Heat Transfer to Water-Oxygen Mixtures at Supercritical Pressure

The constant pressure heat capacity and forced convection heat transfer coefficient of water/oxygen mixtures were measured in a horizontal, smooth, electrically-heated tube. For the supercritical pressure (25 MPa) considered, flow rates (0.76–2.04 kg/min), heat fluxes (21–290 kW/m²) 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. The water/oxygen mixture and the above experimental conditions are relevant to supercritical water oxidation systems (SCWO). At subcritical temperatures the oxygen/water mixture is almost immiscible and the flow is two-phase. Just below the critical temperature, the fluid becomes single-phase. By measuring bulk and surface temperatures, for a given flow rate, heat flux and oxygen content, both the heat capacity and heat transfer coefficient for the mixture were 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. Despite the multi-phase nature of the flow at temperatures well below the critical temperature (i.e., <360°C), the presence of small quantity of oxygen has little effect on the heat transfer. At supercritical temperatures where the flow is single-phase and gas-like, the presence of oxygen has little effect on the heat transfer coefficient. However, at near-critical temperatures, the addition of small amounts of oxygen results in a dramatic change in the heat transfer. Firstly, the magnitude and temperature for the peak heat transfer decrease, consistent with changes in heat capacity. Secondly, heat transfer is deteriorated at moderate heat flux, mostly but not exclusively on the top surface of the tube.