An experimental study of free convection heat transfer from horizontal wires to carbon dioxide at near-critical pressures has been performed. In the experiments, platinum wires ranging in size from 25.4 μm to 100 μm and a nichrome 60/20 wire of 101.6 μm diameter were used. The pressure (P) and bulk temperature (Tb) of the fluid were varied in the range: 6.34 MPa ≤ P ≤ 9.60 MPa and 10 °C ≤ Tb ≤ 33.3 °C, respectively. The wall temperature (Tw) was systematically increased from Tb + 0.1 °C to 250 °C. Visual observations of the fluid flow were made using a high speed camera. The similarity between natural convection heat transfer at Tw < Tsat (for P < Pc) and Tw < Tpc (for P > Pc), as well as the similarity between film boiling at Tw > Tsat (for P < Pc) and natural convection heat transfer at Tw > Tpc (for P > Pc), was demonstrated. The dependence of the heat transfer coefficient on the wire diameter was found to be h$∝$D−0.5, for both P < Pc and P > Pc. The bulk fluid temperature is introduced as a new reference temperature for the calculation of fluid properties. Correlations have been developed to predict the natural convection heat transfer coefficient at both subcritical and supercritical pressures. The developed correlations predict almost all the experimental data from the current study and those reported in the literature to within ±15%.

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