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This work investigated experimentally the photothermal conversion efficiency (PTE) of 10 gold nanofluids in a cylindrical tube under natural solar irradiation conditions, and compared with a 11 developed 3-dimensional numerical model. The PTE of gold nanofluids was found to be much higher 12 than that of pure water, and increased non-linearly with particle concentration, reaching 76% at a 13 concentration of 5.8 ppm. Significant non-uniform temperature distribution was identified both 14 experimentally and numerically, and a large uncertainty can be caused in the PTE calculation by using 15 only one temperature measurement. A mathematical model was also developed to calculate the 16 absorption efficiency without knowing the temperature field, which can be used to predict the 17 theoretical PTE for nanofluids based on their optical properties only. 18 19

Nanofluids are a new class of fluids engineered by dispersing nanometer-size solid particles in base fluids. As a new research frontier, nanofluid two-phase flow and thermal physics have the potential to improve heat transfer and energy efficiency in thermal management systems for many applications, such as microelectronics, power electronics, transportation, nuclear engineering, heat pipes, refrigeration, air-conditioning and heat pump systems. So far, the study of nanofluid two-phase flow and thermal physics is still in its infancy. This field of research provides many opportunities to study new frontiers but also poses great challenges. To summarize the current status of research in this newly developing interdisciplinary field and to identify the future research needs as well, this paper focuses on presenting a comprehensive review of nucleate pool boiling, flow boiling, critical heat flux, condensation and two-phase flow of nanofluids. Even for the limited studies done so far, there are some controversies. Conclusions and contradictions on the available nanofluid studies on physical properties, two-phase flow, heat transfer and critical heat flux (CHF) are presented. Based on a comprehensive analysis, it has been realized that the physical properties of nanofluids such as surface tension, liquid thermal conductivity, viscosity and density have significant effects on the nanofluid two-phase flow and heat transfer characteristics but the lack of the accurate knowledge of these physical properties has greatly limited the study in this interdisciplinary field. Therefore, effort should be made to contribute to the physical property database of nanofluids as a first priority. Secondly, in particular, research on nanofluid two-phase flow and heat transfer in microchannels should be emphasized in the future.

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