Film Boiling Research Articles

Investigation of the Mechanism in Film Boiling Heat Transfer Enhancement by a Honeycomb Porous Plate

Investigation of the Mechanism in Film Boiling Heat Transfer Enhancement by a Honeycomb Porous Plate

Numerical Simulation of Film Boiling Heat Transfer in the Presence of a Uniform Electric Field Using Front Tracking

Numerical Simulation of Film Boiling Heat Transfer in the Presence of a Uniform Electric Field Using Front Tracking

Subpatterns of Thin-Sheet Splash of a Droplet Impact on a Heated Surface

A thin-sheet splash of a droplet impact on a solid surface typically appears as secondary droplets are ejected from a levitated liquid lamella. A recent work (Qin, M.; Tang, C.; Guo, Y.; Zhang, P.; Huang, Z., Langmuir 2020, 36 (18), 4917-4922) identified three subpatterns of a thin-sheet splash on a smooth wall at room temperature (T0). In the present work concerning the high-temperature (TW) surface, we show that subpatterns of the thin-sheet splash can be unified in the three-dimensional phase diagram of Oh-We-TW, where Oh is the Ohnesorge number and We is the Weber number. As TW is sufficiently high, the Leidenfrost effect becomes so prominent that both deposition and thin-sheet splash make a transition to Leidenfrost breakup. For the transition surface temperature TW,cr from thin-sheet splash to deposition, a scaling correlation of TW,cr/T0 ∼ We3/2 is derived based on the analysis of the temperature-dependent destabilizing force on the levitated lamella and agrees well with our experimental data.

Enhanced low-temperature stable combustion of hydrocarbon with suppressing the Leidenfrost effect

Low-temperature stable combustion of liquid hydrocarbons helps improve energy efficiency and alleviate environmental pollution. However, when hydrocarbons (mainly saturated ones) are used as aircraft fuel, they show a long ignition delay time (IDT) and high ignition temperature as a result of Leidenfrost effect and chemical inertness of the CH bond. In especial, the insulated fuel vapor film due to the Leidenfrost effect that inhibits the heat transfer efficiency in the wide Leidenfrost region is unfavorable. Herein, we first propose a strategy that combines low-temperature self-oxidation and enhanced heat conduction to achieve low-temperature stable combustion of hydrocarbon while suppressing the Leidenfrost effect. Specifically, moderately reactive organometallic compound methoxydiethylborane (MDEB) with special radicals and silver nanoparticles (Ag NPs) with high thermal conductivity are preferred as fuel modifiers. By thermodynamic calculation, the heat released by the self-oxidation of MDEB is two orders of magnitude higher than the evaporation latent heat of n-decane, suggesting the applicability of energy conversion supported by enhancing fuel reactivity in suppressing the Leidenfrost effect. Ignition experiment results show that the negative temperature dependence associated with the Leidenfrost effect is eliminated under the specific fuel composition. Encouragingly, compared with n-decane, the IDT of MDEB/Ag/n-decane hybrid fuel is reduced from 4027 to ∼10 ms at a surface temperature of 600 °C. Comparing the ignition of MDEB/Ag/n-decane and MDEB/n-decane hybrid fuels at same temperatures, the influence of Ag NPs on IDT is highlighted by its significant decrease (∼28%). The minimum ignition temperature of n-decane-based hybrid fuel decreases to 112 °C, which is 508 °C lower than that of n-decane. These two ignition characteristic values are the low limit of saturated hydrocarbons to date. Our results thus provide a promising tool for enhancing the low-temperature ignition reliability and combustion stability of liquid hydrocarbons, which potentially decreases the pollution from the low temperature combustion.

Boiling and evaporation model for liquid-gas flows: A sharp and conservative method based on the geometrical VOF approach

A novel phase change model is developed for the direct numerical simulations of the liquid-gas flows, devoting to the evaporation and boiling flows in the framework of the geometrical volume of fluid method. The main focus of the present work is to impose accurate jump conditions for heat and mass transfers across the interface, while the conservative properties of the governing equations are maintained. We tackle this problem by employing an embedded boundary method, which divides the computational domain into the liquid region and the gas region separately, besides, the application of the finite volume method enables us to conserve the energy and the mass precisely. Results are then presented for a series of benchmark problems, and the accuracy and the robustness of the numerical methods are validated. In particular, two very challenging numerical tests are simulated: a droplet levitating above a hot plate or a liquid pool whose temperature is much higher than the boiling temperature. This kind of problem, known as the Leidenfrost effect, is non-trivial in numerical simulations, because the boiling and the evaporation can occur simultaneously on different regions of the same liquid interface. In addition, very careful calculations are required in the thin film of saturated vapor which is entrapped between the bottom of the drop and the plate. We show that the newly implemented numerical methods can accurately reproduce experimental and theoretical results, serving as convincing evidence of the superiority of our method.

Triple Leidenfrost Effect: Preventing Coalescence of Drops on a Hot Plate.

We report on the collision-coalescence dynamics of drops in Leidenfrost state using liquids with different physicochemical properties. Drops of the same liquid deposited on a hot concave surface coalesce practically at contact, but when drops of different liquids collide, they can bounce several times before finally coalescing when the one that evaporates faster reaches a size similar to its capillary length. The bouncing dynamics is produced because the drops are not only in Leidenfrost state with the substrate, they also experience Leidenfrost effect between them at the moment of collision. This happens due to their different boiling temperatures, and therefore, the hotter drop works as a hot surface for the drop with lower boiling point, producing three contact zones of Leidenfrost state simultaneously. We called this scenario the triple Leidenfrost effect.

Influences of Brass Surface Morphology on Leidenfrost Effect during Liquid Nitrogen Cooling

Cooling in liquid nitrogen is a typical service condition of high-temperature superconducting wire, and the variation of boiling stages on the wire protective layers such as the brass layers could be crucial for the quench behavior of superconducting devices. In this study, the influence of brass surface morphology (parameters of surface roughness and fractal dimension) on the Leidenfrost effect (including the wall superheat at critical heat flux and the wall superheat at Leidenfrost point, which are respectively characterized by the temperatures of ΔTCHF and ΔTLP) was studied. The surfaces of brass samples were polished by sandpaper to obtain different morphologies, which were characterized by using white light interferometer images, and the boiling curves were recorded and analyzed by Matlab with lumped parameter method. The experimental results demonstrated that the surface morphology of brass samples could influence the ΔTLP significantly, but had no clear relationship with the ΔTCHF. Moreover, the multi-scaled analysis was carried out to explore the influencing mechanism of surface microstructure, the relationship between ΔTLP and scale was more clear when the scale was small, and the fractal dimension was calculated and discussed together with surface roughness. The findings of this study could be instructive for surface treatment of superconducting wires to suppress quench propagation.

EDITORIAL

The editorial of Thermal Engineering of this issue continues the discussion on scientific research needs in vital areas in which thermal engineering has important participation. The main goal is to motivate the readers, within their specialties, to identify possible subjects for their future research. Natural Convection is present in the most diverse applications of Thermal Engineering, such as controlling and reducing temperatures in electronic systems, reducing the thermal efficiency of cooling in machining processes by the Leidenfrost effect and even in biological systems. With the increasing technological evolution and the development of industrial automation, microelectronics, quantum computing, signal processing, mobile telephony, etc., transmission systems operate increasingly with smaller spacing and higher integration rates between components, with greater power density and heat generation. As a result, there is a growing demand for cooling systems with greater safety, reliability, and efficiency. Therefore, natural convection cooling systems are viable alternatives due to their characteristics of: (A) protection and safety of the transmission system, especially in cases of mechanical and/or electrical failures of the forced cooling system; (B) high reliability and safety of operation; (C) low maintenance costs and (D) no noise. However, due to their low thermal efficiency, such cooling systems are still limited to applications with the low power density and/or combined with forced convection cooling systems. In this sense, the natural convection area is increasingly being researched to create and enable even smaller and more robust high power density transmission systems, with greater economic feasibility (lower costs of acquisition, manufacturing, and maintenance) and exclusively refrigerated (or with minimal use of forced cooling components) by natural convection; all without reducing the efficiency or reliability of these systems. One of the main technologies for thermal optimization of cooling systems researched is the inclusion of geometric surface modifications, through fins (extended surfaces) or corrugated surfaces. The use of corrugated surfaces has been gaining more space in the academic community and industry, standing out for: (A) increasing the area of exposure to the heated surface and the transfer of energy to the circulating fluid; (B) induce changes in the flow in the vicinity of the heated surface, such as the formation of vortices, recirculations, and zones of rarefaction and stagnation; and (C) anticipate and facilitate the flow transition process for the turbulent regime. The study of natural convection – in its most diverse applications and areas of theoretical, applied, and experimental investigation – has been widely explored by Thermal Engineering, arousing more and more the academic community's interest and motivating further research in this area. The mission of Thermal Engineering is to document the scientific progress in areas related to thermal engineering (e.g., energy, oil and renewable fuels). We are confident that we will continue to receive articles’ submissions that contribute to the progress of science. Sílvio Aparecido Verdério JúniorProfessor of Mechanical Engineering

CORRELATION BETWEEN LEIDENFROST TEMPERATURE, COOLING CAPACITY AND MACHINING TEMPERATURE: AN EXPERIMENTAL STUDY OF CUTTING FLUIDS

Much of the energy consumed by machining materials is converted into heat, which causes several technical and economic problems for the process. The cutting fluid application by the conventional method forms a vapor film of low thermal conductivity, which prevents direct contact between the fluid and the heated surface; the so-called Leidenfrost effect, which reduces cooling efficiency. Studies of this phenomenon applied to the machining of materials are still very restricted and scarce. In this sense, the present work experimentally studied the correlation of parameters Leidenfrost temperature, cooling capacity and machining temperatures, applied to the SAE 52100 steel turning process with conventional lubri-coolant, with different cutting fluids. The thermal properties of the studied cutting fluids were taken from the technical-scientific literature. An experimental apparatus was developed for measuring and acquiring machining temperatures. Synthetic cutting fluids were shown to have a better cooling capacity, followed by semi-synthetic fluids and emulsions. There is no relationship between Leidenfrost temperature, cooling capacity, and machining temperatures for the different types of cutting fluids studied.

Pierre Hippolyte Boutigny. The Spheroidal State of Matter Theory

Pierre Hippolyte Boutigny (1798-1884), a French chemist, was the first scientist to study in detail, and for many years, the behavior of a liquid drop thrown over a heated surface. He postulated that the result was an additional state of matter that he named spheroidal state. Today this phenomenon is called the Leidenfrost effect.

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.

Leidenfrost Effect as a Directed Percolation Phase Transition.

Volatile drops deposited on a hot solid can levitate on a cushion of their own vapor, without contacting the surface. We propose to understand the onset of this so-called Leidenfrost effect through an analogy to nonequilibrium systems exhibiting a directed percolation phase transition. When performing impacts on superheated solids, we observe a regime of spatiotemporal intermittency in which localized wet patches coexist with dry regions on the substrate. We report a critical surface temperature, which marks the upper bound of a large range of temperatures in which levitation and contact coexist. In this range, with decreasing temperature, the equilibrium wet fraction increases continuously from zero to one. Also, the statistical properties of the spatiotemporally intermittent regime are in agreement with that of the directed percolation universality class. This analogy allows us to redefine the Leidenfrost temperature and shed light on the physical mechanisms governing the transition to the Leidenfrost state.

High-resolution single-particle cryo-EM of samples vitrified in boiling nitro-gen.

Based on work by Dubochet and others in the 1980s and 1990s, samples for single-particle cryo-electron microscopy (cryo-EM) have been vitrified using ethane, propane or ethane/propane mixtures. These liquid cryogens have a large difference between their melting and boiling temperatures and so can absorb substantial heat without formation of an insulating vapor layer adjacent to a cooling sample. However, ethane and propane are flammable, they must be liquified in liquid nitro-gen immediately before cryo-EM sample preparation, and cryocooled samples must be transferred to liquid nitro-gen for storage, complicating workflows and increasing the chance of sample damage during handling. Experiments over the last 15 years have shown that cooling rates required to vitrify pure water are only ∼250 000 K s-1, at the low end of earlier estimates, and that the dominant factor that has limited cooling rates of small samples in liquid nitro-gen is sample precooling in cold gas present above the liquid cryogen surface, not the Leidenfrost effect. Using an automated cryocooling instrument developed for cryocrystallography that combines high plunge speeds with efficient removal of cold gas, we show that single-particle cryo-EM samples on commercial grids can be routinely vitrified using only boiling nitro-gen and obtain apoferritin datasets and refined structures with 2.65 Å resolution. The use of liquid nitro-gen as the primary coolant may allow manual and automated workflows to be simplified and may reduce sample stresses that contribute to beam-induced motion.

Anomalous Impact of Surface Wettability on Leidenfrost Effect at NanoscaleSupported by the National Key Research and Development Program of China (Grant No. 2018YFE0127800).

Leidenfrost effect is a common and important phenomenon which has many applications, however there is a limited body of knowledge about the Leidenfrost effect at the nanoscale regime. We investigate the impact of substrate wettability on Leidenfrost point temperature (LPT) of nanoscale water film via molecular dynamics simulations, and reveal a new mechanism different from that at the macroscale. In the molecular dynamics simulations, a method of monitoring density change at different heating rates is proposed to obtain accurate LPT under different surface wettability. The results show that LPT decreases firstly and then increases with the surface wettability at the nanoscale, which is different from the monotonous increasing trend at the macroscale. The mechanism is elucidated by analyzing the competitive effect of adhesion force and interfacial thermal resistance, as well as different contributions of gravity on LPT at the nanoscale and macroscale. The investigations can deepen the understanding of Leidenfrost effect at the nanoscale regime and also facilitate to guide the applications of heat transfer and flow transport.

Minimum Leidenfrost Temperature on Smooth Surfaces

During the Leidenfrost effect, a thin insulating vapor layer separates an evaporating liquid from a hot solid. Here we demonstrate that Leidenfrost vapor layers can be sustained at much lower temperatures than those required for formation. Using a high-speed electrical technique to measure the thickness of water vapor layers over smooth, metallic surfaces, we find that the explosive failure point is nearly independent of material and fluid properties, suggesting a purely hydrodynamic mechanism determines this threshold. For water vapor layers of several millimeters in size, the minimum temperature for stability is ≈140 °C, corresponding to an average vapor layer thickness of 10-20 μm.

The Minimum Temperature for Levitating Droplets

For water on hot surfaces, the Leidenfrost effect endures at temperatures much lower than those needed for onset, regardless of surface or fluid properties.

Thermosuperrepellency of a hot substrate caused by vapour percolation

Drop rebound after collision with a very hot substrate is usually attributed to the Leidenfrost effect, characterized by intensive film boiling in a thin vapour gap between the liquid and substrate. Similarly, drop impact onto a cold superhydrophobic substrate leads to a complete drop rebound, despite partial wetting of the substrate. Here we study the repellent properties of hot smooth hydrophilic substrates in the nucleate boiling, non-Leidenfrost regime and discover that the thermally induced repellency is associated with vapour percolation on the substrate. The wetting structure in the presence of the percolating vapour rivulets is analogous to the Cassie-Baxter wetting mode, which is a necessary condition for the repellency in the isothermal case. The theoretical predictions for the threshold temperature for vapour percolation agree well with the experimental data for drop rebound and correspond to the minimum heat flux when spray cooling.

Electric Charge-Induced Active Control of Nucleate and Rapid Film Boiling at the Nanoscale: a Molecular Perspective

An effort has been made to understand the nucleate and the rapid film boiling phenomena under the influence of an electric field using a molecular viewpoint. The behavior of water molecules with a solid copper surface during the film boiling process in the presence of an electric field of different intensities has been studied. The molecular reasoning behind the suppression of the Leidenfrost phenomenon upon application of a uniform electric field along the heating substrate is established. Furthermore, the effect of surface characteristics with different wettabilities on film boiling in the presence of an electric field has also been studied. The electric field produces a finger-like water column besides thinning of the water film over a non-wetting surface. A similar phenomenon is also evident over a hydrophilic surface only after reaching a threshold value of electric charge intensity. Molecular simulations have explained the phenomenon of nucleate boiling of water on hydrophilic or non-wetting surfaces. Finally, the ability to control the bubble formation and suppression at a required location using an electric field has also been demonstrated. The water molecules near the surface experience dispersion at a lower electric field and an attraction force at a higher electric field, mimicking bubble nucleation and suppression, respectively.

Wall Temperature Effects on Ignition Characteristics of Liquid-phase Spray Impingement for Heavy-duty Diesel Engine at Low Temperatures

ABSTRACT The effect of wall temperature on the ignition of vapor-phase spray impingement has been studied extensively, but the mechanism of the liquid-phase case is not clear. Thus, the visualization experiment was conducted at wall temperatures of 363–673 K using Mie-scattering, shadowgraph, and direct photography methods in a constant volume combustion vessel. The results reveal that, compared with the vapor-phase wetting-wall, when the liquid fuel impinges on the cold wall, the threshold temperature of the autoignition increases greatly to 363 K. Furthermore, the wall temperature strongly affects the evaporation and fuel-air mixing under the liquid impingement case. As the wall temperature increases from 423 K to 573 K, the enhanced evaporation results in a significant reduction in the spray radius, height, and area. However, the evaporation is inhibited and the spray parameters increase when the wall temperature further rises to 623–673 K. This characteristic may be caused by the Leidenfrost effect, which is the main difference from the vapor-phase impingement. Too low wall temperature (below 520 K) easily leads to misfire and unstable ignition, and the threshold temperature of the autoignition is greatly increased, while a higher wall temperature exceeding the Leidenfrost temperature is also not conducive to rapid and stable ignition. With the increase of wall temperature, the ignition delay first shortens from 4.1 ms to 3.2 ms gradually and then prolongs to 3.6 ms again, while the flame area and intensity first increase and then decrease, and the transition temperature is 573 K. When liquid wetting-wall inevitably appears in a heavy-duty diesel engine operating at low temperatures, it is necessary to raise the wall temperature moderately to ensure stable ignition. However, it should not exceed the Leidenfrost temperature, otherwise, evaporation will slow down and ignition will worsen due to the Leidenfrost effect.

Experimental study on the physical and mechanical variations of hot granite under different cooling treatments

Geothermal energy arrested in HDR (hot dry rock) is commonly harvested from the reservoir fractured by artificial stimulations. Understanding the physical and mechanical behaviors of HDR after different cooling treatments is critical for selecting most efficient fluid to fracture the rock mass. Here we experimentally examine the physical and mechanical variations of a typical type of HDR, granite, after natural, water, and liquid-nitrogen cooling treatments, followed by a multi-scale investigation on the evolution of cracks. We find that the effect of cooling treatment on the physical and mechanical properties of heated granite specimens is increasingly remarkable as the peak temperature grows. The difference among the influences of different cooling treatments is unnoticeable when the specimens are heated to peak temperatures of 200 °C and 400 °C, but becomes highly pronounced as the peak temperature ascends to 600 °C and 800 °C. Liquid-nitrogen cooling treatment induces most alterations in the granite properties. The cracks in the granite specimens cooled by liquid-nitrogen coalesce together and form complex fracture networks, which are more intensive than those by natural and water cooling treatments. Previous studies claimed that the liquid-nitrogen cooling treatment had weaker influence on the granite specimen than water, possibly due to the Leidenfrost effect that impeded the heat transfer between rock and liquid-nitrogen. Our findings demonstrate that liquid-nitrogen is superior in cracking and thus enhancing the permeability of the granite specimens, particularly for those heated to a temperature over 400 °C. The good performance of liquid-nitrogen is attributed to that the granite specimen is easily cracked by the high tensile stress created jointly by the high temperature difference caused by its ultra-low temperature and large convective heat transfer coefficient induced by violent boiling.