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• Flow boiling of R1233zd(E) under counter-flow interconnected minichannel heat sink was studied experimentally. • Significant neighboring channel cooling dramatically alters flow pattern evolution. • The interconnected structure commendably improves the temperature uniformity of the heat sink. • Effective heat flux of 230.4 W/cm2 with 51.2% enhancement of heat transfer coefficient shown. • Reduction of pressure drop up to 48.5% and remarkable enhancement in coefficient of performance up to 56%. Two-phase mini/micro-channel heat sink is one of the most effective solutions to the heat dissipation of high-power electronic devices in a narrow space. Flow boiling instability and wall temperature nonuniformity are however the two notorious problems of two-phase mini/micro-channel heat sink that have hindered their practical applications to a large degree. This study proposed a counter-flow interconnected minichannel (CFIM) to improve the flow boiling stability and the temperature uniformity of a mechanically machined copper minichannel heat sink. Firstly, a comprehensive comparative study of CFIM and co-current minichannel (CCM) was carried out concerning two-phase flow patterns, boiling heat transfer characteristics, and wall temperature uniformity. Secondly, the effects of mass flux, subcooling and saturation temperature on the boiling heat transfer performance of the two kinds of heat sinks were further investigated. Experiments were carried out at a saturated temperature of 40 ∼ 50°C, mass flux of 291∼ 618 kg/(m2·s), and subcooling of 15 ∼ 30°C, using a new generation of environment-friendly coolant with a low boiling point, R1233zd(E). In particular, no backflow or partial dry-out is observed in the channel throughout all the tested conditions. Due to the mixing of fluid between adjacent channels with counter flow caused by interconnected slots in CFIM, low upstream vapor quality and high downstream vapor quality were avoided. Therefore, this counter-flow interconnected mechanism in the current design can be significantly enhanced in flow boiling stability and temperature uniformity, up to an effective heat flux of 230.4 W/cm2 with 51.2% increment of heat transfer coefficient, 56% increment of coefficient of performance (COP) and 48.5% reduction of pressure drop when compared with those of traditional CCM design. Overall, the research results have good guidance and reference for the application and research of two-phase minichannel heat sinks. [ABSTRACT FROM AUTHOR]

• Flow boiling under non-uniform heat flux is different from that under uniform heat flux. • Typical flow patterns of R245fa were studied and transition models were evaluated. • Effects of parameters on heat transfer coefficient were analyzed. • A new correlation of heat transfer under non-uniform heat flux was provided. The direct vapor generation with parabolic trough solar collector utilizing organic working fluids has a significant potential of large-scale application. In such a system, the non-uniform heat flux commonly exists outside the collector tube which would result in the unique characteristics of two-phase flow and moreover boiling heat transfer. However, few studies pay attention to the effects of the non-uniform heat flux on flow pattern transitions and flow boiling, which have important influence on the long-term stable operation of the system. Therefore, in this study, an experimental system was established to study the two-phase flow boiling of R-245fa in a horizontal tube under the condition of the non-uniform heat flux. The experiment was performed with the mass flow rate from 192.9 to 385.8 kg/(m2 s), heat fluxes from 9.95 to 35.9 kW/m2 and saturation temperatures from 40 to 60 °C. Using a high-speed camera and the image processing, the differences between the flow pattern with non-uniform and uniform heating were analyzed. A comparison study on four available flow pattern I/A transition lines in the literature has been demonstrated as well. The effects of vapor quality, mass velocity, saturation temperature and heat flux on the two-phase heat transfer coefficient were analyzed. Furthermore, based on the experimental data, the correlation of boiling heat transfer of R245fa under non-uniform heat flux was developed. The results indicate that under the same heat addition, the vapor quality where the non-uniform I/A phase transition is lower than the uniform one. Furthermore, the non-uniform heat flux makes R245fa in the tube easier to form nucleation sites, therefore the nucleation boiling can be significantly enhanced. Taking account of the strengthening factor "S" of nucleate boiling, the new correlation of boiling heat transfer can reproduce 94% of data sets under non-uniform heat flux within the uncertainty of ±20%. [ABSTRACT FROM AUTHOR]