| Summary: | In this research, unsteady three-dimensional incompressible Navier−Stokes equations are solved to simulate experiments with the Boussinesq approximation and validate the proposed numerical model for the design of a circular fin-tube heat exchanger. Unsteady time marching is proposed for a time sweeping analysis of various Rayleigh numbers. The accuracy of the natural convection data of a single horizontal circular tube with the proposed numerical method can be guaranteed when the Rayleigh number based on the tube diameter exceeds 400, which is regarded as the limitation of numerical errors due to instability. Moreover, the effective limit for a circular fin-tube heat exchanger is reached when the Rayleigh number based on the fin gap size (<inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mrow> <mi>Ra</mi> </mrow> <mi>s</mi> </msub> </mrow> </semantics> </math> </inline-formula>) is equal to or exceeds 100. This is because at low Rayleigh numbers, the air gap between the fins is isolated and rarely affected by natural convection of the outer air, where the fluid provides heat resistance. Thus, the fin acts favorably when <inline-formula> <math display="inline"> <semantics> <mrow> <msub> <mrow> <mi>Ra</mi> </mrow> <mi>s</mi> </msub> </mrow> </semantics> </math> </inline-formula> exceeds 100.
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