Effect of ultrasound on supercooling characteristics of calcium lactate-ammonium chloride-water composite phase change materials

Phase change materials can absorb or release a large amount of latent heat in the process of phase change. They have the advantages of large energy storage density, stable energy storage, small size and easy control. They are widely used in the fields of heat storage and temperature control. Supercooling degree generally exists in phase change materials and affects the cold storage performance of low-temperature phase change cold storage materials. Severe supercooling phenomenon will lead to the inability to release the cold amount of phase change cold storage materials and reduce the energy utilization efficiency. Solving the problem of supercooling of cryogenic phase change storage materials is the key to its better application. It has been proved that supercooling of phase change materials can be solved by adding nucleating agent. In addition to salt nucleating agents, nanometer nucleating agents have attracted more and more attention because they can reduce the supercooling degree of materials and improve the thermal conductivity of materials. In addition, the dispersion effect and cavitation effect of ultrasound can optimize the supercooling characteristics. In order to solve the problem of high supercooling degree of inorganic phase change materials, ultrasonic shock treatment was carried out on the calcium lactate-ammonium chloride - water phase change system and ultrasonic field was added in the solidification process to study the influence of ultrasonic wave on the solidification characteristics of calcium lactate-ammonium chloride - water phase change system. It was found that after increasing ultrasonic shock, the supercooling degree decreased by 0.62 ℃, the phase transition process lasted for 7 755 s, and the latent heat of phase transition of the solution was 276.2 J/g, increasing by 4.14%. During the solidification of the solution, the ultrasonic phase change system was added and no supercooling occurred. The latent heat of phase change of 8 256 s solution was 283.5 J/g, an increase of 6.9%. Under the action of ultrasonic external field, cavitation bubbles are generated in the phase-change material. With the growth and rupture of cavitation bubbles, micro-jet and local high pressure are caused, which provide local disturbance to the phase-change material, and then induce nucleation of the phase-change material to reduce the subcooling degree. Compared with ultrasonic field, under the effect of ultrasonic phase transformation system of supercooling is instantaneous, that is because the whole liquid region under ultrasonic and cavitation bubbles everywhere at the same time, namely the moment too cold, phase transition time delayed, bubble ceaselessly as the movement of the surrounding medium movement, grew up or suddenly burst, the surrounding liquid into bubbles produced by high temperature and high pressure, and gives off heat. The results showed that adding ultrasonic field in the solidification process of calcium lactate-ammonium chloride-water phase change system could improve the performance of calcium lactate- ammonium chloride-water phase change system.