Gas Phase Distribution Research Articles

Numerical simulation of coal gasification in molten slag: Gas-liquid interaction characteristic

The high quality waste heat recovery of the molten blast furnace slag was necessary and urgent. In this paper, the flow characteristic of the molten slag reactor was investigated by numerical simulation. Firstly, the three-dimensional model was established for investigating the gas-liquid two-phase flow. The gas-liquid flow was modeled to be turbulent, which was described by the RNG k-ε model, and the interface of gas and liquid was conducted by the VOF model. Secondly, the top-submerged cold experiment system was constructed to validate the accuracy of the simulation model. Thirdly, the bubble behavior, gas phase distribution and molten slag motion were investigated. The bubble in the reactor would go through five stages and the maximum gas fraction reached about 4.87% at 0.75s. Meanwhile, the gas phase distribution and molten slag motion were closely related to the bubble behavior. Ultimately, the matrix analysis method was applied to obtain the optimal parameters of the reactor. The optimal condition improved the flow behavior in the molten BFS reactor significantly. The present results of the simulation provided an insight for the gas-liquid two-phase flow in molten slag reactor, which would provide the theoretical guidance for industrial applications.

Investigating the cross-sectional gas holdup distribution in a split internal-loop photobioreactor during microalgae culturing using a sophisticated computed tomography (CT) technique

In this study, an advanced noninvasive gamma-ray computed tomography (CT) technique was used for the first time to visualize and quantify the cross-sectional gas holdup distribution of the multiphase flow of an air–water-microalgae system in a cylindrical split airlift photobioreactor. The gas holdup distribution and their radial profiles were investigated during the culturing of the green microalgae Scenedesmus. The gas phase distribution in the entire reactor, including riser region, downcomer region, distribution regions, as well as their distributions above and below the split plate was also characterized, along with the impact of two superficial gas velocities (0.01 and 0.03m/s), different axial levels, and the changes in the culture from first day until the medium became very dense. The results showed an excellent gas phase distribution in most of the reactor’s regions and had high values around the sparger ring and above the split plate. Moreover, the results indicated that the flow distribution may significantly affect the performance of the photobioreactor, which may have substantial effects on the cultivation process. The results are reliable as benchmark data to validate computational fluid dynamics (CFD) simulations and other models.

Geophysical response to simulated methane migration in groundwater based on a controlled injection experiment in a sandy unconfined aquifer

Geophysical methods have the capacity to detect and characterize gas-phase dynamics in groundwater. Suitable methods can be deployed at surface or within boreholes depending on the required depth of investigation, spatial/temporal resolution, and geologic conditions. While the application of geophysical methods to monitor immiscible phase contaminants in the subsurface has been extensively documented, the effects of hydraulic properties and flow system conditions on the nature of the geophysical responses used to elucidate multi-phase fluid flow remains underdeveloped. A series of numerical 2-dimensional multi-phase flow and geophysical model simulations based on a controlled methane release experiment in the Borden unconfined sand aquifer was carried out to assess the influence of porous media hydraulic properties and flow system conditions on geophysical signatures associated with transient gas-phase saturation and gas migration behaviour. Specifically, the utility of electrical resistivity tomography (ERT) and ground-penetrating radar (GPR) to monitor gas-phase plume dynamics in shallow groundwater flow systems is examined. ERT and GPR responses to gas-phase distribution and migration during a 72-day methane gas injection and subsequent recovery period was calculated using a numerical multi-phase flow model (CFbio) simulating four distinct parameterizations of the sandy aquifer system. Geophysical models showed that ERT was effective at imaging the central position of the plume but was less effective at detecting thinner lateral migration pathways extending beyond the primary high gas saturation bulb. Conversely, GPR was able to detect thin gas pools emanating from the primary gas bulb and small-scale vertical preferential pathways arising from capillary boundaries with contrasting saturations; however, gradational boundaries proved to be more difficult to resolve using GPR. This study demonstrates that ERT and GPR can be very useful tools in combination for longer-term monitoring of stray gas leakage from decommissioned hydrocarbon wells in shallow granular media freshwater aquifers, especially given the likelihood of strong lateral migration.

Distribution performance of gas–liquid mixture in the shell side of spiral-wound heat exchangers

Abstract The non-uniformity of gas–liquid mixture is a critical issue which leads to the heat transfer deterioration of spiral-wound heat exchangers (SWHEs). Two-phase mass flow rate and the content of gas are important parameters as well as structural parameters which have prominent influences on flow distribution uniformity of SWHE shell side. In order to investigate the influences of these parameters, an experimental test system was built using water and air as mediums and a novel distributor named “tubes distributor” was designed. The effects of mass flow rate and the content of gas on two-phase distribution performance were analyzed, where the mass flow rate ranged from 28.4 to 171.9 kg·h−1 and the content of gas changed from 0.2 to 0.8, respectively. The results showed that the mixture mass flow rate considerably influenced the liquid distribution than that of gas phase and the larger mass flow rate exhibited the better distribution uniformity of two-phase flow. It was also found that the tubes distributor had the better two-phase uniformity when the content of gas was around 0.4. Tube diameter played an important role in the distribution of gas phase and slit width was more significant for the uniformity of liquid phase.

A simulation study to enhance the gas production rate by nitrogen replacement in the underground gas storage performance

Underground gas storage (henceforth; UGS) is considered as a common method which resolves the seasonality problem of transportation and distribution of gas phase in the oil and gas industries. The main objective of this study is to simulate one of Iranian’s depleted gas condensate reservoir and compare different injectivity scenarios especially nitrogen injection to enhance the gas production. Based on the anticipated target rate performance of UGS with a different scenario that was analyzed the effect of injection gas, finding optimum original reservoir pressure, an optimum number of wells. Moreover, abandonment pressure is one of the main concerns for UGS process in partially depleted reservoirs especially in the depleted reservoir to decrease the ultimate recovery. It contains insufficient base gas reserves, and if it may not meet the target withdrawal rate, the original reservoir pressure has been estimated at 250 bar. Consequently, it found that this problem can be addressed by injecting a higher volume of gas in the first cycle and due to the existence of new wells, the maximum injection rate was obtained about 6.1∗1010m3.

Numerical Simulation of a Gas-Liquid Centrifugal Pump under Different Inlet Gas Volume Fraction Conditions

In order to study the influence of inlet gas volume fraction (IGVF) on performance of a gas-liquid centrifugal pump, three-dimensional turbulent flow of a single-stage gas-liquid centrifugal pump has been simulated by using computational fluid dynamics (CFD). Both steady and unsteady simulations have been conducted for different inlet gas volume fraction conditions of the pump. The gas phase distribution, the internal flow field and pressure field of the pump were obtained with inhomogeneous two-fluid model. The result showed that many gas accumulation zones accompanied vortices appears in the impeller as IGVF increases, and surge would occur as IGVF reaches a certain value. The change of IGVF would change the magnitude and direction of the impeller radial force.

Fractal characteristics of unsaturated sands − implications to relative permeability in hydrate-bearing sediments

Precise estimation of gas production from hydrate deposits heavily relies on an accurate description of relative permeability to water and gas in hydrate-bearing sediments. Relative permeability innately depicts the competing flow of gas and water phases for the pore spaces in sediments, and thus, is affected by pore characteristics and pore-scale morphology of water and gas. Characterization and quantification of water and gas distribution in pores at various water and hydrate saturation are not always offered. This study aims to establish a fractal theory based relative permeability model considering pore characteristics and the physics of pore-scale water and gas distribution. X-ray computed tomography (CT) images show that the distribution of water and gas phases in quartzitic sands are fractal. The area and tortuosity fractal dimensions, that quantitatively describe the water and gas distribution in the pores, can be computed from the CT images using the box-counting method. These fractal dimensions are further used to establish a fractal theory based relative permeability model. The proposed model implies that relative gas permeability is expected to increase and relative water permeability slightly decreases even the effective water saturation is unchanged during gas production from hydrate deposits, as hydrate dissociation increases the overall pore diameter and lowers capillarity that allows gas to migrate with less effort.

Orthogonal Experiments on the Key Structural Parameters Optimization of Mine Dust Removal Foam Generator by Fluent Numerical Simulation

As an important countermeasure of dust control in mine, the efficiency of foam generator is seriously affected by the key structural parameters itself, including the baffle inclination angle (θ), the disturbance flow distance (L) and the diffusion angle (α). To ascertain the relation of the three parameters, the axial cross section velocity field and gas phase distribution field in the foam generator were simulated by FLUNET software and mixture method, standard κ-ε turbulence model and SIMPLEC algorithm. At the same time, the orthogonal tests were conducted for reducing amount of calculation. The results show that when θ = 30°, L = 22mm, α = 74°, the foam generator is on the optimal working condition. And the importance degree of the key structural parameters is successively θ, L and α. It can play a certain reference role in the actual design of the foam generator for the dust removal technology improvement of the foam generator.

ANALYTICAL SOLUTIONS FOR WATER–GAS FLOW THROUGH 3D ROCK FRACTURE NETWORKS SUBJECTED TO TRIAXIAL STRESSES

This study presented the analytical solutions for water–gas flow through three-dimensional (3D) fracture networks subjected to triaxial stresses. The relationship between fractal dimension for fracture aperture distribution subjected to triaxial stresses and that subjected to no stresses is established, and the analytical solutions for fractal dimensions for aperture distribution and the equivalent permeability of both fluid and gas phases were derived. The results show that the calculated relative permeability of water phase-saturation curves agree well with those reported in the literature, which indicates that the proposed solutions are validate. With the increment of normal stresses applied on the fracture surface, both the maximum aperture and minimum aperture decrease; however, their ratio increases first and then decreases. The fractal dimensions for fracture aperture distribution of water and gas phases with respect to saturation are axisymmetric along saturation [Formula: see text]. With the increase in saturation, the fractal dimension for fracture aperture distribution of water phase increases significantly when the saturation is less than 0.1, and then gently when the saturation is continuously increased by up to 1.0. The normal stress increased by two orders of magnitude for a larger normal stress (i.e. increased from [Formula: see text][Formula: see text]MPa to 10[Formula: see text]MPa) corresponds to smaller variations in equivalent permeability of both water and gas phases for a smaller normal stress (i.e. increased from [Formula: see text][Formula: see text]MPa to [Formula: see text][Formula: see text]MPa).

Experimental and numerical study on CO2 absorption mass transfer enhancement for a diameter-varying spray tower

In this research article, orthogonal experiments were performed in a proposed diameter-varying spray tower system to evaluate the importance of operating parameters and analyse the relationship between factors and the mass transfer evaluation indices (the CO2 removal rate and overall absorption rate). Two different spray methods such as dual-nozzle opposed impinging and spray in the middle method were discussed for performance comparison. Optimal operating conditions for different evaluation indices were obtained by using range analysis. After analysis it was found that the absorption performance was mainly affected by two parameters, monoethanolamine concentration and CO2 concentration. The effects of different operating parameters on the CO2 removal rate and overall absorption rate were also discussed by using trend analysis. Furthermore, gas phase mass transfer coefficient and effective contacting area were determined by experimental data. A comparison with reported conventional reactor systems demonstrated a great application potential for proposed spray tower in CO2 capture. Finally, simulation results were illustrated which shows that the gas phase and liquid phase distributions for the dual-nozzle opposed impinging spray method enhance CO2 absorption performance.

CFD studies on the gas-liquid flow in the swirl generating device

In this work, CFD simulations using the Euler-Euler approach were performed to model the gas-liquid flow in a swirl-generating device. The computational work was based on experiments, which are conducted in a vertical pipe packed with a static swirl element. Measurements of gas volume fractions at several planes within the swirl element were taken using high-resolution gamma-ray computed tomography (HireCT).The simulations were carried out for the experimental conditions with defined inlet gas volumetric flow rates of 5 and 10%. The profile of several key parameters (e.g. pressure, liquid and gas velocities and gas fraction) are used to understand the flow behavior inside the device. The radial gas phase distribution obtained from the simulations assuming different mono-disperse and bi-modal bubble sizes is compared against the experimental results. The significant influence of the selected bubble sizes on the profile is shown and discussed within this paper. In general, the radial profile of gas fraction is well captured by the CFD simulations except in the transition zone where a significant discrepancy to the experiment is observed.

CO2 gas–liquid mass transfer and kLa estimation: Numerical investigation in the context of airlift photobioreactor scale-up

This paper deals with gas–liquid mass transfer in an airlift via CFD simulations in the context of photobioreactor (PBR) scale-up. Two aspects are emphasized. Firstly, since carbon uptake by microalgae is of crucial importance as part of PBRs, CO2 transfer is in focus, and numerical simulations are developed to take into account CO2 gas–liquid transfer and dissociation in the aqueous phase. Secondly, since estimating kLa is of crucial importance when scaling-up PBRs, different ways to evaluate kLa are discussed using numerical experiments. Firstly, kLa may be estimated as the volume average value of the local mass transfer coefficients calculated from steady-state hydrodynamics and Higbie penetration model. Secondly, kLa can be deduced from classical dynamic gassing-out/gassing-in experiments. This second method is simulated for O2, as commonly performed experimentally, and also with CO2 since it is the transferred species in PBRs. Results show that kLa field is strongly heterogeneous, as expected in airlifts where gas is mainly present in the riser. Performed with O2, the gassing-in method leads to quite accurate estimation of the spatial average value of local kLa. But, gassing-in methods performed with O2 and CO2 lead to discordant results. In fact, CFD shows that the CO2 depletion in the gas phase has to be accounted for to predict kLa from CO2 gassing-in method, especially at large scale. This study also puts into evidence the potentialities of CFD which allows to get detailed image of local gas–liquid mass transfer, depending on two-phase hydrodynamics, gas phase distribution and transferred species solubility.

Variation principle in calculating the flow of a two-phase mixture in the pipes of the cooling systems in high-rise buildings

The analytical solution of one of the urgent problems of modern hydromechanics and heat engineering about the distribution of gas and liquid phases along the channel cross-section, the thickness of the annular layer and their connection with the mass content of the gas phase in the gas-liquid flow is given in the paper.The analytical method is based on the fundamental laws of theoretical mechanics and thermophysics on the minimum of energy dissipation and the minimum rate of increase in the system entropy, which determine the stability of stationary states and processes. Obtained dependencies disclose the physical laws of the motion of two-phase media and can be used in hydraulic calculations during the design and operation of refrigeration and air conditioning systems.

Optimal Designing of Mass Transfer Apparatuses with Jet-Film Contact Devices

A design of a jet-film contact device developed for increasing the uniformity of liquid and gas phase distribution and enhancing mass transfer efficiency is described. The influence of the width of the drain cup on the mass transfer efficiency and hydraulic resistance is analyzed.

Pressurized Steam Torrefaction of Biomass: Focus on Solid, Liquid, and Gas Phase Distributions

Torrefaction is a thermal pretreatment of biomass feedstocks aimed at their conversion into a commodity solid fuel with more uniform and standard properties. Pressurized torrefaction—a novel concept—deserves consideration because of its possible advantages with respect to the atmospheric process, in particular the establishment of favorable conditions for generation of valuable condensed products. The objectives of this research are (i) an investigation on the feasibility of the pressurized, steam-assisted, batch torrefaction of some agro-industrial biomass residues and (ii) the conversion of these latter into solid and liquid products to be used in energy production or chemical processes, with improved characteristics with respect to the raw biomass. The results reported in this article prove that the operation under pressure allows maintaining high water vapor pressure in the system, enhancing the solid biomass conversion to liquid products. The recovery of valuable liquid compounds from the solid resid...

Eulerian–Eulerian Modeling of Convective Heat Transfer Enhancement in Upward Vertical Channel Flows by Gas Injection

Two-phase bubbly flows by gas injection had been shown to enhance convective heat transfer in channel flows as compared with that of single-phase flows. The present work explores the effect of gas phase distribution such as inlet air volume fraction and bubble size on the convective heat transfer in upward vertical channel flows numerically. A two-dimensional (2D) channel flow of 10 cm wide × 100 cm high at 0.2 and 1.0 m/s inlet water and air superficial velocities in churn-turbulent flow regime, respectively, is simulated. Numerical simulations are performed using the commercial computational fluid dynamics (CFD) code ANSYS fluent. The bubble size is characterized by the Eötvös number. The inlet air volume fraction is fixed at 10%, whereas the Eötvös number is maintained at 1.0 to perform parametric studies, respectively, in order to investigate the effect of gas phase distribution on average Nusselt number of the two-phase flows. All simulations are compared with a single-phase flow condition. To enhance heat transfer, it is determined that the optimum Eötvös number for the channel with a 10% inlet air volume fraction has an Eötvös number of 0.2, which is equivalent to a bubble diameter of 1.219 mm. Likewise, it is determined that the optimum volume fraction peaks at 30% inlet air volume fraction using an Eötvös number of 1.0.

Experimental investigation of two-phase flow distribution in plate-fin heat exchangers

Flow maldistribution causes declining plate-fin heat exchanger thermal-hydraulic performance. A first-of-its kind experimental facility and the related data acquisition system were constructed for studying liquid–gas flow distribution in a plate-fin heat exchanger. The gas Reynolds numbers ranged from 1880 to about 2600 and the inlet dryness (i.e., quality) from 12% to 41%. Two-phase flow maldistribution among the heat exchanger passages was more widespread compared to that of single-phase flow. More specifically, the liquid-phase distribution was more uneven compared to the gas-phase distribution. The inlet flow rate and dryness were identified as the chief factors affecting the distribution of phases in the heat exchanger. For a given inlet dryness, the two-phase flow distribution became increasingly non-uniform with the inlet gas flow rate, consistent with the behavior observed for single-phase flow. Additionally, the non-uniformity in the gas flow distribution decreased and that in the liquid flow non-uniformity increased with increasing inlet dryness fraction. A novel distributor design, with a complementary fluid cavity was also built and tested. Experimental results show that improving the distributor design is very effective in improving the two-phase flow distribution in plate-fin heat exchangers. Based on heat transfer studies conducted at a single Reynolds number of about 1500 and a dryness of 29.2%, the heat exchanger effectiveness was also correlated as a function of the dryness distribution non-uniformity parameter Sx. The effectiveness was found to reduce as the flow distribution became more uneven, highlighting the importance of accounting for and controlling the flow maldistribution through proper distributor design.

Pore-scale multiphase flow modeling and imaging of CO2 exsolution in Sandstone

This study utilizes synchrotron X-ray micro-tomography and pore scale modeling to investigate the process of gas exsolution and how it affects non-wetting phase relative permeability. Exsolved gas distributions are measured on Domengine and Boise sandstone samples using synchrotron X-ray micro-tomography. Observed gas phase distributions are compared to a new model that simulates the growth and distribution of exsolved gas phase at the pore-scale. Water relative permeability curves are calculated using a Stokes flow simulator with modeled and observed gas distributions, under various conditions, such as rock geometry, and pressure depletion rates. By comparing the actual bubble distributions with modeled distributions, we conclude that exsolved gas is more likely to form and accumulate at locations with higher water velocities. This suggests that convective delivery of CO2 to the gas bubble is a primary mechanism for bubble growth, as compared to diffusive transport through the aqueous phase. For carbonated brine flowing up a fault at half a meter per day, with 5% exsolved gas, the water relative permeability is estimated to be 0.6∼0.8 for various sandstones. The reduction of water mobility reduces upward brine migration when even a small amount of exsolution occurs.

Gas Evolution at Porous Electrodes

Porous electrodes for PEM electrolyser and fuel cell technologies are optimized primarily to provide a high specific surface area for the desired electrochemical reactions. However, the increase in surface area comes at the cost of increased voltage losses due to the transport of reactant and product species through the porous medium. In gas evolving electrolyzer electrodes, the formation and transport of gas bubbles plays an important role: on the one hand, gas in the pores replaces the electrolyte phase, thus hindering ion transport and reducing the effective ionic conductivity in the porous electrode; on the other hand, gas bubbles cover and deactivate a portion of the internal surface area. The impacts of gas formation and removal must be accounted for in the design of electrolyzer electrodes. The focus of the presented work is on the fundamental understanding of the relationships between structure, properties and performance of porous gas-evolving electrodes. The approach combines a macro-scale performance model of the electrode with a micro-scale model of gas evolution. At the macro-scale, a classic porous electrode model describes the transport of ions, electrons and produced oxygen using effective medium theory and accounting for the gas phase volume and distribution. The micro-scale model describes the formation and growth of gas bubbles based on chemical energy considerations. It can explain the experimentally found high oversaturation that is necessary to nucleate bubbles [1]. Additionally, the size of the bubble nucleus in the experiment was estimated. The model was used to study the influence of structural parameters on the operation of porous electrodes and to establish guidelines for an enhanced electrode design. It was found that the transport regime of the dissolved gas, viz. diffusion control vs. transfer control at the liquid-gas interface, determines the bubble growth law. Applications of the model to gas evolving porous electrodes in electrolyzers with liquid electrolyte as well as polymer electrolyte electrolyzers (PEEC) will be discussed. [1] Chen, Q., Luo, L. and White, H. S. Electrochemical generation of a hydrogen bubble at a recessed platinum nanopore electrode. Langmuir 31, 4573–4581 (2015). Figure 1: Bubble growth on an artificial nucleation site at 20 mA/cm2. Comparison of model results with experimental data from C. Brussieux et al., Electrochimica Acta 56 (2011) 7194-7201. Deviation at large bubble radii due to deformation of the bubbles before detachment, which is neglected in the model. Figure 1

An analysis of the velocity field distribution inside the flotation chamber

This paper presents the experimental data of the measured velocity field of a water and air-water system within a laboratory flotation chamber at various rotational speeds of the rotor and aeration. These results were used to verify the gas-phase distribution in the flotation chamber of a process conducted under specified hydrodynamic and physicochemical conditions. The PIV technique was used to determine the flow field in a number of planes; with this method non-invasive measurements of the velocity field in single and two-phase systems were carried out. An analysis of the results allows one to determine the parameters describing the hydrodynamic conditions in the flotation chamber and evaluate the effectiveness of the flotation process.