Overheated Surface Research Articles

Mechanism chaotic movement of Leidenfrost droplets

One of the most effective methods for cooling overheated surfaces is drip irrigation. If the surface temperature exceeds the Leidenfrost temperature, then a vapor film is formed between the droplet and the surface, which leads not only to a decrease in heat transfer intensity but also causes droplet mobility. For a number of applications, the mobility of droplets is an undesirable phenomenon, so the analysis of the factors responsible for their movement is a relevant task. Here we analyze the movement mechanism of the Leidenfrost droplets with variations in the composition and volume of the droplets. The data obtained show that the droplet speed increases with an increase in the droplet volume. However, smaller droplets change direction of motion more often than larger droplets. To substantiate the experimental data, a hypothesis is proposed, according to which the mechanism of movement of Leidenfrost droplets is caused by the reactive force that arises due to the evaporation of liquid. A Leidenfrost droplet changes the direction of its movement due to the deformation of its surface under the influence of gravity and capillary force. To substantiate the experimental data a simple phenomenological model is proposed.

Suppressing the Leidenfrost effect by air discharge assisted electrowetting-on-dielectrics

The Leidenfrost effect for a droplet on an over-heated substrate always results in a superhydrophobic state, significantly hindering the water evaporation for heat dissipation. Here, we demonstrate a strategy of air discharge assisted electrowetting-on-dielectrics (ADA-EWOD), overcoming this challenge. This strategy increases the solid surface free energy by generating air discharge near the three-phase contact line of the droplet and combines it with the electromechanical force to decrease the contact angle, which makes ADA-EWOD have stronger wetting capabilities than traditional electrically control methods that only rely on electromechanical force. The water contact angle on an over-heated surface (above 350 °C) is decreased from nearly 180° down to less than 10°. This superhydrophilicity at high temperature reduces the droplet lifetime by at least 10 times, well inhabiting the Leidenfrost effect. Furthermore, we use ADA-EWOD in droplet evaporation for heat dissipation, where a heated silicon wafer at 600 °C is cooled down to less than 200 °C within 20 s. We believe that the present work provides a perspective on suppressing the Leidenfrost effect, which may have important potential applications in the field of heat dissipation.

Experimental Study of Heat Transfer During Quenching of Steel Cylinders in Boiling Nanofluid

Current study was dedicated to explore cooling performance of TiO2 nanofluid. Preliminary tests have shown the possibility of significant improvement of emergency cooling performance by nanofluids in the case of overheated surfaces. Tests that were held during current investigation were performed with cylinders of stainless steel, which were used as an overheated surface. The temperature was recorded by a digital system of 3 type-K thermocouples. The object, preheated in a furnace to high temperatures (600–800 °C), was immersed into boiling distilled water. Usage of the set of thermocouples allows to determine heat flow in steel cylinder by its temperature distribution. The implementation of emergency cooling was to add a certain amount of nanofluid concentrate to the volume of boiling distilled water. The experiment was continued until the complete cooling of the object (up to 100 °C—the temperature of the surrounding liquid). Calculations confirmed increase of heat transfer coefficient for NF and, respectively, about 40% decrease in cooling time for NanoFluid compared to distilled water was observed. Three method of determination of heat transfer coefficient were compared.

Use of Satellite Images to Determine the Temperature of Urban Surfaces for Landscape Management Purposes, Case Study Bratislava (Slovak Republic)

This contribution deals with the use of data obtained from Landsat 8 satellite imaging to identify surface temperature variability in the example of the city of Bratislava, with an emphasis on identifying hotspots outside the built-up area, for example, on agricultural land—locations which are part of the European Network of Protected Areas. Surface temperature variability is presented in two time periods, on the daytime image taken on 26 July 2021 and on the nighttime image from 28 June 2021. Surface temperature is projected in a profile cut of the area. It vertically illustrates the temperatures of individual types of surfaces. Surfaces are classified by Urban Atlas classes. Areas reflecting the spatial distribution of the residential development in the city of Bratislava have been identified by satellite images in the studied area, and they represent a phenomenon of the urban heat island. Such areas were also identified outside the built-up area, in agricultural areas. The results of our research show that it is important to deal with UHI outside the built-up areas of cities and to orient the attention the territory planning and also to the proposal of measures for the management of these areas. Especially if these areas also include territories of the European system of protected areas, as it is in the case of Bratislava city (e.g., SPA029 Sysľovské polia). The results of reducing the impacts of climate change in cities concern not only the residents. In spatial planning, it is also necessary to address the management of non-built-up areas—localities with a quasi-natural character (e.g., areas with diverse vegetation cover). In order to recognize UHI within residential areas, it is essential to identify areas with significant differences between daytime and nighttime surface temperatures. Large differences between night and daytime surface temperatures can be seen in areas outside the built-up area in Bratislava on arable land where the difference is up to 8.0 °C (in the continuous housing class where the proportion of impermeable surfaces is higher than 80% with a temperature difference of 7.6 °C). Identification of overheated surfaces in the territory makes an important basis for modification of the landscape management and management of nature protection areas. It is important to propose measures related to the reduction in the negative impacts of climate change on the landscape and biodiversity.

Эффект Лейденфроста на струнах

New manifestations of the Leidenfrost effect (levitation of limited volumes of liquid over an overheated surface) on smooth and morphologically complex thin metal strings are described for the first time. Various dynamic modes of Leidenfrost levitation are presented, including a new one - a combined thermohydrodynamic mechanism of levitation as a sum of the effects of a viscous vapor flow under a drop and a convective flow around a drop with a mixture of vapor + air due to natural convection. Keywords: Leidenfrost effect, thin strings, surface morphology, levitation mechanism, vapor layer.

Parameters of temperature disturbance triggering the propagation of the quench front on an extremely overheated surface

The results of computational experiments simulating the triggering of the quench front propagation on the superheated vertically oriented metal plates are presented. The plates are quenched by a gravitationally flowing down liquid nitrogen film. The temperature of the test samples at the beginning of the process was higher than the critical temperature and the Leidenfrost temperature, which means that direct long-term liquid-solid contact is impossible. For this reason, the front is initially motionless. As a result of numerical simulation, a dynamic pattern of the quench front propagation on a high-temperature surface was obtained. Analysis of the results allowed to find the realistic values of heat sink into the cooling medium, as well as the parameters of the local temperature disturbance, its spatial extent and amplitude, at which the conditions are created for triggering the process of quench front propagation on the high-temperature surface. Direct comparison of the numerical simulations results with experimental data on the velocity, geometry of the quench front and on the dynamical pattern of the process confirmed the reliability of the results obtained.

Study of the Leidenfrost Effect on Heterogeneous Surfaces of Complex Structure

This work is devoted to a research of water droplets that are put in-between two parallel metal strings the distance between which is comparable to linear size of the droplet. Strings are heated by Joule heating to temperatures that exceed critical temperatures of nucleate and film boiling. Different configurations of strings’ side surface have been tested: smooth and with winding made of the same material (intermittent and uninterrupted). Experiments have shown that droplets on these types of surface do not boil away quickly or fall down. Instead they displayed behavior that can be described as floating, either stable or with directed motion, depending on surface structure or relief. Multiple experiments have shown that it is quite similar to Leidenfrost effect demonstrated on a flat overheated surface by liquids.

Effects of superheat degree and wettability on droplet evaporation time near Leidenfrost point through Lattice Boltzmann simulation

The evaporation of droplet on a superheated surface is a very important and common phenomenon in industrial production, such as spray cooling and metallurgical treatment. In this paper, a two-dimensional numerical simulation based on Lattice Boltzmann Method has been adopted to analyze the evaporation behavior of liquid droplet on an over-heated surface. The effects of superheat degree (Ja number) and surface wettability on droplet evaporation are analyzed respectively. During the evaporation process, the evaporation pressure at the bottom of the droplet exhibits periodic oscillation corresponding to bouncing movement. The oscillation gradually decays in the case of low Ja numbers, but for high Ja numbers, the oscillation doesn't decay. Leidenfrost vapor layer is more likely to be generated on a hydrophobic surface. Meanwhile, the comprehensive influence of wettability and Ja number on droplet evaporation time was studied systematically. On hydrophilic surfaces, there is a critical Ja value, around which the evaporation time exhibits a U-type tendency. However, the evaporation time continues to decrease as Ja number increases on hydrophobic surfaces.

Peculiarities of the initialization and dynamics of the quench front on an extremely overheated plate cooled by a falling film of a cryogenic fluid

Quenching is a generally accepted term for the process of rapid cooling of a solid body that is superheated above the temperature of the thermodynamic limit of a liquid superheat. The paper presents the results of computational experiments simulating the quenching of a high-temperature vertical copper plate by a gravitationally flowing film of liquid nitrogen. The peculiarities of the initialization and dynamics of the quench front in the transition process are investigated. The initial temperature of the test sample was higher than the Leidenfrost temperature and even higher than the critical temperature, which excluded the possibility of long-term stable liquid-solid contact. An improved numerical model of quenching is implemented, which assumes at the time of the quench front initialization an unsteady local contact of the liquid with the extremely overheated surface in the vicinity of the upper boundary of the heater. A dynamic pattern of the running quench front was obtained numerically, which satisfactorily correlates with that observed in cryogenic experiments by the high-speed digital video camera. The reliability of the results obtained numerically is confirmed by direct comparison with experimental data on the velocity and the shape of the quench front.

Assessment of ASTEC 2.1 capability to predict reactor core behaviour at the late phase of severe accident progression based on QUENCH-12

The presented paper discusses capability of recently developed version 2 revision 1, patch 1 of ASTEC computer code to predict a fuel behaviour at late phase of severe accident progression. The conducted assessment is part of the ASTEC validation based on available information from performed tests in QUENCH program for these purposes. It has been selected QUENCH-12 experiment for simulation of investigated phenomena as fuel cladding, oxidation and hydrogen generation, including temperature escalation during heat up followed by quenching overheated surface fuel cladding.The QUENCH-12 experiment was proposed by FZK together with RIAR Dimitrovgrad and IBRAE Moskow (Russia). The test was conducted at Forschungszentrum Karlsruhe on 27 December, 2006 in the frame of EC-supported ISTC program.The selected QUENCH-12 experiment was performed to extend knowledge gathered in previous QUENCH test to fuel behaviour in other LWRs slightly different from well investigated western type PWR with squared lattice. The fuel assemblies with a hexagonal lattice and containing fuel rod claddings made of Zr1%Nb (E 110) using in the VVER reactors are investigated in the experiment.The developed QUENCH-12 model for ASTEC code successfully predicted the entire sequence from initial heat-up to quench phase of experiment, except hydrogen generation during quenching phase at the end of transient.

Numerical investigation on motion characteristic of a bubble rising along overheated surfaces with different curvatures

In present work, a two-phase thermal LBM model for simulating liquid-gas phase transition in body-fitted coordinate system is proposed. Subsequently, model validation is performed by simulating bubble growth in the superheated liquid without consideration of gravity. After that, the hydrodynamic and thermodynamic behaviors of a bubble during its rising and sliding process along overheated surfaces are analyzed and compared in three cases with different surface axial ratios. The results indicate that bubble experiences the largest deformation and growth in the case with surface axial ratio a/b=1.5 . As for bubble average velocity, its horizontal component increases with surface axial ratio, yet opposite for vertical one. The influences of bubble deformation and growth on temperature field become more significant in the case with a larger axial ratio. Besides, the existence of bubble causes an extremely high temperature gradient on the surface during its sliding stage; such effect is weakened gradually as the bubble departs from surface and moves to the far field.

New nanofluids, based on clay minerals, as promising heat carriers for energetics

ABSTRACTIn an automated installation powered by direct current (DC), the boiling curves and heat-transfer-coefficient (HTC) dependencies on the superheat values (ΔT) under free convection conditions for the water nanodispersions of clay minerals – illite, montmorillonite, palygorskite and genetic mixtures of the latter two – were obtained. The effects of some factors on pool boiling heat transfer were also studied.A significant influence of the shape and anisotropy of nanoparticles (NPs) on the heat-transfer parameters of nanofluids (NFs) was detected. A significant critical heat flux (CHF) enhancement (up to 200–300%) at boiling of the nanofluids studied was established, which is due to nanoparticle deposition on the heater surface during nanofluid boiling. The structure of the nanomaterials deposited is important in the enhancement of heat transfer at boiling of nanofluids and in avoiding boiling crises.The present study showed the effectiveness of clay-mineral nanofluids for extra emergency cooling of overheated surfaces of powerful equipment in the event of the sudden onset of a boiling crisis.

Atomistic Modeling of Short Pulse Laser Ablation of Metals: Connections between Melting, Spallation, and Phase Explosion

The mechanisms of short pulse laser interactions with a metal target are investigated in simulations performed with a model combining the molecular dynamics method with a continuum description of laser excitation, electron−phonon equilibration, and electron heat conduction. Three regimes of material response to laser irradiation are identified in simulations performed with a 1 ps laser pulse, which corresponds to the condition of stress confinement: melting and resolidification of a surface region of the target, photomechanical spallation of a single or multiple layers or droplets, and an explosive disintegration of an overheated surface layer (phase explosion). The processes of laser melting, spallation, and phase explosion are taking place on the same time scale and are closely intertwined with each other. The transition to the spallation regime results in a reduction of the melting zone and a sharp drop in the duration of the melting and resolidification cycle. The transition from spallation to phase e...

Power deposition modelling of the ITER-like wall beryllium tiles at JET

A precise geometric method is used to calculate the power deposition on the future JET ITER-Like Wall beryllium tiles with particular emphasis on the internal edge loads. If over-heated surfaces are identified, these can be modified before the machining or failing that actively monitored during operations. This paper presents the methodology applied to the assessment of the main chamber beryllium limiters. The detailed analysis of one limiter is described. The conclusion of this study is that operation will not be limited by edges exposed to plasma convective loads.

Sodar study of the characteristics of presumable big toroid structures above an overheated surface

Sodar study of the characteristics of presumable big toroid structures above an overheated surface

Sodar study of the characteristics of presumable big toroid structures above an overheated surface

A set of sodars allowed to investigate the spatial variability of meso-scale organized structures under conditions of strong convection and weak wind. The distribution of velocity within the structures and the structure typical sizes have been determined as a result of processing of experimental data.

자유표면변형을 고려한 저에너지밀도 및 고에너지밀도 레이저 용접공정 통합 해석

In this study, a unified numerical investigation has been performed on the evolution of weld pool, key-hole geometry during low-power, high-power density laser welding. Unsteady phase-change heat transfer, fluid flow with the surface tension are examined. The one-dimensional vaporization model is introduced to model the overheated surface temperature, recoil pressure during high-power density laser welding. It is shown that Marangoni convection in the weld pool is dominant at low-power density laser welding,, the keyhole with thin liquid layer, the hump are visible at high-power density laser welding. It is also shown that the transition from conduction welding to penetration welding for iron plate exists when the laser power density is about 10^6W/㎠ .

Combined sheath and thermal analysis of overheated surfaces in fusion devices

The phenomenon of runaway surfaces overheating in fusion experiments due to plasma-sheath/surface-emission coupling has been analyzed. A semi-analytic model has been used as well as the BPHI-3D kinetic sheath code (J.N. Brooks, D. Naujoks, Phys. Plasmas 7 (2000) 2565) coupled with the THERM code which solves the non-stationary heat conduction equation also in 3D geometry. Runaway heating due to initial overheating and subsequent sheath breakdown and superheat has been analyzed for two materials – lithium and carbon. For typical liquid lithium divertor conditions the critical exposure time for thermal runaway is of order 10 ms – generally greater than plasma transient periods (e.g., ELMS) or flowing liquid exposure times. Critical exposure times for carbon are much longer (∼1–2 s), as expected due to thermal property differences, and this may explain various `hot spot' formations in carbon systems. It is also shown that, especially for carbon materials, effects such as flake formation and deterioration of heat conductivity can play a crucial role.

PREDICTION AND RATIONAL CORRELATION OF THERMOPHORETICALLY REDUCED PARTICLE MASS TRANSFER TO HOT SURFACES ACROSS LAMINAR OR TURBULENT FORCED-CONVECTION GAS BOUNDARY LAYERS†

A formulation previously developed to predict and correlate the thermophoretically-augmented submicron particle mass transfer rate to cold surfaces is found to account for the thermophoretically reduced particle mass transfer rate to overheated surfaces such that thermophoresis brings about a 10-decade reduction below the convective mass transfer rate expected by pure Brownian diffusion and convection alone. Thermophoretic blowing is shown to produce effects on particle concentration boundary-layer (BL) structure and wall mass transfer rates similar to those produced by real blowing through a porous wall. The applicability of the correlations to developing BL-situations is demonstrated by a numerical example relevant to wet-steam technology.

Cooling of overheated surfaces by a falling liquid film

A method is presented for calculating the cooling of overheated surfaces by a falling liquid film. Comparison of the results with available empirical data shows that they agree well.