Solidification of phase change material on vertical cylindrical surface in holdup air bubbles

Solidification of phase change material around a vertical cylindrical surface was studied to investigate the performance of ice storage system and stored thermal energy. Air bubbles were generated in the phase change material at various air flow rate as a gas holdup to enhance the heat transfer rate and accelerate the ice layer growth at the solid–liquid interface. The test tube surface was cooled by ethylene glycol–water solution at a flow rate of 40% concentration by weight. The ice layer growth and solidification front velocity at solid–liquid interface were estimated from the temperature–time recorded data of a set of thermocouples fixed in a radial position perpendicular to cooled surface. The ice layer growth at the first instants of solidification process is much higher. Thereafter it decreased gradually according to the increasing of thermal resistance of ice layer. The increasing of ethylene glycol–water solution mass flow rate seems to accelerate the solidification process with small rate. The effect of air bubbles agitation was found to increase the ice layer growth rate and solidification front velocity by about of 20–45%. As a consequence the stored thermal energy was increased by about 55–115% with increasing air bubbles flow according to the attribute of generates turbulence at the solid–liquid interface. The measured data showed that with stirring the bulk water in energy storage tank, the storage time can be reduced by 10–35% of that without stirring.