In-situ investigation of bubble dynamics and two-phase flow in proton exchange membrane electrolyzer cells

Gas bubble dynamics and two-phase flow have a significant impact on the performance and efficiency of proton exchange membrane electrolyzer cells (PEMECs). It has been strongly desired to develop an effective experimental method for in-situ observing the high-speed/micro-scale oxygen bubble dynamics and two-phase flow in an operating PEMEC. In this study, the micro oxygen bubble dynamic behavior and two-phase flow are in-situ visualized through a high-speed camera coupled with a specific designed transparent PEMEC, which uses a novel thin liquid/gas diffusion layer (LGDL) with straight-through pores. The effects of different operating conditions on oxygen bubble dynamics, including nucleation, growth, and detachment, and two-phase flow have been comprehensively investigated. The results show that temperature and current density have great effects on bubble growth rate and reaction sites while the influence of flow rate is very limited. The number, growth rate, nucleation site, and slug flow regime of oxygen gas bubbles increase as temperature and/or current density increases, which indicates that an increase in temperature and/or current density can enhance the oxygen production efficiency. Further, a mathematical model for the bubble growth is developed to evaluate the effects of temperature and current density on the bubble dynamics. A mathematical model has been established and shows a good correlation with the experimental results. The studies on two-phase flow and high-speed micro bubble dynamics in the microchannel will help to discover the true electrochemical reaction at micro-scale in an operating PEMEC.     

Impact of light intensity,CO2 concentration and bubble size on growth and fatty acid composition of Arthrospira (Spirulina) platensis KMMCC CY-007

Studies on lipid accumulation and fatty acid composition of microalgae as a feedstock for biodiesel have been reported in numerous papers in recent years. However, in contrast to microalgae, documentation on cyanobacteria such as Arthrospira platensis with regard to lipid production has been sparse thus far. In this paper, we show the relatively high lipid accumulation potential of A. platensis KMMCC CY-007 in modified Schlösser medium. Four different growth conditions [7.5 candela (cd) and 1% CO₂, 15 cd and 1% CO₂, 7.5 cd and air, and 15 cd and air] were examined over 7 days in order to compare the impact of light intensity. Using two different growth conditions (7.5 cd and 1% CO₂ bulk bubble and 7.5 cd and 1% CO₂ fine bubble), changes in growth, lipid accumulation and fatty acid composition were compared. The highest lipid content of 37.96% was obtained at 7.5 cd and 1% CO₂ fine bubble after 7 days, and the maximum net production (72.56%) of long chain fatty acid methyl esters (C16:0 and C18:0) detected at 7.5 cd and 1% CO₂ bulk bubble. These data suggest the potential of A. platensis for production of crude lipid as a future feedstock for biodiesel production.     

Dual model of bubble growth in vertical rectangular narrow channel

The mechanism of bubble growth has been of particular interest for decades due to its significant contribution to understanding boiling heat transfer. In present work, a visualization experiment was carried out to analyze the bubble growth in a vertical rectangular narrow channel by using water as working fluid under 1 and 3 bar system pressure. The bubble growth in various experimental conditions was obtained by analyzing the recorded pictures. Results show that bubble growth and bubble size are significantly affected by mass flux, heat flux and system pressure, and also by the nucleation site density. To obtain a well prediction for bubble growth, a dual model is proposed in this paper. This model is comprised of a linear model for the inertia controlled stage of bubble growth and a power law model for the heat diffusion controlled stage of bubble growth, and the prediction of the dual model agrees well with the experimental result with an error less than ± 15%.     

Bubble growth rates in pure and binary systems: Combined effect of relaxation and evaporation microlayers

Pohlhausen's equation has been used to determine the initial thickness of the evaporating microlayer beneath a hemispherical vapour bubble on a superheated horizontal wall. Microlayer thickness is proportional to the square root of the distance to the nucleation site during early bubble growth, while a linear relationship exists during advanced growth.A (heat and mass) diffusion-type solution is derived for advanced bubble growth, which accounts for the interaction of the mutually dependent contributions due to the relaxation microlayer (around the bubble dome) and the evaporation microlayer. The entire bubble behaviour during adherence is determined by a combination of this asymptotic solution and the Rayleigh solution, which governs early growth. Also, expressions are derived for both the radius of the dry area and the radius of the maximum contact area between bubble and wall.At low concentrations of the more volatile component in binary systems, the dominating influence of mass diffusion is demonstrated by the following effects: (i) asymptotic bubble growth is slowed down substantially; (ii) the formation of dry areas beneath bubbles is prevented, even at subatmospheric pressures; (iii) the lower part of the bubble is contracted; (iv) the evaporation microlayer contribution to bubble growth is negligible at atmospheric and at elevated pressures.     

Hydrodynamic aspects of interaction between nucleation sites

The dynamics of twin and triple nucleation sites interaction has been investigated. Nonlinear data analysis techniques such as the dimension spectrum and largest Lyapunov exponent have been used for analysis of data recorded in experiment. It has been shown that properties of dynamics of nucleation sites changes nonlinearly together with changing the spacing between cavities. To explain appearance of such properties of nucleation sites interaction the simulation of gas bubble movement in the liquid in the last stage of bubble departures has been made with using the stabilized finite element method and level set method. It has been shown that the bubble departure velocities and structure of velocity field around the bubbles nonlinearly depend on the initial horizontal spacing between bubbles. The obtained results indicate that one of the reasons for nonlinear changes of properties of dynamics of nucleation sites interaction occurring with changes of initial spacing between them is hydrodynamic interaction between departing bubbles.     

Unsteady-state mass transfer from a binary gas bubble with changing volume

Numerical methods are used to investigate the transient mass transfer from a binary gas bubble to an incompressible liquid. The bubble has two components: A – soluble and B – insoluble in the surrounding fluid. The concentration of A inside the bubble is considered spatially uniform but not constant in time. The maximum value of the initial volumetric fraction of A is 0.3. The mathematical model equations were solved numerically in spherical coordinates system. Creeping flow, moderate Re number flow, 10 ? Re ? 100, and potential flow around the bubble were assumed. The computations focused on the influence of the initial fraction of A and Henry number on the mass transfer rate for Pe ? 10⁴.     

Numerical investigation of bubble coalescence characteristics under nucleate boiling condition by a lattice-Boltzmann model

A numerical study was performed to investigate the characteristics of bubble growth, detachment and coalescence on vertical, horizontal, and inclined downward-facing surfaces. The FlowLab code, which is based on a lattice-Boltzmann model of two-phase flows, was employed. Macroscopic properties, such as surface tension (σ) and contact angle (β), were implemented through the fluid–fluid (G_σ) and fluid–solid (G_t) interaction potentials. The model predicted a linear relationship between the macroscopic properties (σ,β) and microscopic parameters (G_σ,G_t). The simulation results on bubble departure diameter appear to have the same parametric dependence as the empirical correlation. Hydrodynamic aspects of bubble coalescence are investigated by simulating the growth and detachment behavior of multiple bubbles generated on horizontal, vertical, and inclined downward-facing surfaces. For the case of horizontal surface, three distinct regimes of bubble coalescence were represented in the lattice-Boltzmann simulation: lateral coalescence of bubbles situated on the surface; vertical coalescence of bubbles detached in a sequence from a site; and lateral coalescence of bubbles, detached from the surface. Multiple coalescence was predicted on the vertical surface as the bubble detached from a lower elevation merges with the bubble forming on a higher site. The bubble behavior on the inclined downward-facing surface was represented quite similar to that in the nucleate boiling regime on a downward-facing surface.     

Bubble growth rates in nucleate boiling of water at subatmospheric pressures

The growth rate of vapour bubbles has been investigated experimentally up to departure in water boiling at pressures varying from 26·7 to 2·0 kPa (the corresponding Jakob number increasing from 108 to 2689).Comparison of the data with existing theory shows the substantial influence of liquid inertia during initial growth, in agreement with previous results of Stewart and Cole [1]on water boiling at 4·9 kPa, the Jakob number varying from 955 to 1112. As an extreme case, at a pressure of 2·0 kPa, large “Rayleigh” bubbles are observed during the entire adherence time. During advanced growth, bubble behaviour is gradually governed by heat diffusion, especially at relatively high (subatmospheric) pressures.Experimental bubble growth in the investigated pressure range is in quantitative agreement with the van Stralen, Sohal, Cole and Sluyter theory [10]. This model combines the Rayleigh solution with a diffusion-type solution, which accounts for the contributions to bubble growth due to both the relaxation microlayer (around the bubble dome) and the evaporation microlayer (beneath the bubble).Finally, a curious bubble cycle is observed at the lowest investigated pressures, which is attributed to the combined action of a high-velocity liquid jet (originating in the wake following a large primary bubble) and a succeeding secondary vapour column (generated at the adjacent dry spot at the heating wall beneath the primary bubble).     

Numerical analysis of boiling on high heat-flux and high subcooling condition using MPS-MAFL

This paper shows the numerical simulation study on the growth of the bubble in the transient pool boiling using moving particle semi-implicit with meshless advection using flow-directional local grid (MPS-MAFL) method. The growth process of a bubble with the different initial radii is calculated in a high heat-flux and high subcooling condition. The smaller initial radius is, the earlier the growth starts. The initial bubble radius has little effect on the growth initiation time and the bubble departure radius.     

Burning velocities of ethanol-isooctane blends

Laminar burning velocities of isooctane, ethanol, and isooctane-ethanol blends in air have been determined over a practical range of mixture strength at various initial mixture temperatures using a constant volume spherical bomb. Measurements were made during the constant pressure combustion period and a detailed density correction scheme was employed for calculation of burning velocities from the measured flame growth rates. A strong promotion of isooctane combustion by ethanol has been observed. Maximum burning rates were found to occur at an equivalence ratio of approximately 1.08, independently of unburned mixture temperature and fuel type. Mixture strength, unburned mixture temperature, and fuel type dependence of burning velocity is represented by empirical functions over the range of Φ = 0.75?1.4, T_u = 350?600K, and up to 20% ethanol (by liquid volume) in isooctane-ethanol blends.     

Premature transition to film boiling with stepwise heat generation 2nd report: Effect of wall material and surface condition

The effects of wall material and surface condition on the behavior of an initial boiling bubble of R113 subjected to transient heating were investigated using a heater with a large heat capacity. The behavior of the initial bubble is closely related to premature transition to film boiling of liquids with high wettability. An initial bubble, which is peculiarly shaped like a “straw hat” and leads to premature transition in saturated liquid nitrogen (as reported in a previous paper), also appears on a heated wall with large heat capacity and grows rapidly to cover the entire wall surface. From the observations using a high-speed video camera, the initial bubble is found to be a coalesced bubble into which small bubbles activated in succession along the heated surface are rolled. The growth rate of the initial bubble along the heated surface is not greatly affected by the thermal conductivity of the wall material but is affected markedly by the surface roughness. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res. 25(1): 51–63, 1996     

Interaction mechanism of double bubbles in hydrodynamic cavitation

Bubble-bubble interaction is an important factor in cavitation bubble dynamics. In this paper, the dynamic behaviors of double cavitation bubbles driven by varying pressure field downstream of an orifice plate in hydrodynamic cavitation reactor are examined. The bubble-bubble interaction between two bubbles with different radii is considered. We have shown the different dynamic behaviors between double cavitation bubbles and a single bubble by solving two coupling nonlinear equations using the Runge-Kutta fourth order method with adaptive step size control. The simulation results indicate that, when considering the role of the neighbor smaller bubble, the oscil-lation of the bigger bubble gradually exhibits a lag in comparison with the single-bubble case, and the extent of the lag becomes much more obvious as time goes by. This phenomenon is more easily observed with the increase of the initial radius of the smaller bubble. In comparison with the single-bubble case, the oscillation of the bigger bubble is enhanced by the neighbor smaller bubble. Especially, the pressure pulse of the bigger bubble rises intensely when the sizes of two bubbles approach, and a series of peak values for different initial radii are acquired when the initial radius ratio of two bubbles is in the range of 0.9～1.0. Although the increase of the center distance between two bubbles can weaken the mutual interaction, it has no significant influence on the enhancement trend. On the one hand, the interaction between two bubbles with different radii can suppress the growth of the smaller bubble; on the other hand, it also can enhance the growth of the bigger one at the same time. The significant en-hancement effect due to the interaction of multi-bubbles should be paid more attention because it can be used to reinforce the cavitation intensity for various potential applications in future.     

水平管道对流化床中气泡成长的影响

利用FLUENT模拟了二维流化床内水平管道和气体在不同进口速度下对气泡成长的影响.结果表明,流化床中存在水平管道对气泡的成长有着显著的影响.与未加水平管道相比,床内由于水平管道的阻碍和干扰,气泡的生成和成长速度相对缓慢,气泡的形状在不同时刻也发生较大的变化.而在同一条件下仅气体的进口速度不同时,当床内存在管列,气体进口...     

Nucleation behavior and kinetics of single hydrogen nanobubble in ionic liquid system

Bubbles generated on the electrode surface have been considered one of important factors limiting the efficiency of hydrogen evolution reaction (HER). However, the bubble formation behaviors have not been clearly defined, especially nanobubbles in ionic liquid system. In this work, single H₂ nanobubbles generation characters are well investigated using platinum nanoelectrodes in ionic liquid (IL) during HER. The crucial factors (i.e., critical condition, geometry, and nucleation rapid of the H₂ nanobubbles) are quantitatively conducted from the voltammograms. The results reveal that the H₂ nanobubbles have a small contact angle (from 152° to 160°) and large bubble height (0.5–1.1 nm), which are favorable for subsequent detachment from nanoelectrode surface. Moreover, higher nucleation energy (7.5–15.3 kT) indicates that IL can suppress bubble formation. This work has gained a fundamental insight into the dynamic formation behaviors of H₂ nanobubble in IL, which can guide us to study H₂ bubbles in the HER influenced by various factors.     

Numerical study of bubble growth and wall heat transfer during flow boiling in a microchannel

A numerical study has been performed to analyze the wall heat transfer mechanisms during growth of a vapor bubble inside a microchannel. The microchannel is of 200 μm square cross section and a vapor bubble begins to grow at one of the walls, with liquid coming in through the channel inlet. The complete Navier–Stokes equations along with continuity and energy equations are solved using the SIMPLER method. The liquid vapor interface is captured using the level set technique. Experiments have been conducted to validate the numerical model. The bubble growth rate and shapes show good agreement between numerical and experimental results. The numerical results show that the wall heat transfer increases with wall superheat but stays almost unaffected by the liquid flow rate. The liquid vapor surface tension value has little influence on bubble growth and wall heat transfer. However, the bubble with the lowest contact angle resulted in the highest wall heat transfer.     

Flow pattern of boiling in micro-channel by numerical simulation

Boiling flows of R-134a and R-22 fluids in a 0.50 mm circular channel have been simulated to analyze bubbly flow, bubbly/slug flow, slug flow and slug/semi-annular flow depending on bubble evolution. The vapor–liquid interface was captured using VOF method. We studied the behavior of bubble growth and coalescence related to flow pattern transitions (bubbly/slug flow to slug flow, slug flow to slug/semi-annular flow) and analyzed the effect of fluid properties on transition lines. Some parameters, including heat flux, mass velocity, ONB point, vapor velocity, bubble lifting diameter, growth rate and generation frequency, have been analyzed in detail. The results show that bubble growth and coalescence are important factors for flow pattern transitions. The flow patterns at the micro-channel outlet predicted by simulation were in agreement with phenomena observed in experiments for bubbly/slug flow, slug flow and slug/semi-annular flow. In addition, the peak bubble frequency at the outlet was predicted and the general shape of the bubble frequency distribution at the outlet from simulation was found to be consistent with the achieved experimental results.     

Lattice Boltzmann simulation to study multiple bubble dynamics

Lattice Boltzmann method (LBM) has been used in this study to understand the behavior of bubble motion and bubble coalescence in liquids. Highly isotropic gradient vectors have been obtained on a lattice for two-phase simulations using LBM. For a fully periodic domain, bubble dynamics and shape for a single bubble and multiple bubbles are dependent on Eotvos number, Reynolds number and Morton number. For single bubble simulations, computations were done for high Eotvos and low to moderate Reynolds numbers, and the results are matched with the experimentally quantified flow visualization chart. The drag coefficient for single bubble motion under buoyancy for both two- and three-dimensional simulations compares well with existing correlations. For multiple bubbles, the bubble dynamics is dictated by the vortex pattern of the leading bubble, which allows the bubbles to coalesce. Coalescence can be described as a three stage process: collision; drainage of the liquid film between adjacent bubbles to a critical thickness; and rupture of this thin film of liquid. Such simulations have also been run for different configurations of the initial bubble distribution for both in-line and staggered bubble configuration to show the effect of vortex shedding on the oscillatory motion of the bubbles and subsequent coalescence.     

Behaviors of micro‐layer in micro‐channel boiling system applying laser extinction method

To elucidate the mechanism and characteristics of boiling heat transfer in a micro‐channel vaporizer, the experimental investigation of the micro‐layer thickness that formed between the heating surface and vapor generated was important. The micro‐layer thickness was measured applying the laser extinction method for channel gap sizes of 0.5, 0.3, and 0.15 mm. It was clarified that the gap size, the rate of bubble growth, and the distance from the incipient bubble site have an effect on the micro‐layer thickness in a micro‐channel boiling system. The initial micro‐layer thickness grew with an increase of the velocity of bubble forefront to moderate the value of the velocity. In the region of greater velocity, the thickness was constant for each gap. The distributions of the initial thickness of micro‐layer on the heat transfer surface were shown. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(1): 35–46, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20096     

Surface wettability control by nanocoating: The effects on pool boiling heat transfer and nucleation mechanism

Experiments were performed to highlight the influence of surface wettability on nucleate boiling heat transfer. Nanocoating techniques were used to vary the water contact angle from 20° to 110° by modifying nanoscale surface topography and chemistry. The bubble growth was recorded by a high speed video camera to enable a better understanding of the surface wettability effects on nucleation mechanism. For hydrophilic (wetted) surfaces, it was found that a greater surface wettability increases the vapour bubble departure radius and reduces the bubble emission frequency. Moreover, lower superheat is required for the initial growth of bubbles on hydrophobic (unwetted) surfaces. However, the bubble in contact with the hydrophobic surface cannot detach from the wall and have a curvature radius increasing with time. At higher heat flux, the bubble spreads over the surface and coalesces with bubbles formed at other sites, causing a large area of the surface to become vapour blanketed. The best heat transfer coefficient is obtained with the surface which had a water contact angle close to either 0° or 90°. A new approach of nucleation mechanism is established to clarify the nexus between the surface wettability and the nucleate boiling heat transfer.     

Theoretical model for fast bubble growth in small channels with reference to startup of capillary pumped loops used in spacecraft thermal management systems

A capillary pumped loop (CPL) is a closed two-phase loop in which capillary forces developed in a wicked evaporator passively pump the working fluid over long distances to dissipate heat from electronic and power sources. Because it has no moving parts and requires minimal power to sustain operation, the CPL is considered an enabling technology for thermal management of spacecraft. While the steady-state operation of a CPL is fairly well understood, its thermal response during startup remains very illusive. During the startup, initial vapor bubble growth in the evaporator is responsible for liquid acceleration that results in a differential pressure spike. A large pressure spike can deprime the evaporator by forcing vapor into the evaporator’s liquid-saturated wick, which is the only failure mode of a CPL other than fluid loss or physical damage to the loop. In this study, a numerical transient 3D model is constructed to predict the initial bubble growth. This model is used to examine the influence of initial system superheat, evaporator groove shape and size, and wick material. A simplified model is also presented which facilitates the assessment of parametric influences by analytic means. It is shown how these design parameters may be optimized to greatly reduce the bubble growth rate and therefore help prevent a deprime.