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This paper investigates the dynamic thermal behavior of phase change material (PCM) melting in a rectangular enclosure at various inclination angles. Lauric acid as a PCM with high Prandtl number (Pr ≈100) is used. The enclosure is heated isothermally from one side while the other walls are thermally insulated. Experiments were performed with hot wall temperatures of 55, 60 and 70°C for different inclination angles of 0°, 45° and 90°. Image processing of melt photographs along with recorded temperatures were used to calculate the melt fractions, Nusselt numbers and the local interfacial heat transfer rates at the solid–liquid interface. Qualitative time-dependent natural convection flow structures were deduced indirectly from the instantaneous shape of the solid–liquid interface which were confirmed by quantitative data from temperature measurements. The results reveal that the enclosure inclination has a significant effect on the formation of natural convection currents and consequently on the heat transfer rate and melting time of the PCM. As the inclination angle is decreased from 90° to 0°, the convection currents in the enclosure increases and chaotic flow structures appear. When melting commences in the horizontally inclined enclosure, the solid–liquid interface line becomes wavy which implies the formation of Benard convection cells in the liquid PCM. For the same hot wall temperatures, a decrease in inclination angle leads to a considerable enhancement in energy transport from the hot wall of the enclosure to the PCM. It is found that the heat transfer enhancement ratio for the horizontal enclosure is more than two times higher than that of the vertical enclosure. [ABSTRACT FROM AUTHOR]

• Critical cell size of a multi-scaled periodic structure has been ascertained. • Lowering porosity or lifting thermal diffusivity lead to a larger critical PPI. • The critical PPI is 5 when SS at 0.75 porosity or Ni at 0.95 porosity. • The occurrence of localised melting has been identified and correlated. The use of metallic periodic structures was considered for melting rate enhancement of a phase change material (PCM) contained in a rectangular enclosure isothermally heated from the side. The critical (optimized) cell size, or pore size, of a periodic structure with fixed porosity, realising the shortest melting time by maximizing the convection and conduction heat transfer rate into the PCM, was studied. Furthermore, the effects of material properties (copper, aluminium, nickel, and stainless steel), enclosure length, wall-melting temperature difference and porosity were numerically investigated. It was observed that increasing porosity and/or reducing thermal conductivity enlarged the critical cell size (i.e. the optimal cell size that minimizes the melting time). The critical PPIs (pores per inch) of copper and aluminium periodic structures for all studied porosities were 10; for nickel, the critical values were 10 PPIs for porosity values of 0.75, 0.8 and 0.85 while it reduces to 5 PPI for the highest porosity considered here being 0.95. Interestingly, showing a different trend, the critical PPI of stainless-steel structures was 5 for the lowest porosity (0.75) and reduced to 3 for higher porosities. The results clearly demonstrated localised melting which was observed in all periodic structures except for the 10 PPI stainless-steel case. Scattered melting islands are observed as opposed to a moving interface when ϕ = (d p / L) α ligament / α PCM > 1. For such cases, localized melting occurs and the PCM is melted at the ligaments away from the heated wall before the melt front reaches those ligaments. [ABSTRACT FROM AUTHOR]

• Critical cell size of a periodic structure with 0.9 porosity has been numerically ascertained for different materials. • Non-brazed scenarios could lead to three times longer melting time, compared to the perfectly brazed case. • The numerically-predicted effective thermal conductivity of the periodic structure agrees well with results available in the literature. To overcome the inherent poor thermal conductivity of most phase change materials (PCM), inserting highly thermally conductive wire nets with periodic structures into them have been proposed. Numerical simulations have been extensively conducted to determine critical cell size and the effect of cell height and interface gap thickness on the heat transfer within the PCM. In addition, predictive correlations of the effective thermal conductivity were put forward. The simulated results indicate that: for structures with the same porosity, the critical cell size gradually decreases as the thermal conductivity of the wire net (ligament material) increases. For the proposed periodic structure embedded in the considered computational domain with 0.90 porosity, the critical pores per inch (PPI) for copper ligaments is approximately 10 PPI, while for stainless steel, it is approximately 3-5 PPI; a shorter cell height with a lower porosity shortens the melting time, therefore, stacking inexpensive metal wire is considered as an interesting alternative to commercially produced metal foams. Moreover, non-brazed scenarios lead to longer melting times, more than three times, compared to a perfectly brazed case. Furthermore, the effective thermal conductivity of the proposed periodic structure has been numerically calculated, which agrees well with some models available in the literature. [ABSTRACT FROM AUTHOR]

• Inverse method used to improve original T-history method equations to determine PCMs latent heat. • The method shown to be suitable method to calculate latent heat of low thermal conductivity bulk PCMs. • New transient modelling is more suitable for calculating latent heat with cooling rates than using original equations. High temperature phase change materials (PCMs) are being proposed as thermal energy storage media in concentrated solar power plants. The thermophysical properties of PCMs, such as the latent heat of fusion/solidification need to be defined to enable system design and evaluate system performance and cost analysis. Several methods have been used to measure the latent heat of fusion/solidification of PCMs including the T-history method. The original assumption of the T-history method is the Biot number should be less than 0.1 where the lumped capacitance method is applicable. However, when the Biot number is more than 0.1 then the original equations are not applicable to estimate the latent heat of PCM. This paper demonstrates how a one-dimensional heat transfer mathematical model can be used by applying an inverse method to overcome the limitation of the lumped capacitance method in the original T-history method to determine the latent heat of PCMs. The mathematical model demonstrates that variation of the temperature distributions inside the samples for the discharge curves. Furthermore, this model shows the errors in the latent heat values if the T-history method is used. It is evident from the discharge curves of low thermal conductivity PCMs that the original simplified equations used for interpreting the T-history test results are not applicable for calculating the latent heat at different ambient temperature. [ABSTRACT FROM AUTHOR]

• The application of foams and fins, both straight and tree-shaped, is examined aiming at accelerate the melting of PCM. • There is an optimal fin number density for a given total fin volume in the storage tank. Moving to non-uniform fin size and un-even fin distribution cannot significantly shorten the melting time. • The fins, if designed properly, can be as efficient as foam while the former is much cheaper. Application of different heat transfer augmentation techniques, including the use of fins or foams, were investigated to enhance the melting rate of a solid phase change material within an annulus where the inner and outer pipes were subjected to constant wall temperature. The carbon fibre fins as well as three commonly-used foams (made of three different materials: nickel, aluminium and copper) were simulated. Firstly, keeping the total fin volume constant, the fin number density effect on the melting rate was investigated. After an optimal fin number density was obtained, three possible strategies (unequal length, uneven intervals and tree-shaped fins) were explored aimed at a more comprehensive understanding of the induced heat transfer enhancement. It was observed that with a fixed fin thickness and volume, the melting time is not a monotonic function of the fin number density and can be optimized. Comparing pure PCM melting, the use of optimized fin number reduced over 60% of melting time, while additional 8% and 4% further time reduction could be achieved by appropriately increasing lengths and decreasing intervals of bottom fins, respectively. The use of tree-like fins resulted in a longer melting time, comparing to that of longitudinal straight fins, which indicates it is not always a good option. Finally, the results, primarily the melting rates, were compared with those obtained through the use of metal foams with different metals. It was observed that the melting time of optimized strategy-1 is rather less than those of Cu and Al foams, and approximately 2200s shorter than that of Ni foams. These results indicate that the fins, if designed properly, can be as efficient as foams. [ABSTRACT FROM AUTHOR]