Rapid Bubble Growth Research Articles

Thermal investigation and flow pattern analysis of a closed-loop pulsating heat pipe with binary mixtures

The present study basically examines thermal performance and relevant flow phenomena of a flat plate closed-loop pulsating heat pipe (FP-CLPHP) filled with binary mixtures. The heat pipe has eight turns, each of which consisting of asymmetrical channel pairs having cross sections as 2 mm × 2 mm and 1 mm × 2 mm (width × height). Binary mixtures are generated as mixtures of ethanol (E) and pentane (P) with different mixing ratios. Mainly, effects of volume mixing ratio (E/P = 1:1, 1:3 and 3:1), vertical (90°) and horizontal orientation (0°) and filling ratio (30% and 60%) on thermal characteristics are investigated under different heat inputs. For examination of flow dynamics, images are obtained by using a high-speed camera at 1000 fps. The results show that variation of volumetric percentage of each component significantly changes thermal performance. Increasing pentane in the mixture improves the thermal performance, such that heat pipe with mixing ratio of E/P = 1:3 can properly operate even at horizontal position. On the other hand, increasing volume of ethanol in mixture leads to collapse of the FP-CLPHP at both orientations (0° and 90°). Generally, the filling ratio of 30% shows better thermal performance. Complex bubble–liquid interactions and dynamics play critical roles on thermal characteristics. Two novel characteristic phenomena are identified for non-uniform PHPs: (1) flooding phenomenon and (2) asymmetrical rapid bubble growth phenomenon.

Deep long period volcanic earthquakes generated by degassing of volatile-rich basaltic magmas

Deep long-period (DLP) earthquakes observed beneath active volcanoes are sometimes considered as precursors to eruptions. Their origin remains, however, unclear. Here, we present a possible DLP generating mechanism related to the rapid growth of gas bubbles in response to the slow decompression of over-saturated magma. For certain values of the gas and bubble content, the elastic deformation of surrounding rocks forced by the expanding bubbly magma can be fast enough to generate seismic waves. We show that amplitudes and frequencies of DLP earthquakes observed beneath the Klyuchevskoy volcano (Kamchatka, Russia) can be predicted by our model when considering pressure changes of ~107 Pa in a volume of ~103–104 m3 and realistic magma compositions. Our results show importance of the deep degassing in the generation of volcanic seismicity and suggest that the DLP swarms beneath active volcanoes might be related to the pulses of volatile-rich basaltic magmas rising from the mantle.

3D Transient heat transfer analysis and flow visualization study in diverging microchannel for instability mitigated two-phase flow: A numerical study

This paper presents a 3D conjugate numerical simulation of the diverging microchannel heat sink, which is performed using the Volume of Fluid (VOF) model coupled with the phase change model. The modeling approach and simulation results are validated with standard correlations and published experimental results. The model is used to analyze the flow boiling and transient heat transfer mechanism through simulation of bubble nucleation, bubble growth pattern, pressure, temperature, and vapor quality variations. Moreover, local and transient Heat Transfer Coefficient (HTC) is calculated and then correlated with the simulated bubble dynamics. Herein, simulations are performed on the expanding microchannel with a varying outlet to inlet width ratio for uniform heat flux 150 kW/m2 at two different mass fluxes 240 kg/m2s and 710 kg/m2s. Further, to understand the effect of wall contact angle, simulations are performed for two different values of wall contact angle of 140° and 65°. The results indicate that the channels with low divergence angle (θ) and low outlet to inlet width ratio (δ) have low nucleation time. Rapid bubble growth and elongated flow due to channel confinement were also observed. Higher values of the heat transfer coefficient are observed in the microchannels with small hydraulic diameter, and as the diameter is increased, a reduction in the HTC values is observed. The transient heat transfer coefficient fluctuation in the channel with time has been studied along with the corresponding flow pattern. Due to the smaller wall contact angle, the bubble separation rate from the wall is low, thereby creating localized hotspots. A comparative study between straight channel and diverging channel has showed that diverging microchannels have a better performance in terms of instability mitigated flow boiling. This study has considerable relevance in electronic chip cooling using expanding microchannel with instability mitigated two-phase flow.

Flow behavior around single nucleate boiling bubble quantitatively grasped by particle tracking visualization

In this study, a single nucleation site was made on the upper surface of heated substrate, and small tracer particles suspended in water for flow visualization. Single vapor bubble dynamics and particle movements were recorded by high speed camera, then the flow field was quantitatively grasped by particle tracking visualization. Resulting of the visualization, in earlier stage of nucleate boiling, rapid growth motion of bubble pushed liquid surrounding bubble interface outwardly. There was no retracting flow into root of boiling bubble though it needed to feed water into root of boiling bubble due to evaporation in case of lower heat flux condition. Liquid flow pushed by rapid bubble growth was explained by combining thermal diffusion equation with mass conservation law, moreover the pressure increase caused this liquid flow was estimated. In bubble departure process, liquid near substrate was horizontally pulled to nucleation site, but the eddy to enhance a vertical mixing of superheated liquid near substrate was not observed. After bubble departure, it was found that a hot liquid plume under a rising bubble was not mainly caused by entrainment of superheated liquid layer formed on heated substrate, but by a relative descending flow around a rising bubble induced by mass conservation law.

Numerical modelling of droplet breakup for flash-boiling fuel spray predictions

Flash-boiling of fuel sprays can occur under injection of superheated fuel into ambient pressure that is lower than the saturation pressure of the fuel and can dramatically alter spray formation due to complex two-phase flow effects and rapid droplet evaporation phenomena. Such phenomena exist in-cylinder at low-load in-city driving conditions where strict engine emission regulations apply, hence the need for faithful flash-boiling fuel spray models by engine designers. To enhance the current modelling capability of superheated fuel sprays, with focus on near-nozzle plume expansion, a flash-boiling breakup modelling approach was developed to introduce the thermal breakup mechanism of droplets caused by nucleation and bubble growth. This model was particularly aimed at sprays where levels of superheat introduced noticeable radial expansion of the plumes upon discharge from the nozzle orifice. The model was able to simulate droplet shattering by introducing Lagrangian child parcels at breakup sites with additional radial velocity components instigated by rapid bubble growth and surface instabilities. Combination of the flash-boiling droplet breakup model with a flash-boiling effective nozzle model that was used as boundary condition for the spray plumes offered a more complete modelling approach, where both in-nozzle phase change effects and near-nozzle flashing through droplet shattering were incorporated into the Eulerian-Lagrangian two-phase computational framework. Sensitivity studies were carried out to investigate important parameters which are inherently difficult to measure experimentally and offered valuable insight into modelling superheated sprays. The model was able to capture important flash-boiling spray characteristics and quantitative validation was achieved through comparison to experimental data in the form of penetration lengths and droplet sizes with a good level of agreement.

An ingenious fluidic capacitor for complete suppression of thermal fluctuations in two-phase microchannel heat sinks

Rapid bubble growth within the confinement of microchannels leads to undesired flow oscillations. The flow regime cycles between single-phase, bubbly/slug, backflow, dryout, and rewetting regimes. The large difference between the heat transfer coefficient (HTC) in these regimes induces significant thermal fluctuations (≈ ± 5 ° C) and the overall HTC is low. The intensity of backflow and the time-span of dryout increases with heat flux. As a result, a gradual deterioration in HTC followed by a premature CHF is encountered. Here we introduce an ingenious pulse-dampener for complete mitigation of two-phase thermofluidic fluctuations in microchannel heat sinks. An inflatable latex bladder in the outlet manifold acts as a fluidic capacitor to accommodate the excess vapor. Such on-demand vapor removal weakens backflow to avoid extended dryout. In sharp contrast to the heat sinks without pulse-dampener, a relatively high and monotonically increasing HTC without any thermal fluctuations is observed in experiments with pulse-dampener. A maximum HTC of ≈270 kW/m2. K at a heat flux of ≈2 MW/m2 was observed with a nanostructured microchannel heat sink. The monotonically increasing HTC suggests that the true potential of this pulse-dampener-based heat sink design is much larger than the maximum heat flux of ≈2 MW/m2 tested in our work.

Viewing internal bubbling and microexplosions in combusting metal particles via x-ray phase contrast imaging

The combustion performance of metal particles for explosives, propellants, pyrotechnics, and bio-agent defeat can be optimized by controlling the chemistry, size, and coatings of the particles. Many pure and composite metal fuels used in these applications have been observed to microexplode during combustion. This causes particles to fragment, revealing fresh surfaces, which may enhance burn rates and efficiencies. Despite these potential benefits, few have attempted to control the bubbling and the microexplosion of metal powders as they combust in molten states. Here, we choose Al:Zr composite powders as a representative system to study bubbling and microexplosions and to identify active mechanisms. Using synchrotron x-rays and phase contrast imaging, we observe molten metal particles of various sizes as they burn in air at temperatures ranging from 2700 to 3500 K. We characterize bubble nucleation and growth in the interior of the particles during the combustion process, and we identify heterogeneous nucleation, slow growth and coalescence of bubbles, as well as rapid bubble growth leading to microexplosions. Bubble growth rates and Laplace pressures are calculated, and we find that during slow expansion growth rates are similar to classical predictions of bubble growth in superheated liquids. For rapid expansions we find that a critical growth rate of ∼0.5 m s−1, independent of the initial particle size, is necessary to enable microexplosions. We calculate the rate of gas generation that is needed to enable this growth and we conclude that nitrogen is the gas most likely driving rapid bubble growth and fragmentation. These results provide the first in situ observations of the mechanisms that control bubbling and microexplosions during the combustion of metal particles.

A combined CFD/visualization investigation of heat transfer behaviors during geyser boiling in two-phase closed thermosyphon

In order to promote the understanding of geyser boiling in two-phase closed thermosyphon, a combined CFD/visualization investigation was carried out in this work. The widely used phase change model, i.e., Lee model, is improved by introducing the consideration of superheat to increase its prediction performance. CFD modellings of heat transfer behaviors during geyser boiling were performed using both the improved and original Lee model based on VOF method. These results were analyzed and compared to the visualized observations from experimental tests. The experimental results show that a periodic geyser boiling with the cycle time at 80–100 s occurs under the conditions that the flow rate of heating water is controlled at 400 mL/min (95 °C) and cooling water at 300 mL/min (30 °C). The corresponding modelling results in accordance with these experimental conditions suggest that the improved Lee model has a better prediction performance. The rapid slug bubble growth in expulsion period, and the surface evaporation and thermally natural convection in liquid pool during the incubation period are in good agreement with the experimental observations. In addition, the temperature and velocity contours during geyser boiling are comprehensively revealed by use of the improved Lee model.

Enhancement of eruption explosivity by heterogeneous bubble nucleation triggered by magma mingling

We present new evidence that shows magma mingling can be a key process during highly explosive eruptions. Using fractal analysis of the size distribution of trachybasaltic fragments found on the inner walls of bubbles in trachytic pumices, we show that the more mafic component underwent fracturing during quenching against the trachyte. We propose a new mechanism for how this magmatic interaction at depth triggered rapid heterogeneous bubble nucleation and growth and could have enhanced eruption explosivity. We argue that the data support a further, and hitherto unreported contribution of magma mingling to highly explosive eruptions. This has implications for hazard assessment for those volcanoes in which evidence of magma mingling exists.

On the bubble-surfactant interaction

Surfactants play an important role in numerous chemical applications. Their effects on dispersion systems, as emulsions and colloids, are vastly studied. Yet, the dissolution (kinetics) of surfactants themselves has not received much scientific attention, despite the high implications for industrial applications. Nevertheless one can acknowledge the rapid growth of new technologies which are being considered to enhance various chemical processes, one being cavitation–the rapid growth and collapse of vapor bubbles, which results in the formation of shockwaves, streaming, free radicals, high local temperatures etc.In this paper, we investigate the influence of cavitation on the dissolution rate of Sodium Dodecyl Benzene Sulfonate. Using a high-speed camera, we determined the physics of the dissolution process. We have shown that the presence of cavitation greatly enhances the dissolution rate of surfactant in water through a process comparable to cavitation erosion.Finally, based on our findings, we also present the newly developed cavitation reactor, which could be potentially used for enhancing the process of washing powder dissolution in home appliances.

Experimental study of bubble dynamics of isolated bubbles in water pool boiling at subatmospheric pressures

The purpose of this work is to expose specific features of bubble dynamics in water pool boiling at low pressure in comparison with the atmospheric pressure. At subatmospheric pressures, the boiling environment is known to be very non-homogeneous, both in terms of pressure and subcooling degree. As a consequence, thermophysical properties of the fluid are not homogeneous either. These non-homogeneities are supposed to be responsible for the particular shape, size and departure frequency of bubble observed at low pressure.Experiments of water pool boiling were conducted at saturation state from atmospheric pressure (Tsat=100°C) down to 4.2kPa (Tsat=30°C) with a single imposed heat flux at the heater. Videos of each experiment were recorded with a high speed camera in order to analyze the bubble dynamics for different pressures.Increases of the bubble size and of the detachment frequency are observed as the pressure decreases while the bubble shape changes: a near-spherical shape at atmospheric pressure to an oblate spheroidal shape in the lowest investigated pressure range through a mushroom shape and/or vapor columns due to the rapid growth of successive bubbles sucked into the wake of the first one in the range of intermediate pressures. Recondensation of bubbles occurs rapidly after departure because of the subcooling degree. A home-made image processing software allowed calculating some quantitative parameters of the bubble dynamics such as the instantaneous volume and the departure frequency from the video records.

Comparison of bubble growth process within a superheated water droplet and in superheated water due to rapid depressurization

This paper reports different characteristics of bubble growth within a superheated water droplet and in superheated water due to rapid depressurization through numerical calculations. Two mathematical models including the classical bubble growth model and the nonequilibrium bubble nucleation model are developed. The classical bubble growth model is based on the momentum equation of bubble growth coupling with the energy conservation equation. The heat transfer due to flash evaporation is considered at the droplet surface. The nonequilibrium bubble nucleation model considers the mechanical nonequilibrium on the formation of bubble nucleus. The numerical results show that a smaller pressure difference (Pv−P∞) leads to a longer initial stage, and the variation of bubble radius simulated by the nonequilibrium model achieves a shorter initial stage and a faster bubble growth rate. The bubble growth is mainly affected by the competition of pressure difference and surface tension, and the prominent character of bubble growth process within a droplet is the occurrence of accelerating stage. With the bubble interface approaching the droplet surface, the boundary layer becomes thinner, leading to a rapid bubble growth rate. The influencing factors of superheat degree, pressure difference and ambient pressure on bubble growth within a water droplet due to depressurization are analyzed. The results reveal that the pressure difference plays an important role on bubble growth. However, bubble grows slower with a higher ambient pressure, even under the same initial pressure difference. The reason is due to a faster bubble interfacial temperature drop under higher pressure environment. The results can provide insight into bubble growth process during spray and atomization.

Suppressing high-frequency temperature oscillations in microchannels with surface structures

Two-phase microchannel heat sinks are attractive for thermal management of high heat flux electronic devices, yet flow instability which can lead to thermal and mechanical fatigue remains a significant challenge. Much work has focused on long-timescale (∼seconds) flow oscillations which are usually related to the compressible volume in the loop. The rapid growth of vapor bubbles which can also cause flow reversal, however, occurs on a much shorter timescale (∼tens of milliseconds). While this high-frequency oscillation has often been visualized with high-speed imaging, its effect on the instantaneous temperature has not been fully investigated due to the typical low sampling rates of the sensors. Here, we investigate the temperature response as a result of the high-frequency flow oscillation in microchannels and the effect of surface microstructures on this temperature oscillation with a measurement data acquisition rate of 1000 Hz. For smooth surface microchannels, fluid flow oscillated between complete dry-out and rewetting annular flow due to the short-timescale flow instability, which caused high-frequency and large amplitude temperature oscillations (10 °C in 25 ms). In comparison, hydrophilic surface structures on the microchannel promoted capillary flow which delayed and suppressed dry-out in each oscillation cycle, and thus significantly reduced the temperature oscillation at high heat fluxes. This work suggests that promoting capillary wicking via surface structures is a promising technique to reduce thermal fatigue in high heat flux two-phase microchannel thermal management devices.

An experimental investigation of saturated flow boiling heat transfer and pressure drop in square microchannels

In this study, saturated flow boiling characteristics of deionized water in parallel microchannels are investigated experimentally. The silicone microchannel heat sink consists of 29 parallel square microchannels having hydraulic diameters of 150 µm. Experiments have been conducted for four different values of the mass flux consisting of 51, 64.5, 78 and 92.6 kg/m2s and heat flux values from 59.3 to 84.1 kW/m2. Inlet temperature of deionized water is kept at 50 ± 1 °C. Heat transfer and pressure drop are examined for varying values of the governing parameters. Simultaneous high-speed video images have been taken as well as temperature and pressure measurements. The flow visualization results lead to key findings for flow boiling instabilities and underlying physical mechanisms of heat transfer in microchannels. Quasi-periodical rewetting and drying, rapid bubble growth and elongation toward both upstream and downstream of the channels and reverse flow are observed in parallel microchannels.

Forces on rapidly growing vapor bubbles on a wall in forced convection with varying angle of inclination

The forces involved in rapid vapor bubble growth are assessed by measurement and analysis of bubble growth in four principal orientations of the boiling surface and channel flow with respect to gravity. The approaching liquid flow is made nearly uniform to warrant the possibility to compute the added mass forces. Almost no difference in shape history is observed between the four cases. This is most remarkable for the case in which the boiling surface is facing downward with respect to gravity. By way of careful shape analysis and extensive force computations, the reason for this remarkable behavior is found to be the crucial role of the inertia force.

Effect of aspect ratio on saturated flow boiling in microchannels

In this study, effect of aspect ratio (AR) on the saturated flow boiling characteristics of deionized water in parallel rectangular microchannels are investigated experimentally. Detailed flow visualizations are conducted as well as temperature and pressure measurements. Effects of heat and mass flux are examined, too. Six different silicon microchannel heat sinks, each of which consists of 29 parallel rectangular microchannels, have been used. In order to determine the effect of the aspect ratio (width to depth), the microchannels having different widths and heights but same hydraulic diameters (100μm) are used. Experiments have been conducted for various values of the mass flux covering 151, 195, 238, 281 and 324kg/m2s, the values of the wall heat flux in the range of 71–131kW/m2 and various values of the aspect ratio including 0.37, 0.82, 1.22, 2.71, 3.54 and 5.00. The inlet temperature of deionized water is kept constant at 50±1°C through the tests. It is obtained that heat transfer coefficient increases with an increase in the aspect ratio up to AR=3.54 and then decreases. AR=1.22 has been appeared as a threshold value for the heat transfer coefficient. There is no regular relationship between the aspect ratio and total pressure drop. The flow visualization results provide key findings for underlying physical mechanism. Quasi-periodical rewetting and drying phenomena, rapid bubble growth and elongation towards both upstream and downstream of the channels and reverse flow are observed in parallel microchannels. In addition, strong interactions are obtained between neighboring channels.

Electrochemical Generation of a Hydrogen Bubble at a Recessed Platinum Nanopore Electrode.

We report the electrochemical generation of a single hydrogen bubble within the cavity of a recessed Pt nanopore electrode. The recessed Pt electrode is a conical pore in glass that contains a micrometer-scale Pt disk (1-10 μm radius) at the nanopore base and a nanometer-scale orifice (10-100 nm radius) that restricts diffusion of electroactive molecules and dissolved gas between the nanopore cavity and bulk solution. The formation of a H2 bubble at the Pt disk electrode in voltammetric experiments results from the reduction of H(+) in a 0.25 M H2SO4 solution; the liquid-to-gas phase transformation is indicated in the voltammetric response by a precipitous decrease in the cathodic current due to rapid bubble nucleation and growth within the nanopore cavity. Finite element simulations of the concentration distribution of dissolved H2 within the nanopore cavity, as a function of the H(+) reduction current, indicate that H2 bubble nucleation at the recessed Pt electrode surface occurs at a critical supersaturation concentration of ∼0.22 M, in agreement with the value previously obtained at (nonrecessed) Pt disk electrodes (∼0.25 M). Because the nanopore orifice limits the diffusion of H2 out of the nanopore cavity, an anodic peak corresponding to the oxidation of gaseous and dissolved H2 trapped in the recessed cavity is readily observed on the reverse voltammetric scan. Integration of the charge associated with the H2 oxidation peak is found to approach that of the H(+) reduction peak at high scan rates, confirming the assignment of the anodic peak to H2 oxidation. Preliminary results for the electrochemical generation of O2 bubbles from water oxidation at a recessed nanopore electrode are consistent with the electrogeneration of H2 bubbles.

Bubble evolution and properties in homogeneous nucleation simulations.

We analyze the properties of naturally formed nanobubbles in Lennard-Jones molecular dynamics simulations of liquid-to-vapor nucleation in the boiling and the cavitation regimes. The large computational volumes provide a realistic environment at unchanging average temperature and liquid pressure, which allows us to accurately measure properties of bubbles from their inception as stable, critically sized bubbles, to their continued growth into the constant speed regime. Bubble gas densities are up to 50% lower than the equilibrium vapor densities at the liquid temperature, yet quite close to the gas equilibrium density at the lower gas temperatures measured in the simulations: The latent heat of transformation results in bubble gas temperatures up to 25% below those of the surrounding bulk liquid. In the case of rapid bubble growth-typical for the cavitation regime-compression of the liquid outside the bubble leads to local temperature increases of up to 5%, likely significant enough to alter the surface tension as well as the local viscosity. The liquid-vapor bubble interface is thinner than expected from planar coexistence simulations by up to 50%. Bubbles near the critical size are extremely nonspherical, yet they quickly become spherical as they grow. The Rayleigh-Plesset description of bubble-growth gives good agreement in the cavitation regime.

Effect of bubble coalescence on the wall heat transfer during subcooled pool boiling

This study experimentally investigates the effects of bubble coalescence on the heat fluxes during nucleate pool boiling. A microheater array was used to generate vapor bubbles in FC-72 liquid with constant surface temperature boundary conditions while the heat flux at selected locations was measured for various superheats using a high speed data acquisition system. The heat flux for boiling with coalescence was found to fluctuate much more than when only a single bubble formed on the surface due to the vaporization of the liquid layer trapped between the bubbles and oscillations of the bubbles after coalescence which resulted in relatively long periods of fairly high heat fluxes. The observations also showed that some coalescence events were not accompanied by an increase in the heat transfer rate as the liquid layer between the bubbles was physically pushed away by the rapid bubble growth during the inertial bubble growth stage instead of evaporating. A mechanistic model was developed to predict the conditions for which the heat fluxes do not increase during coalescence. In many other instances, the coalescence dynamics were such that the only bubble remaining on the surface was swept off by the liquid motion from the previous coalescence event with no nucleation of another bubble. This was followed by a long period without bubble nucleation followed by sudden nucleation and explosive coalescence of two bubbles giving rise to a large bubble which departed violently with a large increase in the heat transfer rate.

FLASHING LIQUID JETS IN LOW-PRESSURE ENVIRONMENT

In the present paper, highly expanded flashing liquid jets in a low-pressure environment are studied experimentally, particularly focusing upon the physics associated with liquid flashing regimes. A long, straight stainless steel capillary with an inner diameter of 0.23 mm and a length of 17.0 mm is used as the nozzle, which is connected with a syringe. Through a solenoid valve, a test vessel is connected with a vacuum chamber with a volume about 800 times bigger than that of the test vessel, in order to keep constant pressure inside the test vessel throughout every experimental run. Distilled water of about 1 mL is filled into the syringe at first, while the syringe is open to the ambient. Then, opening the solenoid valve, the air inside the test vessel will be evacuated quickly, resulting in a quick depressurization and a low backpressure inside the test vessel. The water in the syringe is then driven by the difference between the ambient pressure and the backpressure to form a highly expanded flashing liquid jet into the test vessel. For the case of low initial temperature and high backpressure, there is no evaporation, and then the flow of the liquid jet from the nozzle exit section remains intact and follows a straight path. On the other hand, if the initial temperature is high and/or the backpressure is low enough to lead a superheated exit condition, evaporation will take place, irregular evaporation waves around the liquid core are visible, and the jet shattering occurs. On further decreasing the backpressure, the liquid jet shatters giving rise to a cloud of droplets with a spray angle usually bigger than 90°, indicating a large number of nucleation sites and rapid bubble growth. It is also shown that there is flow choking behavior as the flow rate becomes constant and is insensitive to pressure reduction below some backpressure threshold.