Bubble Nucleation Research Articles

Molecular dynamics simulation of interfacial heat transfer characteristics of CO2, R32 and CO2/R32 binary zeotropic mixture on a smooth substrate

Mixture fluids have the advantage of a flexible adjustment of the physicochemical properties by actively tuning the composition and concentrations, showing promising potential in thermal and power-related applications. However, mixtures are mainly zeotropic with heat transfer deterioration during the boiling process, resulting in a challenge to gain a comprehensive understanding of their microscopic phase change. In this study, molecular dynamics simulations were employed to investigate the nanoscale behavior of CO2, R32 and CO2/R32 mixtures. The equilibrium molecular dynamics (MD) method and the non-equilibrium MD method was utilized to study the vapor-liquid interface characteristics and the boiling heat transfer characteristics of zeotropic mixture, respectively. The results revealed that the thickness of the vapor-liquid interface was linearly related to the temperature, and the surface tension displayed significant fluctuations in both solid-liquid and vapor-liquid interfaces, which indicated a potential barrier for molecular transport. Compared to the pure components, the inception of nucleation boiling and vapor film formation of the mixture fluid was significantly delayed. The heating and evaporation rate, heat flux of mixture were lower, while the thermal resistance was significantly higher. Moreover, the time required for bubble nucleation and the formation of vapor film decreased notably with the enhanced wetting characteristics. The findings in this study provide a molecular-scale insight into zeotropic mixture boiling heat transfer and the possible wettability regulation to improve the heat transfer performance of zeotropic mixtures and promote their wider applications.

Flow boiling characteristics of ethanol in a microchannel with low mass flux

In this study, heat transfer characteristics and dynamic flow boiling instabilities of ethanol were investigated experimentally in a rectangular microchannel with 1000 μm wide, 200 μm deep, and 30 mm long. The local temperature, pressure, and mass flux were recorded at a sampling frequency of 1 kHz in synchronism with high-speed visualization of up to 2000 frames/s enabling transient characterization of the flow boiling behavior. Mass flux and heat flux ranged from 30 to 50 kg m−2 s−1 and 10 to 120 kW m−2, respectively, with the inlet subcooling ∆Tsub = 25 °C. It is concluded that the inhibition of bubble nucleation which is caused by enlarged bubble in the confined space result in the deterioration of heat transfer performance. Flow boiling at higher mass flux present superior heat transfer because of much more effective bubble nucleation. The triggering of micro-channel flow boiling instability show strong dependency on quasi-periodical flow phenomenon with multiphase alternation other than the confinement bubble growth theory. Different frequency and amplitude types of fluctuations co-exist at lower mass flux. The thermo-physical properties of the working liquid significantly affect the fluctuation amplitude.

What determines the rest frame of bubble nucleation?

We revisit the question addressed in recent papers by Garriga et al.: what determines the rest frame of pair nucleation in a constant electric field? The conclusion reached in these papers is that pairs are observed to nucleate at rest in the rest frame of the detector which is used to detect the pairs. A similar conclusion should apply to bubble nucleation in a false vacuum. This conclusion however is subject to doubt due to the unphysical nature of the model of a constant eternal electric field that was used by Garriga et al. The number density of pairs in such a field would be infinite at any finite time. Here we address the same question in a more realistic model where the electric field is turned on at a finite time t 0 in the past. The process of turning on the field breaks the Lorentz invariance of the model and could in principle influence the frame of pair nucleation. We find however that the conclusion of Garriga et al. still holds in the limit t 0 → -∞. This shows that the setup process of the electric field does not have a lasting effect on the observed rest frame of pair nucleation. On the other hand, the electric current and charge density due to the pairs are determined by the way in which the electric field was turned on.

Investigation of nucleate boiling and bubble dynamics at multiple nucleation sites in subcooled boiling flow

Accurate prediction of nucleate boiling heat transfer and phase change is crucial for subcooled boiling flow modeling. Among the nucleate boiling parameters, departure diameter, departure frequency, and active nucleation site density are considered the most important. To fulfill the modeling requirements, the present study established a comprehensive experimental database with simultaneous measurement of departure diameter, departure frequency, and active nucleation site density at multiple nucleation sites. The experimental data were collected in a vertical annular test section using a microscopic high-speed imaging system. Twenty-two experiments were performed at a system pressure of 150 kPa, liquid flow rate of 0.5 m/s, local subcooling ranging from 10 to 30 °C, and heat flux varying from 250 to 700 kW/m2. Stochastic characteristics of departure diameter and departure frequency at single and multiple nucleation sites were investigated. A correction approach was developed to minimize the impact of upstream bubbles on departure frequency measurement. The sample size of single-site measurement was determined based on the analysis of data dispersions. Large variations between nucleation sites were observed, indicating that single-site measurement is insufficient to characterize all nucleation bubbles. A parametric analysis of nucleate boiling parameters over operating conditions was performed based on the multi-site data. The experimental database generated in this study will be utilized for model evaluation and development in future research.

Categorization of iso-pentane flashing spray based on morphology, thermodynamical and mechanical effects

Accidental release of superheated liquids is typically accompanied with hazardous flashing jets. Indeed, flashing jets undergo fragmentation into massive fine droplets, thus forming ideal circumstances for fires and explosion. To gain a deeper understanding of depressurized release, we studied the dynamics of iso-pentane at superheats of 40–80 °C and injection pressures of 0.6–3.0 MPa. The internal flow pattern and external spray morphology are visualized at quasi-steady stage using a high-speed camera. Results show that the flashing patterns can be classified into non-flashing, transitional flashing, and flare flashing arising from the interaction between thermodynamic and mechanical effects. The contribution of thermodynamic effect decreases as mechanical effect intensifies, due to the suppression of bubble nucleation and burst. We propose the index k, based on JaRp = k(WevOh)−1/7, to identify different flashing regions, i.e., k below 211 for non-flashing, k between 212 and 425 for transitional flashing, and k above 425 for flare flashing. Besides, the microscopic characteristics involving droplet velocity and diameter distribution are investigated and a new correlation for drop size is proposed, which is dependent on factors such as nozzle aspect ratio, mechanical force, and thermodynamic force. These findings should help to define policies to decrease the risk associate with flashing jets.

Modulating Force of Nucleated Hydrogen Bubble Adhesion to Boost Electrochemical Water Splitting

AbstractIn electrochemical hydrogen evolution reaction (HER), the produced hydrogen gas bubbles often adhered to the electrode surface and blocked the active catalytic site. While different catalysts are developed to improve the catalytic performance of HER, the design of a durable and universal approach for minimizing the force of nucleated hydrogen gas‐bubble adhesion to prevent blockage of electrocatalytic sites because of bubbles‐adhesion is unprecedented. Generally, buoyancy should outweigh the capillary force to remove nucleated bubbles, which means these forces ratio, Eötvös number Eo > 1. Herein, a chemically reactive multilayer coating on an electrode is reported to chemically modulate the adhesion force of nucleated gas‐bubble on the electrode. A dual modified coating on Ni‐foam provided a non‐adhesive superaerophobicity with nucleated bubble adhesion force of 4.6 ± 0.3 µN and displayed superior HER performance with lower overpotential (333 to 250 mV) at 100 mA cm−2 with respect to bare Ni‐foam. The chemically‐modulated low bubble‐adhesion facilitates the early removal of nucleated tiny hydrogen gas bubbles with a minimum size of 0.64 mm and Eo = 0.05 to keep catalytic sites available for superior electrochemical HER. Such a positive impact of the prepared coating is also noted for various other electrodes.

Investigating the effect of the fluid properties on bubble dynamics and heat transfer in a tapered microgap with multiphase flow modeling

Nucleation and bubble dynamics on a heater surface contribute to high heat transfer rate in pool boiling. Introducing two-phase flow in narrow channels further improves heat transfer. Use of expanding taper microgap geometry further enhances heat transfer, and proper balancing of taper angles and flow lengths leads to self-sustained flow boiling in tapered microgap geometries. This paper focuses on understanding the underlying enhancement mechanism by studying the bubble behavior as they expand and accelerate in the direction of increased taper. The present study conducts a 2D simulation analysis of bubble growth in tapered microgaps with numerical simulations to identify the effect of the fluid properties and tapered angle in the bubble and fluid dynamics behavior. Ansys-Fluent is customized with user-defined-functions (UDFs) accounting for the interfacial heat and mass transport, including a sharp interface and direct calculation of mass transfer with temperature gradients. The study was conducted using air injection and boiling simulation from the conception to the departure of a bubble. The tapered angles were 5°, 10°, and 15°, with flowrates between 3 ml/min to 30 ml/min, 1 mm air inlet, and at 1 mm distance from the convergent end. The departure time of 10 subsequent bubbles was recorded to check the configuration with the quickest bubble removal. A critical flowrate and surface tension region was established for the escape direction of the bubble. In addition, the numerical simulation considered the tapered microgap with a nucleating bubble at atmospheric conditions with a wall superheats of 5 K. The results show that the bubble growing over the heated surface creates fluid circulations and interfacial conditions that suppress the thermal boundary layer leading to an increased local heat transfer coefficient within a range of 1 mm from the interface.

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.