Gas entrainment behavior of vertical plunging liquid jets in terms of changes in jet surface length

For a vertical jet system using various liquids, the shape change of liquid jets discharged from nozzles having different nozzle aspect ratios was evaluated in terms of the surface length l_m of the jet flow, and the relation between the change of l_m and the gas entrainment behavior was investigated. It was found that the way of changing of l_m with varying operating conditions or the liquids corresponded well to that of changing of the gas entrainment rate Q_g, that is, the change of l_m of the jet before plunging could be closely related to the change of Q_g.     

Experimental Study on Bubble Rising and Descending Velocity Distribution in a Slurry Bubble Column Reactor

To determine bubble rising and descending velocity simultaneously, a BVW‐2 four‐channel conductivity probe bubble parameters apparatus and its analysis are used in gas‐liquid and gas‐liquid‐solid bubble columns. The column is 100 mm in internal diameter and 1500 mm in height. The solid particles used are glass beads with an average diameter of 17.82 μm, representing typical particle size for catalytic slurry reactors. The effects of superficial gas velocity (1.0 cm/s ≤ U_g ≤ 6.4 cm/s), solid holdup (0 % ≤ ?_s ≤ 30 %), and radial location (r/R = 0, 0.4, and 0.7) on bubble velocity distributions are determined. It is found that increasing U_g can increase the velocity of bubbles but do not exert much influence on bubble velocity distribution. Solid holdup mainly affects the distribution of bubble velocity while the radial direction affects bubble velocity distribution only slightly. The ratio of descending bubbles to rising bubbles increases from the bubble column center to the wall. It can be proved experimentally that large bubbles do not always rise faster than small bubbles at higher U_g (for example 6.4 cm/s).     

Bubble formation and breakup mechanism in a microfluidic flow-focusing device

The aim of this study is to investigate the bubble formation mechanism in a microfluidic flow-focusing device using a high-speed digital camera and a micro-particle image velocimetry (μ-PIV) system. Experiments were conducted in a PMMA square microchannel with 600 μm wide and 600 μm deep. Gas bubbles were generated in glycerol-water mixtures with several concentrations of surfactant sodium dodecyl sulfate (SDS). Various flow patterns were obtained at the cross-junction by changing gas and liquid flow rates. The formation mechanism of slug bubble at the cross-junction was investigated to gain insight into the effects of liquid and gas flow rates, and viscosity of the liquid phase on the breakup rate of the gas thread, and on the collapse time. The velocity fields in the liquid phase around the thread were determined by μ-PIV measurements. The experimental data of the breakup rate and the collapse time of the gas thread were described as a function of the liquid superficial velocity u_l, the ratio of the gas and liquid flow rates Q_g/Q_l and Reynolds number Re=ρul/μ.     

Flow characteristics of gas–liquid two phase plunging jet absorber–gas holdup and bubble penetration depth

A gas-liquid two phase plunging jet is formed through a gas sucking type multi-jet ejector nozzle. In this study, the effects of various conditions in the multi-jet ejector nozzle, the column diameter, and the liquid jet length on penetration depth of air bubblesl _B and gas holdup h_G in a gas-liquid two phase plunging jet absorber were studied experimentally. Consequently, empirical equations concerningl _B and h_G were obtained, respectively. These equations agree with the experimental data with an accuracy of ±20% forl _B and ±25% for h_G.     

Hydrodynamics in a jet bubbling reactor: Experimental research and mathematical modeling

The radial distribution of liquid velocity in the axial direction of a jet bubbling reactor has been measured by experimentation. Three different typical flow structures controlled by liquid jet, gas bubbling, and liquid jet coupled with bubbling are observed. A tank in series model is established on this basis. Calculated values in each region are in good agreement with measured values in jet, bubbling, and wall effect controlled areas. Axial flow rate, radial exchange rate, and jet controlled volume η are analyzed from energy input aspect under different u_g and u_j. Simulation results indicate that under the synergetic action of the liquid jet and gas bubbling effect, jet controlled area exhibits a “spindle” structure, and its size decreases with the increase of u_g. When gas input power occupies about 67% of total energy consumption, the best synergy of liquid jet and gas bubbling is obtained. © 2017 American Institute of Chemical Engineers AIChE J, 64: 1814–1827, 2018     

Transition velocities and phase holdups at minimum fluidization in gas–liquid–solid systems

Hydrodynamic experiments were performed using a 127‐mm diameter column with 3.2‐mm porous alumina, 3.3‐mm polymer blend, 5.5‐mm polystyrene and 6.0‐mm glass spheres, with water, aqueous glycerol solution and silicone oil as liquids, and air as the gas. The voidage at minimum fluidization fell initially to a minimum, then rose gradually with increasing superficial gas velocity, and was lower for three‐phase systems than for corresponding two‐phase (liquid–solid) fluidized beds. The compaction appears to be due to agitation by gas bubbles near the minimum liquid fluidization condition. The gas holdups agree reasonably well with the correlation of Yang et al. (1993). Curves of minimum liquid fluidization velocity, U_lmf, vs. superficial gas velocity, U_g always show U_lmf decreasing as U_g increases, initially in a concave‐downward manner, but sometimes concave‐upward.     

Gas holdup and gas entrainment of a plunging water jet with a constant entrainment guide

The gas holdup and gas entrainment of a plunging liquid jet with a gas entrainment guide in an air-water system was investigated. The measurement of the gas holdup was performed using an over-flow method. The turbulent jet velocity calculated on an inside nozzle diameter in the range from 4.4-26.5 m/s for this system has been used in our correlations. The gas holdup has been well correlated in terms of 1/H(v₀² + 2gH₁), H₁ d₀ and the gas entrainment in terms of 1/H_w(v₀² + 2gH₁), H₁, d₀. The jet power requirement was also obtained from experimental data.     

Consequences of Inclined and Dual Jet Impingement in Stagnant Liquid and Stratified Layers

Different configurations of bubble entrainment have been explored inside a liquid pool resulting from the impact of plunging jets. Impingement of an inclined jet and the subsequent asymmetries are studied in present effort. The aberrations from the symmetric case have been realized in the genesis of the entrainment, including the formation and collapse of the annular cavity developed immediately after the impact. Furthermore, the three-dimensional association of bubble population has been found to be skewed in the direction of inertia of liquid jets. The density of bubbles along a particular azimuthal plane has been tracked to quantify the intensity of entrainment in case of jet–jet interaction at different configurations. Toward the end, impingement of different fluid jets on a stratified kerosene–water layer is illustrated and complex interfacial shapes are described as an assemblage of emulsions, encapsulations, and liquid fingers. © 2018 American Institute of Chemical Engineers AIChE J, 65: 372–384, 2019     

Gas absorption in the plunging liquid jet reactor

A pseudo first-order gas absorption survey has been made of the contacting system formed when a coherent liquid jet plunges through an ambient reactive gaseous atmosphere into a bath of jet liquid. Using the hypochlorite ion catalysed reaction between pure carbon dioxide and a carbonate ion-bicarbonate ion buffer solution, the reactor has been found to be analogous to a gas sparged stirred tank contactor with the plunging jet acting as both the reactor agitator and gas bubble generator. Resolution of the gas—liquid interfacial area and rate of surface renewal absorption parameters was thus made possible. Specific interfacial areas in the range 20–110 m^?1 and rates of surface renewal in the range 40–160 sec^?1 for the subsurface reactor have been related to the plunging jet surface roughness and velocity or the entrainment rate of the plunging jet.     

Ozone Mass Transfer in the Mixing Zone of a Confined Plunging Liquid Jet Contactor

In a Confined Plunging Liquid Jet Contactor (CPLJC) a jet of liquid is introduced into an enclosed cylindrical column (downcomer) that generates fine gas bubbles that are contacted with the bulk liquid flow. The region where the liquid jet impinges the receiving liquid and expands to the wall of the downcomer is called the Mixing Zone (MZ). In the MZ, the energy of the liquid jet is dissipated by the breakup of the entrained gas into fine bubbles, and the intense recirculation of the two-phase mixture. The study presented here was undertaken to quantify the ozone-water mass transfer performance of the MZ through the determination of the volumetric mass transfer coefficient, k_La (s^?1), and to produce a model for predicting k_La based on the specific energy dissipation rate. It was found experimentally that k_La in the MZ increased with increasing superficial gas velocity. A maximum experimental k_La value of 0.84 s^?1 was achieved which compares well to other contactors used in water treatment. Such a large k_La value combined with the small volume of the reactor, favorable energy requirements and safety features of the system, suggests that the CPLJC provides an attractive alternative to conventional ozone contactors. The relatively large mass transfer rates were found to be a function of the high gas holdup and fine bubble size generated in the MZ, which results in an almost froth-like consistency. A model based on the specific energy dissipation rate of the water jet, E (kg · m^?1· s^?3), and MZ bubble size was used to predict k_La in the MZ. Using E, the number average bubble size was predicted which was then used to calculate the liquid phase mass transfer coefficient k_L. The bubble size was also used with the predicted mixing zone gas holdup to calculate the specific interfacial area, a (m^?1), which was then combined with k_L to determine a predicted value of k_La. The average deviation between experimental and predicted k_La was 6.2%.     

Gas holdup,bubble size distribution,and mass transfer in an airlift reactor with ceramic membrane and perforated plate distributor

The airlift reactor is one of the most commonly used gas–liquid two-phase reactors in chemical and biological processes. The objective of this study is to generate different-sized bubbles in an internal loop airlift reactor and characterize the behaviours of the bubbly flows. The bubble size, gas holdup, liquid circulation velocity, and the volumetric mass transfer coefficient of gas–liquid two-phase co-current flow in an internal loop airlift reactor equipped with a ceramic membrane module (CMM) and a perforated-plate distributor (PPD) are measured. Experimental results show that CMM can generate small bubbles with Sauter mean diameter d₃₂ less than 2.5 mm. As the liquid inlet velocity increases, the bubble size decreases and the gas holdup increases. In contrast, PPD can generate large bubbles with 4 mm < d₃₂ < 10 mm. The bubble size and liquid circulation velocity increase as the superficial gas velocity increases. Multiscale bubbles with 0.5 mm < d₃₂ < 10 mm can be generated by the CMM and PPD together. The volumetric mass transfer coefficient k_La of the multiscale bubbles is 0.033–0.062 s⁻¹, while that of small bubbles is 0.011–0.057 s⁻¹. Under the same flow rate of oxygen, the k_La of the multiscale bubbles increases by up to 160% in comparison to that of the small bubbles. Finally, empirical correlations for k_La are obtained.     

Physical modelling and similitude of air bubble entrainment at vertical circular plunging jets

When a plunging jet impinges into a pool of liquid, air bubble entrainment takes place if the inflow velocity exceeds a threshold velocity. This study investigates air entrainment and bubble dispersion in the developing flow region of vertical circular plunging jets. Three scale models were used and detailed air-water measurements (void fraction, bubble count rate, bubble sizes) were performed systematically for identical inflow Froude numbers. The results highlight that the modelling of plunging jet based upon a Froude similitude is affected by significant scale effects when the approach flow conditions satisfied We₁<1E+3, while some lesser scale effect was noticed for V₁/u_r<10 and We₁>1E+3. Bubble chord time measurements showed pseudo-chord sizes of entrained bubbles ranging from less than to more than with an average pseudo-chord size were between 4 and . However, bubble size data could not be scaled properly.     

Scalar mixing in turbulent concentric round jets

The scalar mixing field of a free, turbulent concentric round jet has been examined using marker nephelometry. The flow conditions included velocity ratios between the centre and annular jet of 0.188, 0.519 and 0.911. The correlation function between the concentration fluctuations in the two jet streams increased from –1.0 near the nozzle to + 1.0 further downstream indicating a tendency to complete mixing between the jets. The initial mixing behaviour between the jets was better for the lower velocity ratio between the jets (U_i/U_o = 0.188) although further downstream there was better transverse mixing between the jets with the higher velocity ratio (U_i/U_o = 0.911).     

Experimental Study of Air Entrainment Rates due to Inclined Liquid Jets

The air entrainment rate due to inclined liquid jet plunging into a pool was investigated experimentally. Three types of fluids with varying physical properties in terms of viscosity and surface tension were utilized. For the impinging jet test section, nozzles with different inner diameters were selected. The inclination angles and liquid jet velocities at the nozzle outlet were varied and the entrained air rate was measured by the soap meniscus method. Taking the falling velocity of the liquid jet as a characteristic velocity, it was found that the air entrainment rate under the present experimental condition largely depended on the Weber number. From the obtained database, a new empirical model dependent on the Weber number and Laplace length scale is proposed which is capable of predicting the air entrainment flow rate at a mean absolute relative deviation of 21.7 %.     

Mass transfer behaviour of gas-liquid jet loop reactors

A special type of jet loop reactor (JLR), designed for continuous operation and short residence times was investigated with regard to its mass transfer behaviour, described by the volumetric mass transfer coefficient k_La. The jet stream and superficial gas velocities are varied in two JLRs of different sizes, equipped with different nozzles. Fully desalinated water, 0.08 molar NaCI solution and solutions of different concentration of carboxymethyl cellulose (CMC) are used as the liquid phase. A steady-state physical method is employed to determine k_La: air oxygen is purged from the liquid phase by gaseous nitrogen. The measurements show that the reactor is characterized by high power density and high mass transfer performance. No limit of mass transfer capacity was observed in the chosen ranges of volumetric gas and liquid flow rates, i.e. at a given jet stream velocity, the relationship between k_La and the superficial gas velocity is nearly linear. The investigations show that the mass transfer contributed by the jet stream largely depends on liquid phase composition.     

Experimental and numerical characterization of viscous flow and mixing in an impinging jet contactor

The laminar flow in an impinging jet contactor is examined as a first step toward the development of new technology for fast mixing of viscous fluids. The flow, velocity, and stretching fields in an impinging jet contactor are quantified for low Reynolds number flow using three-dimensional numerical simulations and particle image velocimetry measurements. Computational and experimental velocity fields are in close agreement, as quantified by the velocity probability density functions. Two steady-state flow regimes are found to exist: for jet Reynolds numbers (Re_j) < 10, the jets do not impinge and the velocity field scales linearly with Reynolds number; for Re_j > 10, the jets begin to impinge and recirculation regions form above and below the impingement point. The magnitude of the rate-of-strain tensor is calculated as a function of Re_j. While areas of essentially zero stretching occupy most of the flow domain, very high rates of stretching occur at specific locations in the flow. The maximum and average rates of stretching in the contactor increase roughly linearly as a function of Reynolds number. Mixing simulations show that no mixing occurs for the steady flow in a symmetric-jet contactor. However, mixing is improved substantially by a slight modification of the impinging jet geometry that disrupts geometric symmetry.     

On the leakage flow around gas bubbles in slug flow in a microchannel

The leakage flow is that liquid does not push gas bubbles and leaks through the channel corners. This leakage flow was confirmed by tracking particles moving in the liquid film with a double light path method and was quantified by tracking the gas–liquid interface movement. The results show that leakage flow varies during bubble formation process. The average net leakage flow Q_net‐leak in a bubble formation cycle at T‐junction can be as large as 62.4% of the feeding liquid flow rate, depending on the liquid properties. Q_net‐leak for regular flow at main channel is much smaller, ranging from about 0 to 30% of the feeding liquid flow rate. The difference between the two leakage flows would lead to an increase in liquid slug length after generation. Finally, the effects of parameters such as phase flow rates, surface tension, and viscosity were investigated. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3964–3972, 2015     

Expansion characteristics of an anaerobic fluidized bed reactor with internal biogas production

More realistic dynamic bed‐expansion experiments using a three‐phase anaerobic fluidized bed reactor (AFBR) with and without internal biogas production were conducted for the establishment of correlation equations for the mean volume ratio of wakes to bubbles (k). A predictive model was also developed for the expansion characteristics of the three‐phase AFBR with internal biogas production. The predicted bed‐expansion heights (H_GLS) deviated by only ±10% from the experimental measurements for the three‐phase AFBR. According to the modeling results, if a three‐phase AFBR is loaded into a carrier with low specific gravity (dry density of carrier, ρ_md = 1.37 g cm^?3; wet density of carrier, ρ_mw = 1.57 g cm^?3) and operated at a high superficial liquid velocity (u_l = 4.0 cm s^?1), the ratio of H_GLS to H_LS at a high superficial gas velocity (u_g = 1.5 cm s^?1) can reach as high as 271%. A higher fluidized‐bed height has a greater effect on the bed‐expansion behavior because of the decrease in liquid pressure (surrounding gas bubbles) along the fluidized‐bed height. From parametric sensitivity analyses, H_GLS is most sensitive to the parameter reactor width (X), especially within a small ΔX/X₀ range of ±10%; sensitive to ρ_mw, diameter of the carrier, ρ_md and total mass of carrier and least sensitive to u_l, biofilm thickness and u_g. Copyright © 2005 Society of Chemical Industry     

The influence of system scale on impinging jet sediment erosion: Observed using novel and standard measurement techniques

Jet impingement as a method for eroding particulate beds and maintaining sediment in suspension is an important process for a host of industries, particularly in nuclear waste processing, where such systems to disperse and mix particulate beds have a number of advantages over other approaches. Existing work has utilised fairly rudimentary techniques for the measurement of erosion depths and here we demonstrate a new technique for measuring both static and dynamic erosion of cohesionless particulates under an impinging jet, using ultrasonic Doppler velocimetry. This approach is tested on both quartz sands and on a range of Mg(OH)₂ particulates that are key simulants for nuclear waste facilities, such as the Highly Active Storage Tanks at Sellafield, U.K. A critical jet height was found to exist that balanced the impingement velocities and total entrained jet volume to maximise erosion. The effect of system scale was also considered by normalising steady-state crater depths and sizes, with erosion being enhanced in the small scale, possibly due to increased turbulent recirculation. Additionally, velocity profiles and acoustic backscatter were used to determine both steady-state crater profiles and kinetic changes in bed-depths with time, and highlighted important differences between static and dynamic measurements of erosion depth.     

On air entrainment in a water pool by impingement of a jet

Air entrainment due to impingement of a water jet on a pool is studied extensively to understand the physics of the initiation and the cluster of bubbles formed below the free surface. Possible outcomes due to the jet impingement in a pool have been identified as smooth free surface without entrainment or formation of rigorous bubble cluster below the jet‐pool contact. Triangular entrained region is found to be a three‐dimensional association of disconnected bubble population continuously breaking and making with the neighbors. A correlation for prediction of maximum entrained height for a range of jet diameters and lengths is proposed. The trajectory of a single bubble is also studied to understand the kinematics of the bubble cluster. Alongside, an electrical conductivity probe has been used to examine the probabilistic presence of the bubble at a given depth in the liquid pool. © 2017 American Institute of Chemical Engineers AIChE J, 2017