Local heat-transfer coefficients are measured for carbon dioxide very near the thermodynamic critical point for flow through an electrically heated circular duct. The experiments cover a range of fluid bulk temperatures spanning the transposed critical temperature and of pressure from 1075 to 1110 psia (reduced pressure from 1.003 to 1.03). The data are correlated by the equation Nuz = 0.0266 Rez0.77 Prw0.55 where the Prandtl number is evaluated at the wall temperature and Nusselt and Reynolds numbers are evaluated at a variable reference temperature. The dominant heat transfer mechanism is found to be turbulent forced convection. There is evidence that free convection effects are also present.

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