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The venation-finned porous network has been demonstrated as a promising method to maximize the thermal transport access. To improve the heat storage efficiency, an innovative venation-finned porous network is employed here to enhance the melting performance of phase change materials (PCMs). The venation-finned porous network is quantitatively described by Murray's law and Voronoi method, and a modified thermal lattice Boltzmann model of PCM melting processes in a square cavity is developed and numerically analyzed to optimize the venation-finned porous network. The melting performance of composite PCMs with three configurations (venation fin, porous network and venation-finned porous network) are compared and analyzed. Moreover, the effects of branching angle and Murray coefficient on melting performance are comprehensively studied. It indicates that venation-finned porous network is favorable to melting performance improvement due to venation's inherent efficient heat transfer paths. Compared to venation fins and porous networks, the melting duration time of venation-finned porous networks is reduced by 78.4% and 21.4%, respectively. Furthermore, the branching angle of 45 ∘ and Murray coefficient of 3 are suggested for maximizing the melting efficiency. Importantly, the melting mechanism is conduction and convective conjugated heat transfer in composite PCMs with venation fins, however, it is dominated by heat conduction for those with porous networks or venation-finned porous networks. [ABSTRACT FROM AUTHOR]

Understanding the fundamental mechanisms of vapor condensation on rough surfaces is crucial to a wide range of industrial applications. A hybrid thermal lattice Boltzmann model of the condensation heat transfer process on downward-facing rough surfaces characterized by the Cantor fractal is developed and numerically analyzed to investigate the condensation phase change behaviors on rough hydrophobic and hydrophilic surfaces. The dynamic behaviors of vapor condensation, including the evolutions of vapor–liquid interface, heat flux, condensate mass, and temperature distribution, on the hydrophilic and hydrophobic rough surfaces are presented and compared with corresponding smooth surfaces. The results indicate that the rough surface preferred a filmwise condensation under hydrophilic conditions but a hybrid dropwise–filmwise condensation under hydrophobic conditions. On the rough hydrophobic surface, the liquid film can rapidly adsorb droplets, maintaining a high-efficiency dropwise condensation. The absorption of droplets accelerates the liquid film growth and detachment process on the rough hydrophobic surface, which reduces the time-averaged thermal resistance of the filmwise region. These two behaviors together enhance condensation heat transfer on the downward-facing rough hydrophobic surface. Besides, stable dropwise condensation could also be formed on smooth hydrophilic surfaces and has better heat transfer performance than corresponding hydrophobic surfaces under the same heat transfer condition. [ABSTRACT FROM AUTHOR]

Inspired by the snowflake structure, an innovative Koch-fractal fin is proposed to optimize the fin geometry of the latent heat thermal energy storage (LHTES) units. A model of unsteady heat transfer accompanied with phase change is developed and numerically analyzed to investigate the effect of fin structure on the discharging process of a LHTES unit. The dynamic response of heat release, the dynamic temperature response and solidification front evolution of a LHTES unit with Koch-fractal fins are discussed and compared with the corresponding radial fins. Furthermore, a comprehensive evaluation of thermal performance of LHTES units is conducted in terms of the TES capacity, TES rate and solidification time. The results indicate that the heat release rate of a LHTES unit with Koch-fractal fins is faster than that with radial fins. Moreover, because the Koch-fractal fins have advantages of higher specific surface area, faster heat flow path from point to surface and smaller thermal resistance arising from the reasonable spatial layout, the evolution of solidification front is faster and the temperature distribution is more uniform. The results of quantitative evaluation show that although the TES capacity is identical, the TES rate of a LHTES unit with Koch-fractal fins is six times that with radial fins. [ABSTRACT FROM AUTHOR]

The fractal geometry provides a new insight for the description for actual pore structure of metal foams. In this paper, the fractal grids are introduced to describe the pore structure of metal foams. A two-dimensional unsteady melting model of phase change material (PCM) in the cross-fractal metal foam is developed and numerically simulated. The transient temperature variation and melting front evolution during the charging process in the cross-fractal metal foam are analyzed. The effects of initial temperature difference, porosity and fractal dimension on the melting heat transfer in fractal metal foam are examined and analyzed. The results indicate that the temperature distribution is more uniform and the melting rate is faster in the cross-fractal metal foam compared with that in the corresponding cavity structure. Interestingly, the fractal metal foam with smaller fractal dimension provides a faster heat flow path and hence enhances the melting performance though the porosity is identical. The melting performance in fractal metal foam can be enhanced when the metal foams have a lower porosity, a smaller fractal dimension and a larger initial temperature difference. [ABSTRACT FROM AUTHOR]

The Sierpinski fractal is introduced to construct the porous metal foam. Based on this fractal description, an unsteady heat transfer model accompanied with solidification phase change in fractal porous metal foam embedded with phase change material (PCM) is developed and numerically analyzed. The heat transfer processes associated with solidification of PCM embedded in fractal structure is investigated and compared with that in single-pore structure. The results indicate that, for the solidification of phase change material in fractal porous metal foam, the PCM is dispersedly distributed in metal foam and the existence of porous metal matrix provides a fast heat flow channel both horizontally and vertically, which induces the enhancement of interstitial heat transfer between the solid matrix and PCM. The solidification performance of the PCM, which is represented by liquid fraction and solidification time, in fractal structure is superior to that in single-pore structure. [ABSTRACT FROM AUTHOR]