•This review explores passive enhancement of pool boiling using surface modification.•Examined are macrosacle, microscale, nanoscale and multiscale (hybrid-scale) techniques.•Addressed are ...performance goals of inhibiting incidence hysteresis and increasing both nucleate boiling heat transfer coefficient and CHF.•Shown is that micro and nano surface features are susceptible to blockage and changes in performance over time.
This paper provides a comprehensive review of published articles addressing passive enhancement of pool boiling using surface modification techniques. They include macroscale, microscale, and nanoscale surfaces, as well as multiscale (hybrid-scale), and hybrid-wettability techniques. Different enhancement methods are assessed in terms of underlying fluid routing mechanisms and ability to achieve three distinct heat transfer goals: eliminating incipient boiling hysteresis, increasing nucleate boiling heat transfer coefficient, and ameliorating critical heat flux (CHF), especially for inert dielectric coolants that are both highly wetting and possess relatively poor thermophysical properties. While different enhancement scales are shown to provide different degrees of success in achieving the three goals, it is shown that both microscale and nanoscale surface features are susceptible to blockage, resulting in deterioration of the enhancement over time. This review also points to scarcity of sufficiently sized databases for a given enhancement scheme in terms of fluid type, surface material, size, and orientation, enhancement shape, pattern, and scale, and operating pressure. This renders available findings less-than-adequate tools for design of practical cooling systems.
•Single-phase liquid and nucleate boiling regimes, and critical heat flux for spray cooling are reviewed.•Discussed are dominant mechanisms, data trends, and predictive correlations and models.•Also ...discussed are enhancement schemes, including surface modifications, additives to the liquid, and use of nanofluids.•It shows future work that must be conducted using many fluids and broad ranges of operating conditions.
This study is the first part of a two-part review of spray cooling, which addresses the relatively high-flux, low-temperature mechanisms and predictive tools associated with the single-phase liquid cooling and nucleate boiling regimes, as well as critical heat flux (CHF). The second part will be focused on the relatively high-temperature transition boiling and film boiling regimes, and the Leidenfrost point, which are encountered in quenching of metal alloy parts. In this part, key spray hydrodynamic parameters influencing heat transfer performance are identified, including volumetric flux, mean droplet diameter, and mean droplet velocity. This is followed by detailed identification of dominant mechanisms, data trends, correlations, and predictive models recommended by different research teams. Also discussed are spray cooling enhancement schemes, including micro and macro modifications to the surface itself, additives to the liquid, and use of nanofluids. Overall, contradictory findings point to a need for future experimental work that must be conducted systematically using many fluids with vastly different thermophysical properties, and broad ranges of operating conditions. There is also a need for further research to investigate parameters that influence CHF, including dissolved gas, spray inclination angle, and interaction between neighboring sprays when using multi-nozzle arrays to cool large surfaces.
•Transition boiling and film boiling regimes, and the Leidenfrost point for spray cooling are reviewed.•Discussed are dominant mechanisms, data trends, and predictive correlations and models.•Shown ...is how to predict the quench curve using spray correlations as boundary conditions.•Also shown is how the spray system can be configured to optimize mechanical properties of a quenched alloy part.
This paper is the second part of a comprehensive two-part review of spray cooling. The first part addressed the mechanisms and predictive tools associated with the relatively low-temperature single-phase liquid cooling and nucleate boiling regimes, as well as critical heat flux (CHF). The present part is focused on the relatively high-temperature transition boiling and film boiling regimes, and the Leidenfrost point. Discussed are dominant mechanisms, data trends, and predictive correlations and models. This information is especially important to the quenching of metal alloy parts from high initial temperature during heat treating. It is shown how correlations for the different spray cooling regimes and transition points can be implemented into boundary conditions for heat diffusion models to predict the temperature-time (quench) curve everywhere within the quenched part. It is also shown how the quench curve can be combined with the alloy’s transformation kinetics to predict mechanical properties. By properly configuring the sprays used to quench complex-shaped parts, it is also possible to greatly enhance the mechanical properties while minimizing residual stresses.
•Different pool boiling CHF mechanisms and parametric trends are reviewed.•Also reviewed are modifications to mechanisms proposed to improve predictive accuracy.•Available models are shown to focus ...on specific parametric influences but not others.•The study points to the need for better understanding of near-wall interfacial behavior.
Critical heat flux (CHF) is arguably the most important design and safety parameter for any heat-flux controlled boiling application. The present two-part study is focused on CHF for pool boiling from flat surfaces. The first part will review different CHF models and associated mechanisms and parametric trends, while the second part will be dedicated to assessment of CHF models and correlations. Aside from Kutateladze’s 1948 pioneering CHF formulation, which is based on dimensional analysis, five different CHF mechanisms are prevalent in the literature: bubble interference, hydrodynamic instability, macrolayer dryout, hot/dry spot, and interfacial lift-off. Additionally, many modifications to these mechanisms have been proposed to improve predictive accuracy in tackling the parametric influences of pressure, surface size and roughness, surface orientation, and contact angle. Among the five mechanisms, Zuber’s hydrodynamic instability theory has received the most attention because of both its mechanistic formulation and theoretical appeal. More recently, the interfacial lift-off mechanism, which is also theoretically based, has received significant experimental validation, and offers the advantage of tackling different surface orientations. Overall, it is shown that, despite the large body of published pool boiling CHF literature, there are major data gaps in the coverage of relevant parameters. This points to a need for more strategically planned future experiments that would also include microphotographic analysis of near-wall interfacial features, in order to validate or dispute proposed CHF mechanisms.
•This review article explores the evolution of liquid drop impact on a liquid film.•Both experimental and numerical methods used in prior studies are highlighted.•Emphasis is placed on high velocity ...impact, and formation of ejecta sheet, crown sheet, and splashing of secondary droplets.•Other topics reviewed are impact on curved wetted surfaces, multi-drop impact, and low-velocity impact.•Recommendations are provided for future research to address poorly understood and/or contradictory issues.
Research on liquid drop impact, especially during the past two decades, has been motivated by a need for better predictive capability in many industries. This paper will review published works concerning mass and momentum interactions during drop impact on a liquid film. First, both experimental and numerical methods for capturing the evolution of the impact will be highlighted. This will be followed by a detailed description of the impact, including formations of the ejecta sheet, crown sheet, and splashing of secondary droplets during high-velocity impact. Other topics reviewed are impact on curved wetted surfaces, multi-drop impact, and the phenomena of spreading, coalescence and rebound in low-velocity impact. Each of these phenomena is discussed in terms of underlying physical mechanisms and predictive correlations and/or models. Despite significant past efforts to understand and characterize these phenomena, it is shown that much uncertainty remains, especially in regards to the interfacial features around the drop-film neck region during the earliest stages of the impact. Recent state-of-art advances in both experimental and numerical methods are shown to play a crucial enabling role in future research. The review is concluded with recommendations concerning future work that is needed to address poorly understood and/or contradictory issues.
Review of drop impact on heated walls Liang, Gangtao; Mudawar, Issam
International journal of heat and mass transfer,
March 2017, 2017-03-00, 20170301, Letnik:
106
Journal Article
Recenzirano
•Drop impact is reviewed in terms of heat transfer mechanisms and predictive tools.•It is segregated into film evaporation, nucleate, transition and film boiling.•Inconsistencies are found regarding ...impact process and ensuing heat transfer.•Recommendations are provided for future research.
This paper provides a comprehensive review of published literatures concerning the fluid mechanics and heat transfer mechanisms of liquid drop impact on a heated wall. The review is divided into four parts, each centered on one of the main heat transfer regimes: film evaporation, nucleate boiling, transition boiling, and film boiling. Each of these regimes is discussed in detail in terms of available depictions of drop deformation and/or breakup, proposed heat transfer mechanisms, predictive correlations and/or models. It is shown that understanding the underlying physics for each heat transfer regime is highly dependent on the experimental methods that investigators have adopted, and broadness of available databases in terms of liquid type, drop size and momentum, impact angle, and wall material and surface roughness. Despite significant advances in experimental, theoretical and computational research in understanding the interfacial behavior of the drop from the moment of impact, there are many inconsistencies concerning some of the most important aspects of the impact process and ensuing heat transfer, especially in regards to critical heat flux, transition boiling, and the Leidenfrost point. This review is concluded with recommendations concerning future work that is needed to address poorly understood and/or contradictory issues.
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•A 3D numerical model with an implement of a random disturbance subjected to Gaussian distribution is built up.•The model is effective to predict droplets splashing process.•A ...geometric model is proposed to demarcate the impact region into semicircle regions and a series of rectangle regions.•Effects of Weber number, film thickness and droplets interval on central liquid sheet height, impact areas and residual film thickness are addressed.
Multiple droplets impinging on a thin liquid film simultaneously is numerically studied using a three-dimensional model with an implement of a random disturbance subjected to Gaussian distribution. This model is very effective to predict droplet splashing under relatively high impingement momentum. Interface evolutions and field distributions are addressed after droplets impact. Besides, a geometric model is built up to demarcate the impact region into two semicircular regions and a series of rectangular regions during simultaneous impact, further to discuss the variations of area and residual film thickness in each region as well as the total area and average film thickness.
•This review explores potential convective heat transfer merits of nanofluids.•Both experimental and numerical findings are reviewed, including macro- and microchannels.•It is shown that heat ...transfer with nanofluids is realized mostly in inlet single-phase region.•Serious practical concerns in deploying nanofluids in cooling situations are identified.
This paper provides a comprehensive review of published literature concerning heat transfer benefits of nanofluids for both macro-channels and micro-channels. Included are both experimental and numerical findings concerning several important performance parameters, including single-phase and two-phase heat transfer coefficients, pressure drop, and critical heat flux (CHF), each being evaluated based on postulated mechanisms responsible for any performance enhancement or deterioration. The study also addresses issues important to heat transfer performance, including entropy minimization, hybrid enhancement methodologies, and nanofluid stability, as well as the roles of Brownian diffusion and thermophoresis. Published results point to appreciable enhancement in single-phase heat transfer coefficient realized in entrance region, but the enhancement subsides downstream. And, while some point to the ability of nanofluids to increase CHF, they also emphasize that this increase is limited to short duration boiling tests. Overall, studies point to many important practical problems associated with implementation of nanofluids in cooling situations, including clustering, sedimentation, and precipitation of nanoparticles, clogging of flow passages, erosion to heating surface, transient heat transfer behavior, high cost and production difficulties, lack of quality assurance, and loss of nanofluid stability above a threshold temperature.
This article provides a comprehensive review of published literature concerning enhancement of channel flow boiling heat transfer by surface modification. Addressed are macro, micro, nano, and hybrid ...multiscale methodologies. While the vast majority of published schemes have shown favorable heat transfer performance, evidenced by earlier onset of boiling, improved flow boiling heat transfer coefficient, and ameliorated critical heat flux (CHF), increased pressure drop is a serious concern shared by most. Nanoscale enhancement remains controversial, given that, unlike macroscale and microscale enhancement features, nanostructure topographies are prone to severe degradation after prolonged boiling. Multiscale enhancement, combining macro, micro, and nano features as well as favorable coolant flow control, is shown highly effective at tackling very high heat flux situations. Also included in this review is discussion of effective means for mitigating flow instabilities. It is concluded that the most serious obstacle to adopting surface modification techniques is absence of generalized predictive tools or robust computational models for different channel shapes, sizes, and enhancement configurations, and different fluids and operating conditions, such as tools presently available for two-phase flow and heat transfer in conventional micro-channel heat sinks.
This paper addresses interfacial phenomena associated with droplet array impact on solid surface experimentally, with aid of high-speed photography. Aside from the simultaneous case, droplet array ...impact is more represented by various non-simultaneous cases, which can be classified into three major subcases: edge/edge interaction with large droplet horizontal spacing, edge/droplet interaction with intermediate droplet spacing, and film/droplet interaction with small spacing. Outcomes include droplet coalescence induced at low velocities and central liquid sheet generated due to droplets interaction at relatively high impact velocities. In quantitative analysis, increasing droplet vertical spacing leads to decreases in both spreading factor and height of central liquid sheet due mainly to increased viscous dissipation. While both the two parameters decrease with increasing droplet horizontal spacing, its effect is weakened with increasing impact velocity, which acts as a positive role for evolution of the two parameters. Although the central sheet continues to descend under gravity at later stage, a sheet height shoulder raises attention because of cusp formation at top of the central sheet. This study provides a fundamental understanding for practical applications involving droplets impingement.
