Investigations of flow boiling in highly viscous fluids show that heat transfer mechanisms in such fluids are different from those in fluids of low viscosity like refrigerants or water. To gain a better understanding, a modified standard apparatus was developed; it was specifically designed for fluids of high viscosity up to 1000 Pa∙s and enables heat transfer measurements with a single horizontal test tube over a wide range of heat fluxes. Here, we present measurements of the heat transfer coefficient at pool boiling conditions in highly viscous binary mixtures of three different polydimethylsiloxanes (PDMS) and n-pentane, which is the volatile component in the mixture. Systematic measurements were carried out to investigate pool boiling in mixtures with a focus on the temperature, the viscosity of the non-volatile component and the fraction of the volatile component on the heat transfer coefficient. Furthermore, copper test tubes with polished and sanded surfaces were used to evaluate the influence of the surface structure on the heat transfer coefficient. The results show that viscosity and composition of the mixture have the strongest effect on the heat transfer coefficient in highly viscous mixtures, whereby the viscosity of the mixture depends on the base viscosity of the used PDMS, on the concentration of n-pentane in the mixture, and on the temperature. For nucleate boiling, the influence of the surface structure of the test tube is less pronounced than observed in boiling experiments with pure fluids of low viscosity, but the relative enhancement of the heat transfer coefficient is still significant. In particular for mixtures with high concentrations of the volatile component and at high pool temperature, heat transfer coefficients increase with heat flux until they reach a maximum. At further increased heat fluxes the heat transfer coefficients decrease again. Observed temperature differences between heating surface and pool are much larger than for boiling fluids with low viscosity. Temperature differences up to 137 K (for a mixture containing 5% n-pentane by mass at a heat flux of 13.6 kW/m²) were measured.

对高粘度流体中流动沸腾的研究表明，此类流体中的传热机制与低粘度流体（如制冷剂或水）中的传热机制不同。为了更好地理解，开发了一种改进的标准装置；它专为高达 1000 Pa∙s 的高粘度流体而设计，可使用单个水平试管在较宽的热通量范围内进行传热测量。在这里，我们展示了三种不同聚二甲基硅氧烷 (PDMS) 和n 的高粘性二元混合物在池沸腾条件下的传热系数测量值。-戊烷，它是混合物中的挥发性成分。进行了系统测量以研究混合物中的池沸腾，重点是温度、非挥发性组分的粘度和挥发性组分在传热系数上的分数。此外，使用具有抛光和砂光表面的铜试管来评估表面结构对传热系数的影响。结果表明，混合物的粘度和组成对高粘度混合物的传热系数影响最大，因此混合物的粘度取决于所用 PDMS 的基础粘度、n的浓度-混合物中的戊烷，以及温度。对于核态沸腾，试管表面结构的影响不如在低粘度纯流体的沸腾实验中观察到的明显，但传热系数的相对增强仍然显着。特别是对于挥发性成分浓度高且池温高的混合物，传热系数随着热通量的增加而增加，直至达到最大值。在进一步增加热通量时，传热系数再次降低。观察到的加热表面和水池之间的温差比低粘度沸腾流体大得多。温差高达 137 K（对于含有 5%质量正戊烷的混合物，热通量为 13.6 kW/m ²) 进行测量。