Film Boiling Research Articles

Combustion enhancement and soot reduction of kerosene-based fuel under engine-relevant conditions enabled by fuel reactivity modulation

Liquid fuels incorporating biomass components (e.g., bio-alcohols) offer a great potential for improving combustion performance and energy efficiency. However, the insufficient reactivity and Leidenfrost effect of these fuels are two long-standing challenges for its stable combustion under wide engine-relevant conditions. Herein, the effect of methoxydiethylborane (MDEB) on the spray combustion characteristics promotion of kerosene and binary kerosene/ethanol-based fuels is reported, where the suppression of Leidenfrost effect under air conditions is demonstrated. The synergistic effect of MDEB on oxidation heat release and active radical formation not only enable the resultant kerosene/ethanol-based suspended droplets to present high evaporation rate (∼44 % reduction in evaporation time), excellent ignition performance, e.g., elevating ignition reliability and ignition delay time dropping to 1051 ms (∼67 % reduction) at an ambient temperature of 820 °C, and soot reduction property (few residual black particles displayed in suspension unit) compared to neat fuel droplet, but also make single kerosene-based hybrid fuels exhibit improved atomization and jet combustion characteristics. Notably, the negative growth behavior of evaporation time associated with the Leidenfrost effect that is dominated by low thermal conductivity air and local fuel vapor layer can be eliminated by introducing MDEB. Further, the jet dynamic reflects the improvement of gas phase diffusion and fuel–air mixing in jet and radial directions, and the increased flame propagation rate and heat transfer efficiency, which highlights the applicability of fuel reactivity modulation strategy for the enhancement of combustion stability under wider engine-relevant conditions. The findings thus provide a promising way to engineering scalable liquid fuels for advanced aviation engines, and realizing pollutants control for efficient energy conversion and management.

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.

Marangoni effect of surfactant droplet in transition boiling and formation of secondary droplet

The dynamic processes of surfactant droplets impacting onto substrates of varied temperatures have been widely studied in heat transfer, cooling and printing. In this work, we observe the impacting process of aqueous droplets of surfactants SDS, CTAB, and Triton X-100 on a hot aluminum plate via a high-speed camera, in order to study the dynamics of different surfactant droplets impacting on a hot aluminum substrate. Experimentally, it is discovered that the surfactant droplets in transition boiling produce a secondary droplet of non-wetting state in the final stage of evaporation. The analysis demonstrates that after the droplet impacts the substrate, a temperature gradient is created between the top of the droplet and the triple-phase contact line, increasing the surfactant concentration near the triple-phase contact line as compared with that of the top. The top liquid is maintained by the Marangoni effect, which is caused by the concentration gradient. In the final stage of the evaporation process, the residual droplet gradually shrinks into a sphere. It is detached from the substrate and taken off under the impulse force of the bubble explosion at the bottom, generating the secondary droplet. The radius of the secondary drop increases with the raising of initial concentration of the drop, but ultimately reaches the saturation size. This work explains the role of surfactants in forming secondary droplets. Additionally, this work provides a reference for understanding the physical mechanism of Leidenfrost effect and the controlling of boiling heat transmission.

Aerodynamic Super-Repellent Surfaces.

Repelling liquid drops from engineering surfaces has attracted great attention in a variety of applications. To achieve efficient liquid shedding, delicate surface textures are often introduced to sustain air pockets at the liquid-solid interface. However, those surfaces are prone to suffer from mechanical failure, which may bring reliability issues and thus limits their applications. Here, inspired by the aerodynamic Leidenfrost effect, we present that impacting drops are directionally repelled from smooth surfaces supplied with an exogenous air layer. Our theoretical analysis reveals that the synchronized nonwetting and oblique bouncing behavior is attributed to the aerodynamic force arising from the air layer. The versatility and practicability of our approach allow for drop repellency without the aid of any surface wettability treatment and also avoid the consideration of mechanical stability issues, which thereby provides a promising candidate for the applications that necessitate liquid shedding, e.g., resolve the problem of tiny raindrop adhesion on the automobile side window during driving.

Experimental study on droplet behavior and mechanisms during reflooding in a Single-rod channel

• The complete vapor-liquid interface behavior during reflooding is observed. • Discovery and interpretation of the bimodal distribution of droplets. • The three main mechanisms of droplet formation and breakup are discussed. A high-temperature surface reflooding visualization experiment in a single-rod channel was conducted to investigate the transient droplet behavior, which is considered to be critical for heat transfer in Dispersed Flow Film Boiling (DFFB). After the zirconium alloy cladding was heated to the preset temperature, the specimen was subjected to a continuous flow of deionized water under varying input heat flux, coolant subcooling, and apparent reflooding velocity. The temperature history during reflooding was recorded, and the quench behavior was visualized using a high-speed camera. The experimental results show the typical transition mode of inverted annular flow-dispersive flow; at the same time, although the size distribution of droplets conforms to the form of log-normal distribution, the probability of large droplets will increase significantly at higher input heating power. Furthermore, to provide physical support for building droplet distribution models containing more physical mechanisms in the future, several droplet formations and breakup mechanisms, which include the Helmholtz instability, capillary breakage, and rewetting area sputter, have been discussed in this paper.

Drag Reduction and Leidenfrost Effect on Submerged Ratcheted Cylinder

This paper presents experimental investigations of submerged self-propulsion due to the Leidenfrost effect. We aim to identify and quantify the forces governing the free-fall of heated cylinders at temperatures between 25 °C and 550 °C. Understanding the forces involved within three-phase systems is essential when designing sensitive instruments, such as microelectronic devices. Cylinders with sizes in the range of tens of millimeters are released into a vertical column filled with Novec 7000. Experiments were conducted to investigate five topological designs, and bored-out hollows within the cylinders forced vertical free-fall. The control cylinder was smooth, while the others had ratchet teeth engraved on their surface. Ratcheted cylinders were manufactured in pairs to compare ratchet directionality. An in-house tracking tool reduced experimental error and measurement noise. The time taken to reach terminal velocity increases with initial temperature while terminal velocity decreases, respectively. The depth at which terminal velocity occurs also decreases with increasing initial temperature. The existence of submerged self-propulsion has been identified by comparing ratcheted cylinders of opposing directionality. A theoretical mechanism for the action of the viscous friction force that drives self-propulsion has been developed. The viscous friction force has been quantified to be between 0.04–3.28 mN.

Versatility of the Preparation Method for Macroporous Cryogel Particles Utilizing the Inverse Leidenfrost Effect.

We have investigated the versatility of a two-step preparation method, without a detergent, that combines both the inverse Leidenfrost effect and the cryogelation technique by using the macroporous particles of different kinds of monomers (four vinyl monomers) or a natural polymer (agarose). First, the precursor of polymers was dropped into liquid nitrogen to prepare the spherical frozen droplet by the inverse Leidenfrost effect. Second, the frozen droplets were cryo-polymerized at the frozen temperature; then, cryogel particles were prepared after thawing. Subsequently, the basic characteristics of the macroporous polymer particles obtained above were compared, focusing on the appearances, porous morphologies, and mechanical properties. It was found that the similar polymer particles could be obtained by the two-step preparation method, while there was a slight difference in their characteristics, depending on the type of monomers. Especially for the mechanical properties, the cryogel particles of the hydrophilic polymer exhibited a shape memory function with sponge-like elasticity, whereas the hydrophobic polymer particles were observed to be cracked after compression (i.e., no shape memory function). This work provides a versatile method of adopting various kinds of monomers and natural polymers for the preparation of macroporous particles. Hence, the method possibly has a potential to prepare and design "tailor-made" macroporous polymer particles for the application purpose.

Evidence for the Leidenfrost Effect at the Nanoscale

Evidence for the Leidenfrost Effect at the Nanoscale

Study of Cryogenic Film Boiling in the Presence of External Electric Field Using a Variant of Volume of Fluid-Based Interface Tracking Algorithm

Study of Cryogenic Film Boiling in the Presence of External Electric Field Using a Variant of Volume of Fluid-Based Interface Tracking Algorithm

Investigation of the Behaviors of Methanol Spray Impingement and Wall Wetting

Port fuel injection is an important technical route in methanol engines. To obtain a theoretical basis for injector arrangement and injection strategy development in methanol engines, an optimal experimental platform based on diffuse back-illumination and the refractive index matching method (RIM) was designed and built in this study. The experiments on the behavior of low-pressure methanol spray-wall impingement and wall film were carried out and the influence of the three boundary conditions of spray distance (Dimp), wall temperature (Twall), and injection pressure (Pinj) were analyzed comprehensively. Results showed that with the increase of Dimp, the overall shape of spray before impinging the wall changed from conical to cylindrical. The impinging spray height Hi and impinging spray width Wi increased with the decrease of Dimp and the increase of Pinj. Adhesive fuel film mass Mf increased with the increase of Dimp due to the decrease of kinetic energy during wall impact. In addition, the increase of the wall temperature Twall reduced Mf due to evaporation, but when Twall reached 423 K, Mf rebounded due to the Leidenfrost effect. The results of this study are helpful to improve the accuracy of the numerical methanol engine model.

Numerical Study on Spreading and Vaporization Process of Liquid Nitrogen Droplet Impinging on Heated Wall

Micro-structured surfaces can affect heat transfer mechanisms because of enlarged specific surface areas. However, employing the Leidenfrost effect during liquid nitrogen (LN2) droplet cooling of a heated micro-structured surface possessing a fin with a spacing much smaller than the diameter of the droplet has not yet been explored. In the present work, a direct numerical simulation (DNS) is carried out to investigate heat transfer mechanisms of the LN2 droplet, whose diameter is sufficiently larger than the structured spacing of fin, impinging on a micro-structured surface with variable velocities. For a comparative study, a smooth surface is also employed in numerical simulations. The spreading mechanisms and vaporization behavior of the droplet along with liquid film morphology at various conditions are investigated. Results show that a smaller fin size inhibits LN2 in entering into the grooves between the fins and left the surface untouched by the droplet completely, and eventually, a thinner liquid film is spread out in contrast to the smooth surface. Notably, at a low Weber number, the droplet can be shrunk or even rebounded away from the wall after impinging on the wall. The fastest vaporization behavior for both surfaces, namely smooth and micro-structured, is obtained at a Weber number of 180. Additionally, an effective heat transfer upon the micro-structured surface is observed at a low impinging velocity of the droplet.

Numerical analysis of heat transfer between a wall and diesel spray: Effect on ignition characteristics

The heat transfer between the wall and spray has an important effect on the in-cylinder ignition characteristics of the diesel engine, especially during cold start conditions. To better understand the effect of heat transfer between the wall and spray at different wall temperatures on the ignition characteristics, a numerical analysis was conducted with a large-eddy simulation (LES) based on optical experiments. The results show that the spray/wall interaction led to a smaller Sauter mean diameter (SMD) and more small and large droplets compared to the free spray. Furthermore, the total surface area of droplets increases, and heat transfer between droplets and the surrounding hot gas is promoted, which is conducive to the ignition process. On the other hand, when the wall temperature is low, the spray is cooled by the wall during impingement, thus inhibiting ignition. As a result of this competing effect, the air entrainment rate increases with the increase of wall temperature due to the cooling effect weakening and gradually changing to a heating effect. However, when the wall surface temperature increases to a high temperature, the heating effect is suppressed and the local heat transfer coefficient decreases due to the Leidenfrost effect. It is worth emphasizing that the Leidenfrost effect only occurs near the stagnation point in this study, and the heat transfer between the wall and the vapor-phase spray still impacts the ignition process, resulting in a shorter ignition delay time (IDT) as the wall temperature increases. Interestingly, the low-temperature reactions of both the free spray and spray/wall interaction cases started at the same moment, and the transition period from the low-temperature ignition delay time (τ1) to IDT shows a significant difference, indicating that the interaction between the wall and spray primarily affects the heat accumulation of the low-temperature reaction (LTR).

Theoretical model of the Leidenfrost temperature.

The Leidenfrost effect is a phenomenon in which a liquid, poured onto a glowing surface significantly hotter than the liquid's boiling point, produces a layer of vapor that prevents the liquid from rapid evaporation. Rather than making physical contact, a drop of water levitates above the surface. The temperature above which the phenomenon occurs is called the Leidenfrost temperature. The reason for the existence of the Leidenfrost temperature, which is much higher than the boiling point of the liquid, is not fully understood and predicted. For water we prove that the Leidenfrost temperature corresponds to a bifurcation in the solutions of equationsdescribing evaporation of a nonequilibrium liquid-vapor interface. For water, the theoretical values of obtained Leidenfrost temperature, and that of the liquid-vapor interface which is smaller than the boiling point of liquid, fit the experimental results found in the literature.

Experimental constraints on the stability and oscillation of water vapor film—a precursor for phreatomagmatic and explosive submarine eruptions

Pre-mixing of magma and external water plays a key role in driving explosive phreatomagmatic and submarine volcanic eruptions. A thin film of water vapor forms at the magma–water interface as soon as hot magma comes in direct contact with the cold water (Leidenfrost effect). The presence of a stable vapor film drives efficient mixing and mingling between magma and water, as well as magma and wet and water-saturated sediments. Such mixing occurs before explosive molten fuel–coolant type interactions. Using high-temperature laboratory experiments, we investigate the effect of magma and water temperatures on the stability of vapor film, which has not been performed systematically for a magmatic heat source. The experiments were performed with re-melted volcanic rock material, from which spherically-shaped rock samples were produced. These samples were heated to 1,110°C and then submerged in a water pool with a constant temperature (3–93°C). The experiments were recorded on video, and, synchronously, sample and water temperatures were measured using thermocouples. The time-dependent thickness of the vapor film was measured from the video material. The vapor film tends to oscillate with time on the order of 102 Hz. We find that the vertical collapse rates of vapor films along the sample–water interfaces are 13.7 mm s−1 and 4.2 mm s−1 for water temperatures of 3.0°C and 65°C, respectively. For a given initial sample temperature, the thickness and stability time scales decrease with decreasing water temperature, which has implications for the efficiency of pre-mixing required for explosive eruptions. Using thermodynamics and previously measured material parameters, it is shown that a sudden collapse of the vapor film can start brittle fragmentation of the melt and thus serves as the starting point of thermohydraulic explosions.

Universal Correlations for Post-CHF Saturated and Superheated Flow Film Boiling Heat Transfer Coefficient, Minimum Heat Flux and Rewet Temperature for Cryogenic Fluids in Uniformly Heated Tubes

Despite the many experiments conducted throughout the globe during the past sixty years for a variety of cryogens to determine film boiling heat transfer coefficient (HTC) in a uniformly heated round tube, experimental data are either rarely published or published only for a few cryogens, with majority of the data remaining in archives of original authors, or in obscure technical reports of an organization or other inaccessible sources. In the present study, a very comprehensive data mining effort is undertaken to develop a consolidated database for Post Critical Heat Flux (CHF) flow boiling HTC for cryogens from world literature dating back to 1959. With 1730 local Dispersed Flow Film Boiling (DFFB) and 1310 local Inverted Annular Film boiling (IAFB) HTC data points for LHe, LH2, LN2, and LCH4, it represents the largest cryogen post-CHF HTC database assembled to date. Using this database, new universal cryogen correlations for DFFB and IAFB HTCs are constructed and verified in terms of both predictive accuracy and trend. Similar efforts are carried out to collect a relatively small Minimum Heat Flux (MHF) and local Re-wet Temperature (RW) database, and construct correlations for both.

Micro Sponge Balls: Preparation and Characterization of Sponge-like Cryogel Particles of Poly(2-hydroxyethyl methacrylate) via the Inverse Leidenfrost Effect

Micro Sponge Balls: Preparation and Characterization of Sponge-like Cryogel Particles of Poly(2-hydroxyethyl methacrylate) via the Inverse Leidenfrost Effect

Droplet impact on a heated porous plate above the Leidenfrost temperature: A lattice Boltzmann study

In the past few decades, the droplet impact on a heated plate above the Leidenfrost temperature has attracted immense research interest. The strong hydrophobicity caused by the Leidenfrost effect leads to the droplet bouncing from a flat plate at a given contact time predicted by the classical Rayleigh theory. Numerous investigations were conducted to break the theoretical Rayleigh's limit to reduce the interfacial contact time. Recently, a droplet was observed to form a pancake shape and bounce as it impacted nanotube or micropost surfaces above the Leidenfrost temperature. This led to a significant reduction in droplet contact time. However, this unique bouncing phenomenon is still not fully understood, such as the influence of the plate configuration and the relationship between the droplet rebound time and evaporation mass loss. In this study, we carry out a numerical study of the droplet impact dynamics on a heated porous plate above the Leidenfrost temperature, using a multiphase thermal lattice Boltzmann model. Our model is constructed within the unified lattice Boltzmann method framework and is first validated based on theoretical and experimental results. Then, a comprehensive parametric study is performed to investigate the effects of the impact Weber number, the plate temperature, and the plate configurations on the droplet bouncing dynamics. Results show that higher plate temperature, larger Weber number, and smaller pore intervals can accelerate the droplet rebound and promote the droplet pancake bouncing. We demonstrate that the occurrence of the pancake bouncing is attributed to the additional lift force provided by the vapor pressure due to the evaporation of liquid inside the pores. Moreover, the droplet maximum spreading time and maximum spreading factor can be described by a power law function of the impact Weber number. The droplet evaporation mass loss increases linearly with the impingement Weber number and the plate opening fractions. This study provides new insights into the Leidenfrost droplet impingement on porous plates, which may potentially facilitate the design of novel engineering surfaces and devices.

A general prediction model of minimum film boiling temperature during quenching propagation in narrow rectangular channel

During quenching, the Leidenfrost effect deteriorates the heat transfer at high temperatures before the arrival of the minimum film boiling point. Accurate prediction of the minimum film boiling temperature (Tmin) is significant in assessing cooling efficiency and providing a reference for optimization design. The experimental investigation on the Tmin is carried out in multi-narrow rectangular channels containing internal and external coolant channels. The effects of initial surface temperature and wall heat flux on Tmin are discussed in detail. The experimental results indicate that Tmin increases with the increase of initial wall temperature and wall heat flux, by an average of 6.5% and 7.7%, respectively. Under high wall temperature and heat flux conditions, the ratio of Tmin to the initial wall temperature at the inlet is much larger and then remains nearly constant with the propagation of the quench. Furthermore, an improved dimensionless prediction model of Tmin is proposed considering the effects of high initial wall temperature and wall heat flux. The comparison results illustrate good agreement with relative errors within ± 15%. Meanwhile, the current model predicts most of the published experimental data with an uncertainty of ± 15%.

Suspended water droplet confined laser shock processing at elevated temperatures

The temperature-assisted laser shock process has application prospects in various manufacturing fields such as forming and surface strengthening. However, the large-scale application of this process is limited by the instability and failure of confinement medium at high temperatures (>300 °C). To address this problem, this paper proposes a novel laser shock strategy based on the Leidenfrost effect, wherein suspended droplets are used as the confinement medium. The dynamic behaviors of the droplets during laser shock process were studied. The focusing enhancement effect of the droplets was comprehensively explored. By combining the dynamic behaviors and focusing enhancement effect of the droplets, a theoretical model of the laser shock pressure under droplet confinement was established. Finally, a typical application of the droplet-confined laser shock process in the fields of forming and strengthening was demonstrated to verify its effectiveness and feasibility at high temperatures. The proposed process provides a flexible, convenient, and green plasma confinement strategy that allows stable operation of laser shock at high temperatures. Moreover, the droplet confinement strategy broadens the application prospects of laser shock, and provides a technical support for enabling enhanced ductility and dynamic strain aging of alloys at high temperatures.

Film Boiling Conjugate Heat Transfer during Immersion Quenching

Boiling conjugate heat transfer is an active field of research encountered in several industries, including metallurgy, power generation and electronics. This paper presents a computational fluid dynamics approach capable of accurately modelling the heat transfer and flow phenomena during immersion quenching: a process in which a hot solid is immersed into a liquid, leading to sudden boiling at the solid–liquid interface. The adopted methodology allows us to couple solid and fluid regions with very different physics, using partitioned coupling. The energy equation describes the solid, while the Eulerian two-fluid modelling approach governs the fluid’s behaviour. We focus on a film boiling heat transfer regime, yet also consider natural convection, nucleate and transition boiling. A detailed overview of the methodology is given, including an analytical description of the conjugate heat transfer between all three phases. The latter leads to the derivation of a fluid temperature and Biot number, considering both fluid phases. These are then employed to assess the solver’s behaviour. In comparison with previous research, additional heat transfer regimes, extra interfacial forces and separate energy equations for each fluid phase, including phase change at their interface, are employed. Finally, the validation of the computational approach is conducted against published experimental and numerical results.