Bubble Nucleation Research Articles

Thermal-hydraulic characterization of manifold microchannel heat sink with diverging channels and uniform heating

This study presents the design and experimentation of a manifold microchannel heat sink with diverging channels, employing ammonia as the working fluid to evaluate its thermal–hydraulic performance under uniform heating conditions. The impact of heat flux, inlet subcooling, saturation temperature, and outlet thermodynamic quality were investigated. Under single-phase conditions, the heat sink exhibited low sensitivity to variations in heat flux with constant thermal resistance and pressure drop. However, with the onset of boiling, the heat transfer capability and pressure drop rapidly increased. A lower inlet subcooling boosted boiling inside the heat sink, leading to decreased thermal resistance and increased pressure drop. Under near-saturated inlet conditions, a higher saturation temperature enhanced heat transfer owing to smaller bubble nucleation and detachment size. The outlet thermodynamic quality for engineering applications should be moderately low because it provides a balance between high heat transfer capability and low pressure drop. Moreover, the results demonstrated excellent thermal–hydraulic performance, with a heat flux of up to 636.9 W/cm2 efficiently dissipated under uniform heating conditions while maintaining the heating surface temperature and pressure drop below 79.3 °C and 150.3 kPa, respectively.

Nucleation of surface nanobubbles in electrochemistry: Analysis with nucleation theorem

The formation of single bubbles at nanoelectrodes during electrochemical reactions allows to accurately identify the critical nucleus for bubble formation. As demonstrated before, combining nanoelectrode experiments and an analysis approach based on classical nucleation theory (CNT) delivers useful insight into bubble nucleation. In this work we propose an alternative approach to analyze the critical nuclei by applying the nucleation theorem (NT), which is able to overcome the inherent shortcomings of CNT. The size of the critical nucleus can be calculated more accurately by fitting experimental data in a simple form of the NT. Simulating the local gas concentration using a finite element approach, and considering the effect of gas oversaturation on the interfacial tension and the real gas compressibility, we obtain a more realistic estimation of the critical nuclei morphology. With the NT-based analysis presented, we re-analyze the nucleation data reported before. The properties of the critical nuclei obtained here are roughly consistent with those obtained from the CNT-based approach. In addition, we confirm that the critical nucleus for bubble formation in high gas oversaturation is featured with a contact angle much larger than Young’s contact angle.

Imaging Single H2 Nanobubbles Using Off-Axis Dark-Field Microscopy.

A robust and detailed physicochemical description of electrochemically generated surface nanobubbles and their effects on electrochemical systems remains at large. Herein, we report the development and utilization of an off-axis, dark-field microscopy imaging tool for probing the dynamic process of generating single H2 nanobubbles at the surface of a carbon nanoelectrode. A change in the direction of the incident light is made to significantly reduce the intensity of the background light, which enables us to image both the nanoelectrode and nanobubble on the electrode surface or the metal nanoparticles in the vicinity of the electrode. The correlated electrochemical and optical response provides novel insights regarding bubble nucleation and dissolution on a nanoelectrode previously unattainable solely from its current-voltage response.

Characterization of homogeneous nucleation of superheated liquid droplets by nonlinear analysis

The homogeneous bubble nucleation in superheated liquid droplets can be initiated by energetic particles or radiation. The nonlinearity in the heat transfer introduces chaos in the temperature time series of a boiling system. In the present study, the acoustical pulses from the homogeneous nucleation of superheated liquid droplets by neutrons and gamma-rays have been collected. Nonlinear time series analysis methods like 3D attractor reconstruction and Fast Fourier transformation (FFT) have been performed on the collected pulses. The analysis predicts the possibility of the existence of chaos, and the positive value of the largest Lyapunov exponent confirms it. The nonlinearity in the energy deposition by both the recoil nuclei and electrons from the neutrons and gamma-rays respectively, introduce chaos. A qualitative difference has been observed in the 3D attractor and Rescaled Range (R/S) plots between the neutron and gamma-ray-induced pulses, and the present differences are useful for the identification of these pulses.

Molecular dynamics study of bubble nucleation on trigonometric nanostructured surfaces

Nanostructures have a crucial impact on bubble nucleation. In this paper, different nano-grooved surfaces are constructed using complex trigonometric functions, and the nucleation process of liquid argon on the surfaces is investigated using molecular dynamics simulations. The nanostructures affect the bubble nucleation location, and the results show that the bubble nucleation always occurs in the groove region during the boiling process. In contrast, the bubbles on the flat surface appear randomly. In addition, bubble growth is also affected by the nanostructure. Increasing nanostructure size can reduce the bubble nucleation time and enhance the bubble growth rate. The nanostructure increases the solid-liquid interface area, which enhances the heat transfer between solid and liquid. Finally, the reasons for bubble nucleation on different nanostructured surfaces are analyzed. The heat transfer between solid and liquid is the leading factor for bubble nucleation when the difference in liquid atoms’ potential energy is slight. As the nanostructure size increases, the substrate attracts more argon atoms to form a non-evaporating liquid layer, improving heat transfer and promoting bubble nucleation.

Effect of friction stir welding on microstructure and foaming mechanism of rolled-state anisotropic powder

Friction stir welding (FSW) has a significant impact on welding aluminum foam sandwich (AFS) precursor. Yet, the understanding of the microstructure and foaming process on rolled-state powder of welded precursor remains limited. This study exploited the single-pass double-sided forming welding approach to weld the AFS precursor, while the impact of different process parameters of FSW on the microstructure of rolled-state powder was evaluated. Meanwhile, the foaming process of anisotropic powder at different directions was observed in-situ using synchrotron radiation. The results display that FSW transforms gap-like voids, which were ∼20 µm in width and 100–400 µm in length, into circular voids with a diameter of ∼6 µm in the powder at the weld zone (WZ). The welding process eliminates powder agglomeration aggregation and increases the relative density (>97%) of rolled-state powder. However, the low thermal conductivity of the rolled-state powder (117 W·m−1·K−1) leads to a tunnel defect at the root of the WZ. In addition, in-situ observation experiments of the foaming process found that the bubble nucleation of rolled-state powder exhibits anisotropic characteristics, and the welding process causes a transition in the types of bubble nucleation in the WZ region. This study demonstrates the feasibility of improving the anisotropy of powders caused by processing methods to obtain an isotropic pore formation and foam expansion by FSW technology.

Effects of continuous wettability on the pool-boiling bubble dynamics and heat transfer characteristics of a triangular structure-roughened surface

A phase transition lattice Boltzmann model is used in the current study to simulate the pool boiling process on a triangular structure-roughened surface with conjugate heat transfer effect. Through the verification of the Laplace law and the theoretical film boiling results, the adopted model is ensured to predict pool boiling phenomena accurately with excellent stability. The bubble dynamics, including the nucleation, growth, coalescence and detachment of the bubbles on the triangular structure-roughened surface with continuous wettability, is discussed. Little research has been conducted on the continuous wettability effect on the pool boiling with triangular structures-roughened surfaces. The results show that a hydrophilic surface inhibits bubble growth but facilities bubble detachment with bubble nucleation at the concave corners; a hydrophobic surface increases bubble growth but inhibits bubble detachment with bubble nucleation at hydrophobic areas. It is found that when the direction of the self-drive favors bubble detachment from the surface, the surface which from contact angle θBottom=120° to θTop=90° under the study can achieve better heat transfer over a wider range of superheat, while setting the continuous wettability from θBottom=120° to θTop=60° can achieve a better heat flux density at lower superheat.

Effects of grain boundaries on the evolution of radiation-induced bubbles in polycrystalline tungsten: A phase-field simulation

Tungsten (W) is widely recognized as a promising material for plasma-facing components in nuclear fusion reactors, and the evolution of bubbles in polycrystalline tungsten significantly impacts its properties. This study developed a phase-field model considering elastic inhomogeneity to investigate the evolution of radiation-induced bubbles. The Gibbs free energy of helium bubbles was derived using the Van der Waals equation. An explicit nucleation algorithm accounting for the position of grain boundaries (GBs) was coupled to the model. Furthermore, the model also considered the inequality between the internal pressure of the bubble and the surface tension. The growth kinetics of a single bubble with elastic heterogeneity were examined, and based on these findings, the evolution of bubbles under irradiation was simulated, yielding results consistent with experimental observations. Furthermore, the influence of GBs on radiation-induced bubbles was investigated using the developed model. The simulation results demonstrated that both GBs, as preferential nucleation regions and as sinks for vacancy absorption, had an impact on bubble nucleation and growth. The superior radiation resistance observed in nanocrystalline tungsten was dominantly attributed to the absorption of vacancies by GBs.

Insight into pH-controlled bubble dynamics on a Pt electrode during electrochemical water splitting

Bubbles adhering to the electrode surface are the major factors causing the decreased efficiency of water electrolysis. In this work, the dynamic behaviors of oxygen bubbles on vertical platinum sheet electrodes were investigated by a high-speed camera and their corresponding current densities were measured simultaneously using an electrochemical workstation. The impact of wide range of electrolyte pH values (i.e., 1–13) on both the bubble nucleation number and the dynamics behaviors of bubbles was investigated under different applied voltages. When the applied voltage was increased from 1.6 to 1.7 V vs the saturated calomel electrode, the radius of bubbles upon detachment increased in acidic environments (pH = 1–7) and decreased in alkaline environments (pH = 7–13). In an alkaline environment, the Marangoni force plays an essential role here. Furthermore, the bubble detachment radius predicted by the force balance model is well matched with our experimental results. Our results demonstrate that bubble detachment is not favorable in a strongly acidic environment, whereas oxygen bubbles exhibit rapid detachment from the electrode surface in a strongly alkaline environment.

Towards Controlled Transfer of (001) β-Ga2O3 to (0001) 4H-SiC Substrates

We demonstrate successful surface blistering of He-implanted (001) β-Ga2O3 substrates, bonding to (0001) 4H-SiC, and initial results towards large-area transfer of (001) β-Ga2O3 to 4H-SiC. Surface blistering of unbonded, implanted substrates is an important indication of successful exfoliation and transfer of films, which is achieved by initiating He bubble nucleation during a low temperature anneal followed by bubble growth at a high temperature anneal. Prior to annealing, implanted substrates were bonded to (0001) 4H-SiC at room temperature using a thin ~5 nm Ti interlayer. However, the β-Ga2O3 substrate did not wafer split from the bonded structure after annealing. Instead, small area transfers up to ~200 μm were achieved (~7% of the total bonded area transferred while the entire structure remained bonded). Further optimization of implant parameters is underway. These are promising results towards achieving large wafer-scale (001) β-Ga2O3 composite wafers suitable for β-Ga2O3 devices with efficient thermal management characteristics.

Controlled inertial nano-cavitation above 100 MHz

The maximum size and lifetime of an acoustically nucleated cavitation bubble is inversely proportional to the driving frequency and has achieved a limit of approximately 10 MHz. Smaller cavitation bubbles that are critical to microscopic applications require shorter lifetimes that correspond to higher oscillation frequencies. Here, we demonstrate that acoustic cavitation in the 100 MHz range and beyond can be achieved through wave propagation in a solid rather than in a liquid. The cavitation bubble is nucleated at a nano-sized fracture on a glass substrate, and its expansion is driven by a leaky Rayleigh wave, while the inertial collapse is induced by a trailing shock wave. As both waves travel at different velocities, the time interval between these two events is a function of the distance to the source. In this way, we demonstrate experimentally control of the lifetime of the bubbles in a range between 6 and 80 ns, corresponding to oscillation frequencies between 13 and 166 MHz. Our results agree with finite-volume fluid–structure interaction simulations.

Pool-boiling enhancement on periodic micro/nano ripple-structured surfaces fabricated by femtosecond laser

A study was conducted on whether periodic micro/nano ripple structures generated on metal surfaces using femtosecond laser processing could improve pool boiling heat transfer. Depending on the laser irradiation conditions on the metal surface, a surface structure with different geometrical characteristics is created. Changes in sample surface morphology caused by the laser irradiation intensity are examined using scanning electron microscopy and confocal laser microscopy. Pool boiling tests are conducted to compare three laser irradiation conditions fabricated samples and the bare copper surface. The working fluid is deionized water. The pool boiling curves of each sample surface were compared with each other. Visualization of bubble nucleation is performed using a high-speed camera. The superheat, bubble departure diameter, and bubble growth period of each sample are measured and compared at the onset of nucleate boiling (ONB). The most efficient periodic micro/nano ripple-structured surface reduces the superheat at the ONB by 6.13 °C in comparison with that at the bare Cu surface. The enhanced surface structure reduced the bubble departure diameter and renewal period by 60.5% and 55.6% in comparison with that of the smooth surface. Further, the structured surface showed 178.5% enhancement in heat transfer coefficient and 39.1% in critical heat flux, respectively. Thus, the combination of increased nucleate site density, high nucleation site activation, and a wicking effect in the periodic micro/nano ripple structure significantly improves the pool boiling heat transfer.

Effect of H and Pd atoms on the migration of He atoms in 3C–SiC

3C–SiC is an important structural material for tri-structural isotropic coated fuel particles in high-temperature gas reactors. It serves as a vital pressure vessel for gas fission products and a shielding layer for metal fission products. However, the accumulation of interstitial He atoms can lead to the formation of He bubbles, which threaten the safe service of materials. A study of how interstitial atoms affect the migration of He atoms can help us understand the nucleation of He bubbles in SiC cladding materials. The effects of H and Pd interstitials on the solution energy and migration behavior of He atoms were investigated. It was found that H favored He atoms to approach it, while Pd has the opposite effect, causing He atoms to move away from it. Under the synergistic effect of H and Pd atoms, He atoms tend to move away from the Pd interstitial, similar to the effect of the Pd interstitial alone. Furthermore, the external H interstitial strengthens the effect of Pd atoms on He atoms, drastically reducing the migration energy of He atoms away from Pd.

Experimental investigation on the evolution of bubble behavior in subcooled flow boiling in narrow rectangular channel based on bubble tracking algorithm

The nucleate subcooled boiling is an efficient heat transfer form and plays an important role in many cooling applications. The size and distribution of bubbles in subcooled boiling have considerable influence on boiling heat transfer. In this paper, subcooled flow boiling experiment is carried out to investigate the nucleation point density and detachment frequency of bubbles under different system pressure, and the test section is full transparent. Since the whole body of the test section is composed of transparent materials, it can be observed from different directions to obtain high quality images. A bubble tracking algorithm has been developed, which can effectively determine the diameter and position of detached bubbles, thereby inferring the nucleation point density and detachment frequency of bubbles. Besides, the distinctive properties of bubble nucleation within narrow rectangular channel are verified by comparing bubble detachment diameters with existed models. Finally, models for bubble nucleation point and detachment frequency under different operating conditions were proposed and verified through experimental results.

Novel classification of multistage flash desalination via finite and infinite flashing: Investigating thermofluid dynamics under varying inlet flow rates employing validated multiphase CFD simulations

This research presents a comprehensive investigation into the flashing process within the evaporation zone of a multistage flashing (MSF) desalination chamber. It introduces a novel classification of the flashing process as ideal, finite, and infinite, aiming to advance the understanding of this critical phenomenon. A validated volume-of-fluid (VOF) multiphase computational model is employed to accurately predict the shape of the free surface and elucidate the complex phase change phenomena occurring within the MSF chambers. By systematically varying the inlet brine velocities and rigorously analysing the thermofluid behaviour in the flashing chamber under both finite and infinite flashing conditions, this study provides a comparative analysis of design factors, including flashing rate and maximum bubble nucleation. Additionally, non-equilibrium losses and flashing efficiency are examined as pivotal metrics for evaluating the flashing process. The findings reveal distinct behaviours of the evaporation zone for finite and infinite flashing processes. Intriguingly, alterations in the flow rate lead to simultaneous enhancements and detriments in thermal efficiency due to the intricate interplay of turbulence promotion, improved mixing, and reduced residence time. Furthermore, the study highlights the significant impact of the inlet flow rate on phase change size, flashing rate, and bubble nucleation frequency within the bubble formation region. This investigation emphasizes the critical importance of comprehending the flashing phenomena and its consequential effect on the thermal efficiency of MSF systems. Leveraging advanced computational models, this study provides a solid foundation for identifying innovative methodologies to optimize the flashing process and enhance the overall performance of MSF systems. The ground-breaking insights presented in this research contribute significantly to the scientific understanding of the field and pave the way for future advancements in MSF desalination technology.

Experimental Analysis of Bubble Behavior and Critical Heat Flux During Pool Boiling on Vertical Circular Tubes

Abstract The heat transfer during pool boiling incorporates a higher rate of heat dissipation capability at low-temperature differences. This technique is widely used in the nuclear industry for thermal management. In this study, the effect of tube diameter and length on critical heat flux (CHat atmospheric conditions in saturated water during pool boiling) is analyzed. The tubes of SS 304 are kept in the vertical orientation. The diameter of the tubes ranges from 1.2 mm to 9 mm. The experiments are performed with tubes having lengths varying from 50 mm to 1000 mm. It has been noted that tubes of smaller diameter show a greater magnitude of critical heat flux (CHF) for the given length. Compared to other tubes, the magnitude of CHF for a 1.2 mm diameter is higher. For a given diameter, a longer tube is found to have lower CHF than the ones having lesser length. The variation in the CHF magnitude is negligible for tubes with a diameter of more than 2.5 mm beyond a length of 200 mm. The location of occurrence of CHF is near the bottom end of the vertical tube. The study illustrates the behavior of bubble nucleation for various tube dimensions and heat fluxes. The inception and detachment of bubbles for different tubes are analyzed. The pool boiling regime is categorized and studied basing the behavior of the incepted and departed bubble while maintaining uniform heat flux. A mathematical relation that empirically accounts for the effect of tube dimensions , i.e., length and diameter on pool boiling CHF is proposed. The experimental CHF data obtained during pool boiling are tabulated toward contributing to the CHF databank.

Influence of step-down pressure on wettability-controlled heterogeneous bubble nucleation of sparingly soluble gases in water

The liberation of dissolved gas from a liquid typically occurs either by diffusion through the gas–liquid interface and/or by bubble nucleation at the solid–liquid interface. The goal of this research was to study the influences of step-down pressure (degree of supersaturation) and wettability on the onset pressure for bubble nucleation of sparingly soluble gases, methane and nitrogen, in water. Bubble nucleation experiments were carried out with the gas-supersaturated liquids in the hydrophilic (untreated) and hydrophobic (treated) glass vials. The gas-saturated liquids were prepared by saturating water with methane or nitrogen in a glass vial placed in a pressure cell at 6,000 mbar for a minimum of 16 or 15 h, respectively. To initiate the pressure-driven bubble nucleation process, 100-, 500-, or 6,000-mbar step-down pressures were applied to the pressure cell until the bubble nucleation was initiated. Bubble nucleation did not occur in the hydrophilic vial with any of the gas-supersaturated waters, irrespective of the step-down pressures. Unexpectedly, 100-mbar step-down pressure did not cause neither methane nor nitrogen bubble nucleation in the hydrophobic vials. Whereas, both methane and nitrogen bubble nucleations occurred in the hydrophobic vials when the step-down pressure was increased to 500 mbar. The above experimental findings revealed the importance of step-down pressure and surface wetting nature on sparingly soluble gas bubble nucleation. To further understand the experimental findings, an analytical method was developed to estimate the effective supersaturations using Fick’s 2nd law of diffusion and Henry’ law, which indicated that at any given pressure the 500-mbar step-down pressure creates higher supersaturation compared to 100-mbar step-down pressure.

Primordial black holes from strong first-order phase transitions

We study the formation of primordial black holes (PBHs) in strongly super-cooled first-order phase transitions. The mechanism is based on the presence of remnants dominated by the false vacuum that scale slower with the expansion of the Universe than their surroundings where this energy was already converted into radiation. We compute the PBH formation from these remnants including the contribution from the false vacuum and the bubble walls, by estimating the collapse using the hoop conjecture and by considering both regions collapsing immediately when entering the horizon and sub-horizon regions that collapse as their compactness grows. We show that for exponential bubble nucleation rate, Γ ∝ eβt, the primordial black hole formation implies β/H ≳ 3.8, where H denotes the Hubble rate, if the potential energy of the false vacuum is ∆V ≲ (1012 GeV)4, as otherwise a too large abundance of long-lived PBHs forms. The observed dark matter abundance can be formed in asteroid mass PBHs if β/H ≃ 3.8 and 105 GeV ≲ ∆V1/4 ≲ 108 GeV. Finally, we consider also the effect of the second order correction to the exponential nucleation rate showing that the PBH abundance is mainly determined by the average radius of the true vacuum bubbles.

The microstructure and He+ ion irradiation behavior of novel low-activation W-Ta-Cr-V refractory high entropy alloy for nuclear applications

Microstructure and nanohardness of a nearly equimolar W-Ta-Cr-V high entropy alloy (HEA), as well as its irradiation response under He+ irradiation, were investigated. The single-phase body-centered cubic nanostructured alloy with a 1 µm thick layer was fabricated on a silicon substrate using a magnetron sputtering method. The HEA film has a complex microstructure consisting of micrometric domains that exhibit internal nanostructure controlled by their crystal orientation. The measured nanohardness of the W-Ta-Cr-V alloy is 13 ± 2 GPa, which significantly exceeds the hardness of nanocrystalline tungsten as a result of the high solid-solution strengthening effect. In order to evaluate the irradiation resistance of the HEA film, the material was irradiated with 200 keV He+ ions at room temperature, with two different ion fluences: 1 × 1016 and 5 × 1016 ions/cm2. Using transmission electron microscopy, a high density of extremely fine He bubbles is observed that were uniformly distributed in the matrix. The increase of He+ ion fluence increased the density of bubbles, whereas their size remained at a similar level, which indicates that the damage proceeds by the nucleation of additional He bubbles, not by their growth.

Heat transfer incipience of capillary-driven liquid film boiling

High-power electronic devices bring both unprecedented challenges and great opportunities to two-phase cooling technologies for efficient thermal management. Capillary-driven liquid film boiling with wicking structures combines the advantages of capillary evaporation and nucleate boiling, and thus could be one of the most promising cooling methods. However, a comprehensive understanding of liquid film boiling mechanism is lacking. For example, the current prediction of liquid film boiling incipience is based on the bubble nucleation model utilized in pool boiling, which assumes that bubbles nucleate inside the thermal boundary layer with linear temperature distribution due to heat conduction. In liquid film boiling, heat transfer processes are quite different, including both heat conduction inside the liquid film and evaporation at the liquid-vapor interface. In this work, based on the thermal resistance network considering evaporation at thin-film region near the tri-phase line, we develop a model to predict liquid film boiling incipience. Both visualization and heat transfer experiments are conducted using copper micromesh to validate this model. The model predictions of critical liquid film thickness, surface superheat, and heat flux at the onset of nucleate boiling (ONB) are compared with the experimental data on various wicking structures in the literature, including silicon micropillar, copper micropowder, and copper micromesh. The results show that the heat flux and superheat reduce as the wick thermal conductivity decreases, the wick thickness increases, and the wick porosity increases.