Increase In Superficial Liquid Velocity Research Articles

Characteristics of flow fields in the gas–liquid mini‐bubble columns with particle image velocimetry measurements

AbstractAs a new type of gas–liquid microreactors, the gas–liquid mini‐bubble column has potential applications. However, few studies on the flow fields in the mini‐bubble column can be found at present. In this work, particle image velocimetry (PIV) was used to visually study the velocity fields, vorticity fields and bubble dynamics in the gas–liquid mini‐bubble columns with column inner diameters of 1–3 mm and mini‐bubble diameters ranged from 0.7 to 1.3 mm. It is found that with the increase of superficial liquid velocity, bubbles rose from almost straight line to Z‐shaped or S‐shaped trajectory, and the bubble trajectory changed from one‐dimension to three‐dimension; when the bubble velocity changed, the bubble size and gas holdup decreased; bubble terminal velocity was controlled by bubble buoyancy and flow resistance, and increased slightly with bubble coalescence. These findings may provide basic reference for the design and scale‐up of such a mini‐bubble column reactor.

An Experimental Study of Pressure Drop Characteristics and Flow Resistance Coefficient in a Fluidized Bed for Coal Particle Fluidization

Liquid–solid fluidized beds have a wide range of applications in metallurgical processing, mineral processing, extraction, and wastewater treatment. Great interest on their flow stability and heterogeneous fluidization behaviors has been aroused in research. In this study, various fluidization experiments were performed by adjusting the operating conditions of particle size, particle density, and liquid superficial velocity. For each case, the steady state of liquid–solid fluidization was obtained, and the bed expansion height and pressure drop characteristics were analyzed. The time evolution of pressure drop at different bed heights can truly reflect the liquid–solid heterogeneous fluidization behaviors that are determined by operating conditions. With the increase in superficial liquid velocity, three typical fluidization stages were observed. Accordingly, the flow resistance coefficient was obtained based on the experimental data of bed expansion height and pressure drop. The flow resistance coefficient experiences a decrease with the increase in the modified particle Reynolds number and densimetric Froude number.

Wax Deposition Law under a Gas-Liquid Bubbly Flow Pattern.

Under the high-pressure and low-temperature environment of the deep sea, wax deposition will occur in the wellbore. This study discusses the effect of gas–liquid two-phase bubbly flow on wax deposition. In this study, wax deposition experiments of a gas–liquid two-phase flow in vertical pipelines were carried out using waxy white oil and air as the experimental medium. The results show that under the bubble flow pattern, within a short period of time from the start of wax deposition, it rapidly accumulates to the peak, then rapidly cuts off part of it, and then presents a simple harmonic dynamic trend until the final stability. The average wax-deposit thickness decreases first and then increases with the increase of superficial gas velocity, and it increases first and then decreases with the increase of superficial liquid velocity. The average wax-deposit thickness decreases with the increase of oil temperature. The average wax-deposit thickness was substantially constant and then decreases with the increase of ambient temperature. The wax-deposit thickness increases with the increase of wax content in oil flow. This research is useful for people to further understand the changes in the wax deposition process.

Evaluation of Liquid Film Thickness in Gas-Liquid Annular Flow in Horizontal Pipes Using Three Methods

Experimental investigations on annular flow film thickness were conducted using a closed-loop horizontal pipe with an internal diameter of 2-inch (0.0504m). The aim is to progress the understanding of such flow and facilitate the optimum design of hydrocarbon production systems were such flow is encountered. Liquid film thickness was extensively investigated using three methods: the conductance probe sensors installed at the bottom of the pipe, conductivity ring sensors and triangular relationship model. From these methods, liquid film thickness was proven to decrease with increase in superficial gas velocity, while increases with increase in superficial liquid velocity. In comparison, the predicted triangular relationship liquid film thickness matched better with the liquid film thickness obtained from conductance probe sensors at all the flow conditions in the experiments, while the conductivity ring sensor results matched closely at superficial liquid velocity of 0.0505m/s and 0.0714m/s but overestimated at superficial liquid velocity of 0.0903m/s and 0.1851m/s. This has shown the impact of high superficial gas velocity on conductivity ring sensors in accounting for liquid film thickness.

Experimental investigation of the surfactant effect on liquid removal in vertical pipes

In order to investigate the effect of surfactant on liquid removal in vertical pipes, an experimental study has been conducted in a 50 mm inner diameter vertical pipe. The superficial gas velocities ranged from 0.06 m/s to 27 m/s to include bubble, slug, churn, and annular flow. The superficial liquid velocities were 0.01 m/s and 0.1 m/s. The surfactant concentration was 2000 ppm. In the presence of surfactant, foaming postpones film reversal dramatically and makes the flow much more regular at low superficial gas velocities, due to the decrease of film density and increase of interfacial-friction factor. However, the slug-churn and bubble-slug transitions are barely influenced with surfactant injecting. At high superficial gas velocities, adding surfactant in the air-water flow creates an increased frictional pressure gradient, leading to an increase in the total pressure gradient. When the superficial gas velocity decreases to some point, there is a limit where the pressure gradients in the flow with and without surfactant are equal. A higher superficial liquid velocity leads to a smaller value for this equality. Thus, proper timing for surfactant injection is of significant importance in gas wells. As the superficial gas velocity decreases, the lifting efficiency of the surfactant increases at first and subsequently decreases dramatically, due to the interaction effect of liquid holdup and hydrodynamic agitation. An optimal area in lifting efficient is in the churn flow near the slug-churn transition. Furthermore, a decrease in lifting efficiency is observed with the increase of superficial liquid velocity and thus we infer that the transition to dispersed bubble flow is the limit for superficial liquid velocity to prevent foaming.

Investigation of slug-churn flow induced transient excitation forces at pipe bend

Numerical simulations of two-phase flow induced fluctuating forces at a pipe bend have been carried out to study the characteristics of multiphase flow induced vibration (FIV). The multiphase flow patterns and turbulence were modelled using the volume of fluid (VOF) method and the k−ϵ turbulence model respectively. Simulations of seventeen cases of slug and churn flows have been carried out showing the effects of superficial gas and superficial liquid velocities. The simulations results show good agreement of the volume fraction fluctuation frequencies of slug and churn flows with the reported experiment. In addition, the vibration characteristics of the excitation force have been accurately captured. The simulation results show that the predominant frequency of fluctuations of force decreases and the RMS of force fluctuation increases with the increase of superficial gas velocity. On the other hand, both predominant frequency and the RMS of force fluctuations increases with the increase of superficial liquid velocity. Increase of gas fraction narrows the range of frequency ranges, while increasing the liquid expands the frequency ranges of force fluctuations.

Experimental study on local interfacial parameters in upward air-water bubbly flow in a vertical 6 × 6 rod bundle

This paper presents a complete database of the local two-phase flow parameters for upward adiabatic air-water two-phase flows in a vertical 6 × 6 rod bundle flow channel. A total of 16 flow conditions were selected and investigated at 16 measuring points in the octant triangular measuring region of a cross section at the position with height-to-diameter ratio of 149 (z/DH = 149) in the rod bundle experiment. The local void fraction, interfacial area concentration (IAC), bubble diameter and bubble velocity vector were measured by using the four-sensor optical probe measuring technique. In the measured cross-sectional distributions of local void fraction and IAC, both core-peaking and wall-peaking distribution shapes were observed and the distribution pattern trended to change from the core-peaking shape to the wall-peaking shape with the increase of superficial liquid velocity (〈jf〉) and the decrease of superficial gas velocity (〈jg〉). The radial distributions of local bubble diameter are nearly flat, which explains the similarity of the radial local void fraction and IAC distributions. The measured local bubble diameters increase with the increase of 〈jg〉 and the decrease of 〈jf〉. The bubbles move along the rod bundle flow channel with their velocity component in the main flow direction (vgz) showing a typical radial core-peaking profile and their velocity components in the cross section showing a significant movement from the central region to the channel box wall region especially under high superficial liquid velocities. The cross-sectional area-averaged results of the local parameters were obtained by a measuring-point-based graphical integration. The obtained area-averaged void fraction and IAC are compared with the predictions of the drift-flux model with its available 2 frequently-used distribution parameter and drift velocity correlations and the 2 recently-developed IAC correlations respectively. The applicability of the 2 drift-flux correlations and the 2 IAC correlations to the rod bundle flow channel is evaluated and analyzed.

Modeling of co-current and counter-current bubble columns with an extended EMMS approach

Bubble columns can be operated in semi-batch, co-current or counter-current mode. This study attempts to extend the dual-bubble-size (DBS) model, viz., the energy-minimization multi-scale (EMMS) model for gas–liquid systems, to co-current and counter-current bubble columns. With the increase of superficial liquid velocity, the predicted total gas holdup and regime transition are in accordance with experiments. A DBS drag model was then developed and incorporated into the CFD simulation. The total gas holdup and local gas holdup radial distribution in both the co-current and counter-current bubble columns are well estimated. The findings in the present work demonstrate that developing closure laws through the stability condition is of great significance in modeling complicated multi-scale flow in bubble columns.

Effects of Liquid Velocity on Pressure Gradient, Slip and Interfacial Friction Factor in Annular Flow in Horizontal Pipe

Experimental investigations on annular flow behaviour in two-phase (air/water) flow in horizontal pipe were conducted using 2-inch (0.0504m) with a total length of 28.68m closed loop system. The emphasis from the experiments were on pressure gradient, slip and interfacial friction factor in annular flow. For interfacial friction factor, the entrainment, gas quality, the droplets and slip mixture density values were obtained through the experimental results which were substituted to determine it. In all, effects of liquid velocity were felt, as increase in superficial liquid velocity, increases the interfacial friction factor and pressure gradient in annular flow in horizontal pipes. More so, increase in superficial gas velocity, reduces the interfacial friction factor. Thus, interfacial friction factor decreases with increases in superficial gas velocity, while the pressure gradient increases with increase in superficial liquid velocity. The lower the superficial liquid velocity, the higher the slip but the lower the pressure gradient. Likewise, the lower the superficial liquid velocity, the more ripple waves obtained while the higher the superficial liquid velocity, the more disturbance waves in annular flow in horizontal pipe from the experiments.

Effects of Liquid Velocity on Pressure Gradient, Slip and Interfacial Friction Factor in Annular Flow in Horizontal Pipe

Experimental investigations on annular flow behaviour in two-phase (air/water) flow in horizontal pipe were conducted using 2-inch (0.0504m) with a total length of 28.68m closed loop system. The emphasis from the experiments were on pressure gradient, slip and interfacial friction factor in annular flow. For interfacial friction factor, the entrainment, gas quality, the droplets and slip mixture density values were obtained through the experimental results which were substituted to determine it. In all, effects of liquid velocity were felt, as increase in superficial liquid velocity, increases the interfacial friction factor and pressure gradient in annular flow in horizontal pipes. More so, increase in superficial gas velocity, reduces the interfacial friction factor. Thus, interfacial friction factor decreases with increases in superficial gas velocity, while the pressure gradient increases with increase in superficial liquid velocity. The lower the superficial liquid velocity, the higher the slip but the lower the pressure gradient. Likewise, the lower the superficial liquid velocity, the more ripple waves obtained while the higher the superficial liquid velocity, the more disturbance waves in annular flow in horizontal pipe from the experiments.

Settling/rising of a foreign particle in solid-liquid fluidized beds: Application of dynamic mesh technique

The modeling of moving objects has been the focus of many studies and has succeeded to attract sufficient attention by researchers. However, commonly used modeling approaches such as discrete element modeling (DEM), direct numerical simulations (DNS) lack simplicity and have been computationally intensive. In the present work, simple method of dynamic mesh in the framework of computational fluid dynamics has been employed. Eulerian-Eulerian simulations of a monodisperse solid-liquid fluidized bed (SLFB) have been carried out. A foreign particle (settling particle or rising bubble) was inserted in the system to study the effect of turbulence in SLFB on the motion of settling particle. The operating and geometrical parameters have been chosen based on the experiments performed by Ghatage et al. (2013). The results showed that the model can satisfactorily predict the settling velocity for low voidage fluidization in 2D as well as 3D simulations. Computational fluid dynamics (CFD) simulations at higher values of superficial liquid velocity showed liquid bubbles confirming the transition to heterogeneous regime. These liquid bubbles directed the settling particle to move zig-zag resulting in lower settling velocity. The size and number of the bubbles increase with an increase in the liquids velocity indicating increased heterogeneity. However, CFD predicted larger and higher number of bubbles than experimentally noted. This resulted in an increase in the deviation of predicted settling velocities from experimentally observed with an increase in superficial liquid velocity. In case of bubbles, it was observed that the dynamic mesh method is greatly dependent on the regime of operation in the column and works only in the range of low voidage when the fluidized bed is homogeneous and does not contain liquid bubbles.

Simulation of flow behavior of liquid and particles in a liquid–solid fluidized bed

Flow behavior of liquid and solid phases is simulated by means of DEM-CFD in a liquid–solid fluidized bed. The lubrication force is considered. A detailed description of the model equations used has been presented. The distributions of velocity and volume fraction are predicted at the different superficial liquid velocities, liquid viscosity and solids densities in the bed. The granular temperature is computed from simulated particle velocity. Predicted solid axial velocities are in agreement with experiments. Simulations indicate that axial velocities of particles increase with the increase in the superficial liquid velocity. The bed expansion height is increased with an increase of superficial liquid velocity and liquid viscosity and decreases with the increase of particle density. The lubrication force reduces granular temperature in the liquid–solid fluidized beds.

Prediction of gas holdup in a combined loop air lift fluidized bed reactor using Newtonian and non-Newtonian liquids

Many experiments have been conducted to study the hydrodynamic characteristics of column reactors and loop reactors. In this present work a novel combined loop airlift fluidized bed reactor was developed to study, the effect of superficial gas and liquid velocities, particle diameter, fluid properties on gas holdup by using Newtonian and non-Newtonian liquids. Compressed air was used as gas phase. Water, 5% n-butanol, various concentrations of glycerol (60 % and 80 %) were used as Newtonian liquids, different concentrations of Carboxy Methyl Cellulose (0.25 %, 0.6 % and 1.0 %) aqueous solutions were used as non-Newtonian liquids. Different sizes of Spheres, Bearl saddles and Raschig rings were used as solid phases. From the experimental results it was found that the increase in superficial gas velocity increases the gas holdup, but it decreases with increase in superficial liquid velocity and viscosity of liquids. Based on the experimental results a correlation was developed to predict the gas holdup for Newtonian and non-Newtonian liquids for a wide range of operating conditions at a homogeneous flow regime where the superficial gas velocity is approximately less than 5 cm/s.

Clustering behaviour in gas–liquid–solid circulating fluidized beds with low solid holdups of resin particles

AbstractThe flow in a gas–liquid–solid circulating fluidized bed is self‐organised and manifests itself with clustering of particles and bubbles. The clustering behaviour in the fluidized bed at low solid holdups of resin particles was experimentally investigated with a high‐speed image measurement and treatment technique of complementary metal oxide semiconductor to enhance the fundamental understanding on such a flow. Several new physical quantities were suggested to characterise such ordered flow structures. The main findings are as follows. The clusters of solid particles largely exist as doublets and triplets, the mixed groups of particles and bubbles mostly exist as one bubble carrying two to four particles. Increasing superficial liquid velocity, particle diameter or density weakens the aggregation degrees of both particle and mixed clusters in the riser and downer, except that the increase of superficial liquid velocity enhances the mixed clustering behaviour in the riser. The climbing of the auxiliary liquid velocity or liquid phase viscosity intensifies the aggregation behaviour, except that the increase of liquid phase viscosity reduces the mixed clustering degree in the riser. The influences of superficial gas velocity and surface tension of liquid phase on the clustering behaviour seem to be a little complex and the trends are not simply increasing or decreasing. The life cycle of solid particle clusters in the GLS riser is not sensitive to the operation conditions, being around 0.07 s. The mixed clusters' life cycle is more sensitive to the conditions and physical properties of phases, changing from 0.02 to 0.07 s.

Axial Mixing in a Novel Perforated Plate Bubble Column

This investigation reports the experimental and theoretical results carried out to evaluate the axial dispersion number for an air-water system in a novel hybrid rotating and reciprocating perforated plate bubble column for single phase and two phase flow conditions. Axial dispersion studies are carried out using stimulus response technique. Sodium hydroxide solution is used as the tracer. Effects of superficial liquid velocity, agitation level and superficial gas velocity on axial dispersion number were analyzed and found to be significant. For the single phase (water) flow condition, it is found that the main variables affecting the axial dispersion number are the agitation level and superficial liquid velocity. When compared to the agitation level, the effect of superficial liquid velocity on axial dispersion number is more predominant. The increase in superficial liquid velocity decreases the axial dispersion number. The same trend is shown by agitation level but the effect is less. The rotational movement of the perforated plates enhances the radial mixing in the section; hence, axial dispersion number is reduced. For the two phase flow condition, the increase in superficial liquid velocity decreases the axial dispersion number, as reported in the single phase flow condition. The increase in agitation level decreases the axial dispersion number, but this decreasing trend is non-linear. An increase in superficial gas velocity increases the axial dispersion number. Correlations have been developed for axial dispersion number for single phase and two phase flow conditions. The correlation values are found to concur with the experimental values.

Mass Transfer Studies in a Novel Perforated Plate Bubble Column

This investigation reports the experimental and theoretical results carried out to evaluate the volumetric mass transfer coefficient (kLa) in a novel hybrid rotating and reciprocating perforated plate bubble column. Countercurrent condition is performed. kLa is studied by the absorption of oxygen from air into deoxygenated water at room temperature (27 ± 1°C). Effects of agitation level, superficial gas velocity, superficial liquid velocity and plate spacing on kLa were analyzed and found to be significant. With an increase in agitation level at a constant superficial gas and liquid velocities, the breakage process of gas bubbles starts to be more pronounced and intensive oxygen mass transfer occurs. Hence, kLa increases sharply. kLa increases with an increase in superficial gas velocity, due to higher gas holdup and the enhanced breakup of bubbles. Similarly, kLa increases with an increase in superficial liquid velocity and the effect is found to be significant. When plate spacing is decreased (by increasing the number of plates), it is observed that the kLa increases at higher superficial gas velocity and agitation level. Correlation is developed for the determination of kLa and found to concur with experimental results. This correlation can be used for the determination of kLa for this hybrid column with 95% accuracy within the range of variables investigated in this present study.

An investigation on near wall transport characteristics in an adiabatic upward gas–liquid two-phase slug flow

The near-wall transport characteristics, inclusive of mass transfer coefficient and wall shear stress, which have a great effect on gas–liquid two-phase flow induced internal corrosion of low alloy pipelines in vertical upward oil and gas mixing transport, have been both mechanistically and experimentally investigated in this paper. Based on the analyses on the hydrodynamic characteristics of an upward slug unit, the mass transfer in the near wall can be divided into four zones, Taylor bubble nose zone, falling liquid film zone, Taylor bubble wake zone and the remaining liquid slug zone; the wall shear stress can be divided into two zones, the positive wall shear stress zone associated with the falling liquid film and the negative wall shear stress zone associated with the liquid slug. Based on the conventional mass transfer and wall shear stress characteristics formulas of single phase liquid full-pipe turbulent flow, corrected normalized mass transfer coefficient formula and wall shear stress formula are proposed. The calculated results are in good agreement with the experimental data. The shear stress and the mass transfer coefficient in the near wall zone are increased with the increase of superficial gas velocity and decreased with the increase of superficial liquid velocity. The mass transfer coefficients in the falling liquid film zone and the wake zone of leading Taylor bubble are lager than those in the Taylor bubble nose zone and the remaining liquid slug zone, and the wall shear stress associated falling liquid film is larger than that associated the liquid slug. The mass transfer coefficient is within 10−3 m/s, and the wall shear stress below 103 Pa. It can be concluded that the alternate wall shear stress due to upward gas–liquid slug flow is considered to be the major cause of the corrosion production film fatigue cracking.

Experimental study on hydrodynamic characteristics of upward gas–liquid slug flow

To clarify the impacts of the hydrodynamic boundary layer and the diffusion boundary layer in the near wall zone on gas–liquid two-phase flow induced corrosion in pipelines, the hydrodynamic characteristics of fully developed gas–liquid slug flow in an upward tube are investigated with limiting diffusion current probes, conductivity probes and digital high-speed video system. The Taylor bubble and the falling liquid film characteristics are studied, the effects of various factors are examined, and the experimental results are compared with the data and models available in literature. The length of Taylor bubble, the local void fraction of the slug unit and the liquid slug, the shear stress and mass transfer coefficient in the near wall zone, are all increased with the increase of superficial gas velocity and decreased with the increase of superficial liquid velocity, whereas the length of liquid slug and the liquid slug frequency are changed contrarily. The alternate wall shear stress due to upward gas–liquid slug flow is considered to be one of the major causes for the corrosion production film fatigue cracking. A normalized formula for mass transfer coefficient is obtained based on the experimental data.

Axial Liquid Dispersion Characteristics in Magnetically Stabilized Bed

Axial liquid dispersion was experimentally studied in liquid-solid and gas-liquid-solid magnetically stabilized beds using the ferromagnetic catalyst of SRNA-4 as the solid phase. The effects of operating factors and fluid characters, such as superficial liquid velocity, superficial gas velocity, magnetic field intensity, liquid viscosity and surface tension, on axial dispersion coefficients of liquid were investigated. The dispersion coefficients increased with the increase of superficial liquid velocity and superficial gas velocity, and decreased with the increase of liquid viscosity, liquid surface tension and magnetic field intensity. A correlation equation of Peclet number was obtained for both liquid-solid and gas-liquid-solid magnetically stabilized bed.