Vapor Bubble Growth Research Articles

Numerical simulation of the growth and interaction of vapour bubbles in superheated liquid jets

Operating fluids for steering and propulsion of orbital manoeuvring systems are to be changed from toxic substances to environmentally less harmful alternatives. Liquid oxygen (LOX) can be used as oxidizer but the near vacuum conditions of outer space lead to a fast expansion into a superheated state when LOX is injected into the combustion chamber. The disintegration of the liquid jet is driven by bubble nucleation and growth, which is called flash boiling. These processes typically occur at small length and time scales and need to be modelled in macroscopic simulations of the entire combustion chamber. The goal of the present work is a quantification of bubble growth rates and bubble-bubble interactions that can be used to improve the existing submodels needed for full-scale simulations of the entire thruster. We use direct numerical simulations (DNS) of bubble clusters and compare the resulting growth rates to standard models for single bubble growth. The DNS solver is based on a discontinuous Galerkin approach and combined to a level-set equation to transport the interface between liquid and vapour. A modified HLLC Riemann solver accounts for the phase transfer. The computations show that vapour bubbles grow more slowly in the center of a jet than at its surface. The bubble radii exponentially decrease with distance from the liquid jet interface and growth rates are reduced by more than 70% in the center of the jet such that their volumetric expansion can be neglected for the computation of the jet expansion. The reduced growth can be associated with the interactions of the pressure fields surrounding the bubbles as the liquid pressure increases due to bubble growth and evaporation. The degree of superheat is locally reduced and bubbles grow at a smaller rate. The growth rates of individual bubbles can be parameterised with the local degree superheat, which may serve as a potential sub-scale model. These findings hold for various operating conditions that are characteristic for LOX flash evaporation under vacuum conditions.

Experiments to compare the dynamics and thermal impact of single vapor bubble subjected to upward and downward flow boiling configurations

ABSTRACT Experiments on subcooled flow boiling have been carried out to investigate the dynamical parameters and thermal impact of a growing vapor bubble under vertically upward and downward flow configurations with water as the fluid medium. Bubble growth and detachment phenomena have been visualized in real-time using high-speed rainbow schlieren technique. Schlieren-based imaging technique simultaneously maps both the bubble dynamics and thermal gradients field along the heater surface. Experimental data have been compared for a range of Reynolds number (Re = 3600–7200) at a constant subcooling level of ΔTsub = 4°C. The thermal aspects have been identified in the form of real-time visualization of thermal boundary layer and its scavenging as the bubble detaches/lifts-off from the heater surface. Time variation of various bubble dynamic parameters have been determined from the quantitative processing of the schlieren images. Significant differences in the bubble trajectory after departure from the nucleation site were observed for downward and upward flow configurations. For Re ≤ 6000, comparatively higher evaporative heat flux dissipation was obtained for upward flow, while at Re ≥ 6000, the evaporative heat flux dissipation was found to be more or less similar for both upflow and downflow configurations.

On the transition between contact line evaporation and microlayer evaporation during the dewetting of a superheated wall

We present experimental results on the transition between contact line evaporation and microlayer evaporation during the dewetting of a superheated wall. The length and thickness of the liquid microlayer is measured, both showing a dependency on the wall superheat, dewetting velocity, and heating power.This work falls within the continuing efforts of understanding the formation of the so called microlayer: a thin liquid film, that is observed for example during boiling underneath a growing vapor bubble. The short lived and small scale nature of the phenomenon make accurate measurements difficult, therefore a novel experimental set-up is proposed that allows high resolution measurements in a steady-state manner. The time resolved formation of the microlayer is described in detail and the influence of the dewetting velocity, wall superheat, and heating power is discussed. Microlayer formation is observed only if a critical velocity is exceeded for a given combination of wall superheat and heating power. Below this threshold velocity, contact line evaporation dominates the heat transfer. The dependency of the microlayer length and thickness on the dewetting velocity is described using power laws. Instability effects near the contact line are observed at high dewetting velocities and high wall superheat. It is concluded that microlayer evaporation and contact line evaporation exist in separate regimes, obviously separated by a parameter combination of dewetting velocity, wall superheat, and heating power.

Conjugate heat transfer analysis of liquid-vapor two phase flow in a microtube: A numerical investigation

Flow boiling in microchannel is a promising way of high heat flux removal. In account of this, numerous experimental and numerical studies have been performed on flow boiling in conventional as well as in microchannels focusing on heat transfer, bubble dynamics and flow pattern while studies on conjugate heat transfer in flow boiling are yet to be explored. This work presents a systematic three-dimensional numerical study of conjugate effect of heat transfer in two-phase flow in a microtube subjected to constant wall heat flux. Volume of fluid (VOF) model along with phase change model are used to capture bubble shape and flow pattern. Water enters the microtube at a temperature of 300 K. The effect of (i) wall to fluid conductivity ratio (ksf), (ii) wall thickness to inner radius ratio (δsf), and (iii) inlet mass flux (G) in unsteady flow boiling condition are explored. Transformation of nucleating bubbles into elongated bubbles are characterised in terms of wall thickness (δsf) and its material conductivity (ksf). The results indicate that bubble length increases with increasing wall material conductivity but decreases with increasing wall thickness as well as mass flux. Growth and motion of vapor bubbles due to phase change are also explored. Comparison between single and two-phase flow under conjugate heat transfer scenario is also discussed.

Jet dynamics during vaporization of water drops in hot oil films

We experimentally investigate the vaporization of water droplets immersed in a hot oil film, with moderate temperaturedifferentials of 30–50 °C. In this regime, only a fraction of the water droplet vaporizes, but results in a symmetric vertical jet of oil. We provide detailed high-speed imaging of the physical process and quantitatively describe the vapor bubble growth, jet formation, jet speed and satellite droplets resulting from the jet break-up. The principal findings are that the jet speeds (vjet∼O(1) m/s) are inversely correlated to the oil film thickness (δ∼O(1) mm), whilst the surface tension-driven break-up results in primary satellite droplets whose normalized diameters also scale inversely with film thickness. Lastly, we provide observations of a water ‘peeling’ mechanism which occurs inside the vapor bubble, leading to a re-entrant jet that ultimately causes a secondary vaporization event.

Numerical Investigation of Heat Transfer During the Cooling of a Metal Cylinder by a Flow of a Gas-Liquid Medium in an Annular Channel

A mathematical model of conjugate heat transfer in the heterogeneous system "solid–gas–liquid medium" with account of the description of bubble vaporization in a liquid flow has been proposed. Consideration has been given to a two-dimensional nonstationary formulation taking account of the axisymmetry of cooling-medium flow about the longitudinal axis of a cylinder. Results of numerical calculations of thermophysical parameters during the cooling of a high-temperature metal cylinder manufactured from structural-grade steel by a flow of a gas–liquid medium in an annular channel have been given. The influence of the flow velocity on the value of the concentration of the vapor in the liquid and the regularities of cooling of the metal cylinder have been assessed. An analysis has been made of the influence of the formation, growth, and motion of vapor bubbles in the liquid on the distribution of the heat flux over the surface of the conjugate boundary of the metal cylinder and the gas–liquid medium.

Numerical simulation of secondary atomization of an emulsion fuel droplet due to puffing: Dynamics of wall interaction of a sessile droplet and comparison with a free droplet

Physical mechanisms of explosive boiling of an ethanol-in-decane emulsion droplet are investigated by numerical simulation. Vapor bubble growth due to explosive boiling leads to secondary breakup of the droplet. The ethanol mass ratio is chosen in the regime of puffing (vapor ejection and partial breakup), which is generally likely to occur for a fuel spray in a combustor. Using both free and sessile droplet configurations and varying the initial number of boiling bubbles and the depth of the bubble formation in the dispersed sub-droplets, the droplet breakup and vapor mixing processes are discussed. For both free and sessile configurations, the basic dynamics of puffing is the same. When the bubble depth is deeper in the sub-droplet, the bubble growth is eventually larger. When the number of bubbles is more, puffing occurs several times, or the bubbles coalesce leading to larger bubble size, larger breakup and enhanced vapor mixing. In the sessile droplet configuration, the vapor ejection dynamics is essentially similar, but particularly when puffing is strong, the droplet bounces from the wall, first pushed toward the wall by the ejected vapor and then pulled from the wall due to the repelling and recoiling motion. Under such a condition, the initially attached droplet finally detaches from the wall and returns into the ambient gas. This phenomenon may transiently influence the mixture formation in a combustor when a fuel spray collides with the combustor wall. The present study serves as a first step toward a goal of quantitatively evaluating the puffing/microexplosion effect on a real fuel turbulent spray in internal combustion engines as to how large it can contribute to improving combustion performance, and insights have been obtained for future modeling which is expected to fill the gap between the droplet-scale and spray-scale knowledge.

Single vapour bubble growth under flash boiling conditions using a modified HLLC Riemann solver

Common fuel-oxidizer combinations in orbital manoeuvring systems consist of toxic substances but will be replaced in the future. Liquid oxygen (LOX) is one potential oxidizer but it rapidly superheats under the initial low-pressure conditions in the combustion chamber. Bubble nucleation and growth dominate the efficient disintegration of the liquid jet, which is called flash boiling. An investigation of the small scale bubble dynamics will help to improve existing models for the break-up of the LOX jet and the mixing with the fuel. Direct numerical simulations (DNS) can be used to analyse the underlying processes if the solver handles phase transition effects at extreme ambient conditions. In this paper we use a fully compressible discontinuous Galerkin solver combined to a level-set equation and to a modified HLLC Riemann solver. The main goal of our investigation is to assess closure conditions for the mass transfer models which accurately represent the physics of vapour bubble growth. We couple three models for the estimation of vaporisation mass fluxes to the interface Riemann solver. The Hertz-Knudsen relation and a kinetic relation predict the volumetric expansion well but cannot represent the instantaneous mass flux. A sub-grid scale heat flux model predicts the mass flux qualitatively better but the volumetric bubble expansion matches only at late time intervals. The dependency of the calibrated coefficients on the physical conditions is similar for the different vaporisation models.

Nucleate pool boiling enhancement by ultrafast water permeation in graphene-nanostructure

Nucleate boiling efficiency can be significantly enhanced using the nanostructured graphene surface. The wettability of the graphene nanoplatelets (GNPs) coated surface is tunable using thermal curing process. The uncured GNPs surface is hydrophobic in nature while the cured GNPs surface manifests a hydrophilic characteristic and the latter performs better than the former in the nucleate boiling regime. Ultimately the ultrafast water permeation property of the cured GNPs enhances the efficiency of nucleate pool boiling, leading to a maximum enhancement of 151% in the boiling heat transfer coefficient and 154% in the vapor mass flow rate. The boiling performance of the uncured GNPs surface is also enhanced despite its hydrophobicity. Hence, the enhancement in boiling is not solely attributed to the surface wettability. The enhancement is attributed to the fascinating ultrafast water permeation property of graphene on top of its nanoporous structure. The unimpeded fast water transport within the nanochannel-network of GNPs facilitates the fast absorption of latent heat of vaporization by the water molecules, leading to a substantial increase in the nucleation, growth and departure of vapor bubbles. We propose a new explanation on the role of graphene coating on nucleate pool boiling enhancement. This study provides important insights into the effects of ultrafast water permeation property of graphene on the nucleate boiling heat transfer.

Mechanical damage induced by the appearance of rectified bubble growth in a viscoelastic medium during boiling histotripsy exposure

In boiling histotripsy, the presence of a boiling vapour bubble and understanding of its dynamic behaviour are crucially important for the initiation of the tissue fractionation process and for the control of the size of a lesion produced. Whilst many in vivo studies have shown the feasibility of using boiling histotripsy in mechanical fractionation of solid tumours, not much is known about the evolution of a boiling vapour bubble in soft tissue induced by boiling histotripsy. The main objective of this present study is therefore to investigate the formation and dynamic behaviour of a boiling vapour bubble which occurs under boiling histotripsy insonation. Numerical and experimental studies on the bubble dynamics induced in optically transparent tissue-mimicking gel phantoms exposed to the field of a 2.0 MHz High Intensity Focused Ultrasound (HIFU) transducer were performed with a high speed camera. The Gilmore-Zener bubble model coupled with the Khokhlov-Zabolotskaya-Kuznetsov and the Bio-heat Transfer equations was used to simulate bubble dynamics driven by boiling histotripsy waveforms (nonlinear-shocked wave excitation) in a viscoelastic medium as functions of surrounding temperature and of tissue elasticity variations. In vivo animal experiments were also conducted to examine cellular structures around a freshly created lesion in the liver resulting from boiling histotripsy. To the best of our knowledge, this is the first study reporting the numerical and experimental evidence of the appearance of rectified bubble growth in a viscoelastic medium. Accounting for tissue phantom elasticity adds a mechanical constraint on vapour bubble growth, which improves the agreement between the simulation and the experimental results. In addition the numerical calculations showed that the asymmetry in a shockwave and water vapour transport can result in rectified bubble growth which could be responsible for HIFU-induced tissue decellularisation. Strain on liver tissue induced by this radial motion can damage liver tissue while preserving blood vessels.

Molecular Dynamics on the Effect of Vaporization Rate near the Wall Surface on the Vapor Bubble Growth

Molecular Dynamics on the Effect of Vaporization Rate near the Wall Surface on the Vapor Bubble Growth

Characteristics of Boiling Heat Transfer on Hydrophobic Surface

The paper presents the results of an experimental study of the effect of hydrophobic fluoropolymer coating on the multiscale characteristics of heat transfer at water boiling. New experimental data on dynamics of vapor bubble growth and detachment, evolution of contact line, nucleation site density, heat transfer coefficient were obtained using high-speed imaging techniques, including infrared thermography and video recording from the bottom side of transparent ITO heater. It was shown, that the using of hydrophobic fluoropolymer coating leads to heat transfer enhancement, to decrease of the superheat temperature at the onset of boiling, to increase of the active nucleation site density and to significant change in the dynamics of growth and departure of vapor bubbles and the evolution of the triple contact line.

A laser interference/high-speed photography method for the study of triple phase contact-line movements and lateral rewetting flow during single bubble growth on a small hydrophilic heated surface

The laser interferometric method and high-speed camera techniques are used simultaneously to study dynamic characteristics of a single vapor bubble growth and departure from an ITO heater surface submerged in ethanol. Two high-speed cameras are used simultaneously in this experiment: one is used to record interference fringes of the microlayer at the bubble's bottom and the other is used to record the growth and detachment of the single bubble synchronously from the side view. By comparing these images frame by frame, it is found that the bubble's growth time can be divided into two periods sequentially: the microlayer period and the lateral rewetting period. Variations of dry spot radius, bubble contact radius, bubble volume and diameter, dynamic contact angles during these two periods are illustrated.

Numerical investigation of bubble dynamics and heat transfer in subcooling pool boiling under low gravity

Abstract The numerical simulation of a single vapor bubble growth in subcooling liquid under different gravity conditions has been carried out. In the numerical model, a thin superheated layer and the thermocapillary convection caused by the surface tension variation along the surface are considered. The continuity equation and energy equation are modified to allow for the phase change. In addition, the thermocapillary convection effect has been included in the momentum equation. The vapor-liquid interface is captured by the phase field method. The results show that the bubble behavior in the numerical model agrees well with previous experiments conducted in high subcooling liquid under microgravity. The effects of gravity level, contact angle and wall superheat on the bubble growth, critical subcooling (the liquid subcooling under the condition that the evaporation rate of a bubble is equal to its condensation rate), together with heat transfer have been investigated. The growth period and departure radius both reduce with the increase in gravity level, while the critical subcooling increases slightly. Large contact angle at the three-phase contact line augments the departure radius. However, the critical subcooling decreases as contact angle increases. With the wall superheat increasing, the growth period reduces rapidly, while the departure radius and the critical subcooling increase. What’s more, the non-departing bubble adhering to the surface would prevent heat transfer with a dry spot, which may damage the heating element in application.

Direct numerical simulations of incompressible multiphase magnetohydrodynamics with phase change

A new phase change model has been developed for the simulation of incompressible multiphase magnetohydrodynamics based on the Volume-of-Fluid method. To decrease the pressure oscillations when large density contrasts are present between the liquid phase and the vapor phase, a smooth distribution of sharp mass transfer rate within a narrow region surrounding the interface is adopted, and a ghost-cell approach is used to impose the saturating temperature at the liquid–vapor interface when solving the energy equation. After that, the method has been implemented in an incompressible multiphase magnetohydrodynamics solver developed in our previous work (Zhang and Ni (2014) [3]). Moreover, when computing the electromagnetic fields, a cut-cell approach is implemented to keep the sharpness of the interface, which is treated as an electrically insulating boundary as it translates and deforms with the fluid. The phase change model has been verified for a series of one-dimensional, two-dimensional and three-dimensional problems, while the numerical results agree well with either the theoretical solutions or the experimental data. In particular, by simulating the vapor bubble rising in superheated liquid under nonzero gravity in presence of external magnetic field, the magnetohydrodynamics effect on the vaporization of the rising bubble is investigated and we observe the magnetic fields to suppress the vapor bubble growth during the phase change. At last, both two-dimensional and three-dimensional film boiling simulations are conducted, which show good qualitative agreement with heat transfer correlations, and the vapor bubble is found to elongate along the direction of the magnetic field during its growth, moreover, the time instant for the vapor bubble to detach from the film is also delayed.

A Numerical Investigation of Nucleate Boiling on Enhanced Surfaces by Lattice Boltzmann Method

To study the effect of enhanced surface geometries on nucleate boiling heat transfer, the numerical simulation of nucleate boiling on enhanced surfaces are performed by lattice Boltzmann method. For validation of the present model, a vapor bubble growth on and departure from plain surface is simulated and compared to experimental results. The numerical results are in good agreement with existing experimental results. By means of present model, the processes of bubble growth on and departure from triangular and rectangular structure surface are simulated, respectively. The numerical results demonstrate that the rectangular structure surface possesses the highest performance for heat transfer during nucleate boiling. The plain surface has the lowest heat transfer efficiency. The mechanisms contributing to the high thermal performance of enhanced surfaces are discussed. In addition, through the analysis of bubble shapes and temperature fields, it is found that the simulation results support the transient micro-convection model and the thin film evaporation model.

Study on Local Heat Transfer in the Vicinity of the Contact Line under Vapor Bubbles at Pool Boiling

The results of an experimental study of the dynamics of local heat transfer at nucleate pool boiling of liquids are presented. Experimental data on the nucleation site density and the evolution of the temperature field underneath individual vapor bubbles were obtained by high-speed infrared thermography with high spatial and temporal resolutions. Deionized water and ethanol at the saturation line under atmospheric pressure were used as working liquids. Evolution of the distribution of the local heat flux rate in the region of an individual nucleation site has been constructed based on numerical simulation. It has been shown that the maximum rate of the local heat flux is observed in the region of the liquid microlayer during the period of vapor bubble growth and reaches a value exceeding the average heat flux rate by 15–20 times. Based on the results, the thickness of the microlayer underneath the vapor bubble during the period of the bubble growth was estimated. The estimates satisfactorily agree with experimental literature data obtained with the use of laser interferometry.

Ignition and combustion behavior of mechanically activated Al–Mg particles in composite solid propellants

Metallized propellants typically produce large agglomerates that result in two-phase flow losses. Tailoring composite metal fuel particles can improve ignition and combustion characteristics while reducing product droplet sizes. In this work, mechanically activated (MA) aluminum (Al) and magnesium (Mg) powders are synthesized, characterized and compared to magnalium (Mag) alloy, neat Al, neat Mg and their physical mixtures (PM) at the same 1:1 mass ratio as MA and Mag powders. CO2 laser ignition tests showed that the MA powders are more reactive than those of Mag and exhibit particle fragmentation upon ignition. Mag powders only show fragmentation and microexplosions at high heating rates. The burning rates of the ammonium perchlorate/hydroxyl-terminated polybutadiene composite propellants containing MA powders were the highest, compared to Mag, neat Al, PM and neat Mg in decreasing order. High-speed imaging of the propellants and product collection showed that MA, Mg and Mag powders produce much smaller agglomerates than neat Al or PM at lower pressures due to fragmentation and Mg vaporization. However, the microexplosion tendency of the MA and Mag particles in the propellants was reduced at higher pressures due to reduced vapor bubble growth. Out of all the materials investigated, MA particles provide the best combination of burning rate and product sizes.

Bubble dynamics and atomization mechanisms in burning multi-component droplets

We examine the complete sequence of events associated with the transition in the topology of a single droplet into multiple fragments of secondary droplets in the context of burning multi-component miscible mixtures. The multi-component blends consist of tetradecane as a lower volatile component, while butanol and acetone-butanol-ethanol (A-B-E) are used as the higher volatile constituents. In addition to the widely recognized theory of bubble growth via micro-bubble coalescence, we reveal that the vapor bubble growth also occurs through the merging of large bubbles during the combustion of droplets. The initial bubble growth (Regime I) and collapse cycles were found to increase the rate of bubble nucleation in the droplet, which in turn leads to the growth and merging of two or more vapor bubbles into a single larger bubble (Regime II). The final stage of bubble growth (Regime III) is associated with the Rayleigh-Taylor (RT) instability at the vapor-liquid interface. After the inception of the RT instability, capillary wave propagation is also witnessed on the droplet surface. The breakup of a vapor bubble results in the creation of a ligament that subsequently undergoes pinch-off into one or more secondary droplets. The ligament pinch-off mechanisms are categorized into two types, i.e., tip breakup and tip-base breakup, which govern the diameter and velocity of secondary droplets along with succeeding volumetric shape oscillations in the parent droplet. In particular, the ligament tip-base pinch-off mechanism results in a bimodal distribution of secondary droplets. After the initial breakup event, a vapor bubble may grow either in the secondary droplet or inside the developing ligament, leading to a sequential cascade of breakup events.

Validation of a dynamic model for vapor bubble growth and collapse under microgravity conditions

A complete set of ordinary differential equations, based on modifications of existing models, is used to investigate bubble growth and collapse under microgravity conditions in this paper. As in previous work, effects of inertia, surface tension and viscosity are taken into consideration in the momentum equation of the liquid phase outside of vapor bubble, while effects of “moving interface”, interface curvature and thermal resistance of surrounding liquid are considered in the evaporation of the vapor bubble. A dimensionless fitting constant b is introduced to account for area change of the moving vapor/liquid interface and the diffusive nature of the interface layer. The values of these fitting constants for bubble growth and bubble collapse in water and ethanol are obtained by matching predicted temporal variations of bubble radii with experimental data. The predicted interfacial dynamics during bubble growth and bubble collapse is analyzed. Different stages during the bubble growth process are characterized. During the early stage of bubble collapse, the simulated bubble radii show some “fluctuations”, which can be attributed to the “rebound effect” of pressure balance in the bubble owing to the initial condition of a sudden drop in temperature.