Numerical approach to non-spherical vapour bubble dynamics

An explicit finite difference analysis has been applied to the growth and collapse of a vapour bubble in sub-cooled water. It is assumed that only mass transfer across the liquid-vapour interface is important and that heat transfer can be neglected. The results of this computational analysis are compared with experimental results from the literature and shown to be in good agreement at sub-cooling of 1 C and 10 C.

For these experimental conditions the analysis shows that the bubble grows in a roughly spherical fashion but collapses in highly non-spherical manner.     

Temperature variation and collapse time at the condensation of vapour bubble

The collapse of vapour bubble in subcooled liquid is studied in view of the fact that the predicted duration is delayed in comparison with the recorded graphs of size reduction. Deduced from well-known physical laws, a hypothesis was elaborated by which the delay is attached to neglecting the vapour temperature variation during the process, on the one hand, and disregarding the effect of the interfacial surface energy, on the other hand. The temperature rise of vapour bubble during its collapse, tending to and ending in the critical state, is explained, as well as the interrelation of the interfacial surface tension and latent heat is yielded. The hypothesis seems to agree well with the character of experimental results, and to be iustified by the studies on the mechanism of cavitation damage. Boänjakovic's heat balance equation and other functions are produced by means of dimensionless groups adequate for bubble.     

Formation of a liquid jet after detachment of a vapour bubble

Originated by the action of Laplace-pressure at the moment of break-off of a vapour bubble, a liquid jet can form, penetrate the bubble and pierce the bubble vertex at a relatively high velocity. The prehistory of the jet formation, including the detachment process of a single vapour bubble are described in the paper. Photographs of a vapour bubble, generated on a horizontal heated surface, show clearly the process of bubble detachment, the formation and the development of a liquid jet immediately after the bubble break-off. Refrigerant R113 containing 1% (mass) of oil was used as the test liquid.     

The dynamics of spherical bubble growth

Spherically symmetric bubble expansion in uniformly superheated infinite pools of liquid have been simulated numerically. Bubble growth curves have been generated for a range of Jakob numbers, 3?Ja?3167, by altering the initial metastable state of the system facilitated by changes in the initial superheat and system pressure. The detailed physics of vapour bubble growth is studied through delineation of the parameters governing the changes in the growth dynamics from surface tension, to inertia dominated, to diffusion controlled, and the domains between them.     

The characteristic functions of multiphase fluid systems and the thermodynamic interpretation of vapour bubble collapse

The effects of phase interface and surface tension in multiphase fluid systems are investigated. The modification of the entropy, free energy, internal energy, enthalpy, free enthalpy, and their differentials are deduced for both droplet and vapour bubbles of uniform and nonuniform temperature and size distribution. The investigation yields also the latent heat, the Clausius-Clapeyron equation, and the polytropic exponent, adapted for disperse state. The relationships deduced for one-component systems are generalized for two-and more-component systems. It is presented that in the course of bubble size reduction both the vapour temperature and pressure arise much over the critical values taken in the usual sense, since the saturation state, the coexistence curve, and the critical state are significantly modified because of the bubble size in submicroscopic order of magnitude. Also the interdependence of bubble annihilation and cavitation damage is explained.     

Molecular dynamics simulation on bubble formation in a nanochannel

Molecular dynamics simulations are carried out to examine the bubble behavior confined in a nanochannel with particular emphasis on the nucleation phenomenon. Simple Lennard-Jones fluids are under consideration and nano-sized bubbles are observed under different conditions of solid–liquid interfacial wettability. It is found that the bubble nucleation behavior shows a marked dependence on the solid–liquid interfacial interaction. In particular, it is found that bubbles appear in the bulk liquid homogenously for a hydrophilic surface, but grow directly on a hydrophobic solid surface. Also, a bubble will not form on a non-wetting surface. A nanobubble exists stably under the equilibrium state and the number density distribution of the curved liquid–vapor interface is examined. It is also found that there are few vapor atoms in the nano-sized bubble and the internal vapor pressure of the nanobubble is much lower than that required from the Young–Laplace equation. The disagreement with the prediction of the Young–Laplace equation can be attributed to the fact that the liquid–vapor interface region plays an important role on the force balance at the curved liquid–vapor interface of a nanobubble.     

Microscale heterogeneous boiling on smooth surfaces—from bubble nucleation to bubble dynamics

In this investigation, boiling incipience and bubble dynamics on a microheater with a geometry of 100 μm × 100 μm fabricated with MEMS technology are evaluated using a high-speed digital camera. For the purpose of comparison with conventional boiling heat transfer, boiling incipience and bubble dynamics are also studied on a carefully selected microheater with a fabricated defect (i.e., a microcavity on the heater surface). Of industrial interest are the effects of dissolved gases on boiling incipience and bubble dynamics, which are also discussed in detail. The possible nucleation temperature (or incipience temperature) is analyzed and discussed from the perspective of the measured bulk temperature of the microheater and a 3D heat conduction numerical model. The time-resolved bubble dynamics (i.e., the bubble size evolution, interface velocity and interface acceleration) are all presented along with high-speed digital images. Based upon this investigation, it is clear that explosive boiling can take place on a smooth surface no matter how slow the heating rate, and dissolved gases have a significant influence on the incipience temperature and bubble behavior. Furthermore, this study illustrates that the classical kinetics of boiling can explain the explosive boiling occurring on a smooth surface in principle and can provide a useful guide for the design of microscale heat transfer and/or MEMS devices. Although unexpected, due to the gravitational effects, Marangoni flow on the vapor–liquid interface induced by the temperature gradient was also observed.     

Comparison of methanol-based working fluid combinations for a bubble pump-operated vapour absorption refrigerator

Miniaturization of an alcohol-based absorption refrigerator requires an air-cooled absorber and condenser and the replacement of customary solution pumps by the bubble pump. Evaluation of such a refrigerator requires thermodynamic (specific heat and heat of mixing) and thermophysical (vapour pressure, density, viscosity, surface tension and solubility) properties of refrigerant–absorbent solution. These property correlations for five alcohol-based working combinations, majority of them obtained by curve fitting, have been complied and presented in this paper along with their validity ranges and percentage of error. The working fluids have been analyzed and compared with reference to the solution density governing the hydrostatic height, viscosity and specific heat affecting the heat and mass transfer and solubility to avoid crystallization. Further the variations of performance parameters like cut-off temperature, circulation ratio, coefficient of performance and efficiency ratio of these working fluids with respect to various operating conditions are discussed. © 1998 John Wiley & Sons, Ltd.     

Numerical simulation of bubble dynamics in the gravitational and uniform electric fields

A coupled volume-of-fluid, level set, and smoothed physical parameter (VOF+LS+SPP) method based on FLUENT is used to simulate bubble dynamics in the gravitational and uniform electric fields. Both of the bubble and surrounding medium are assumed to be perfect dielectrics with constant but different permittivities. The effects of electric Bond number, permittivity ratio, Morton number, and Eotvos number on the deformation and rising motion of a single bubble are systematically investigated. Simulation results show that a vertical electric field elongates the bubble along the electric field direction and accelerates the bubble rising. The electric Bond number has a much greater effect on bubble deformation and rising velocity than the permittivity ratio. The bubble behaviors in the electric field are similar for different Morton numbers but totally different for various Eotvos numbers. The influence of electric field on bubble Reynolds number changes a little for different Morton numbers but decreases distinctly with the increase of Eotvos number.     

Lattice Boltzmann simulation to study multiple bubble dynamics

Lattice Boltzmann method (LBM) has been used in this study to understand the behavior of bubble motion and bubble coalescence in liquids. Highly isotropic gradient vectors have been obtained on a lattice for two-phase simulations using LBM. For a fully periodic domain, bubble dynamics and shape for a single bubble and multiple bubbles are dependent on Eotvos number, Reynolds number and Morton number. For single bubble simulations, computations were done for high Eotvos and low to moderate Reynolds numbers, and the results are matched with the experimentally quantified flow visualization chart. The drag coefficient for single bubble motion under buoyancy for both two- and three-dimensional simulations compares well with existing correlations. For multiple bubbles, the bubble dynamics is dictated by the vortex pattern of the leading bubble, which allows the bubbles to coalesce. Coalescence can be described as a three stage process: collision; drainage of the liquid film between adjacent bubbles to a critical thickness; and rupture of this thin film of liquid. Such simulations have also been run for different configurations of the initial bubble distribution for both in-line and staggered bubble configuration to show the effect of vortex shedding on the oscillatory motion of the bubbles and subsequent coalescence.     

On the dynamics and heat transfer of bubble train in micro-channel flow boiling

The dynamics and heat transfer characteristics of flow boiling bubble train moving in a micro channel is studied numerically. The coupled level set and volume of fluid (CLSVOF) is utilized to track interface and a non-equilibrium phase change model is applied to calculate the interface temperature as well as heat flux jump. The working fluid is R134a and the wall material is aluminum. The fluid enters the channel with a constant mass flux (335 kg/m² 1 s), and the boundary wall is heated with constant heat flux (14 kW/m²). The growth of bubbles and the transition of flow regime are compared to an experimental visualization. Moreover, the bubble evaporation rate and wall heat transfer coefficient have been examined, respectively. Local heat transfer is significantly enhanced by evaporation occurring vicinity of interface of the bubbles. The local wall temperature is found to be dependent on the thickness of the liquid film between the bubble train and the wall.     

Transient heat transfer and bubble dynamics in a pressurized fluidized bed

A flow visualization technique for studying the gas bubble dynamics in a pressurized fluidized bed was developed and used to quantify these dynamics at the surface of a vertical tube submerged in the bed. Transient heat flux measurements were made and correlated with bubble motion. As a result, it is concluded that the heat transfer process is strongly affected by bubble dynamics and is much more complex than any of the generally accepted models can predict. It is also shown that the overall bed operating conditions are the primary driver for local bubble/particle motion around the tube which significantly affects the time-dependent fluctuation in local heat transfer.     

On the profile of the liquid wedge underneath a growing vapour bubble and the reversal of the wall heat flux

Combining mass and energy balances, a differential equation for the profile of a liquid wedge underneath a vapour bubble that is growing on a solid surface is derived. It connects the spatial and temporal changes of the film (wedge) thickness with the spatial temperature changes and velocity of liquid at the interface. Specifying particular conditions, the equation reduces to those from the literature. The paper brings further an illustrative explanation of why the wall heat flux in the wedge region may reverse its direction.     

Confined growth of a vapour bubble in a capillary tube at initially uniform superheat: Experiments and modelling

Bubble growth was triggered in a capillary tube closed at one end and vented to the atmosphere at the other and initially filled with uniformly superheated water. Measurements of the rate of axial growth and the varying pressure at the closed end were used to test under these simplified conditions assumptions employed in one-dimensional models for bubble growth applicable to the more complex conditions of confined-bubble flow boiling in micro-channels. Issues included the thickness of the liquid films round confined bubbles and changes in saturation temperature due to the changes in pressure generated by bubble motion. Modelling features requiring further attention were identified, such as the possibility of “roll-up” of the liquid film due to a large dynamic contact angle.     

High resolution measurements of wall temperature distribution underneath a single vapour bubble under low gravity conditions

Heat transfer performance in nucleate boiling crucially depends on a circular thin film area near the foot of a vapour bubble where high heat fluxes and thus high local evaporation rates occur. The corresponding wall temperature drop close to the tiny thin film area is computed using an existing nucleate boiling model. To verify the predicted temperature distribution, an experiment is designed with a thin electrically heated wall featuring two-dimensional, high resolution temperature measurement using unencapsulated thermochromic liquid crystals. By means of temperature measurements during a parabolic flight under low-g conditions, the validity of the model used to calculate the temperature distribution in the tiny thin film area could be confirmed.     

A molecular dynamics study of bubble nucleation in liquid oxygen with impurities

The present study investigates bubble nucleation in liquid oxygen with dissolved impurities (nitrogen or helium molecules) using molecular dynamics simulations. When the mole fraction of impurities is 0.05, there is a fundamental difference in the bubble nucleation mechanism between the two dissolved impurities cases; vaporization in the homogeneous bulk makes a bubble in the case of a nitrogen‐dissolved liquid while phase separation of impurities and liquid molecules makes a nucleus in the case of a helium‐dissolved liquid. Fluctuations can cause local voids, which in turn can grow to be bubbles, and this effect is stronger in the case of a helium‐dissolved liquid with a lower mole fraction (0.01) than in the case of a nitrogen‐dissolved liquid with a higher mole fraction (0.05). From these results, we conclude that helium molecules have a much stronger action to raise the bubble formation pressure compared with nitrogen. In this paper, the kinetically‐defined critical nucleus, which is a very important factor in quantitatively evaluating the nucleation mechanism, is also estimated through the calculation of the size change rate of each nucleus. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(7): 514–526, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20082     

The effects of liquid motion induced by phase change and thermocapillarity on the thermal equilibrium of a vapour bubble

A theoretical estimate is made of the liquid motion induced in the vicinity of a vapour bubble on a heated solid surface by evaporation and condensation at the bubble surface and by thermocapillarity effects. These results are used to examine the thermal equilibrium of the vapour bubble.