Simulation of pool boiling of nanofluids by using Eulerian multiphase model

In the present work, a new simulation of nanofluid/vapor two-phase flow inside the 2-D rectangular boiling chamber was numerically investigated. The Eulerian–Eulerian approach used to predict the boiling curve and the interaction between two phases. The surface modification during pool boiling of silica nanofluid represented by surface roughness and wettability is put into the account in this simulation. New closure correlations regarding the nucleation sites density and bubble departure diameter during boiling of silica nanofluid were inserted to extend the boiling model in this work. Besides, the bubble waiting time coefficient which involved in quenching heat flux under heat flux partitioning HFP model was corrected to improve the results of this study. The numerical results validated with experimental works in the literature, and they revealed good agreements for both pure water and nanofluids. The results found that when improving the heat flux partitioning model HFP by considering the surface modification of nucleate pool boiling parameters, it will give more mechanistic sights compared to the classical model, which is used for predicting of boiling heat transfer of pure liquid.

     

The detachment of particles from coalescing bubble pairs

This paper is concerned with the detachment of particles from coalescing bubble pairs. Two bubbles were generated at adjacent capillaries and coated with hydrophobic glass particles of mean diameter 66 μm. The bubbles were then positioned next to each other until the thin liquid film between them ruptured. The particles that dropped from the bubble surface during the coalescence process were collected and measured. The coalescence process was very vigorous and observations showed that particles detached from the bubble surfaces as a result of the oscillations caused by coalescence. The attached particles themselves and, to some extent the presence of the surfactant had a damping affect on the bubble oscillation, which played a decisive role on the particle detachment phenomena. The behaviour of particles on the surfaces of the bubbles during coalescence was described, and implications of results for the flotation process were discussed.     

Pool boiling enhancement with environmentally friendly surfactant additives

Boiling heat transfer is used in variety of industrial processes and applications, such as refrigeration, vapor cycle power generation, heat exchangers, petroleum refining, and chemical manufacturing. Enhancements in boiling heat transfer processes are critical for making these applications more energy efficient. The aim of this paper is to demonstrate the water pool boiling phenomena under the influence of environmentally friendly surfactant additives. The test setup used in this study has multiple benefits. First, the test setup enhances teaching in variety of classes through in-class demonstrations and student experiments. An experiment is described to allow the students to visualize and qualify different modes of boiling heat transfer. In addition, the test setup provides a platform for research in boiling enhancement. Using surfactant additives in boiling causes increased number of nucleation sites and decreased level of wall superheat. For determining surfactant effects, different concentrations of sodium lauryl sulfate (SLS), ECOSURF? EH-14, and ECOSURF? SA-9 are added to pure water and enhancement through surfactants is quantified. When times until boiling (liquid temperature reaches the boiling point) are measured, 17, 10.3, and 19.6 % lower times found (for SLS, EH-14, and SA-9, respectively) compared to pure water. Wall temperature reduction is measured for 50 ppm SLS 9.48 %, for 300 ppm EH-14 11.3 %, and for 200 ppm SA-9 10.43 %. It can be concluded from this study that a reduction in surface tension leads to a higher nucleation site density and more small bubbles on the boiling surface.     

Entrainment of cold gas into thermal plasma jets

There is increasing evidence that the entrainment of cold gas surrounding a turbulent plasma jet is more of an engulfment type process rather than simple diffusion. A variety of diagnostic techniques have been employed to determine the development of turbulence in a plasma jet and to measure concentration and temperatures of the cold gas entrained into atmospheric-pressure argon plasma jets in ambient argon or air. The results indicate that the transition to turbulence causes a rapid drop of the axial jet velocity due to entrainment of the cold gas surrounding the plasma jet. Dissipation of the cold engulfed gas bubbles by molecular diffusion is relatively slow if molecular gases (for example air) are entrained, as indicated by conditional sampling and CARS measurements. Temperature measurements using emission spectroscopy and enthalpy probes show strong discrepancies in the jet fringes.     

On air entrainment in coatings

A series of experiments that clarify how air bubbles become entrained into coatings are described. The contact line dynamics at the air-liquid interface surrounding a fiber is characterized for a typical coating die operating under atmospheric and pressurized conditions. Glycerin and other viscous liquids are used to reveal that a critical fiber speed exists at which air entrainment begins. The observations confirm that the critical capillary number Ca(c) depends on the physical properties of the coating material, in the form of the Morton number. When the liquid supply is pressurized, the experiments show that adjusting the pressure can stabilize the displaced free surface interface at a prescribed location. Controlling the meniscus location in this way eliminates air entrainment. The threshold occurs when the applied pressure balances the shear exerted on the coating by the moving fiber. Using this approach it is possible to eliminate air entrainment and attain stable wetting at very large values of the capillary number, e.g., Ca congruent with 50.     

Multibubble Sonoluminescence from a Theoretical Perspective

In the present review, complexity in multibubble sonoluminescence (MBSL) is discussed. At relatively low ultrasonic frequency, a cavitation bubble is filled mostly with water vapor at relatively high acoustic amplitude which results in OH-line emission by chemiluminescence as well as emissions from weakly ionized plasma formed inside a bubble at the end of the violent bubble collapse. At relatively high ultrasonic frequency or at relatively low acoustic amplitude at relatively low ultrasonic frequency, a cavitation bubble is mostly filled with noncondensable gases such as air or argon at the end of the bubble collapse, which results in relatively high bubble temperature and light emissions from plasma formed inside a bubble. Ionization potential lowering for atoms and molecules occurs due to the extremely high density inside a bubble at the end of the violent bubble collapse, which is one of the main reasons for the plasma formation inside a bubble in addition to the high bubble temperature due to quasi-adiabatic compression of a bubble, where “quasi” means that appreciable thermal conduction takes place between the heated interior of a bubble and the surrounding liquid. Due to bubble–bubble interaction, liquid droplets enter bubbles at the bubble collapse, which results in sodium-line emission.     

Volumes of critical bubbles from the nucleation theorem

A corollary of the nucleation theorem due to Kashchiev [Nucleation: Basic Theory with Applications (Butterworth-Heinemann, Oxford, 2000)] allows the volume V(*) of a critical bubble to be determined from nucleation rate measurements. The original derivation was limited to one-component, ideal gas bubbles with a vapor density much smaller than that of the ambient liquid. Here, an exact result is found for multicomponent, nonideal gas bubbles. Provided a weak density inequality holds, this result reduces to Kashchiev's simple form which thus has a much broader range of applicability than originally expected. Limited applications to droplets are also mentioned, and the utility of the p(T,x) form of the nucleation theorem as a sum rule is noted.     

A finite-temperature density functional study of electron self-trapping in 3He and 4He

We introduce a compact finite-temperature density functional model to study electron self-trapping in both liquid and vapor (3)He and (4)He. This model can quantitatively reproduce the most essential thermodynamic properties of (3)He and (4)He along their liquid-vapor coexistence lines. The structures and energetics of self-trapped electron bubbles on the 1S ground state and 1P excited state are particularly investigated. Our results show that 1S and 1P bubbles exist in liquid at any temperature, whereas 1S bubbles exist in vapor only above 1.6 K in (3)He and above 2.8 K in (4)He, 1P bubbles exist in vapor only above 2.5 K in (3)He and 4.0 K in (4)He. An initially spherical 1P bubble is unstable against deformation towards a peanut shape. In liquid, a peanut-shaped 1P bubble is held from fission by surface tension until reaching the liquid-vapor critical point, whereas in vapor it always splits into two smaller bubbles. The existence of 1P bubbles in finite-temperature liquid helium and their fission instability in helium vapor reveal interesting physics in this system.     

A volume-amending method to improve mass conservation of level set approach for incompressible two-phase flows

A volume-amending method is developed both to keep the level set function as an algebraic distance function and to preserve the bubble mass in a level set approach for incompressible two-phase flows with the significantly deformed free interface. After the traditional reinitialization procedure, a vol-ume-amending method is added for correcting the position of the interface according to mass loss/gain error until the mass error falls in the allowable range designated in advance. The level set approach with this volume-amending method incorporated has been validated by three test cases: the motion of a single axisymmetrical bubble or drop in liquid, the motion of a two-dimensional water drop falling through the air into a water pool, and the interactional motion of two buoyancy-driven three- dimensional deformable bubbles. The computational results with this volume-amending method in-corporated are in good agreement with the reported experimental data and the mass is well preserved in all cases.     

'Bubble chamber model' of fast atom bombardment induced processes

A hypothesis concerning FAB mechanisms, referred to as a 'bubble chamber FAB model', is proposed. This model can provide an answer to the long-standing question as to how fragile biomolecules and weakly bound clusters can survive under high-energy particle impact on liquids. The basis of this model is a simple estimation of saturated vapour pressure over the surface of liquids, which shows that all liquids ever tested by fast atom bombardment (FAB) and liquid secondary ion mass spectrometry (SIMS) were in the superheated state under the experimental conditions applied. The result of the interaction of the energetic particles with superheated liquids is known to be qualitatively different from that with equilibrium liquids. It consists of initiation of local boiling, i.e., in formation of vapour bubbles along the track of the energetic particle. This phenomenon has been extensively studied in the framework of nuclear physics and provides the basis for construction of the well-known bubble chamber detectors. The possibility of occurrence of similar processes under FAB of superheated liquids substantiates a conceptual model of emission of secondary ions suggested by Vestal in 1983, which assumes formation of bubbles beneath the liquid surface, followed by their bursting accompanied by release of microdroplets and clusters as a necessary intermediate step for the creation of molecular ions. The main distinctive feature of the bubble chamber FAB model, proposed here, is that the bubbles are formed not in the space and time-restricted impact-excited zone, but in the nearby liquid as a 'normal' boiling event, which implies that the temperature both within the bubble and in the droplets emerging on its burst is practically the same as that of the bulk liquid sample. This concept can resolve the paradox of survival of intact biomolecules under FAB, since the part of the sample participating in the liquid-gas transition via the bubble mechanism has an ambient temperature which is not destructive for biomolecules. Another important feature of the model is that the timescale of bubble growth is no longer limited by the relaxation time of the excited zone ( approximately 10(-12) s), but rather resembles the timescale characteristic of common boiling, sufficient for multiple interactions of gas molecules and formation of clusters. Further, when the bubbles burst, microdroplets are released, which implies that FAB processes are similar to those in spraying techniques. Thus, two processes contribute to the ion production, namely, release of volatile solvent clusters from bubbles and of non-volatile solute from sputtered droplets. This view reconciles contradictory views on the dominance of either gas-phase or liquid-phase effects in FAB. Some other effects, such as suppression of all other ions by surface-active compounds, are consistent with the suggested model.     

Statistical Regularities of Homogeneous Boiling-Up of Gas-Supersaturated Solutions

Statistical regularities of homogeneous boiling-up of liquid solutions supersaturated with gas are studied. The local gas concentration and local rate of homogeneous boiling-up of the solution are found in the vicinity of the bubble emerging in the solution at low gas solubility. The probability of the emergence of new bubbles in addition to the bubble originally emerged in the solution is calculated. The mean distance between two neighboring bubbles and the relative statistical scatter of this distance are found. The mean number of bubbles in a solution unit volume by the end of the stage of their emergence is estimated. The mean expectation time of the emergence of the neighboring bubble estimating the duration of the stage of bubble emergence is established. The proposed theory is extended to the case of homogeneous nucleation in the vapor–gas medium.     

Formation of mesoscopic silver particles at micro- and nano-liquid/liquid interfaces

Silver particles have been deposited at externally polarised 1,2-dichloroethane (DCE)/water interfaces supported at the tip of micro- and nanopipettes. The electrochemical process involved the reduction of silver ion in the aqueous phase by an organic-phase electron donor (butylferrocene). The silver nucleation and growth process was investigated using potential step chronoamperometry, and the resulting current–time transients were analysed to extract nucleus numbers. At larger pipettes, with diameters of several micrometers, multi-particle nucleation was observed and optical microscopy provided direct evidence for metal electrodeposition at the liquid/liquid interface. For pipettes with radii of 0.5 μm or smaller, the current–time behaviour was consistent with single particle generation. Under some conditions, detachment of the particle from the liquid/liquid interface was inferred from the current–time characteristics, and it is suggested that controlled-detachment from micropipettes could provide a method for the deposition of small metal structures on surfaces.     

Nucleation and cavitation of spherical, cylindrical, and slablike droplets and bubbles in small systems

Computer simulations are employed to obtain subcritical isotherms of small finite sized systems inside the coexistence region. For all temperatures considered, ranging from the triple point up to the critical point, the isotherms gradually developed a sequence of sharp discontinuities as the system size increased from approximately 8 to approximately 21 molecular diameters. For the smallest system sizes, and more so close to the critical point, the isotherms appeared smooth, resembling the continuous van der Waals loop obtained from extrapolation of an analytic equation of state outside the coexistence region. As the system size was increased, isotherms in the chemical potential-density plane developed first two, then four, and finally six discontinuities. Visual inspection of selected snapshots revealed that the observed discontinuities are related to structural transitions between droplets (on the vapor side) and bubbles (on the liquid side) of spherical, cylindrical, and tetragonal shapes. A capillary drop model was developed to qualitatively rationalize these observations. Analytic results were obtained and found to be in full agreement with the computer simulation results. The analysis shows that the shape of the subcritical isotherms is dictated by a single characteristic volume (or length scale), which depends on the surface tension, compressibility, and coexistence densities. For small reduced system volumes, the model predicts that a homogeneous fluid is stable across the whole coexistence region, thus explaining the continuous van der Waals isotherms observed in the simulations. When the liquid and vapor free energies are described by means of an accurate mean-field equation of state and surface tensions from simulation are employed, the capillary model is found to describe the simulated isotherms accurately, especially for large systems (i.e., larger than about 15 molecular diameters) at low temperature (lower than about 0.85 times the critical temperature). This implies that the Laplace pressure differences can be predicted for drops as small as five molecular diameters, and as few as about 500 molecules. The theoretical study also shows that the extrema or apparent spinodal points of the finite size loops are more closely related to (finite system size) bubble and dew points than to classical spinodals. Our results are of relevance to phase transitions in nanopores and show that first order corrections to nucleation energies in finite closed systems are power laws of the inverse volume.     

Bubble formation in a quiescent pool of gold nanoparticle suspension

This paper begins with an extensive review of the formation of gas bubbles, with a particular focus on the dynamics of triple lines, in a pure liquid and progresses into an experimental study of bubble formation on a micrometer-sized nozzle immersed in a quiescent pool of aqueous gold nanofluid. Unlike previous studies of triple line dynamics in a nanofluid under evaporation or boiling conditions, which are mainly caused by the solid surface modification due to particle sedimentation, this work focuses on the roles of nanoparticles suspended in the liquid phase. The experiments are conducted under a wide range of flow rates and nanoparticle concentrations, and many interesting phenomena are revealed. It is observed that nanofluids prevent the spreading of the triple line during bubble formation, i.e. the triple line is pinned somewhere around the middle of the tube wall during the rapid bubble formation stage whereas it spreads to the outer edge of the tube for pure water. A unique ‘stick-slip’ movement of the triple line is also observed for bubbles forming in nanofluids. At a given bubble volume, the radius of the contact line is found to be smaller for higher particle concentrations, but a reverse trend is found for the dynamic bubble contact angle. With the increase of particle concentration, the bubble frequency is raised and the bubble departure volume is decreased. The bubble shape is found to be in a good agreement with the prediction from Young-Laplace equation for given flow rates. The influence of nanoparticles on other detailed characteristics related to bubble growth inside, including the variation of bubble volume expansion rate, the radius of the curvature at the apex, the bubble height and bubble volume, is revealed. It is suggested that the variation of surface tensions and the resultant force balance at the triple line might be responsible for the modified dynamics of the triple line.     

Nanosecond imaging of microboiling behavior on pulsed-heated au films modified with hydrophilic and hydrophobic self-assembled monolayers

Fast transient microboiling has been characterized on modified gold microheaters using a novel laser strobe microscopy technique. Microheater surfaces of different hydrophobicity were prepared using self-assembled monolayers of hexadecane thiol (hydrophobic) and 16-mercaptohexadecanol (hydrophilic) as well as the naturally hydrophilic bare gold surface. The microheater was immersed in a pool of water, and a 5-micros voltage pulse to the heater was applied, causing superheating of the water and nucleation of a vapor bubble on the heater surface. Light from a pulsed Nd:Yag laser was configured to illuminate and image the sample through a microscope assembly. The timing of the short duration (7.5 ns) laser flash was varied with respect to the voltage pulse applied to the heater to create a series of images illuminated by the flash of the laser. These images were correlated with the transient resistance change of the heater both during and after the voltage pulse. It was found that hydrophobic surfaces produced a bubble that nucleated at an earlier time, grew more slowly to a smaller maximum size, and collapsed more rapidly than bubbles formed on hydrophilic surfaces.     

The influence of environment on the drop size — contact angle relationship

Advancing contact angles are reported for the water-PTFE, water-copper, water-stainless steel, water-PMMA andn-decane-PTFE systems for a range of liquid drop sizes.The angles were determined at 25 °C in an air or nitrogen-saturated atmosphere and compared with those measured at the boiling point in an environment only of its vapor.For water-PTFE and water-PMMA systems, a decrease in contact angle with decreasing drop size was observed in an air-saturated environment at 25 ° confirming the data of Good and Koo. An increase in contact angle however occurred with decreasing drop size at the boiling point in the pure vapor atmosphere confirming the results of Boyes and Ponter. The introduction of nitrogen decreased the contact angle although the trend remained the same.     

Microbubble formation using asymmetric Shirasu porous glass (SPG) membranes and porous ceramic membranes—A comparative study

We have recently proposed a new method for generating uniformly sized microbubbles from Shirasu porous glass (SPG) membranes with a narrow pore size distribution. In this study, to obtain a high gas permeation rate through SPG membranes in microbubble formation process, asymmetric SPG membranes were used. At the transmembrane/bubble point pressure ratio of less than 1.50, uniformly sized microbubbles with a bubble/pore diameter ratio of approximately 9 were generated from an asymmetric SPG membrane with a mean pore diameter of 1.58 μm and a skin-layer thickness of 12 ± 2 μm at a gaseous-phase flux of 2.1–24.6 m³ m⁻² h⁻¹, which was much higher than that through a symmetric SPG membrane with the same pore diameter. This is mainly due to the much smaller membrane resistance of the asymmetric SPG membrane. Only 0.27–0.43% of the pores of the asymmetric SPG membrane was active under the same conditions. The proportion of active pores increased with a decrease in the thickness of skin layer. In contrast to the microbubble formation from asymmetric SPG membranes, polydispersed larger bubbles were generated from asymmetric porous ceramic membranes used in this study, due to the surface defects on the skin layer. The surface defects were observed by the scanning electron microscopy and detected by the bubble point method.     

Gas-vapor bubble nucleation--a unified approach

In a solution which is saturated with gas near the superheat limit, one might expect a bubble formed from both dissolved gas and vapor molecules to appear. The integration of the surface-energy concepts, that are postulated on completely different physical bases for gas and vapor bubble formation is a major issue. In this paper, we reformulate gas and vapor bubble nucleation by a scaling transformation, which turns the surface energy for the bubble formation from both dissolved gases and vapor molecules to the translational energy of a molecule, (3/2)kBT. With this unified approach, one could estimate the dissolved gas effect on the superheat limit of the liquid. The driving force and the molecular volume are important quantities for determining the number of gas and vapor molecules composed of a critical cluster. This approach, of course, can predict pure gas bubble formation, as well as pure vapor bubble formation, as limiting cases. Also, this approach makes it possible to find that the possible occurrence of gas bubble nucleation by dissolved gases prevents measuring the theoretical superheat limit of water at atmospheric pressure, 300 degrees C.     

Spreading of a fluid phase on a spherical surface

The adsorption between a liquid drop and a micro-particle in an air or an air bubble and a micro-particle in water is dominated by liquid-solid or air-solid interfacial tension and wetting area of the liquid or air on the particle surface. The wetting area is determined by the spreading of the liquid drop or the bubble on the micro-particle. To explore this spreading, a wetting model of a fluid phase on a spherical particle was built. According to the theoretical results, the contact angle is constant when a fluid phase spreads on a spherical solid surface; the micro-particle can not submerge under a fluid when only interfacial tensions are involved and the wetting is not a complete wetting. The corresponding experiments were performed to confirm the theoretical results.