Detachment Diameter Research Articles

Effects of ultrasound on bubble dynamic behavior of flow boiling in microchannel

Bubble dynamics is paramount in comprehending the heat transfer mechanisms of flow boiling in the microchannel within ultrasonic field, which is regarded as a promising method to confront challenges of thermal management posed by microelectronic devices. Nevertheless, the impact of ultrasound on bubble behaviors and its underlying mechanisms remain largely unexplored. This study first delves into the effect of ultrasonic parameters on bubble dynamic behaviors and associated mechanisms, subsequently further analyzing the forces acting on bubbles through the constructed force model. The findings suggest that although growth force serves as the significant resistance, the primary Bjerknes force dominates the rapid detachment of bubbles. The secondary Bjerknes force results in the bubble only sliding along the bottom wall rather than lifting off. Furthermore, the elevated ultrasonic pressure amplitude resulting from augmenting ultrasonic power induces a substantial increase in the critical detachment diameter and growth rate by 55.49% and 59.42%, respectively. The enhanced primary Bjerknes force, attributed to the rise in ultrasonic frequency, leads to a 71.42% increase in sliding velocity and a 46.45% reduction in growth time. The positive impacts arising from ultrasonic power and frequency are anticipated to notably enhance the thermal performance of microchannels. Besides, surface tension acts as the resistance and diminishes slightly with an augmentation of the boiling number, resulting in a moderate variation in sliding velocity and growth time.

Insight into the electrolyte concentration impact on single hydrogen bubble evolution dynamics at a microelectrode during electrochemical water splitting

Bubbles generated during electrochemical water splitting could adhere to the electrode surface and therefore impede the reaction. Thus, understanding and manipulating the evolution dynamics of bubbles is crucial for enhancing electrolysis efficiency. In this study, we investigated the evolution mechanism and forces acting on individual hydrogen bubble on Pt microelectrode surface by employing different H2SO4 electrolyte concentrations (0.2–1.0 M) under various applied voltages [−2 to −6 V vs saturated calomel electrode (SCE)]. We focused on bubble detachment diameter, average current, and bubble lifetime and subsequently established relational equations over these variables. At −6 V vs SCE, the growth coefficient has a maximum value of 14.42 × 10−4 m/s0.333 when the concentration of electrolyte is around 0.6 M. Gas production at 0.6 M increased by 63.4% compared to 0.2 M and by 11.2% compared to 1.0 M. Therefore, choosing the appropriate electrolyte concentration can maximize gas production and bubble detachment efficiency. Additionally, a force balance model incorporating the Marangoni force for single bubbles on the microelectrode surface was established across varying concentrations of H2SO4 electrolyte. At −4 V vs SCE, the solutal Marangoni force starts to dominate when electrolyte concentrations above 0.4 M. The results demonstrate the critical role of the solutal Marangoni force beyond a certain value of electrolyte concentration.

In Situ Observation and Electrical Signal Analysis of Different Bubble Detachment Modes on Microelectrode Surface

The deactivation of the reaction surface caused by bubble covering is an important factor that causes the decrease in the efficiency of water electrolysis. By combining an electrochemical workstation with a high-speed camera, the evolutionary properties of hydrogen bubble and potential were obtained on a horizontal microelectrode with a diameter of 100 μm. During the electrolysis, individual bubbles evolved periodically on the electrode surface. Under the influence of electrode adsorption, the detachment diameter of the bubble is independent of current density, but with the increase of current density, the bubble detachment frequency increases. With the increase of ethylene glycol (EG) concentration, the bubble detachment frequency increased. When the concentration of EG is greater than 7.5 g l−1, the surface tension of the solution is lower than that of water, at which time the detachment mode of the bubble changes. Separating the resistance caused by the bubbles from the total resistance, it was found that the additional resistance caused by the bubbles rapidly became smaller as the EG concentration increased until the EG concentration reached 7.5 g l−1. The total resistance decreases first and then increases when the EG concentration is greater than 3 g l−1.

Research on the influence of aeration manners on the mass transfer characteristics of bubbles in multi‐column airlift photobioreactor

AbstractMass transfer of bubbles is important in microalgae cultivation. In this study, aiming to improve the microalgae culturing efficiency, computational fluid dynamics (CFD) was adopted to study the influence of the aeration manners on mass transfer of bubbles within ascending columns in multi‐column airlift photobioreactor (PBR) and the hydrodynamic conditions within the PBR under different aeration manners. In addition, the bubble generation time, the detachment diameter, and the average volumetric mass transfer coefficient of bubbles in ascending columns were analyzed. Furthermore, the experimental results were compared with the simulation results obtained from microalgae cultivation. The results showed that the whole aeration manner yields the lowest mixing strength and mass transfer efficiency of bubbles at an aeration rate of 0.2 vvm. Conversely, both the mixing strength of the liquid and mass transfer coefficient of bubbles were enhanced under the half and alternate aeration manners. However, the results demonstrated that the distribution of the flow field was not uniform under the half aeration manner and there were obvious high‐speed and low‐speed zones. In contrast, the flow field distribution in the PBR was more uniform under the alternate aeration manner, which was suitable for microalgae cultivation at high concentration. This study effectively enhanced the mixing strength and CO2 transfer rate in the PBR.

Boiling mechanism of biphilic surfaces based on Helmholtz instability and Taylor instability

This study demonstrated that combination hydrophilic copper and hydrophobic nano-silver surfaces simultaneously enhanced the critical heat flux (CHF) and heat transfer coefficient (HTC) in pool boiling heat transfer (BHT) with deionized water as the employed working fluid. To further investigate the influence of geometric factors on heat transfer, this study designed three patterns denoted as squares/stripes/networks, ranging from 50 μm to 3000 μm. The effects of pattern size d, spacing p, and pitch ratio p/d on heat transfer results were obtained. Based on bubble visualization, the study further analyzed the reasons for inflection points in the boiling curve and the mechanisms caused by porous hydrophobic coating. Finally, the study proposed a numerical model suitable for predicting CHF of biphilic surfaces based on Helmholtz instability and Taylor instability. The model began with vapor columns escaping toward far-field and the fluid supplied to the heating surface. Concrete model derivation and adjustments are categorized according to the relationship between pattern size and bubble detachment diameter. For surfaces where the bubble detachment diameter exceeds the pattern size, this study originated from the perspective of Helmholtz instability on the vapor column surface. The resulting model is anticipated to quantitatively solve for the former and modulate the wavelength of Taylor instability on biphilic surfaces. Regarding surfaces with bubble detachment diameters smaller than the pattern size, this research begins with the existence time of the micro-liquid layer. The derived model validates the predominant role of p/d in the CHF values on micron-scale biphilic surfaces.

Visual experimental study of nanofluids application to promote CO2 absorption in a bubble column

The addition of dispersed-phase nanoparticles in the liquid phase can enhance the gas–liquid transfer process as the suspended nanoparticles affect the transfer process inside the fluid through micro-disturbance or micro-convection effects. In this paper, a high-speed digital camera was used to visualize the bubble behavior of CO2 in pure water and nanofluids to examine the effects of CO2 gas flow rate, nanoparticle solid content and type on the bubble behavior in the fluids. The CO2 absorption performance in three water-based nanofluids were compared in a bubbler. And the mass transfer characteristics during CO2 bubble absorption and the reasons for the enhanced gas–liquid mass transfer effect of nanoparticles were analyzed. The results showed that the presence of nanoparticles affected the formation process of bubbles in the fluid, shortened the bubble detachment time, reduced the detachment diameter, effectively increased the gas–liquid contact area, and improved the bubbles detachment frequency. The system with MCM-41 corresponded to a higher overall mass transfer coefficient. Uncalined MCM-41 contained surfactant which enhanced foaming behavior in water. This prevented the transfer of CO2 to some extent, and the CO2 absorption by uncalined MCM-41/H2O was 5.34% higher than that by pure water. Compared with SiO2 nanoparticles with the same particle size and the same composition, MCM-41 had a higher adsorption capacity and better hydrophilicity due to its larger specific surface area and rich porous structure, which was more favorable to accelerate the collision between nanoparticles and CO2 bubbles to cause micro-convection. Under the condition of 0.1% (mass) solid content, the enhancement of CO2 absorption process by MCM-41 nanoparticles was more significant and improved by 16.9% compared with pure water.

A Comparative Study of Flow Boiling Heat Transfer and Pressure Drop Characteristics in a Pin-Finned Heat Sink at Horizontal/Vertical Upward Flow Orientations

Abstract With the continuous development of power electronic devices toward miniaturization and compactness, it is necessary to develop more efficient flow boiling heat transfer technologies. In this work, the flow boiling heat transfer and pressure drop characteristics of Novec649 in a pin finned channel under two kinds of flow orientations (horizontal and vertical upward) are experimentally investigated. Heat flux, inlet flow velocity, and inlet subcooling are considered as the variable parameters. The results show that among all boiling operating conditions, the heat transfer performances between two orientations are basically consistent, while the pressure drop of vertical upward pin finned channel is relatively lower, indicating that the comprehensive flow boiling heat transfer performance of vertical oriented channel is better. Subsequently, a series of flow visualization experiments are performed in vertical upward pin finned channel. With the increase of heat flux, four kinds of flow pattern are discovered in the order of dispersed bubble flow, bubble flow, homogeneous flow, and annular flow. In the region of annular flow, although a vapor flow has already formed in the channel, there is still a large amount of liquid phase surrounding the wall and pin fins. Therefore, no obvious heat transfer deterioration was observed in the pin finned channel. Along the flow direction, the diameter of bubbles will increase first, and then present obvious oscillation. As the heat flux increases, both the average bubble detachment diameter and the frequency increase correspondingly. As the fluid velocity increases, the average bubble detachment diameter presents a downward trend, while the average bubble detachment frequency presents an upward trend.

Experimental study on subcooled pool boiling heat transfer under hypergravity

The pool boiling heat transfer experiment was conducted with pure water under subcooled and saturation conditions, both in hypergravity (1–3 g) and normal gravity (1 g) at atmospheric pressure. A turntable was employed to simulate the hypergravity environment. Subcooling was varied at four different levels: 2 °C, 5 °C, 8 °C, and 10 °C, corresponding to gravity levels ranging from 1 g to 3 g. Heat transfer enhancement was observed in the fully developed region under saturation conditions, whereas it was less pronounced in the natural convection region. Specifically, increasing the subcooling degrees leads to an expansion of the natural convection region, where the effects of hypergravity become noticeable at higher heat flux levels. The effect of hypergravity diminishes as the bubble detachment diameter in the fully developed region decreases with increasing subcooling degree, implying that the main effect of hypergravity on heat transfer is due to its influence on the bubble behavior. Additionally, a detailed analysis is presented to further explain the effect of hypergravity on heat transfer, focusing on the bubble dynamical behavior and the impact of buoyancy under hypergravity conditions.

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.

Optical coherence tomography-based misdiagnosis and morphological distinction in pachychoroid neovasculopathy vs. polypoidal choroidal vasculopathy

PurposeTo evaluate the rate of misdiagnosis of aneurysmatic pachychoroid type 1 choroidal neovascularization/polypoidal choroidal vasculopathy (PAT1/PCV) among cases diagnosed as non-aneurysmatic pachychoroid neovasculopathy (PNV) and to define optical coherence tomography (OCT) features facilitating their distinction.MethodsThe database of the Department of Ophthalmology, Ludwig-Maximilians University Munich, was screened for patients diagnosed with PNV. Multimodal imaging was screened for the presence of choroidal neovascularization (CNV) and aneurysms/polyps. Imaging features facilitating the diagnosis of PAT1/PCV were analysed.ResultsIn total, 49 eyes of 44 patients with a clinical PNV diagnosis were included, of which 42 (85.7%) had PNV and 7 (14.3%) represented misdiagnosed PAT1/PCV. SFCT was comparable (PNV: 377 ± 92 vs. PAT1/PCV: 400 ± 83 µm; p = 0.39). Whereas no difference was detected in total pigment epithelium detachment (PED) diameter (p = 0.46), maximum PED height was significantly higher in the PAT1/PCV group (199 ± 31 vs. 82 ± 46, p < 0.00001). In a receiver operating characteristic (ROC) analysis, the optimum cutoff for defining “peaking PED” was 158 µm with an area under the curve of 0.969, a sensitivity of 1.0 (95% CI: 0.59–1.0), and a specificity of 0.95 (95% CI: 0.84–0.99). Sub-retinal hyperreflective material (SHRM; p = 0.04), sub-retinal ring-like structures (SRRLS; p < 0.00001), and sub-RPE fluid (p = 0.04) were significantly more frequent in eyes with PAT1/PCV.ConclusionA relevant percentage of eyes diagnosed with PNV might instead suffer from PAT1/PCV. The detection of a maximum PED height (“peaking PED”) exceeding approximately 150 µm, SHRM, SRRLS, and sub-RPE fluid might greatly aid in the production of a more accurate diagnosis.

Study on the behavior of single oxygen bubble regulated by salt concentration in photoelectrochemical water splitting

The increase of electrolyte resistance near the electrode due to bubble evolution is the main obstacle to significantly improve the efficiency of photoelectrochemical water splitting. The oxygen bubble evolution on the TiO2 nanorod photocatalytic electrode was observed in situ by micro high-speed camera, and its corresponding current curve was measured synchronously by electrochemical workstation. The effect of single oxygen bubble evolution on current under different Na2SO4 electrolyte salt concentrations (0.01 M–1.0 M) was studied. When the salt concentration increases from 0.01 M to 0.8 M, the current in the bubble growth stage increases by about 2.5 times, while the bubble period and detachment diameter decrease by about 78.7% and 28.6%, respectively. When the salt concentration further increases, the current and bubble detachment diameter and period remain almost unchanged. The bubble growth coefficient reaches the optimum at 0.5 M, while the gas production rate reaches the optimum at 0.6 M. As the increase of electrolyte salt concentration, the increase rate of photocurrent density is lower than that of mass transfer coefficient, resulting in the decrease of solutal Marangoni force and the smaller bubble detachment diameter. The results show that regulating the concentration of electrolyte is an effective method to remove bubbles on the photocatalytic electrode.

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.

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.

Dynamics of growth and detachment of single hydrogen bubble on horizontal and vertical microelectrode surfaces considering liquid microlayer structure in water electrolysis

The dynamics of the growth and detachment of a single hydrogen bubble on both the horizontal and vertical microelectrode surfaces in water electrolysis were synthetically investigated by combining the numerical simulation, force balance analysis, and available experimental data. Approximately, multiple steady simulation cases with different bubble diameters for different growth instances were conducted to state the actual unsteady bubble growth and detachment behavior. The numerical simulations of the temperature distribution considering the heat transfer caused by the liquid microlayer and induced Marangoni convection effects were performed. Then, a force balance model for predicting the bubble detachment diameter was developed by fully utilizing the simulated multi-physical field parameters and the experimental results of some key bubble geometric parameters. The presented numerical model and the force balance model were validated by comparing them with previous experimental data on the potential and the bubble detachment diameter, respectively. The simulation results indicate a significantly larger potential value occurs within the microlayer, and hence, the Joule heat of the electrolyte is mainly generated in the microlayer and then transferred to the bulk flow region. Obviously, the temperature gradient distribution is formed at the bubble interface, causing unstable Marangoni convection structure. The distribution patterns and evolutions of the electrolyte temperature, Marangoni convection velocity, and microlayer thickness for the horizontal and vertical microelectrode systems are significantly different. The present force balance model presents higher prediction accuracy for the bubble detachment diameters. Moreover, the in-depth force analysis results reveal that some dominant forces influence the bubble growth and detachment.

Growth characteristics and the mass transfer mechanism of single bubble on a photoelectrode at different electrolyte concentrations.

In the photoelectrochemical water splitting reaction, the bubble attached to the working electrode is an essential factor affecting the reaction resistance, current density and gas-liquid mass transfer. An experimental measurement system based on an electrochemical workstation synchronously coupled with a high-speed microscopic camera was proposed and used to systematically study the growth kinetics and mass transfer mechanism of single oxygen bubbles at different electrolyte concentrations (Na2SO4, 0.1-2.0 M) on the TiO2 photoanode surface. Under constant voltage and constant current control conditions, when the electrolyte concentration increases, the bubble detachment diameter and the bubble detachment frequency gradually decrease. The bubble coverage equation expressed in terms of gas evolution efficiency is proposed and is associated with the photocurrent and bubble radius. The average bubble coverage and average gas evolution efficiency decrease when the electrolyte concentration is increased. According to the Sherwood dimensionless number, various mass transfer coefficients during bubble growth were calculated. The results show that the average total mass transfer coefficient is positively correlated with the change trend of the electrolyte concentration, and the mass transfer coefficient of single-phase natural convection is one order of magnitude larger than the mass transfer coefficient of bubble-induced convection. Finally, a conclusion on the transient mass transfer process in the bubble evolution process was obtained, that is, the mass transfer coefficient of single-phase natural convection and the total mass transfer coefficient remain high during the first growth stage, and gradually decrease during the second growth stage. Therefore, regulating the electrolyte concentration can effectively promote the gas-liquid mass transfer in the photoelectrochemical water splitting reaction.

Distribution Characteristics of Multiphysics around the Bubble on the Surface of Photoelectrode

In the heterogeneous photocatalytic reaction systems, the evolution of bubbles on the catalyst surface is a complex process that depends on multiple factors, such as the solid–liquid interface structure, solution properties of the reaction system, and multiphysics interactions, and directly affects energy and mass transport processes. This study investigated the multiphysics distribution around a bubble and the bubble detachment characteristics on the surface of a TiO2 photoelectrode during photoelectrocatalytic water splitting. The bubble diameter had an effect on the distribution of the light field in the system, affecting the temperature and flow distributions of the electrolyte around the bubble. Marangoni convection was induced by the inhomogeneous temperature distribution along the bubble interface, which enhanced the heat and mass transfer near the reaction site. It was also demonstrated that an increase in laser power increased Marangoni convection on the bubble surface, thus delaying bubble detachment. A force model for predicting the bubble detachment diameter that considered the coupling effect between the light, temperature, and flow fields was developed. The predicted bubble detachment diameters agreed well with the experimental results within an error of ±10%.

Experimental and theoretical investigation on the effect of surface porosity on the nucleate flow boiling heat transfer

The surface porosity of accident tolerant fuel cladding materials is different from the fuel material inservice at nuclear power plant, which leads to differences in the heat transfer characteristics. An experiment was conducted to investigate the effect of surface porosity on the nucleate flow boiling heat transfer. The results showed that bubbles on the SiC surface appear to be more uniform and remain isolated longer after bubble generation, and the average diameter of bubbles on the SiC surface tends to be smaller than that on the FeCrAl alloy surface. The nucleation site density is power-law dependent on porosity and superheat, and negative power-law dependent on critical cavity diameter and coolant subcooling. For the same degree of temperature increase on both surfaces, the number of nucleation sites on the surface with larger porosity increases much more than that on the surface with lower porosity, while the coolant subcooling degree inhibits the bubble nucleation and shrinks the bubble detachment diameter accordingly. A new correlation was proposed based on dimensional analysis, and the predictions were compared with the present experimental data and the other research data, with an 86% confidence interval of ± 50% for the consolidated datasets. Furthermore, the side-by-side comparison of the performance on these correlations showed that the present correlation, which includes surface porosity and roughness factors, significantly reduces the prediction error compared to the previous correlations.

Enhanced pool boiling heat transfer of multilayer copper-nanoparticle-packed beds by copper mesh coating

Pool boiling heat transfer has drawn a wide interest in dissipating high density heat flux produced by electronic products etc. In this work, the advantages of the copper mesh and the multilayer nanoparticle-packed beds in improving pool-boiling heat transfer was combined to give a good performance. The copper-nanoparticle-packed beds were firstly prepared, and then the effects of the layer thickness and porosity in the bed on the pool boiling heat transfer were explored. After obtaining the optimal thickness and porosity of the multilayer beds, the copper mesh was further coated on the multilayer beds to further enhance the pool-boiling heat transfer. Results turn out that: The increased thickness of the beds has a negative effect on enhancing heat transfer coefficient (HTC) because of the increased bubble-escape resistance, however, this effect will be set-off by the positive effect of the increased wetted area when the thickness was increase to be larger than 3 mm in this work. The onset of nucleate boiling of the optimal 4-layer beds without coating is about 1.7 °C which is much lower than most reported values, and the HTC is about 9 × 104 W/(m2·k) which is 9 times higher than that of the smooth surface. With the optimal bed further coated with copper mesh, the HTC can be further increased to 9.8 × 104 W/(m2·k). This enhancement arrives from the fact that the copper mesh prevents the continuous merging of bubbles, thus reducing the bubble detachment diameter by 30% and increasing detachment frequency of bubbles by 0.5 times.

Experimental and theoretical investigation of bubble dynamics on vertical surfaces with different wettability for pool boiling

Surface wettability is generally employed to manipulate the bubble dynamics to enhance the performance of boiling heat transfer. This work conducted an experimental and theoretical investigation of bubble dynamics on vertical surfaces with different wettability for pool boiling. The side-by-side comparison for bubble growth, bubble detachment diameter and frequency on superhydrophilic, superhydrophobic, and neutral surfaces were visualized, the bubbles detachment diameter and growth time in a negative exponential dependence with the subcooling degree, even a tiny difference in subcooling, such as 1–2K, contributes significantly to the shrinking of bubble volume. When the fluid was in a saturated state or the subcooling of the fluid can be neglected, the bubble was tightly adhered to the wall and cannot detach from the superhydrophobic surface, the bubbles would gather to a certain volume and then slide upward, and the first time of bubble detachment was observed when the coolant subcooling reached 2K. An energy balance model was developed to evaluate the bubble diameter in the bubble growth period, with 94% reproduction of the consolidated database within the error band of ± 40%. In addition, a new correlation is proposed for bubble detachment diameter, the new correlation performed significantly better in a broad range of contact angle, with 82% confidence interval of the consolidated datasets was ± 30%.

Removal of bubbles from eletrodes in a planar cyclonic electrolyzer

Alkaline water electrolysis is the most mature and most extensive in-scale hydrogen production technology. For scaled-up electrolyzers, the negative effect of the bubbles formed on the electrodes contributes to high energy consumption and hinders the further expansion of this technology. This study developed a planar cyclonic electrolyzer to improve bubble detachment. The effects of the hydrodynamics on the bubble detachment diameter and current density were studied. According to a proof-of-concept experiment, an increase in the electrolyte velocity led to a gradual decrease in the bubble detachment diameter and a significant increase in the current density. At an applied voltage of 5 V, the detached bubble size decreased from 160 to 82 microns when the electrolyte velocity increased from 0.33 to 2.33 m/s. Meanwhile, a marked rising in the current density from 0.43 to 1.71 A/cm2 was observed. The significance of bubble detachment from an electrode on electrolysis efficiency was discussed. Furthermore, the diaphragm-free separation of hydrogen and oxygen bubbles can be possibly achieved with this novel design.