations, whereas practical latent heat thermal energy storage (LHTES) units incorporating PCM are subjected to uncontrollable mechanical vibrations. An instrumented rectangular LHTES unit that incorporates a heated plate connected to two aluminum end plates normal to the heated plate was built. The remaining two plates were made of Plexiglas to accommodate visual observations. In effect, the PCM was held in the space defined by the aluminum-Plexiglas side walls. The set-up can be oriented as needed with respect to the gravitational field. Two sets of experiments corresponding to (i) no mechanical vibrations and (ii) mechanical vibrations were conducted. For cases with no mechanical vibrations, melting experiments corresponding to the inclination angles of 90, 60, 45, 30 and 0 degrees were conducted. For the inclination angle of 90 degrees (horizontal heater plate at bottom), expedited melting was observed next to the two aluminum walls, whereas wavy liquid-solid interface (LSI) was present in the vicinity of the mid plane signifying presence of a thermally-unstable layer. It was also noted that strong three dimensionality of the liquid-solid interface was persistent contrary to information available at the observation plane. Temperature recordings on both heat conducting aluminum walls also exhibited lack of plane symmetry. As the inclination angle was raised, distinction among thermocouples readings among the two batches were clearly observable. For the inclination angle of 0 degrees (vertical heater plate), the wavy liquid-solid interface was observed next to the bottom heated aluminum plate where a thermally-unstable layer is present. Among the cases with no vibration, the melting rate based on the estimation of the LSI at the observation window has increased as 3 many as 45% with the increase of the vibration frequency. In explaining the LSI waviness, the effect of the Rayleigh number on intensification of the thermally unstable layer being formed between the bottom heated aluminum plate and the receding PCM’s liquid-solid interface was elucidated. The unit was also subjected to mechanical vibrations at an acceleration level of 1 g subject to 50 and 100 Hz for the inclination angle of 0 degrees. In view of greater leakage of liquid from the unit, observing melting enhancement is promising.