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This paper presents a numerical study of transient buoyancy-induced fluid flow and heat transfer between two horizontal, differentially heated pipelines inside a circular, air-filled enclosure. Numerical simulations based on the finite difference method were conducted to investigate the flow mode transition of the buoyant airflow and its effects on the heat transfer characteristics of the pipelines. The results indicate that the fluid flow complexity and the heat transfer of air between the pipelines are strongly affected by the Rayleigh number. When Ra = 6 × 105 and 1.2 × 106, both the flow field and the temperature distribution exhibit periodic variations with different patterns. The former (Ra = 6 × 105) is a complete alteration of the flow direction from clockwise to counterclockwise, whereas the latter is a variation in the flow field strength that varies between strong and weak. The latter has a lower variation frequency than that of the former. [ABSTRACT FROM AUTHOR]