Review of pool boiling enhancement by surface modification

Owing to advances in micro- and nanofabrication methods over the last two decades, the degree of sophistication with which solid surfaces can be engineered today has caused a resurgence of interest in the topic of engineering surfaces for phase change heat transfer. This review aims at bridging the gap between the material sciences and heat transfer communities. It makes the argument that optimum surfaces need to address the specificities of phase change heat transfer in the way that a key matches its lock. This calls for the design and fabrication of adaptive surfaces with multiscale textures and non-uniform wettability. Among numerous challenges to meet the rising global energy demand in a sustainable manner, improving phase change heat transfer has been at the forefront of engineering research for decades. The high heat transfer rates associated with phase change heat transfer are essential to energy and industry applications; but phase change is also inherently associated with poor thermodynamic efficiency at low heat flux, and violent instabilities at high heat flux. Engineers have tried since the 1930s to fabricate solid surfaces that improve phase change heat transfer. The development of micro and nanotechnologies has made feasible the high-resolution control of surface texture and chemistry over length scales ranging from molecular levels to centimeters. This paper reviews the fabrication techniques available for metallic and silicon-based surfaces, considering sintered and polymeric coatings. The influence of such surfaces in multiphase processes of high practical interest, e.g., boiling, condensation, freezing, and the associated physical phenomena are reviewed. The case is made that while engineers are in principle able to manufacture surfaces with optimum nucleation or thermofluid transport characteristics, more theoretical and experimental efforts are needed to guide the design and cost-effective fabrication of surfaces that not only satisfy the existing technological needs, but also catalyze new discoveries.

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Surface engineering for phase change heat transfer: A review

Publisher Summary Condensation represents the change of phase from the vapor state to the liquid state because of cooling. It is considered one of the most important heat-transfer processes in many energy-conversion systems, such as electric power generation plants. This chapter emphasizes on the areas of condensation heat transfer that have made progress in the past 15 years. It introduces various types of condensation and examines the wettability of the surface. The transport process at the vapor-liquid interface and the arguments on whether the condensation coefficient takes the value of unity are discussed. Furthermore, the chapter reviews dropwise condensation and film condensation in detail. It also describes the techniques of enhancement of condensation heat transfer. The usage of surface tension force is one of the most sophisticated ways for augmentation of condensation because it does not require extra energy. The chapter concludes with discussion of future trends in research on condensation heat transfer.

Advances in Condensation Heat Transfer

Dropwise condensation (DWC) heat transfer depends strongly on the maximum diameter (Dmax) of condensate droplets departing from the condenser surface. This study presents a facile technique implemented to gain control of Dmax in DWC within vapor/air atmospheres. We demonstrate how this approach can enhance the corresponding heat transfer rate by harnessing the capillary forces in the removal of the condensate from the surface. We examine various hydrophilic-superhydrophilic patterns, which, respectively, sustain and combine DWC and filmwise condensation on the substrate. The material system uses laser-patterned masking and chemical etching to achieve the desired wettability contrast and does not employ any hydrophobizing agent. By applying alternating straight parallel strips of hydrophilic (contact angle ∼78°) mirror-finish aluminum and superhydrophilic regions (etched aluminum) on the condensing surface, we show that the average maximum droplet size on the less-wettable domains is nearly 42% of the width of the corresponding strips. An overall improvement in the condensate collection rate, up to 19% (as compared to the control case of DWC on mirror-finish aluminum) was achieved by using an interdigitated superhydrophilic track pattern (on the mirror-finish hydrophilic surface) inspired by the vein network of plant leaves. The bioinspired interdigitated pattern is found to outperform the straight hydrophilic-superhydrophilic pattern design, particularly under higher humidity conditions in the presence of noncondensable gases (NCG), a condition that is more challenging for maintaining sustained DWC.

Enhancing dropwise condensation through bioinspired wettability patterning.

In this review we cover recent developments in the area of surface-enhanced dropwise condensation against the background of earlier work. The development of fabrication techniques to create surface structures at the micro- and nanoscale using both bottom-up and top-down approaches has led to increased study of complex interfacial phenomena. In the heat transfer community, researchers have been extensively exploring the use of advanced surface structuring techniques to enhance phase-change heat transfer processes. In particular, the field of vapor-to-liquid condensation and especially that of water condensation has experienced a renaissance due to the promise of further optimizing this process at the micro- and nanoscale by exploiting advances in surface engineering developed over the last several decades.

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Dropwise Condensation on Micro- and Nanostructured Surfaces

Heat transfer enhancement of dropwise condensation on a vertical surface with round shaped grooves

The effect of polyvinylidene chloride coating thickness on promotion of dropwise steam condensation

This article describes work aimed at obtaining higher filmwise condensation heat flux by distributing dropwise condensation surfaces of optimal width promoted by an organic coating among filmwise surfaces, and to get higher mean overall heat transfer coefficients in condensing systems. Several different spacings were examined for the horizontal orientation, arranging a dropwise section above a filmwise one, to make clear the effect of the drops falling down to the filmwise section. The heat flux of the filmwise part increased with increasing the height of the dropwise part up to 2 mm, but then decreased above that. The extent of the filmwise part that was augmented by drops was also tested by changing the width of the filmwise section sandwiched between dropwise sections of constant width. The heat flux of the filmwise part increased abruptly at a width between 5 and 3 mm. Consequently, it was shown that there exists an optimum width for each section for enhancing condensation heat transfer in the filmwis...

On the enhancement of filmwise condensation heat transfer by means of the coexistence with dropwise condensation sections

The local and average coefficients of heat transfer from a rotating cylinder in a uniform flow were measured by Mach-Zehnder interferometer with visual observation of flow patterns. The experiments covered the range of rotating and cross-flow Reynolds numbers (Re d ) from 0 to 3000

Heat transfer from a rotating cylinder with and without cross flow

Microbubble emission boiling (MEB) is the regime that occurs in the subcooled transition boiling region. This experimental investigation reveals that this phenomenon includes two regimes, i. e., the stormy and calm microbubble emission boilings. This study also shows temperature fluctuations of a heat transfer surface and pressure fluctuations in the liquid near the surface to classify MEB into the above regimes. The temperature fluctuations in either regime are smaller than those at the critical heat flux. The pressure fluctuations are larger in the stormy MEB, and smaller in the calm MEB as compared with those at the critical heat flux. The liquid subcooling and flow velocity approaching the surface are important aspects by which to generate those regimes. A number of experiments lead to the map of MEB regimes that consists of liquid subcooling and flow velocity. An oscillatory character between the two regimes appears under several conditions.

Satoshi Kumagai

Papers

Heat transfer enhancement of dropwise condensation on a vertical surface with round shaped grooves

The effect of polyvinylidene chloride coating thickness on promotion of dropwise steam condensation

On the enhancement of filmwise condensation heat transfer by means of the coexistence with dropwise condensation sections

Heat transfer from a rotating cylinder with and without cross flow

Occurrence and Stability of Microbubble Emission Boiling.