Heat Transfer Enhancement

Heat dissipation in high-heat flux micro-devices has become a pressing issue. One of the most effective methods for removing the high heat load of micro-devices is boiling heat transfer in microchannels. A novel approach to flow pattern and heat transfer recognition in microchannels is provided by the combination of image and machine learning techniques. The support vector machine method in texture characteristics successfully recognizes flow patterns. To determine the bubble dynamics behavior and flow pattern in the micro-device, image features are combined with machine learning algorithms and applied in the recognition of boiling flow patterns. As a result, the relationship between flow pattern evolution and boiling heat transfer is established, and the mechanism of boiling heat transfer is revealed.

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Computer Vision and Machine Learning Methods for Heat Transfer and Fluid Flow in Complex Structural Microchannels: A Review

Increased cooling requirements of many compact systems involving high heat fluxes demand the development of high-performance cooling techniques including immersion cooling utilizing pool boiling. This study presents the functionalization of copper surfaces to create interfaces for enhanced pool boiling heat transfer. Three types of surface structures including a crosshatch pattern, shallow channels and deep channels were developed using nanosecond laser texturing to modify the surface micro- and nanomorphology. Each type of surface structure was tested in the as-prepared superhydrophilic state and superhydrophobic state following hydrophobization, achieved through the application of a nanoscale self-assembled monolayer of a fluorinated silane. Boiling performance evaluation was conducted through three consecutive runs under saturated conditions at atmospheric pressure utilizing water as the coolant. All functionalized surfaces exhibited enhanced boiling heat transfer performance in comparison with an untreated reference. The highest critical heat flux of 1697 kW m−2 was achieved on the hydrophobized surface with shallow channels. The highest heat transfer coefficient of 291.4 kW m−2 K−1 was recorded on the hydrophobized surface with deep channels at CHF incipience, which represents a 775% enhancement over the highest values recorded on the untreated reference. Surface microstructure was identified as the key reason for enhanced heat transfer parameters. Despite large differences in surface wettability, hydrophobized surfaces exhibited comparable (or even higher) CHF values in comparison with their hydrophilic counterparts, which are traditionally considered as more favorable for achieving high CHF values. A significant reduction in bubble departure diameter was observed on the hydrophobized surface with deep channels and is attributed to effective vapor entrapment, which is pointed out as a major contributing reason behind the observed extreme boiling heat transfer performance.

/pdf/nanosecond-laser-textured-copper-surfaces-hydrophobized-with-1cz1dk6j.pdf

Nanosecond Laser-Textured Copper Surfaces Hydrophobized with Self-Assembled Monolayers for Enhanced Pool Boiling Heat Transfer

Water–copper is one of the most common combinations of working fluid and heating surface in high-performance cooling systems. Copper is usually selected for its high thermal conductivity and water for its high heat transfer coefficient, especially in the two-phase regime. However, copper tends to suffer oxidation in the presence of water and thus the heat flux performance is affected. In this research, an experimental investigation was conducted using a cooper bare surface as a heating surface under a constant mass flux of 600 kg·m−2·s−1 of deionized water at a subcooled inlet temperature ΔTsub of 70 K under atmospheric pressure conditions on a closed-loop. To confirm the heat transfer deterioration, the experiment was repeated thirteen times. On the flow boiling region after thirteen experiments, the results show an increase in the wall superheat ΔTsat of approximately 26% and a reduction in the heat flux of approximately 200 kW·m−2. On the other hand, the effect of oxidation on the single phase is almost marginal.

https://www.mdpi.com/2673-3161/4/1/2/pdf?version=1674009769

Heat Transfer Deterioration by the Copper Oxide Layer on Horizontal Subcooled Flow Boiling

Heat pumps and refrigeration are key technologies to realize carbon neutrality, and active research is being conducted around the world [...]

https://www.mdpi.com/2076-3417/12/20/10630/pdf?version=1666324654

Special Issue on Sciences and Innovations in Heat Pump/Refrigeration: Volume II

Passive methods to increase the heat flux on the subcooled flow boiling are extremely needed on modern cooling systems. Many methods, including treated surfaces and extended surfaces, have been investigated. Experimental research to enhance the subcooled flow boiling using high sintered fiber attached to the surface was conducted. One bare surface (0 mm) and four porous thickness (0.2, 0.5, 1.0, 2.0 mm) were compared under three different mass fluxes (200, 400, and 600 kg·m−2·s−1) and three different inlet subcooling temperature (70, 50, 30). Deionized water under atmospheric pressure was used as the working fluid. The results confirmed that the porous body can enhance the heat flux and reduce the wall superheat temperature. However, higher porous thickness presented a reduction in the heat flux in comparison with the bare surface. Bubble formation and pattern flow were recorded using a high-speed camera. The bubble size and formation are generally smaller at higher inlet subcooling temperatures. The enhancement in the heat flux and the reduction on the wall superheat is attributed to the increment on the nucleation sites, the increment on the heating surface area, water supply ability through the porous body, and the vapor trap ability.

Kenji Orito

Papers

Subcooled Flow Boiling Heat Flux Enhancement Using High Porosity Sintered Fiber

Heat Transfer Deterioration by the Copper Oxide Layer on Horizontal Subcooled Flow Boiling