A multiphase lattice Boltzmann model is used to explore the presence, evolution, and behavior of nanobubbles. The existence and behavior of nanobubbles has been a recent area of interest since the presence of nanobubbles challenges classical nucleation theory which dictates that bubbles below the critical radius should collapse. Nanobubbles have many areas of interest including cleaning of surfaces, nucleate boiling in microchannels, and nucleation on nanostructured materials. Multiphase Lattice Boltzmann methods (LBM) have been demonstrated to be an effective mesoscale approach to modeling multiphase flows and phase-change processes. These methods provide accurate macroscopic results while accounting for microscopic interactions without invoking an extraordinary computational cost. In this study, an LBM is used to model the evolution of nanobubbles with diameters ranging from 5 to 50 nanometers. LBM results are provided for a variety of real physical conditions that are of interest for exploring nanobubble existence within a nanoporous layer. In addition to the single nanobubble analysis, the effects of bubble interaction with smooth surfaces and within nanostructured surfaces are also presented. The results show that the hydrophilic nature of the surfaces is likely the cause of suppression in the onset of nucleate boiling which is often seen in hydrophilic nanoporous layers. The implications of these results on heat transfer applications including multiphase flows and nucleate boiling in roughened nanostructured surfaces are discussed.

It is well known that a high-power laser could breakdown liquid [1, 2]. Laser-induced breakdown of liquids is characterized by fast plasma formation after evaporation of liquid and subsequent vapor expansion accompanied by shock wave emission [2]. The bubble wall velocity after the shock departure has been found to be sufficiently high to produce emission of light at the collapse point [3]. Recently, bubble formation on the surface of gold nanoparticles irradiated by a high-power laser in water [4, 5] has been studied for medical applications such as cancer diagnosis and possible therapy [5]. However, it is very hard to perform these experiments and to obtain good data from the bubble formation on the surface of laser-irradiated nano-particles because the nanoparticles dispersed in liquid cannot be controlled properly. In this study, laser-induced bubble formation on a micro gold particle levitated at the center of a spherical flask under ultrasound was investigated experimentally. The obtained results are compared with the results for laser cavitation without the gold particle, i.e., typical laser-induced cavitation. Figure 1 shows a schematic of the experimental setup used to investigate the laser-induced bubble formation on a micro gold particle. Two disk-type lead zirconate titanate (PZT) transducers (Channel Industries Inc.; 15 mm in diameter and 5.0 mm in thickness) attached to the side of the wall of the cell produced a velocity stagnation point near the center of the flask. The driving frequency of the PZT transducers was approximately 27.0 kHz which was close to the resonance frequency of the LRC circuit (Its capacitor unis is PZT.) and the acoustic resonance frequency of the water-filled flask. A drop of water containing gold particles with an average diameter of 10 μm are dispersed was injected into a 100-ml pyrex spherical flask filled with degassed water. When the body force of a gold particle in liquid is slightly lower than the Bjerknes force [6] induced by ultrasound, the particle will stay near the pressure antinode, i.e., the center of the flask. A Q-switched Nd:Yag laser delivered a single pulse of 0.5 ns in width with an energy of 7.5 mJ at a wavelength of 1064 nm to the gold particle or liquid at the center of the cell. The laser light was focused at the center of the flask using a lens with an effective focal length of 36.3 mm. Bubble formation and subsequent growth and collapse were visuallized by a high-speed camera (V2511, Phantom, USA) with 0.45 Mfps (million frames per second). The time-dependent radius was also obtained by the light scattering method. The scattering angle chosen was 80 degree where one-to-one relationship exists between the scattered intensity and the bubble radius [7]. The scattered intensity from a bubble illuminated by a 5-mW He-Ne laser was received by a photomultiplier tube (PMT: Hamamatsu, R2027) and was recorded in an oscilloscope. The scattering data were calibrated using the maximum radius for different bubble, which was obtained by high-speed camera. The shock strength during the expansion stage of bubbles was measured by a calibrated needle hydrophone (HPM1, Precision Acoustics, UK) at various distances from the center of the cell for different bubbles. The hydrophone can measure acoustic signals ranging from 1 kPa to 20 MPa. The hydrophone was attached to a three-dimensional micro stage so that fine control of the positioning of the hydrophone was possible.

The open-source software OpenFOAM has been employed to better understand the transport, deformation, and collapse of single, isolated bubbles in subcooled boiling. High pressure steam-water was selected as the fluid system of interest due to its relevance to the power generation industry. Ranges of bubble size, subcooling, pressure/saturation temperature, and mesh refinement have been employed. Both quantitative (bubble rise velocity and collapse times) and qualitative (bubble shape) results are discussed across the parameter ranges studied.

We present our recent study on spreading and solidification of micro-droplets of alumina impacting onto patterned surfaces textured by micron-size obstacles. We employed an in-house, three-dimensional computational tool that solves the flow and energy equations and takes into account the solidification. We investigated the spreading dynamics, heat transfer, and solidification of the droplets as a function of the height and spacing of the obstacles as well as the impact velocity. The results show that, independent of the obstacle height, the droplet assumes a disk-shape geometry when the obstacles are either packed tightly or are very distanced. The results at intermediate obstacle spacings exhibit the most significant deformations, where the droplet develops long fingers. A quantitative relationship shows the collapse of the final spread diameter of the droplet normalized by the obstacle spacing when plotted against the spacing for different impact velocity as well as the obstacle height.

Numerical simulations of the thermal inkjet (TIJ) droplet ejection process are performed. The computational approach is based on a volume of fluid (VOF) formulation. This method allows determining the coupled flow and thermal fields in the firing chamber in addition to the phase change processes that take place during the drive bubble formation, expansion, and collapse. The drive bubble pressure is a result of the phase change heat transfer during the heating pulse and is not imposed by a pressure heuristic approach. A commercially available TIJ architecture was chosen as a baseline to assess the computational model predictions of ejected droplet volume and droplet velocity during a firing cycle. These computational model predictions were compared to experimental results demonstrating an excellent agreement. The transient histories of pressure in the vapor bubble, temperature, and heat transfer rate to the fluid are analyzed to explain some of the relevant physical processes observed.

This work deals with the dynamics of rapid-boiling of a droplet, at medium-high superheat, rising in a host liquid environment. It considers the heat transfer, the superheat consumption and the hydrodynamics of the droplet as it boils. In the course of the research water-column experiments were conducted, and results are shown. Superheating was implemented by the sudden depressurization of the ambient liquid. Boiling was very rapid, concluding within several milliseconds, and high heat fluxes across the interface were obtained. Additionally, certain critical times in the boiling process were predicted and defined, and a novel criterion for the end of rapid boiling (liquid film collapse), is proposed. These defined critical times agree well with measured points of change in the boiling dynamics. From these results and analysis a deeper understanding of the three-fluid rapid boiling at medium-high superheat has been established, for the first time. In addition, various initial conditions were tested and their effect established qualitatively. This form of boiling, though being very rapid and sustaining high heat transfer rates, is non-explosive in nature, and therefore more designable and widely applicable.

The behavior of rewetting on high superheated surface, focusing on rewetting temperature during collapse of saturated and subcooled film boiling was investigated experimentally and analytically. Saturated and subcooled film-boiling experiments were conducted by using a Silicon wafer with 20 mm length, 20 mm width and 0.5 mm thickness and pure water at atmospheric condition. Both pool boiling and impingement jet experiments were preformed in the present study. Then, the present experimental results were examined by using hydrodynamics instability on liquid-vapor interface (Rayleigth-Tayor instability) and a heat conduction model (rewetting model under a falling-flow of a liquid layer).

We have developed hybrid numerical simulation codes to investigate the dynamics of nanobubbles. The idea is based on a combination of a molecular dynamics (MD) technique and a continuum dynamics with the CIP method. The MD technique enables us to examine rapid change of the bubble surface and the inside region of nanobubbles at molecular scale. The CIP method enables us to trace the mass and energy transfer processes far from the bubble surface at continuum scale. In the hybrid simulation, the simulation cell is divided into two parts. The inner region containing a bubble (or bubbles) consists of sufficiently large number of particles and is treated with the MD method. The outer region is treated with a computational fluid dynamics (CFD) scheme. The boundary between the inner and outer regions is movable and driven with the pressure difference between the two regions. To deal with the moving boundary, we adopt the CIP scheme. Two different codes have been developed, i.e., one-dimensional CFD with assuming a spherical symmetry, and full three-dimensional CFD. Examples will be given in the paper to demonstrate each code. The first example is the oscillating dynamics of a spherical bubble with one-dimensional CFD. The bubble is initially located at the center of the MD region with no translational motion. The outer (continuum) region is treated one dimensionally, with the assumption of spherical symmetry. Under the equilibrium condition, where there is no pressure difference between the two regions, we give a sudden pressure increase in the continuum region far from the bubble. The spherical pressure wave propagates through the continuum region, and the pressure difference drives the boundary to shrink the MD region. As the MD region shrinks, the bubble inside the region starts to collapse. The collapsed bubble bounces back. We have analyzed the oscillation dynamics under several different conditions, such as different initial pressures and the state of gas inside the bubble. The second one is the deformation of a non-spherical bubble. For that purpose, the continuum region is treated with full three-dimensional CIP scheme. Furthermore, we use the level set method in order to capture the interfaces (boundary) between the MD region and the continuum region.

At approximately 3:41 AM on the morning of April 29, 2007, a tractor-trailer rig carrying 8,600 gallons (32.6 m 3 ) of fuel overturned on Interstate 880 in Oakland, CA. The resultant fire weakened the surrounding steel superstructure and caused a 50-yard (45.7 m) long section of the above connecting ramp from Interstate 80 to Interstate 580 to fail in approximately 18 minutes. In this study, we performed a loosely-coupled thermal-structural finite element analysis of the freeway using the LLNL Engineering codes NIKE3D, DYNA3D and TOPAZ3D. First, we applied an implicit structural code to statically initialize the stresses and displacements in the roadway at ambient conditions due to gravity loading. Next, we performed a thermal analysis by approximating the tanker fire as a moving box region of uniform temperature. This approach allowed for feasible calculation of the fire-to-structure radiative view factors and convective heat transport. We used a mass scaling methodology in the thermal analysis to reduce the overall simulation time so an explicit structural analysis could be used, which provided a more computationally efficient simulation of structural failure. Our approach showed structural failure of both spans due to thermal softening under gravity loading at approximately 20 minutes for a fixed fire temperature of 1200°C and fixed thermal properties. When temperature-dependent thermal properties were applied, the south and north spans collapsed at approximately 10 minutes and 16 minutes, respectively. Finally, we performed a preliminary fully-coupled analysis of the system using the new LLNL implicit multi-mechanics code Diablo. Our investigation shows that our approach provides a reasonable first-order analysis of the system, but improved modeling of the transport properties and the girder-box beam connections is required for more accurate predictions.

A numerical study of the drawing of hollow capillary rods over a range of draw speeds and feed speeds, with application to the drawing of photonic crystal fibers is presented. In this study the axisymmetric Navier-Stokes equations are solved numerically using the finite element method. In the numerical study inertia, gravity, surface tension and internal pressurization of the capillary is included and the effect of each of these parameters on the neck down profile is presented. The free surface which defines the neck down profile, is not assumed but is determined using a surface force balance. The results of isothermal and non-isothermal analysis results are compared with experimental, numerical and analytical results from the literature for validation. In the non-isothermal analysis, arbitrary temperature profiles are assumed to represent the temperature distribution in the neck down region and the effect of different temperature distributions on the neck down profile is discussed. The effect of the draw parameters (draw speed and draw down ratio) and the temperature distribution on the collapse of the capillaries is examined. A qualitative discussion on the onset of collapse of the capillary is presented and the effect of the draw parameters on the draw tension is discussed. For isothermal cases, the feed speed dominated the determination of collapse, while the draw speed had a relatively small effect on the collapse. It was found that collapse can be avoided with higher feed speeds and lower temperatures, while the draw tension can be minimized at lower feed speeds and higher temperatures.

The dynamics of Thermal Ink Jet (TIJ) micro-bubble life cycles is characterized through bubble area measurements for small planar heaters (41 μm × 45 μm). A laser visualization technique is used to measure the bubble area viewed from above. The bubbles are generated in water by 2–4 μs heating pulse and survive for 8–10 μs after the pulse. The bubble life-cycle consists of a short initiation time, fast growth, a period of maximum size, and a rapid collapse. Effect of the power inputs (2–4 W) total energy releases (8.8–13.3 μJ) and operational frequencies of 2–3 kHz is observed. Direct observations of bubble area indicate that bubble initiation time and initial growth rate correlates to the input power. The maximum bubble size and overall longevity correlates to the total energy release.

Air bubble trapped in water emits light stably at its collapse point when proper forcing amplitude and frequency of ultrasound are applied to the bubble. We investigated the Rayleigh-Taylor instability mechanism that causes deviation from sphericity of wall of the bubble at the collapse point in the sonoluminescence (SL) regime with rigorous analytical treatment. Our study revealed that the RT instability due to the bubble wall acceleration occurs when the gas density inside bubble exceeds a certain limit. Lagging motion of bubble to the ultrasound due to the viscoelastic behavior of bubble rather than viscous damping was a crucial parameter in determining the upper threshold of SL. The calculation results for the upper limit of stable sonoluminescence regime agreed well with experimental data.

The comparison of the structures of the boundary layer with and without suction of the same momentum thickness Reynolds number R θ have been made in a turbulent boundary subjected to concentrated suction, applied through a short porous wall strip. The results indicate that, relative to σ = 0, the mean velocity collapses reasonably well but there are some discrepancies in the Reynolds stresses distributions. These discrepancies are also noted in the distributions of the anisotropy invariant tensor, skewness and flatness factors. The result would suggest that the differences are a result of the difference in the initial boundary condition, which influences the flow structures to a significant streamwise location.

Instead of CW (continuous wave) mode, pulsed mode laser welding has been popularly used in industry especially for Nd: YAG lasers. In pulsed mode laser keyhole welding, pores have been frequently observed near the root of the solidified weld. Our previous studies have indicated that the formation of porosity is caused by two competing mechanisms during the keyhole collapse process, and they are 1) the speed of solidification process for the melt surrounding the keyhole and 2) the speed of melt backfilling the keyhole. If the solidification process is too fast and completed before the keyhole is filled, pores will be formed. A technique to control the laser power trailing when it is turned off during the keyhole collapse process has been proposed and experimentally validated to postpone the solidification speed and, as a result, to prevent the porosity formation. However, this method fails for a “deep” keyhole. In this study, an electromagnetic force is used to control the melt backfill flow which is proved to be very effective in preventing porosity from occurring. A mathematical model has been developed to calculate the transient heat transfer and fluid flow during the keyhole formation and collapse processes in pulsed laser welding. The continuum model is used to handle the entire domain including solid phase, liquid phase and mush zone. The enthalpy method is employed to handle the absorption and release of latent heat during melting and solidification. The laser induced plasma inside the keyhole due to the Inverse Bremsstrahlung (IB) absorption is considered and the temperature distribution inside the keyhole is calculated. Both the Fresnel absorption and multiple reflections of laser beam energy at the keyhole walls are also considered. Parametric studies to determine the desired strength of the electromagnetic force and its duration under several laser welding conditions have been conducted. Computer animations showing the keyhole formation and collapse, metal flow, and possible formation of pores will be presented.