Superheated Surface Research Articles

Vapor-mediated impact of droplet on superheated powder bed

Droplet impacting on a sufficiently heated powder bed resembles those on a superheated solid surface, as the surface deformation is mediated by the spontaneously generated vapor flow from the bottom surface of the droplet. This emerged impacting behavior is denoted as vapor-mediated impact to differentiate from the wetting impact, which involves the wetting and absorption of the particles due to capillarity. We systematically vary the impacting velocity and the temperature of the powder bed to characterize the impacting dynamics for these two behaviors. For the vapor-mediated impact, the contact time and the maximum spreading diameter are found to have the same scaling laws derived for impact on the superheated surface. We construct a phase diagram of the impacting behaviors based on experimental observation, and propose a simplified model to predict the transition between these two behaviors. The predicted values match well with the experimental results, suggesting the proposed model captures the physical mechanism of the vapor-mediated impact.

Predicting Leidenfrost Temperature and Effects of Impact Conditions on Droplet Size Distribution in Film-Boiling Regime

Abstract The interaction of a droplet with a solid wall is relevant in various engineering applications. The properties of the resulting secondary droplets are determined by the wall temperature, ambient pressure, impact momentum, and impact angle. This paper presents a comprehensive characterization of drop-wall interactions and the subsequent atomization as a function of the combined effects of such parameters. A drop-wall interaction model is derived for F-24 droplets using smoothed particle hydrodynamics (SPH). The model can predict different impact outcome regimes (deposition, rebound, contact-splash, and film-splash) for different ambient pressures, wall temperatures, and impact parameters. The model also addresses the effect of ambient pressure on the Leidenfrost behavior. Size distributions of secondary droplets are compared for vertical and nonvertical impacts of F-24 droplets on superheated surfaces in the film-boiling regime. The non-dimensional Sauter mean diameter (SMD) of the secondary droplets varies based on the position in the impact plane for all the nonvertical impacts but remains almost unchanged for vertical impacts. The leading arc for nonvertical impact consists of larger secondary droplets, and the size decreases toward the trailing arc. An empirical relation is proposed to represent this trend. This research sheds light on successive droplet impacts by studying the effects of impact frequency on SMD evolution. The results are compared to single droplet impact cases for different fuels and Weber numbers. The size of secondary droplets for successive impacts is observed to be nearly indistinguishable from that of single-droplet vertical impacts.

Thermal hysteresis in wettability and the Leidenfrost phenomenon

The Leidenfrost temperature (TL), at which the liquid drop lifetime peaks on a superheated surface, is believed to be wettability dependent. Here, we show that the wettability effect on TL is subject to the history of the surface temperature. Observing a water drop evaporating on a polished stainless-steel surface heated from 100 to 400∘C in argon gas, we find TL≈265∘C. We then repeat the experiment along decreasing temperature and find a TL increase by 10 K, i.e., TL≈275∘C. This thermal hysteresis is due to a reduced contact angle during heating. Once hydrophilized, the hysteresis disappears until the contact angle recovers. Similar observations are made in the air where oxidation is possible. Published by the American Physical Society 2024

The Recovery from Grade II Burns in Test Mice (Mus Musculus Linn) After Using Iodine Leaf Latex Gel (Jatropha Multifida Linn)

Burn injuries are emergencies that pose a severe threat and require immediate action to save lives. The saponins, tannins, alkaloids, and flavonoids in iodine leaf latex gel (Jatropha multifida Linn) are known to speed up the healing process for burn wounds. The research aimed to investigate the effect of iodin leaf latex gel on the recovery of grade II burn wounds in male mice (Mus musculus Linn). The research employed a true-experimental laboratory method with-a-post-test-only control design group at the Kusuma Husada University Laboratory in Surakarta. A total of 24 test mice were manipulated and divided into three groups: the positive control group, the negative control group, and the iodine leaf latex gel group. Burn wounds utilized a 1,5 cm superheated mental surface applied to the backs of the mice for 5-10 seconds to induce burn injuries. The observed parameters included the burn wound area, the percentage of burn wound healing, and the duration of burn wound healing. Data analysis operated One-Way ANOVA. The research revealed that iodine leaf latex gel had a restorative time of 8 days and healing percentage of 30,75%. It was better than the other control groups. The statistical analysis indicated significance (<0,05) in the burn wound recovery percentage. In conclusion, a 6% concentration of iodine leaf latex gel had a more favorable effect on burn wound healing than other control groups.

Femtosecond laser-produced heterogeneous wettability surfaces for turning Leidenfrost drop spinning

Liquid droplets on superheated surfaces produce the Leidenfrost effect. This phenomenon might lead to droplet manipulation and control strategies in microfluidics and thermal management. However, Leidenfrost droplets move randomly and irregularly on superheated surfaces and the manufacturing of special surfaces to control Leidenfrost droplet movement poses great challenges. Here, we propose a simple and environment-friendly method to create heterogeneously wetting surface structures to control the spin motion of droplets on superheated brass using femtosecond laser patterning. The water contact angle of the superhydrophobic area on the surface was ∼160°, and the superhydrophilic area showed ∼7°. A z-shaped pattern was fabricated, which segmented the vapor film and influenced gas flow, and it resulted in the spinning of oval-shaped droplets analogous to a spinning egg. We used simulation to explain this phenomenon and also expanded the application of this droplet control in accelerating dissolution of solids and mechanical driving. This study provides the basis for a creative control method using the Leidenfrost droplet phenomenon, which has broad implications in steam-driven droplet motion and future fluid manipulation.

Experimental study on the dynamics of a liquid nitrogen droplet impacting a surface under Leidenfrost conditions

Liquid nitrogen droplet impacting a superheated surface is a fundamental phenomenon of liquid nitrogen spray cooling, whereas the mechanisms behind which are still unclear. We designed and developed a visual experimental system to investigate the dynamics of a liquid nitrogen droplet impacting a superheated surface under cryogenic conditions. The impact dynamics of the liquid nitrogen droplet at the Leidenfrost state are captured, and the effects of Weber number (We) and surface temperature on the spreading and rebound characteristics of the droplet are analyzed. The findings show that the droplet exhibits spreading, retraction and rebound at a low We. Droplet spreading and rebound characteristics are mainly affected by We while insensitive to surface temperature. The maximum spreading coefficient exhibits a power-law increase with We, while the maximum rebound coefficient shows an upward and then downward trend with We. The dimensionless maximum spreading time, dimensionless residence time, and dimensionless maximum rebound time show power-law increase with We. Corresponding fitting correlations for these factors for liquid nitrogen droplets are also proposed. This study contributes to an in-depth understanding of the impact dynamics of cryogenic liquid droplet under cryogenic conditions.

Numerical study of droplet impact on a superheated surface under an electric field based on perfect and leaky dielectric theories

AbstractThis paper investigates the hydrothermal behavior of leaky dielectric and perfect dielectric droplets impacting a superheated wall within a specific range of Weber numbers under an electric field. Through this investigation, we aim to provide a more comprehensive understanding of the dynamics involved in droplet‐superheated surface interactions under electric fields, which can be useful in various applications, such as the design of cooling systems and combustion chambers. The study utilizes the level‐set and ghost fluid techniques to capture the interface accurately. Under an electric field, different behaviors are observed during the impact process, depending on the electrical properties of the droplet. A perfect dielectric droplet experiences a reduction in spreading extent and an increase in contact time. However, no remarkable enhancement in total heat removal occurs in this case. For the leaky dielectric droplet exhibiting prolate deformation at the stationary state, increasing the electric field magnitude results in a slight decrease in the droplet spreading extent, while the droplet contact time and total heat removal from the surface increase. At an electric capillary number of 1.55E − 2 and a Weber number of 25, the enhancement in the contact time and total heat removal is about 43% and 15%, respectively. For the leaky dielectric droplet with oblate deformation at the stationary state, the spreading extent and total heat removal increase, with negligible changes in contact time. At the above‐mentioned electric capillary and Weber numbers, the enhancement in the spreading extent and total heat removal is about 7.5% and 15%, respectively.

A Phenomenological Thermal Spray Wall Interaction Modeling Framework Applied to a High-Temperature Ignition Assistant Device

Abstract Airborne compression ignition engines must operate with reliable ignition systems to achieve proper ignition at every cycle, particularly at high altitudes. Glow-plug-based ignition-assistant (IA) devices can provide the necessary energy to preheat the fuel and ensure ignitability of the fuel-air mixture. Ignitability of liquid sprays can be facilitated via direct impingement onto the hot IA surface, however this comes with adverse effects on the IA durability. Therefore, optimizing an IA's design requires detailed understanding of the physics of fuel spray impingement of superheated surfaces. While spray impingement on relatively low wall temperatures has been extensively studied and appropriate numerical models have been proposed through the years, fundamental understanding of high-speed liquid spray impingement on superheated walls is still elusive. This work aims to formulate a phenomenological thermal spray-wall interaction framework for modeling the film-boiling-induced heat transfer, atomization, and dispersion of fuel spray droplets impinging onto a superheated IA device. A qualitative comparison of the new phenomenological model is performed against optical experiments from the literature of an F-24 fuel spray injected onto an IA device located 12 mm away from the injector tip. The temperature of the IA was set at 1400 K. The fuel injection pressure was 400 bar, while the ambient gas pressure and temperature were 30 bar and 800 K, respectively. The performance of the phenomenological model is evaluated in comparison with two other state-of-art models from the literature. A qualitative analysis of the different spray and fuel-air mixture characteristics is performed to outline the differences in the predictions offered by the new phenomenological model and the two state-of-art spray-wall interaction models.

Constrained Vapor Bubble Experiment (CVB) in the Light Microscopy Module (LMM)

Abstract This short article describes the major findings from the CVB experiment performed in the LMM on the International Space Station from 2010–2012. CVB was the first experiment to run in the new facility and focused on understanding the heat transfer and fluid mechanics occurring inside a wickless miniature heat pipe. The LMM was used to map the location of the vapor-liquid interface inside the device and to measure the film thickness profile on the walls of the device. Several interesting and unexpected phenomena were observed in microgravity including flooding of the heater end with liquid as the heat input increased, explosive nucleation of vapor bubbles at the heater end in the shortest version of the heat pipe tested, condensation on highly superheated surfaces, and the spontaneous formation of rip currents as the device tried to enhance the contact line area available for evaporation of the liquid.

Boiling regimes of a single droplet impinging on a superheated surface: Effect of the surrounding medium

Impinging droplets on a hot surface is a significant phenomenon in many industrial applications, such as spray cooling. Previous studies reported various morphological behaviours affecting the droplet's wetting, heat transfer, and evaporation characteristics. However, there is an ambiguity in understanding the influence of the surrounding medium on the resulting boiling regimes. The present work aims to study this aspect through an experimental investigation using high-speed photography and infrared thermography. During the experiments, the degassed FC-72 liquid droplets have impinged onto a heated chromium-plated calcium fluoride (CaF2) surface with a constant impact velocity. Three ambient conditions, such as saturated vapour (heated vapour), air at liquid saturation temperature (heated air), and air at room temperature (ambient air), are used. The surface temperature is varied in each ambience until the complete droplet dewetting, i.e., the Leidenfrost phenomenon has appeared. In this study, the stages of boiling are perceived as film evaporation, excessive and nominal bubbly boiling, transition, and Leidenfrost. With different surrounding media, the onset of a regime and its boiling morphology are distinct. During the film evaporation regime, the droplet heat transfer is high for the impact in ambient air, and the contact line evaporation is dominant in the vapour medium. In contrast, heat transfer is improved in a vapour medium during the bubbly boiling process, reaching up to 30% enhancement compared to the droplets in air surroundings. It is attributed to increased nucleation sites and excessive bubbling compared to air media. Correlations from the literature are also revisited to understand the onset of bubbly boiling and the droplet breakup at the Leidenfrost point. An overall comparison of droplet heat transfer during the boiling regimes in surrounding media is discussed in this paper.

Evaporation of ferrofluid drop in magnetic field in Leidenfrost mode

When a droplet evaporates in the Leidenfrost mode, the intensity of heat transfer is significantly reduced compared to the boiling mode, which is highly undesirable for a number of applications. Here we demonstrate a method for intensifying heat transfer during droplet evaporation in the Leidenfrost mode. The essence of this method is that if a magnetic nanofluid is used as a cooling liquid, then with the help of an inhomogeneous magnetic field it is possible to forcibly press a levitating drop to a superheated surface, which contributes to the intensification of the drop evaporation. An aqueous suspension of Fe3O4 is used here as a ferrofluid. The experimental results show that this method can reduce the drop evaporation time by several times. When the magnetic field gradient increases, the drop evaporation time decreases according to the power law. A simple phenomenological model is proposed to substantiate the observed effects.

EMERGENCY COOLING OF SUPERHEATED SURFACES BY NANOFLUIDS ADDITIVES IN STOP- AND NON-STOP MODES OF HEAT LOAD RISE

Early [Technical Physics Letters, 2016, Vol. 42, P. 677–681. — https://doi.org/10.1134/ S106378501607004X] we have shown the possibility of emergency cooling of an overheated Ni/Cr surface using additives of aluminosilicate nanofluids (AlSi-NF) at the time of developed film boiling of water (crisis), provided that the increase in thermal load is stopped and its fixation at the level of Q » 1.0 MW/m2, exceeding the critical heat flux (CHF) of water (0.7 MW/m2). However, in real operating conditions of cooling systems (especially for nuclear reactors), emergency situations sometimes arise in which it is very problematic to immediately turn off the heat load supply or maintain it at a certain predetermined level. In this regard, in this work, on a fully automated stand, the peculiarities of eliminating the water boiling crisis and emergency cooling of the overheated surface of the mini-reactor heater by injecting a portion of hot AlSi-NF in conditions of film boiling of water and a steady increase in heat load were studied, and the results were compared with previously obtained in the mode of stopping the rise of the thermal load. The test was carried out on an aqueous AlSi-NF nanofluid obtained on the basis of a natural mixture of aluminosilicates montmorillonite + palygorskite (Ukraine). The boiling-overheating-cooling curves, as well as the time dependences of the heat transfer coefficient and the heating surface temperature, were recorded in automatic real-time mode. Emergency cooling of the overheated surface (from 600 to 125 °C) after the introduction of a portion of hot AlSi-NF occurred in a matter of minutes due to a sharp increase in the heat transfer coefficient α up to 55,000 W/(m2.K). Such a phenomenon of a sharp intensification of heat transfer and a 3-fold increase in the specific heat flux (qsp) during boiling of AlSi-NF compared to the base liquid (water) is explained by the deposition on the heating surface of a gel-like layer of nanoparticles with high hydrophilicity and mobility, which can sharply increase nucleate boiling and convection. Regardless of the mode of supplying the heat load, the principal possibility of overcoming the boiling crisis and emergency cooling of the superheated surface with the addition of AlSi-NF nanofluid has been established, for a time sufficient to eliminate the accident. Bibl. 27, Fig. 4, Tab. 1.

Experimental investigation on boiling regime transformation when the binary-droplet impact on the superheated surface

This experimental study focuses on the phenomena induced by thermal effects after a two-component droplet of propylene glycol water impinges on a hot substrate. The visualization experiment revealed that this phenomenon is characterized by a shift in the boiling regime of the droplets, which exhibits different transformation patterns with changes in surface temperature and propanediol concentration. On the basis of high-speed camera sequence images, a temperature-concentration regime map is created. Qualitative analysis is performed on the causes of the boiling transformation, as well as the wall temperatures and concentration intervals at which it might occur. The time at which the boiling regime transformation occurs can be determined by analyzing the secondary droplets. In addition, the effects of various boiling regimes and propylene glycol concentrations on the size and velocity distribution of secondary droplets are briefly investigated.

Effects of Superheated Surface on the Deposition Behavior of Na2SO4 in Supercritical Water

The reduced solubility of inorganic salts in supercritical water has a significant impact on the stable operation of desalination facilities as it may lead to surface fouling due to salt deposition. In this study, the solubility of Na2SO4 was experimentally determined to be 0.04–15.34 mmol/kg water at 23–25 MPa and 390–420 °C. To investigate the precipitation behavior of Na2SO4 in supercritical water, a reactor with a heating bar was designed and the deposition effect of salt on the superheated surface in an autoclave was tested at a temperature of 390 °C and a pressure of 23 MPa. Then, the deposition mechanism of salt in the autoclave was analyzed and the temperature field in the reactor was simulated using CFD commercial software. The experimental results showed that Na2SO4 was present on both the heating rod and the bottom of the autoclave with a loose salt layer. The simulation results indicated that the temperature near the heating rod was significantly higher than the bulk fluid temperature and it provided the temperature condition where the inorganic salt preferentially nucleated and precipitated. Nickel foam was chosen as the porous media carrier to investigate the selective precipitation of salt on different superheated surfaces. The results showed that nickel foam could collect a large amount of salt on the heating rod and change the state of the compact salt layer on the kettle bottom wall, providing a technical choice for salt recovery under supercritical conditions.

Interfacial heat transfer and boiling transition of the droplets on superheated surface with Leidenfrost effects

This study, a novel three-dimensional (3D) method is developed to address the Leidenfrost effect issues with a focus on the synergies of droplet velocity (v) and surface temperature (Tw). Here, we proposed a 3D computational fluid dynamics numerical simulation that combines a VOF model and an amended evaporation model and performed high-speed video testing for capturing vapor dynamics details and distinguish anisotropic heat transfer on a high-temperature surface near the Leidenfrost point. The results revealed that the law of the dynamic Leidenfrost point (TDLT) was predominated by changes in the thickness and shape of the vapor layer. Additionally, coexistence of v and Tw produced a vapor layer, instantly disentangling the boiling state and initiating a gradual change from contact boiling to film boiling, with v affecting the boiling state transition and delaying buildup of the vapor layer aggregation. We determined the empirical correlation of TDLT with the Weber number (We) as follows: TDLT=376.65+84.45×We0.22. Moreover, the high Tw promoted vapor layer formation, thereby decreasing the heat transfer efficiency, whereas the large v inhibited vapor layer formation and enhanced interfacial heat transfer. These findings contribute to a better understanding of the theoretical basis for inhibiting the Leidenfrost effect.

Drop impact on superheated surfaces: from capillary dominance to nonlinear advection dominance

Ambient air cushions the impact of drops on solid substrates, an effect usually revealed by the entrainment of a bubble, trapped as the air squeezed under the drop drains and liquid–solid contact occurs. The presence of air becomes evident for impacts on very smooth surfaces, where the gas film can be sustained, allowing drops to bounce without wetting the substrate. In such a non-wetting situation, Mandre & Brenner (J. Fluid Mech., vol. 690, 2012, p. 148) numerically and theoretically evidenced that two physical mechanisms can act to prevent contact: surface tension and nonlinear advection. However, the advection dominated regime has remained hidden in experiments as liquid–solid contact prevents rebounds being realised at sufficiently large impact velocities. By performing impacts on superheated surfaces, in the so-called dynamical Leidenfrost regime (Tran et al., Phys. Rev. Lett., vol. 108, issue 3, 2012, p. 036101), we enable drop rebound at higher impact velocities, allowing us to reveal this regime. Using high-speed total internal reflection, we measure the minimal gas film thickness under impacting drops, and provide evidence for the transition from the surface tension to the nonlinear inertia dominated regime. We rationalise our measurements through scaling relationships derived by coupling the liquid and gas dynamics, in the presence of evaporation.

The local heat transfer coefficient determination during boiling a subcooled liquid on a superheated surface by the gradient heatmetry

The local heat transfer coefficient determination during boiling a subcooled liquid on a superheated surface by the gradient heatmetry

Enhancing Boiling Heat Transfer on a Superheated Surface by Surfactant-Laden Droplets.

Boiling, one of the most common phase-change heat transfer methods, is widely used in nuclear power plants, spacecraft, integrated circuits, and other situations, where rapid and efficient heat transfer is crucial. However, boiling heat transfer is efficient only in a specific surface temperature range when a droplet impacts a superheated surface. Here, we enhance the boiling heat transfer and extend this temperature range by adding a tiny amount of surfactant. We find that surfactants can weaken the Kelvin effect of boiling bubbles, and thus reduce the onset of boiling driven temperature and significantly enhance the maximum vaporization rate of the droplet effectively. In particular, different from previous studies, we find that the surfactants at lower concentrations can increase the Leidenfrost temperature of the droplets. All the above effects jointly expand the temperature range of effective boiling heat transfer. This study sheds new light on the role of surfactants in the boiling process and offers a new medium to promote heat-transfer applications.

Numerical study on subcooled water jet impingement cooling on superheated surfaces

The present study aims to numerically investigate the rapid cooling heat transfer characteristics of the superheated solid surfaces when the subcooled water jet impinges. The computational fluid dynamics (CFD) simulation was carried out by considering boiling and condensation heat transfer to estimate key design parameters such as wall heat flux, heat transfer coefficient, and surface temperature variation using the volume-of-fluid (VOF) model. The simulated results agreed well with the experimental data of the surface temperature and the wall heat flux at the stagnation point. The water vapors formed near the stagnation point and rapidly propagated radially after impact. Also, strong vorticity was found in a radial direction, resulting in a vapor blanket. The result showed that the vapor blanket prevented the liquid flows from directly contacting the heated surface, decreasing the heat transfer. In particular, the surface temperature in the radial direction cooled down more rapidly than that in the vertical direction because of higher boiling heat transfer within a wetting radius where the heat transfer coefficient became higher owing to the liquid wetting.

Boiling Transitions During Droplet Contact on Superheated Nano/Micro-Structured Surfaces.

Manipulating surface topography is one of the most promising strategies for increasing the efficiency of numerous industrial processes involving droplet contact with superheated surfaces. In such scenarios, the droplets may immediately boil upon contact, splash and boil, or could levitate on their own vapor in the Leidenfrost state. In this work, we report the outcomes of water droplets coming in gentle contact with designed nano/microtextured surfaces at a wide range of temperatures as observed using high-speed optical and X-ray imaging. We report a paradoxical increase in the Leidenfrost temperature (TLFP) as the texture spacing is reduced below a critical value (∼10 μm) that represents a minima in TLFP. Although droplets on such textured solids appear to boil upon contact, our studies suggest that their behavior is dominated by hydrodynamic instabilities implying that the increase in TLFP may not necessarily lead to enhanced heat transfer. On such surfaces, the droplets display a new regime characterized by splashing accompanied by a vapor jet penetrating through the droplets before they transition to the Leidenfrost state. We provide a comprehensive map of boiling behavior of droplets over a wide range of texture spacings that may have significant implications toward applications such as electronics cooling, spray cooling, nuclear reactor safety, and containment of fire calamities.