An impact regime map for water drops impacting on heated surfaces

Bertola, Volfango

doi:10.1016/j.ijheatmasstransfer.2015.01.084

Cited by 151 publications

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References 26 publications

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“…On even hotter surfaces, however, beyond the socalled Leidenfrost temperature T L , the droplet interface becomes smooth again without any bubbles inside it. In this regime the droplet lives much longer, as now it levitates on its own vapor layer: the well-known Leidenfrost effect [9,10].In order to determine the Leidenfrost temperature T L and its dependence on the impact velocity U, phase diagrams have been experimentally produced for various impacting droplets with many combinations of substrates and liquids: water on smooth silicon [8], water on microstructured silicon [11], FC-72 on carbon nanofiber [12], water on aluminium [13], and ethanol on sapphire [14]. All of these phase diagrams show a weakly increasing behavior of T L with U.…”

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Dynamic Leidenfrost Effect: Relevant Time and Length Scales

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When a liquid droplet impacts a hot solid surface, enough vapor may be generated under it to prevent its contact with the solid. The minimum solid temperature for this so-called Leidenfrost effect to occur is termed the Leidenfrost temperature, or the dynamic Leidenfrost temperature when the droplet velocity is non-negligible. We observe the wetting or drying and the levitation dynamics of the droplet impacting on an (isothermal) smooth sapphire surface using high-speed total internal reflection imaging, which enables us to observe the droplet base up to about 100 nm above the substrate surface. By this method we are able to reveal the processes responsible for the transitional regime between the fully wetting and the fully levitated droplet as the solid temperature increases, thus shedding light on the characteristic time and length scales setting the dynamic Leidenfrost temperature for droplet impact on an isothermal substrate. DOI: 10.1103/PhysRevLett.116.064501 Boiling and spreading of droplets impacting on hot substrates have been extensively studied since both phenomena strongly affect the heat transfer between the liquid and the solid. Applications include spray cooling [1], spray combustion [2], and others [3].At room temperature, an impacting droplet spreads on a solid surface and entraps a bubble under it [4][5][6]. At temperatures higher than the boiling temperature T b , vapor bubbles appear which disturb and finally rupture the free surface, resulting in the violent spattering of tiny droplets [7,8]. On even hotter surfaces, however, beyond the socalled Leidenfrost temperature T L , the droplet interface becomes smooth again without any bubbles inside it. In this regime the droplet lives much longer, as now it levitates on its own vapor layer: the well-known Leidenfrost effect [9,10].In order to determine the Leidenfrost temperature T L and its dependence on the impact velocity U, phase diagrams have been experimentally produced for various impacting droplets with many combinations of substrates and liquids: water on smooth silicon [8], water on microstructured silicon [11], FC-72 on carbon nanofiber [12], water on aluminium [13], and ethanol on sapphire [14]. All of these phase diagrams show a weakly increasing behavior of T L with U. When theoretically deriving T L , one needs to determine the vapor thickness profile. In the case of a gently deposited droplet, this can be accomplished since the shape of the droplet is fixed except for the bottom surface, which reduces the problem to a lubrication flow of vapor in the gap between the substrate and the free surface [15][16][17][18][19][20]. For impacting droplets on an unheated surface at high Weber number We ≡ ρU 2 D 0 =σ (here, D 0 is the equivalent diameter of the droplet and ρ and σ are the density and the surface tension of the liquid, respectively), it is known that the neck around the dimple beneath the impacting droplet rams the surface. In this cold impact case, the neck propagates outwards like a wave [21]. For impact on a superheated s...

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Dynamic Leidenfrost Effect: Relevant Time and Length Scales

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“…The data was also processed in Matlab environment to generate impact regime maps, showing the impact behaviour as a function of surface temperature and Weber number. More details about the experimental apparatus and procedure can be found in previous works [13,17,18].…”

Section: Methodsmentioning

confidence: 99%

“…Results about Newtonian drop impact on heated surfaces reported in the literature are generally consistent with one another, to the exception of one recent work presenting an IRM based upon the impact of water and FC-72 drops upon silicon and sapphire surfaces, for temperatures between 200°C and 600°C and the Weber numbers between 0.5 and 600 [14]. Whilst one may argue the differences between these results and the rest of the literature are due to the high smoothness of the surfaces used, a more careful analysis suggests they may be caused by the design of the experimental setup, which leads to overestimate the surface temperature [13]. The impact of non-Newtonian drops on heated surfaces received comparatively less attention.…”

Section: Introductionmentioning

confidence: 92%

“…A comprehensive review on single droplet impact research and its relevance to understanding fuel sprays impingement can be found in [12]. To rationalise the rich variety of impact morphologies observed for Newtonian drops impinging on heated surfaces, it was proposed to identify "simple" impact regimes, displaying one distinctive feature (deposition, rebound, splashing/breakup) and "mixed" regimes, resulting from the combination of simple regimes with secondary atomisation [13]. Such unifying classification on one hand embraces the different impact morphologies reported in the existing literature, and on the other hand is simple enough to be used for practical purposes; in addition, it allows one to derive simple models for transition boundaries.…”

Section: Introductionmentioning

confidence: 99%

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Dilute polymer solution drops impacting on heated surfaces: New impact morphologies and impact regime maps

Lakhanpal¹,

Bertola²

2017

Proceedings ILASS–Europe 2017. 28th Conference on Liquid Atomization and Spray Systems

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The impact morphology of dilute polymer solution drops on a heated surface is studied experimentally by means of high-speed imaging, with respect to the following parameters: surface temperature; impact Weber number; polymer concentration; polymer molecular weight. In addition to impact morphologies observed in Newtonian drops (deposition, rebound, secondary atomisation and breakup/splashing), three new impact regimes have been identified: (i) a single satellite droplet ejected in the direction of bouncing but tethered to the main drop by a thin liquid filament; (ii) a splashing-like behaviour (semi-splashing), where the rim instability generates satellite droplets tethered to the lamella by thin liquid filaments; (iii) a spray-like behaviour (semi-spray), where a fine secondary atomisation generated upon impact is quickly absorbed back into the drop globule. Experiments were carried out using drops of aqueous polyethylene oxide (PEO) solutions, with mass concentrations of 100 ppm, 200 ppm and 400 ppm, and PEO molecular weights of 2 MDa, 4MDa, and 8MDa. The impact morphology on a polished aluminium surface with temperatures ranging between 160°C and 400°C was investigated for impact Weber numbers between 20 and 170, taking side view images of impacting drops at a rate of 1,000 frames per second. KeywordsDrop Impact; Heated Surfaces; Polymer Solutions; Impact Regime Map. IntroductionThe dynamics of liquid drops impinging on a heated surface is a phenomenon of interest in many areas of engineering, including spray cooling, inkjet printing for advanced manufacturing applications, quenching of alloys in the steel industry and nuclear reactor safety. Despite several decades of research, this phenomenon is understood only to a limited extent and requires further research; specifically, the development of impact regime maps (IRMs) is of practical interest because they provide a consolidated understanding of the impact behaviour and represent a tool of practical use in the design of nozzles, print heads, etc. A quantitative and comprehensive IRM also presents an important test case for the development and validation of theoretical models describing specific features, such as transitions between different impact regimes. Currently, an extensive body of literature on the behaviour of Newtonian drops impacting on heated surfaces exists. The first IRM produced [1] identified most of the impact regimes, however lacked clear quantitative description on the initiation and extent of these regimes. This work was soon followed by a detailed investigation that allowed production of three distinct IRMs corresponding to Weber numbers of 20, 60 and 220 and for surface temperatures ranging from 100° to 300°C [2]. Although this work included substantial information on the spreading behaviour of a drop and its heat transfer characteristics, the IRMs were far from being fully comprehensive, particularly lacking information on all possible impact regimes that can be realised as well as the transitional boundaries. This was partly ...

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“…In order to determine the Leidenfrost temperature T L and its dependence on the impact velocity U , phase diagrams have been experimentally produced for various impacting droplets with many combinations of substrates and liquids: water on smooth silicon [8], water on micro-structured silicon [50], FC-72 on carbon-nanofiber [67], water on aluminium [68], and ethanol on sapphire [69]. All these phase diagrams show a weakly increasing behaviour of T L with U .…”

Section: Introductionmentioning

confidence: 99%

The dynamics of Leidenfrost drops

Limbeek¹

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An impact regime map for water drops impacting on heated surfaces

Cited by 151 publications

References 26 publications

Dynamic Leidenfrost Effect: Relevant Time and Length Scales

Dynamic Leidenfrost Effect: Relevant Time and Length Scales

Dilute polymer solution drops impacting on heated surfaces: New impact morphologies and impact regime maps

The dynamics of Leidenfrost drops

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