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Ebullition is an important route of methane emission from aquatic ecosystems. Ebullitive CH4 emissions from the wetlands, particularly the mountain wetlands of Eastern Himalayan, are poorly understood. To gain insights into the role of ebullition in CH4 emissions and understand the factors influencing CH4 ebullition, we conducted field measurements of the spatial and temporal variation of ebullition in a freshwater wetland area in floating national park (40 sq. km, 780 m amsl, maximum depth < 4.5) in Northeast India. The average ebullitive CH4 flux ranged from 220.24 to 1889.35 mg m−2 d−1, while the overall CH4 fluxes varied widely ranging from 345.81 to 2240.56 mg m−2 d−1. Methane constituted 90.18% of the gas bubbles produced from the sediment, with CO2 comprising 8.82% of the total sediment gas in the wetland. This suggests that CH4 emission through ebullition plays an important role in transporting biogenic CH4 to the atmosphere. The ebullition rate was markedly higher during summer and lower during winter and exhibited a significant seasonal variation. At a spatial scale, the sites with dense aquatic vegetation growth increase CH4 emission where plants derived autochthonous sediment organic matter, substantiating the supply of carbon substrate for CH4 production. Linear mixed-effect models revealed that water temperature, organic matter, organic carbon and dissolved organic matter are the important factors affecting the ebullitive methane flux. Our results indicate that mountainous wetlands with organic-rich sediments may be potential hotspots for CH4 ebullition. However, the lack of information on these wetlands in the scientific literature emphasizes the need for further research. [ABSTRACT FROM AUTHOR]

Nucleate boiling is a phenomenon of significant importance in a broad range of industries. Increasing boiling performance parameters could lead to more efficient power plants and better electronics thermal management. However, difficulties associated with studying this extremely complex phenomenon have prevented a meaningful progress in the area of boiling heat transfer enhancement. In this paper, we implement a laser-based controlled bubble generation technique to enable accurate phenomenological studies of the boiling process. We present details of the transient focused-laser heating mechanism used to nucleate a microscale vapor embryo on the boiling surface. We present high-speed optical imaging data showing how this vapor embryo grows into a bubble using electrically applied background heat flux. Unlike the currently available artificial bubble generation approaches, which either generate unphysical bubbles or are extremely difficult to implement, we show that the laser-nucleated controlled single bubble demonstrates bubble ebullition characteristics closely representative of naturally occurring bubbles in nucleate pool boiling. [ABSTRACT FROM AUTHOR]

The description of boiling and condensation phenomena presents a challenging problem. We propose a Lagrangian incompressible–compressible mesh-free particle method for modeling multiphase flows involving boiling and condensation. This multiphase scheme combines the moving particle semi-implicit method with the smoothed particle hydrodynamics method, while incorporating a phase transformation model based on heat transfer to simulate gas–liquid phase transformation. Gas volume expansion and shrinkage are accounted for through particle splitting and merging techniques. Numerical investigations demonstrate the effectiveness and accuracy of this multiphase method and phase transformation model, including simulations of multiphase dam-break flow, rising bubble, Stefan problem, and sucking problem. Our approach successfully simulates the boiling growth and condensing collapse of vapor bubbles, which is validated against numerical and experimental results. Stable and relatively accurate simulations can be achieved for bubble collapse processes under different water subcooling degrees, gas overheating degrees, surface tension values, and bubble sizes. The complex process of boiling before condensation can be accurately reproduced. [ABSTRACT FROM AUTHOR]

Bubble growth, departure and sliding in low-pressure flow boiling has received considerable attention in the past. However, most applications of boiling heat transfer rely on high-pressure flow boiling, for which very little is known, as experimental data are scarce and very difficult to obtain. In this work, we conduct an experiment using high-resolution optical techniques. By combining backlit shadowgraphy and phase-detection imaging, we track bubble shape and physical footprint with high spatial (6µm) and temporal (33µs) resolutions, as well as bubble size and position as bubbles nucleate and slide on top of the heated surface. We show that at pressures above 1MPa bubbles retain a spherical shape throughout the growth and sliding process. We analytically derive non-dimensional numbers to correlate bubble velocity and liquid velocity throughout the turbulent boundary layer and predict the sliding of bubbles on the surface, solely from physical properties and the bubble growth rate. We also show that these non-dimensional solutions can be leveraged to formulate elementary criteria that predict the effect of pressure and flow rate on bubble departure diameter and growth time. [ABSTRACT FROM AUTHOR]

Boiling of the coolant at the hot surface provides relatively better cooling by absorbing latent heat along with convection heat transfer as compared to heat transfer under single-phase conditions. In boiling, the orientation of heated surface also plays a crucial role. Downward facing boiling is complex than upward facing boiling, as the detachment of the bubble inhibited due to the heater surface orientation. Consequently, the bubble residence time and interaction with other bubbles are different in such boiling conditions. Our experiments on a large downward facing flat surface (100 × 400 mm2) revealed unexplored boiling phenomena. The boiling process is dominated by a complex engulfing phenomenon, which is rarely reported in the past. The engulfing phenomena have been captured using high-speed photography, wherein, at low heat fluxes, it is observed that larger bubbles engulf small bubbles by opening their mouth and swallowing the small bubbles. However, at higher heat fluxes, this phenomenon disappears. A larger vapor blanket is formed due to engulfing of bubbles, which may lead to departure from nucleate boiling. This engulfing behavior depends on the heat flux and subcooling. With the increase in heat flux, it is found that the rate of vapor engulfing increases. We have attempted to explain the science behind such engulfing phenomenon based on the capillary pressure difference. These results are consistent at various subcooling. This research provides new insights into nucleate boiling and may help in developing advanced mathematical models for accurate heat transfer prediction on downward facing nucleate boiling. [ABSTRACT FROM AUTHOR]

[Display omitted] Despite the flourishing studies of droplet interfacial boiling, the boiling upon vibratory surfaces, which may cause vigorous liquid–vapor-solid interactions, has rarely been investigated. Enhanced boiling normally can be gained from rapid removal of vapor and disturbance of liquid–vapor interface. We hypothesize that the vibratory surfaces enhance both effects with new intriguing phenomena and thus, attain an enhanced boiling heat transfer. We experimentally investigated the impacting fluid dynamics and coupled heat transfer patterns of multiple droplets and a single droplet impinging on still and vibratory surfaces of various materials and different wettability. The boiling under vibratory surfaces with increased vibration velocity amplitude and enhanced wettability can be enhanced by 80% in heat transfer coefficient and Nusselt number, which is attributed to several reasons: shortened bubble lifespan, thinner and smaller bubbles, and enhanced disturbances in liquid–vapor interfaces. The vibration also delays the Leidenfrost point when the droplet impacts a descending surface, which shows that the droplet impact moment (vibration phase angle) is particularly crucial. The descending surface releases the generated vapor actively and facilitates liquid–solid contact, thereby delaying the Leidenfrost. From fundamentals to application, this article strengthens our understanding of vibrated interfacial boiling in scenarios closer to multiple natural processes and practical industries. [ABSTRACT FROM AUTHOR]

Gravity plays a crucial role in influencing bubble behavior and heat transfer in flow boiling, and its impact can vary significantly in aerospace settings. Therefore, this study aims to numerically investigate flow boiling heat transfer in a rectangular mini-channel under hyper-gravity conditions, specifically at 12 times the normal gravity. To accomplish this, a coupled volume-of-fluid and level set method is employed, taking into account fluid-solid conjugated heat transfer as well as a nucleus site density model derived from experimental data. By reproducing the flow pattern and heat transfer characteristics under different heat flux and flow rate conditions, the study unveils the effects of hyper-gravity on flow boiling heat transfer. When the flow rate is lower under hyper-gravity conditions, a notable phenomenon occurs wherein numerous bubbles detach from the heating wall and coalesce into a vapor film at the top of the mini-channel due to increased buoyancy. In contrast, under normal gravity, bubbles merge and slide on the heating wall, leading to the formation of a dry patch below. Consequently, hyper-gravity results in a lower wall superheat, and the disparity in average wall superheat between normal and high gravities escalates as the added heat flux rises. Notably, in the hyper-gravity environment, the frequent detachment of bubbles in the middle and downstream sections of the mini-channel leads to an initial increase in wall superheating, followed by a plateau along the flow direction. As the flow rate increases, the inertial force intensifies. However, intriguingly, the discrepancy in flow boiling heat transfer between normal and high gravities does not exhibit a monotonic decrease with the increasing flow rate. This behavior can be attributed to the pressing of more bubbles onto the heating wall under normal gravity, resulting in the formation of dry patches at high velocities. [ABSTRACT FROM AUTHOR]

This work investigates the bubble dynamics during pool boiling from isolated cavities on a horizontal silicon substrate with three surface coatings, silicon oxide, Perfluorodecyltrichlorosilane, and silica nanoparticles. The experiments were conducted with FC-72 under various superheat degrees. In the case of vertical coalescence, coalescences at the boundary between the departed bubble and the subsequent nucleated bubble were observed and analyzed. Vertical coalescence appears with lower superheat degrees on Perfluorodecyltrichlorosilane coated surfaces due to the faster initial bubble growth than on the other surfaces. However, the vertical coalescence has a limited effect on the bubble departure diameter, which is nearly kept constant at around 0.6 mm. In the case of horizontal coalescence, bubbles growing on the silica nanoparticles coated surface are able to coalesce for small cavity spacing with lower superheat degree than other surfaces. Coalescing bubbles from cavities spaced 0.50 mm apart can detach just after coalescence, while for a shorter spacing of 0.25 mm, bubbles remain attached to the surface and further growth is needed for their detachment. Based on the analysis of energy removal per unit of area, the optimal heat transfer performance may be achieved when the cavities spacing is approximate to the single bubble departure diameter. [ABSTRACT FROM AUTHOR]

It is very important to accurately analyze the heat transfer characteristics of a working fluid in the cooling system based on subcooled flow boiling. In contrast to previous computational fluid dynamics (CFD) studies for the subcooled flow boiling of water or synthetic refrigerants in a vertical tube, we study the subcooled flow boiling characteristics of CO2 in a horizontal tube at -30 ℃. Our model is built based on Eulerian multi-phase model combined with the interfacial area concentration and wall heat flux partitioning models, and it is solved by the CFD solver, ANSYS FLUENT 19.3. This paper determines a reasonable bubble waiting time coefficient (CWT), critical Weber number (Wecri), and turbulence model (TM) for modelling of flow boiling heat transfer of CO2 by combining with uniform design (UD), Taguchi method, multiple quadratic regression (MQR) model and grid search method. As the result, the reasonable values of CWT and Wecri, and reasonable TM were determined as CWT = 0.8, Wecri = 0.7, and RNG k–ε model. This work has a significant contribution to study the subcooled flow boiling heat transfer characteristics of CO2 at low temperature. The proposed approach could be actively applied to improve the solution accuracy of the heat transfer of a working fluid in various heat transfer system. [ABSTRACT FROM AUTHOR]

Dissolved oxygen plays a central role for all organisms dwelling in water. However, the flux of oxygen by ebullition has not received much attention in environmental science. For a better quantitative understanding of the oxygen flux due to ebullition, we conducted a series of laboratory experiments, where we forced macrophytes to produce photosynthetic gas bubbles. Raising the CO2 concentration in the water greatly increased bubble formation. Depth was varied to compare the results with theoretically predicted composition of photosynthetic bubbles forming at minimum required gas pressure. Oxygen concentrations lay between this theoretical line as lower boundary (ca. 21% O2 at 0.3 m depth and 45% of O2 at 4.5 m) and 45% of oxygen as the purely empirical upper limit for all depths. As a consequence, no bubble formation was observed at depths below 4.5 m. Plain Language Summary: Under light, green plants produce oxygen, when they grow. Submerged plants can produce bubbles under these conditions. Some of these bubbles rise to the surface and hence transport oxygen out of the water into the atmosphere. We studied this process by measuring the composition of such bubbles in a laboratory experiment. Adding carbonic acid to the water boosted the bubble production a lot. Depending of depth, there is a minimum of gases required for bubble formation, which yields a theoretical minimum concentration for oxygen. In all experiments, the oxygen concentration of bubbles was above atmospheric percentage of 21%, but always below 45%. Key Points: Photosynthetic gas bubbles from submerged macrophytes contain other gases beyond oxygenThe depth‐dependent lower limit can be derived from minimum gas pressure consideration for the formation of bubblesEmpirically we find an upper limit of 45% of oxygen in the collected gas bubbles [ABSTRACT FROM AUTHOR]

Bubble nucleation is the initial stage of flow boiling and plays an important role in boiling heat transfer. However, bubble nucleation occurs at a microscopic scale, rendering it challenging for the macroscopic computational fluid dynamics method to realistically simulate this intricate process. In this paper, based on the coupled volume-of-fluid and level set method, a heterogeneous nucleation model is improved and conducted to simulate the subcooled flow boiling in a rectangular mini-channel, considering these conditions both in the presence and absence of a microlayer. The coefficient of the original heterogeneous nucleation model is adjusted across a range from 0.1 to 10.0 times its previous value to establish multiple new nucleation models for illustrating their effects on flow patterns and heat transfer characteristics. For flow boiling without a microlayer, when the coefficient of the original heterogeneous nucleation model is halved, the nucleate boiling intensity upstream of the channel diminishes, resulting in a reduction in the heat transfer coefficient. Nevertheless, this alteration mitigates the formation of slug flow and the appearance of dry patches near the channel outlet, consequently averting a sharp increase in outlet wall superheat. Quantitatively, relative differences of 23.83% and 90.48% in average and local maximum wall superheat are observed, respectively. In contrast, the presence of a very thin microlayer beneath the growing and slipping bubble in flow boiling with a microlayer is notable. This microlayer quickly evaporates, dissipating more than 77% of the input heat flux and substantially expanding the bubble volume. Consequently, under identical wall superheat conditions, the influence of variations in the number of activated bubbles induced by different heterogeneous nucleation models on heat transfer and flow patterns in flow boiling is significantly attenuated. Specifically, when the difference in nucleus site density remains within a tenfold range, the differences in the average and maximum wall superheat are limited to just 16.78% and 33.86%, respectively. Concerning flow boiling in a mini-channel featuring a microlayer, the simulation results verify that large deviations in the activated bubble number have few effects on the flow pattern and wall superheat, greatly reducing heterogeneous nucleation model requirement and promoting the numerical study of flow boiling. [ABSTRACT FROM AUTHOR]

In this study, deionized water pool boiling experiments were performed on austenitic stainless steel AISI 304 steel balls with different initial temperatures. The disturbance and rupture of the vapor film during the quenching process are obtained by a high-speed camera. The temperature profile of the steel ball center during cooling was obtained by thermocouple temperature measurement. Based on the inverse heat conduction method, the inverse heat transfer program was independently developed to solve the surface temperature of the steel ball, the interfacial heat transfer coefficient and the heat flux. The results show that there is no film boiling when the steel ball temperature is 650 °C. There are four stages of film boiling, transition boiling, nucleate boiling and natural convection when the steel ball temperature is between 700 and 1000 °C. The thickness of the vapor film increases with the increase of the initial quenching temperature. At an initial temperature of 1000 °C, the thickness of the vapor film in the equatorial direction is 3.5 to 3.6 mm. With the increase of the initial temperature, the duration and rupture time of the vapor film were prolonged, and the central and boundary LFP temperatures gradually decreased. When the initial temperature is between 700 ~ 1000 °C, the vapor film lasts about 6.5 ~ 32.1 s, and the LFP temperatures at the center are between 612.3 °C and 514.7 °C, respectively. When the initial temperature is 650 °C, the maximum IHTC value is about 11,700 W/(m2·℃), and the critical heat flux is 1545.85 kW/m2. As the initial temperature is 1000 °C, the maximum IHTC value is about 14,900 W/(m2·°C), and the critical heat flux is 1508.43 kW/m2. [ABSTRACT FROM AUTHOR]

[Display omitted] Despite the flourishing studies of Leidenfrost droplet motion in its boiling regime, the droplet motion across different boiling regimes has rarely been focused on, where bubbles are generated at the solid–liquid interface. These bubbles are probable to dramatically alter the dynamics of Leidenfrost droplets, creating some intriguing phenomena of droplet motion. Hydrophilic, hydrophobic, and superhydrophobic substrates with a temperature gradient are designed, and Leidenfrost droplets with diverse fluid types, volumes, and velocities travel from the hot end to the cold end of the substrate. The behaviors of droplet motion across different boiling regimes are recorded and depicted in a phase diagram. A special phenomenon of Leidenfrost droplets that resembles a jet engine is witnessed on a hydrophilic substrate with a temperature gradient: the droplet traveling across boiling regimes repulsing itself backward. The mechanism of repulsive motion is the reverse thrust from fierce bubble ejection when droplets meet nucleate boiling regime, which cannot take place on hydrophobic and superhydrophobic substrates. We further demonstrate that conflicting droplet motions can occur in similar conditions, and a model is developed to predict the occurring criteria of this phenomenon for droplets in diverse working conditions, which agrees well with the experimental data. [ABSTRACT FROM AUTHOR]

The physical parameters exerting an effect on the density of the heat flux transported by vapor bubbles during nucleate boiling are considered. It is shown that this density depends on the radius of bubble departure from the substrate, the rate of formation of departing bubbles per unit area of the substrate, the thermophysical properties of the liquid, and the saturation temperature. It has been established that the density of the energy flux transported by bubbles increases exponentially with the saturation temperature and depends strongly on the bubble departure radius. It is shown that the thickness of the superheated liquid layer, inside which bubbles grow, does not exceed several hundred microns and depends on the total energy flux. In this case, if the average distance between the bubbles is comparable to the departure radius, bubble coagulation leads to a boiling crisis. [ABSTRACT FROM AUTHOR]

Immersion cooling is widely used for thermal management of servers. The two-phase immersion cooling, which transfers heat by boiling, possesses efficient temperature control ability under intensive heat generation. In the process of temperature control through boiling, the generation and transportation of bubbles play a crucial role in calculating the heat-transfer capacity. Therefore, it holds immense significance to obtain a profound understanding of the mechanisms underlying bubble formation and detachment. Currently, numerous mechanistic explanations and empirical correlations have been proposed to elucidate the various parameters of bubbles during the boiling process. These findings were considered to be valuable references when selecting appropriate boiling media and designing efficient heating surfaces. To comprehensively present the progress of bubble formation and heat transfer in the boiling system, the forces exerted on the bubbles are highlighted in this article. A meticulous review of bubble-force analysis and correlation formulae pertaining to various relevant parameters (e.g., nucleation sites density, bubble growth rate, bubble growth period, and detachment frequency) was conducted. This review article was also expected to provide a novel foundation for further exploration of enhanced boiling heat transfer. [ABSTRACT FROM AUTHOR]

A wide range of recent applications require high-quality sprays that are characterized by extremely small-sized droplets, a narrow droplet size distribution, and a short breakup length. Fuel injection systems in propulsion units, energy storage, medical implementations, printings, and coatings are just a few examples. Flash-boiling atomization is a unique method that was extensively developed during the past two to three decades and has been proven to generate high-quality demanded sprays. In flash-boiling atomization, the liquid is forced to reach a metastable superheated state by either rapid heating or rapid pressure drop, where vapor bubbles nucleate, become fast-growing, and subsequently break down the liquid into a fine spray in a very short time. This present article focuses on flash-boiling atomization via rapid depressurization, which is presently more relevant to energy systems. The field of flash-boiling atomization has seen rapid growth and popularity in the past two decades. The aim of this article is to quantitatively portray the landscape and evolutionary trajectory of flash-boiling atomization research and applications and to detect new research frontiers and emerging trends in the literature on flash-boiling atomization. We briefly review the basic theories of the flash-boiling atomization mechanism present a comprehensive overview of the field, from its birth in approximately the 1970s to the present, and provide a database comprising 386 articles published on the topic of flash-boiling atomization. We visualize the intellectual structure of flash-boiling atomization research and applications and track its evolvement over the past five decades, thus providing a global overview and a comprehensive understanding of the development of flash-boiling atomization research and emerging applications. [ABSTRACT FROM AUTHOR]

Optimizing heat transfer in the evaporator tubes is essential for obtaining optimal coefficients. Deviation from ideal nucleated boiling configurations can lead to rapid transitions to film boiling conditions, compromising the exchange coefficients. Detecting critical bubble flow conditions is invaluable in preventing thermal overloads and safeguarding operational integrity. The aim is to detect the type of bubble motion established in the ducts using non-intrusive techniques. To achieve this aim, accelerometric devices detect the vibrations induced by the passage of bubbles. The bubble's movement is also recorded contextually by a fast video camera capable of detecting images through a vertical transparent quartz duct. The accelerometric data, processed using discriminant analysis theory and then associated with the recorded images, give a good relationship, more than 95%, between the motion of the bubbles and their behavior characteristics. The latter in this work is represented on the plane of the phases. [ABSTRACT FROM AUTHOR]

Amidst the background of "double carbon", energy saving and emission reduction is a popular direction in the current refrigeration industry. Therefore, the research on the boiling heat transfer of gas–liquid two-phase flow is helpful to strengthen the heat transfer and design a more efficient heat exchanger. In this paper, a research method combining numerical simulation and experimental verification is adopted. Firstly, an experimental platform used for the subcooled flow boiling of refrigerant in casing tubes is introduced and experiments are carried out to obtain experimental data, which provides a theoretical basis for the development of numerical simulation and verifies the feasibility of numerical simulation. A numerical model of subcooled flow boiling in R22 was established and the grid independence test was carried out. Based on the simulation results, three factors affecting the boiling heat transfer of R22 are analyzed: First, the boiling heat transfer coefficient of R22 increases with the increase of the mass flow rate of R22, but the increase decreases when the mass flow rate increases from 0.018 kg/s to 0.020 kg/s. Second, the boiling heat transfer coefficient of R22 increases significantly with the increase of hot water flow rate. Third, the influence of R22 subcooling on boiling heat transfer is more complex. When the subcooling is 5 °C and 1 °C, heat transfer can be enhanced; high subcooling at 5 °C can enhance convective heat transfer and low subcooling at 1 °C can accelerate the arrival of saturated boiling. In this paper, three kinds of bubble behaviors affecting heat transfer in supercooled flow boiling, including sliding, polymerization, and bounce are also studied, which provides a basis for further research on heat transfer mechanism of supercooled flow boiling. [ABSTRACT FROM AUTHOR]

Droplet evaporation has great scientific significance in industrial and agricultural production as well as natural meteorological processes. Among many factors that affect evaporation, wettability and wall temperature are among the most important. The Lattice Boltzmann Method (LBM) was used to explore the effects of evaporative wall wettability and wall temperature on the Constant Contact Angle (CCA) model. We constructed a dual-distribution gas–liquid phase transition model and found that the higher the wall temperature, the more rapid the evaporation; however, nucleate boiling was more likely to occur at high temperatures, making the CCA (and thus evaporation) unstable. Compared to hydrophobic surfaces, hydrophilic surfaces have larger droplet spreading area, and generally higher evaporation rate; as the contact diameter decreased, droplet diffusion occurred and evaporation to dryness was observed. Droplets on hydrophobic surfaces tended to form nucleus of boiling, thus generating bubble and gas films. After the droplets shrunk and stabilized, the contact diameter reduced and evaporation slowed. Droplet evaporation can be regulated to improve energy efficiency depending on different applications. [ABSTRACT FROM AUTHOR]

Understanding the detailed dynamics of bubble ebullition cycle is central to effectively exploit coolant phase change in subcooled flow boiling. In the present study, the bubble growth rate and the bubble departure mechanism in subcooled flow boiling conditions are investigated. The bubble growth rate is estimated by employing an energy balance model, which ensures that the applied wall heat flux contribution to components, such as (i) microlayer evaporation, (ii) conduction through superheated layer region, and (iii) condensation heat transfer, are accounted. The bubble departure diameter and the type of departure (i.e., sliding or lift-off) are predicted using a simple force balance model on the vapor bubble. The implemented models are thoroughly validated against the benchmark experimental data. Furthermore, the influence of operating conditions, viz., pressure (1 – 3 bar), mass flux (200 – 1000 kgm − 2 s − 1 ), heat flux (200 – 500 kWm − 2 ), and the degree of subcooling (20–30 K) on the bubble dynamics, is investigated for subcooled flow boiling conditions. Based on the parameters considered, a flow regime map is developed to identify the bubble departure type and diameter for a given set of operating conditions. It was noticed that the bubble departure diameter was maximum at low pressure (1 bar), low mass flux (200 kgm − 2 s − 1) , high heat flux (500 kWm − 2) , and low degree of subcooling (20 °C). For all the values of pressure and degree of subcooling, the sliding mode of departure was noticed at low and high mass flux values, whereas bubble departs by lift-off for moderate values of mass flux. [ABSTRACT FROM AUTHOR]

This paper presents the results of an experimental study on the evolution of a nonstationary temperature field during ethanol pool boiling in a pressure range of 12–101.2 kPa. Experimental data were obtained using infrared thermography with high temporal and spatial resolutions, which made it possible to reconstruct the distribution of the heat flux density and to study the influence of pressure reduction on the local heat transfer rate in the vicinity of the triple contact line under vapor bubbles for the first time. It is shown that, for all studied pressures, a significant heat flux density is removed from the heating surface due to microlayer evaporation, which exceeds the input heat power by a factor of 3.3–27.7, depending on the pressure. Meanwhile, the heat transfer rate in the area of the microlayer evaporation significantly decreases with the pressure reduction. In particular, the local heat flux density averaged over the microlayer area decreases by four times as the pressure decreases from 101.3 kPa to 12 kPa. Estimates of the microlayer profile based on the heat conduction equation were made, which showed the significant increase in the microlayer thickness with the pressure reduction. [ABSTRACT FROM AUTHOR]

The numerical method was used to study bubble sliding characteristics and dynamics of R134a during subcooled flow boiling in a narrow gap. In the numerical method, the volume of fraction (VOF) model, level set method, Lee phase change model and the SST k − ω turbulent model were adopted for the construction of the subcooled flow boiling model. In order to explore bubble sliding dynamics during subcooled flow boiling, the bubble sliding model was introduced. The bubble velocity, bubble departure diameter, sliding distance and bubble sliding dynamics were investigated at 0.2 to 5 m/s inlet velocities. The simulation results showed that the bubble velocity at the flow direction was the most important contribution to bubble velocity. Additionally, the bubble velocity of 12 bubbles mostly oscillated with time during the sliding process at 0.2 to 0.6 m/s inlet velocities, while the bubble velocity increased during the sliding process due to the bubble having had a certain inertia at 2 to 5 m/s inlet velocities. It was also found that the average bubble velocity in flow direction accounted for about 80% of the mainstream velocities at 0.2 to 5 m/s. In the investigation of bubble sliding distance and departure diameter, it was concluded that the ratio of the maximum sliding distance to the minimum sliding distance was close to two at inlet velocities of 0.3 to 5 m/s. Moreover, with increasing inlet velocity, the average sliding distance increased significantly. The average bubble departure diameter obviously increased from 0.2 to 0.5 m/s inlet velocity and greatly reduced after 0.6 m/s. Finally, the investigations of the bubble sliding dynamics showed that the surface tension dominated the bubble sliding process at 0.2 to 0.6 m/s inlet velocities. However, the drag force dominated the bubble sliding process at 2 to 5 m/s inlet velocities. [ABSTRACT FROM AUTHOR]

In the present study, three-dimensional numerical simulations are performed to examine the effect of applied direct current electric fields on the subcooled flow boiling heat transfer in a vertical minichannel. The volume of fluid model is used to capture the liquid–vapor interfaces, and the conservation laws, together with the electrohydrodynamic (EHD) equations, are solved using the finite volume method. Two mass transfer models of Lee and Fourier are evaluated, and a new seeding algorithm is developed based on the physics of the bubble formation and departure on the heated walls, integrated with the Fourier model. The early transition from slug to churn/annular flow regimes, due to EHD forces, is observed in the numerical solutions with both models, in agreement with the available experiment. Nevertheless, the Lee model tuned for the electric-free condition fails to predict the EHD-induced heat transfer enhancement, while the Fourier model with the new seeding algorithm captures this phenomenon with reasonable accuracy. Based on the present numerical results, this effect can be attributed to the migration of small bubbles toward the walls under the effect of electric fields, forming recirculating flow regions near the walls, augmenting the flow mixing, and mitigating hot spots. [ABSTRACT FROM AUTHOR]

The current work aims to experimentally evaluate the effect of the size of circular superhydrophobic regions of biphilic surfaces on the bubble dynamics under pool boiling conditions. Biphilic surfaces are structured surfaces with tunable wettability, presenting an array of hydrophobic small spots in a hydrophilic surface or vice versa. The factors that affect the bubble dynamics are of geometric nature such as the diameters of the bubbles, their volume, and the height of the centroid, and of more complex nature such as the departure frequency of the bubbles and the rate of evaporation mass transfer. In this study, the bubble dynamics and boiling performance were evaluated by adjusting the diameter of the single circular superhydrophobic regions. A stainless steel AISI 304 foil was used as the base hydrophilic region, and the superhydrophobic regions were made by spray coating the NeverWet® superhydrophobic solution over well-defined masks. The main conclusion was that the bubble dynamics are clearly affected by the diameter of the superhydrophobic spots. The smaller spots favored the generation of more uniform and stable bubbles, mainly due to the border surface tension forces' dominance. With the increase in the diameter of the bubbles, the surface tension acting at the border with the much larger hydrophilic region impacts the process less. Thus, the smaller superhydrophobic regions had higher evaporation mass transfer rates. The region with the best pool boiling performance along with improved bubble dynamics was the superhydrophobic region with an 0.8 mm diameter, corresponding to a superhydrophobic area to total area ratio of 0.11%. Moreover, this experimental work confirmed that the bubble dynamics' impacting factors such as the diameter at the various stages of development of the bubbles can be modulated according to the final objectives of the design and fabrication of the biphilic surfaces. The research significance and novelty of this work come from the comprehensive study of the geometrical pattern of the heat transfer surface in pool boiling conditions and its impact on the bubble dynamics and heat transfer capability. We also suggest further studies considering nanoscale superhydrophobic spot arrangements and the future usage of different working fluids such as nanofluids. [ABSTRACT FROM AUTHOR]

Due to the importance of the boiling phenomena and the need of studying the bubble nucleation in different conditions, a series of experiments have been performed in the present study to investigate a F e 3 O 4 magnetic nanofluid subcooled flow boiling phenomenon inside an annulus. The test section is made of outer glass and inner stainless-steel tubes. The effect of three important parameters, including mass flux, inlet subcooling temperature, and pressure on the bubble liftoff diameter and frequency in the absence and presence of a magnetic field caused by quadrupole magnets, have been considered. The experiments were visualized by using a superzoom camera to study the bubble behaviors on the heating surface. The obtained results show that increasing the mass flux, subcooling temperature, and pressure contributes to decreasing bubble liftoff diameter both in the absence and presence of the magnetic field during the flow boiling. The bubble liftoff diameter decreases by applying the magnetic field, and the bubbles lift from the surface faster. Moreover, the bubble liftoff frequency showed stochastic behavior such as dependence on heat transfer surface, bubble nucleation site properties, and fluid velocity. The obtained results show that the bubble liftoff frequency decrease with increasing the fluid mass flux. [ABSTRACT FROM AUTHOR]

We present the results of a study aimed at investigating the effects of electric fields on quasi-static bubble departure dynamics during pool boiling of perfluorohexane (FC-72) in microgravity conditions. Analysis was performed through an alternative formulation of the bubble momentum balance in which the contribution of non-uniform electric stress distributions at the bubble interface can be quantified through high-speed video measurements without having to numerically solve the laws of electrostatics. Data used in this study were obtained in the scope of the Multiscale Boiling Project, which included advanced single bubble growth experiments performed aboard the International Space Station. Our results confirm that bubble departure counterintuitively begins before the force resulting from electric stresses starts to pull the bubble up from the wall. When this occurs, it is shown that the shrinking process of the contact line accelerates, in agreement with known theoretical results. It is concluded that the electric force is essentially determined by the electric stress distribution at the bubble cap above the contact area. Furthermore, we show that the electric stress at the bubble interface is also responsible for the increase in bubble internal overpressure, which explains the early departure of the bubble while increasing the intensity of the electric field. The results of this study provide an important step in achieving a more comprehensive understanding of the bubble behavior at the heated surface in the presence of an electric field, which is essential to optimally design electrodes and two-phase heat transfer devices for future space applications. [ABSTRACT FROM AUTHOR]

Droplet impact on a heated substrate is an important area of study in spray cooling applications. On substrates significantly hotter than the saturation temperature, droplets immediately hover on its vapor cushion, exhibiting the Leidenfrost phenomenon. Here, we report the phenomena wherein addition of Al2O3 nanoparticles to water significantly increases the onset of dynamic Leidenfrost temperature (TDL) and suppresses the overall Leidenfrost regime. We experimentally revealed that the onset of TDL delays with increasing the nanoparticle concentration of the colloidal dispersions at a particular Weber number (We). But, for a constant concentration, the onset of TDL decreases with an increase in impact We. In contrast to water droplets, the colloid droplets exhibit vigorous spraying behavior due to the nanoparticulate residue deposition during the spreading and retraction stages. Further, the residue on the heated substrate changes the departure diameter of the vapor bubbles during boiling, prevents bubble coalescence and vapor layer formation, and reduces the propensity to attain dynamic Leidenfrost regime. With the aid of scaling analysis of TDL with impact We, we have explored the thermo-hydrodynamic behavior of impacting colloid droplets on a superheated substrate. Finally, we have also segregated the different boiling regimes of colloid droplets over various impact We. [ABSTRACT FROM AUTHOR]

In this paper, by using a phase-change lattice Boltzmann (LB) model coupled with an electric field model, we numerically investigate the performance and enhancement mechanism of pool boiling of dielectric fluids on pillar-structured surfaces under an electric field. The numerical investigation reveals that applying an electric field causes both positive and negative influences on the pool boiling of dielectric fluids on pillar-structured surfaces. It is found that under the action of an electric field, the electric force prevents the bubbles nucleated in the channels from crossing the edges of the pillar tops. On the one hand, such an effect results in the bubble coalescence in the channels and blocks the paths of liquid supply for the channels, which leads to the deterioration of pool boiling in the medium-superheat regime. On the other hand, it prevents the coalescence between the bubbles in the channels and those on the pillar tops, which suppresses the formation of a continuous vapor film and, therefore, delays the occurrence of a boiling crisis. Meanwhile, the electric force can promote the departure of the bubbles on the pillar tops. Accordingly, the critical heat flux (CHF) can be improved. Based on the revealed mechanism, wettability-modified regions are applied to the pillar tops for further enhancing the boiling heat transfer. It is shown that the boiling performance on pillar-structured surfaces can be enhanced synergistically with the CHF being increased by imposing an electric field and the maximum heat transfer coefficient being improved by applying mixed wettability to the pillar-structured surfaces. [ABSTRACT FROM AUTHOR]

In boiling phenomena, the heater surface gets covered by a vapor, which results in a decrease in the overall heat transfer coefficient (HTC). The curvature of the surface helps to reduce this vapor blanketing by vapor sliding over the curved surface. The sliding generates fluid circulation and makes the heater surface available for boiling liquid. Therefore, in the present work, the vapor dynamics, nucleation and growth, pinch‐off of the bubble, and heat transfer characteristics over smooth and uniform surfaces with different curvature ratios (δ = 0, 0.01, 0.05, 0.15, and 0.25) are numerically studied. Also, the effect of the degree of superheat (ΔT = 10, 20, 30, and 40 K) on the bubble formation, vapor sliding, and fluid recirculation over the curved surface have been observed. Furthermore, investigated the dome formation and multiple pinch‐off of the bubble on the heated curved surfaces. The dominance of the curvature and degree of superheat on the bubble departure has been investigated using PDF of FFT of vapor void fraction at the surface. At higher curvature of the surface, there is less significance of the degree of superheat on the departure frequency of vapor bubbles. Furthermore, it is found that there is an increase in the average HTC with an increase in the curvature ratio, while the pinch‐off time of the bubble decreases. The HTC is enhanced by 86% with curved surfaces as compared to flat surfaces. Furthermore, the curved surface with a curvature (δ) of 0.15 showed better performance than the curved surface with δ = 0.25. Boiling over a curved surface leads to vapor sliding and recirculation of the fluid, leading to enhancement in surface renewal and an increase in HTC. [ABSTRACT FROM AUTHOR]

This study focuses on the numerical illustration of bubble growth and addresses the inherent interface deformation problem associated with phase change in the Volume of Fluid (VOF) interface tracking method. To avoid the interface deformation, this study proposes a cell alternation algorithm that could simulate bubble growth with a sharp interface. The proposed algorithm identifies two types of cells near the vicinity of the interface, which include the primary-interface-cell and small-interface-cell. This algorithm computes mass transfer associated with a phase change at the primary interface cell and relocates it to a nearby cell full of vapor. The predictive accuracy of the proposed method is assessed with a one-dimensional Stefan phase change problem. In the case of the Stefan phase change problem, the results of simulations agree well with the results of the theoretical solution, and the predictive accuracy increases as the grid are refined. In addition, the proposed method is verified with two engineering problems which include bubble growth on a constant temperature surface and bubble growth in a microchannel. The simulation results show a good agreement with the results of the experiment in terms of bubble growth rate, bubble shape, and other parameters associated with experiment. Also, an insight into the temperature and velocity fields is provided in simulation which cannot be achieved directly through experiment. [ABSTRACT FROM AUTHOR]

In the present study, the response surface methodology is used to predict the bubble departure frequency of pure liquids using experimental data. Water, ethanol and methanol pure liquids were used as the testing fluid. The effects of vapor and liquid density difference, vapor and liquid viscosity, surface tension, thermal conductivity, heat flux on the vapor bubbles departure frequency on the heat transfer of boiling pool of pure liquids were investigated by response surface methodology. The results showed that the output of the Response surface methodology had a good overlap with the data of bubbles departure frequency of pure liquids. Also, the results for the bubble departure frequency show a good overlap between the models presented by the researchers and the experimental data and have good accuracy. In this research, a new model for the prediction of vapor bubble departure frequency, based on the Buckingham theory, in nucleate boiling is proposed, which predicts the experimental data with a satisfactory accuracy (9%). [ABSTRACT FROM AUTHOR]

Under a low flow rate, gravity may become prominent for bubble behavior and heat transfer of flowing boiling because of the weakness of drag force from liquid, and its effect changes with the inclination angle of the mini-channel but without consensus. In this paper, based on a reasonable nucleus site density model and considering conjugate heat transfer, the coupled volume-of-fluid and level set method is adopted to study the subcooled flow boiling in an inclined three-dimensional rectangular mini-channel (0°–180°) with a characteristic size of 1.0 mm at a low flow rate of 88.8 kg m−2 s−1. The inclination angle is found to have a slight effect on the flow boiling, which is different from the conclusion drawn based on the traditional-macro channel. A bubbly flow appears when a heat flux of 300 kW/m2 is added. An unconventional impact force is proposed, which presses large bubbles to slip along the heating wall, with slight differences in the flow pattern under different inclination angles. When the inclination angle is close to 0°, the upstream small/medium bubbles leave the heating wall under gravitational effects, which is conducive to heat transfer. As the inclination angle approaches 90°, gravity pushes the large bubbles downstream to leave the channel, favoring the rewetting of the dry patches below. These two positive effects fail as the inclination angle approaches 180°, leading to slightly worse overall heat exchange efficiency. However, the maximum differences in the average and local wall superheating of the mini-channel are only 8.4% and 22.5%, respectively, across the range of inclination angles because the flow pattern remains similar under the effect of the impact force. In addition, the effect of inclination angle on flow boiling becomes weaker with the increase in heat flux because of happening of slug flow. [ABSTRACT FROM AUTHOR]

A multimaterial car body design, using light materials in combination with steel, has become widespread in recent years in order to reduce a vehicle's weight. The joining of dissimilar materials is one of the most critical issues in this area. In our previous research, we reported on a new dissimilar laser lap joining method, using a cold-spray process. In that procedure, a steel coating was first sprayed onto an aluminum surface, after which a steel sheet, together with the coating, was laser welded. However, it was found that porosity occurred in the weld metal due to melting of the coating. In this study, we investigated the effects of welding parameters on porosity formation in this joining process as well as the behavior patterns of the flow and bubbles in a molten pool. The porosity increased as the laser input energy decreased. It was also discovered that the porosity could be reduced drastically by the melting of the aluminum substrate, together with the steel sheet and coating, even at lower input energy. Electron probe microanalyzer results showed that both the solidification rate, related to the input energy, and aluminum's melting were strongly associated with the generation of flow from the bottom to the surface of the molten pool. This was captured by means of an x-ray in situ observation, using tungsten tracers, behind the keyhole. The bubbles, moving upward and reaching the surface of the pool, were also observed there in the welding conditions, where only a little porosity occurred. These results demonstrated that this flow helped the bubbles move up and go outside the molten pool and, hence, prevented porosity formation. [ABSTRACT FROM AUTHOR]

Solar-driven green hydrogen production is a promising sustainable technology though large-scale solar water splitting remains challenging due to expensive electrocatalyst. Herein, we propose a facile and low-cost approach to fabricate nanoengineered hierarchically porous Ni 5 P 4 electrocatalyst (p-Ni 5 P 4), which provides high surface area and enables fast gas diffusion. By coating p-Ni 5 P 4 on Ni foam (NF), p-Ni 5 P 4 @NF has pores ranging from 50-500 nm to 200–600 μm and enlarges the electrochemically-active surface area by 5 times. p-Ni 5 P 4 @NF offers abundant cavities for gas nucleation and fast growth of H 2 bubble, and small bubbles (10–50 μm) depart quickly owing to the reduced contact line. By enhancing gas transfer and reducing bubble overpotential over 29%, p-Ni 5 P 4 @NF achieves excellent electrocatalytic performance with a low HER (145 mV), OER (197 mV) and overall water splitting potential (1.54 V) at 10 mA cm−2. Powered by multijunction PV cell, the full electrolytic cell records a high solar-to-hydrogen efficiency of 14.5%. [Display omitted] • Hierarchically porous Ni 5 P 4 electrocatalyst is fabricated with a facile and low-cost approach. • Bubble dynamics at electrode-electrolyte interface affect transport overpotential. • Nanoscale pores of Ni 5 P 4 accelerate bubble departure and reduce bubble overpotential over 29%. • p-Ni 5 P 4 @NF achieves excellent performance as HER/OER bifunctional electrocatalyst. • PV-driven electrolytic cell achieves an outstanding solar-to-H 2 efficiency of 14.5%. [ABSTRACT FROM AUTHOR]

In the boiling process, the growth, separation, and movement of bubbles are expeditious. The visualization experiment of nucleate boiling was carried out with the help of high-speed photography. The evolution of the entire bubble life cycle is clearly observed at the nucleation site without interference from the leading and neighboring bubbles. Bubble behavior at the local heating surface has strong randomness due to the influence of the wall micro-structure, convection intensity, heating surface geometry configuration, heat flux density, and so on, but bubble behavior also has a certain regularity. In this paper, the behavior characteristics of bubbles were analyzed, with a particular focus on the evolution of bubbles. Under lower load (ΔTsat = 8~9 °C) in study conditions, nucleation sites have a long enough time interval. In addition, the bubble separation and rising velocity obviously increase due to the change of pool boiling flow characteristics in the restricted space. The setting of confined space increases the bubble escape velocity and the rising velocity, and decreases the diameter of bubbles escaping from the wall. The results will provide some help for the understanding of bubble behavior mechanisms and numerical research. [ABSTRACT FROM AUTHOR]

Bubble lift-off diameter is characterized as the size of a bubble rising from a wall, which is vital in the boundary condition of heat transfer model and interfacial area transport equation. In this paper, mechanistic force analysis was conducted to explore a predictive model for bubble lift-off diameter in a vertical channel of subcooled boiling flow. Specifically, the component of growth force normal to the wall and the shear lift force lead to the lift-off of a bubble on the vertical surface. Through force analysis, we found that bubble lift-off diameter is arranged to be related to wall superheat, latent heat, liquid velocity, fluid properties, bulk liquid subcooling, etc. To account for the contribution of the influencing factors, the dimensionless bubble lift-off diameter was correlated with dimensionless parameters, including the Prandtl number, the Reynolds number, the Jacob number, and dimensionless subcooling. The proposed correlation was assessed according to experimental data and the predictions showed good agreement with the data. [ABSTRACT FROM AUTHOR]

The free energy of formation of a vapor bubble in a conical cavity of a substrate is calculated. An approximate formula is obtained for the characteristic length of the critical bubble. It has been established that the length of such a bubble depends on the angle of wetting and is inversely proportional to the overheating of the liquid. For freon R495, the calculated dependences of the free energy of the critical bubble on the cone apex angle and the wetting angle are given. It is shown that in the case of the same heating in a fairly wide range of these angles, the free energy of bubble formation is much less than in a homogeneous liquid. [ABSTRACT FROM AUTHOR]

This study develops robust numerical algorithms for the simulation of three-phase (solid-liquid-gas) boiling and bubble formation problems in rectangular channels. The numerical algorithms are based on the Finite Volume Methods (FVM) and implement both the volume-of-fluid (VOF) methods for liquid-gas interface tracking as well as the volume-fraction methods to account for the concentration of embedded solid nano-particles in the liquid phase. Water is used as the base-liquid and the solid phase is modelled via metallic nano-particles (both aluminium oxide and titanium oxide nano-particles are considered) that are homogeneously mixed within the liquid phase. The gas phase is considered as a vapour arising from the bolling processes of the liquid-phase. The finite volume methodology is implemented on the OpenFOAM software platform, specifically by careful modification and manipulation of existing OpenFOAM solvers. The governing fluid dynamical equations, for the three-phase boiling problem, take into account the thermal conductivity effects of the solid (nano-particle), the momentum and energy equations for both the liquid-phase and the gas-phase, and finally the decoupled mass conservation equations for the liquid- and gas- phases. The decoupled mass conservation equations are specifically used to model the phase change between the liquid- and gas- phases. In addition to the FVM and VOF numerical methodologies for the discretization of the governing equations, the pressure-velocity coupling is resolved via the PIMPLE algorithm, a combination of the Pressure Implicit with Splitting of Operator (PISO) and the Semi-Implicit Method for Pressure-Linked Equations (SIMPLE) algorithms. The computational results are presented graphically with respect to variations in time as well as in the nano-particle volume fractions. The simulations and results accurately capture the formation of vapour bubbles in the two-phase (particle-free) liquid-gas flow and additionally the computational algorithms are similarly demonstrated to accurately illustrate and capture simulated boiling processes. The presence of the nano-particles is demonstrated to enhance the heat-transfer, boiling, and bubble formation processes. [ABSTRACT FROM AUTHOR]

Flow boiling is a common phenomenon in modern technology and industries. A finite difference Front Tracking method is used to study film boiling on a flat plate subject to a constant heat flux. Large density ratios are examined (1000, 2000). The flow structure including the circulation zones, the vortex developing areas, and vortex pair generation are studied in detail. The numerical method is validated by comparing the result with the analytic solution for a simple problem. Large bubbles comparable to the computational domain are formed at large density ratios. The bubble either breaks up and departs from the rest of the gas, or a jet is formed depending on the heat flux imposed. It is found that the Nusselt number depends on the heat flux on the wall. Specifically, the Nusselt number decreases as the heat flux is raised. The Morton number also affects the Nusselt number in agreement with experimental correlations. The Nusselt number increases when the Morton number is raised. The results obtained agree with experimental correlations at relatively low heat fluxes. Agreement gets worse at large heat fluxes. Experimental correlations estimate larger Nusselt numbers compared to the present study. The flow enhances along the direction of the gravitational acceleration at large density ratios (2000). A wide neck is formed that stabilizes at a specific position inside the flow. The microstructure of the flow shows a vortex pair that develops beneath the root of the bubble at a large density ratio (2000). [ABSTRACT FROM AUTHOR]

Having published some of the most widely cited publications over the last 30 years, Professor Satish G. Kandlikar has been among the most prominent researcher in the field of heat transfer. Throughout these years, he has coauthored over 200 journal paper in various areas of heat transfer. This paper provides a comprehensive look at Professor Kandlikar's research work over the years. The research work has been broadly categorized into (1) flow boiling correlations, (2) fluid flow and heat transfer in microchannels, (3) roughness effect at microscale, (4) pool boiling heat transfer and critical heat flux modeling, (5) surface enhancements for pool boiling, (6) numerical modeling of bubble growth in boiling, (7) water transport in proton exchange membrane (PEM) fuel cells, and (8) infrared imaging to detect breast cancer. The research conducted in each of these areas has produced some landmark findings, some of the most widely used theoretical models and an abundance of high-quality experimental data. The focus of this paper is to collate major finding and highlights some of the common themes that guided the research in Professor Kandlikar's group. This will help the readers gain a broad understanding of each of the areas of study in Professor Kandlikar's group and place the findings of the papers in a larger context. [ABSTRACT FROM AUTHOR]

In the liquid hydrogen storage and delivery, cavitation and boiling bubbles are prone to occur, which reduces the safety and economy of the liquid hydrogen delivery. For the bubble in liquid hydrogen, its growth process is different from that of room temperature media owing to the thermodynamic properties. In this paper, a single bubble growth model in liquid hydrogen is developed considering temperature distribution inside the bubble. The growth of single bubble in liquid hydrogen is described and predicted by solving Rayleigh-Plesset equation, thermal diffusion equation, thermal equilibrium equation, and heat conduction equation in semi-infinite space simultaneously. The growth trend of bubble radius, radius growth rate, vapor pressure, thermal boundary layer thickness and temperature difference between boundary and center are investigated by the model. The influence of superheat and ambient pressure on the growth of single bubble in liquid hydrogen is investigated by analysis of variance (ANOVA) and range analysis method. The mechanism of the single bubble transform from dynamic growth to thermal growth is clarified by comparing the critical time of the above physical indicators. • A cryogenic cavitation model is developed considering temperature at bubble center. • The growth of single bubble in liquid hydrogen is described and predicted. • Influence of factors on critical times of bubble growth is investigated. • The relationship among critical times of bubble growth is drawn by statistics. [ABSTRACT FROM AUTHOR]

Pool boiling surface modification by pin fin array has demonstrated a practical but effective cooling enhancement. Whilst the main challenge is to determine the best configuration of the pin fin dimensions and positioning, it is known that the result varies among the working conditions. To obtain a more general formulation, it is utterly vital to obtain a deep understanding of the pool boiling mechanism during the phase-change process. This study aimed to analyze the role of pin fin array configuration on the pool boiling phenomenon by generating visualization and non-dimensional analysis. The results illustrated three different regimes occurring during the boiling process: natural convection, isolated bubble, and merged bubble. For all regimes, the average heat transfer performance was enhanced as the fin gap rose to 17% and 63% for circular and rectangular pin fins, respectively. Furthermore, Grashof (Gr) and Bond (Bo) numbers were calculated to quantitatively describe the effect on the bubble dynamics. From these approaches, it was found that the insulated bubble regime provides a better means of cooling by around 60% due to the better bubble dynamics. [ABSTRACT FROM AUTHOR]

The benefits of nucleate pool boiling phenomena and their potential applications on thermal management of various microelectronic devices have triggered the development of new approaches that augment the magnitude of heat transfer rate. To implement these approaches, an accurate estimation of the boiling heat transfer coefficient between the fluid and the heated surface is often required. The acquisition of the boiling heat transfer coefficient must follow a better understanding of the bubble ebullition cycle because of its inherent coupling with heat transfer mechanisms involved in this cycle. Bubble ebullition occurs by periodic bubble nucleation on a boiling surface, bubble growth, and subsequent bubble departure from the surface. Different parameters related to the dynamics of the bubble ebullition cycle, including bubble departure diameter, bubble waiting period, active nucleation site density, bubble growth period, bubble departure frequency, and bubble growth rate govern the heat transfer rate in the nucleate pool boiling. Thus, numerous empirical correlations that determine the boiling heat transfer coefficient have been proposed by many researchers according to different bubble dynamics parameters. To accurately predict the boiling heat transfer coefficient and boiling heat flux based on the bubble ebullition cycle, understanding bubble growth mechanisms and associated dominant parameters is crucial. In this review, different bubble growth mechanisms during nucleate pool boiling are thoroughly reviewed. Then, bubble dynamics parameters used in different correlations for determining the boiling heat transfer coefficient are discussed. Semi-empirical and empirical correlations for determining these parameters are also extensively provided. Additionally, a detailed review of factors affecting bubble dynamics parameters is provided. Next, different applications of nucleate boiling in cooling systems are reviewed. Overall, this review includes various correlations from experimental and numerical data, which can be used to better predict the heat transfer during nucleate boiling. [ABSTRACT FROM AUTHOR]

Based on the one-component multiphase lattice Boltzmann method, a novel solid–liquid conjugate boiling heat transfer pseudo-potential lattice Boltzmann (LB) model is tentatively proposed in this paper. By respectively introducing the physical property parameters of solids and liquids into the relaxation time τ T of the temperature distribution equation, different energy transfer rates in solid, liquid, and vapor regions can be successfully predicted. After verifying the accuracy, stability, and reasonability of this model, the bubble detaching behavior and boiling heat transfer performance on the rectangular cavity structure are analyzed through setting different contact angles of the cavity surface and plane heating surface. The results show that the hydrophobic cavity surface can initialize bubble nucleation earlier and obviously increase the bubble detaching frequency because of its gas-bounding character, while the hydrophilic plane heating surface can restrict the expansion of bubbles and delay the appearance of film boiling. Moreover, for uniform wettability surfaces, the bubble detaching period varies in the quadratic equation with the surface contact angle due to the interaction of surface tension and buoyancy, and there is a minimum detaching period. While for the mixed wettability surfaces, the bubble detaching period also has a minimum value with the decrease in the contact angle the cavity surface, but the average bubble detaching diameter basically does not change with the cavity surface contact angle; moreover, the cavity surface contact angle corresponding to the minimum detaching period also increases with the increase in the plane heating surface contact angle. In addition, for the boiling heat transfer surface with cavity structure, the maximum heat flux and temperature gradient occur on the cavity surface, and the local heat flux of the hydrophobic cavity surface is higher than that of the hydrophilic cavity surface. This work will provide useful help for the further development of the conjugate boiling heat transfer LB model and clarify the mechanism of enhanced boiling heat transfer on a mixed wettability surface. [ABSTRACT FROM AUTHOR]

Subcooled flow boiling was experimentally investigated in a horizontal annulus with a temperature-controlled boiling surface and transparent outer pipe facilitating visualization. Boiling occurs on a copper tube with a diameter of 12 mm in an annulus with a 2 mm gap. Refrigerant R245fa is used as a working fluid. The focus of this study is to explore the effect of heat flux variation on the boiling flow patterns at approximately constant inlet flow conditions of the working fluid (fixed mass flux and inlet fluid temperature). Subcooled flow boiling is recorded by a high-speed camera, images are analyzed by a neural network to determine the bubble size distributions and their variation with the heat flux. The experimental setup being a part of the laboratory THELMA (Thermal Hydraulics experimental Laboratory for Multiphase Applications) at the Reactor Engineering Division of Jožef Stefan Institute, analysis methods and measurement results are presented and discussed. [ABSTRACT FROM AUTHOR]

The authors propose a calculation model for the process of inflow of a liquid from a microfilm to dry spots above vapor bubbles, which occurs in nucleate boiling and is due to the action of capillary forces (Laplace head) in a porous coating on the heating surface. In comparing the value of the heat flux from the heating surface to the interface in the zone of intense evaporation of the liquid and the value of the heat flux to evaporate the liquid sucked through a porous substrate to a dry spot, it turns out that these values are comparable, which explains the effect of rise in the critical density of the heat flux on porous surfaces compared to "smooth" surfaces. A comparison of the results of quantification of this effect with the proposed model and the experimental data obtained during the boiling of water on a porous surface that was formed by nanoparticles precipitated from a nanolfluid has shown their mutual agreement. [ABSTRACT FROM AUTHOR]

This work proposes an improved three-dimensional double multiple relaxation time lattice Boltzmann model that can correctly recover the macroscopic temperature equation. Using this model, the impact of a droplet on a heated surface with pillars is simulated, and the effects of pillar width (w) and distance between pillars (b) are analyzed in detail. Four boiling droplet types are numerically reproduced, namely, deposition boiling, contact boiling, transition boiling, and film boiling. In the deposition boiling regime, no vapor bubble generation is observed within the droplet, but secondary droplets are generated at the periphery of the droplet due to the sharp geometric profile of the pillars and the differences in the rate of evaporation. The difference between contact boiling and transition boiling lies in whether the droplet is always in contact with the surface. For conditions with large w values, contact boiling is not observed, due to the high-pressure peaks from below, while for smaller w, a transition from deposition boiling to transition boiling, then to contact boiling, and subsequently to transition boiling can be observed. Two obvious vortices are found in the center of the liquid film with decreased b, which results in bubble nucleation even at low Ja values. In addition, boiling may take place, in which a stable vapor layer is formed between the droplet and the surface by increasing Ja, which leads to a significant reduction in the heat transfer efficiency. [ABSTRACT FROM AUTHOR]

Nucleate boiling has significant applications in earth gravity (in industrial cooling applications) and microgravity conditions (in space exploration, specifically in making space applications more compact). However, the effect of gravity on the growth rate and bubble size is not yet well understood. We perform numerical simulations of nucleate boiling using an adaptive moment-of-fluid (MoF) method for a single vapor bubble (water or Perfluoro-n-hexane) in saturated liquid for different gravity levels. Results concerning the growth rate of the bubble, specifically the departure diameter and departure time, have been provided. The MoF method has been first validated by comparing results with a theoretical solution of vapor bubble growth in superheated liquid without any heat-transfer from the wall. Next, bubble growth rate, bubble shape, and heat transfer results under earth gravity, reduced gravity, and microgravity conditions are reported, and they are in good agreement with experiments. Finally, a new method is proposed for estimating the bubble diameter at different gravity levels. This method is based on an analysis of empirical data at different gravity values and using power-series curve fitting to obtain a generalized bubble growth curve irrespective of the gravity value. This method is shown to provide a good estimate of the bubble diameter for a specific gravity value and time. [ABSTRACT FROM AUTHOR]

Local and temporal temperature mapping of the liquid phase around boiling bubbles and above the heat-transfer surface reveals that convective heat transfer plays a significant role in boiling heat transfer. To investigate this, we employ one of the most accurate methods for detecting temperature: the laser-induced fluorescence (LIF) technique. Previously, ratiometric LIF has been conducted mainly at temperatures well below 70 °C, which is too low for our purposes. We used Rhodamine B (RhB) and Rhodamine 110 (Rh110) as fluorescent dye pairs. We found that Rh110 excited by a 532-nm laser (rather than the blue laser used in some earlier studies) exhibited a temperature dependence: the combination of RhB and Rh110 could be used as a temperature indicator up to 88 °C. The calibration curve was extended beyond the superheating temperature range, permitting liquid-phase temperature visualization of boiling. A thin film heater sandwiched between two transparent plates was used to gain optical access to the liquid–vapor structure. This method successfully captured characteristic phenomena such as superheated liquid layer formation and thermal plumes beneath the rising boiling bubbles. [ABSTRACT FROM AUTHOR]