engleski

Implications of boundary conditions on natural convective heat transfer of molten phase change material inside enclosures

Considering the significance of appropriate selection of boundary conditions in modelling step for practical engineering applications, this study presents a numerical work focusing on the impacts of boundary conditions on the natural convection of molten phase change material (PCM) used for cooling of photovoltaics (PV). A rectangular enclosure loaded with liquid PCM (Pr=41.22) is considered at an inclination angle of θ=30° to simulate a PV-PCM system. Heat flux is applied to the top wall, and impacts of two different boundary conditions at the bottom wall, namely isothermal and convective boundary conditions, on the flow and heat transfer characteristics are compared by taking six different Biot numbers (0.1≤Bi≤100) into account, while the reference case is considered as isothermal boundary condition. Furthermore, effects of various aspect ratios (AR=1, 2 and 4) and Rayleigh numbers (Ra=104, 105 and 106) are also included. The results revealed that Biot number has a significant effect on mean Nusselt number. Compared to the isothermal case, low Biot numbers (Bi<10) significantly restrict convection motions inside the enclosure and result in remarkably different mean Nusselt numbers, while high Biot numbers give significantly similar results due to low thermal resistance outside the bottom wall. Moreover, the heat transfer enhancement by increasing AR and Ra is considerably high at high Biot numbers, while it is remarkably constricted at low Biot numbers. As a result of comprehensive analyses, it is deduced that utilization of isothermal boundary condition instead of convective boundary condition for easiness of modelling can be a reasonable approach providing that Biot number is sufficiently large (Bi>10). Although the inspiration of the present study is PV/PCM systems, the results can be generalized for any kind of fluids used in similar natural convection applications.

Biot number ; Convective boundary condition ; Enclosure ; Natural convection ; PV-PCM system ; Aspect ratio

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