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The convection heat transfer of n-decane at supercritical pressures in 0.95 mm inner diameter and 2.0 mm inner diameter vertical tubes was experimentally investigated for various heat fluxes, pressures and inlet Reynolds numbers. The effects of buoyancy and flow acceleration due to heating were analysed. This study shows that:(1)The experimental results of the convection heat transfer of n-decane at supercritical pressures in the 0.95 mm inner diameter tube show that NLG-8189 the influences of buoyancy and acceleration are insignificant for the studied conditions. For these conditions, the convection heat transfer is mainly influenced by the thermophysical properties of n-decane.(2)The experimental results of the convection heat transfer of n-decane at supercritical pressures in the 2.0 mm inner diameter tube show that for high inlet Reynolds numbers (e.g. 7000), Bo∗ was less than 2.0 × 10−7 and Kv was less than 6 × 10−7, and the interneurons local wall temperature increased rapidly in the entrance region, and then increased monotonically in the latter region, without abnormal temperature distribution for both upward and downward flow. On the other hand, for the lower inlet Reynolds numbers (e.g. 2700–4000), buoyancy may significantly deteriorate heat transfer for upward flow and increase heat transfer for downward flow. A threshold for Bo∗ was obtained as 2.0 × 10−7, above which buoyancy influence heat transfer obviously.(3)Two new local Nusselt number correlations for turbulent heat transfer of n-decane at supercritical pressures flowing in vertical small circular tubes with and without buoyancy effects were developed.