Coalesced Bubble Regime Research Articles

Two-dimensional model of low-pressure PEM electrolyser: Two-phase flow regime, electrochemical modelling and experimental validation

Based on proton conduction of polymeric electrolyte membrane (PEM) technology, the Polymer Electrolyte Membrane Water Electrolyser (PEMWE) offers an interesting solution for efficient hydrogen production. During the electrolysis of water in PEMWE, water is split into oxygen, protons and electrons at the anode and a water-gas two-phase flow results. The aim of this study is to investigate the link between the two-phase flow at the anode side and cell performance under low-pressure conditions. We have developed a two-dimensional stationary PEMWE model that takes into account electrochemical reaction, heat transfer, mass transfer (bubble flow) and charge balance through the Membrane Electrodes Assembly (MEA). In order to take into account the changing electrical behaviour, our model combines two scales of descriptions: at microscale within anodic active layer and MEA scale. The water management at both scales is strongly linked to the Not Coalesced Bubble regime (NCB regime) or the Coalesced Bubble regime (CB regime). Therefore, water content close to active surface areas depends on two-phase flow regimes. Our simulation results demonstrate that the coalesced phenomenon is associated with improvement of mass transfer, a decrease in ohmic resistance and an enhancement of the PEMWE efficiency. At low and medium current density values, the model has been validated using two separate experiment electrolysis cells.

Experimental study of the transition between unsteady flow regimes in a partially aerated two-dimensional bubble column

The experiments performed in this article led to the identification of the characteristic flow patterns generated in a rectangular bubble column when a partially aerated plate is used. Considering the traditional time-averaged approach and based on the flow regimes identified by several authors, it is concluded that the flow regime existing at all values of the superficial gas velocity ( U G from 1.2 mm/s to 21.3 mm/s) is the coalesced bubble regime. Furthermore, homogeneous and turbulent regimes are not observed and the prevailing flow regime is the 3-region vortical regime. However, focusing on both the local and instantaneous values of the most characteristic flow variables, it is possible to identify different flow regimes from those previously described as a function of U G. Based on this approach, it is proposed for the first time the existence of three new flow patterns included in the generic vortical flow regime: the dispersed bubble vortical flow (DBVF), the transition vortical flow (TVF) and the fully developed vortical flow (FDVF). The main characteristics of these new flow regimes and the hydrodynamic variables that determine the transition between them are presented.

Prediction of The Kolmogorov Entropy Derived from CT Data in a Bubble Column Operated under The Transition Regime and Ambient Pressure

AbstractThe Kolmogorov entropy (KE) algorithm was successfully applied to single source γ‐ray Computed Tomography (CT) data measured by three scintillation detectors in a 0.162 m‐ID bubble column equipped with a perforated plate distributor (163 holes × ∅︁ 1.32 · 10–3 m). The aerated liquid height was set at 1.8 m. Dried air was used as a gas phase, while Therminol LT (ρL = 886 kg m–3, μL = 0.88 · 10–3 Pa s, σ = 17 · 10–3 N m–1) was used as a liquid phase. At ambient pressure, the superficial gas velocity, uG, was increased stepwise with an increment of 0.01 m s–1 up to 0.2 m s–1. Based on the sudden changes in the KE values, the boundaries of the following five regimes were successfully identified: dispersed bubble regime (uG < 0.02 m s–1), first transition regime (0.02 ≤ uG < 0.08 m s–1), second transition regime (0.08 ≤ uG < 0.1 m s–1), coalesced bubble regime consisting of four regions (called 4‐region flow; 0.1 ≤ uG < 0.12 m s–1), and coalesced bubble regime consisting of three regions (called 3‐region flow; uG > 0.12 m s–1). The KE values derived from three scintillation detectors in the first transition regime were successfully correlated to both bubble frequency and bubble impact. The latter was found to be inversely proportional to the bubble Froude number. The KE model implies that the bubble size in this particular flow regime is a weak function of the orifice Reynolds number (db = 7.1 · 10–3Re0–0.05).

Flow Regime Identification in a Bubble Column Based on Both Statistical and Chaotic Parameters Applied to Computed Tomography Data

AbstractThe Kolmogorov entropy (KE) algorithm was applied successfully to single source γ‐ray Computed Tomography (CT) data measured in a 0.162 m ID bubble column equipped with a perforated plate distributor (163 holes · ∅︁ 1.32 mm). Dried air was used as the gas phase and Therminol LT (ρL = 886 kg m–3, μL = 0.88 · 10–3 Pa s, σ = 17 · 10–3 N m–1) was used as a liquid phase. Three different pressures, P, of 0.1, 0.4, and 1.0 MPa were examined. At each pressure the superficial gas velocity, uG, was increased stepwise by steps of 0.01 m s–1 up to 0.2 m s–1. The average absolute deviation (AAD) was also used as a robust statistical criterion for regime transition. At all three pressures, based on the sudden changes in both the AAD and KE values, the boundaries of the following five regimes were identified: dispersed bubble regime, first and second transition regimes, coalesced bubble regime consisting of four regions (called 4‐region flow), and coalesced bubble regime consisting of three regions (called 3‐region flow). The existence of these regimes has already been documented. As the pressure increases, the transition velocity between the dispersed bubble and first transition regimes and the transition velocity between coalesced bubble (4‐region flow) and coalesced bubble (3‐region flow) regimes shift to higher uG values. On the other hand, at P = 0.4 MPa the second transition regime starts earlier. In addition, at P = 1 MPa the transition to coalesced bubble (4‐region flow) is delayed.

Enhancements of Nucleate Boiling and Critical Heat Flux Under Microgravity Conditions

Two means are presented for enhancing nucleate boiling and critical heat flux under microgravity conditions: using microconfigured metal-graphite composites as the boiling surface and using dilute aqueous solutions of longchain alcohols as the working fluid. In the former, thermocapillary force induced by the temperature difference between the graphite-fiber tips and the metal matrix plays an important role in bubble detachment. Thus, boiling heat transfer performance does not deteriorate in a reduced-gravity environment. In the latter case, the surface-tension-temperature gradient of the long-chain alcohol solutions turns positive as the temperature exceeds a certain value. Consequently, the Marangoni effect does not impede, but rather aids in bubble departure from the heating surface. This feature is most favorable in microgravity conditions. As a result, the bubble size of departure is substantially reduced at higher frequencies. Based on the existing experimental data, and a two-tier theoretical model, correlation formulas are derived for nucleate boiling on the copper-graphite and aluminum-graphite composite surfaces, in both the isolated and coalesced bubble regimes. In addition, performance equations for nucleate boiling and critical heat flux in dilute aqueous solutions of long-chain alcohols are obtained.

Effects of macroscopic hydrodynamics on heat transfer in bubble columns

The hydrodynamics and heat transfer of two-dimensional bubble columns operated in various flow regimes are simultaneously studied using particle image velocimetry and heat transfer probe, respectively. With increasing gas velocity, the flow structures change from dispersed bubble regime to coalesced bubble regime divided into the 4- and 3-region flows. At low gas velocity (<1 cm/ s) , the main flow structure is the dispersed bubble regime with the uniform distributions of gas holdup, average bubble size, liquid velocity, and heat transfer in the radial direction except near sidewalls. For gas velocity between 1 and 3 cm/ s , the 4-region flow regime begins to take over with the bubble coalescence and interaction in the flow. Macroscopic structures operated in the 4-region flow regime comprise descending, vortical, fast bubble, and central plume regions. In this regime, the fast bubble region has the maximum gas holdup, average bubble size, liquid velocity as well as heat transfer rate due to bubble inducing the strongest turbulent intensity. The vortex in the vortical flow region acts like a close cell to prevent interacting with its surrounding, and leads to unfavorable heat transfer. The heat transfer in the central plume region is better than that in the dispersed bubble regime because it has higher gas velocity to generate larger bubbles from the injector. When gas velocity is over 3 cm/ s , the regime becomes the 3-region flow with a large fast bubble stream, no central plume. The path of the center of descending vortex has the minimum heat transfer rate. On the whole, the 3-region flow regime has the highest heat transfer rate in all three regimes. It is found that the flow and heat transfer are profoundly dominated by the macroscopic hydrodynamics structures.

Gas disengagement technique in a slurry bubble column operated in the coalesced bubble regime

The gas disengagement technique is discussed in detail and is applied in a two-dimensional bubble column and slurry bubble column to obtain the bubble size distribution. Flow characteristics including velocities of each phase and dispersed phase size distribution during the gas disengagement process are obtained by using an advanced image analysis tool including a particle image velocimetry technique. It is shown that in the coalesced bubble regime, the bubble and flow characteristics during the gas disengagement process are complex, i.e., there are strong bubble–bubble interactions as well as interactions between the liquid flow and bubbles. Several different analogies of the disengagement processes assuming a bimodal bubble size distribution are discussed in terms of the behavior of large and small bubbles and their influence over one another. The small bubbles, which disengage within the fast bubble flow region, must be taken into account to evaluate the bimodal bubble size distribution. The applicability of the gas disengagement technique to obtain a multimodal bubble size distribution is also elaborated upon.

Effect of Clearance between Two Sensor-Probes on Measured Values of Bubble Properties in a Bubble Column.

The effect of probe clearance on the rising velocity and chord length of bubbles in a bubble column was examined in measurements with the two-channel U-shaped optical fiber probe method. The probe clearances ranged from 0.2 to 1.5 cm. A few restrictions required in the signal processing to obtain the reasonable data. With increasing probe clearance, the observed values of the bubble velocities slightly increase; however, those of the bubble chord lengths drastically increase to 160% in the coalesced bubble regime, though they are almost constant in the dispersed bubble regime. A simple theory is proposed considering the probability of bubbles contacting with the probes, and used to successfully explain the experimental results.

High‐pressure three‐phase fluidization: Hydrodynamics and heat transfer

AbstractThe phase holdups and the heat‐transfer behavior were studied experimentally in three‐phase fluidized beds over a pressure range of 0.1–15.6 MPa. Bubble characteristics in the bed are examined by direct flow visualization. Pressure effects on the bubble coalescence and breakup are analyzed mechanistically. The study indicates that the pressure affects the hydrodynamics and heat‐transfer properties of a three‐phase fluidized bed significantly. The average bubble size decreases and the bubble‐size distribution becomes narrower with an increase in pressure. The bubble‐size reduction leads to an increase in the transition gas velocity from the dispersed bubble regime to the coalesced bubble regime, an increase in the gas holdup, and a decrease in the liquid and solids holdups. The pressure effect is insignificant above 6 MPa. The heat‐transfer coefficient between an immersed surface and the bed increases to a maximum at pressure 6–8 MPa and then decreases with an increase in pressure at a given gas and liquid flow rate. This variation is attributed to the pressure effects on phase holdups and physical properties of the gas and liquid phases. A mechanistic analysis revealed that the major heat‐transfer resistance in high‐pressure three‐phase fluidized beds resides in a liquid film surrounding the heat‐transfer surface. An empirical correlation is proposed to predict the heat‐transfer coefficient under high‐pressure conditions.

Performance of a three‐phase fluidized bed as a reactive distillation device

AbstractA new reactive distillation device, the multistage gas/liquid/solid three‐phase fluidized bed, has been developed. The flow regimes of the multistage three‐phase fluidized bed have been studied and the regimes can be divided into the liquid leakage regime, the dispersed bubble regime, and the coalesced bubble regime. Liquid velocity has a much smaller effect on phase holdups in this device than in conventional three‐phase fluidized beds. The three phase fluidized bed is used as a reactive distillation device for the hydrolysis of methyl acetate. Much higher reaction conversion than the equilibrium value and high catalyst‐contacting efficiency are obtained. Different methods of feeding the water into the reactive distillation section are studied.

Quantitative analysis and computation of two‐dimensional bubble columns

AbstractExperiments conducted quantify the macroscopic hydrodynamic characteristics of various scale 2‐D bubble columns, which include dispersed and coalesced bubble regimes characterized by two flow conditions (4‐ and 3‐region flow) with coherent flow structures. Hydrodynamic behavior is analyzed based on flow visualization and a particle image velocimetry (PIV) system. Columns operated in the 4‐region flow condition comprise descending, vortical, fast bubble and central plume regions. The fast bubble flow region moves in a wavelike manner, and thus the flow in the vicinity of this region is characterized macroscopically in terms of wave properties. In columns greater than 20 cm in width, the transition from the dispersed bubble flow regime to the 4‐ and then to 3‐region flow in the coalesced bubble regime occurs progressively with gas velocities at 1 and 3 cm/s, respectively. The demarcation of flow regimes is directly related to measurable coherent flow structures. The instantaneous and time‐averaged liquid velocity and holdup profiles provided by the PIV system are presented in light of the macroscopic flow structure in various 2‐D bubble columns. Numerical simulations demonstrate that the volume of fluid method can provide the time‐dependent behavior of dispersed bubbling flows and account for the coupling effects of pressure field and the liquid velocity on the bubble motion. Comparison of computational results with PIV results for two different bubble injector arrangements is satisfactory.

Transient flow structure in the entrance region of a bubble column using particle image velocimetry

The transient flow phenomena in the entrance region of a three-dimensional bubble column system were investigated over the dispersed bubble and coalesced bubble regimes. Two different gas distributors (porous plate and perforated plate) were studied to elucidate their effects on the overall hydrodynamic behavior of the system and on the hydrodynamic behavior of the entrance region. The instantaneous and averaged flow information was obtained through the use of a Particle Image Velocimetry (PIV) system. The PIV system is a non-intrusive technique capable of simultaneously providing the full-field flow information of both the gas and liquid phases. The overall gas holdup of the bubble column system was used to demarcate the flow conditions, in order of increasing gas velocity, into the dispersed bubble regime and the coalesced bubble regime which consisted of the vortical—spiral and turbulent flow conditions. The instantaneous flow fields and averaged liquid velocity and gas holdup profiles obtained through the PIV system provide a better understanding of the mechanisms responsible for the development of the flow in each regime. The averaged results were also used to determine the length required for the flow to become fully developed. The results show that the entrance effect are significant in the dispersed bubble regime but diminish with increasing velocity in the coalesced bubble regime, represented by the vortical—spiral and turbulent flow conditions.

HEAT TRANSFER AND HYDRODYNAMIC INVESTIGATIONS IN A BAFFLED BUBBLE COLUMN: AIR-WATER-GLASS BEAD SYSTEM

Measurements of heat transfer coefficient between an immersed surface and surrounding two- and three-phase dispersions have been conducted as a function of gas velocity in a cylindrical bubble column equipped with either a single axial tube or a seven-tube bundle at 309 K. Gas holdup is also measured under the same conditions. In particular, the systems that have been investigated are air-water and air-water-glass beads. For the latter system, glass beads of three different sizes (50-143 μm) have been used and slurry concentration has been varied up to thirty percent by weight. It is observed that the seven-tube bundle configuration leads to higher heat transfer coefficient and gas holdup values over the entire air velocity range than the single axial tube arrangement. The influence of particle size and slurry concentration is nominal at least as long as it is less than ten weight percent. The presence of solids in the column lowers the gas holdup and raises the heat transfer coefficient. These qualitative variations have been discussed in terms of the bubble dynamics, and liquid or slurry movement in the bubble column that prevails as discrete bubbling, bubble coalescing and coalesced bubble regimes are encountered by increasing the air velocity. Gas holdup and heat transfer coefficient data have been correlated

Hydrodynamics of gas‐liquid‐solid annular fluidization

AbstractIn a strict sense, operation of the three‐phase ebullated bed reactor used in the H‐Oil process is not characterized by conventional three‐phase fluidization, but by annular three‐phase fluidization. In annular three‐phase fluidized beds differences in the hydrodynamic behavior over that found in conventional beds occur through changes in the bubble behavior. The addition of a concentrically located inner column in a gas‐liquid‐solid fluidized bed promotes bubble coalescence. This increase in coalescence can be observed either through changes in the liquid velocity of transition from the coalesced bubble regime to the dispersed bubble flow regime or through changes in holdup behavior. A hydraulic diameter defined for the annulus region proves suitable when comparing different annular systems having the same outside column diameter, but unsuitable when comparing systems having different outside column diameters.

Hydrodynamic behaviour of a draft tube gas-liquid-solid spouted bed

Experiments were conducted using various types of solid particles to investigate the hydrodynamic properties of a gas-liquid-solid spouted bed with a draft tube. The hydrodynamic properties under study include flow modes, pressure profile and pressure drop, bubble penetration depth, overall gas holdup, apparent liquid circulation rate and bubble size distribution. Three flow modes were classified: a packed bed mode, a fluidized bed mode and a circulated bed mode. It was found that the friction factor accounting for the friction loss in the bed varies linearly on a logarithmic scale with the Reynolds number defined based on the apparent liquid circulation rate. The bubble penetration depth in the annular region, overall gas holdup and apparent liquid circulation rate increase with an increase in gas or liquid velocity. At high gas flow conditions an optimal solids loading exists which yields a maximum apparent liquid circulation rate. A model was proposed to describe the liquid circulation behaviour in the draft tube three-phas spouted bed. The average bubble size in the draft tube region is higher than that in the annular region for both the dispersed bubble regime and the coalesced bubble regime in the draft tube region.