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• A theoretical model is proposed for the freezing of micro-sized water droplet. • Level-Set method is adopted to capture the ice front precisely. • The freezing patterns of micro-sized droplets are revealed. • The effects of location and number of nucleation site are taken into consideration. The freezing of mini-sized supercooled water droplet includes two successive stages, rapid porous dendritic ice formation in recalescence stage and gradual ice growth in solidification stage. However, the freezing of supercooled micro-sized water droplet is significantly different and remains ambiguous. By implementing crystallization kinetics into heat and mass transfer analysis in the present study, a 3-dimensional numerical model is built to depict the fast freezing dynamics of micro-sized water droplets with diameters of 10 and 1 μm. The temporal evolutions of temperature and solid mass fraction during freezing process are elaborated, revealing two specific freezing patterns of the micro-sized droplets. For the water droplet with d = 10 μm, the propagation of porous ice decelerates due to strong heat conduction within the droplet. While for the water droplet with d = 1 μm, the cold energy is abundant for ice freezing due to strong heat exchange with external cold air stream, leading to the consecutive propagation of compact ice with solid mass fraction of 1.0. Furthermore, the influences of the location and number of the nucleation site on the freezing process are discussed. Limited by the external heat transfer, the freezing time is independent of the number of nucleation site for the water droplet with d = 10 μm. For the water droplet with d = 1 μm with abundant cold energy, multiple nucleation sites can accelerate the freezing of the droplet. [ABSTRACT FROM AUTHOR]

• A three-dimensional theoretical model for droplet freezing is proposed. • Crystallization kinetics is taken into the temporal and spatial thermodynamic analysis. • The dynamic behavior of ice-water phase front in recalescence stage is revealed. • Theoretical and experimental results show a good agreement. A three-dimensional (3-D) theoretical model is proposed to take the crystallization kinetics into the consideration of heat transfer, which allows for the description of the entire freezing process of a supercooled water droplet with the inclusion of the description of recalescence. The temporal evolution of the temperature and solid mass fraction at various stages during freezing are numerically studied, which shows a good agreement with the experimental results of the water droplet frozen in cold nitrogen gas stream. In particular, the ice-water phase front dynamics initiated from a single ice nucleation site is numerically investigated in recalescence stage, which shows that the ice-water phase front spreads over the entire water droplet with a diameter of 1.4 or 2.0 mm within about 16.0 or 23.0 ms governed by crystallization kinetics, leading to the drastic increase of the temperature and solid mass fraction of the droplet under the effect of thermodynamics. The present theoretical model can be useful to describe the dynamic behavior of ice crystallization in many other relevant cases. [ABSTRACT FROM AUTHOR]

• A high-performance solid-liquid phase change thermal diode is presented. • Thermal rectification ratio of around 3.0 has been achieved. • Theoretical and experimental results find a great agreement. Phase change materials (PCMs) are highly effective to enhance the sustainability of thermal energy storage, management and control systems. The quest to find other novel applications of PCMs diverts attention towards phase change thermal diodes. However particularly, solid-liquid phase change thermal diodes (SL-PCTDs) suffer from fairly low thermal rectification ratio, hampering their implementation as thermal management and temperature control units. Herein, a SL-PCTD prototype is reported that is capable to access thermal rectification ratio of 3.0, being the highest in the family of SL-PCTDs. With the help of a theoretical model, the design was optimized in terms of bi-terminal length, providing the guideline to physically fabricate and experimentally execute the SL-PCTD. Paraffin wax and calcium chloride hexahydrate have been employed to assemble phase change bi-terminals. In operation, the heat flux was manipulated within a wide temperature bias of 10–40 °C, in both the forward and reverse directions. Also, theoretical model includes the natural convection and liquid-height-dependent effective thermal conductivity of calcium chloride hexahydrate, helping to meet the prerequisites of a high-performance design. [ABSTRACT FROM AUTHOR]

• The non-uniform melting of an ice particle in free ascending has been investigated. • The effects of supercooling and superheating have been discussed. • The morphological variations of the melting ice particle have been obtained and analyzed. In order to achieve a thorough understanding of the flow and heat transfer characteristics of phase change slurry, melting process of a spherical ice particle during free ascending in resting water can provide fundamental knowledge. The motion of the ice particle is depicted by the momentum equation while the flow and heat transfer are described using the enthalpy-porosity model to take the phase change into account. The ascending and melting characteristics at different superheating degrees of water as well as supercooling degrees of the ice particle are investigated. The results show that the spherical ice particle first accelerates to the maximum velocity of approximately 0.04 m s−1 under the effect of buoyancy, and then gradually decelerates as the ice particle melts. The superheating degree of water has a significant influence on the movement and melting characteristics of the ice particle while the supercooling degree imposes negligible effect. The variations in morphology and the local heat transfer coefficient of the ice particle are obtained and analyzed. It is found that the local heat transfer coefficient on the surface of the ice particle significantly influences the morphology of the ice particle during melting, which appears as sphere, ellipsoid, barrel shape and oval shape, sequentially. [ABSTRACT FROM AUTHOR]