Leidenfrost Transition Research Articles

Drop impact on superheated surfaces: short-time dynamics and transition to contact

When a volatile drop impacts on a superheated solid, air drainage and vapour generation conspire to create an intermediate gas layer that delays or even prevents contact between the liquid and the solid. In this article, we use high-speed synchronized reflection interference and total internal reflection imaging to measure the short-time dynamics of the intermediate gas film and to probe the transition between levitation and contact. We observe that the substrate temperature strongly affects the vertical position of the liquid–gas interface and that the dynamic Leidenfrost transition is influenced by both air and vapour drainage (i.e. gas drainage), and evaporation, the latter giving rise to hitherto unreported vertical oscillations of the gas film that can trigger liquid–solid contact. We first derive scaling relations for the height of the gas film trapped under the drop's centreline, called the dimple height, and the minimum film thickness at short times. The former is set by a competition between gas drainage and liquid inertia, similarly as for isothermal impacts, while the latter strongly depends on the vapour production. The gas pressure, at the location where the minimum thickness is reached, is determined by liquid inertia and vapour production and ultimately balanced by the increasing interfacial curvature, determining the minimum thickness. We show that, in the low impact velocity limit, the transient stability of the draining gas film remarkably makes dynamic levitation less demanding than static levitation. We characterize the vertical gas film oscillations by measuring their frequency and monitoring their occurrence in the parameter space spanned by surface temperature and impact velocity. Finally, we model the occurrence of these oscillations and account for their frequency through a hydrodynamic mechanism.

The Leidenfrost transition of liquid droplets impinging onto a superheated surface

The Leidenfrost transition of liquid droplets impinging onto a superheated surface

The Leidenfrost transition of water droplets impinging onto a superheated surface

Water droplets impinge on a sapphire wall heated to a temperature ranging from 300∘C to 700∘C. Advanced measurement techniques are used to characterize the thermal processes associated with the drop impact. IR thermography, implemented by coating the impacted surface with an opaque and emissive material in the IR domain, makes it possible to measure the temperature of the solid surface during the impact process. Laser-induced fluorescence imaging is used to characterize the temperature field in the spreading droplet. At the onset of film boiling, the temperature distribution on the solid surface is marked by the formation of a fingering pattern. This latter corresponds to spatial fluctuations in the thickness of the vapor film. When a water droplet hits an overheated wall with a significant impact velocity, the thermal contact is so rapid and intense that the liquid temperature can largely overtake the saturation temperature and reach the spinodal temperature, i.e. the highest temperature at which water can exist in the liquid state. In this situation, experiments show that the dynamic Leidenfrost point is directly linked to the spinodal temperature. A superheating of the liquid by several hundred of ∘C and the subsequent homogeneous nucleation, have to be considered to describe the heat transfer in the film boiling regime.

Can Wicking Control Droplet Cooling?

Wicking, defined as absorption and passive spreading of liquid into a porous medium, has been identified as a key mechanism to enhance the heat transfer and prevent the thermal crisis. Reducing the evaporation time and increasing the Leidenfrost point (LFP) are important for an efficient and safe design of thermal management applications, such as electronics, nuclear, and aeronautics industry. Here, we report the effect of the wicking of superhydrophilic nanowires (NWs) on the droplet vaporization from low temperatures to temperatures above the Leidenfrost transition. By tuning the wicking capability of the surface, we show that the most wickable NW results in the fastest evaporation time (reduction of 82, 76, and 68% compared with a bare surface at, respectively, 51, 69, and 92 °C) and in one of the highest shifts of the LFP of a water droplet (5 μL) in the literature (about 260 °C).

Leidenfrost heat engine: Sustained rotation of levitating rotors on turbine-inspired substrates

The prospect of thermal energy harvesting in extreme environments, such as in space or at microscales, offers unique opportunities and challenges for the development of alternate energy conversion technologies. At microscales mechanical friction presents a challenge in the form of energy losses and wear, while presence of high temperature differences and locally available resources inspire the development of new types of heat engines for space and planetary exploration. Recently, levitation using thin-film boiling, via the Leidenfrost effect, has been explored to convert thermal energy to mechanical motion, establishing the basis for novel reduced-friction heat engines. In the Leidenfrost effect, instantaneous thin-film boiling occurs between a droplet and a heated surface, thereby levitating the droplet on its own vapor. This droplet state provides virtually frictionless motion and self-propulsion, whose direction can be designed into the system by asymmetrically texturing the substrate. However, sustaining such thermal to mechanical energy conversion is challenging because the Leidenfrost transition temperature for water on a smooth metal surface is ∼220 °C and, despite the low thermal conductivity of the vapor layer, the droplet continuously evaporates. Further challenges include effective transfer of thermal energy into rotational, rather than linear motion, and driving solid components and not simply droplets.Here we present a Leidenfrost rotor, where a solid component is coupled to a rotating liquid volume using surface tension and levitated in continuous operation over a turbine-inspired substrate. We address two key challenges: we show how the liquid can be replenished to achieve the continuous operation of the device; and we show how a superhydrophobic coating to the substrate can broaden the temperature range of operation and the stability of the rotor. Because the liquid acts as a working substance by extracting heat from the substrate to produce useful work in the form of rotation of the coupled solid component, our results demonstrate that a Leidenfrost engine operating in a closed thermodynamic cycle is possible.

On modeling of the initial stage of nonstationary nucleate boiling for the high heat fluxes

During recent years, there have been made significant achievements in the numerical description of the bubble boiling, particularly, in the calculation of the bubble growth dynamics, the nucleation density, and the bubble boiling threshold [1, 2]. However, the numerical prediction is mostly based on the empirical correlations, the accuracy of which does not exceed 20%. Boiling is a complicated process, where each parameter affects not only the general outcome but other parameters, too. The non-stationary heat release is most difficult for modeling, because many of the existing researches are based on analytical expressions for the fluid temperature distribution. The basic stages of the explosive boiling are: (i) heating of the wall to the nucleation temperature; (ii) nucleation of the isolated bubbles; (iii) merging of bubbles and coverage of the entire surface by the steam phase; (iv) heating of the surface to the Leidenfrost temperature and transition to the boiling crisis. This paper presents results of the experimental study for the initial stages of the explosive boiling (i) and (ii), as well as an attempt to simulate them in order to clarify whether the existing approaches can be extended to the case of the nonstationary heat release.

Universality of oscillating boiling in Leidenfrost transition.

The Leidenfrost transition leads a boiling system to the boiling crisis, a state in which the liquid loses contact with the heated surface due to excessive vapor generation. Here, using experiments of liquid droplets boiling on a heated surface, we report a phenomenon, termed oscillating boiling, at the Leidenfrost transition. We show that oscillating boiling results from the competition between two effects: separation of liquid from the heated surface due to localized boiling and rewetting. We argue theoretically that the Leidenfrost transition can be predicted based on its link with the oscillating boiling phenomenon and verify the prediction experimentally for various liquids.

Leidenfrost transition temperature for stainless steel meshes

On surfaces well above 100°C water does not simply boil away. When there is a sufficient heat transfer between the solid and the liquid a continuous vapour layer instantaneous forms under a droplet of water and the drop sits on a cushion of vapour, highly mobile and insulated from the solid surface. This is known as the Leidenfrost effect and the temperature at which this occurs if known as the Leidenfrost transition temperature. In this report, an investigation of discontinuous surfaces, stainless steel meshes, have been tested to determine the effect of the woven material on the Leidenfrost phenomenon. It was found that with increasing the open area of the mesh pushes up the Leidenfrost temperature from 265°C for an open area of 0.004mm2 to 315°C for open area of 0.100mm2. This allows suppression of the Leidenfrost effect as it can be increase to over 300°C from 185°C for a stainless steel surface.

Influence of surface contamination on the wettability of heat transfer surfaces

The wettability of heat transfer surfaces plays an important role in liquid–vapor phase change phenomena, including boiling incipience, the critical heat flux, the Leidenfrost transition, and condensation. The influence of adsorbed surface contamination at the nanoscale, though seldom considered, can have a profound impact on wetting behavior. This study quantitatively investigates the impact of contaminant layer thickness on wettability. Various cleaning treatments are explored on zirconium and 6061 aluminum to determine the effect on contaminant and oxide layer thickness. Angle-resolved X-ray photoelectron spectroscopy can be used to measure the thickness of oxide and contaminant layers, which is then correlated to wettability by measuring the equilibrium contact angle. Results indicate that even after solvent cleaning, the contact angle of water on practical heat transfer surfaces is dominated by a hydrocarbon contaminant overlayer around five nanometers thick.

Molecular-Level Modeling of Interfacial Phenomena in Boiling Processes

The thermophysics of liquid–vapor interfaces has long been recognized as a critical element in the physical mechanisms of boiling processes. This article will describe results of recent molecular dynamics simulation studies that explore the structure and stability of liquid–vapor interfacial regions using a hybrid analysis scheme that combines new formulations of capillarity theory with molecular dynamics simulations that use similar interaction potentials. Two forms of this type of hybrid scheme have been developed: one for non-polar fluids based on a Lennard–Jones interaction potential and a second specifically for water using a modified treatment of the extended simple point charge interaction potential that accounts for water dipole interactions. The hybrid model has the advantage that the capillarity theory provides theoretical relationships among parameters that govern interfacial region structure and thermophysical behavior, while the companion molecular dynamics simulations allow more detailed molecular level exploration of the interfacial region thermophysics. Predictions of interfacial region structure indicated by this kind of hybrid modeling will be described for pure non-polar and water liquid–vapor interfaces and for water with dissolved ionic solutes (i.e., salts). Extension of the methodology to thin liquid films will also be described. Rupture of a free liquid film dictates merging of adjacent bubbles, which is particularly important in nucleate boiling heat transfer, bubbly two-phase flow in small tubes, and the mechanisms that dictate the Leidenfrost transition. To understand the mechanisms of bubble merging in nanostructured boiling surfaces and in nanotubes, it is useful to explore film stability and the onset of rupture at the molecular level. Results obtained with the hybrid model indicate that wave instability predominates as an onset of rupture mechanism for free liquid films of macroscopic extent, but for free liquid films with nanoscale lateral extent (e.g., in nanostructured boiling surfaces), lack of film core stability is more likely to be the mechanism. Predictions of the hybrid models will be compared to results of experimental studies of the effects of ionic solutes on interfacial tension and bubble merging. The implications of the molecular dynamics model predictions for boiling processes in micro-channels and boiling in nanostructured surfaces are also discussed.

Thermophysics of the Interfacial Region and Its Impact on Instability and Rupture of Thin Free Liquid Films

Merging of adjacent bubbles is a particularly important process in nucleate boiling heat transfer, bubbly two-phase flow in small tubes, and the mechanisms that dictate the Leidenfrost transition. The merging process is ultimately associated with the rupture of the liquid film separating two bubbles, and the stability of the liquid film dictates whether coalescence is likely to occur. This paper examines two methods of exploring how the molecular-level features of the thin film and its interfaces affect the stability of the film and its subsequent impact on bubble merging. One method is an extended molecular capillarity model that has been developed for a free liquid film bounded by two liquid–vapor interfacial regions. This model predicts that as film thickness diminishes, a point is reached at which the mean centerline density in the film is between the spinodal limits for the fluid, implying that the film core lacks intrinsic stability. This suggests that loss of intrinsic stability may be the mechanism for film rupture in some cases. The second method examined here is use of molecular dynamics simulations to examine film structure and stability. Simulations using a Lennard–Jones interaction potential for non-polar fluids and the SPC/E potential for water and salt water are examined. Results of these explorations indicate that for water–NaCl solution films, surface tension increases with increasing NaCl concentration. These predictions are shown to be consistent with measured surface-tension data for such solutions. The results also indicate that for pure water and NaCl solutions, below a threshold thickness, the surface tension decreases with film thickness. Wave interface stability theory implies that this tends to make the film less stable. The implications of the results of these investigations for bubble merging in two-phase flow and boiling processes are also examined.

Hybrid modeling of interfacial region thermophysics and intrinsic stability of thin free liquid films

The film rupture process that dictates merging of adjacent bubbles is particularly important in nucleate boiling heat transfer, bubbly two-phase flow in small tubes, and the mechanisms that dictate the Leidenfrost transition. To understand the mechanisms of bubble merging in nanostructured boiling surfaces and in nanotubes, it is useful to explore film stability and onset of rupture at the molecular level. This paper reports the results of such an investigation using a hybrid analysis scheme that combines a new formulation of capillarity theory for free liquid films with molecular dynamics (MD) simulations that use similar interaction potentials. Two forms of our molecular film capillarity theory are developed here: one for non-polar fluids based on a Lennard-Jones interaction potential, and a second specifically for water using a modified treatment of the SPC/E interaction potential that accounts for water dipole interactions. The hybrid model has the advantage that the capillarity theory provides theoretical relationships among parameters that govern film structure and thermophysical behavior, while the companion MD simulations allow more detailed molecular level exploration of the film thermophysics. Results obtained with the hybrid model indicate that wave instability predominates as an onset of rupture mechanism for liquid films of macroscopic extent, but for free liquid films with nanoscale lateral extent (in, for example, nanostructured boiling surfaces), lack of core stability is more likely to be the mechanism. The implications of these predictions for film rupture and bubble merging in nanostructured surfaces and nanotubes are examined in detail.

The effects of dissolved salt on the Leidenfrost transition

In this study, Leidenfrost experiments were conducted for water, NaCl, and KCl aqueous solutions at atmospheric pressure. In our tests, a 1.1 g test liquid was gently deposited on a horizontal heated aluminum surface. The evaporation time at various surface temperatures was recorded and plotted as evaporation curves. To examine the relationship between bubble coalescence, dissolved salt and the Leidenfrost transition, test surfaces were fabricated with arrays of small holes serving as artificial nucleating sites. Cavity spacing is 1 mm or 2 mm. It is demonstrated that the dissolved salt increases the Leidenfrost temperature. Mechanisms associated the Leidenfrost transition, such as suppression of bubble coalescence, variation in properties and salt deposition during the initial liquid–solid contact, are explored and accessed.

Temperature and Volumetric Fraction Measurements in a Hot Gas Laden With Water Droplets

Two temperatures can be detected in a hot gas laden with water droplets. The lower one is the temperature read by a sensor immersed in the flow. This measurement is affected by significant evaporative cooling resulting in readings much below the real gas temperature. The higher one is the temperature of the gas. The difference between these two temperatures is proportional to the water volumetric fraction in the flow. On this basis, a new technique for the measurement of the water volumetric fraction is proposed. The results are compared with a conventional measurement technique based on optical methods. A novel approach to the gas temperature measurement is also introduced. The sensing device is kept at temperatures exceeding the Leidenfrost transition and therefore it is insensitive to the presence of the water droplets. Independent measurement techniques developed at the Evaporative Cooling Sensor Accuracy Test (ECSAT) facility provide the data to validate the measurements obtained with the Above Leidenfrost Temperature and Evaporative Cooling (ALTEC) sensor.

Evaporation of a Water Droplet Deposited on a Hot High Thermal Conductivity Surface

Many studies have been performed to quantify the vaporization process for both single droplets and multiple-droplet arrays impacting on hot surfaces. For the studies found in the published literature, the full span of the droplet vaporization processes is usually reported. These would include evaporation, nucleate boiling, film boiling, and Leidenfrost transition. The present investigation is concerned with the evaporation of a droplet on a hot surface The study reports detailed results for spital and temporal variation of the heat flux at the exposed surface of the droplet, and for temporal variation of the droplet volume. Limiting the study to evaporation implies that conditions are maintained under which nucleate boiling is fully suppressed.

A Numerical Solution to Dropwise Evaporation

A numerical solution is presented for spray evaporation on the surface of a spray evaporator. Excess temperatures below the Leidenfrost transition temperature are considered along with the nonisothermal wall condition. The combination of a high thermal conductivity and low heat capacity for the evaporator wall material, as well as the time-dependent boundary conditions, dictates a quasi-steady-state solution to this evaporator phenomenon. A numerical solution to the conduction equation and boundary conditions is given. A dimensionless evaporation time is computed, and from this a flooding index is developed. The flooding index is shown to be related exponentially to the droplet diameter and wetting ratio. The functional correlation between the heat transfer coefficient for the spray and the wetting ratio is verified from previously published experimental data.