The constant pressure heat capacity and forced convection heat transfer coefficient was measured in a horizontal, smooth, electrically-heated tube. For the supercritical pressures considered, flow rates and temperatures (330–430 °C), the flow in the 6.2 mm ID tube was fully turbulent. The fluid was distilled water and up to 9 wt% oxygen. This mixture and the experimental conditions are found in supercritical water oxidation systems. At subcritical temperatures, the oxygen and water are almost immiscible, but just below the critical temperature, the fluid becomes single-phase. By measuring bulk and surface temperatures, knowing the mass and heat flux, both the heat capacity and heat transfer coefficient could be measured. The water-oxygen system is a highly non-ideal mixture, and small amounts of oxygen significantly reduce the temperature at which maximum heat transfer occurs. The changes in heat capacity appear to dominate the effect of oxygen on heat transfer, however, the mixtures do exhibit heat transfer deterioration at slightly subcritical temperatures, at flows and heat fluxes for which pure water shows nothing similar.

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