This paper investigates fluid flow and turbulent natural convection heat transfer in an enclosure embedded with an isothermal cylinder. A relaxation time method with the stability constants is introduced to solve natural convection problems. The thermal lattice Boltzmann based on D2Q9 and D3Q19 methods without any turbulent models was purposed to simulate the flow and thermal fields. The effects of high Rayleigh number on the fluid flow and heat transfer analysis have been investigated for different geometric aspect ratios. The flow behaviors and the average rate of heat transfer in terms of the Nusselt number NuNu have effectively changed by different controlling parameters such as Rayleigh number (103≤Ra≤109 and 103≤Ra≤109) and aspect ratio (0.5≤AR≤20.5≤AR≤2). The present results provide a good approximation for choosing effective parameters of designing the thermal system.

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