High Gas Superficial Velocities Research Articles

Experimental Investigation of Film Thickness in Wastewater Airlift Pumps by an Image Processing Method

The airlift pump is a key part of wastewater treatment and is employed as an innovative and feasible collection tool. However, as one of the key factors in the lifting capability of airlift pumps, film thickness in the gas–liquid two-phase flow operating in pumps is still an unknown topic because it is difficult to measure. This paper proposes a visualization method for measuring film thickness and investigates the film thickness when operating under gas flow with a high rate in airlift pumps using experiments. Firstly, a simulation experiment platform was built, and the images of the film structure were acquired by a high-speed camera. Then, image-processing technology and an image distortion correction were proposed to extract the gas–liquid interface for studying the thickness of the film. The experimental results demonstrated that a large film thickness ranging from 0.15 D to 0.24 D was found in airlift pumps and that its film thickness kept a constant value, even under a high gas superficial velocity, maintaining a large output liquid flow from airlift pumps. As wastewater was carried by wastewater treatment, a larger film thickness of the annular film will benefit the high lifting rate of wastewater. The works in this paper offer valuable insights for the higher performance of working airlift pumps and wastewater treatment efficiency.

Experimental measurement of bubbly two-phase flow in a 4x4 rod bundle with wire mesh sensor

• Bubbly flow in a 4x4 rod bundle is measured with wire mesh sensor to investigate the effect of mixing-vane spacer grid on void fraction distribution and bubble size. • Effects of bubble injection size on void fraction can hardly be eliminated in the bare rod bundle even when the two-phase flow becomes fully developed. • Bubble entrapment into the swirl core at the tips of mixing vanes produces high localized void fraction and enhances bubble interaction. • Effects of bubble injection size can be eliminated with mixing-vane spacer grid, especially for those with relatively small bubble size. In order to investigate two-phase bubbly flow through fuel assembly of pressurized water reactor (PWR), the wire mesh sensor (WMS) is employed to measure air-water flow in a up-scaled 4x4 rod bundle based on PWR fuel assembly. Measurement is carried out in the bare rod bundle and downstream of mixing-vane spacer grid. Air bubbles are injected equally into 16 subchannels at the inlet with a standalone bubble generator which adjusts the injected bubble size. Distribution of void fraction and bubble size across the rod bundle are derived based on the WMS data. In the bare rod bundle, small-size bubbles (<3mm) are inclined to coalesce and medium-size bubbles (3∼8mm) tends to break up at low gas superficial velocity, while at high gas superficial velocities bubble coalescence produces large-size bubbles (>8mm). High liquid superficial velocity reduces the bubble size. Void fraction distribution of bubble size groups shows that bubbles with diameter less than 4mm mainly reside near the solid walls, while large bubbles with diameter larger than 6mm mainly appear in the subchannel center, which is consistent with Tomiyama’s lift force correlation. The effect of injected bubble size can hardly be eliminated along with flow development in bare rod bundles. When installed with mixing-vane spacer grid, the vortices produced at the tips of mixing vanes induce bubble entrapment into the swirl core, which significantly enhances the interaction between bubbles, as well as the interphase interaction. Such enhancement is even more remarkable in the low-void-fraction cases. As a consequence, the bubble size distribution downstream of the spacer grid shows relatively weak dependence on the injected bubble size. Besides, the vortices lead to strong inter-subchannel void drift of small-size bubbles, which results in void fraction difference between subchannels. The effects of mixing-vane spacer grid decay comparatively fast in the high void fraction cases.

PENGARUH TEGANGAN PERMUKAAN PADA TEBAL FILM CAIRAN DAN KARAKTERISTIK GELOMBANG ANTARMUKA ALIRAN DUA FASE STRATIFIED HORIZONTAL

This experimental study aims to examine the effect of the surface tension on the characteristics of unidirectional air-liquid interface waves on a horizontal pipe. The pipe used is an acrylic pipe with a diameter of 26 mm. To vary the surface tension, the working fluid consisted of 100% water and water mixed with 2% and 5% butanol. with surface tension (σ) values ​​of 72, 58, and 42.5 mN/m, respectively. The wave characteristics studied in this research include the interface wave structure which is analyzed from visual observations and parameters by image processing. The superficial velocities of gas and liquid were varied in the range of 4-16 m/s and 0.02-0.1 m/s, respectively. Wave velocity and frequency were determined by using cross-correlation and power spectra density (PSD) from time-series data of liquid film thickness. Generally, velocity increases as the gas superficial velocity and surface tension decrease, the high gas superficial velocity the dominant frequency is high.

Experimental investigation on flow-induced vibration of a flexible catenary riser conveying severe slugging with variable gas superficial velocity

A flexible catenary riser conveying gas–liquid mixture may experience the severe slugging induced vibration (SSIV) within a specific gas and liquid superficial velocity range. In this work, the SSIV of a flexible catenary riser with aspect ratio of 200 and a horizontal upstream pipeline was experimentally investigated to shed some light on the nonlinear multiphase flow-induced vibration and the correlation with the dynamic behaviour and the flow evolution. The non-intrusive optical measurement with high-speed cameras was employed to capture the vibration displacements of evenly distributed markers along the riser as well as the flow characteristics of gas–liquidmixture in the riser. The first and second types of severe slugging (SSI and SSII) were observed in the standard case (vsg = 0.177 m/s and vsl = 0.094 m/s) with an occurrence frequency ratio of 1:3. Large amplitudes are excited in the slug formation and gas blowout stages, and the latter presents more violent vibration with a faster travelling waves’ propagation speed. When the gas superficial velocity increases while maintaining the liquid superficial velocity, the transition stage is significantly elongated, characterized by the alternate appearance of partially filled gas–liquid mixture and short liquid slugs. Nevertheless, each liquid slug, no matter it is long or short, makes its contribution to the vibration response, presenting the coincidence of the recurrence frequency and the vibration frequency. The response amplitude is associated with the liquid slug length as well as the location of liquid inventory. The contributions of the three regimes (SSII, short liquid slugs and partially filled mixtures) are mainly associated with their occupied time. The severe slugging is the predominant contributor at vsg≤ 0.708 m/s, while its contribution is negligible at higher gas superficial velocities.

Investigation of mass transfer characteristics under turbulent condition in adsorption separation process for CO2 capture

Turbulence caused by high superficial gas velocity has significant impact on the CO2 capture process by adsorption. However, the poor consideration of turbulent diffusion in existing methods limits the further analysis of mass and heat transfer in CO2 adsorber. In this study, a novel computational fluid dynamics (CFD) model was introduced for CO2 capture with fixed bed, which is based on the methodology of computational mass transfer (CMT)[1]. Compared with traditional CFD models, it rigorously considers turbulent diffusion and can more accurately predict the mass and heat transfer behavior of CO2 capture processes at high gas superficial velocity while reducing computer resources. Through the comparison of simulation with breakthrough experiment, the feasibility of proposed approach was verified. In addition, the simulated results indicated that the turbulent mass diffusion causes a more uniform distribution of CO2 in the radial direction of adsorption bed. As Reynolds number decreases, turbulent mass diffusivity gradually weakens, and it could be neglected when the Reynolds number is below 10. Besides, adsorbent size and adsorber size was found to have significant influences on turbulent mass diffusion. They affect turbulent diffusion by changing the condition of fluid flow, then having influences on the mass transfer characteristics in CO2 capture process. Our results can provide a valuable guide for the structural design and optimization of industrial CO2 adsorber.

Gas–liquid–hydrate flow characteristics in vertical pipe considering bubble and particle coalescence and breakage

The gas–liquid flow behavior laden by hydrate particles is significant for the flow assurance of submarine natural gas hydrate production. An integrated multiphase flow model considering the coalescence and breakage of bubbles and hydrate particles was developed to investigate the gas–liquid–hydrate flow problem in a vertical pipe. The population balance theory was adopted to describe the evolution of the bubble and hydrate particle sizes in a flow field. The effects of particle behavior on flow pattern transition were analyzed. The results indicated that the phase distribution became inhomogeneous during the flow process, and bubbles tended to aggregate at the central region of the pipe. Most hydrate particles agglomerated in the liquid film region, and the particle size increased. The coalescence and breakage of bubbles induced a flow pattern transition from a bubbly to slug and churn flows. However, the agglomeration of hydrate particles increased the viscosity of the liquid phase, further reduced the turbulent intensity, and enhanced the stability of the bubbles. Thus, a higher gas superficial velocity was required for the formation of slug and churn flows. An annular flow was easier to form because the hydrate particles entrained in the liquid phase stabilized the liquid film. This study contributes to the flow assurance of submarine natural gas hydrate production and relevant multiphase flow modeling studies with hydrate particles.

Void Fraction and Flow Pattern of R-290 in A Horizontal Evaporator: Effects of Pipe Diameter and Cooling Capacity

Void fraction and flow pattern play important roles in the performance of evaporator in an air conditioning system. In this paper, the behaviors of void fraction and flow pattern of refrigerant R-290 in a horizontal evaporator are discussed. To simplify the analysis, the evaporator was divided into 10 segments. The void fraction was calculated based on the refrigerant flow quality and the flow pattern was determined on the basis of superficial gas and liquid velocity plotted in the flow pattern map. The calculation was carried on air conditioning machines with nominal capacities of 2.64 kW, 3.62 kW, and 5.28 kW using the evaporator pipe diameters of 3/8 in and 5/16 in. Generally, the lower evaporating temperature results in the higher void fraction, higher gas superficial velocity, and lower liquid superficial velocity. For all ranges of evaporator diameter and cooling capacity, annular flow is found to be the dominant flow pattern. The wavy flow is only found in the lower cooling capacity and larger evaporator diameter. Meanwhile, the slug flow occupies 16.7% to 25.8% of evaporator segment.

Computational Modeling of the Impact of Solid Particles on the Gas Hold-up in Slurry Bubble Columns

Three-phase bubble column reactors, despite their broad diffusion in chemical process, are often difficult to describe given the intricate mutual phase interactions. Moreover, validation of computational models is likewise complex, since it is considerably challenging to obtain exhaustive and precise experimental data from these systems. The aim of this work is to predict reliably the effect of disperse solid particles on the overall fluid dynamics of a bubble column by computational fluid dynamics.The small size of particles allowed us to approximate the solid-liquid mixture as a single pseudo-homogeneous phase in the Euler-Euler framework. The simulation settings were tuned on two-phase gas-liquid systems and then extended to the three-phase gas-solid-liquid columns. Experimental data from different setups and correlations for the global gas hold-up in slurry systems were used to validate the model. In particular, what emerges from experiments is that the presence of the solid reduces gas hold-up, as a consequence of the higher density of the slurry in comparison to the pure liquid and the increased coalescence of the bubbles.Results show that this model is able to predict the reduction of the hold-up with a solid volumetric loading up to 20%, achieving good agreement with experimental data. Moreover, what stands out from the simulations is that the addition of the solid also changes the shape of the flow map curve (global hold-up as a function of the gas superficial velocity), switching to a smoother transition between the behavior at low and high gas superficial velocity.

Experimental Analysis of Gas\u2013Liquid\u2013Solid Three-Phase Flows in Horizontal Pipelines

The dynamics of three-phase flows involves phenomena of high complexity whose characterization is of great interest for different sectors of the worldwide industry. In order to move forward in the fundamental knowledge of the behavior of three-phase flows, new experimental data has been obtained in a facility specially designed for flow visualization and for measuring key parameters. These are (1) the flow regime, (2) the superficial velocities or rates of the individual phases; and (3) the frictional pressure loss. Flow visualization and pressure measurements are performed for two and three-phase flows in horizontal 30 mm inner diameter and 4.5 m long transparent acrylic pipes. A total of 134 flow conditions are analyzed and presented, including plug and slug flows in air–water two-phase flows and air–water-polypropylene (pellets) three-phase flows. For two-phase flows the transition from plug to slug flow agrees with the flow regime maps available in the literature. However, for three phase flows, a progressive displacement towards higher gas superficial velocities is found as the solid concentration is increased. The performance of a modified Lockhart–Martinelli correlation is tested for predicting frictional pressure gradient of three-phase flows with solid particles less dense than the liquid.

Assessment of the Resilience against Liquid Maldistribution of Monolith Packings under Offshore Floating Conditions

The study provides insights for adapting commercially available packings to work on floating vessels in the marine context aiming at preserving performances similar to land-based packed-bed reactors. Specifically, liquid distribution for cocurrent gas-liquid downflow (Ug = 0.0065-0.131 m/s and Ul = 0.001-0.0078 m/s) inside monolith packings embarked on a hexapod emulator was compared against glass-bead containing trickle bed and open-cell foam packings using wire-mesh sensors under vertical, inclined and rolling column conditions. Sensitivity of the tested internals with regard to liquid maldistribution was diagnosed and analyzed in terms of a maldistribution factor for the vertical/inclined and rolling columns. Liquid maldistribution was found to prevail at high gas and low liquid superficial velocities for the tested three packings regardless of column conditions. Monolith packed beds demonstrated superior robustness to counteract liquid maldistribution induced by bed static inclinations and roll motion...

Population balance modeling for air–water bubbly flow in a vertical U-bend

Bubbly flow in U-bend is widely encountered in two-phase flow systems because of its compactness and high heat transfer coefficient. The modeling of phase distributions, velocity fields, and interfacial area concentration in the U-bend is crucial for the analysis of mass, momentum, and energy transportation processes in the equipment. However, this subject has not received enough attention yet. In this paper, the combination of population balance model and two-fluid model was used in the simulation of air–water bubbly flow in a U-bend with 24 mm inner diameter and 96 mm curvature. The homogeneous multiple size group model was used to solve the population balance equation and reconstruct the bubble size distribution function. The phase distribution at 0°, 90°, and 180° was predicted and the results showed that the superficial velocities of gas and liquid phase were the control parameters. Under higher gas superficial velocity, the buoyant force is dominant and makes the bubbles concentrate on the outer side of the tube wall; while under lower gas superficial velocity, the centrifugal force is dominant and makes the bubbles concentrate on the inner side of the tube wall. These results met well with the experimental results of Usui.

Numerical analysis of the effects of air on light distribution in a bubble column photobioreactor

Light distribution inside photobioreactors (PBR) is a crucial parameter for the determination of growth of phototropic microorganisms and reactor productivity. In order to compute the light propagation inside PBR, scattering due to the presence of microorganisms is often neglected, since it is difficult to measure experimentally and it is not trivial to handle numerically. Moreover, absorption is usually assumed constant, but it is affected by the concentration of microorganisms and the presence of gas bubbles. In the present contribution we study how the flow hydrodynamics and local gas fractions inside a bubble column PBR affect the light distribution. First, we perform numerical simulations of a bubble column flow at different gas superficial velocities. Afterwards, we use instantaneous air volume fractions to calculate the effective scattering and absorption coefficient of the mixture, as well as the effective scattering phase function. Finally, we compute the polychromatic light distribution inside the PBR by means of a Lattice-Boltzmann solver. On the one hand, we find that gas bubbles affect both spatial distribution and magnitude of the light intensity field and their impact increases at higher gas superficial velocity. On the other hand, we also observe that the biomass counteracts these effects already at concentrations less than 1 kg/m3 so that the role of the gas phase on light fields seems to be of minor importance in PBR.

Understanding the formation process of the liquid slug in a hilly-terrain wet natural gas pipeline

In the present work, the liquid slug formation in a hilly-terrain pipeline is simulated using the Volume of Fluid model and RNG k-ε turbulence model. The numerical model is validated by the experimental data of the horizontal slug flow. The influence of the pipe geometric structure and flow condition on the liquid slug formation is discussed including pipe diameter, inclination angle, gas superficial velocity and liquid holdup. The results show that the pipe is blocked by the liquid slug at the moment of slug formed. The pipe pressure suddenly increases, and then decreases gradually in the process of liquid slug formation and motion. The pipe pressure drop and liquid holdup decrease along with the increasing inclination angle of ascending pipe. On the contrary, they rise with the increase of the inclination angle of descending pipe. Higher gas superficial velocity and liquid holdup result in a larger pressure drop in the formation of a liquid slug, and correspondingly induces a slug flow more rapidly in the hilly-terrain pipelines.

CO2 capture from air by Chlorella vulgaris microalgae in an airlift photobioreactor

In this work, hydrodynamics and CO2 biofixation study was conducted in an airlift bioreactor at the temperature of 30±2°C. The main objective of this work was to investigate the effect of high gas superficial velocity on CO2 biofixation using Chlorella vulgaris microalgae and its growth. The study showed that Chlorella vulgaris in high input gas superficial velocity also had the ability to grow and remove the CO2 by less than 80% efficiency.

Local gas distribution and mass transfer characteristics in an annulus-rising airlift reactor with non-Newtonian fluid

Abstract Many chemical and biochemical processes operated in airlift reactors are limited by the gas-liquid mass transfer of the reactants. Therefore, it is essential to have good understanding of the mass transfer characteristics and gas distribution in airlift reactors to be able to design effective reactors and to optimize aeration conditions. To accomplish this goal, experimental studies on the mass transfer characteristics, i.e. gas holdup, bubble size, interfacial area, volumetric mass transfer coefficient and liquid-side mass transfer coefficient were performed in an annulus-rising airlift reactor with Newtonian and non-Newtonian fluids. A 3D electrical impedance tomography (EIT) technique was used to measure local gas holdup and visualize gas distribution. The influences of aeration rate and liquid viscosity on local gas holdup and interfacial area were investigated at three axial positions along the riser. The EIT results show that the uniformity of gas distribution reduces with increasing liquid viscosity. Bubble size was observed to have a bimodal distribution in highly viscous liquids at high gas superficial velocities due to the intensified coalescence and breakup of bubbles. The maximum value of the local interfacial area is found at the fully developed flow zone in the riser. The liquid-side mass transfer coefficient was found to depend on liquid viscosity and bubble size for non-Newtonian liquids. Empirical correlations based on the experimental data obtained in this work predict well the local gas holdup and volumetric mass transfer coefficient.

Kinetic study of vapor-phase Beckmann rearrangement of cyclohexanone oxime over silicalite-1

The intrinsic kinetics for vapor-phase Beckmann rearrangement of cyclohexanone oxime (CHO) to caprolactam (CPL) over fresh silicalite-1 was studied at atmospheric pressure by an improved approach that combines the differential and the integral methods. To keep the reaction zone temperature constant at high flow rate and obtain homogeneous mixture of CHO and methanol largely different in boiling point, a special fixed-bed tubular reactor was designed. The relationship of deactivation rate with weight hourly space velocity (WHSV) was investigated in a wide range of CHO concentrations under high gas superficial velocity. The rate equation was directly deduced by the differential method on the basis of the dependence of CHO disappearance rate on CHO and CPL partial pressures. The reaction mechanism and rate-limiting step were determined by in situ infrared spectroscopy and comparison of the catalytic results of fresh and partially poisoned silicalite-1. The kinetic parameters were estimated and statistically tested by the integral method. It is shown that the vapor-phase Beckmann rearrangement of CHO to CPL occurs via a single-site surface reaction route with activation energy of 94.69kJ/mol, and desorption of CPL is the rate-limiting step.

Measuring maldistribution of two-phase flows in multi-parallel microchannels

Liquid flow distributions were measured in nine parallel microchannels with inner diameters of 0.8 mm connected to a common header, fed by air–water flows through a single tube. Flow maldistribution in the microchannels was analyzed considering the combined effects of the two-phase flow characteristics (flow pattern, gas quality, mass fluxes and superficial velocities), microchannel-header arrangements (horizontal or vertical), and microchannel flow and header feeding orientations (horizontal, vertical upward or vertical downward). The header was loaded with two-phase mass fluxes of 72, 144 and 216 kg/m2 s and the gas quality ranged from 0 to 0.75. An increase in the flow rate homogeneity was noted for the arrangement horizontal header and horizontal microchannels, when the header was loaded parallel to the microchannels or perpendicularly from the header bottom. Highly heterogenous flow distributions occurred for the arrangement vertical header and horizontal microchannels. Preferential liquid feeding occurred in the microchannels far from the inlet feeding tube for inlet flows with high gas quality (over 10%) and high gas superficial velocity (over 20 m/s) and with the inlet tube parallel to the microchannels. Preferential liquid feeding occurred in the microchannels close to the inlet feeding tube for inlet flows with a horizontal header and upward flows through the microchannels. The results and conclusions of this study represent an important contribution to this field, which could enhance the design of evaporators and condensers in compact heat exchangers.

Measurements of liquid film and droplets of annular two-phase flow on a rod-bundle geometry with spacer

Measurements have been conducted to simultaneously consider both liquid films and droplets of the annular flow on a 3×3 simulating BWR fuel rod-bundle test-section with spacers. The optical system of a high speed camera and a tele-microscope was used to record the backlight images at the gap between a corner rod and a side rod of the bundle at high time and space resolutions. The data obtained from the liquid film showed that the mean film thickness, wave height, power spectral density, and wave velocity at the corner rod are larger than those at the side rod, and that the influences of the spacer are different in the cases of low and high gas superficial velocities. Simultaneously, the data containing size and spatial distributions as well as the axial velocity distribution of liquid droplets were obtained. In the case of lower gas flow rates, the spacer generates not only a large number of small droplets but also big droplets whose size exceeds the maximum droplet diameter at upstream. At further downstream, the spatial distribution of the droplets indicates an asymmetry characteristic, which emphasizes the contribution of the droplet impingement mechanism to the entrainment phenomena. Moreover, a close-up observation at right up- and downstreams of the spacer was conducted to describe the interactions between the two-phase flow and this structure. By using these new experimental arrangements, the interaction mechanisms among the wavy liquid film, droplets and spacer were discussed.

Detailed analysis of phase distributions in a vertical riser using wire mesh sensor (WMS)

This paper looks into the results of an experimental study concerned with the phase distributions of gas–liquid multiphase flows experienced in a vertical riser. Scale experiments were carried out using a mixture of air and silicone oil in a 6m long riser pipe with an internal diameter pipe of 67mm. A series of pipe flow experiments were performed for a range of injected air superficial velocities over the range 0.05–4.73m/s, whilst the liquid superficial velocities ranged from 0.05 to 0.38m/s. Measurements of cross-sectional void fraction and radial time averaged void fraction across a pipe section located 4.92m from the pipe flow injection were obtained using a capacitance wire mesh sensor (WMS). The data were recorded at a frequency of 1000Hz over an interval of 60s. For the range of flow conditions studied, the average void fraction was observed to vary between 0.1 and 0.83. An analysis of the data collected concluded that the observed void fraction was strongly affected by the gas superficial velocity, whereby the higher the gas superficial velocity, the higher was the observed average void fraction. The average void fraction distributions observed were in good agreement with the results obtained by other researchers. The accuracy and performance of void fraction correlations were carried out in terms of percentage error and Root Mean Square (RMS) error. Reasonably symmetric radial void fraction profiles were obtained when the air–silicone oil was fully developed, and the shape of the symmetry profile was strongly dependent on the gas superficial velocity. The data for air/water and air/silicone oil systems showed reasonably good agreement except at gas superficial velocity of 0.05m/s. A comparison of the experimental data was performed against a published model to investigate the flow structure of air–water mixtures in a bubble column. A satisfactory report was observed for radial void fraction profile (mean relative error is within 5.7%) at the higher gas superficial velocities.

Hydrodynamic modeling of downward gas–solid flow. Part II: Co-current flow

The one-dimensional model of accelerating turbulent downward co-current gas–solid flow of coarse particles was formulated and experimentally verified by measuring the pressure distribution along the transport tube. The continuity and momentum equations were used in the model formulation and variational model was used for the prediction of the fluid–particle interphase drag coefficient.The experiments were performed by transporting spherical glass particles 1.94mm in diameter in a 16mm i.d. acrylic tube at constant solid mass flux of 392.8kg/m2s. Tube Reynolds number ranged from 880 to 11,300 and the slip Reynolds number from 32 to 670. At these conditions, the loading ratio Gp/Gf was in the range from 395 to 31. Experimental data for the static fluid pressure distribution along the transport tube agree quite well with the model predictions.The results measured at a distance of 1.51m from the transport tube inlet show that the particle velocity and the mean voidage increase with the increase in superficial gas velocity. The slip velocity changes from negative values at low gas superficial velocities to positive values at high gas superficial velocities. The same trend was observed for the change of the pressure gradient in the system.The values of the pressure gradient, porosity, particle velocity and slip velocity along the tube were calculated according to the formulated model. The distance from the transport tube inlet at which the slip velocity changes its sign from positive to negative is the function of the gas superficial velocity. At positive slip velocity both gravity and drag contribute to particle acceleration. At negative slip velocity the drag force acts in upward direction resisting the particle acceleration. In downward co-current gas–solid flow acceleration length is relatively long, about two times longer compared to the upward co-current gas–solid flow.