Bubble Evolution Research Articles

Phase-field study of the effect of stress field and fission rate on intragranular Xe bubble evolution in U3Si2 nuclear fuel

Due to the superior thermal conductivity and high uranium density, U3Si2 is an excellent candidate for conventional UO2 nuclear fuel and shows great potential application in accident-tolerant fuel (ATF) assembly of light water reactors (LWRs). Currently, the behavior of Xe bubbles with internal or applied stress is rarely investigated, restricting further understanding of swelling in U3Si2. The mesoscopic phase-field method has been developed in this work to study the spatial and temporal Xe bubble evolution in U3Si2. The results show that the bubble density and its average size increase as the fission rate increases. Applied stress accelerates the nucleation and growth of gas bubbles, reshaping the bubbles’ morphology from spherical in a stress-free state into elongated along the applied direction in a stressed state. The gas bubbles in a local dislocation stress field nucleate preferentially at stress-concentrated sites and spread over the whole system in succession, and the bubble coarsening is controlled by the stress overlap of the dislocation pair. The results show a practical phase-field method for Xe bubble evolution study in U3Si2, which can be expanded into swelling behavior investigation in other fuels and lay a solid foundation for the development of ATF assembly.

Elements of beaked whale anatomy and diving physiology and some hypothetical causes of sonar-related stranding

A number of mass strandings of beaked whales have in recent decades been temporally and spatially coincident with military activities involving the use of midrange sonar. The social behaviour of beaked whales is poorly known, it can be inferred from strandings and some evidence of at-sea sightings. It is believed that some beaked whale species have social organisation at some scale; however most strandings are of individuals, suggesting that they spend at least some part of their life alone. Thus, the occurrence of unusual mass strandings of beaked whales is of particular importance. In contrast to some earlier reports, the most deleterious effect that sonar may have on beaked whales may not be trauma to the auditory system as a direct result of ensonification. Evidence now suggests that the most serious effect is the evolution of gas bubbles in tissues, driven by behaviourally altered dive profiles (e.g. extended surface intervals) or directly from ensonification. It has been predicted that the tissues of beaked whales are supersaturated with nitrogen gas on ascent due to the characteristics of their deep-diving behaviour. The lesions observed in beaked whales that mass stranded in the Canary Islands in 2002 are consistent with, but not diagnostic of, decompression sickness. These lesions included gas and fat emboli and diffuse multiorgan haemorrhage. This review describes what is known about beaked whale anatomy and physiology and discusses mechanisms that may have led to beaked whale mass strandings that were induced by anthropogenic sonar. Beaked whale morphology is illustrated using Cuvier’s beaked whale as the subject of the review. As so little is known about the anatomy and physiology of beaked whales, the morphologies of a relatively well-studied delphinid, the bottlenose dolphin and a well-studied terrestrial mammal, the domestic dog are heavily drawn on.

Effect of laser spot diameter on oxygen bubble behavior in photoelectrochemical water splitting

The detrimental effect caused by the long-term adhesion of bubbles on the gas production efficiency has been the main bottleneck limiting the efficiency improvement of photoelectrochemical water splitting. The evolution of oxygen bubbles on the fixed photoelectrode surface of TiO2 films was observed in-situ using a combination of a synchronized high-speed microscopic camera and an electrochemical workstation. The effect of oxygen bubble evolution on the current during the photoelectrochemical reaction was investigated under different laser spot diameters. When the spot diameter increased 1.68 times from 700 μm to 1176 μm with a laser power of 5 mW, the bubble detachment diameter and growth period increased 2.7 times and 20.2 times, respectively, yet the gas production efficiency decreased by 34.7%. Meanwhile, the peak current caused by the bubble detachment gradually increased. However, the valley current at the nucleation waiting stage of the bubble did not have obvious changes. The model for the gas evolving based on supersaturation was adopted. It illustrated that the concentration gradient of dissolved gas perpendicular to the photoelectrode surface increased with the increase of spot diameter, leading to an increase in the bubble surface tension gradient, and causing the Marangoni force to inhibit the detachment of bubbles. A force balance model was developed to evaluate the Marangoni forces and the buoyancy forces. The growth rate of the Marangoni force was found to be proportional to the spot diameter. Therefore, reducing the laser spot diameter is a means to effectively remove the bubbles from the photoelectrode surface.

Acceleration Waves in Cylindrical Shrinking Gas Bubbles

The paper studies the case of shrinking cylindrical gas bubbles acting as a radial piston and generating acceleration waves. The behavior of such waves and their improbable transformation into shocks are illustrated theoretically, as well as through some examples inspired by experimental data. The use of rational extended thermodynamics enables us to highlight the relevance of the dissipation and the possible role played by dynamic pressure and stress tensor in bubble evolution or shock formation. These results constitute an extension and a completion of a previous work dedicated to the analysis of acceleration waves generated in oscillating spherical bubbles.

Influence of focus positions on underwater femtosecond laser dicing of silicon wafer

This paper investigates the effects of laser-induced bubbles on the surface morphology of silicon wafer at three focus positions: above the focus (positive-defocus), at the focus and below the focus (negative-defocus). Firstly, the behaviors of the bubbles are observed from both the side view and top view of the laser ablation process at the three focus positions. Next, time-resolved shadowgraph images were used to examine evolution of the cavitation bubbles. The experimental results show that laser positive-defocus machining was accompanied by many bubbles, which aggregated to form a large bubbles barrier on the silicon wafer surface and disturbed the subsequent laser beam, and resulted in poor surface morphology of the groove. Compared to the laser machining at focus, laser positive-defocus machining reduced the groove depth by 77.77 %, groove aspect ratio by 80.98 %, cavitation bubbles size by 52.08 %, and cavitation bubbles lifetime by 58.82 %. Laser negative-defocus machining produced few bubbles that moved in the opposite direction of laser scanning. The bubbles did not interfere with the subsequent laser pulses and showed little effect on the surface morphology of the machined groove. Compared to the laser machining at focus, the laser negative-defocus machining increased the groove depth by 88.61 %, groove aspect ratio by 45.65 %, cavitation bubbles size by 65.64 %, and cavitation bubbles lifetime by 80.08 %. The results show that the negative-defocus machining method has application prospects in underwater laser machining.

Effect of Electrolyte pH on Oxygen Bubble Behavior in Photoelectrochemical Water Splitting

The issue of increased reaction resistance due to bubble growth has always been a major bottleneck limiting the efficiency improvement of photoelectrochemical water splitting. In this study, we developed a synchronized measurement system with a micro-high-speed camera and an electrochemical workstation to observe oxygen bubble evolution on the surface of a fixed TiO2 film electrode in situ. The intrinsic relationship between the nucleation and growth of oxygen bubbles and photocurrent at different pH values (1.0–13.0) was investigated. The results indicate that higher pH can promote bubble nucleation at lower potentials. Additionally, increasing pH from 1.0 to 13.0 at 0.1 V vs Ag/AgCl, the photocurrent in the bubble growth stage increases by about 35 times, and the average period of bubble growth decreases by about 15 times. Compared with pH = 9.0, the gas production rates of pH = 1.0 and pH = 13.0 are improved by 13 times and 22 times at 0.71 V vs RHE, respectively. Then, we developed a force balance model for oxygen bubbles at the anode surface, and the predicted bubble detachment diameters are in good agreement with the experimental results. The Marangoni force induced by the nonuniform distribution of dissolved oxygen was found to be increased with pH, which leads to the larger detachment diameter of bubbles. The results show that the strong alkali environment is an effective means to remove oxygen bubbles from the surface of the photoelectrode.

Experimental investigation of bubble dynamics and flow patterns during flow boiling in high aspect ratio microchannels with the effect of flow orientation

Multiphase flow and boiling phase-change within microchannels are of paramount importance to thermal management and electronics cooling, among others. The present experimental study investigated the bubble dynamics and flow patterns during flow boiling in a high aspect ratio microchannel at different channel orientations. Hydrofluoroether (HFE−7000) was used as the working fluid within a transparent glass microchannel with a width-to-height aspect ratio of 10 and a hydraulic diameter of 909 μm. A tantalum layer coating was applied to the channel, allowing visual observation to be conducted whilst imposing uniform heating along the channel. Under horizontal and vertical upward flow, the mass fluxes were varied between 14 kg m−2 s−1 and 42 kg m−2 s−1 for Reynolds numbers between 28.4 and 85.2, while the heat fluxes were set from 2.8 kW m−2 to 18.7 kW m−2. The results show that single bubble growth can be divided into three stages, namely: bubble-free growth, partially confined bubble growth, and fully confined growth. The effects of mass flux and heat flux on bubble behaviour and transition points between the defined stages are discussed. Equally important, the channel orientation influences the bubble behaviour in terms of bubble evolution, bubble shape, and bubble nose velocity, with no significant differences observed for the flow patterns when comparing horizontal and vertical upward cases. The comparison of the present work with previous flow pattern map results from the literature shows that earlier flow pattern transition models cannot accurately predict the observed flow patterns in the present configuration. Two types of instabilities are observed, which are associated with the dominant flow pattern during flow boiling and hence a function of the mass flux to heat flux ratio. The first type of instability, high amplitude and long period oscillation, is found during slug flow which is characterised by the presence of bubble rapid expansion along the channel. In addition, a second instability ensues when churn and annular flow create short period oscillations due to the high number of nucleation events, bubble coalescence, and the breakage of liquid slug or liquid film.

A comprehensive atomistic investigation on the cascade induced helium bubble motion in bcc iron for neutron irradiated RAFM steels

Cascade induced by the primary knock-on atom could significantly influence the evolution of helium bubbles in RAFM steels. In this work, the helium bubble migration and helium bubbles coalescence under cascade overlapping, i.e., when the helium bubble was overlapped by the thermal spike of one cascade, in bcc Fe was investigated by molecular dynamics simulation. The influence of He-to-vacancy (He/V) ratio as well as cascade energy, temperature, bubble size and bubbles distance were investigated based on the statistical results of 4700 cases. A method which could quantitatively estimate the overlapping extent between the cascade and helium bubbles was put forward to confirm that the cascade did overlap with the helium bubble in all cases. It was found that under cascade overlapping, the helium bubble tends to migrate toward the thermal-vacancy-rich center. The mechanisms of migration change from vacancy diffusion to He atoms migration with the increase of the He/V ratio, and the corresponding migration distance could decrease one order of magnitude. The cascade induced coalescence of two close helium bubbles is associated with the synergy effect of migration, dissociation and joint strain field of the two bubbles. Especially, it was found that the joint strain field of two close helium bubbles could significantly promote the bubbles’ coalescence by fostering the two bubbles’ directional migration towards each other under cascade overlapping. It was also found that the stable He/V ratio of helium bubble under cascade overlapping is around 0.5.

Multi‐Source Perturbations in the Evolution of a Low‐Latitudinal Equatorial Plasma Bubble Event Occurred Over China

AbstractIn this paper, multi‐source perturbations during the evolution of an equatorial plasma bubble (EPB) event at low latitudes in China are studied by means of multi‐ground‐based instruments, including an all‐sky airglow imager, a very high frequency (VHF) radar and eight digisondes. We found that EPB event initially evolved from bottom perturbations (∼600 km scale) seeded by atmospheric gravity waves in a form of large‐scale wave‐like structure, accompanying smaller‐scale perturbations (∼150 km scale) mostly by collision‐shear instability (CSI); once formed, those seed perturbations further evolved into the ionospheric topside by the plasma instability. Observed and analyzed are two different instabilities: one is the Rayleigh‐Taylor instability (RTI) driven by a prereversal enhancement of the zonal electric field (PRE) occurred near sunset; the other is an equatorward wind‐induced secondary E × B gradient drift instability (GDI) around midnight. Accompanying the PRE‐induced RTI are freshly‐generated depletions with larger poleward (upward) velocities. The PRE‐driven RTI could elevate the bottom perturbations directly to form fast‐moving depletions/structures at the ionospheric topside. The E × B GDI was trigged by a vertical upward plasma jet caused by a seasonal equatorward wind in regions as far as 10°N (20°N) from the geomagnetic (geographic) equator. This equatorward wind‐induced E × B GDI continuously forced topside structures of those drifting‐type EPB depletions to extend poleward more slowly, resulting in active 3.2‐m irregularities around midnight. Besides, we present evidence that a westward polarization electric field generated in an adjacent trough region of the faster‐growing cluster‐type depletions inhibited the neighboring slower‐growing cluster‐type depletions.

Numerical investigation of oxygen bubble condensation in subcooled quiescent liquid oxygen based on height function method

Bubble condensation is a fundamental issue in the direct contact condensation (DCC) process, which is extremely hard to predict due to the intense mass transfer and flow instability. Aiming at the cryogenic oxygen bubble evolution and pressure oscillation, a modified energy balance mass transfer model is adopted based on the height function method. The height function method is used to calculate the real-time interfacial curvature, modifying the mass transfer rate that emerged in CFD simulation. The modified model is validated to be convincing and accurate by a steam bubble condensation simulation. The results indicate that the cryogenic bubbles condense faster with a more intense mass transfer process, compared with the near-room-temperature mediums. The corresponding amplitude of pressure oscillation for the cryogenic mediums is approximately 3 ∼ 4 times as larger as that for near-room-temperature mediums. It is found that both the condensation rate and the frequency of pressure oscillation are repressed with the increase of bubble volume. That means that a big bubble condensation might produce a low-frequency pressure oscillation. In addition, both the condensation rate and the intensity of the pressure oscillation increase with the increase of subcooling. The average amplitude of pressure oscillation has a power function law with subcooling. These conclusions provide a theoretical basis for a deep understanding of the cryogenic DCC process.

Experimental evidence of the effect of solute concentration on the collective evolution of bubbles in a regular pore-network

The dissolution of bubbles confined in porous media is relevant to applications such as carbon sequestration and soil remediation. Recent numerical work indicates that a rich variety of collective dissolution behaviors can be obtained depending on the initial solute concentration, the size distribution of bubbles and the structure of the porous network. However, there is only sparse experimental evidence that supports these findings. Here, we present an experimental study that uses optical microscopy to track the dissolution of CO2 bubbles in a two-dimensional porous network etched on a microfluidic chip filled with CO2–saturated water. We consider two distinct level of initial liquid supersaturation for situations involving a single isolated bubble and small bubble clusters, and observe dissolution, growth or a combination of these processes. A pore-network model is used to complement the experimental observations with information on local concentration development. The model captures qualitatively the evolution of the bubble size in each case tested experimentally and enables shedding light on the interplay between the inter- and intra-pore diffusive fluxes in driving the dissolution process.

Helium diffusion and bubble evolution in single-phase tungsten-based W-Ta-Cr-V complex concentrated alloy

Tungsten-based complex concentrated alloy (CCA) W-Ta-Cr-V films with bcc single-phase have been investigated for its exceptional radiation tolerance. This work aims to investigate the formation of helium (He) bubble and He retention behavior in the CCA film under 100 eV He plasma at a fluence of 2.02 × 1025 He/m2. The results show that the size of He bubbles in the CCA film is smaller than that in the pure W film, and the depth distribution is shallower, which indicates that the evolution of He bubbles is inhibited. As for the underlying mechanism of resistance to the formation of He bubbles in CCAs, it is believed that the characteristic of low He mobility reduces the diffusion depth of He, and the sluggish diffusion of He inhibits the aggregation of He atoms or He clusters to a certain extent. In other words, the unique energy and microstructure characteristics also inhibit the diffusion of He in the CCA film, which affects the nucleation and growth of He bubbles. Furthermore, the release of He is mainly from He adsorption of the near surface, and the desorption peaks shift to lower temperatures in the CCA film within the desorption temperature of 1273 K.

Experimental study of gas–liquid two-phase operating characteristics and bubble evolution law of a semi-open mixed-flow pump

This work experimentally studied the influence of gas and liquid distribution on a semi-open mixed-flow pump performance. High speed photography technology was contributed to obtain the evolution law of bubbles in the impeller. The results show that the inlet gas volume fraction (IGVF) and liquid flow rate are the main factors affecting the performance of the pump. The pressurization capacity linearly decreases with the increase in IGVF when the liquid flow rate is close the best efficiency point. However, this factor suddenly drops when the IGVF reaches a certain critical value under the condition of a low liquid flow rate. The spatiotemporal evolution characteristics of the isolated bubbles and gas column in the segregated flow were investigated. In addition, the bubbles are evenly distributed, and the flow pattern is mainly bubbly flow (BF) or aggregated BF (ABF) in the impeller inlet area without blades. Under small liquid flow rate conditions, the bubbles tend to coalesce into larger-sized gas-pocket with an increase in IGVF. The tip leakage vortex increases the turbulence intensity of the flow field. The results show that the bubbles downstream of the tip leakage vortex in the impeller channels are more easily broken into smaller bubbles and form a BF or ABF.

PHANGS–JWST First Results: A Statistical View on Bubble Evolution in NGC 628

The first JWST observations of nearby galaxies have unveiled a rich population of bubbles that trace the stellar-feedback mechanisms responsible for their creation. Studying these bubbles therefore allows us to chart the interaction between stellar feedback and the interstellar medium, and the larger galactic flows needed to regulate star formation processes globally. We present the first catalog of bubbles in NGC 628, visually identified using Mid-Infrared Instrument F770W Physics at High Angular resolution in Nearby GalaxieS (PHANGS)–JWST observations, and use them to statistically evaluate bubble characteristics. We classify 1694 structures as bubbles with radii between 6 and 552 pc. Of these, 31% contain at least one smaller bubble at their edge, indicating that previous generations of star formation have a local impact on where new stars form. On large scales, most bubbles lie near a spiral arm, and their radii increase downstream compared to upstream. Furthermore, bubbles are elongated in a similar direction to the spiral-arm ridgeline. These azimuthal trends demonstrate that star formation is intimately connected to the spiral-arm passage. Finally, the bubble size distribution follows a power law of index p = −2.2 ± 0.1, which is slightly shallower than the theoretical value by 1–3.5σ that did not include bubble mergers. The fraction of bubbles identified within the shells of larger bubbles suggests that bubble merging is a common process. Our analysis therefore allows us to quantify the number of star-forming regions that are influenced by an earlier generation, and the role feedback processes have in setting the global star formation rate. With the full PHANGS–JWST sample, we can do this for more galaxies.

Observation of Scintillation Enhancements and Large‐Scale Structures Within the Equatorial Ionization Anomaly During a Sudden Stratospheric Warming Event

AbstractTotal electron content (TEC) and L‐band scintillations measured by several networks of GPS and GNSS receivers that operate in South and Central America and the Caribbean region are used to observe the morphology of the equatorial ionization anomaly (EIA), examine the evolution of plasma bubbles, and investigate the enhancement of L‐band scintillations that occurred on February 12 and 13, 2016. A few weak and short magnetic storms developed these days, and a minor sudden stratospheric warming (SSW) event was initiated a few days before. During these unusual conditions, TEC maps reported a split of the otherwise continuous crests of the EIA and the formation of a large‐scale (thousands of kilometers) almost‐circular structure. The western part of the southern crest faded, and a north‐south aligned segment developed near the center of the South American continent, joining the north and south crests of the EIA, forming an anomaly that resembled a closed loop on the eastern side of the continent. Concurrently with the anomaly events, several GPS stations reported increases in the L‐band scintillation index from 0.4 to values greater than 1. We analyzed TEC values from receivers between ±6° from the magnetic equator to identify and follow TEC depletions associated with plasma bubbles when they reach different stations. Although the magnetic activity was moderate (Kp = 3°), we believe that the anomaly redistribution and the scintillation enhancements are not related to a prompt penetration electric field but to enhancing the semidiurnal lunar tide propitiated by the onset of the minor SSW event. We found that depending on the lunar tide phase cycle, the neutral wind's meridional component can augment sub‐km scale irregularities and enhance L‐band scintillations through the wind gradient instability when U·n < 0 or the action of wind gradients (U) within the bubbles. Our observations imply that the SSW event enables prominent changes in the thermosphere wind system at F‐region altitudes.

Microbubble collapse near a fiber: Broken symmetry conditions and a planar jet formation

The collapse of microbubbles near a fiber is an example often encountered in water treatment situations and cavitation fibrillation processes. However, due to the broken symmetry conditions, this process has not been studied in detail experimentally or numerically, making it difficult to precisely measure or simulate the rapid bubble evolution during collapse. In this work, we present a novel experimental method, allowing for precisely repeatable cavitation events observation, combined with numerical simulations offering insight into pressure and velocity fields distribution developments in time. Both experimental and numerical works focused on small distances between the bubble and the fiber, where the physical interaction between subjects is the strongest. Four different bubble offsets were considered within the scope of this work, and very good agreement of numerical simulations with experiments was found in all cases. Two modes of bubble collapse were identified, leading to mushroom-shaped bubbles at positions closest to the fiber and a pear-shaped bubble at the farthest position. It is noteworthy that in all four cases, a planar jet formation toward the fiber was observed. The formed jet initially assumes an elongated shape, whereas its stability depends on the mode of bubble collapse. Numerical analysis of the planar jet as the defining feature of the collapse defined lower bounds for the actual values of peak jet velocities, ranging between 250 and 330 m/s, and the resulting impact pressures, which range from 100to 500 MPa.

A flow-through electrode for hydrogen production from water splitting by mitigating bubble induced overpotential

Bubble evolution induced overpotential for hydrogen production by water electrolysis is an important issue and the internal/external surface of the electrode can be covered by the generated bubbles. Herein, flow-through electrodes with in-situ restructured Co-based nanosheets loaded in Ni foam pores are presented. The adhesion between H2 bubbles and the superaerophobic catalyst layer is reduced from 14.2 μN to about zero and H2 bubbles size is reduced from 41.76 μm to 23.17 μm. Under the condition of flow-through mode, the catalysts can be adequately exposed because the generated bubbles can be timely removed from the catalyst surface. The overpotential can be reduced by about 100 mV for hydrogen evolution reaction (HER) and 57 mV for oxygen evolution reaction (OER) under the condition of 300 mA cm−2 and electrolyte flux of 339 m3 m−2 h−1. A novel electrolyzer assembled with flow-through electrode for hydrogen production by alkaline water electrolysis is conducted. The cell voltage can be decreased by 50 mV at 200 mA cm−2 at 6 M KOH, 338 K, and 1 atm, with the energy of 4.42 kW h Nm−3 required. The results provide insights into the design of various electrodes for electrolyzer setups that can be used for gas-involving reactions.

Helium-driven element depletion and phase transformation in irradiated Ti3SiC2 at high temperature

Future nuclear reactors and advanced power generators require materials with good stability and damage tolerance under harsh conditions, including high temperatures and high-dose radiation. Ti3SiC2 MAX phase has good physical properties and mechanical strength. It can remain crystalline under serious microstructure damage due to the nanolaminate structure. In this study, the effects of helium in irradiated Ti3SiC2 at up to 1100 °C were investigated by microstructural and chemical composition analysis. The concentrated helium can grow into large bubbles without significant confinement or capture by the nano-laminated layers. A new hexagonal to fcc phase transformation mechanism, driven mainly by the evolution of the helium bubbles accompanied by Si diffusion and depletion, is found and investigated. Si interstitials are forced to move out from the peak helium region by the helium evolution and segregate at the outermost surface, forming a thin Si-O layer, at 1100 °C. The formation of the fcc phase is the result of chemical compositional changes and local compressive stress contributed by He bubbles.

Deterioration of irradiation resistance of ODS-F/M steel under high concentration of helium

Simultaneous Fe+He ion irradiations of China Low Activation Martensitic (CLAM) steel and oxide-dispersion-strengthened ferritic/martensitic (ODS-F/M) steel were carried out at 350 °C-550 °C to the damage dose of 11 dpa with different He concentrations to investigate the effect of oxide particles on the evolution of He bubbles and dislocation loops. The microstructures of all irradiated samples were observed by transmission electron microscopy (TEM). The average size of oxide particles decreased and the density increased after irradiation. The aggregation of He bubbles was observed at the edge of oxide particles. As the He concentration increased, the aggregation of helium bubbles appeared at the edge of more oxide particles. Furthermore, He bubbles were not observed inside ODS-F/M steel grains or at grain boundaries, which suggests that the oxide particle interface is more capable of capturing helium atoms than other sinks. The average dislocation loop size of ODS-F/M steels is only 58% of that in CLAM steel at 450 °C, showing that ODS-F/M steel has excellent inhibition on the growth of dislocation loops. Nevertheless, with the increase of temperature and He concentration, the number density and size of dislocation loops in ODS-F/M steels gradually approach those in CLAM steels. The ability of ODS-F/M steels to inhibit irradiation hardening relative to CLAM steels increases with increasing temperature and decreases with increasing He concentration. The result of first-principles calculations shows that the formation energy of interstitial Fe atom at the interface increases with the increasing of the number of He atoms, which means that the aggregation of excessive He atoms at the interface will inhibit superior irradiation resistance of ODS-F/M steel.

Enhanced boiling heat transfer of HFE-7100 on copper foams under overflow conditions

Boiling of dielectric liquids on structured surfaces is a powerful option for thermal management of electronic devices, whose cooling performance depends strongly on liquid supply and vapor departure. Herein, saturation boiling of HFE-7100 on copper foams with a pore density of 30 pores per inch (PPI) under overflow conditions is proposed to enhance liquid supply and facilitate vapor departure. Liquid HFE-7100 is fed to the copper foam by a peristaltic pump and un-vaporized liquid is discharged by gravity. The higher flowrate leads to higher critical heat flux (CHF) and heat transfer coefficient (HTC) only if the copper foam is not flooded. Visual observations reveal evolution of vapor bubbles with heat flux on the copper foam under overflow conditions. Dependence of CHF and HTC on foam thickness is experimentally evaluated and an optimal thickness of 4.7 mm is derived from the experimental data. The optimal foam thickness is related to submergence level of bulk flow and capillary height. CHF and HTC are enhanced by the forced liquid supply in combination with the reduced hindrance of hydrostatic pressure to vapor departure, reaching 860 kW m−2 and 47.3 kW m−2 K−1, respectively. The overflow boiling offers an efficient way to improve heat transfer of boiling on porous medium for efficient thermal management.