Dynamics of thermal and wetting footprint of a volatile droplet during Leidenfrost transition (original) (raw)
Related papers
The Leidenfrost transition of liquid droplets impinging onto a superheated surface
International Conference on Liquid Atomization and Spray Systems (ICLASS), 2021
Heat transfer during the impact of a droplet on a sapphire substrate is investigated by means of infrared thermography. For that, the sapphire is coated with a very thin layer of TiAlN having a high emissivity in the IR domain. Spatially and time resolved measurements of the temperature at the front face of the solid wall are obtained. Results obtained for water droplets show that the dynamic Leidenfrost point (LFP) is close to 450°C and coincides with the onset of a fingering pattern. Approaching the dynamic LFP, despite the cooling by the droplet, the wall surface temperature never decreases below 310°C which is about the temperature of the spinodal for water, i.e. the maximum temperature at which water can still exist in the liquid state. Considering that a wetting contact is taking place below the dynamic LFP, wall temperature measurements demonstrate that the drop impact comes with a very strong superheating of the liquid. The liquid touching the wall is heated up to the spinodal temperature. Based on the idea that the dynamic LFP could correspond to the wall temperature, for which the contact temperature at a solid/liquid interface is equal to the spinodal temperature, a model is proposed for the dynamic LFP. This model considers the thermal effusivities of the liquid and the wall, as well as the liquid flow in the spreading lamella.
Heat transfer for Leidenfrost drops bouncing onto a hot surface
Experimental Thermal and Fluid Science, 2013
When droplets impinge onto a hot wall, different regimes can be observed depending on the wall temperature and Weber number. In the case of interest, the temperature of the wall is more than the Leidenfrost temperature and the Weber number based on normal velocity of the droplet is less than the threshold value leading to the splashing regime (We < 80). We particularly focused on the perfect bouncing regime (We < 30) for which an impinging droplet levitates on a thin layer of its own vapour. This vapour is instantaneously created between the base of the deforming droplet and the heated surface so that direct contact with the hot solid is avoided. For these low Weber numbers, the droplet surface energy is high enough compared to its kinetic energy to permit the rebound, so that the droplet recovers its initial shape without breaking up after the bounce. Although the generated vapour insulates the droplet, some heat is exchanged with the wall during the interaction (i.e. during the resident time). In this paper, we report on experimental measurements of heat transfer due to droplet impact in the Leidenfrost regime. The energy released by the wall and measured using an inverse conduction method leads to an estimation of the heat transfer coefficient during impact in the Leidenfrost regime. For that, the time evolution of the droplet base surface is estimated using a simple modelling validated on experimental data. Finally, the energy measured is compared to existing models.
Dynamic Leidenfrost Effect: Relevant Time and Length Scales
Physical Review Letters, 2016
When a liquid droplet impacts a hot solid surface, enough vapor may be generated under it as 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/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.
Heat transfer and dynamics of the droplet on a superheated surface
WSEAS Transactions on Heat and Mass Transfer
The conditions governing the collapse of a "Leidenfrost drop", e.g. a liquid drop supported by a vapour film on a heated surface was studied analytically. Robust analytical model for the phenomenon has been developed and numerical simulation has been done. The model was represented by two second-order non-linear differential equations for the radius of evaporating drop and its distance over the heated surface. The results obtained have shown the high-frequent oscillations of the drop over the hot plate until complete evaporation of the drop occurs. In contrast with existing precise complex models, the mathematical model developed was simple and could be used for the qualitative estimation of different parameters and quantitative estimation of the integral behaviours of the drop such as time for complete drop evaporation. The effects of surfactants on the Leidenfrost phenomenon and its industrial applications were discussed
Triple Leidenfrost Effect: Preventing Coalescence of Drops on a Hot Plate
Physical Review Letters, 2021
We report on the collision-coalescence dynamics of drops in Leidenfrost state using liquids with different physicochemical properties. Drops of the same liquid deposited on a hot concave surface coalesce practically at contact, but when drops of different liquids collide, they can bounce several times before finally coalescing when the one that evaporates faster reaches a size similar to its capillary length. The bouncing dynamics is produced because the drops are not only in Leidenfrost state with the substrate, they also experience Leidenfrost effect between them at the moment of collision. This happens due to their different boiling temperatures, and therefore, the hotter drop works as a hot surface for the drop with lower boiling point, producing three contact zones of Leidenfrost state simultaneously. We called this scenario the triple Leidenfrost effect.
High jump of impinged droplets before Leidenfrost state
Physical Review E
Unlike the traditionally reported Leidenfrost droplet which only floats on a thin film of vapor, we observe a prominent jump of the impinged droplets in the transition from the contact boiling to the Leidenfrost state. The vapor generation between the droplet and the substrate is vigorous enough to propel the spreading droplet pancake to an anomalous height. The maximum repellent height can be treated as an index of the total transferred energy. Counterintuitively, a stronger vaporization and a higher jump can be generated in the conditions normally considered to be unfavorable to heat transfer, such as a lower substrate temperature, a lower droplet impact velocity, a higher droplet temperature, or a lower thermal conductivity of the deposition on the substrate. Since the total transferred energy is the accumulation of the instantaneous heat flux during the droplet contacting with the substrate, it can be deduced that a longer contact time period is secured in the case of a lower instantaneous heat flux. The inference is supported by our experimental observations. Moreover, the phase diagrams describe the characteristics of the high repellency under different substrate temperatures, droplet subcooling temperatures, and Weber numbers. It allows us to manipulate the droplet jump for the relative applications.
Energy balance of droplets impinging onto a wall heated above the Leidenfrost temperature
International Journal of Heat and Fluid Flow, 2013
This work is an experimental study aiming at characterizing the heat transfers induced by the impingement of water droplets (diameter 80-180 lm) on a thin nickel plate heated by electromagnetic induction. The temperature of the rear face of the nickel sample is measured by means of an infrared camera and the heat removed from the wall due to the presence of the droplets is estimated using a semi-analytical inverse heat conduction model. In parallel, the temperature of the droplets is measured using the twocolor Laser-Induced Fluorescence thermometry (2cLIF) which has been extended to imagery for the purpose of these experiments. The measurements of the variation in the droplet temperature occurring during an impact allow determining the sensible heat removed by the liquid. Measurements are performed at wall conditions well above the Leidenfrost temperature. Different values of the Weber numbers corresponding to the bouncing and splashing regimes are tested. Comparisons between the heat flux removed from the wall and the sensible heat gained by the liquid allows estimating the heat flux related to liquid evaporation. Results reveal that the respective level of the droplet sensible heat and the heat lost due to liquid vaporization can vary significantly with the droplet sizes and the Weber number.
Inverted Leidenfrost-like effect during condensation
Langmuir, 2015
Water droplets condensing on solidified phase change materials such as benzene and cyclohexane near their melting point show in-plane jumping and continuous "crawling" motion. The jumping drop motion has been tentatively explained as an outcome of melting and refreezing of the materials surface beneath the droplets and can be thus considered as an inverted Leidenfrost-like effect (in the classical case vapor is generated from a droplet on a hot substrate). We present here a detailed investigation of jumping movements using high-speed imaging and static crosssectional cryogenic focused ion beam scanning electron microscope imaging. Our results show that drop motion is induced by a thermocapillary (Marangoni) effect. The in-plane jumping motion can be delineated to occur in two stages. The first stage occurs on a millisecond time scale and comprises melting the substrate due to drop condensation. This results in droplet depinning, partial spreading, and thermocapillary movement until freezing of the cyclohexane film. The second stage occurs on a second time scale and comprises relaxation motion of the drop contact line (change in drop contact radius and contact angle) after substrate freezing. When the cyclohexane film cannot freeze, the droplet continuously glides on the surface, resulting in the crawling motion.
International Journal of Heat and Mass Transfer, 2010
The objective of this work is to investigate the coupling of fluid dynamics, heat transfer and mass transfer during the impact and evaporation of droplets on a heated solid substrate. A laser-based thermoreflectance method is used to measure the temperature at the solid-liquid interface, with a time and space resolution of 100 µs and 20 µm, respectively. Isopropanol droplets with micro-and nanoliter volumes are considered. A finite-element model is used to simulate the transient fluid dynamics and heat transfer during the droplet deposition process, considering the dynamics of wetting as well as Nomenclature c drop liquid vapor concentration [kg m -3 ] c p specific heat [J kg -1 K -1 ] d diameter of the drop [m]