Bubble Detachment Frequency Research Articles

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.

Effect of free particles in porous media on pool boiling heat transfer performance

We conducted a comprehensive investigation into the mechanisms the enhancement of pool boiling heat transfer. Created a porous medium composite free particle reinforced structure (FPPM). FPPM is synthesized with copper foam (Cu foam) with a porous structure as the base material; it is characterized by free particles inside the cavity. We used deionized water (DI water) as the working fluid to study the heat transfer performance of pool boiling on different surfaces under atmospheric pressure. The boiling mechanism was verified by bubble dynamics. The findings showed that the addition of free particles to the copper foam sample significantly reduced the superheating of its onset of nucleate boiling (ONB). Compared to the polished surface, a maximum of 2.57 times the heat transfer coefficient was achieved; in addition, a maximum of 1.59 times the heat transfer coefficient was achieved compared to the copper foam surface without free particles. The visualization results showed the formation of nucleation sites on the surface with free particles, which decreased the energy barrier to easy nucleation. During the boiling process of the pool, the high thermal conductivity of free particles and the disturbance to the gas–liquid layer significantly reduced the bubble detachment frequency, resulting in good heat transfer performance over the whole range of heat fluxes.

Experimental investigation on the boiling heat transfer characteristic of copper surfaces with different hydrophilicity

Surface hydrophilicity has a significant effect on boiling heat transfer. In order to systematically investigate the influence of hydrophilic degree on the boiling heat transfer performance, seven different surfaces with gradually changing contact angles (CAs) were prepared using the chemical etching technology. Meanwhile, Visual studies on pool boiling of four of these different hydrophilic surfaces were conducted using a high-speed camera at atmospheric pressure, with deionized water as the working fluid. The influence laws of the hydrophilicity coefficient and relative wall superheat on the enhancement of boiling heat transfer are systematically summarized. It is shown that the boiling heat transfer coefficient (HTC) and critical heat flux (CHF) of the heating surface increased as the CA decreased. Owing to its vertically and horizontally interlaced nanorods and nanoflower structures, the surface oxidized by NaOH/(NH4)2S2O8 has higher surface roughness and larger phase change heat transfer area, resulting in a super-hydrophilic property of the copper surface and a faster bubble detachment frequency. The CHF and boiling HTC were 1.16 and 1.59 times that of the untreated surface, respectively, and the onset of nucleate boiling was advanced by 3.1 K, greatly enhancing boiling heat transfer and exhibiting better heat transfer performance.

Saturated Boiling Enhancement of Novec-7100 on Microgrooved Surfaces with Groove-Induced Anisotropic Properties

The effects of the anisotropic properties (wettability and roughness) of microgrooved surfaces on heat transfer were experimentally investigated during pool boiling using Novec-7100 as a working fluid. The idea for introducing the concept of anisotropic wettability in boiling experiments draws inspiration from biphilic surfaces. The investigation is also motivated by two-phase immersion cooling, which involves phase-change heat transfer, using a dielectric liquid as a working fluid. Very few studies have focused on the effects of surfaces with anisotropic properties on boiling performance. Thus, this study aims to examine the pool-boiling heat transfer performance on surfaces with microgroove-induced anisotropic properties under the saturation condition. A femtosecond-laser texturing method was employed to create microgrooved surfaces with different groove spacings. The results indicated that anisotropic properties affected the heat transfer coefficient and critical heat flux. Relative to the plain surface, microgrooved surfaces enhanced the heat transfer performance due to the increased number of bubble nucleation sites and higher bubble detachment frequency. An analysis of bubble dynamics under different surface conditions was conducted with the assistance of high-speed images. The microgrooved surface with a groove spacing of 100 μm maximally increased the BHTC by 37% compared with that of the plain surface. Finally, the CHF results derived from experiments were compared with related empirical correlations. Good agreement was achieved between the results and the prediction correlation.

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 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.

Bubble management in PEM water electrolysis via imprinting patterned grooves on catalyst layer

The demand for cost reduction in proton exchange membrane water electrolyzer (PEMWE) is driving it to increase the operating current density and higher current densities require better bubble management. In this work, we demonstrate that introducing patterned grooves to the catalyst layer surface (CL) is able to enhance the bubble evolution and improve electrolyzer performance. The patterned grooves microstructure on CL provides additional bubble nucleation sites and faster bubble evolution. Smaller average bubble size and higher bubble detachment frequency during water electrolysis are observed through a bubble evolution visualization system and further confirmed by a frequency analysis method. In addition, patterned grooves also improve the hydrophilicity and aerophobicity of CL surfaces. In the PEM water electrolyzer cell, the patterned grooves on CL boost the cell performance and reach high current densities up to 5.0 A cm-2. With better bubble management, patterned grooves significantly reduce the mass transport overpotential at each current density. At 5.0 A cm-2, the mass transport overpotential is reduced by 59%. This work provides a new idea of bubble management in PEMWE and is highly informative for designing CLs for PEMWE and other gas-evolving electrochemical energy conversion applications.

A unifying criterion of the boiling crisis

We reveal and justify, both theoretically and experimentally, the existence of a unifying criterion of the boiling crisis. This criterion emerges from an instability in the near-wall interactions of bubbles, which can be described as a percolation process driven by three fundamental boiling parameters: nucleation site density, average bubble footprint radius and product of average bubble growth time and detachment frequency. Our analysis suggests that the boiling crisis occurs on a well-defined critical surface in the multidimensional space of these parameters. Our experiments confirm the existence of this unifying criterion for a wide variety of boiling surface geometries and textures, two boiling regimes (pool and flow boiling) and two fluids (water and liquid nitrogen). This criterion constitutes a simple mechanistic rule to predict the boiling crisis, also providing a guiding principle for designing boiling surfaces that would maximize the nucleate boiling performance.

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.

Modification and pool boiling performance elevation of copper foam wicks for high power applications

Surface modification is an effective method to improve boiling heat transfer performance and has the feasibility of application in enhancing equipment cooling and energy conversion efficiency. A magnetron sputtering (MS) method is employed to modify the skeleton surface of copper foam for further maximizing the boiling performance in this study. After modification, the heat transfer coefficient (HTC) and critical heat flux (CHF) of copper foam wicks have been significantly improved, with a maximum increase of 75.6 % and 17.3 % compared to the original copper foam, respectively. The greatly enhanced boiling performance is attributed to the structure of “cauliflower-like” and gradational wettability along the thickness direction, which provide more nucleation sites and increase the bubble detachment frequency. Meanwhile, the heat transfer correlation for modification samples is developed, and the results show an average error equal to 11.2 %. In addition, the beat sample is assembled in the evaporator of heat pipe (HP) as wick which increased the heat transfer coefficient by 144.3 ∼ 333.4 %. All results demonstrated that the surface modification of copper foam based on MS can greatly increase heat and mass transfer performances, which is promising in the high power applications.

Numerical and experimental investigation of convective flow boiling in vertical annulus using CFD code: Effect of mass flow rate and wall heat fluxes

In this paper a numerical and experimental analysis of convective flow boiling in a vertical annular channel has been performed. As the best substitute for water, a refrigerant Trichlorofluoromethane CCl3F (R11) has been selected for the working fluid in the experiment to evaluate the capability of the Computational Fluid Dynamic (CFD) codes to predict the heat transfer rates, transitions points and the boiling flow regime. The experiments cover a wide range of mass fluxes (435–1443 kg/m2 s) and heat fluxes (1.81–11.5 W/cm2), which enabled to study the influence of different boundary conditions on the development of local boiling phenomena up to CHF occurrence. The contribution of each of the mechanisms for transfer of heat to the liquid – forced convection and quenching, as well as the energy transport associated with vapor generation has been quantified in terms of nucleation site densities, bubble departure, bubble detachment frequency, flow parameters like velocity, inlet subcooling and wall superheat. The qualitative and quantitative trends of the parameters studied experimentally are also supported by the analysis and observation of the various phenomena visualized. A sensitivity analysis of several modeling parameters on the radial distribution of flow quantities has highlighted the importance of correct description of the boiling model. Also, some measurements have been compared to the predictions by the three-dimensional two-fluid model of subcooled boiling flow carried out with the computer code. The computational method has been tested against experimental data. The comparison shows a good agreement between the predicted and experimental results.

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%.

Experimental study of multilayer gradient copper foam effect on pool boiling heat transfer performance and gas-liquid behavior characteristics

The rapid growth of the internet of things has induced the integration of many microelectronic devices in physical objects, yet the generated heat decreases the performance and stability of microelectronic devices. Therefore, new materials such as gradient metal foams (GMFs) have been recently designed to improve heat transfer. In this paper, an experimental visualization setup was built to investigate the effect of the GMFs gradient layers number and the arrangement order on the pool boiling heat transfer performance. Results show that increasing the number of gradient layers enhances the heat transfer when the copper foam pore density is low. By contrast, at high pore density of 50 PPI, increasing layers hardly changes heat transfer. The bubbles dynamic behavior on the metal foams surface of with different gradient structures is also different. When bubbles detach upward, the temperature of the metal foam is lower, and the temperature gradient is higher. When bubbles detach sideward, the bubble escape is much shorter, and the bubble detachment frequency and size increase. Combined with the theoretical research, the metal foam gas-liquid flow heat transfer model were constructed. The advantages and disadvantages of GMFs with different structures and the applicable scenarios are analyzed.

A parametric study of subcooled flow boiling of Al2O3/water nanofluid using numerical simulation and artificial neural networks

ABSTRACT Utilizing an Euler-mixture three-dimensional numerical simulation for Al2O3/water nanofluid subcooled flow boiling in a mini channel, we study the effects of pressure, heat flux, nanoparticle concentration, surface roughness, and subcooled temperature on heat transfer quantities (average and local heat transfer coefficient, average and local vapor volume fraction, and average and local wall temperature) and bubble dynamics quantities (bubble departure diameter, bubble detachment frequency, bubble detachment waiting time, and nucleation site density). The numerical results demonstrate that the nanoparticles particularly impact the bubble dynamics significantly by increasing wettability and decreasing contact angle. In order to reduce the computational burden of such an expensive multiphase flow simulation, we also present a machine learning approach based on artificial neural networks (ANN). The numerical experiments show that using the ANN model, we can achieve highly accurate results with much less computational time and resources.

Experimental study on R-410A subcooled flow boiling heat transfer and bubble behavior inside horizontal annuli

The present research is concerned with the experimental work on R-410A subcooled flow boiling in horizontal annuli. Firstly, boiling curves and boiling heat transfer coefficients are used to explore effects of input heat flux (q), liquid mass flux (G), inlet refrigerant subcooling (ΔTsub), saturation temperature (Tsat) and annular gap (δ). The results indicate the obvious temperature drop occurs during onset of nucleate boiling (ONB) and increasing G and ΔTsub enlarges the temperature undershoot. With the increasing q, the heat transfer coefficient is raised apparently. Higher ΔTsub and larger δ both lead to lower boiling heat transfer coefficients, but varying G and Tsat has small influence on the boiling heat transfer. Secondly, the visualization of bubble behavior discloses increasing q improves bubble size, formation, and departure. With the increase in G, ΔTsub, Tsat and δ, the bubble growth is suppressed, leading to the reduction in bubble detachment diameter (dp). Moreover, the bubble detachment frequency (fb) and mean density of active nucleation site (Nac) are reduced with either the increase in ΔTsub and δ or the decrease in Tsat. Finally, empirical correlations are proposed for dp, fb, Nac, and total heat flux, which yields satisfactory agreement with our measured data.

Bubble dynamics and deformation of free liquid surface in aerated liquid storage tanks

OpenFOAM was utilized for analyzing bubble behavior and deformation of free liquid surface due to bubble formation and bursting in a rectangular container. Influence of three system parameters, orifice diameter, number of orifices and spacing between orifices, on various bubble dynamics and deformation of gas-liquid interface is presented. The study also incorporates information on bubble formation, bubbling frequency, and orientation of bubbles. Considering different orifice spacing, bubbling synchronicity is also reported. Details regarding interaction of wakes during bubble coalescence for single, double and triple inlet orifices are presented. The deformation of free liquid surface due to bubble formation and bursting is quantified using a new parameter called deformation index (D L * ). The analyses indicate that the frequency of bubble detachment is augmented with increase in orifice diameter and number of orifices. However, bubble detachment frequency is reduced when orifice spacing increased. Orientation of detached bubbles keeps on changing for larger orifice spacing. Results show that variations of these geometric parameters have substantial influence on free liquid surface deformation due to bubble bursting and other bubble behavior. Using results of these studies, one can develop a bubble-generating device for optimal performance.

Interface-Resolved Simulation of Bubbles–Metal–Slag Multiphase System in a Gas-Stirred Ladle

A grid-filtered scale interface-resolved volume of fluid (VoF) model coupling a sub-grid scale large eddy simulation (LES) was used to directly simulate bubble behavior and the evolution of multiphase interfaces and free surfaces in a bubble–metal–slag multiphase system. The model has been applied to an industrial 150-ton gas-stirred ladle. The results show transient behavior of the slag eye, with large variations over time of the eye size. Bubble detachment frequency and rise characteristics (including coalescence and breakup) directly affect the size, position and variation of the slag eye. The effects of the argon flow rate, slag thickness, slag viscosity, and slag–steel interfacial energy were investigated; the latter two have little effect. The calculated average slag eye size agrees with previously reported results for industrial ladles. A new correlation for the time-averaged slag eye area, based on the modified Froude number, is proposed for industrial ladles.

Different nanofluids effect on bubble characteristics at the isothermal bubble column

AbstractIn this study, the effect of different nanofluids such as SiO2, Fe3O4, and Al2O3 on bubble characteristics is studied experimentally. The nanoparticles concentration for the SiO2 nanofluid is 0.05 wt% and for other nanofluids is 0.005 wt%. Bubbles are formed by injection of air at a constant gas flow rate (between 600‐1200 mL/h) into a stagnant isothermal liquid column. Experimental data of formation, growth, and detachment of the air bubbles were recorded by a high‐speed digital camera (1200 fps), and the image processing method was used to analyze the bubble characteristics. In the present study, bubble characteristics such as diameter, size, aspect ratio, and detachment frequency were studied in four different liquids. The results show that a bubble has the biggest size in the pure water and adding nanoparticles to the pure water decreases bubble size. Also, the variation of detachment frequency has an inverse relation with the bubble size behaviour. Between different nanoparticles, Fe3O4 has maximum and SiO2 has minimum effect on bubble features. By adding Fe3O4 to pure water, bubble diameter decreases nearly 7%‐8% and bubble detachment frequency increases nearly 15%‐20%. In order to analyze the variation of the bubble characteristics, a force modelling on bubbles during the growth was proposed. Moreover, a correlation has been proposed for the prediction of bubble diameter at detachment time in three different nanofluids and in the pure water. The mean absolute error of this correlation is 2.65% and has a good agreement with experimental results.

Compound analysis of gas bubble trajectories with help of multifractal algorithm

Three-dimensional trajectories of gas bubbles were the subject of study in this article. Authors carried out experiments in two glass tanks with dimensions of 300 × 300 × 1500 mm and 300 × 150 × 500 mm, respectively, filled with distilled water with a temperature equal to T = 21 ± 1 °C. The tests were carried out for an air volume flow rate, values from 0.00492 l/min to 0.0645 l/min. Basic characteristics of recorded measurement data like: analysis of the bubble detachment frequency from the nozzle, gas bubble size analysis have been shown. Based on the conclusions from those characteristics the multifractal algorithm analysis has been presented as one that allows comprehensive analysis of such a process. The results as a comparison of the main points on the air volume flow rate showed that: the trajectories of gas bubbles in liquid are multifractal and the width of the multifractal spectrum, and thus the complexity of the process being examined, depends on the air flow, the diameter of the bubble generating nozzle and the height of the liquid column in which the phenomenon is examined. The results of the analysis show that the key impact on the overall observed phenomenon is the deformation of the bubble surface, oscillation of the bubble column and observation of the two-phase flow spatially (three-dimensional).

High-speed characterization of two-phase flow and bubble dynamics in titanium felt porous media for hydrogen production

Bubble dynamics and two-phase flow phenomena are closely related to the performance of proton exchange membrane electrolyzer cells (PEMECs). This paper reports an in-situ study of the oxygen bubble behavior and associated multiphase evolutions in the anode side of PEMECs with titanium (Ti) felt liquid gas diffusion layers (LGDLs) by a high-speed visualization system. The micro oxygen bubble dynamics was captured and analyzed at different locations and virous operating conditions. The results show that the bubble detachment frequency and detachment diameter greatly increase with the operating current density. Additionally, they are significantly impacted by the local pore structure and morphology of Ti felt LGDLs. In the flow channels, there exist only several discrete micro bubbles at a low current density (0.04 A/cm2) and a large flow velocity (133 mm/s). At a current density (0.2 A/cm2) and a flow velocity (67 mm/s), a number of gas slugs are formed in the follow channels, in addition to discrete micro bubbles. At a high current density (1 A/cm2) and a flow velocity (67 mm/s), more bubbles appear in the channel, and the flow field is dominated by slug or annular flows. These investigations can help to better understand the two-phase flow and bubble detachment mechanism, and provide a foundation for electrochemical reaction, multiphase flow studies and optimize the design of gas diffusion layers and flow fields for PEMECs in the future.