Film Boiling Research Articles

Promoted stable combustion of alcohol-based fuel accompanied by inhibition of Leidenfrost effect in a wide temperature range

Combustion of bio-alcohols helps alleviate energy crisis and environmental pollution. However, the Leidenfrost effect (LE) of alcohols triggered by the adiabatic vapor layer greatly reduces the heat transfer efficiency, which makes alcohols exhibit a long ignition delay time (IDT) at low temperatures, hindering energy efficiency and clean combustion. Here, we first develop a scalable strategy to inhibit LE via self-propagating oxidation heat release and concomitant in-situ active radical generation. Reliable ignition and stable combustion of alcohol-based fuels are achieved in a wide temperature range by using a reactive organometallic compound methoxydiethylborane (MDEB). The theoretically released heat of MDEB self-oxidation is two orders of magnitude greater than the evaporation latent heat of ethanol or n-decane by thermodynamic calculation. Experiments show that the flammable temperature limits of MDEB/ethanol and MDEB/ethanol/n-decane are extended to 225 and 125 °C, which dropped by 370 and 465 °C, respectively. Moreover, the IDT of MDEB/ethanol/n-decane hybrid fuel significantly decreases to 27 ms. The 100% ignition achievement and disappearance of negative temperature behavior highlight the reliable start-up in the Liedenfrost region. This work thus provides a promising tool for promoting the ignition reliability and combustion stability of alcohols for ICEs at low temperatures, as well as pollutant control.

Visual analysis of density and velocity profiles in dense 3D granular gases

Granular multiparticle ensembles are of interest from fundamental statistical viewpoints as well as for the understanding of collective processes in industry and in nature. Extraction of physical data from optical observations of three-dimensional (3D) granular ensembles poses considerable problems. Particle-based tracking is possible only at low volume fractions, not in clusters. We apply shadow-based and feature-tracking methods to analyze the dynamics of granular gases in a container with vibrating side walls under microgravity. In order to validate the reliability of these optical analysis methods, we perform numerical simulations of ensembles similar to the experiment. The simulation output is graphically rendered to mimic the experimentally obtained images. We validate the output of the optical analysis methods on the basis of this ground truth information. This approach provides insight in two interconnected problems: the confirmation of the accuracy of the simulations and the test of the applicability of the visual analysis. The proposed approach can be used for further investigations of dynamical properties of such media, including the granular Leidenfrost effect, granular cooling, and gas-clustering transitions.

Prediction of the Minimum Film Boiling Temperature of Quenching Vertical Rods in Water Using Random Forest Machine Learning Algorithm

A great amount of research is focused, nowadays, on experimental, theoretical, and numerical analysis of transient pool boiling. Knowing the minimum film boiling temperature (Tmin) for rods with different substrate materials that are quenched in distilled water pools at various system pressures is known to be a complex and highly non-linear process. This work aims to develop a new correlation to predict the Tmin in the above process: Random forest machine learning technique is applied to predict the Tmin. The approach trains a machine learning algorithm using a set of experimental data collected from the literature. Several parameters such as liquid subcooling temperature (Tsub), fluid to the substrate material thermophysical properties (βf/βw), and system saturated pressure (Psat) are collected and used as inputs, whereas Tmin is measured and used as the output. Computational results show that the algorithm achieves superior results compared to other correlations reported in the literature.

Growth dynamics of mist-CVD grown ZnO nanoplatelets

In this study, ZnO nanoplatelets (NPs) grown on the SiO2 layer using mist chemical vapor deposition (mist-CVD) growth method were investigated for different growth temperatures. Structural and surface properties of the grown ZnO NPs have been characterized by x-ray diffraction (XRD), atomic force microscopy (AFM), and scanning electron microscopy (SEM) measurements. ZnO NPs have preferred to grow in the [101‾2] and [112‾0] growing directions with two-dimensional (2D) nanosheet like behavior. Furthermore, it is observed from XRD results that possible beta phase willemite (Zn2SiO4) structure has been grown for the first time using mist-CVD method. It is shown that the higher growth temperature is decreasing the growth rate of ZnO NPs on the substrate where the Leidenfrost effect is seemed to occur. The characteristic vibration modes related to wurtzite ZnO have been shown together with confocal Raman spectroscopy measurement.

On the Leidenfrost effect of water droplet impacting on superalloy plate surface

The boiling of liquid water on the superalloy surface is important in applications such as steam and gas turbines, for which it is necessary to explore the mechanisms of the Leidenfrost phenomenon. The molecular dynamics simulation method provides an effective way to study this phenomenon at the nanoscale. Previous studies, however, have not revealed the Leidenfrost effect of liquid molecules on a superalloy surface. This work investigated the effects of surface temperature and impact velocity on the nanoscale Leidenfrost phenomenon of water droplet over the Fe-Cr superalloy plate. When the surface temperature is higher than 798 K, the Leidenfrost phenomenon occurs on the superalloy surface with apparent temperature, stress, and pressure differences. The results indicate that the Leidenfrost phenomenon is related to the high surface temperature, rather than its impacting velocity. Furthermore, it is found that the Leidenfrost phenomenon is the combined results from the temperature and pressure differences exist in the vapor film, as enough kinetic energy can be generated by the thermal motion to cause the bounce phenomenon of droplet. The revealed mechanisms are critical for the design of a gas turbine, in which vapor film could be dominant in determining the film cooling effectiveness of gas/vapor on the blade surface.

Leidenfrost droplet trampolining

A liquid droplet dispensed over a sufficiently hot surface does not make contact but instead hovers on a cushion of its own self-generated vapor. Since its discovery in 1756, this so-called Leidenfrost effect has been intensively studied. Here we report a remarkable self-propulsion mechanism of Leidenfrost droplets against gravity, that we term Leidenfrost droplet trampolining. Leidenfrost droplets gently deposited on fully rigid surfaces experience self-induced spontaneous oscillations and start to gradually bounce from an initial resting altitude to increasing heights, thereby violating the traditionally accepted Leidenfrost equilibrium. We found that the continuously draining vapor cushion initiates and fuels Leidenfrost trampolining by inducing ripples on the droplet bottom surface, which translate into pressure oscillations and induce self-sustained periodic vertical droplet bouncing over a broad range of experimental conditions.

Erratum: Leidenfrost effect: Accurate drop shape modeling and refined scaling laws [Phys. Rev. E 90 , 053011 (2014)

This corrects the article DOI: 10.1103/PhysRevE.90.053011.

Modeling and Analysis of Porous Platinum Nanolayer Used in Thin Film Boiling by Resistor Network Approach

Utilizing nanoporous membranes can realize a new thin film boiling regime with ultrahigh heat flux of over 1 kW/cm2. In thin film boiling experiments, a nanoscaled platinum (Pt) layer coating serves as both heater and temperature sensor and therefore plays a fundamentally important role. However, river-shaped micro cracks were discovered in the Pt layer after experiments, of which the origin and the influence are still unclear. In this paper, based on the microscopic observation, the porous Pt layer was abstracted as a resistor network. A comprehensive model was set up to quantitatively analyze the Pt layer by obtaining the resistance and temperature of all the resistors in the network. With this model, the formation of the river-shaped cracks was successfully simulated and the influence of the thickness uniformity of Pt layer was analyzed in detail. It was found that the river-shaped cracks were formed by the successive melting of resistors, which started from the thinner or thicker resistor and its adjacent resistors that achieved critical heat flux (CHF) earlier than the rest resistors, and then spread to the other part of the Pt layer in a way similar to a chain reaction. For ideal Pt layer with uniform thickness, it could eliminate the river-shaped cracks, achieve higher CHF and endure a wider voltage variation. The in-depth modeling on Pt layer may not only help the fundamental study of the thin film phase change heat transfer, but also contribute to the research of other nanoscaled conductive layers.

Janus Vitrification of Droplet via Cold Leidenfrost Phenomenon.

Janus particles with asymmetric crystals show great importance in optoelectronics and photocatalysis, but their synthesis usually requires complicated procedures. Here, an unexpected Janus vitrification phenomenon is observed in a droplet caused by the Leidenfrost effect at a cryogenic temperature, which is commonly regarded as symmetric. The Leidenfrost phenomenon levitates the droplet when it comes in contact with liquid nitrogen causing different cooling conditions on the droplet's top and bottom surfaces. It induces asymmetric crystallization in the droplet, forming a Janus vitrified particle with an asymmetric crystallization borderline after cooling, as further evidenced by cryotransmission electron microscopy (cryo-TEM) experiments. Theoretical analysis and experimental study indicate that the position of the asymmetric crystallization borderline is determined by the droplet radius and density, and the observation window of asymmetric crystallization borderline is determined by the chemical concentration. The finding reveals the asymmetric crystallization phenomenon in droplet vitrification for the first time, and provides a new insight for creating Janus particles through the Leidenfrost phenomenon.

Hot surface ignition of a wall-impinging fuel spray: Modeling and analysis using large-eddy simulation

The accidental ignition of liquid fuels is an industrial safety concern due to the storage and transport of pressurized flammable liquids near components at elevated temperatures. In this work, a liquid n-dodecane fuel spray is considered that impinges on a hot surface, undergoing thermal ignition. Surface temperatures above the minimum hot surface ignition temperature (MHSIT) for n-dodecane ignition at atmospheric pressure are considered. At these temperatures, the interaction of the spray with the hot surface is governed by the Leidenfrost effect, resulting in inelastic reflection of impinging droplets. Large-eddy simulations are employed with finite-rate chemistry using a realistic 54-species chemical mechanism with low-temperature ignition chemistry. An Eulerian-Lagrangian approach is taken to the describe the spray dynamics. Ignition kernel formation and propagation are discussed in physical and composition spaces, and the flow field structure is compared to theory. At temperatures near the MHSIT, the secondary flow resulting from the spray impingement in the form of a toroidal vortex is shown to enhance scalar mixing. Low-temperature ignition within the vortex is seen to significantly precede high-temperature ignition near the wall and subsequent rapid flame propagation. At higher wall temperatures, the ignition delay is greatly reduced such that high temperature ignition occurs prior to the establishment of mixing structures, and hence the extent of flame propagation is diminished, with transition occurring rapidly to a steady-burning regime. This study provides fundamental understanding of the physical phenomena involved in the thermal ignition of impinging sprays in different temperature regimes toward the goal of improved industrial safety.

Self-Similarity in the Problem of Laminar Film Boiling on a Vertical Surface Submerged in a Large Volume of a Liquid

Self-Similarity in the Problem of Laminar Film Boiling on a Vertical Surface Submerged in a Large Volume of a Liquid

Dry ice hoverboard: Friction reduction by the Leidenfrost effect.

Friction reduction is a major issue in multiple domains, and lubrication is often used in order to achieve it. Gas lubrication is a very efficient way to increase slipperiness, reducing the friction coefficient to almost zero. The main challenge with gas lubrication is to keep the gas inside the contact area due to the fact that it is easily squeezed out because of its low viscosity. Here we use the Leidenfrost effect to form a lubricating gas layer in between a disk of dry ice and a substrate, thus leading to lubricated friction. The gas is continuously provided by sublimation due to the temperature difference between dry ice and substrate. We perform different experiments on dry ice, measuring friction and parameters inside the gas layer. We then chart the crossover from high to low friction as a function of pressure and temperature, and we reveal the role of gas layer thickness. The substrate temperature and macroscopic pressure are found to strongly affect the friction, and very low friction is reached only in particular conditions. These conditions are easily controlled through external parameters, which allows us to use the Leidenfrost effect to efficiently modify friction.

Beyond Leidenfrost levitation: A thin-film boiling engine for controlled power generation

Overcoming friction between moving components is important for reducing energy losses and component wear. Hydrodynamic lubrication via thin-film boiling provides an opportunity for reduced friction energy and mass transport. A common example of such lubrication is the Leidenfrost effect, where a liquid droplet levitates on a cushion ofits own vapor on a surface heated to temperatures above the liquid's boiling point. An asymmetry in this vapor flow, self-propels the droplet on the surface due to viscous drag, converting thermal energy to mechanical motion, like a heat engine. Although levitation significantly reduces friction, the induced self-propulsion depends on substrate geometry and material properties, which limits dynamic propulsion control. Therefore, the ability to control the power output is a significant challenge in realizing operational mm and sub-mm scale virtually frictionless engines. Here, we present a thin-film boiling engine where we control the power output mechanically. The rotor, which comprises of a working liquid coupled to a non-volatile solid, is manually positioned over a heated turbine-inspired stator in a thin-film boiling state. We show that by controlling the position of the rotor over the substrate the power output from the rotation can be controlled above and below the Leidenfrost temperature (~250 °C). We explain these experimental observations using a hydrodynamic analytical model. Additionally, we achieve propulsion outputs almost 4 times higher than levitation-based propulsion systems. The ability to control the rotation characteristics of such virtually frictionless engines allows potential applications in extreme environments such as at microscales or for space and planetary exploration.

Application of a Magnetic Field in Saturated Film Boiling of a Magnetic Nanofluid (MNF) under Reduced Gravity

To overcome the problem of abnormally large bubbles and the large reduction of heat flux under low gravity, the computational model of magnetic nanofluid (MNF) boiling flow was used to systematically study the thermodynamic characteristics of an MNF-saturated film boiling with and without the magnetic field. This study found that in the absence of a magnetic field, the decrease of the gravity level makes the bubble size increase and the bubble departure time increase, and the lower the gravity level, the worse the boiling heat transfer. However, after applying the magnetic field, bubble size decreases significantly and the bubble departure time is shortened. As the magnetic field intensity increases, the difference in bubble size and heat transfer characteristics between different gravity levels becomes smaller and smaller, which shows that for the boiling flow of MNF under low gravity levels, applying a magnetic field can effectively avoid the appearance of abnormally large bubbles, enhance heat transfer, and improve the safety of related heat transfer equipment.

Leidenfrost Motion of Water Microdroplets on Surface Substrate: Epitaxy of Gallium Oxide via Mist Chemical Vapor Deposition

AbstractIt is hypothesized that gravity‐driven sedimentation and gliding of water microdroplets on a substrate surface due to the Leidenfrost effect explain the deposition behaviors of epilayers via mist chemical vapor deposition (mist CVD). The deposition rate, thickness uniformity, and surface morphology are dependent on the velocity, incident angle, incident probability, gliding distance, and the lifetime of the droplets on the surface of the substrate. Practically, the deposition of corundum structured gallium oxide (α‐Ga2O3) epilayers on a tilted 2‐inch c‐plane sapphire substrate validated the hypothesis. A high substrate tilt angle and a low mist stream velocity are favorable for a high deposition rate. In contrast, a low substrate tilt angle and a high mist stream velocity are suitable for high uniformity and stable deposition of the α‐Ga2O3 thin films via mist CVD.

Levitation and Self-Organization of Droplets

We review studies of levitating droplets over liquid–gas interfaces and dry solid surfaces with a focus on the physical mechanisms of levitation under different conditions. A fascinating physical phenomenon of self-organization of levitating droplets into large arrays is described and explanations for this unusual behavior are reviewed. Closely related topics of nonisothermal flotation and levitation of evaporating droplets over a pool of nonvolatile liquid, as well as recent advances in the study of the Leidenfrost effect, are also discussed.

How ambient conditions affect the Leidenfrost temperature.

By sufficiently heating a solid, a sessile drop can be prevented from contacting the surface by floating on its own vapour. While certain aspects of the dynamics of this so-called Leidenfrost effect are understood, it is still unclear why a minimum temperature (the Leidenfrost temperature TL) is required before the effect manifests itself, what properties affect this temperature, and what physical principles govern it. Here we investigate the dependence of the Leidenfrost temperature on the ambient conditions: first, by increasing (decreasing) the ambient pressure, we find an increase (decrease) in TL. We propose a rescaling of the temperature which allows us to collapse the curves for various organic liquids and water onto a single master curve, which yields a powerful tool to predict TL. Secondly, increasing the ambient temperature stabilizes meta-stable, levitating drops at increasingly lower temperatures below TL. This observation reveals the importance of thermal Marangoni flow in describing the Leidenfrost effect accurately. Our results shed new light on the mechanisms playing a role in the Leidenfrost effect and may help to eventually predict the Leidenfrost temperature and achieve complete understanding of the phenomenon, however, many questions still remain open.

Thermophobic Leidenfrost.

We report that a volatile liquid deposited on a hot substrate with a gradient of temperature does not only levitate (Leidenfrost effect), but also spontaneously accelerates to the cold. This thermophobic effect is also observed with sublimating solids, and we attribute it to the ability of temperature differences to tilt (slightly) the base of the "object", which induces a horizontal component to the levitating force. This scenario is tested by varying the drop size (with which the acceleration increases) and the substrate temperature (with which the acceleration decreases), showing that the effect can be used to control, guide and possibly trap the elusive Leidenfrost drops.

Effect of the coefficient of restitution and friction in the granular Leidenfrost effect in the absence of gravity. A numerical study

In this study the granular Leidenfrost effect in the absence of gravity is investigated numerically by means of the discrete element method. Apart from identifying the phenomena, a parametric study to quantify the influence of the coefficient of restitution and friction in the packing fraction of the granular media is carried on numerically. Surprisingly, both the coefficient of restitution and the coefficient of friction exhibit an influence of the same magnitude in the packing fraction of the granular system, which has not been reported in experiments and simulation of granular Leidenfrost regime under gravity or microgravity conditions.

Electrohydrodynamic analysis of bubble burst in large Leidenfrost droplets

A thin vapor gap forms underneath a liquid drop on a sufficiently hot surface, which prevents solid–liquid contact (the Leidenfrost effect). This vapor gap can be partly eliminated by applying an electrical potential difference across the vapor gap to electrostatically suppress the Leidenfrost state. An interesting hydrodynamics-related phenomenon that can occur in Leidenfrost droplets is the formation of a vapor dome and subsequent bubble burst at the center of the droplet. This work reports a comprehensive study of vapor dome formation and bubble burst in large Leidenfrost droplets under the influence of an electric field. First, a detailed numerical model (non-linear thin film lubrication equation) is developed to analyze the evolution of the vapor dome and bubble burst. Second, a simplified stability analysis is conducted to analytically estimate the critical droplet diameter (for bubble burst) under the influence of an electric field. Third, experiments are conducted to measure the critical diameter of Leidenfrost droplets for bubble burst under the influence of electric fields. The results from the numerical modeling and stability analysis show very good agreement with experimental measurements. The critical diameter for bubble burst and the time period between consecutive vapor bursts reduce with the applied electric field. Comparisons are made between the presently studied vapor burst and film boiling; similarity in the underlying hydrodynamic phenomena results in the length and time scales for bubble burst being similar to those encountered in film boiling.