Gas Velocity Profiles Research Articles

Determining the Efficiency of Packed Gas Separators of Droplets Taking into Account the Nonuniformity of the Gas Velocity Profile

Determining the Efficiency of Packed Gas Separators of Droplets Taking into Account the Nonuniformity of the Gas Velocity Profile

ОЧИСТКА ДЫМОВЫХ И ТЕХНОЛОГИЧЕСКИХ ГАЗОВ В ПРЯМОТОЧНЫХ ЦИКЛОНАХ

In this work, gas velocity profiles in the separating chamber of a direct-flow cyclone are obtained, and a method for calculating its efficiency is proposed. The method allows us to trace the influence of the geometric parameters of the chamber, the particle size and the gas flow rate on the degree of its purification. The calculation results are confirmed by numerous experiments

Development and validation of an open-source, disposable, 3D-printed in vitro environmental exposure system for Transwell culture inserts.

Accessible in vitro models recapitulating the human airway that are amenable to study whole cannabis smoke exposure are needed for immunological and toxicological studies that inform public health policy and recreational cannabis use. In the present study, we developed and validated a novel three-dimensional (3D)-printed in vitro exposure system (IVES) that can be directly applied to study the effect of cannabis smoke exposure on primary human bronchial epithelial cells.Using commercially available design software and a 3D printer, we designed a four-chamber Transwell insert holder for exposures to whole smoke. COMSOL Multiphysics software was used to model gas distribution, concentration gradients, velocity profile and shear stress within IVES. Following simulations, primary human bronchial epithelial cells cultured at the air–liquid interface on Transwell inserts were exposed to whole cannabis smoke using a modified version of the Foltin puff procedure. Following 24 h, outcome measurements included cell morphology, epithelial barrier function, lactate dehydrogenase (LDH) levels, cytokine expression and gene expression.Whole smoke delivered through IVES possesses velocity profiles consistent with uniform gas distribution across the four chambers and complete mixing. Airflow velocity ranged between 1.0 and 1.5 µm·s−1 and generated low shear stresses (<<1 Pa). Human airway epithelial cells exposed to cannabis smoke using IVES showed changes in cell morphology and disruption of barrier function without significant cytotoxicity. Cannabis smoke elevated interleukin-1 family cytokines and elevated CYP1A1 and CYP1B1 expression relative to control, validating IVES smoke exposure impacts in human airway epithelial cells at a molecular level.The growing legalisation of cannabis on a global scale must be paired with research related to potential health impacts of lung exposures. IVES represents an accessible, open-source, exposure system that can be used to model varying types of cannabis smoke exposures with human airway epithelial cells grown under air–liquid interface culture conditions.

The investigation of arc fluctuations in thermal plasma torch using 3D modeling approach

Two and three-dimensional transient models of a thermal plasma torch are presented to analyse the effect of operating conditions on arc root fluctuations for various types of anode configuations. The computational fluid dynamics code, OpenFOAM(c), is utilised in a modified form to model the arc and plasma flow inside the argon plasma torch. Modeling a thermal plasma requires a combination of mutually related fluid dynamics and electromagnetic phenomena which dictate the size and stability of the arc column. While the 2D axisymmetric model is effective in depicting the average energy and average velocity profiles of plasma gas, it fails to describe the fluctuations in arc behaviour and its mode of attachment on the anode. In a 3D model, the arc movement is captured in both axial and azimuthal directions wherein the arc-root attachment on anode surface seems to fluctuates in a periodic manner and the voltage fluctuations mirrors the arc-root attachment behaviour within the system. We have here investigated the effect of operating conditions on the frequency of this arc reattachment, finding that the input current and gas flow rate have a significant effect on arc behaviour. We have also reported the effect of arc voltage perturbations on the exit plume properties (velocity and average temperature) and finally the effect of anode nozzle geometry on the fluctuations and plume behavior is presented.

Study on bubbly and cap-bubbly flow in a square channel using dual wire-mesh sensors

The experiments of co-current vertical upstream air-water two-phase flow in a 66 mm × 66 mm square channel were carried out. The superficial liquid velocity ranges from 0.765 m/s to 1.53 m/s and the superficial gas velocity ranges from 0.0215 m/s to 0.313 m/s, including bubbly flow and cap-bubbly flow. A dual wire-mesh sensors (WMSs) was used to measure the bubbles in the two-phase flow. The local superficial gas velocity from the dual WMSs was compared with the rotameters and most results were within the deviation of 5%. It can ensure the measurement accuracy of the WMSs. The experimental results include the void fraction projection, sectional averaged void fraction variation, time-averaged void fraction profile, gas velocity profile, and the bubble size distribution. Within the range of flow conditions of this paper, the void fraction was relatively uniformly distributed among the cross-section in the bubbly flow and core peak in the cap-bubbly. Both cross-correlation velocity and bubble velocity were used to calculate the gas velocity profiles. Since the bubble lateral displacement, the cross-correlation method trend to miss the correlation caused by small bubbles and get a higher velocity. Bubble velocity was recommended to be used for determining gas velocity profiles. The results of void fraction profiles, gas velocity profiles, and bubble size distributions can be used for the validation of two-phase models of CFD codes. The void fraction predictions of two drift-flux correlations were compared against the experimental data and both models provided reasonable prediction accuracy.

Predicting below‐surface horizontal pore gas velocity in porous media from above‐surface wind conditions and medium gas permeability

AbstractBelow‐surface horizontal pore gas velocity profiles in response to above‐surface wind conditions were measured for 16 combinations of wind speed (1.4–5.7 ms−1) and wind gust frequency (0–1 Hz) in four porous media with different particle size (0.6–10 mm), yielding 64 combinations of wind condition and porous medium using a recently developed experimental gas tracer tracking method. Experiments were carried out under controlled conditions in a wind tunnel. Gusty wind conditions were created using a wind‐driven propeller operating at variable rotation frequencies. A recently presented empirical model for predicting horizontal pore gas velocity in porous media in response to above‐surface wind conditions was validated against the experimental data. The model uses one empirical parameter (B) together with the wind speed at the surface as input parameters. Further analyses were carried out to identify relations between B, above‐surface wind conditions and porous medium properties. Results indicate that it is possible to accurately estimate B and thus predict below‐surface pore gas velocity profiles based on porous medium properties and surface wind speed. B was found inversely proportional to above‐surface wind speed, porous medium particle diameter and porous medium gas permeability, all in a linear manner.Highlights Horizontal pore gas velocity profiles measured for 64 porous medium–wind speed combinations Recent two‐parameter empirical model for pore gas velocity evaluated using the experimental data Model accurately approximated experimental pore gas velocity data. Model parameters could be predicted from medium gas permeability and above‐surface wind speed

A hybrid multiphase flow model for the prediction of both low and high void fraction nucleate boiling regimes

The improvement of a hybrid (a combination of Volume-of-Fluid (VOF) and Eulerian model) multiphase flow solver for the numerical prediction of high- and low-void fraction flow boiling regimes is presented in this article. These flow regimes could simultaneously occur in core-catcher and in-vessel retention-external reactor cooling systems during a severe accident in nuclear power plant. To enhance the prediction of the low void fraction boiling regime, a turbulent rough wall function model is implemented in the hybrid model to reproduce the impact of coarseness induced by the existence of growing bubbles along the heating wall on the liquid velocity profile. With this wall function, a more accurate prediction of the radial velocity profile is achieved within the uncertainty of the velocity measurements. Moreover, an improved prediction of the radial void fraction is achieved using the model proposed by Lopez de Bertodano for turbulence dispersion force without compromising the prediction of the radial gas velocity profile and radial liquid temperature profile. Although the hybrid model shows potential in capturing the interface and dynamic behavior of large-scale bubbles (vapor slug) for high void fraction regime, the predicted wall superheat is higher than the measured values. This highlighted the need for the extension of the present wall boiling model to cover flow boiling involving sliding vapor slugs on the heated wall.

Mathematical Modeling of the Stefan Flow Effect on Process in a Catalyst Grain Using Mathcad

Weekman and Gorring [2] considered the problem of the influence of the Stefan flow in a catalyst grain for mth order gas-phase chemical reaction A → nB. Special numerical methods were used to solve the problem. This paper proposes a simple method for solving the problem of a first-order gas-phase chemical reaction A → 2B using the Mathcad program. As an illustration of the method, the profiles of dimensionless reagent concentration and gas velocity of the Stefan flow at the Thiele parameter ψ = 1 and the molar fraction of reagent A outside the grain θ = 1, the dependence of the catalyst effectiveness factor on the parameters ψ and θ in the ranges ψ = 0.1–10 and θ = 0.1–1, and the dependence of a ratio between the catalyst effectiveness factor at θ = 1 and an analogous parameter in the absence of the Stefan flow on the Thiele parameter were calculated.

On the near wake structure of a supersonic coaxial gas-liquid jet

The work is devoted to a complex experimental study of the near wake of supersonic coaxial gas-liquid jets with a high concentration of the dispersed phase. At the various liquid flow rates, wave structure of a gas phase was visualized, droplet velocity profiles were obtained by the LDA method, and the spray concentration was measured. The effect of the liquid phase on the supersonic underexpanded jet was studied, in particular, a change in the general wave structure, Mach disk transformation, etc. A gas velocity profile was obtained in a two-phase core of a gas-liquid jet by the method of droplet velocity profile correction taking into account their velocity relaxation.

Mass transfer in the homogeneous flow regime of a bubble column

A model for the evaluation of the mass transfer coefficient in bubble column reactors in the homogeneous regime is proposed. This approach combines three-dimensional two-phase flow simulations using OpenFOAM with mass transfer models in order to evaluate the average mass transfer coefficient in the reactor. The accuracy and limitations of the simulations are discussed based on comparisons with existing experimental data and correlations from the literature. Radial gas holdup and velocity profiles at different operating conditions are in reasonable agreement at low gas velocities. A sensitivity analysis was performed to determine the best interfacial force model among several possibilities. Schiller-Naumann’s drag force model appeared to be the most appropriate, a swarm correction has a limited impact. Overall, the predicted global mass transfer coefficients fell within the range of values obtained from existing correlations in the literature. The level of accuracy of the proposed approach is, however, difficult to estimate as the predictions from the correlations in the literature varied over a 5-fold range. Despite the limitations of the proposed model (e.g., low gas velocities), the approach could be used to estimate the mass transfer in columns with internal packings, baffles or inserts, for which no correlation exists in the literature.

Prediction of flow distribution of vertical upward co-current adiabatic annular flow in 8 × 8 rod bundle

Annular flow in rod bundle occurs frequently in boiling water reactor (BWR) under normal operation and in pressurized water reactor (PWR) under accident conditions. When annular flow occurs, rod film thickness is one of the most important parameters for safety concern. In order to calculate rod film thickness distribution inside rod bundle, both gas and liquid velocity distribution inside rod bundle is necessary. Also, flow distribution inside rod bundle is of importance for calculation of interfacial shear stress and other parameters in each sub-channel. However, entrained droplets in gas core can sharpen the gas velocity profile while existing rods can flatten the gas velocity profile, making it different from that of single phase flow. The liquid velocity distribution inside rod bundle is hard to obtain from experiment because of the complex structure of its geometry. Due to these reasons, this paper deducts gas and liquid velocity profiles from existing adiabatic rod film thickness data. Even the gas velocity profiles for different flow conditions are different, there exists a gas velocity profile that can be applied to all current flow conditions with minimum uncertainties. Based on that gas velocity profile, liquid velocity profile can be calculated. From these flow velocity profiles, film thickness on each rod and interfacial shear stress in each sub-channel can be calculated.

Application of a novel model for bubble-induced turbulence to bubbly flows in containers and vertical pipes

The baseline model for the CFD-simulation of turbulent bubbly flows in the Euler-Euler framework was proposed several years ago. It defines a fixed set of closures including all constants to avoid case by case tuning and to have a basis for sustainable model development. The present paper extends the baseline model by improving the modelling of bubble-induced turbulence. The closure terms in the transport equations of the k-ω SST model are revisited and replaced with the model recently proposed by Ma et al. (Ma et al., 2017. Physical Review Fluids 2, 034301) which is based on an analysis of the turbulent kinetic energy budget obtained from direct numerical simulation data. Detailed validation results for various flow configurations with a wide range of gas and liquid volumetric fluxes are presented. In case of vertical pipe flow significant improvements in the predicted gas volume fraction and velocity profiles are obtained, especially in high gas volume fraction cases where bubble-induced turbulence is dominant. Simulations of other configurations, such as uniform and non-uniform bubble columns, show that the model of Ma et al. (2017) results in an also for these cases overall improvement. Therefore, the baseline model is now updated to include the new model for bubble-induced turbulence.

Particle shape effect on bubble dynamics in central air jet pseudo-2D fluidized beds: A CFD-DEM study

Bubbles considerably influence the characteristics of gas-solid fluidized bed, hence play an important role in determining process performance. This paper presents a CFD-DEM study on the effect of particle shape on the bed microstructure and bubble properties in a pseudo-2D bubbling gas-solid fluidized bed, operated with a continuous central jet. The bubble formation process is successfully generated, where bubbles rise through the bed and burst at the top of the bed. The numerical results show that ellipsoids have slightly different flow patterns from those of spheres. However, the mechanisms of bubble splitting and coalescence are found strongly dependent on particle shape. For spheres, the bubble trajectories mainly follow the bed centreline, whereas the bubbles for ellipsoids are widely distributed on both sides of the bed centreline. This result suggests that the lateral drift of bubbles is high for ellipsoids because such particles prefer to orient their longest major axis in the direction of the fluid flow. At the lower part of the bed, both gas and particle velocity profiles are found axially similar to a Gaussian distribution. In contrast, at the upper part of the bed, their peaks become flatter and broader for ellipsoids. Additionally, the bubble equivalent diameters are higher for ellipsoids while bubbles become more circular for spheres. Both bubble frequency and bubble velocity for ellipsoids are lower than spheres. The results obtained from this study can improve the understanding of bubble dynamics in the fluidization of non-spherical particles.

A Numerical Model for Bubble Column Reactors: Prediction of the Fractional Gas Holdup by the Implementation of the Drift-Flux Model

Comprehensive knowledge of hydrodynamics inside a reactor is crucial for the design and scale-up of bubble columns. Fractional gas holdup (αg) is an important parameter that should be obtained for the design of bubble column reactors. The estimation of this parameter depends mainly on experimental procedures. Drift-flux theory is one of the most practical and accurate models for calculating the gas holdup. Although many researchers have studied bubble column reactors, because of the limits of the experimental setting, there are few studies that have operated over a wide range of superficial gas velocities. In this work, a transient 3-D numerical simulation of upward air-water flow in the bubble column was performed over a wide range of superficial gas velocities (0.025-0.4 m/s) using the Eulerian-Eulerian model. The effect of the superficial gas velocity on the flow pattern was simulated, and two-phase flow regimes were classified into homogeneous, transition and heterogeneous regimes. Considering the importance of the drift-flux model, the values of the distribution parameter and the drift velocity were computed according to their definitions using the cross-sectional gas holdup and velocity profiles obtained via computational flow dynamic simulation. The results were verified against the experimental data, and a correlation is proposed for predicting the gas holdup.

Measuring horizontal pore gas velocity profiles in porous media in response to near‐surface wind speed and gustiness

SummaryA simple method for experimental determination of horizontal wind‐induced, near‐surface pore gas velocities in porous media is presented. This method uses traditional tracer gas tracking methodology, but is designed for applications where mass loss of tracer gas from the experimental domain occurs at an unknown rate (as is the case in near‐surface, wind‐exposed porous media), making traditional inverse transport modelling inapplicable. The method was applied to a dry, granular porous medium consisting of crushed basalt (2–4 mm grain size), using CO2 as the tracer gas. Experiments were conducted under controlled (wind tunnel) conditions, at average near‐surface wind speeds ranging from 0 to 5.6 ms−1, for both gusty and non‐gusty winds. Velocity profiles of near‐surface horizontal pore gas in the top 15 cm of the porous medium were determined for 10 different combinations of average wind speed and wind gust frequency. Pore gas velocities of up to 0.5 cms−1 were observed that depended on depth and near‐surface wind conditions. Results indicate that horizontal gas flow can occur in wind‐exposed porous media under both non‐gusty and gusty wind conditions, and that wind gustiness has a significant effect on the velocity profiles of horizontal pore gas inside porous media. Results show further that pore gas velocities were almost linearly proportional to near‐surface average wind speed, regardless of porous medium depth.HighlightsDeveloped a novel method for measuring wind‐induced pore gas velocity in porous media. Near‐surface wind action induced horizontal pore gas velocities. Pore gas velocity profiles depended strongly on wind gustiness (gust frequency). Pore gas velocities were directly proportional to average near‐surface wind speed.

Gas Transport in Shale Nanopores with Mobile High-Viscosity Water Film

An analytical model for calculating gas velocity profiles and predicting gas apparent permeability enhancement factors in nanopores of shale with nanometer scale characteristic dimensions of different geometries (slit pores and circular pores) is proposed. The proposed model considers the presence of a mobile high-viscosity water film by modifying boundary conditions at a liquid–solid interface and a gas–liquid interface has good agreements with experimental data, and confirms that a mobile high-viscosity water film enhances gas flow capacity. The importance of a mobile high-viscosity water film is further evaluated with a varying pore size, pressure, and surface wettability. In the case of smaller pores and higher pressure, a mobile high-viscosity water film makes more positive contributions to both the gas velocity and a gas apparent permeability enhancement factor. Increasing the contact angle at a solid–water interface implies a reduction in molecular attractions and a decrease in gas flow resistance ...

WALL EFFECT OF A PACKED BED WITH PELLET PARTICLES

Fluid flow profile is a dominate role in the performance of packed bed reactor. In small ratio of column-to-particle diameter, velocity pattern is strongly affected by voidage distribution, which depends on radial coordinate, flow rate and bed height. In this study, effects of voidage distribution to gas velocity profile in a packed bed with pellet particles was empirically investigated. Uniformity of local velocity at the top of the bed was clearly observed with decreasing of bed height and flow rate. For 400 mm of bed height, the measured velocities are a well fitting to Fahien and Stankovich model for any expected flow rate.

Modeling of bubble flows in vertical pipes with the N-phase compressible algebraic slip model

In the current work, the N-phase compressible algebraic slip model (ASM) framework is presented. The framework is implemented in TransAT© which is a commercial finite-volume CFD solver specialized in the modeling of multiphase flows. It is applied to bubbly flows in vertical pipes where compressibility of the gas phase is important. Effects of compressibility of the gas phase namely, the change in volume of the individual bubbles, the reduction in the liquid volume fraction, and the change in bubble size due to almost constant number density are identified and illustrated with the help of an analytical one-dimensional model. It was shown that the volume effect was the strongest, although the effect of the change in the bubble radius is also important though to a lesser extent.The framework implemented in TransAT© does not solve for the gas number density and assumes a fixed radius for the bubbles. In the first step the implementation is verified with the help of the one-dimensional model. In the next step, the implementation is tested against the selected test cases from the TOPFLOW database for different types of bubbly flow namely the wall-peaked, the transition and the central-peaked. The simulation results are able to predict the overall behavior of the different types of bubbly flow. The radial gas velocity profiles for all the test cases showed a good match with the experimental data. Unrealistic accumulation of small bubbles close to the walls is observed for the wall-peaked test cases. The results showed that the unrealistic behaviour could be removed tuning the wall lubrication force model. The results highlight the inadequacy of the current wall lubrication force models and show that the wall lubrication force should depend on the velocity of the continuous phase probably in an analogous way to the lift force.

Hydrodynamics and bubble size in bubble columns: Effects of contaminants and spargers

The simulation of bubble columns operating under the heterogeneous regime is an ambitious challenge, due to the difficulty of predicting accurately hydrodynamics and bubble size distributions, that requires experimental data for model validation. Gas fraction distributions, liquid and gas velocity profiles and bubble size distributions across bubble columns are deeply interconnected in these systems and only a comprehensive study allows the links between them to be understood. This work reports experimental data obtained by measuring bubble sizes with an innovative technique based on the cross correlation between two optical probes. Particular attention is given to the role of additives and impurities with a view to suppressing bubble coalescence. Initially experiments are carried out with demineralized water; subsequently they are repeated with tap water and adding small quantities of ethanol. Results show that contaminants and alcohol addition suppress bubble coalescence and induces a decrease of mean bubble sizes. Furthermore, alcohol addition delays the transition from homogeneous to heterogeneous regimes and increases the gas hold-up under the heterogeneous regime. Gas distribution is studied through two different perforated spargers. Changing the sparger it is possible to modify the bubble size in the lowest part of the column significantly. A perforated sparger with big holes causes the formation of big bubbles close to the holes of the sparger and promotes bubble breakage in the lower part of the column. By combining ethanol addition and sparger modification, bubble coalescence and bubble breakage can be decoupled in a controlled manner and interesting conclusions concerning these processes can be drawn. Beyond the novelty of bubble size measurements at high gas fraction, the experimental data collected are very useful to validate and develop computational fluid dynamics simulations coupled with population balance models suitable for heterogeneous bubbly flows.

Pressure-dependent transport characteristic of methane gas in slit nanopores

The transport mechanism of methane gas through the nanopores is a fundamental concern in the shale gas extraction. Shale gas is mainly stored in nanopores of shale formations over a wide range of pressure which is a fundamental thermodynamic variable dominating the properties of substances, rendering that it is important to understand the pressure-dependent transport characteristic of gas molecules in nanopores. Toward this end, we performed molecular dynamics (MD) simulations to investigate the transport characteristic and its mechanism of methane gas in slit nanopores under different pressures. A modified bin method considering local gas density was proposed to reveal the mass velocity profile of methane gas in nanopores. Combining the trajectories of representative molecules, we found that the underlying mechanism of methane gas transport characteristic behaves as the competition between gas intermolecular collisions and gas-wall interactions. The transport characteristic of methane gas in slit nanopores is dependent on the pressure. Meanwhile, the pore width can significantly influence the mass flux contributions of different transport mechanisms. Under the high pressure, viscous flow is the primary transport mechanism due to the frequent gas intermolecular collisions. Under lower pressure and in larger nanopores, Knudsen diffusion becomes the dominating transport mechanism due to the enhancement of gas-wall interactions (collisions). Narrower nanopores can significantly increase the mass flux contribution of surface diffusion due to the increased proportion of absorbed gas molecules. In particular, we proposed a theoretical model coupling multiple transport mechanisms, which can accurately predict the apparent permeability of methane gas in slit nanopores. Under lower pressure and in narrower nanopores, the enhancement of surface diffusion and Knudsen diffusion can increase the apparent permeability of methane gas by as much as 50 times or more, demonstrating the sensitivity of pressure and pore size for methane gas transport.