Behavior Of Vapor Bubbles Research Articles

Engulfing behavior of vapor bubbles in downward facing heated surface boiling

Boiling of the coolant at the hot surface provides relatively better cooling by absorbing latent heat along with convection heat transfer as compared to heat transfer under single-phase conditions. In boiling, the orientation of heated surface also plays a crucial role. Downward facing boiling is complex than upward facing boiling, as the detachment of the bubble inhibited due to the heater surface orientation. Consequently, the bubble residence time and interaction with other bubbles are different in such boiling conditions. Our experiments on a large downward facing flat surface (100 × 400 mm2) revealed unexplored boiling phenomena. The boiling process is dominated by a complex engulfing phenomenon, which is rarely reported in the past. The engulfing phenomena have been captured using high-speed photography, wherein, at low heat fluxes, it is observed that larger bubbles engulf small bubbles by opening their mouth and swallowing the small bubbles. However, at higher heat fluxes, this phenomenon disappears. A larger vapor blanket is formed due to engulfing of bubbles, which may lead to departure from nucleate boiling. This engulfing behavior depends on the heat flux and subcooling. With the increase in heat flux, it is found that the rate of vapor engulfing increases. We have attempted to explain the science behind such engulfing phenomenon based on the capillary pressure difference. These results are consistent at various subcooling. This research provides new insights into nucleate boiling and may help in developing advanced mathematical models for accurate heat transfer prediction on downward facing nucleate boiling.

Numerical simulation of vapor explosion bubbles in the presence of a non-condensable gas and a phase change

The complex dynamics of two flow problems of vapor explosion bubbles near a free surface and between two parallel plates are simulated by a compressible three-phase flow code. Firstly, a coupled high-order monotonicity-preserving (MP) interface capturing scheme is developed for the numerical computation of compressible three-phase flows, which is followed by validation with previous numerical and experimental data. Phase changes are taken into consideration and the empirical condensation coefficient is calibrated with the experimental data. For both two flow problems, good agreement between the numerical results and reference data is obtained. Most physical phenomena, including the free surface spike, the liquid jet before the impact and the complex interface deformations during multiple expanding and collapsing oscillations, are reasonably well reproduced. Afterwards, the effects of the initial vapor pressure and the non-condensable gas volume fraction in the surrounding water on the behavior of vapor bubbles are analyzed and discussed. It is found that both the initial vapor pressure and the non-condensable gas volume fraction have a substantial influence on the behavior of the bubble. Higher initial vapor pressure does indeed tend to result in larger bubbles. The presence of non-condensable gases tends to hinder the transfer of heat, slowing down condensation and potentially resulting in larger bubbles. When the initial vapor pressure is in the range of 10 to 20 atmospheres and the gas volume fraction is in the range of 0 to 0.2, the maximum vapor bubble volume is found to increase linearly with an increase of the amount of non-condensable gas.

Study on single-phase and two-phase flow and heat transfer characteristics of HFE-7100 in manifold microchannel heat sink with corrugated bottom

To solve the problem of high heat flux heat dissipation in microelectronic devices, a manifold micro-channel heat sink with corrugated bottom (CB-MMC) is pro-posed on the basis of the manifold microchannel heat sink (MMC). The flow and heat transfer characteristics of HFE-7100 in MMC and CB-MMC are investigated numerically. The results show that CB-MMC reduces the pressure loss and enhances the heat transfer performance in single-phase flow. The orthogonal test method is used to obtain structural design solutions with optimal thermal performance. It is observed that the temperature reduction is always at the expense of the increase of the pressure drop. In addition, the optimization parameters combination obtained through comprehensive evaluation of temperature and pressure drop through weight matrix ? optimized solution 19 (wavelength A = 800 ?m, am-plitude B = 40 ?m, channel depth C = 180 ?m, outlet width D = 300 ?m, channel width E = 25 ?m). Its Tave has decreased by 6.89?C, ?P decreased by 10.27 kPa. Moreover, the subcooled boiling flow and heat transfer performance in MMC and CB-MMC are comparatively studied. The results demonstrate that the dynamic behavior of vapor bubbles causes large pressure fluctuations, which further leads to small temperature fluctuations, and thus reduces the stability of the flow and boiling heat transfer. Compared with MMC, CB-MMC exhibits more stable two-phase flow and better boiling heat transfer.

Subcooled Pool Boiling on Hierarchical Micro- and Nanostructure-Modified Copper Surfaces in HFE-7100 Dielectric Liquid

ABSTRACT Hierarchical surfaces comprised both microscale and nanoscale structures have been previously studied as a means of targeting multiple length scales to achieve superior pool boiling performance. However, preceding studies have focused almost exclusively on high surface tension working fluids, while technologically important low surface tension fluids have remained largely unexplored. Due to their significantly lower surface tension these liquids tend to push out the air trapped in surface cavities of the heating surface, resulting in fewer nucleation sites compared to the same surface in water at low to moderate superheats. Thus, developing effective surface modification techniques for pool boiling in dielectric liquids and understanding the multiphase physics behind them is a pressing need in order to overcome these performance limitations and accelerate their adoption. In this work, we utilize scalable manufacturing techniques to realize four separate surface types (planar, nanoscale-modified, microscale-modified, and hierarchical) and experimentally determine their respective pool boiling performance within the low surface tension commercial working fluid HFE-7100. A maximum heat transfer enhancement of 125% at 38 K of superheat was observed for the best performing samples, which interestingly were nanoscale-modified and not those of the hierarchical type. Visual observations via high-speed video analysis of vapor bubble behavior are utilized to explain the underlying multiphase physics as to why these samples performed so well and future directions for achieving surface optimization across multiple length scales.

Investigation on visualization and heat transfer performance study of the mini-channel flow boiling

In recent years, mini-channel water flow boiling has been proven to have favorable cooling capability, especially for electronic component heat dissipation. Nevertheless, the fundamental principles related to flow boiling in mini-channel water lack clear comprehension, and the relevant experimental data on heat transfer performance and CHF visualization need to be supplemented. The behavior of vapor bubbles was explored in detail through visualization experiments to elucidate the mechanism of flow boiling and heat transfer in mini-channels. Meanwhile, the effects of mass flux, heat flux and tilt angle on the flow boiling of water and the CHF characteristics were revealed clearly though a series of crossover experiments. The results showed that in mini-channel water flow boiling, mass flux and heat flux will have effects on vapor bubble behavior, especially the merging of vapor bubbles, and then modify the heat transfer coefficient, and the mass flux in the optimal range exists. The tilt angle has less effect on the heat transfer coefficient, but affects the rate of vapor bubble growth. After local dryness the heat transfer mechanism transitions directly to film boiling instead of transition boiling, the boiling curve grows linearly with the wall superheat, and the CHF increases with increased mass flux. The research content of this paper helps to fill the gap in the experimental study of flow boiling in mini-channel and provides guidance for related practical applications.

A review on techniques to alter the bubble dynamics in pool boiling

• Influence of force fields on the behaviour of vapour bubbles were studied. • Effects of nanofluids, additives, micro/nanoporous structured surfaces, electric and magnetic fields on the bubble dynamics were assessed. • Effects of hybrid techniques and smart surfaces on bubble dynamics were studied. • The enhancement mechanisms behind the passive and active techniques on boiling heat transfer were studied. Boiling phenomena exhibits a complex nature in many ways with chaotic bubble dynamics being one of them. Understanding the dynamic behavior of vapor bubbles in pool/flow boiling is an issue of great importance in many engineering applications. Despite their ambiguity, contemporary researchers acknowledged bubbles as a powerful scientific tool. In this regard, many strategies have been devised to exploit the bubble dynamics for various applications. Here, we aim to review recent strategies adopted for tailoring the pool boiling bubble dynamics. In particular, this article overviews the effects of nanofluids, additives, micro/nanoporous structured surfaces, electric and magnetic fields on the bubble dynamics, and critical heat flux (CHF). We assess these strategic impacts on vapor bubble behavior through the themes of force field and mechanics. This review article also includes a compilation of prominent models and equations for predicting the forces acting on the vapor bubble. Further, we highlight the possible physical mechanisms governing the critical heat flux with vivid illustrations. In the pursuit of knowledge, we summarise the current interdisciplinary approaches adopted for manipulating the bubble dynamics. Finally, the key challenges and future research directions in bubble dynamics are also discussed.

Numerical study on the behavior of vapor bubbles during boiling with surface acoustic wave (SAW)

In this study, the dynamic mechanism of vapor bubble during boiling process under the effect of acoustic field was numerically investigated, by coupling the phase field method and thermo-viscous acoustic module. The purpose is to use the surface acoustic wave (SAW) to interfere with vapor bubbles in the boiling process, so as to realize control of boiling on the temporal and spatial scales while improving the boiling heat transfer capability. It is observed that the effect of SAW on bubble behaviors is related to the amplitude of SAW and the initial bubble size. For pool boiling case, at the critical displacement amplitude, tiny vortices form at the root of bubble during the bubble growth stage. The velocity amplitude induced by SAW in the liquid phase around bubble can reach up to 2.1 m/s, which is nearly twofold comparing to that without SAW effect. The vortices at the root of bubble disrupt the thermal boundary layer to a certain extent by creating ‘micro-turbulence’, thereby reducing the thermal resistance. Furthermore, the strong SAW effect for the large amplitude case causes one bubble to collapse into two separate bubbles after the bubble detaches from the heating surface. For flow boiling case, the SAW can break the growing bubbles along the flow direction, and the collapsed bubbles flow downstream with a maximum speed of 5 m/s, which is almost 12.5 times of that without acoustic excitation. The acoustic streaming can interrupt the growth of the boundary layer in the single-phase region, so that the subcooled fluid can rewet the microchannel wall in time. Thus, the growth of bubbles and flow instability are effectively inhibited, and flow boiling heat transfer is promoted. It is revealed that the SAW is an effective approach for the active control and enhancement of boiling heat transfer.

Numerical Study on the Behavior of Vapor Bubbles During Boiling with Surface Acoustic Wave (Saw)

Numerical Study on the Behavior of Vapor Bubbles During Boiling with Surface Acoustic Wave (Saw)

Spatially periodic vapor bubble activity during subcooled pool boiling on 1D aluminum alloy micro-fin arrays

Within the broad field of micro- and nano- scale surface modification to improve pool boiling performance, micro-fin arrays are especially attractive due to their ability to significantly increase wetted surface area with minimal increase in size or weight compared to macroscopic fins. In addition, their regular and repeating geometry enables greater potential for parametric optimization, uniformity, and repeatability as compared to more randomized alternatives. However, the presence of a regular, repeating microscale pattern on a heated surface has previously been shown to create alternating regions of liquid and vapor flow at small length scales which can strongly affect the macroscopic heat transfer performance of the surface. In this work, a combination of pool boiling experiments, high-speed imaging, and numerical modeling were used to investigate vapor bubble behavior and pool boiling heat transfer characteristics from one-dimensional (1D) aluminum alloy micro-fin arrays in the dielectric coolant HFE-7100. Results showed that the presence of 1D micro-fin arrays can significantly enhance pool boiling performance above that of a planar, unpatterned baseline surface but with important and sometimes nonintuitive dependence on micro-fin height. Experimental data indicated that a simple increase in surface area does not necessarily correlate to improved heat transfer performance, while high-speed imaging revealed that the presence of the micro-fins induces a periodic series of upward and downward flow patterns, in addition to distinct vapor bubble migration behavior. These results expand the range of surface types and working fluids for which spatially periodic and distinct liquid/vapor transport pathways created via microscale surface modification have been reported.

Post-breakdown dielectric recovery characteristics of water for high-repetition-rate switch

Water plays a pivotal role in pulse switches, owing to its fast recovery characteristic and high dielectric strength. In this paper, we empirically studied the effect of behavior of vapor bubbles on the dielectric recovery characteristics of water during the post-breakdown. The experimental medium is distilled water, and highly pressurized CO 2 in its supercritical phase is used as a comparative medium to test the voltage recovery ratio. The dielectric recovery characteristics of water was explained by alternating expansion-contraction bubble behaviors, including bubble density variation in the second oscillation stage. The experimental results showed that water showed a shorter recovery time (2 ms), and it could be regarded as a promising medium for high-repetition-rate switch. The motion of second oscillating bubbles resulted in a considerably short recovery time.

Numerical analysis of bubble condensation behavior under high-pressure flow conditions

Heat transfer during subcooled flow boiling has a pivotal role in pressurized water reactors; it also occurs in boiling water reactors prior to the onset of saturated nucleate boiling. We examined the condensation behavior of vapor bubbles in the subcooled liquid phase using the fully compressible two-phase homogeneous mixture method, solved by an implicit dual-time preconditioned method. The continuous surface force method was applied to determine the surface tension between the phases in the simulation. To predict the empirical coefficient, we conducted a sensitivity study using Lee’s mass transfer model. For nuclear applications, we simulated high-pressure vapor–water conditions under higher mass flow conditions. The comparison of the numerical simulation and experimental results showed that the proposed model accurately predicted the condensation behavior of the bubble. Additionally, we investigated single bubble condensation behavior at different operating pressures, subcooling temperatures, bubble diameters, and bulk velocities. We also investigated the effects of high-pressure condensation on bubble shape. At lower subcooling temperatures, the condensation rate increased as pressure increased; however, at higher subcooling temperatures, pressure had no significant impact on the condensation rate.

Dynamics of vapour bubbles induced during the boiling of superfluid helium under microgravity conditions

The presented research aims to establish a theoretical framework for the analysis of the transient behaviour of vapour bubbles resulting from superfluid helium boiling under the conditions of microgravity. As long as the volume of a bubble is small in comparison with the volume of a container, the dynamics of the surrounding liquid and interfacial momentum transfer seem to be the dominant factors determining the pace of bubble growth. This notion leads to a relatively clear mathematical description of the system. A pair of 1st order ODEs was formulated on the basis of the mass and momentum conservation principles applied to the liquid phase as well as the assumption of the perfect spherical symmetry of a bubble. In order to obtain the empirical data necessary for the validation of the model, an experimental campaign has been conducted at the ZARM Drop Tower. A dedicated cryostat equipped with optical windows has been utilised in order to achieve superfluidity. The boiling was procured using a 1.88 mm long segment of manganin wire, measuring 50 μm in diameter. The vapour-liquid interface was tracked using a high-speed camera equipped with two separate heads positioned along two perpendicular axes. A reasonable agreement between the model predictions and the empirical data has been observed, indicating the suitability of the framework for the analysis of superfluid helium boiling.

Effects of mass transfer on damping mechanisms of vapor bubbles oscillating in liquids

The damping mechanisms play an important role in the behavior of vapor bubbles. In the present paper, effects of mass transfer on the damping mechanisms of oscillating vapor bubbles in liquids are investigated within a wide range of parameter zone (e.g. in terms of frequency and bubble Péclet number). Results of the vapor bubbles are also compared with those of the gas bubbles. Our findings reveal that the damping mechanisms of vapor bubbles are significantly affected by the mass transfer especially in the regions with small and medium bubble Péclet number. Comparing with the gas bubbles, the contributions of the mass-transfer damping mechanism for the vapor bubble case are quite significant, being the dominant damping mechanism in a wide region.

Thermal performance analysis of pool boiling on an enhanced surface modified by the combination of microstructures and wetting properties

Characterized by low temperature difference and high heat flux, pool boiling heat transfer has been widely applied in various engineering technical fields. The aim of this study was to explore the thermal performance of pool boiling heat transfer on an enhanced surface modified by the combination of microstructures and wetting properties. The two-dimensional transient volume of fluid model and numerical simulation were selected to investigate each case which was established. Variation of the vapor bubble behavior, the velocity field of computational domain, the temperature at the liquid/solid interface and heat transfer coefficient (HTC) of each case were analyzed. The heat transfer performances of mixed hydrophilic and hydrophobic microstructures were all superior to those of hydrophilic microstructures only, except the flat. With respect to hydrophilic microstructures, the average growth rates of HTC for mixed hydrophilic and hydrophobic microstructures were 17.59%, 21.59%, 2.74% and 3.81% when the heights of microstructures were 3 μm, 5 7, 7 μm and 9 μm, respectively. Moreover, the average growth rate of HTC for mixed hydrophilic and hydrophobic microstructures decreased by 12.44%, 5.31% and 5.21% with increasing width.

Subcooled boiling-induced vibration of a heater rod located between two metallic walls

The phenomenon that a heating structure vibrates itself due to the behavior of vapor bubbles generated under subcooled boiling has been known as “Subcooled Boiling-induced Vibration (SBIV)”. As one of such a heating structure, fuel assemblies for Boiling Water Reactors (BWR) are utilized in subcooled boiling of water, and those for Pressurized Water Reactors (PWR) may face unexpected subcooled boiling conditions in case of sudden drop of the system pressure or loss of water flow, though they are utilized in single phase of water under normal operating conditions. As studies on SBIV, some researchers have conducted demonstrative experiments with a partial array of fuel rods simulating the actual BWR fuel assembly in a flow test loop, which showed no significant influences of the SBIV to degrade the integrity of the fuel rods. In addition, in order to investigate the fundamental phenomenon of the SBIV, pool boiling experiments of the SBIV on a single heater rod were performed in other studies with a simplified apparatus of a water tank in laboratory size under atmospheric pressure. In the experiments, behavior of bubbles generated under various degree of subcooling were observed, and the acceleration of the SBIV of the heater rod was measured. The present study, as a series of the above experiments for the fundamental phenomenon of the SBIV, the two thin walls made of stainless steel were installed in parallel to interleave the heater rod with the gap distance of 1mm or 3mm to each of the two walls, which was expected to provide useful results to investigate effects of the parallel walls configuration to the SBIV of the heater rod. The results obtained in the experiments suggested that bubbles growing largely in the gap influenced on the SBIV of the heater rod, with the remarkable behavior such as slide of the large bubbles on the surface of the heater rod towards the open region, interference and coalescence between large bubbles.

Prediction of bubble departure in forced convection boiling: A mechanistic model

In the context of computational fluid dynamic simulations of boiling flows using time-averaged Eulerian multi-phase approaches, the many sub-models required to describe such a complex phenomena are of particular importance. Of interest here, wall boiling requires calculation of the contribution of evaporation to global heat transfer, which in turn relies on determination of the active nucleation site density, bubble departure diameter and frequency of bubble departure. In this paper, an improved mechanistic model for the bubble departure diameter during flow boiling is developed. The model is based on the balance of forces acting on a bubble at a single nucleation site, with a new equation governing bubble growth proposed. The formulation accounts for evaporation of the micro-layer under the bubble, heat transfer from superheated liquid around the bubble surface, and condensation on the bubble cap due to the presence of sub-cooled liquid. Validation of the growth equation is provided through comparison against experiments in both pool boiling and flow boiling conditions. Introduction of condensation on the bubble cap allows reproduction of the growth of the bubble for different sub-cooling temperatures of the surrounding liquid. In addition, a sensitivity study guarantees dependency of the bubble departure diameter on relevant physical quantities such as mass flow rate, heat flux, liquid sub-cooling and pressure, with any inclination of the channel walls correctly accounted for. Predictions of bubble departure diameter and bubble lift-off are validated against three different databases on sub-cooled flow boiling with water and an additional database on saturated boiling with refrigerant R113. The whole data set guarantees validation is performed over a range of parameters and operating conditions as broad as possible. Satisfactory predictive accuracy is obtained in all conditions. The present formulation provides an appropriate starting point for prediction of the behaviour of vapour bubbles under more general conditions which include lift-off after sliding, the frequency of bubble departure, bubble merging and bubble shrinking and collapse due to condensation.

Numerical Simulation of Droplets Impacting on a Liquid Film with a Vapor Bubble Growing

A numerical model of multiphase flow is developed to investigate the relative contributions of droplet parameters to spray cooling heat transfer. The 2-D model takes into account the effects of surface tension, gravity and viscosity. The heat and mass exchanges of free surface are defined to study vapor bubble behavior in liquid films. The multiphase flow and heat transfer are discussed for the three droplet parameters: initial droplet position, initial droplet temperature, and droplet impact frequency. The heat transfer mechanisms of the three cases are discussed in detail.

D114 細線沸騰における局所の蒸気泡発生・崩壊過程と熱伝達特性(OS-15: 沸騰・凝縮伝熱および混相流の最近の進展(1))

A series of experiments on subcooled pool boiling on a thin wire are carried out. We focus on the condensation and collapse processes of vapor bubbles generated on the heated wire. We find two major patterns of the vapor bubble behaviors resulting in the emission of the microbubble around the thin wire by employing high-speed observation with frame rate up to 150,000 fps. Effect of the surface roughness of the wire on the vapor bubble behavior and heat transfer coefficient is discussed.

CHF Phenomena by Photographic Study of Boiling Behavior due to Transient Heat Inputs

The transient boiling heat transfer characteristics in a pool of water and highly wetting liquids such as ethanol and FC-72 due to an exponentially increasing heat input of various rates were investigated using the 1.0 mm diameter experimental heater shaped in a horizontal cylinder for wide ranges of pressure and subcooling. The trend of critical heat flux (CHF) values in relation to the periods was divided into three groups. The CHF belonging to the 1st group with a longer period occurs with a fully developed nucleate boiling (FDNB) heat transfer process. For the 2nd group with shorter periods, the direct transition to film boiling from non boiling occurs as an explosive boiling. The direct boiling transition at the CHF from non-boiling regime to film boiling occurred without a heat flux increase. It was confirmed that the initial boiling behavior is significantly affected by the property and the wettability of the liquid. The photographic observations on the vapor bubble behavior during transitions to film boiling were performed using a high-speed video camera system.

Numerical study of vapor bubble effect on flow and heat transfer in microchannel

Flow boiling in a microchannel is characterized by nucleation and dynamic behavior of vapor bubbles in the channel. In the present study, the effect of vapor bubble on fluid flow and heat transfer in a microchannel is investigated via lattice Boltzmann (LB) modeling. With respect to boiling flow in a single microchannel, the bubble nucleation, growth, and departure are simulated by using an improved hybrid LB model. Relating bubble behavior with fluid flow and boiling heat transfer provides some insight into the relevant fundamental physics on flow boiling in the microchannel. It is found that the bubble growth before its departure from the wall induces an obvious resistance to the fluid flow. The processes of nucleation and motion of different bubbles interact, leading to an alternate, either enhanced or weakened, effect of bubble behavior on the flow boiling.