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Microchannel flow boiling is an efficient cooling solution for high-power-density miniaturized systems. Many studies on microchannel flow boiling focused on the dynamics of single vapor bubbles, while neglecting the interaction between bubbles, which is important in relevant applications. Here, numerical simulations are carried out to study the interaction between multiple vapor bubbles in microchannel flow boiling. The results show that for different numbers of bubbles in the microchannels with the same initial size and position of leading bubbles, the bubble size in a single-bubble microchannel is larger compared to the leading bubble of multiple-bubble cases because of heat absorption by the vaporization at the rear bubbles. As the initial volume ratio between the leading bubble and the rear bubble decreases, the leading bubble size in the downstream becomes smaller because of the reduced contact with the superheated thermal boundary layer. With increasing the Reynolds number, both the leading and the trailing bubbles increase slightly in size in the upstream of the heated region, because the bubbles at higher Reynolds number move faster and firstly get in contact with the superheated fluid. The increase in the bottom wall thickness increases the growth rate of the multiple bubble sizes with earlier bubble coalescence because of the higher upstream wall temperature by heat conduction in the solid wall. • Bubble in single-bubble scenario is larger than leading bubble of multiple bubbles. • Heat absorption of rear bubbles affects the expansion of the leading bubble. • Initial volume ratio influences contact with superheated thermal boundary layer. • Reynolds number influences bubble growth by affecting time to reach heated region. • Wall thickness influences bubble growth by affecting upstream wall temperature. [ABSTRACT FROM AUTHOR]

Evaporation of liquid nitrogen in another immiscible liquid occurs in many industrial applications. Existing oversimplified one-dimensional (1D) quasi-steady models, although can quantitatively predict the evaporation rate by introducing an empirical fitting parameter, rely on configurations inconsistent with experimental observation so more rigorous models are required to get in-depth physical insights and improve modeling capability. This study proposes a 2D quasi-steady-state theoretical model, free of fitting parameters, that predicts the bubble growth rate and estimates the heat transfer rate for a liquid nitrogen droplet evaporating inside a liquid pool within the spherical bubble regime. The droplet's shape and position within a spherical bubble are determined by the equilibrium between the gravitational force and the upward pressure force resulting from the vapor flow between the droplet and the pool. The vapor layer thickness is calculated to be on the order of 10 microns. Notably, the primary contribution to heat transfer arises from the lower portion of the droplet, leading to local heat flux values up to approximately six times higher at the bottom compared to the top. The predicted bubble growth is quantitatively consistent with experimental data within the capillary spherical bubble regime. Furthermore, the overall heat transfer rate Q exhibits a distinct scaling relationship with the volume ratio between the bubble and droplet, yielding Q ∼ (V b / V d) − 0.1. • Propose a fitting-parameter-free 2D model for evaporating liquid nitrogen droplet in an immiscible liquid pool. • The scaling law of droplet shrinkage rate, bubble growth rate, and heat transfer rate are presented. • The model predicted results are compared with open literature data revealing a good agreement. [ABSTRACT FROM AUTHOR]

• A superimposed model applicable for predicting the growth rates of both stationary and sliding bubbles is proposed. • The force balance model for predicting bubble departure diameter and lift-off diameter is modified. • Present modified model predicts both departure and lift-off diameters with mean absolute percentage errors of 25.0 % and 24.0 %, respectively. The calculation of heat flux for each component in the wall heat flux distribution model involves a crucial parameter, namely the characteristic bubble diameter. The bubble departure diameter and lift-off diameter are two of the most significant characteristic bubble diameters. In this paper, the bubble growth rate and bubble force analysis in subcooled flow boiling under various inclination angles of the heating surface are investigated, and the bubble departure and lift-off diameters are calculated based on force balance. According to the energy balance method, a superimposed bubble growth model is proposed that incorporates the effects of superheated layer evaporation around the bubble, microlayer evaporation, and condensation at the top of the bubble on the bubble growth rate. This paper proposes a force balance model for heating surfaces with varying inclination angles, based on accurate calculations of wall superheat. The model has been validated by a large amount of experimental data and exhibits superior accuracy and stability compared to existing models. [ABSTRACT FROM AUTHOR]

Highlights • A phase change model based on pseudo-potential Lattice Boltzmann model is proposed. • The overheat degree is introduced to replace the thermodynamic temperature. • The proposed model is tested in the simulation of the phase change problems with large density ratios. Abstract This paper proposes a phase-change pseudo-potential lattice Boltzmann model based on the bubble dynamic theory. To maintain the stability of the liquid during the phase change process, the phase change of the model is driven by the superheat degree in the interface area instead of the thermodynamic temperature in the equation of state (EOS). The presented model is verified by the simulations of liquid film evaporation and is applied to the simulation of the bubble growth in superheated fluid and on the wall. The simulation results show that the simulation density ratio over 100 can be reached. The influence of the bubble curvature on the superheat degree in the bubble can also be reflected with the proposed model. [ABSTRACT FROM AUTHOR]

• Elucidation of the nucleate boiling process in different positions of the barrel. • Research on the boiling phase change heat mechanism of the machine barrel. • The tail vortex disturbance enhances the boiling heat transfer from the barrel. • The rounded bottom of the barrel can improve the thermal capacity of the barrel. Gravity heat pipe barrels produce gas films during the processing of thermoplastic materials, which affect the heat transfer, so there is an urgent need to study the nucleate boiling process and mechanism at different locations in the barrel. The results showed that the local heat flux at the vapor-liquid-solid three-phase contact was the largest, and different process conditions had different effects on the departure diameter and departure time of boiling bubbles. The tail vortex disturbance generated by bubble departure drives the merging and growth of bubbles to departure, which in turn improves the heat transfer. The rounded bottom of the barrel reduced the air film and increased the heat transfer efficiency compared to the horizontal bottom of the barrel. This study can deepen the understanding of the mechanism of bubble boiling heat exchange in gravity heat pipe-type barrels and provide theoretical support for more relevant application scenarios. [ABSTRACT FROM AUTHOR]

The formation of heterogeneous vapour bubbles is widely studied due to its importance to two-phase thermal management systems, ultrasonic cleaning, and turbomachinery performance. However, the role that the surface plays in determining the growth of a bubble is still poorly understood. Currently, theoretical understanding of heterogeneous vapour bubble growth is limited to hemispherical bubbles or completely spherical bubbles next to a surface. We have previously developed an inertio-thermal model to accurately predict how heat transfer from the surrounding fluid affects homogeneous vapour bubble growth from the nanoscale to the macroscale (Sullivan et al. J. Fluid Mech. 948(A55):1-15, 2022). By accounting for the presence of the surface and its wettability on both the geometry of the bubble and on the available thermal energy in the surrounding fluid, we extend our model to capture heterogeneous vapour bubble growth. Using molecular simulations, we show not only how the strength of fluid-solid interaction affects the growth rate, but also how the formation of an adsorbed fluid layer under the bubble on lyophilic surfaces plays a vital role in determining the bubble shape and subsequent dynamics. These insights have potential to improve the performance of systems involving a change of phase from liquid to vapour by better understanding the role of surface wettability on this process. • Analytical model for heterogeneous vapour bubble growth developed. • Good agreement is observed between model predictions and molecular simulation results. • Formation of non-evaporating layer of molecules on highly wetting surfaces alters apparent contact angle of vapour bubbles. [ABSTRACT FROM AUTHOR]

• We have successfully accelerated the growing vapor bubble by solving the force balance equation for bubble velocity in a pool of superheated water. • The bubble growth and acceleration has been solved in two modes - isothermal growth (ODE solver) and non-isothermal growth model (PDE-ODE solver). • The energy balance at bubble surface includes the effect of bubble size on its motion. Our model computes bubble radius and velocity simultaneously. • The isothermal growth model can better explain the bubble growth at the initial stage, and the non-isothermal growth model at the final stage. • The non-isothermal growth of bubble is accompanied by the slowdown of the bubble translation Q3 in the superheated liquid. In the present work, we have numerically simulated the effect of bubble translation on the growth of a vapor bubble in a pool of superheated liquid based on the isothermal and non-isothermal model. The isothermal growth uses the ODE solver and the non-isothermal growth the PDE-ODE solver. The overall model at present considers the forces acting on the bubble to compute the bubble motion and the energy balance at the bubble surface to include the effect of bubble size on its motion. Thus, it computes equivalent bubble radius and bubble velocity simultaneously. The numerical simulation shows a remarkable improvement by our method over the previous numerical model while validating with the experimental and numerical results on bubble growth with the phase motion. The improved model has significantly reduced the deviation from the experiments reported in literature on the growth of a moving bubble. The aspect ratio of the bubble is computed to see the shape change of the bubble and is used to model the drag force accurately. It has been shown that the model of the isothermal growth can better explain the bubble growth at the initial stage, and the model of the non-isothermal growth at the final stage. The non-isothermal growth of bubble is accompanied by the slowdown of the bubble translation in the superheated liquid. [ABSTRACT FROM AUTHOR]

Highlights • Bubble growth model for two phase droplets in an immiscible liquid is developed. • Bubble is taken to be growing concentrically within the evaporating droplet. • Initial temperature profile in the droplet determines dictates growth profile. • Jacobs number and thermal conductivity ratio significantly affects growth rate. Abstract A numerical bubble growth model for two phase droplets in an immiscible superheated liquid is developed by solving the mass and energy conservation equations. The model is applicable for concentric two-phase droplets (where the bubble is completely surrounded by the volatile liquid) like those seen during evaporation of refrigerants suspended in water. The effect of pressure driven growth in the initial phases of bubble growth is shown to be minimal and is therefore ignored to simplify the model. While previous works assumed that the droplet is initially uniformly superheated, the current model assumes a more accurate temperature profile obtained by solving the radial heat diffusion equation in the droplet and surrounding liquid. The model is validated by measuring bubble growth rate within an FC-72 droplet introduced in a water bath and it is seen that the results are in close agreement with the model predictions. Experimental results from various cases in literature are also found to be well represented by the model. The effect of the liquid superheat and initial bubble diameter on bubble growth rate is also studied. Finally, a parametric study on the effect of droplet and bulk liquid properties on bubble growth rate is conducted to highlight the relative importance of the thermal conductivity and thermal mass ratios of the two liquids. [ABSTRACT FROM AUTHOR]

Highlights • Identified the minimum set of dimensionless parameters controlling microlayer formation. • Numerically reproduced the hydrodynamics of hemispherical bubble growth at a wall. • Generated a numerical database that resolves microlayer formation for various conditions of interest. • Identified remaining gaps in building a generally applicable model for microlayer formation. Abstract As a bubble grows outside of a cavity during nucleate boiling, viscous effects can be large enough compared to surface tension to impede liquid motion and trap a thin liquid layer, referred to as the microlayer, underneath the growing bubble. Numerical simulations of nucleate boiling typically resolve the macroscopic liquid-vapor interface of the bubble, but resort to subgrid models to account for micro scale effects, such as the evaporation of the microlayer. Evaporation models require initialization of the microlayer shape and extension, but models for microlayer formation are either physically incomplete or purely empirical. In this work, the Volume-Of-Fluid (VOF) method, implemented in the Gerris code, is used to numerically reproduce the hydrodynamics of hemispherical bubble growth at the wall, and resolve the formation of the microlayer with an unprecedented resolution. The simulations are validated against the latest experimental data and compared to existing analytical models. Lastly, remaining gaps in building a generally applicable model for the formation of the microlayer are presented. [ABSTRACT FROM AUTHOR]

Highlights • A novel model was developed to study bubble growth in superheated binary mixture. • Temperature and concentration were assumed quadratic within boundary layers. • PR equation and NRTL equation were adopted to estimate VLE at the bubble surface. • The maximum mass diffusion limitation on bubble growth rate was proposed. Abstract A novel model was developed to investigate the fundamental heat transfer and mass diffusion mechanisms of bubble growth in uniformly superheated ethanol–water mixture. In the proposed model, the energy equation was applied coupling with the quadratic temperature distribution within the thermal boundary layer. The mass diffusion effect was accounted by the introduction of species conservation equation in combination with the quadratic concentration distribution within the concentration boundary layer. Peng-Robinson equation of state and activity coefficient calculation were also adopted for the estimation of vapor-liquid equilibrium. In the present study, the maximum mass diffusion limited growth rate was proposed to quantify and illustrate the effect of mass diffusion on bubble growth. The results show that the bubble growth process in a binary mixture can be divided into three distinctive stages. The later stage of bubble growth is mainly subject to mass diffusion and partly to heat transfer at low ethanol concentrations. [ABSTRACT FROM AUTHOR]

Development of an accurate bubble growth model is central to the prediction of heat transfer coefficient in component scale wall-boiling formulations. The bubble growth models available in the literature are not generic enough to be applicable over a wide range of pressures. For example, pressurized water reactors operate at high pressures, where the experimental correlations are sparse. In this study, a framework for modeling wall bubble growth is developed, for water. This generalized model is synthesized in a form, which takes into account the factors that contribute to the bubble thermal layer deformation in a physically consistent way. These factors have been systematically evolved to account for a wide range of conditions (i) pressures of 1–180 bar, (ii) pool as well as flow boiling conditions, (iii) low as well as high subcooling, (iv) horizontal and vertical test section orientations, etc. Bubble growth predictions from the present model have shown very good agreement across a wide range of pressures. It was observed that, for pool boiling, the wake effect at the apex of the bubble has influenced the overall growth rate. On the contrary, for flow boiling, the flow induced distortions to the thermal layer were found to be dominant both at the base as well as the apex. In the latter case, bubble growth rate was found to be significantly dependent on the magnitude of these individual distortions. [ABSTRACT FROM AUTHOR]

The hydrodynamic formation of a microlayer at the base of a vapor bubble growing on a heated wall was experimentally and theoretically studied. Single bubble nucleate boiling experiments were conducted in a pool of saturated water under atmospheric pressure. A complete picture of the bubble geometry was obtained, including the three-phase contact line, microlayer and macroscopic bubble. This was performed using integrated laser interferometry, infrared thermometry and shadowgraph techniques. Existing models of the initial microlayer thickness that use an idealized hemispherical bubble shape and neglect surface tension force significantly overestimate microlayer thickness measured in experiment. The visualization results revealed that the non-hemispherical shape and surface tension of the bubble play a critical role in determining initial microlayer thickness. Theoretical analysis also indicated that the short-lived residual flow could play a key role in microlayer formation. Finally, a sophisticated model of initial microlayer thickness, developed to take into account the three identified mechanisms, provides predictions in agreement with experiments for different fluids. [ABSTRACT FROM AUTHOR]

A novel thermodynamic model was developed to calculate the bubble growth in the cavitation region of an injector nozzle. The influence of liquid pressure on bubble growth process was evaluated for both homogenous nucleus and heterogeneous nucleus. In this model, the energy equation coupling with the quadratic temperature distribution within the thermal boundary layer was applied with consideration of temporal variation of bubble pressure, radius, velocity and the thickness of the thermal boundary layer. The results show that the evolution of the bubble growth can be divided into three stages, i.e., surface tension controlled stage, transitory stage, and heat transfer controlled stage. The evolution processes of bubble growth from a homogeneous nucleus or a heterogeneous nucleus are much alike other than in the surface tension controlled stage. [ABSTRACT FROM AUTHOR]

Simulations of phase change rely on methods to compute heat and mass transfer at the interface. Current methods estimate the mass transfer with interpolation functions or by assuming a local temperature difference between saturation and interface. This work reports a method that uses a single cell around the interface to find the interfacial temperature gradient, and a linear interpolation normal to the interface to find the temperature of the mixture cell (cell with an interface). The one-cell algorithm for sharp interface and mass transfer (OCASIMAT) simplifies the estimation of the mass transfer and mixture cell temperature and improves accuracy. The proposed approach leads to a more realistic representation of the heat and mass transfer at the interface with a sharp discontinuity on the thermal properties at the interface and with mass transfer only at the cells with an interface. Simulations of planar interface evaporation and spherical bubble growth demonstrate the application of the proposed approach. Results indicate that OCASIMAT accurately predicts the temperature distributions near the interface and the interface displacements. [ABSTRACT FROM AUTHOR]

Bubble formation in flow boiling undergoes an ebullition cycle in which nucleation is followed by bubble growth, departure and a waiting period. Bubble growth rate plays an important role in bubble dynamics and also affects the whole ebullition cycle. It is difficult to make analytical calculation of bubble growth rate and departure diameter due to involvement of various forces and heat transfer processes on a nucleating bubble. In this paper, an analytical technique is proposed to calculate the growth rate and departure diameter of a nucleating bubble in subcooled flow boiling. Both force and energy balance approaches have been applied to calculate the growth and departure diameter of the bubble. Heat transfer contributions from evaporative microlayer beneath the bubble, superheated liquid surrounding the bubble and condensation from the bubble cap have been considered to calculate the bubble growth rate. In place of using a constant coefficient to represent the fraction of bubble in subcooled bulk liquid by existing analytical models, the present model correlates the coefficient with the operating parameters of flow boiling. An effort has also been made for accurate calculation of temperature distribution in heated surface and flowing liquid. Bubble growth rate and departure diameter have been calculated for different values of coolant mass flux, system pressure, applied heat flux, surface orientation angle and liquid subcooling. The heat transfer contribution from individual layer towards the growth rate of the bubble has been quantified and analyzed. The present analytical model has been validated with a wide range of experimental data and it produces better agreement with the experimental data compared to existing analytical models. [ABSTRACT FROM AUTHOR]

A series of new flash boiling sub-models inside a droplet were proposed in this study. Compared to the previous flash boiling model, the proposed model includes the classical homogeneous nucleation sub-model, the Rayleigh bubble growth sub-model, and a new bubble explosion sub-model. Both the energy and momentum balances inside a droplet are considered. By coupling with the variation of the fuel thermo-physical properties with temperature, the criterion of the bubble nucleation, growth, and explosion criteria are well predicted. The model is validated by the experimental data of superheated water. The computational results indicate that the flash boiling process can be divided into three stages, i.e., the initial stage controlled by the surface tension of the fuel, the rapid growing stage controlled by the heat diffusion in the droplet, and the final explosion stage. Based on the present model, the boiling explosion time is quantitatively provided at wide ranges of fuel temperature and ambient pressure which have the vital significance in predicting the secondary breakup of the droplet. [ABSTRACT FROM AUTHOR]

• Pool boiling heat transfer at atmospheric and sub-atmospheric pressure. • Bubble growth mechanism and characteristics in pool boiling. • Pool boiling heat transfer mechanisms. • Effect of pressure on bubble growth and heat transfer mechanisms in pool boiling. • Forces affecting bubble departure at atmospheric and sub-atmospheric pressures. Bubble growth rate is one of the most important parameters required for the development of accurate mechanistic nucleate boiling heat transfer models. It is also very important for understanding the hydrodynamic forces and the mechanism of bubble departure. This paper presents an experimental study on bubble growth measurements in saturated pool boiling of deionized water on a plain copper surface at atmospheric and sub-atmospheric pressure. The measurements were conducted using a high-speed, high-resolution camera with a microscopic lens. The mechanisms of bubble growth are discussed, while the microlayer evaporation mechanism has been evaluated and discussed using the measured bubble growth curve. The estimated contribution of microlayer evaporation to a single bubble growth is about 70 %, while the contribution of latent heat transfer (evaporation) to the total heat transfer rate from the surface is about 30 %. The remaining 70 % is due to other mechanisms, i.e. conduction and convection. These values were obtained based on the analysis of the bubble growth curve only and agreed with some researchers who conducted local heat transfer measurements using integrated sensors or infrared thermography. These detailed measurement techniques cannot be used with the thick copper block tested in the current study, which was also tested by many researchers in literature and is representative of industrially used surfaces. It was also found that the bubble departure mechanism at atmospheric pressure is due to a static balance between surface tension and buoyancy forces while at sub-atmospheric pressure, it was between buoyancy and liquid inertia forces. The pressure did not have a significant effect on the characteristics of the dynamic contact angle, which was also measured from the instantaneous images of the bubble. It was concluded also that the force balance required for the accurate prediction of departure diameter should be conducted when the two forces are equal, which occurred at time less than the departure time and dynamic contact angle of about 45. In most bubble departure models, researchers recommended the balance to be conducted at the moment of departure when the bubble forms a neck with contact angle of 900 (underestimation to the surface tension force). The analysis of one of the commonly used homogeneous growth models indicated that for homogeneous bubble growth models to be applicable in nucleate boiling, an allowance must be made for the fact that the degree of superheat varies with time during a bubble growth period. [ABSTRACT FROM AUTHOR]

Simulations of the formation of small steam bubbles indicate that the rate of growth of bubbles is very sensitive to the rate of evaporation of the micro-layer of liquid beneath the bubble. Such evaporation is rapid, and is modelled as being driven by the large heat flux through the thin liquid layer caused by the difference in temperature between the solid–liquid interface, and the saturation temperature in the interior of the bubble. However, application of this approach to recent experimental measurements of Jung and Kim generated anomalous results. In this paper we demonstrate that a model of the micro-layer heat flux that includes an allowance for the finite evaporative thermal resistance is able to eliminate these anomalies. This evaporative thermal resistance is a consequence of near-interface molecular dynamics, characterised by a quantity termed ‘evaporation coefficient’. Whilst in most engineering applications evaporative thermal resistance is small compared to conductive resistance, here, with the micro-layer thickness ranging from a few microns down to zero, it becomes of considerable importance. Selection of a molecular ‘evaporation coefficient’ to restore consistency to the anomalous measurements allows a plausible numerical value to be inferred. For the several times and multiple locations studied, a fairly consistent value of between 0.02 and 0.1 is indicated, (for saturated water in laboratory conditions), which itself is consistent with earlier literature values of this rather difficult quantity. It is shown that the evaporative resistance always represents a large fraction of the conductive resistance, and for important phases of the process dominates it. The need for inclusion of this phenomenon in the micro-layer models used in bubble analysis is clear. [ABSTRACT FROM AUTHOR]

This work explores single bubble dynamics on superheated superhydrophobic surfaces and the corresponding heat transfer mechanism of water pool boiling. The superhydrophobic surfaces are prepared by coating a thin layer of polytetrafluoroethylene on top of silicon substrates with electroless etched silicon nanowires. It is observed that a vapor film covers the whole superhydrophobic surface when it is immersed in water. Once the surface is superheated, single bubble forms and departs from the vapor film. We find that the bubble growth rate shows little dependence on surface superheat and applied heat flux at large superheats due to the presence of the vapor layer, which seriously limits the heat transfer from the superheated superhydrophobic surface. The bubble departure diameter is mainly determined by the static force equilibrium, but local disturbance and surface superheat also play important roles. There is a plateau of bubble departure frequency at large superheats, which is a result of the constant departure velocity limit. Accompanying the continuous bubble formation and departure, a smooth boiling curve is observed. The corresponding heat transfer coefficient (HTC) first increases and then slowly decreases as the superheat increases, showing a peak at a superheat of ∼50 K. Such HTC change is explained by the heat flux partitioning model, suggesting latent heat transport due to water vaporization and bulk liquid water convection after bubble departure contribute together to the heat removal on superhydrophobic surfaces. Insights gained from pool boiling on superhydrophobic surfaces might provide design guidelines for new surfaces excellent in heat removal or hydrodynamic drag reduction. [ABSTRACT FROM AUTHOR]

Bubble behaviors are closely related to the heat transfer performance during flow boiling in microchannel, however, the effect of channel cross-section decreasing on the bubble growth is still not fully understood at present. In this work, an experimental investigation is conducted to investigate the bubble growth characteristics during flow boiling in a single microchannel with 0.5 mm × 1 mm rectangular cross-section, and the heat transfer performance of flow boiling and its influencing factors are studied. Experiments are conducted with subcooled deionized water and the bubble behaviors are visualized by a high speed CCD camera installed upon the test section. Depending on the heat flux, different growth features are observed in the bubble growth process. Two kinds of bubble growth model are identified: the power law model in initial growth period and the linear law model in later period. The confinement effect of the microchannel is deemed as the mechanism causing the alteration of bubble growth models during its growth process. The deformation features of confined bubble are discussed to illustrate the intensification of evaporation on the liquid–vapor (LV) interface at bubble root, which increases the growth rate of bubble in its confined growth period as well as the heat transfer capability of bubble. Therefore, the maximum local heat transfer coefficient along the channel is found in the region where confined bubble and/or short elongated bubble flow pattern are dominant. Moreover, the heat flux is found to have great influence on the overall heat transfer performance of flow boiling in microchannel, but the effect of mass flux is much less. [ABSTRACT FROM AUTHOR]

The pool boiling heat transfer of deionized water on copper foams was experimentally studied under saturation conditions. Bubble growth behavior was captured by a high-speed camera. Two opposing factors, namely, surface area and bubble escaping resistance, governed the pool boiling heat transfer performance. The former dominated at large pore sizes, whereas the latter prevailed at small pore sizes. An experimental correlation was proposed for the heat transfer coefficient, which was applied at a porosity range of 0.9-0.98, pore density range of 5-40 PPI, and foam thickness of 5-7 mm. The bubble departure diameter and frequency of the high pore density (130 PPI) foam sample with V-shaped grooved arrays were further examined. The departure of the lotus-shaped bubble cluster caused by grooves was observed. Bubble cluster pulsation occurred at a high heat flux. The average departure diameter and frequency increased with the increase in heat flux. A typical bubble cluster pulsation period included four stages, namely, suction of the previous bubble cluster, fresh liquid supplement, liquid vaporization, and bubble cluster departure. The relationships between bubble cluster diameter and growth frequency were consistent in the boiling heat transfer performance of the three groove-arrayed foam surfaces. [ABSTRACT FROM AUTHOR]

Abstract: As microfluidic devices are used with high-pressure fluids, the effects of liquid compressibility become important for understanding system behavior. Bubble growth in a microfluidic channel, for instance, is qualitatively different in a rigid fixed-volume high pressure microfluidic device than it is in a flexible or open microfluidic device. With a fixed volume, bubble growth must be accompanied by compression of the surrounding liquid, increasing the system pressure. Increased pressure decreases the diffusion drive, slowing bubble growth. A nonlinear equation for system pressure is derived relating these coupled effects. This equation is linearized in the limit of small supersaturation, yielding a closed form solution. For larger supersaturations, the governing equations are solved numerically. Experiments on bubble growth in gas saturated liquid agree well with this one-dimensional model. [Copyright &y& Elsevier]

Abstract: In the present investigation, adiabatic gas bubble growth from a submerged orifice has been numerically simulated. The growth of the bubble is described by the full Navier–Stokes equations which have been solved by the finite-volume method using the commercial software package TransAT. The numerical simulations have been validated against detailed experimental measurements including the position of the centre of gravity, the curvature profiles and the departure volume. Subsequent to this the CFD platform was used to study some aspects of the influence of gravity level on bubble growth dynamics. In particular, the influence of gravity in the range of 0.1⩽g/g∗ ⩽1.5 is investigated and the subsequent influence on the bubble formation and departure characteristics are discussed. [Copyright &y& Elsevier]

Abstract: A microchannel test section comprised of parallel square microchannels with a 25×25μm and 50×50μm cross section was manufactured. Boiling of perfluorinated dielectric fluid FC-72 and water in microchannels was studied. Troublesome occurrences associated with flow boiling in microchannels were reduced or eliminated with inlet/outlet restrictors, inlet/outlet manifolds and potential nucleation cavities incorporated in the array of microchannels. The gradual reduction of channel cross section in the manifolds ensured a uniform distribution of the working fluid among the microchannels. The flow restrictors provided a higher upstream pressure drop in comparison with the downstream pressure drop which favors vapor flow in the downstream direction and consequentially suppresses the vapor backflow present in flow boiling. The superheat of the microchannel wall necessary for the onset of boiling was decreased significantly with the incorporation of properly sized artificial cavities. Experimental results confirmed the benefits of the etched features, as there was (i) an even working fluid distribution (ii) without dominating backflows of vapor (iii) at a low temperature of the onset of boiling. Bubble growths as well as other events in the microchannels were visualized with a high-speed imaging system which captured images at over 87,000 frames per second. Results exhibit boiling hysteresis dependence of the working fluid and its mass flux through the microchannels. The temperature of the onset of boiling is highly dependent on the working fluid, microchannel size and its roughness. [Copyright &y& Elsevier]

Abstract: Growth of an isolated bubble and horizontal coalescence events between bubbles of dissimilar size were examined during pool nucleate boiling of water on a horizontal, electrically-heated titanium foil 25μm thick. Wall temperature measurements on the back of the foil by high-speed IR camera, synchronized with high-speed video camera recordings of the bubble motion, improved the temporal and spatial resolution of previous observations by high-speed liquid crystal thermography to 1ms and 40μm, respectively, leading to better detailed maps of the transient distributions of wall heat flux. The observations revealed complex behaviour that disagreed with some other observations and current modelling assumptions for the mechanisms of heat transfer over the wall contact areas of bubbles and interactions between bubbles. Heat transfer occurred from the entire contact area and was not confined to a narrow peripheral triple-contact zone. There was evidence of an asymmetrical interaction between bubbles before coalescence. It was hypothesised that a fast-growing bubble pushed superheated liquid under a slow-growing bubble. Contact of this liquid with regions of the wall that had been pre-cooled during bubble growth caused local reductions in the wall heat flux. During coalescence, movement of liquid under both bubbles caused further changes in the wall heat flux that also depended on pre-cooling. Contraction of the contact area caused a peripheral reduction in the heat flux and there was no evidence of a large increase in heat flux during detachment. Boiling on very thin foils imposes special conditions. Sensitivity to the thermal history of the wall must be taken into account when applying the observations and hypotheses to other conditions. [Copyright &y& Elsevier]

Abstract: The effect of ambient pressures below atmospheric, liquid subcooling and heating rate on bubble dynamics associated with rapid evaporation in highly superheated water is investigated. Platinum films of various aspect ratios are electrically heated under partial vacuum and the average metal film temperature monitored during a power pulse of duration of several tens of microseconds. At high liquid subcooling, the surface temperature of the pulse-heated thin platinum strips is shown to oscillate with a frequency on the order of 0.5MHz. The oscillation frequency increases with ambient pressure in the range 0.02MPa and 0.101MPa, and decreases with increasing ambient temperature at a fixed pressure. An unusual dynamic instability is observed in which the surface temperature is constant before relaxing into oscillations, and the delay to oscillations increases with decreasing pressure. High speed imaging shows that the bubble growth phase of the oscillation is associated with surface heating and the collapse phase with surface cooling in the cyclic temperature history. These effects are explained by a bubble growth theory that includes a heat loss mechanism to the surrounding liquid. The nucleation temperature is unaffected by pressure in the sub-atmospheric range examined and it approaches the theoretical superheat limit for water at a heating rate of about 109 °C/s. [Copyright &y& Elsevier]

Abstract: Wall temperatures were measured by high-speed IR thermography during the sequential chain coalescence of bubbles from four nucleation sites in 4ms to form a single large hovering bubble during pool boiling of water at atmospheric pressure on an electrically-heated titanium foil 25μm thick. Interactions between bubbles before and during coalescence were significantly affected by asymmetries in bubble growth rate or size, and by the thermal response of the thin wall to previous heat transfer. Liquid was redistributed over the contact area of a coalesced bubble, maintaining heat transfer. Outward movement of the shared wall contact line at the instant of horizontal coalescence of two attached bubbles caused localized increases in wall heat flux but continuing oscillations following coalescence and during declining coalescence between an attached bubble and a detached bubble did not affect the wall heat flux in these studies. During the contraction of the contact area leading to detachment, there was a reduction in heat flux at the contact line and sometimes a reversal in heat flux at the contact line. [Copyright &y& Elsevier]

Abstract: In the present study, the bubble growth, departure and the following flow pattern evolution during flow boiling in the mini-tube were visualized and quantitatively investigated, along with the simultaneous measurement of the local heat transfer coefficient around a specified nucleation site. Liquid nitrogen was employed as the working fluid and the test section was a segment of vertically upward quartz glass tube with the inner diameter range of 1.3–1.5mm, which was coated by a layer of transparent ITO film as the heater on the outer surface. The growth rates of bubbles had similar and constant growth rate in two periods of time, i.e., before and after the bubbles departing from the nucleation site, which indicated the bubble growth was primarily governed by the inertial force. The bubble departure diameter and bubble period were investigated and the corresponding correlation was obtained based on the experimental data, which showed that the tube size of the mini-tube had no notable effect on the bubble departure and the trend of the bubble departure was similar to that in macro-tubes. Whereas the following flow pattern evolution was apparently confined due to the size effect, which presented desirable heat transfer performance in mini-tubes. The heat transfer coefficients for different flow patterns along the mini-tube were obtained in terms of bubbly, slug, annular flow and the flow regimes of flow reversal and post dryout. It was found that the dominant heat transfer mechanism was the liquid film evaporation which offered desirable heat transfer capability. The heat transfer performance would be deteriorated in the post dryout regime, while flow reversal could somewhat enhance the heat transfer upstream of the nucleation site. Boiling curves around the specified nucleation site were recorded and analyzed based on the recorded flow patterns. [ABSTRACT FROM AUTHOR]

Abstract: A phase-field model is presented for gas bubble growth and flow in a supersaturated liquid inside a Hele–Shaw cell. The flows in the gas and liquid are solved using a two-phase diffuse interface model that accounts for surface tension, interfacial mass transfer, and density and viscosity differences between the phases. This model is coupled with a phase-field equation for interface motion and a diffuse interface conservation equation for gas species transport. The model is first validated for a planar gas layer and a spherical bubble growing in a supersaturated liquid. It is shown that the results converge to the solution of the corresponding sharp interface model in the thin-interface limit. The model capabilities are demonstrated in several examples involving the growth and flow of multiple bubbles, including bubble coarsening and coalescence. [Copyright &y& Elsevier]

Abstract: A phenomenological theory of heat and mass transfer is applied to model the absorption of the R134a gas bubble in liquid R134a–dimethyl formamide (DMF) solution. Required properties of R134a–DMF liquid solution namely liquid density, surface tension, diffusion coefficient, thermal conductivity, specific heat, etc., have been calculated using experimental correlations. Results have been obtained by modeling on a computer using MATLAB. The bubble dynamics during bubble growth are studied using this model. Liquid concentration, temperature, heat and mass transfer rates, local heat and mass transfer coefficients are estimated at the bubble interface. Absorption rate, coupled heat transfer rate, average heat and mass transfer coefficients over the entire bubble life span are also calculated. This model results are validated using experimental results of ammonia–water solution. The maximum bubble radius determined from this study is compared with available experimental correlations and it is found that agreement is good. Heat and mass transfer rates obtained from this model are compared with the literature experimental results. The results are in good agreement with the predictions of the model. [Copyright &y& Elsevier]

Abstract: Using the VOF multiphase flow model, numerical simulations are conducted to investigate the nucleate boiling of water in micro-channels. The Marangoni heat transfer through the bubble surface is analyzed, and is compared with the incipient heat flux at the onset of nucleate boiling in micro-channels. The bubble growth in the channel is divided into two stages. At the initial stage, bubble growth is controlled by surface tension, while at the second stage the incipient heat transfer dominated the boiling process. In the results, the full process of bubble generating, growing, departing, combining, and shrinking in the channel is displayed. The simulated results with similar condition are agreed well with some experimental results in references. The method and discussion in the paper are helpful to the investigation of the mechanism of micro-scale two-phase flow and heat transfer. [Copyright &y& Elsevier]

Abstract: Microboiling is commonly used in thermal inkjet atomizers (TIJ) and microelectromechanical (MEM) devices. The TIJ and MEM devices performance is closely related to the dynamics of the bubble used to operate them; therefore, it is important to determine the conditions of input energy and power leading to specific bubble dynamics. The objective in this work is the characterization, in a confined space, of the bubble dynamics on a range of input conditions of energy and power and what is the effect of the input conditions on the bubble extractable mechanical efficiency. Mechanical efficiency is defined by the ratio of the integral of the mechanical work (work done by the bubble expansion due to the elevated internal pressure relative to atmospheric pressure minus the increase in bubble surface energy) to the total energy input to the microheater. Bubbles are generated with energies of 7–17μJ under high heating rates and short pulses in deionized water. Resulting nucleation temperature measurements are consistent with homogeneous nucleation. The bubble lifecycle shows strong dependence on the input heater energy and input heating rate. This work presents new results in bubble growth where growth–shrink–growth derived from specific energy conditions. The bubble growth–shrink–growth may be due to subcooled fluid, local variation in the pressure field, and by the surface tension driven change in curvature of the bubble. Mechanical bubble efficiencies result in small values suggesting most of the energy applied to the heater is distributed in other processes which may include increasing the internal energy of the heater film and the fluid. [Copyright &y& Elsevier]

Abstract: Growth and detachment of carbon dioxide bubbles on a horizontal porous surface in water is investigated in this paper. Uniform carbon dioxide injection on the porous surface is considered. The Young–Laplace equation is solved with the geometry method to yield the bubble shape. The dynamic pressure on the bubble surface due to bubble expansion is neglected. Multi-solution modes are found. Based on the characteristics of the solution modes, it is postulated that the bubble grows with a monotonically increasing base area until the maximum value is reached according to the fundamental solution mode. After that, the bubble jumps toward the secondary solution mode at a constant volume, and then detaches from the surface. The increasing rate of the bubble volume due to mass diffusion/convection is evaluated by solving the momentum and concentration equations. Evolution of the bubble shape then is determined corresponding to the variation of the bubble volume. The numerical results indicate that hydrophobic surface produces large bubble and thus gives rise to better efficiency for dissolved gas removal. [Copyright &y& Elsevier]

Abstract: This work investigates the nucleation and growth of CO2 bubbles due to chemical reactions of sulfuric acid and sodium bicarbonate in three types of microchannels: one with uniform cross-section, one converging, and another one diverging. The Y-shaped test section, composed of main and two front microchannels, was made of P-type 〈100〉 orientation SOI (silicon on insulator) wafer. Bubble nucleation and growth in microchannels under various conditions were observed using a high-speed digital camera. The theoretical model for bubble dynamics with a chemical reaction is reviewed or developed. In the present study, no bubble was nucleated at the given inlet concentration and in the range of flow rate in the converging microchannel while the nucleation and growth of bubbles were observed in the diverging and uniform cross-section microchannels. Bubbles are nucleated at the channel wall and the equivalent bubble radius increases linearly during the initial period of the bubble growth. The bubble growth behavior for a particular case, without relative motion between the bubble and liquid, shows that the mass diffusion controls the bubble growth; consequently, the bubble radius grows as a square root of the time and agrees very well with the model in the literature. On the other hand, for other cases the bubbles stay almost at the nucleation site while growing with a constant gas product generation rate resulting in the instant bubble radius following the one-third power of the time. [Copyright &y& Elsevier]

Abstract: Nucleate pool boiling analyses were performed using experimental results previously obtained at sub-atmospheric pressure and analytical results formerly proposed by the authors of this paper. The main objective was to reveal the differences between bubble growth behavior at atmospheric pressure and sub-atmospheric pressure. A secondary objective was to show the effect of the system pressure on bubble growth behavior. Experimental data were correlated using the non-dimensional characteristic radius and time scale parameters proposed in a previous study. The growth behavior at sub-atmospheric pressure was noticeably different from that at atmospheric pressure during saturated nucleate pool boiling. [Copyright &y& Elsevier]

Abstract: A model of the suction evaporation mode in nucleate boiling from tunnel and pore structures is presented. The model is based on the analysis by Nakayama et al. [W. Nakayama, T. Daikoku, H. Kuwahara, T. Nakajima, Dynamic model of enhanced boiling heat transfer on porous surfaces – Part II. Analytical model, ASME J. Heat Transfer 102 (3) (1980) 451–456] and L.H. Chein and R.L. Webb [A nucleate boiling model for structured enhanced surfaces, Int. J. Heat Mass Transfer 41 (14) (1998) 2183–2195]. Additionally, a detailed phenomenological model of liquid refill has been developed. It has been shown that the process of liquid refill and the time needed for it is strongly dependent on pool height. Effect of liquid pool height on bubble frequency has also been discussed. Finally, a generalized methodology is given for the prediction of boiling data from a structured surface. [Copyright &y& Elsevier]

The processes of gas evolution at electrodes in electrochemical reactors and of boiling belong to strongly different fields in chemical engineering and for a long time were investigated separately. Nonetheless, they exhibit numerous common features. The analogies of both processes have been made use of giving rise to transfer findings obtained in one field to the other one. However, the analogy is limited, and the limitations have not yet attracted sufficient interest. A brief review on the analogies is given. The discrepancies in the fields of initial nucleation, the upper bound of operation and the different mechanisms controlling the transport of substance and of heat in both processes are discussed. [Copyright &y& Elsevier]

• The conjugate heat transfer in flow boiling simulation plays an important role and cannot be ignored. • Various bottom wall materials and thicknesses are analyzed during the bubble growth in microchannel. • There exists an optimum thickness for each solid material in the phase-change application. The conjugate heat transfer of bubble growth during flow boiling in microchannel has a significant effect on the flow field and heat transfer performance but few studies analyzed it before. In this study, the volume of fluid (VOF) method, Hardt's phase-change model, conjugate heat transfer between solid and fluid domains are adopted within an OpenFOAM solver to investigate the bubble growth and heat transfer performance in a microchannel with changed wall thickness from 5 μm to 160 μm and materials including silicon, aluminum, and copper. The results reveal that even if uniform heat flux is applied to the bottom wall, heat flux is not uniform at the solid-fluid interface due to the phase-change process in the channel. Conjugate heat transfer between the fluid and solid domain plays an important role in transferring the uniform heat flux from the bottom wall to the solid-fluid interface and homogenizing the solid-region temperature distribution, which cannot be ignored in the simulation of the phase-change phenomenon. When using different wall thicknesses, the bubble growth period differs by over two times. An optimum thickness exists for each material because the increasing wall thickness leads to a faster bubble growth rate but higher thermal resistance. With the same bottom wall thickness, the solid material with higher thermal diffusivity owns a faster bubble growth rate, thus a higher heat transfer coefficient. The optimum thickness decreases with increasing thermal diffusivity of the solid-domain material. [ABSTRACT FROM AUTHOR]

• For a spherical bubble, the direct numerical integration of Young-Laplace equation works very well in computing the bubble shape. • The approximate analytical solution computes a non-spherical shape of a bubble at a high superheat much better than the direct numerical integration of Young-Laplace equation non-spherical bubbles. • The shape of a bubble has been found to be influenced by superheat and Bond number Bo. • The Young-Laplace equation seems to include the growth force to produce the hemispherical shape of non-spherical bubbles. • A very low Bo results in a train of small bubbles that coalesce to a big bubble with increase in Bo. In this work, we have developed an improved numerical model to simulate the dynamics of bubble shape and departure in nucleate pool boiling at a heated surface at low and high superheats. The numerical model computes the bubble shape solving Young-Laplace equation directly and considering the effect of radial variation of film pressure in our numerical model of microlayer evaporation. The numerical results of bubble shape and departure have also been compared with the CFD-results in literature and with the results of our experiments. For a spherical bubble, the direct numerical integration of Young-Laplace works very well in computing the bubble shape. For a non-spherical bubble, an approximate analytical solution for bubble shape proposed in literature works better than the numerical solution of Young-Laplace equation. The shape of a bubble has been found to be influenced by superheat and Bond number Bo. We have also analyzed the forces acting on the bubble surface while departing the heated surface. The Young-Laplace equation seems to include the growth force to produce the hemispherical shape of non-spherical bubbles. A very low Bo results in a train of small bubbles that coalesce to a big bubble with increase in Bo. [ABSTRACT FROM AUTHOR]

• Visualized microbubble dynamics in boiling water droplets on textured substrates. • Number and size distributions of the nucleated bubbles within a single droplet. • Analysis of bubble growth & number change with time within a drop as it boils. • Estimated latent heat-flux removal during growth of single and multiple bubbles. Dissipating large heat fluxes from a surface is critically important in numerous industrial and natural applications. Boiling based spray cooling and surface texturing are two of the most promising methods being investigated to address this problem. Although our understanding on these topics has significantly improved over past decades, critical gaps remain in the knowledgebase stymieing the realization of their full potential. As an example, while bubble growth in pool boiling have been investigated in detail, comparatively little is known about how the bubbles evolve inside boiling drops. In the present work, we have investigated for the first time, the microbubble dynamics inside water droplets boiling on superhydrophilic textured substrates using high-speed X-ray phase contrast imaging (XRPCI). Our observations show that the transient bubble density variation follows similar characteristics irrespective of the texture spacing at a given surface temperature. For an example microstructure, we found that the number of discrete bubbles on the surface decreases as temperature is increased although their growth rate increases. We observe that bubble growth is highly non-uniform during the lifetime of a drop on the surface. Initially, bubbles grow under diffusion-limited regime, but at later times they grow as ~ t 1.45 due to combined effects of coalescence and evaporation. In some conditions, we found that bubbles shrink dramatically after the initial growth spurt presumably due to severe quenching of the surface, and migration of bubbles on the surface. Using the bubble sizes, for the first time we analyzed the heat flux removed by a single bubble and also by all the bubbles at a given time. We find that the highest dissipation through latent-heat component (~600 W/cm2) occurs just in the beginning and thereafter it decreases. We expect that our findings and the analysis would guide further work on the topic and will aid in the overarching goal of engineering surfaces that are more efficient in boiling heat transfer. [Display omitted] [ABSTRACT FROM AUTHOR]

• Adiabatic bubble growth and departure simulations are performed considering the dynamic wetting characteristics of the surface • The receding contact angle is shown to be the dominant wetting characteristic governing maximum contact diameter and departure diameter • The advancing contact angle governs the departure mechanism and has a modest influence on the departure diameter • Correlations are established for the maximum contact diameter and departure diameter based on the advancing and receding contact angles • Boiling surface wetting classifications of hygrophilic, hygrophobic and ambiphilic are defined based on characteristic bubble dynamics Surface wettability is known to have a major influence on the ebullition characteristics of a bubble growing from a solid surface. Yet, simplistic static characterization of the wetting behavior is still relied upon to indicate performance characteristics during boiling. In this study, a theoretical framework is developed for the wetting and dewetting processes occurring during bubble growth based upon the dynamic contact angles. This framework is incorporated into adiabatic volume-of-fluid simulations to capture the influence of the surface wettability on contact line and contact angle dynamics during bubble growth and departure. The simulations span a large range of dynamic wetting behaviors and fluid properties. The receding contact angle is shown to govern the early stages of bubble growth as the contact line recedes outward from the bubble center and is the dominant wetting characteristic that determines the maximum contact diameter and departure size. The advancing contact angle dictates the departure morphology as the contact line retracts inward and has a secondary role in determining the departure size. Following, improved reduced-order models are developed that establish fluid-property-independent correlations for the maximum contact diameter and departure diameter as a function of the dynamic contact angles. The results call for the need to redefine wettability classifications based on dynamic contact angles rather than static contact angle in the context of boiling. Hygrophilicity and hygrophobicity are redefined in this context, and an additional classification, ambiphilicity, is introduced for boiling surfaces exhibiting low receding contact angles and high advancing contact angles. [Display omitted] [ABSTRACT FROM AUTHOR]

• Adiabatic bubble growth and departure simulations are performed considering the dynamic wetting characteristics of the surface • The receding contact angle is shown to be the dominant wetting characteristic governing maximum contact diameter and departure diameter • The advancing contact angle governs the departure mechanism and has a modest influence on the departure diameter • Correlations are established for the maximum contact diameter and departure diameter based on the advancing and receding contact angles • Boiling surface wetting classifications of hygrophilic, hygrophobic and ambiphilic are defined based on characteristic bubble dynamics Surface wettability is known to have a major influence on the ebullition characteristics of a bubble growing from a solid surface. Yet, simplistic static characterization of the wetting behavior is still relied upon to indicate performance characteristics during boiling. In this study, a theoretical framework is developed for the wetting and dewetting processes occurring during bubble growth based upon the dynamic contact angles. This framework is incorporated into adiabatic volume-of-fluid simulations to capture the influence of the surface wettability on contact line and contact angle dynamics during bubble growth and departure. The simulations span a large range of dynamic wetting behaviors and fluid properties. The receding contact angle is shown to govern the early stages of bubble growth as the contact line recedes outward from the bubble center and is the dominant wetting characteristic that determines the maximum contact diameter and departure size. The advancing contact angle dictates the departure morphology as the contact line retracts inward and has a secondary role in determining the departure size. Following, improved reduced-order models are developed that establish fluid-property-independent correlations for the maximum contact diameter and departure diameter as a function of the dynamic contact angles. The results call for the need to redefine wettability classifications based on dynamic contact angles rather than static contact angle in the context of boiling. Hygrophilicity and hygrophobicity are redefined in this context, and an additional classification, ambiphilicity, is introduced for boiling surfaces exhibiting low receding contact angles and high advancing contact angles. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: The contribution of the microregion (vicinity of the triple gas–liquid–solid contact line) to the force balance of a vapor bubble is discussed. The force balance of a bubble attached to the heater includes the capillary adhesion force that involves the contact angle. From the theoretical point of view, the microscopic value of the contact angle should be used in this expression. However in the experimental literature on the bubble departure, the apparent contact angle is commonly used instead. This usually leads to a substantial increase of the bubble adhesion and provides a good agreement with experimental data. A theoretical justification for such a substitution is proposed. The difference between the effective capillary adhesion force (that involving the apparent contact angle) and its true value is provided by the hydrodynamic stress caused by the liquid flow towards the contact line which is also at the origin of the difference between the microscopic and apparent contact angles. [Copyright &y& Elsevier]

• The dynamics of bubble growth and departure in pool boiling at surface in low pressures considering both macrolayer and microlayer evaporation. • We propose a new asymptotic solution for bubble growth in low pressures. • The effect of superheat on the shape of growing bubbles. In this work, we investigate numerically the dynamics of bubble growth and departure in pool boiling at surface in low pressures using the asymptotic solution derived from our semi analytical method and considering both macrolayer and microlayer evaporation. Our asymptotic solution for bubble-growth kinetics is capable of computing bubble departure diameter D d and growth time t g more accurately in higher superheats at atmospheric pressure, in which the existing asymptotic growth-kinetics are not adequate. It does not apply in low pressures, particularly below atmospheric pressure. We therefore propose a new asymptotic solution of growth kinetics for computing bubble departure in low pressures. A steady-state analysis of force balance equation in low pressure yields a power-law variation of D d with Jakob number and pressure. The bubble departure with and without microlayer evaporation reveal that the method with dynamic microlayer evaporation predicts D d and t g more accurately. The effect of superheat on the shape of growing bubbles reveals that spherical bubbles form at low superheat and hemi-spherical bubbles at high superheat. [ABSTRACT FROM AUTHOR]

• Two single bubble growth processes have been recorded in a visual experiment. • Bubble heat flows have been estimated according to experimental data. • A numerical simulation has been conducted based on heat flow distributions. Two growth processes of individual bubble on heating surface in subcooled boiling water were numerically studied based on the heat flows derived by experiment. In the experiment, a high speed camera was used to photograph the growth and condensation process of a single bubble in low subcooling degree water under normal pressure. The bulk fluid temperature and the temperature of heating wall at the nucleation point were recorded. Based on the experimental data of the bubble condensation process, the convective heat transfer formula at the bubble cap was derived and the heat flow distributions during the whole process were obtained. Then a numerical simulation of the whole process was conducted according to the heat flows. The bubble size, bubble shape and temperature fields were obtained. The simulation results can confirm the rationality and correctness of the theoretical analysis, for the bubble size is highly close to the experiment data. The results also indicated that the evaporation at the bubble base accounts for more than 45% of the total evaporation, which is mainly contributed by microlayer. [ABSTRACT FROM AUTHOR]

• Drop impact onto a heated substrate under depressurized environment is captured. • New drop outcomes named magic carpet breakup and tiptoeing are identified. • Drop is repelled from the surface violently and spread out to a flattened shape. • We developed a model based on single bubble growth correlated with drop spreading. • The estimations on the bubble burst time and upward velocity agree with experiment. Drop impact onto a hot substrate leads to various hydrodynamic outcomes depending on the substrate temperature and the impact parameters: deposition, drop dancing, thermal atomization, and finally rebound at temperatures above the dynamic Leidenfrost condition. In the present experimental study this parameter space has been extended to conditions of a depressurized environment (1–101 kPa), which are relevant to space applications such as a reentry spaceship or a rocket engine. The impact onto a superheated smooth substrate with relatively small impact velocities (0.1–0.46 m/s) has been captured using a high-speed video camera system. Under these conditions two new modes of drop outcome have been identified; magic carpet breakup , in which the drop splashes immediately after the impact, leading to extensively large lateral deformation and upward drop movement, and tiptoeing , in which the drop bounces back with a time scale similar to capillary rebound (so-called rebound) accompanied by a significant fluctuation of the outer shape and secondary droplet generation triggered by successive attach-and-detach cycles. These new modes occur at a temperature range between the deposition regime and the capillary rebound regime. Detailed regime maps indicating the type of drop outcome for different pressures and substrate temperatures have been constructed. Moreover, a hypothesis on the mechanism of magic carpet breakup is presented and a theoretical model, in which a classical bubble growth model for pool boiling is extended to bubble growth in a drop and correlated with drop kinematics, is developed. The numerical solution is in good qualitative agreement with the experimental data for the bubble burst time and for the change in drop velocity. [ABSTRACT FROM AUTHOR]

• Sliding bubble dynamics characteristics are investigated visually. • The probability density distribution of bubble diameter and velocity are obtained. • The correlation to predict the mean bubble diameter is obtained. • The models for two typical sliding bubbles growth characteristics are proposed. Narrow rectangular channels are widely used in many fields due to its characteristics of boiling heat transfer enhancement, and the bubble behaviors have been in particular interest for decades because of its significant contribution to understanding the mechanism of heat transfer enhancement. In the present work, visualization experiments in upward subcooled flow boiling in a narrow rectangular channel under natural circulation condition were carried out. A sequence of image processing algorithms was used to deal with the original bubble images to get relevant bubble parameters, including bubble diameter and bubble velocity, etc. The experiments were performed at pressures ranging from 0.1 MPa to 0.3 MPa, with inlet subcooling ranging from 20 to 60 K and heat flux ranging from 100 kW/m2 to 300 kW/m2. Effects of the liquid subcooling and heat flux on the bubble stochastics and mean characteristics were investigated. In natural circulation, the bubble diameter distribution approximately conforms to lognormal distribution while the bubble velocity distribution agrees well with normal distribution. These are in contrast to both of the bubble diameter and velocity follow normal distribution in forced circulation. The present experiment data shows a good linear relationship between the mean bubble velocity and mean bubble diameter. A correlation for the mean bubble diameter as a function of local subcooling, heat flux, mass flux, and gap width size were obtained. Moreover, we found that all the bubbles will slide along the heating wall after generation in narrow rectangular channel, and two types of sliding bubbles can be observed in our experiments. The growth curve of the Lift-type bubbles and the Sliding-type bubbles conform to a dual model and a power model, respectively. [ABSTRACT FROM AUTHOR]

• Mutual effect of surface characteristics and bulk flow on bubble dynamics is studied. • High resolution imaging technique captured the life-cycle of a nucleated bubble. • Bubble departure diameters (D d) are greater for roughness 0.108 μm ≤ Sq ≤ 0.218 μm. • Significance of surface characteristics for D d reduces at low liquid velocities. • A bubble departure criterion is derived. The present study reports the mutual effect of heater surface wettability, roughness and bulk liquid velocity on the bubble dynamics and departure in nucleate boiling. Boiling experiments were conducted at atmospheric pressure with degassed-deionized water at low subcooling (1.9 ± 0.25 K) for vertically oriented stainless steel heaters. Self-assembled monolayer (SAM) coating and wet-etching technique were used to alter the heater surface wettability and roughness. Liquid contact angle hysteresis (θ hys) and root mean square roughness (Sq) of the heater surfaces were adjusted between 42.32 ° ≤ θ hys ≤ 68.56 ° and roughness 0.01 μm ≤ Sq ≤ 0.549 μm. High resolution optical shadowgraphy has been used to record the bubble life cycle. Experimental results show that higher bulk liquid velocity yields smaller bubble departure diameters for all heater surface characteristics. Bubble departure diameters are greater for low wetting surfaces. The bubble growth rate and departure diameter were found maximum for an intermediate surface roughness Sq between 0.108 and 0.218 μm. The corresponding roughness height is referred to as the 'optimal roughness height' in this study. Eventually, a bubble departure criterion was derived from the expressions of forces which act on a nucleating bubble throughout its growth cycle. 90% of the departing bubbles satisfy the bubble departure criterion with ±25% deviation. [ABSTRACT FROM AUTHOR]