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Engineering nano-structured surfaces with mixed/thermo-responsive wettability offer a new approach to improve the boiling performances of advanced thermal systems, such as the solar system and the heat dissipation systems in nuclear power plants, where more efficient cooling and higher safety limits are extremely desirable. In this study, five groups of surfaces: a) plain surfaces, b) nanofilm coated surfaces, c) patterned surfaces with superhydrophilic nanograss, d) patterned surfaces with superhydrophobic nanograss, f) patterned surfaces with thermo-responsive wettable nanograss are investigated for their boiling performances. It is found that the nanofilm coated surfaces show improved maximum heat transfer coefficient (HTCmax) as well as critical heat flux (CHF) compared with the plain surface. The patterned surfaces shift the boiling curves to left, and the CHF increases with increasing nanograss cover density. The surfaces with thermo-responsive wettability, which responses to the external heating/cooling stimuli by gradually increasing or decreasing the wettability, show the most optimal CHF enhancement. This study serves as a proof-of-concept for efficient heat transfer through carefully fabricated nano-structured wettability-enhanced surfaces.

Experimental investigation for a copper vertical tubular bubble absorber was done with R124 (2-chloro-l,l,l,2,-tetrafluoroethane)-DMAC (N′,N′-dimethylacetamide) as working fluid. The objective of the investigation was to explore key parameters affecting heat and mass transfer characteristics of the bubble absorber and propose new correlations for heat and mass transfer coefficients respectively for the R124-DMAC bubble absorption. The parameters include vapor and solution flow rates, solution inlet temperatures and mass fractions, nozzle orifice diameters and cooling water inlet temperatures. The results showed that to reduce the solution, cooling water inlet temperature and nozzle orifice diameter or to increase vapor and solution flow rate could enhance the heat and mass transfer performance of the absorber. Finally, correlations of Nusselt number and Sherwood number for calculating heat and mass transfer coefficients were proposed, respectively. Both correlations could predict about 90% of the experimental values at a margin of error less than 20%.

With the increasing power consumption in the worldwide energy-intensive sectors, a more efficient thermal heat transfer during the boiling process is pressingly needed. The fundamental understanding of the boiling mechanism is essential for the enhanced heat transfer and subsequently the improvement of heat utilization. The present study decouples the contributions of the intrinsic surface wettability from the hierarchical (dual-layer) structure on the boiling enhancement by growing nanograss as the substructure and the micro flowers with different cover density as the superstructure. The structured surfaces show maximum critical heat flux (CHF) enhancement by 68%. While for the multi-orientated (from 0° to 180°) substrates, the surface orientation will influence the boiling performance through different physical mechanisms. It discloses that the departure time and the thickness of the fully developed vapor film of the inclined surfaces increase with increasing surface orientation, resulting in impeded boiling performance of the downward-facing surface. Moreover, enhanced critical heat flux and heat transfer coefficient can be observed for the nanograss surface with the downward-facing orientations. In addition, new correlations regarding the downward-facing CHF prediction based on the horizontal CHF are proposed for the future multi-oriented surface design in advanced heat-transfer applications.