
 The heat dissipation density of electronic devices is increasing dramatically, which causes a serious heat bottleneck in electronics. Operating temperature over its rated temperature results in performance deterioration and even device damage. With the development of micro-machining technologies, microchannel heat sinks have become one of the best ways to remove the considerable amount of heat generated by the high-power electronics. It shows the advantages of large specific surface area, small size, saving coolant and high heat transfer coefficient. This paper comprehensively overviews the research progress in microchannel heat sinks and generalizes the hotspots and bottlenecks of this area. The heat transfer mechanisms and performances of different channel structures, coolants, channel materials and some other influence factors are reviewed. Besides, this paper classifies the heat transfer enhancement technology and reviews the related studies on both the single-phase and phase-change flow and heat transfer. The comprehensive review is expected to provide theoretical reference and technical guidance for further research and application of microchannel heat sinks in the future.

A comprehensive review on microchannel heat sinks for electronics cooling

Free-falling objects impacting onto water pools experience a very high initial impact force, greatest at the moment when breaking through the free surface. Many have intuitively wondered whether throwing another object in front of an important object (like oneself) before impacting the water surface may reduce this high impact force. Here, we test this idea experimentally by allowing two spheres to consecutively enter the water and measuring the forces on the trailing sphere. We find that the impact acceleration reduction on the trailing sphere depends on the dynamics of the cavity created by the first sphere and the relative timing of the second sphere impact. These combined effects are captured by the non-dimensional ‘Matryoshka’ number, which classifies the observed phenomena into four major regimes. In three of these regimes, we find that the impact acceleration on the second sphere is reduced by up to 78 % relative to impact on a quiescent water surface. Surprisingly, in one of the regimes the force on the trailing sphere is dramatically increased by more than 400 % in the worst case observed. We explain how the various stages of cavity evolution result in the observed alterations in impact force in this multi-body water entry problem.

/pdf/impact-force-reduction-by-consecutive-water-entry-of-spheres-xogwmibllt.pdf

Impact force reduction by consecutive water entry of spheres

Liquid–vapor phase change phenomena such as boiling and condensation are processes widely implemented in industrial systems such as power plants, refrigeration and air conditioning systems, desalination plants, water processing installations and thermal management devices due to their enhanced heat transfer capability when compared to single-phase processes. The last decade has seen significant advances in the development and application of micro and nanostructured surfaces to enhance phase change heat transfer. Phase change heat transfer enhancement mechanisms on micro and nanostructures are significantly different from those on conventional surfaces. In this review, we provide a comprehensive summary of the effects of micro and nanostructure morphology and surface chemistry on phase change phenomena. Our review elucidates how various rational designs of micro and nanostructures can be utilized to increase heat flux and heat transfer coefficient in the case of both boiling and condensation at different environmental conditions by manipulating surface wetting and nucleation rate. We also discuss phase change heat transfer performance of liquids having higher surface tension such as water and lower surface tension liquids such as dielectric fluids, hydrocarbons and refrigerants. We discuss the effects of micro/nanostructures on boiling and condensation in both external quiescent and internal flow conditions. The review also outlines limitations of micro/nanostructures and discusses the rational development of structures to mitigate these limitations. We end the review by summarizing recent machine learning approaches for predicting heat transfer performance of micro and nanostructured surfaces in boiling and condensation applications.

/pdf/advances-in-micro-and-nanoengineered-surfaces-for-enhancing-jpm057p5.pdf

Advances in micro and nanoengineered surfaces for enhancing boiling and condensation heat transfer: a review

Advancements and challenges in ultra-thin vapor chambers for high-efficiency electronic thermal management: A comprehensive review

Free-falling objects impacting onto water pools experience a very high initial impact force, greatest at the moment when breaking through the free surface. Many have intuitively wondered whether throwing another object in front of an important object (like oneself) before impacting the water surface may reduce this high impact force. Here, we test this idea experimentally by allowing two spheres to consecutively enter the water and measuring the forces on the trailing sphere. We find that the impact acceleration reduction on the trailing sphere depends on the dynamics of the cavity created by the first sphere and the relative timing of the second sphere impact. These combined effects are captured by the non-dimensional `Matryoshka' number, which classifies the observed phenomena into four major regimes. In three of these regimes, we find that the impact acceleration on the second sphere is reduced by up to 78\% relative to impact on a quiescent water surface. Surprisingly, in one of the regimes, the force on the trailing sphere is dramatically increased by more than 400\% in the worst case observed. We explain how the various stages of cavity evolution result in the observed alterations in impact force in this multi-body water entry problem.

/pdf/impact-force-reduction-by-consecutive-water-entry-of-spheres-3j5ozmh50h.pdf

Recent advances in the optimization of evaporator wicks of vapor chambers: From mechanism to fabrication technologies

Many engineering technologies such as electronic cooling and thermal desalination exemplify the enhancement of evaporation and boiling heat transfer by surface modification. Nevertheless, the core parameters of heat transfer such as critical heat flux and heat transfer coefficient are associated with surface wettability and morphology. Herein, for the first time, a metal–organic framework (MOF) film, viz. HKUST-1, was integrated into a metallic woven mesh (macroporous support) for enhancing liquid rewetting and capillary-driven evaporation and boiling heat transfer. Compared to bare copper mesh, this architecture was found to significantly increase the critical heat flux by 205% and the heat transfer coefficient by 90%. The complex coupled two-phase (liquid and gas) transport process involving capillary wicking, evaporation, adsorption and desorption were critically examined by analysing the dynamics of multiple interfaces during horizontal wicking. Relying upon visible colorimetric changes, HKUST-1 sustained on the copper woven mesh could expedite quantitative analysis of the coupled capillary evaporation process. In principle, this is primed to offer fundamental insights into the mechanisms of transport phenomena. Introduction of such previously unreported hierarchical porous structures could also potentially advance the state-of-the-art of passive thermal management technologies. In essence, a new route to elicit superhydrophilic surfaces emerges, paving new ways for understanding the intrinsic mechanisms of phase-change heat transfer.

A superhydrophilic metal–organic framework thin film for enhancing capillary-driven boiling heat transfer

Composite nanofiltration (NF) membranes prepared using interfacial polymerization (IP) have gained significant attention in the field of wastewater treatment. In this study, sodium camphor sulfonate (CSA-Na) and tetraethylammonium chloride (TEAC) were employed as aqueous phase additives to regulate the diffusion of piperazine (PIP) molecules through electrostatic interactions. The dissociated CSA-Na and TEAC in the aqueous solution formed an organic structure at a certain concentration, restricting the interfacial transport behavior of PIP monomers. The results show that when the content of CSA-Na is 2% w/v, TEAC is 3.9% w/v, that is, the material dosage ratio is 1 : 3, and the NF membrane shows the best performance, with a water flux of 55.61 L m−2 h−1 (test pressure is 0.5 MPa), and MgSO4 rejection rate of more than 98%.

Enhancing the water permeability of composite NF membranes through the incorporation of organic ions in the aqueous phase

Spontaneously Grown Boehmite Structures Improve Pool Boiling Heat Transfer on Aluminium Surfaces

The impact of solid bodies on the free surface of liquid pools is involved in many practical applications—such as bullets and air-to-sea anti-torpedo defense systems, or the water entry of athletes in water sports—aimed at improving the performance through a control of cavity dynamics. This work reports an experimental investigation of the impact of a linear array of hydrophilic (H) and superhydrophobic (SH) spheres on a deep water pool. The array consisted of ten magnetic spheres, with different permutations of H and SH spheres. Using high speed shadowgraphy, we captured the underwater kinematics of the array for different permutations of H and SH spheres. In particular, we observed the evolution or absence of an air cavity attached to the array as a function of the position of the H and SH spheres. The position of the first SH sphere from the leading edge of the array (ZSH) emerged as a key parameter that alters the characteristics of cavity evolution. The appearance and pinch-off characteristics of a wake cavity behind the trailing edge were governed by the wetting properties of the leading and trailing surfaces of the array. The position of the first SH surface, as well as the wetting characteristics of the leading and trailing surfaces, are potential control parameters to alter underwater cavity evolution during solid surface impact on deep water pools.

Impact of a Linear Array of Hydrophilic and Superhydrophobic Spheres on a Deep Water Pool

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