Visualization of the phase change behavior of sodium acetate trihydrate for latent heat storage

• The velocity and temperature during the solidification of a PCM were visualized.
• The temperature around crystals was lower than the melting point with the convection.
• The temperature increased to the melting point after the solidification.
• Solution flow into the needle-shaped crystals was observed.

A latent heat storage system has higher storage capacity than a sensible heat storage system. Sodium acetate trihydrate has large latent heat at its melting point of 58 °C, which is suitable for a hot-water supply system. The crystal growth rate, convective flow and temperature fields during the phase change of sodium acetate trihydrate were quantitatively visualized using PIV analysis with temperature sensitive particles (TSPs) which has enabled a simultaneous measurement of the velocity and temperature fields. The velocity field was obtained by the PIV analysis of TSPs and the temperature field was simultaneously visualized from the intensity decay of TSPs. The visualized temperature of the solid–liquid multiphase flow increased slowly as crystals grew. The temperature around the crystal was lower than the melting temperature during the phase change process with the convection because the crystal was needle-shaped and a liquid phase was also present between crystals. After the convective velocity rapidly decreased due to the solidification, the temperature started to increase to around the melting point. Buoyancy-driven convection occurred under certain conditions of supercooling temperature and concentration of sodium acetate trihydrate. However, convection could not be fully developed over a certain range of concentration and supercooling temperature because the crystal growth rate was too fast and solidification was completed quickly.

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