Visualization of boiling bubble dynamics using a flat uniformly heated transparent surface

Bubble dynamics in saturated pool boiling of R-123 with and without an applied electric field have been investigated using a novel, flat, transparent heated surface. This method allows viewing and measurement of bubble dynamics from the entire heater surface without interference from the fluid or other bubbles. The data have been used to quantify the effect of an electric field on the latent heat contribution to the total heat flux and to demonstrate the effectiveness of this experimental technique. For a given heat flux, the application of the electric field reduces the surface temperature, thereby suppressing boiling and reducing the latent heat contribution.     

Holographic interferometric study of heat transfer to a sliding vapor bubble

Heat transfer associated with a vapor bubble sliding along a downward-facing inclined heater surface was studied experimentally using holographic interferometry. Volume growth rate of the bubbles as well as the rate of heat transfer along the bubble interface were measured to understand the mechanisms contributing to the enhancement of heat transfer during sliding motion. The heater surface was made of polished silicon wafer (length 185 mm and width 49.5 mm). Experiments were conducted with PF-5060 as test liquid, for liquid subcoolings ranging from 0.2 to 1.2 °C and wall superheats from 0.2 to 0.8 °C. The heater surface had an inclination of 75° to the vertical. Individual vapor bubbles were generated in an artificial cavity at the lower end of the heater surface. High-speed digital photography was used to measure the bubble growth rate. The temperature field around the sliding bubble was measured using holographic interferometry. Heat transfer at the bubble interface was calculated from the measured temperature field. Results show that for the range of parameters considered the bubbles continued to grow, with bubble growth rates decreasing with increasing liquid subcooling. Heat transfer measurements show that condensation occurs on most of the bubble interface away from the wall. For the parameters considered condensation accounted for less than 12% of the rate heat transfer from the bubble base. In this study the heater surface showed no drop in temperature as a result of heat transfer enhancement during bubbles sliding.     

Study of the behaviour of a bubble in an electric field: steady shape and local fluid motion

The aim of this study is to investigate the effects of a uniform electric field on bubbles. Numerical analyses have been carried out in order to determine the shape of an axisymmetric conducting bubble immersed in an isothermal dielectric fluid. A detailed analysis of the interfacial electric stresses acting on the liquid–vapour conducting interface is discussed. This study shows a deformation of the bubble in the electric field direction and also electroconvective movements within and around the conducting bubble. The electroconvective movements are analysed on the basis of the creeping flow approximation. A comparison between the conducting bubble shape and the dielectric bubble one is also presented.     

A Study of Bubble Behavior and Boiling Heat Transfer Enhancement under Electric Field

The effect of a D.C. electric field on nucleate boiling heat transfer for refrigerants, R11, R113, and FC72, was investigated experimentally in a single-tube shell/tube heat exchanger by using the temperature control method of wall superheat. Also the behavior of bubble under nonuniform electric field produced by wire electrodes was studied by numerical calculation. For R11, the electrohydrodynamic (EHD) enhancement for boiling heat transfer was observed for all ranges of wall superheat tested. However, the enhancement in boiling heat transfer disappeared if the wall superheat exceeded 13°C for R113, and no electric field effect on the boiling heat transfer was observed for FC72. An application of approximately 5 kV was enough to eliminate the boiling hysteresis for R11 and R113. Numerical study of the electric field in a single medium has hinted that the bubbles are forced away from the heating surface and toward the electrostatic stagnation point by the dielectrophoretic force. Such modified bubble motion turns out to promote the boiling heat transfer if one uses proper electrode configuration.     

Investigation of heat transfer and bubble dynamics in a boiling thin liquid film flowing over a rotating disk

Nucleate boiling heat transfer and bubble dynamics in a thin liquid film on a horizontal rotating disk were studied. A series of experiments were conducted to determine the heat transfer coefficient on the disk. At low rotation and flow rates, vigorous boiling increased the heat transfer coefficients above those without boiling. Higher rotational speeds and higher flow rates increased the heat transfer coefficient and suppressed boiling by decreasing the superheat in the liquid film. The flow field on the disk, which included supercritical (thin film) flow upstream of a hydraulic jump, and subcritical (thick film) flow downstream of a hydraulic jump, affected the type of bubble growth. Three types of bubble growth were identified. Vigorous boiling with large, stationary bubbles were observed in the subcritical flow. Supercritical flow produced small bubbles that remained attached to the disk and acted as local obstacles to the flow. At low rotational rates, the hydraulic jump that separated the supercritical and subcritical regions produced hemispherical bubbles that protruded out of the water film surface and detached from the disk, allowing them to slide radially outward. A model of the velocity and temperature of the microlayer of water underneath these sliding bubbles indicated that the microlayer thickness was approximately 1/25th of that of the surrounding water film. This microlayer is believed to greatly enhance the heat transfer rate underneath the sliding bubbles.     

电场作用下冷气泡行为试验研究

气泡的生成、成长、脱离和上升等行为均是决定电场强化传热机理的主要过程,为获得电场作用下气泡生长的动态过程,研究了电场作用下冷态氮气泡的行为特性,利用高速摄像机拍摄了不同电场强度下的气泡生长试验图像,并对气泡脱离形态、周期和速度及加速度进行对比分析。试验结果表明,冷态氮气泡沿着场强方向拉长,呈圆柱体形状脱离壁面,气泡的脱离周期、速度与场强成正比,而加速度与场强成反比。     

On the transition from partial to fully developed subcooled flow boiling

The subject of the present study is to relate the boiling heat transfer process with experimentally observed bubble behaviour during subcooled flow boiling of water in a vertical heated annulus. It presents an attempt to explain the transition from partial to fully developed flow boiling with regard to bubble growth rates and to the time that individual bubbles spend attached to the heater surface.Within the partial nucleate boiling region bubbles barely change in size and shape while sliding a long distance on the heater surface. Such behaviour indicates an important contribution of the microlayer evaporation mechanism in the overall heat transfer rate. With increasing heat flux, or reducing flow rate at constant heat flux, bubble growth rates increase significantly. Bubbles grow while sliding, detach from the heater, and subsequently collapse in the bulk fluid within a distance of 1-2 diameters parallel to the heater surface. This confirms that bubble agitation becomes a leading heat transfer mode with increasing heat flux. There is however, a sharp transition between the two observed bubble behaviours that can be taken as the transition from partial to fully developed boiling. Hence, this information is used to develop a new model for the transition from partial to fully developed subcooled flow boiling.     

Interaction of two cavitation bubbles in a tube and its effects on heat transfer

When two cavitation bubbles exist in a confined space, the interaction between the bubbles significantly affects the characteristics of bubble dynamic behaviors. In this paper, a three-dimensional (3D) model is established to study the growth and collapse of two cavitation bubbles in a heated tube and its effects on heat transfer. The liquid and gas phases throughout the calculation domain are solved by a set of Navier-Stokes equations. It is assumed that the gas inside the bubble is compressible vapor, and the surrounding liquid is incompressible water. The mass transfer between two phases is ignored. The calculated bubble profiles were compared to the available experimental data, and a good agreement has been achieved. Then, the relationship among the bubble motion, flow field and pressure distributions was analyzed. On this basis, the effects of bubble interaction on the heat transfer between the wall surface and sounding liquid were discussed. It is found that heat transfer in the centre wall region is enhanced owing to the vortex flow and micro-jet induced by the bubble contraction and collapse. In contrast, the highest surface temperature appears in the surrounding region, which is mainly attributed to the thermal resistance induced by the bubble. The present study is helpful to understand the heat transfer phenomenon with cavitation in the liquid.     

Lateral coalescence of bubbles in the presence of a DC electric field

In this work, the influence of electrohydrodynamic forces on lateral bubble coalescence during nucleate pool boiling is investigated. An experimental pool boiling test facility was used with n-pentane as the working fluid. Boiling took place atop a polished copper surface on which two artificial nucleation sites were fabricated. The nucleation sites were 180 μm in diameter and 500 μm deep with a centre-to-centre spacing of 660 μm. Two diametrically opposed windows allowed for illumination and high speed videography of the bubble growth process from the two nucleation sites. For the saturated boiling tests considered here, bubbles only formed at the two artificial nucleation sites allowing their coalescence behaviour to be scrutinized. A screen electrode above the boiling surface and a high voltage DC power supply facilitated the establishment of the electric field which was varied between 0 and 34.5 kVcm^− 1. Observation of the high speed videos has revealed that bubble coalescence is influenced in such a way that it is delayed in the presence of the electric field to such an extent that, at the highest electric field strength tested, it is avoided all together. To help explain the observed results, a simple numerical model is solved showing that bubbles in close proximity to one another create an electric field distribution with high intensity between them. The overall result is net polarization forces that push the bubbles apart, and the closer they are together the larger this repulsive force becomes.     

Analysis of vapor–gas bubbles in a single artery heat pipe

The noncondensable gas supported bubbles in an arterial heat pipe are studied. The governing conservation equations are solved to study the growth/collapse of spherical bubbles under different conditions by using the finite element method. The criterion used in the design of the venting pores to prime the artery is explained. The diffusion-limited bubble collapse in the condenser and bubble growth due to the phase change in the evaporator are both studied. A theoretical explanation for the capability of venting bubbles under different scenarios is provided. The experimental results, including rapid startup and condenser cooldown, are also presented to prove the ability of the heat pipes to vent vapor–gas bubbles.     

Numerical Simulation of Bubble Dynamics in a Uniform Electric Field by the Adaptive 3D-VOSET Method

This article presents a three-dimensional numerical simulation of the effect of a uniform electric field on the dynamics of bubbles in a viscous fluid. The two-phase interface is captured utilizing a coupled volume-of-fluid and level set (VOSET) method by solving the full Navier–Stokes equations coupled with electric field equations. To track the interface more accurately, the dynamically adaptive octree grids are used to refine the grids around the interface. The effects of different parameters such as the electric Bond number, the ratio of electrical permittivity, the gravitational Bond number, and the Reynolds number on the motion and deformation of the bubble are investigated. According to the computational results, it is found that the electric field has a significant influence on the bubble dynamic behavior. Increase of the electric Bond number or the ratio of electrical permittivity results in the larger deformation and rising velocity of the bubble. For a higher electric Bond number and the Reynolds number, separations of the tail of the bubble are observed. In this case, the jet above the bubble is strong enough to turn the spherical bubble to a toroidal shape.     

Numerical study on bubble behavior in magnetic nanofluid used for waste heat recovery power generation concept

Electrical energy can be generated by the bubble motion inside the magnetic nanofluid under the influence of an external magnetic field. The relative movement of the magnetic particles dispersed in the magnetic fluid is induced through the movement of the bubbles rising by buoyancy force. This disturbs the external magnetic field associated with the generator coil, and electrical energy can be generated. The bubble movement in this complex physical environment was studied through 2D numerical analysis. Commercial magnetic fluids EFH1 and EFH3, manufactured by Ferrotec, were selected as the working fluid for the investigation. A level set method was used to analyze the 2‐phase flow of bubbles motion in the magnetic fluid. The effect of magnetic particle concentration on the behavior of bubbles and the change of bubble flow patterns through interaction between bubbles were observed by analysis. In addition, the influence of the magnetic force caused by the external magnetic field on the behavior of the bubble was also investigated. The following results can be obtained through the analysis of this study. The high concentration of magnetic particles increases the viscosity and attenuates the rising velocity and the lateral oscillation of the bubbles. The interaction of the 2 bubbles depends on the initial relative distance. Merging occurs only between 2 bubbles within a certain initial distance, which maximizes disturbance of the surrounding magnetic fluid. The magnetic force exerted by the permanent magnets externally applied is relatively small in comparison with gravity. Therefore, the effect on the rise behavior of the bubble is not significant. In consideration of the overall external force and flow conditions, the pattern of the bubble flow that maximizes the efficiency in the present electric energy generation concept was found.     

Coalescence of bubbles in nucleate boiling on microheaters

In this paper, an experiment was performed which is based on a heating surface consisting of microheaters where the temperature of each heater can be individually controlled by an electronic feedback loop. The power consumed by the heaters throughout the cycle of individual bubble growth, coalescence, detachment and departure was measured at high frequencies, thus the heat flux and its variation were obtained. By a careful timing and control of two individual microheaters, we were able to produce two individual bubbles side-by-side. The coalescence would takes place when they grow to a certain size that allows them to touch each other. We have recorded two major heat flux spikes for a typical cycle of boiling with coalescence. The first one corresponds to the nucleation of bubbles; the second one is for the coalescence of the two bubbles. We found that the heat flux variation is closely related to the bubble dynamics and bubble-bubble interaction. By comparing with the single bubble results without coalescence, we also found that the heat transfer is highly enhanced due to the coalescence.     

Bubble growth and horizontal coalescence in saturated pool boiling on a titanium foil,investigated by high-speed IR thermography

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 25 μm 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 1 ms and 40 μm, 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.     

Interaction mechanism of double bubbles in hydrodynamic cavitation

Bubble-bubble interaction is an important factor in cavitation bubble dynamics. In this paper, the dynamic behaviors of double cavitation bubbles driven by varying pressure field downstream of an orifice plate in hydrodynamic cavitation reactor are examined. The bubble-bubble interaction between two bubbles with different radii is considered. We have shown the different dynamic behaviors between double cavitation bubbles and a single bubble by solving two coupling nonlinear equations using the Runge-Kutta fourth order method with adaptive step size control. The simulation results indicate that, when considering the role of the neighbor smaller bubble, the oscil-lation of the bigger bubble gradually exhibits a lag in comparison with the single-bubble case, and the extent of the lag becomes much more obvious as time goes by. This phenomenon is more easily observed with the increase of the initial radius of the smaller bubble. In comparison with the single-bubble case, the oscillation of the bigger bubble is enhanced by the neighbor smaller bubble. Especially, the pressure pulse of the bigger bubble rises intensely when the sizes of two bubbles approach, and a series of peak values for different initial radii are acquired when the initial radius ratio of two bubbles is in the range of 0.9～1.0. Although the increase of the center distance between two bubbles can weaken the mutual interaction, it has no significant influence on the enhancement trend. On the one hand, the interaction between two bubbles with different radii can suppress the growth of the smaller bubble; on the other hand, it also can enhance the growth of the bigger one at the same time. The significant en-hancement effect due to the interaction of multi-bubbles should be paid more attention because it can be used to reinforce the cavitation intensity for various potential applications in future.     

Nucleate boiling in two types of vertical narrow channels

To explore the mechanism of boiling bubble dynamics in narrow channels, we investigate 2-mm wide I- and Z-shaped channels. The influence of wall contact angle on bubble generation and growth is studied using numerical simulation. The relationships between different channel shapes and the pressure drop are also examined, taking into account the effects of gravity, surface tension, and wall adhesion. The wall contact angle imposes considerable influence over the morphology of bubbles. The smaller the wall contact angle, the rounder the bubbles, and the less time the bubbles take to depart from the wall. Otherwise, the bubbles experience more difficulty in departure. Variations in the contact angle also affect the heat transfer coefficient. The greater the wall contact angle, the larger the bubble-covered area. Therefore, wall thermal resistance increases, bubble nucleation is suppressed, and the heat transfer coefficient is lowered. The role of surface tension in boiling heat transfer is considerably more important than that of gravity in narrow channels. The generation of bubbles dramatically disturbs the boundary layer, and the bubble bottom micro-layer can enhance heat transfer. The heat transfer coefficient of Z-shaped channels is larger than that of the I-shaped type, and the pressure drop of the former is clearly higher.     

Heat transfer in simulated boiling

An experimental study was undertaken to determine the variations of heat-transfer coefficient on a submerged heating surface while air bubbles were injected into the liquid through an orifice in the plate. The results indicated that heat transfer is most intensive during the time that the bubble detaches from the surface. This casts doubts on boiling heat-transfer correlations based on bubble growth or rising phase considerations. In conclusion, it is suggested that the “agitation” and “latent heat” views of boiling heat transfer may be combined in a unified model.     

电场作用下气泡内外的速度场分析

在蠕动流假设的基础上计算了电场作用下气泡内外的速度场分布,并与无电场作用但有来流时气泡内外的速度场进行了比较。结果表明,无电场作用时,来流速度只能使气泡内部形成两个球涡,而作用电场后即使无来流,气泡内部的流体运动仍然加剧,可以形成四个球涡。这表明外电场作用也有利于加剧气泡内部流体的运动,从而有助于核态沸腾换热过程中气泡底部微液层液体的蒸发。     

Bubble growth rates in nucleate boiling of water at subatmospheric pressures

The growth rate of vapour bubbles has been investigated experimentally up to departure in water boiling at pressures varying from 26·7 to 2·0 kPa (the corresponding Jakob number increasing from 108 to 2689).Comparison of the data with existing theory shows the substantial influence of liquid inertia during initial growth, in agreement with previous results of Stewart and Cole [1]on water boiling at 4·9 kPa, the Jakob number varying from 955 to 1112. As an extreme case, at a pressure of 2·0 kPa, large “Rayleigh” bubbles are observed during the entire adherence time. During advanced growth, bubble behaviour is gradually governed by heat diffusion, especially at relatively high (subatmospheric) pressures.Experimental bubble growth in the investigated pressure range is in quantitative agreement with the van Stralen, Sohal, Cole and Sluyter theory [10]. This model combines the Rayleigh solution with a diffusion-type solution, which accounts for the contributions to bubble growth due to both the relaxation microlayer (around the bubble dome) and the evaporation microlayer (beneath the bubble).Finally, a curious bubble cycle is observed at the lowest investigated pressures, which is attributed to the combined action of a high-velocity liquid jet (originating in the wake following a large primary bubble) and a succeeding secondary vapour column (generated at the adjacent dry spot at the heating wall beneath the primary bubble).     

On the rise paths of single vapor bubbles after the departure from nucleation sites in subcooled upflow boiling

A photographic study was carried out for the subcooled flow boiling of water to elucidate the rise characteristics of single vapor bubbles after the departure from nucleation sites. The test section was a transparent glass tube of 20 mm in inside diameter and the flow direction was vertical upward; liquid subcooling was parametrically changed within 0–16 K keeping system pressure and liquid velocity at 120 kPa and 1 m/s, respectively. The bubble rise paths were analyzed from the video images that were obtained at the heat flux slightly higher than the minimum heat flux for the onset of nucleate boiling. In the present experiments, all the bubbles departed from their nucleation sites immediately after the inception. In low subcooling experiments, bubbles slid upward and consequently were not detached from the vertical heated wall; the bubble size was increased monotonously with time in this case. In moderate and high subcooling experiments, bubbles were detached from the wall after sliding for several millimeters and migrated towards the subcooled bulk liquid. The bubbles then reversed the direction of lateral migration and were reattached to the wall at moderate subcooling while they collapsed due to the condensation at high subcooling. It was hence considered that the mechanisms of the heat transfer from heated wall and the axial growth of vapor volume were influenced by the difference in bubble rise path. It was observed after the inception that bubbles were varied from flattened to more rounded shape. This observation suggested that the bubble detachment is mainly caused by the change in bubble shape due to the surface tension force.