Energy balance of droplets impinging onto a wall heated above the Leidenfrost temperature

Dunand, Pierre; Castanet, Guillaume; Gradeck, Michel; Maillet, Denis; Lemoine, Fabrice

doi:10.1016/j.ijheatfluidflow.2013.05.021

Cited by 41 publications

(40 citation statements)

References 26 publications

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“…The heating increases with W e, but there is a saturation, meaning that W e has almost not influence on the final liquid temperature in the splashing regime contrary to the bouncing regime. This trend has already been observed by Castanet et al [6] and Dunand et al [4], who also measured an heating of about 40°C in the case of splashing droplets with an initial temperature of 20°C. The duration of the heating decreases with W e. At low impact velocity, the heating period covers both the spreading and the recoiling phases.…”

Section: Recoiling and Bouncingsupporting

confidence: 79%

“…The heat flux density is maximal shortly before the end of the spreading phase and vanishes progressively thereafter. The obtained values for the heat flux, a few MW/m 2 , compare with the literature [3,4]. The images of the solid surface temperature show some finger-like structures.…”

Section: Recoiling and Bouncingsupporting

confidence: 79%

“…Reynolds, Weber and Ohnesorge numbers. Regarding heat transfers, almost all the studies were focused around the heat removal from the solid surface, whose temperature was monitored using either thermocouples embedded in the wall thickness [2] and more recently by IR thermography [3,4]. Coupled with an inverse model, these measurements made quantification of the heat removed from the wall and the cooling efficiency [4] possible .Recently, Jung et al [3] used IR thermography to characterize the temporal evolution of the heat flux at the wall during the impact of millimeter-size water droplets.…”

mentioning

confidence: 99%

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Instantaneous heat transfers at the impact of a droplet onto a hot surface in the film boiling regime

Chaze¹,

Castanet²,

Caballina³

et al. 2017

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

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Heat and mass transfers at the impact of a droplet onto a hot solid surface are investigated experimentally. Millimetersized water droplets impinges onto a perfectly flat sapphire surface heated at 600°C. The temperature of the liquid inside the droplet is measured using the two-color laser-induced fluorescence (2cLIF) technique. Water is seeded with a temperature-sensitive fluorescent dye, while a nanosecond pulsed laser is used for the excitation of the fluorescence. The ratio of fluorescence signal detected in two appropriate spectral bands allows to determine the liquid temperature. One advantage of this non-intrusive optical technique is that it eliminates adverse effects associated with signal variations caused by droplet shape during its impact. In parallel, the temperature of the solid surface is characterized using infrared thermography. The latter measurements are made possible by the deposition of a nanosize coating of titanium aluminium nitride (TiAlN) on the upper surface of the sapphire window. Thanks to the high frame rate of the IR camera, the time evolution of the heat flux distribution at the solid surface can be reconstructed. A comparison of IR and 2cLIF techniques enable to correlate the heating of the liquid with the cooling of the wall. This reveals that most of the heat removed from the solid surface is devoted to the heating of the liquid, the energy used for liquid vaporization being significantly lower. IntroductionMany industrial applications require a rapid cooling of surfaces from high temperatures. Among the cooling technologies, spray cooling is certainly one of the most attractive for the thermal management of high heat flux systems. Compared to jet impingement, it has the capability of cooling a relatively wider surface area with a single nozzle. It also has an unrivalled cooling efficiency, meaning that significant quantities of coolant liquid can be saved to remove the same amount of heat. These features explain why it is widely employed in many industrial applications, especially in metal production and processing industry. However, while it is applied for decades, its integration remains a complex and cumbersome process because of still incomplete knowledge of the fluid flow and heat transfer characteristics. In particular, scientific investigations focused on individual droplets are still required to understand the underlying physics behind the interactions between droplets and a hot solid surface. When a drop impacts a hot wall, different behaviours are observed. The drop can spread over the solid surface and remain attached to it due to wettability forces. It can splash and creates several smaller secondary droplets or simply rebounds. Extensive experimental investigations were carried out in the past to characterize the parameters influencing the drop behavior at the impact. Among them, some can be related to the dynamic of the impacting droplets (velocity, diameter, etc.), the physical properties of the liquid (viscosity, surface tension, etc.), and the solid su...

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Section: Recoiling and Bouncingsupporting

confidence: 79%

Section: Recoiling and Bouncingsupporting

confidence: 79%

mentioning

confidence: 99%

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Instantaneous heat transfers at the impact of a droplet onto a hot surface in the film boiling regime

Chaze¹,

Castanet²,

Caballina³

et al. 2017

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

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“…This idea is backed up by recent measurements which show that drop heating is limited to a few • C during a rebound. [36][37][38] In the following, liquid properties are taken at the injection temperature T 0 = 23 • C to calculate W e and Re. The experimental conditions are summarized in Table I and cover the rebound regime including partial rebounds (Fig.…”

Section: Maximum Spreading Diameter: Scaling Analysismentioning

confidence: 99%

“…As a result, ϕ ≈ L vṁ ′′ v , where L v is the latent heat of vaporization of the liquid. In the case of a drop impact, the transient heating of the droplet cannot be ignored given that the sensible heat flux entering the droplet usually compares with the heat flux of vaporization as shown in recent experiments by Dunand et al 38 The geometry of the vapor film (a thin and long slot) naturally sets the conditions for the lubrication approximation meaning that the vapor escapes radially with a Poiseuille flow. Assuming a no-slip condition at the solid and liquid interfaces (the smallness of the slip velocity at the vapor/liquid interface in comparison with the mean vapor velocity can be checked afterwards), the viscous stress at the interface can be evaluated as…”

Section: Modeling Of the Lamella Entrainment By The Vapor Flowmentioning

confidence: 99%

Drop spreading at the impact in the Leidenfrost boiling

2015

Self Cite

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Although the Leidenfrost effect has been extensively studied in the past, one challenge for the modeling of this phenomenon remains, namely, how to determine the effect induced by the presence of a vapor film on the frictions exerted on the drop. To address this issue, experiments are carried out on liquids with very different viscosities including water, ethanol, and several mixtures of water and glycerol. The deformation of droplets of a few hundred micrometers, impinging a perfectly smooth solid surface heated above the Leidenfrost temperature, is observed by shadowgraphy using a high-speed camera. Experimental results are compared to a theoretical model which is based on an inviscid asymptotic solution for the flow inside the lamella. This model also considers a lamella thickness which does not depend on the viscosity, the surface tension, and thus on the Reynolds and Weber numbers. This description of the lamella is valid if Weber and Reynolds numbers are high enough. Mass and momentum balances applied to the rim bounding the spreading lamella yield an equation for the rim motion which is then solved numerically. This equation accounts for the momentum transferred to the rim by the liquid coming from the lamella, the capillary forces, and the viscous stress at the separation between the lamella and the rim. The comparison between the model and the experiments suggests that the liquid at the bottom edge of the lamella is dragged by the vapor film given that the vapor velocity in the vapor film is significantly larger than that of the liquid. This process significantly increases the drop spreading for the low viscosity liquids. An analysis of the viscous boundary layer which develops at the bottom edge of the lamella is found to confirm this scenario.

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Experimental investigation of heat transfer characteristics of UWS spray impingement in diesel SCR

Liao

Eggenschwiler

2016

Proceedings

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Energy balance of droplets impinging onto a wall heated above the Leidenfrost temperature

Cited by 41 publications

References 26 publications

Instantaneous heat transfers at the impact of a droplet onto a hot surface in the film boiling regime

Instantaneous heat transfers at the impact of a droplet onto a hot surface in the film boiling regime

Drop spreading at the impact in the Leidenfrost boiling

Experimental investigation of heat transfer characteristics of UWS spray impingement in diesel SCR

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