Superficial Air Velocity Research Articles

Impact of geometric variables on the functionality of downdraft biomass gasifiers across diverse dimensions

This study offers an extensive examination of the fundamental design parameters that influence the performance of biomass downdraft gasifiers. The impact of design parameters such as reactor dimensions, throat diameter, length of converging and diverging section and ratio of maximum to minimum diameter (dmax/dmin) were analysed using FORTRAN code, which was initially developed for 4 kW Imbert gasifier. Simulation results highlight that the optimal range of hearth loads for gasifier operation is influenced by both the gasifier’s size and the particle size of the feedstock. Increasing gasifier size from 20 mm to 100 mm throat diameter, while maintaining the same superficial air velocity (SAV), escalates hearth load and superficial velocity due to heightened biomass consumption, leading to higher hearth temperature and improved gas calorific value. Similar trends are observed across varying SAV values (0.35 m/s to 1.8 m/s), suggesting a need for lower SAV values in larger gasifiers, possibly with larger particle sizes. Additionally, altering the length of the converging and diverging sections impacts gas quality, with deviations from recommended values affecting ignition and overall performance, while dmax/dmin variations show no significant impact.

High potency of application on an open direct-contact thermal storage using humid air

To reuse low-temperature wasted heat as a thermal resource for high temperature, a direct-contact adsorption thermal storage was focused using humid air and zeolite 13X particles as the working fluid and adsorbent, respectively. Only a few previous studies have chosen the working fluid in gaseous form because it is unavailable for latent heat in generating heat sources. Recovering waste heat in humid air to generate hotter steam is unique and becomes an originality of our present work. The time required to regenerate zeolite particles and the maximum temperature of the generated steam were investigated assuming a warm-up device for a vehicle. The time required to regenerate zeolite was investigated by changing the dew point, temperature, and superficial velocity of the inlet humid air. It was mainly affected by the temperature of the inlet air. The absorbent was regenerated within 30 min when the humid air preheated to 200 °C was supplied. On the other hand, the maximum steam temperature was investigated by changing the superficial velocity and temperature of saturated inlet humid air. As one of the significant and novel finding in this work, the steam of >200 °C was obtained as a high-temperature heat source even with saturated humid air unavailable latent heat. Moreover, as theoretical knowledge, it was revealed that the maximum temperature of the heat source can be estimated by the relationship between the heat balance on the packed bed and adsorption equilibrium.

Bubble Size Characterization in the HydroFloat® Fluidized-Bed Flotation Cell Using Tap Water and Seawater

This research aims to analyze the behavior of bubble size distribution in the HydroFloat® with seawater and tap water. The study characterized bubble size in a two-phase gas–water system in a fluidized-bed flotation cell. The impact of seawater was compared to tap water using two frothers, MIBC and polyglycol F507. The experimental design was used to investigate the influence of various parameters such as superficial air velocity, superficial liquid velocity, frother concentration, and seawater concentration on bubble size. The results indicate that the critical coalescence concentration followed the order of MIBC > F507. Bubble size decreases with increasing superficial liquid velocity, while the superficial gas velocity and frother/seawater concentration have the opposite effect. ANOVA results reveal that all linear factors are significant, the quadratic terms of the frother and seawater concentrations are significant, and the interaction term for the superficial air velocity–superficial liquid velocity is nonsignificant for bubble size. Global sensitivity analysis demonstrates that the variables significantly affecting bubble size are frother concentration and seawater concentration, followed by superficial water velocity. The superficial gas velocity has minimal impact on bubble size under the conditions studied.

Experimental Investigation of Threshold Velocities for Air-Water Two-Phase Flow in a Vertical Tube and Annular Channels

This work presents an experimental study of five threshold velocities for air-water flow in three different vertical channels. The measured threshold velocities included onset flooding (OF), end flooding (EF), onset deflooding (OD), end deflooding (ED), and minimum pressure (MP) velocities. The experimental system includes a transparent vertical tube of 52.5 mm inner diameter and 1500 mm length. The test channel can be easily changed from a tube to an annular shape by inserting a cylindrical test element. A counter-current or concurrent upward flow was achieved by blowing air upward from the channel’s bottom and flowing water from its top. The threshold velocities were determined by analyzing the pressure drop versus air superficial velocity. Findings revealed evident hysteresis between the end flooding and onset deflooding velocities. In contrast, the end deflooding and onset flooding velocities were found to be identical. The end flooding velocity was indifferent to the water’s superficial velocity for all three channel geometries. A generalized gas-liquid flow state diagram for vertical channels is developed based on the present empirical analysis for different threshold velocities.

Bubble Separation in a Gas–Liquid Swirling Pipe Flow

Gas–liquid swirling pipe flow plays an essential role in phase separators. Bubble motions and flow patterns transitions in the flow are crucial for further investigation of this flow. However, much attention has been paid to these in non-swirl flow instead of swirl flow. In this work, bubble separation generated by a vane-typed swirler in a vertical pipe with an inner diameter of 62 mm was experimentally studied. Superficial air velocities (0.036 − 1.41 m/s) and superficial water velocities (0.02 − 0.95 m/s). The results show that dispersed bubbles gradually transform to gas column flow with increasing superficial water velocities (0.093 − 0.640 m/s) in the swirl flow; gas column flow gradually transforms to swirling intermittent flow with increasing superficial air velocities (0.05 − 0.40 m/s), and one peak increases to two peaks in the probability density function curve of pressure drop; slug flow can transform to swirling intermittent flow. Finally, transition criteria for bubble separation were proposed: when the turbulent fluctuations are strong enough to overcome centrifugal force, then the bubbles move to the pipe center and coalescence with each other, lead to the transition of gas column flow/swirling intermittent flow.

Measurement of air-water counter-current flow rates in vertical annulus using multiple differential pressure signals and machine learning

Abstract Gas–liquid counter-current flow in vertical annulus is involved in several industrial fields such as petroleum engineering. For example, in coalbed methane wells with liquid pump drainage, obtaining the real-time flow rate of gas–liquid two-phase in the annulus is crucial for the development management of coalbed methane wells. However, due to complex flow conditions, this demand is difficult to achieve through traditional flow metering means. Therefore, this paper proposes a flow prediction method based on multi-group differential pressure signals and machine learning technology. Air-water two-phase flow experiments were carried out on a vertical annulus pipeline with inner and outer diameters of 75 mm/125 mm and adjustable eccentricity. The probability density function and power spectrum density of 3 groups of differential pressure signals collected at different height intervals in the annulus were used as model inputs. A gas–liquid two-phase flow prediction model was constructed based on the artificial neural network model, and the model hyper-parameters were optimized using Bayesian optimization. The results show that on the 440-group test dataset under the combined conditions of air superficial velocity of 0.06–5.04 m s−1, water superficial velocity of 0.03–0.25 m s−1, and pipeline eccentricity of 0, 0.25, 0.5, 0.75, 1, etc. This model can achieve average prediction errors of 9.12% and 29.34% for gas and water flow rates respectively. This method can be applied to the non-throttling, non-invasive measurement of phase flow rate under the condition of annulus air-liquid counter-current flow.

MFiX-TFM simulation of hydrodynamics of a dual fluidized bed system

The abundance and replenishable nature of biomass make it a suitable fuel for power generation. Among the various technologies, dual fluidized bed gasification (DFBG) is a suitable technology to generate high-quality syngas from biomass. However, the complexity of hydrodynamics and heat transfer with bed geometry, flow conditions as well as feedstock demands effective research for the design of such system. This study, therefore, focuses on the simulation of hydrodynamics of a dual fluidized bed gasification system. The 2-D two-fluid model (TFM) simulations were performed using Multiphase Flow with Interphase eXchanges (MFiX) software by varying the superficial air velocities for both the gasifier and riser. The influence of superficial air velocity on static pressure, pressure drop, axial and radial voidage, solid velocities, and granular temperature was evaluated. From the simulations, it was observed that with the increase in superficial air velocities, there was an increase in the effective height of pressure drop (66%), voidage, and solid velocities of the gasifier. For the simulation of the riser, similar trends in the profile of hydrodynamic parameters were also observed. The optimum velocity for the gasifier and riser has been suggested to be between 0.15 and 0.35 m/s and 6–7 m/s, respectively.

Investigation of the Static Pressure Drop and Production that Occurs due to Flow Patterns in a Perforated Horizontal Wellbore

This study examined the intricate interaction between flow patterns and production within a perforated horizontal wellbore. The study precisely assessed the behavior of static pressure drop by utilizing an array of flow regimes encompassing bubble, dispersed bubble, transitional bubble/slug, slug, stratified, transitional slug/stratified wave, and stratified wave. Remarkably, an upward trend in static pressure drop was observed with increasing water phase presence, while the converse was true for the air phase. Besides, the air phase superficial velocity exhibited a direct correlation with the magnitude of pressure drop fluctuations. The liquid production demonstrated a peak during bubble and slug flow regimes, followed by a descent during the transition to stratified and stratified wave flow. This decline can be attributed to mixing pressure drops localized during the perforations. Furthermore, an upward trend in average liquid production was observed with increasing mixture superficial velocity, primarily due to the dominant presence of the water phase. Additionally, the percentage of liquid production was positively associated with the water's superficial velocity when the air's superficial velocity was held constant. While the experimental and numerical results were in agreement for slugs and structured flows, there were discrepancies in the behavior of static pressure for bubbles, small bubbles, and structured waves.

Experimental study of swirling flow pattern at the swirler outlet using a Wire-Mesh sensor

Entrained droplets and flow pattern change adversely affects separation performance of swirl-vane separators. Investigating gas–liquid distribution at swirler outlet, an experiment was conducted using a wire-mesh sensor positioned. Air and liquid superficial velocities ranged from 8.3 to 25.6 m/s and 0.1 to 1.4 m/s, respectively, covering swirling annular and swirling churn flow at swirler outlet. In swirling annular flow, liquid phase primarily exists as a stable thin film which facilitates efficient separation. In swirling churn flow, wall liquid film has unstable large amplitude disturbance waves. Disrupting liquid film leads to entrained droplets forming in gas center, increasing separation distance and hindering gas–liquid separation. The transition boundary from swirling churn to swirling annular is crucial. Increasing swirler angle shifts the transition boundary towards higher gas velocity at the same liquid velocity. A theoretical model based on interfacial shear stress and gas core blockage predicts the flow pattern transition at swirler outlet.

Experimental analysis on flow characteristics of particulate materials conveyed by airslides

Increasing demands on efficient airslides conveying in the fields of construction, agriculture, pharmacy and power require deep understanding of the flow characteristics of materials being conveyed. This study designs a full-scaled airslide test rig, in conjunction with an experimental method, to investigate the flow behaviour of two different materials, namely sand and flyash under vent and non-vent conditions. The flow modes of the sand and flyash are predicted based on the measured material properties and fluidisation. Furthermore, tests are conducted using the designed airslide, from which the flow is characterised by analysing the variations of the key parameters, including the mass flow rate, the superficial air velocity, the pressure inside the airslide and the flowing material bed height. The work shows the developed test rig and method are capable of characterising fluidised flow for airslide systems.

Numerical modelling of two-phase flow hydrodynamics of column flotation - validation against ERT data

ABSTRACT In mineral processing, column flotation is being used extensively for fine particle beneficiation. Although a few attempts were made to develop computational fluid dynamics (CFD) models for column flotation in the past, validation against comprehensive experimental data is yet to be demonstrated. This work employs an Eulerian-Eulerian model coupled with the standard k-ε turbulence model for modeling the hydrodynamics in the column flotation. The two-phase simulations were performed on both laboratory and industrial-scale column flotations. The two-fluid model was further modified with the appropriate interphase forces (Ishii-Zuber drag force and Tomiyama lift force) and population balance method (PBM) to accurately simulate and predict the bubble dynamics. The axial and radial gas holdup variations along with the axial liquid velocity are predicted at different air and liquid superficial velocities. The numerical predictions were then validated against the Electrical Resistance Tomography (ERT) data in a 4-inch laboratory column flotation and conductivity probe data in an industrial column flotation. The modified two-fluid model was able to predict accurate hydrodynamics close to ERT data. The gas holdup was found to increase with the superficial air velocity, liquid height, and liquid velocity. The gas holdup found uniformly dispersed radially at low superficial air velocities and heterogenous dispersed bubble flow at higher superficial velocities. A parabolic profile of liquid flow observed at higher superficial air velocities.

Hold-up formation in bubble channel reactors: A bubble-scale investigation

Experiments were carried out to measure the air volume fraction and the hold-up in a bubble channel reactor fed by porous pipes.A new processing method was proposed for the signal acquired by means of a resistivity probe for the air volume fraction. This method avoids the loss of the bubble residence time on the probe tip, which affects the most used signal processing procedures. The probe measurements were compared with other studies regarding bubble curtains.The measured air hold-up was in agreement with correlations available in the literature and formulated for bubble columns fed by a porous sparger. A bubble-scale model was proposed to explain the functional form of the dependence of the hold-up on the air superficial velocity. Moreover, a link between the hold-up and the Koide equation for bubble dimensions was established.The experiments were reproduced by means of CFD simulations. The air volume fraction measurement method and the hold-up model found a confirmation in the numerical results. Moreover, the latter showed how the bubble plumes interact inside the reactor and the consequences on the air hold-up.

Long-term operational performance and membrane fouling mechanisms of AGMBR treating municipal wastewater under different superficial air velocities

The granulation and stability of aerobic granular sludge (AGS) as well as membrane fouling in AGS-membrane bioreactor (AGMBR) are significantly influenced by superficial air velocity (SAV). However, there is a lack of relevant research on this topic. Therefore, this study aims to investigate the specific impacts of SAV on AGMBR performance and membrane fouling. The results showed that the experimental SAVs (1.6, 2.1, and 2.6 cm/s) were found to provide sufficient dissolved oxygen for the removal of organics and ammonium nitrogen at efficiencies exceeding 94.50% and 99.40%, respectively. The larger SAV also extended the overall lifespan of the AGMBR, while altering both the content of foulants on fouled membranes and their associated microbial community structure. Moreover, the cake layer exhibiting the highest degree of porosity was observed at an SAV of 2.6 cm/s, which corresponded to the largest pore blocking fouling resistance (28.81% of the total fouling resistance) and the presence of fluorine on the fouled membrane. Additionally, the results from extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory were consistent with the increase in transmembrane pressure. Polysaccharides (PS) played a more significant role than proteins in membrane fouling, and the PS present in soluble microbial products exhibited a strong correlation with fouling resistance distribution. This study aims to optimize the treatment performance and minimize membrane fouling in AGMBR, thereby establishing a mutually beneficial scenario that is essential for the practical implementation of wastewater reuse in engineering.

Self-cleansing velocity in upward three-phase steady pipe-flow

The solids transport and the conditions required to begin the transport of granular particles, or to avoid their deposition, in three-phase turbulent flows of mixtures of gas–liquid–solids, in upward inclined pipes, are complex phenomena whose governing equations and corresponding solutions can be approximated via experimental investigation and numerical computation. An experimental installation for establishing steady flow conditions of air–water-solids in an upward transparent acrylic pipe of 84 mm was built and prepared to allow the measurement of the transported flow rates of air, water, sand and fine gravel, with particle diameters between 0.425 and 7.20 mm. Two full set of experiments with water-solids, and air–water-solids, under comparable conditions, were performed in Laboratory, in order to analyse the influence of the gas phase. Three pipe angles between 30° and 58°, and four solid particle ranges with intermediate sizes forming a bed were tested. The average water superficial velocity demonstrates to be the most relevant variable for the solids transport beginning, and the presence of air has a positive influence, even without the mobilisation of the water flow rate increase due to air-lift pumping. A model relating a modified Shields parameter (Shmixc) with a modified Reynolds number of the particles (Remixc), both defined for the critical average flow rates of three-phase mixtures under steady flow, for which a residual mass of solid particles begins to be transported, is proposed. The resulting equation follows a power law of the generic type Shmixc = a Remixc−b, where a and b are positive constants experimentally obtained for the different angles of inclination of the upward pipe, with coefficients of determination well above 90%. The mathematical model proposed in this work allows the explicit computation of the self-cleansing velocity required for two-phase flows. The critical average air superficial velocity and subsequent average velocity of the mixture required for the solids transport in steady three-phase flows, when the average water superficial velocity is below the two-phase self-cleansing velocity, are computed using the proposed model by numerical iterative processes.

Investigating the Relationship between the Time Constant Ratio and Plug-Flow Behaviour in the Pneumatic Conveyance of Biomass Material

This study introduces a novel methodology to evaluate the behaviour of biomass material by examining the ratio of aeration and deaeration time constants. To this end, a series of tests were conducted on four different materials, namely, cottonseed, wood chips, wood pellets, and wheat straw, in order to investigate their aeration and deaeration behaviours. The study derives the aeration and deaeration pressure drop equations, and discusses the corresponding time constant expression. Subsequently, the four materials were conveyed in 12 m long batch-fed and continuous pneumatic conveying pipelines to examine their behaviour in longer pipelines. The results indicate that the aeration and deaeration time constants increased with an increase in air superficial velocity. However, the ratio of the aeration and deaeration time constants was identified as a unique number, where a value close to 1 indicates a higher likelihood of plug flow. On the basis of the results, cottonseed, with the lowest ratio of time constant, was more likely to form a stable plug flow in both batch-fed and continuous pneumatic conveying. Given the unique properties of biomass and the limited research on the pneumatic conveyance of biomass, this methodology represents a novel approach for predicting modes of flow in materials with complex properties.

Predictive modelling approach for cottonseed plug velocity applying a circuit theory analogy

In this study, a circuit theory analogy was used to investigate plug behaviour in dense phase pneumatic conveying of cottonseeeds. It was found that the time series aeration/deaeration pressure drop of a fixed bed exhibits exponential behaviour, similar to the charging/discharging of a capacitor in a circuit. Using this analogy, the pressure drop below transition superficial air velocity was considered equivalent to the potential voltage across a capacitor over time. By extending this analogy beyond transition superficial air, a potential pressure drop was introduced and found to separate from the measured pressure drop at the moment the plug moves. It was demonstrated that the difference between the potential and measured pressure drop can be scaled to the plug velocity, indicating a linear relationship between these variables. This approach provides a novel method for understanding and predicting plug behaviour in pneumatic conveying, which may have practical applications for various types of other materials.

Hydrodynamic study of pressure swing rotating bubble cap column with gas-liquid systems

ABSTRACT The present investigation on a hydrodynamic study focused on the RPM and pressure drop in a pressure swing rotating bubble cap (PSRBC) sparger column. PSRBC is one of its kind, which has the capability of producing bubbles and effective mixing in the sparged column. In this experimental work, a plastic PSRBC sparger of cylindrical shape has been designed and used for the supply of compressed gas (air) through the stationary liquid (water-glycerine solution) pool. The system variables viz. superficial air velocity, stationary liquid height, column diameter, the viscosity of liquid and sparger outlet opening area are considered for this study. Model equations have been developed for predicting RPM for the PSRBC sparger and pressure drop of the column in dimensionless form. The calculated values from predicted models have been compared with the experimental values and the coefficients of correlations are found to be more than 0.97. The present hydrodynamic study relates to RPM and pressure drop and may be useful for the design of gas–liquid systems viz. bubble columns, deaerators, bio-reactors (without stirrer) etc. where the interfacial area of gas–liquid has a distinct role in the performance.

Effect of nanobubbles on flotation of El-Maghara coal

ABSTRACT To extensively explore the advantage of using nanobubbles in the El-Maghara coal flotation process, the effect of nanobubbles on both column and mechanical flotation was investigated under different operating parameters such as diesel oil collector dosage, MIBC frother concentration, superficial feed velocity, superficial air velocity, and superficial wash water velocity in column flotation; besides the slurry flow rate through nanobubbles generator into a 25-liter mechanical flotation cell. The representative coal flotation feed acquired from the El-Maghara deposit located in Sinai, Egypt with chemical characterization using proximate analysis containing 25.27% mineral matter forming ash during coal combustion and with particle size distribution measurement using laser particle size analyzer is 57 µm d90. Also, the flotation kinetic experiments were done to show the influence of nanobubbles on the flotation time required to obtain high-quality coal products with high combustible recovery. Nanobubbles enhanced the flotation performance and kinetics by up to 24% combustible recovery based on the operation parameters reducing flotation time from 4 to 2.25 min for 80% combustible recovery.

Froth carry rate and mass recovery of a semi-continuous flotation system with acoustic sound

Column flotation is widely used to upgrade ores in fine and ultrafine size fractions. One of the challenges for column flotation operations is to increase the froth carry rate without the need for extensive capital and additional operating expenditure. The present work addresses this challenge by dynamic stabilization of the froth using acoustic sound at a frequency of 400 Hz. Quartz particles (with an 80% passing size of 40.3 µm) were used as model particles to demonstrate this approach, with the experimental variables being feed solids content, feed volumetric flowrate and superficial air velocity, and flotation tests were conducted at laboratory scale, in semi-continuous mode, with and without acoustic sound. The results showed that applying the acoustic sound to the column flotation could increase froth carry rate by as much as 20% and mass pull by up to 19%.

Bioaugmentation and enhanced formation of biogranules for degradation of oil and grease: Start-up, kinetic and mass transfer studies

Biogranulation technology is an emerging biological process in treating various wastewater. However, the development of biogranules requires an extended period of time when treating wastewaters with high oil and grease (O&G) content. A study was therefore conducted to assess the formation of biogranules through bioaugmentation with the Serratia marcescens SA30 strain, in treating real anaerobically digested palm oil mill effluent (AD-POME), with O&G of about 4600 mg/L. The biogranules were developed in a lab-scale sequencing batch reactor (SBR) system under alternating anaerobic and aerobic conditions. The experimental data were assessed using the modified mass transfer factor (MMTF) models to understand the mechanisms of biosorption of O&G on the biogranules. The system was run with variable organic loading rates (OLR) of 0.69–9.90 kg/m3d and superficial air velocity (SAV) of 2 cm/s. After 60 days of being bioaugmented with the Serratia marcescens SA30 strain, the flocculent biomass transformed into biogranules with excellent settleability with improved treatment efficiency. The biogranules showed a compact structure and good settling ability with an average diameter of about 2 mm, a sludge volume index at 5 min (SVI5) of 43 mL/g, and a settling velocity (SV) of 81 m/h after 256 days of operation. The average removal efficiencies of O&G increased from 6 to 99.92%, respectively. The application of the MMTF model verified that the resistance to O&G biosorption is controlled via film mass transfer. This research indicates successful bioaugmentation of biogranules using the Serratia marcescens SA30 strain for enhanced biodegradation of O&G and is capable to treat real AD-POME.