Abstract
In this paper, natural convection melting in a square cavity with gradient porous media is numerically studied at pore-scale level by adopting the lattice Boltzmann method. To generate the gradient porous media, a Monte Carlo technique based on the random sampling principle is used. The effects of several factors, such as Rayleigh number, gradient porosity structure, gradient direction, and particle diameters on natural convection melting are investigated in detail. Based on the numerical data, it is observed that the thermal performance of the gradient porous media always depends on the Rayleigh number and, specifically, as the Rayleigh number is set to 106, the total melting time obtained for the case of the negative gradient porous media is always shorter than the cases of positive gradient and uniform porous media. However, if the Rayleigh number is equal to 104, at which the heat transfer is dominated by the heat conduction, it is noted that the performance of the positive gradient porous media is better than the other cases. To have a better understand on this point, various simulations are also performed and we found that there usually exists a critical value of Rayleigh number to determine the thermal performance of the gradient porous media. Moreover, our numerical results also show that the influence of the particle diameter on the liquid fraction is insignificant as Rayleigh number is set to 104, while it has a great impact on the liquid fraction when Rayleigh number equals 106.
Highlights
Thermal energy storage (TES) technology has attracted wide attention due to its ability to solve the temporal and spatial imbalance between energy supply and energy demand [1,2]
We turn to study the natural convection melting in a square enclosure with gradient porous media, and the influences of Rayleigh number, gradient pore structure, gradient direction, as well as the particle diameter are investigated in detail
It is noted that the heat transfer performance obtained for negative gradient in case A is usually better than those gained for positive and uniform pore structures, which is caused by the fact that the remanent space near the hot wall is larger than the other cases such that the convection is more to be activated at this Rayleigh number of 106
Summary
Thermal energy storage (TES) technology has attracted wide attention due to its ability to solve the temporal and spatial imbalance between energy supply and energy demand [1,2]. The results show that the three-level gradient and the two-level gradient have comparable thermal performance, which are 4.4 times and 4.0 times stronger than the thermal performance of the uniform structure, respectively These exiting works show that the pore structure of the porous media has a significant impact on solid–liquid phase change heat transfer, and it can be served as an effective mean to enhance PCM melting. We noted that some previous works stated that the employing of positive gradient pore structures is always a good choice in enhancement of solid–liquid phase change [24,25], some researchers have stated that the negative gradient porous media has a better melting performance than the positive one [26], and these contradictory statements most likely arise from the various parameters used in REV model.
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