In a project which deals with degassing of polymer solutions using pipe-bundle heat exchangers, noticeable discrepancies have been observed between experimental results for boiling highly viscous mixtures and predictions of available correlations. These differences revealed shortcomings in understanding and modeling of the process. An experimental project is presented, which focuses on fundamentals of heat and mass transfer in boiling highly viscous mixtures. Effects that have been predicted for boiling in such mixtures are discussed. In order to distinguish between the influence of viscosity and marginal vapor pressure of the highly viscous component, an extremely wide-boiling model system with low viscosity will be investigated. A standard apparatus for measuring heat transfer in an equilibrated pool is modified for measurements on such systems. The design of the modified apparatus is described. Additional modifications necessary to adapt a standard apparatus to wide-boiling highly-viscous mixtures are briefly discussed. Results are presented for mixtures of n-pentane and poly dimethyl siloxane (silicon oil) with a viscosity of 10 m Pa s boiling in a simple pod-boiling equipment at ambient pressure. Temperature distributions in the pool are reported and nucleation mechanisms are discussed. A new mechanism is observed for bubble formation above the test tube.

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.