E. Teodori

Enhancement of pool boiling heat transfer by surface micro-structuring

The present paper addresses the use of surfaces structured with arrays of square micro-cavities to enhance pool boiling heat transfer. The heat transfer performance, obtained with the structured surfaces is evaluated based on the measured boiling curves and on the heat transfer coefficients. Two new parameters are suggested to relate the bubble dynamics (and consequently the surface topography) with the heat transfer coefficients: the modified dimensionless cavity spacing and the dimensionless distance, which cover the governing parameters of the phenomena. Correlations of these parameters with the heat transfer coefficients allowed to identify the best performing patterns, from those tested so far. Based on this progress it is expected that optimization of these relations will lead to precise relations which allow a systematic optimization of the surface pattern leading to an effective heat transfer enhancement, for situations involving high heat fluxes.

Application of bioinspired superhydrophobic surfaces in two-phase heat transfer experiments

This paper addresses the potential to use Lotus leaf bioinspired surfaces in applications involving heat transfer with phase change, namely pool boiling and spray impingement. Besides describing the role of bioinspired topographical features, using an innovative technique combining high-speed visualization and time-resolved infrared thermography, surface durability is also addressed. Water is used for pool boiling and for spray impingement systems (simplified as single droplet impact), while HFE7000 is used in a pool boiling cooler for electronic components. Results show that surface durability is quickly compromised for water pool boiling applications, as the chemical treatment does not withstand high temperatures (T > 100 °C) during long time intervals (3 h - 4 h). For HFE7000 pool boiling (depicting lower saturation temperature - 34 °C), heat transfer enhancement is governed by the topography. The regular hierarchical pattern of the bioinspired surfaces promotes the heat transfer coefficient to increase up to 22.2%, when compared to smooth surfaces, while allowing good control of the interaction mechanisms until a distance between micro-structures of 300 µm - 400 µm. Droplet impingement was studied for surface temperatures ranging between 60 °C - 100 °C. The results do not support the use of superhydrophobic surfaces for cooling applications, but reveal great potential for other applications involving droplet impact on heated surfaces (e.g. metallurgy industry).

2 phase microprocessor cooling system with controlled pool boiling of dielectrics over micro-and-nano structured Integrated Heat Spreaders

The present work addresses a microprocessor cooling technique based on pool boiling of a dielectric fluid, HFE-7000 with a compact closed loop thermosyphon, which requires no pumping or auxiliary components to operate. Aiming at modern desktop CPU cooling, the devised system is modular to infer on the optimization of several parameters influencing the system performance. The evaporator bottom surface is enhanced with micro-structured cavities to increase the liquid/solid contact area and optimize nucleation and bubble dynamics within the heterogeneous nucleation process. Optimization of surface structuring must account for several interaction mechanisms and assure that the flow near the surface maximizes the heat transfer mechanisms present in pool boiling heat transfer. This optimization is based on the minimization of steady-state overall thermal resistance of the system and on transient power conditions to control the onset of nucleate boiling and the inherent temperature overshoot upon regime transition at start-up. The condenser tilt angle is optimized as well as the effect of evaporator dimensions, orientation (horizontal and vertical positioning) and liquid fill charges. Based on the outcomes of this exploratory research, a cooling system is implemented in a working computer, cooling a modern CPU, mounted vertically.

Bubble dynamics and heat transfer for pool boiling on hydrophilic, superhydrophobic and biphilic surfaces

This paper proposes a detailed analysis of bubble dynamics to describe pool boiling heat transfer in extreme wetting scenarios (superhydrophobic vs hydrophilic). A mechanistic approach, based on extensive post-processing allows quantifying the relative advantage of the superhydrophobic surfaces to endorse the onset of boiling at very low superheats (1-2K) vs their worse heat transfer performance associated to the swift formation of an insulating vapour film. Based on this analysis, a simple biphilic surface is created. The results suggest that for high heat fluxes, bubble dynamics is dominated by the emission of very small bubbles, which seems to affect the interaction mechanisms, precluding the emission of the large bubbles from the surface, thus compromising the good performance of the biphilic surfaces.

Time Resolved Infrared Analysis of Droplet Impacts onto Heated Surfaces Under Extreme Wetting Scenarios

The authors are grateful to Fundacao para a Ciencia e Tecnologia (FCT) for partially financing the research under the framework of the project RECI/EMS-SIS/0147/2012 and for supporting P. Pontes with a research fellowship. A. S. Moita acknowledges FCT for financing her contract through the IF 2015 recruitment program (IF 00810- 2015) and E. acknowledges FCT for supporting his PhD fellowship (SFRH/BD/88102/2012).