Review on Pool Boiling Heat Transfer of Wide Boiling Binary Refrigerant Mixtures
In many refrigeration or heat pump applications, heat transfer at gliding temperature is needed. Significant temperature glides can be realized in boiling or condensation by using refrigerant mixtures with wide distances of vapour-liquid saturation conditions for the pure components. In mixture boiling, heat transfer is reduced below the level of the pure components in most cases by enrichment of the higher boiling component near the heated wall, which is accompanied by additional mass (and heat) transfer resistance. The effect increases the higher the saturation pressure and heat flux and the wider the gap between the saturation temperatures of the pure components. This may end up with a reduction of heat transfer with increasing saturation pressure, instead of the well-known increase of nucleate boiling heat transfer with pressure. Starting from recent measurements by Stier (2003) with Methane/Ethene mixtures boiling on a horizontal tube and adding data in the literature for other binary systems it is shown in the paper that the negative pressure dependence of boiling heat transfer of mixtures is not a peculiarity, but a result of continuous, relative deterioration of nucleate boiling heat transfer below the values for the pure components with increasing pressure, heat flux and temperature difference between the components at saturation conditions.