Low Gas Velocities Research Articles

Modeling of hydrocarbon-free potassium flowing-gas diode-pumped amplifier

А 3D CFD model coupled with wave optics model for laser beam propagation is used to calculate the performance of a hydrocarbon-free K end pumped flowing-gas diode pumped alkali amplifier (DPAA). The K DPAA efficiency and output beam quality were calculated as a function of various parameters and the results were compared with those calculated for end pumped Rb DPAA with hydrocarbon-free buffer gas. For moderate pump intensity, both the efficiency and beam quality deteriorate at low gas velocity. The efficiency strongly depends on the pressure of the buffer helium, reaching a maximum at pressure close to atmospheric and falling with a further increase in pressure. The maximum efficiency of the K DPAA (∼0.45) is approximately 4 times higher than that of the Rb one, achieved at the same optimal pump power of 5 kW. Addition of CH4 to the He buffer in the K DPAA decreases the amplifier efficiency and reduces the beam quality.

Experimental investigation on long hydrodynamic slugs in offshore pipeline

In the process of liquid-dominated offshore oil and gas transportation, long hydrodynamic slugs are likely to occur in subsea pipelines at high liquid holdup. A large number of long hydrodynamic slug data at high liquid velocity and low gas velocity were obtained by using visual images in a 46 mm ID, 1657 m long pipeline. The flow pattern map close to the industrial parameters is plotted, and slug flow can take place at a lower superficial gas velocity (VSG = 0.045 m/s) in the long pipeline. The maximum slug length is 2.5 times the average slug length, and the longest liquid slug is 969 D near the transition boundary of slug flow. When VSG < 1.00 m/s, slug length increases rapidly and slug frequency decreases significantly with the decrease of superficial gas velocity at the same superficial liquid velocity. The prediction correlations of slug velocity, length, and frequency are developed for long-distance offshore pipelines at high liquid velocity and low gas velocity, and the maximum error is less than 25%.

Analyzing Microstructural Features, Surface Topography, and Scratch Resistance of Innovative Nano-Composites Coated with High Velocity Air-Fuel Technology

New developments in thermal spraying processes may offer higher-quality alternatives to hard chrome plating and possibilities for hard chrome plating in a range of coating applications. These include spraying with high-velocity air fuel (HVAF) and new spray consumables. The low operating temperatures and accelerated particle velocity of the HVAF process enable investigation and development of a wide range of novel coating materials and applications. The High-velocity Air Fuel Process' quality and efficiency are primarily due to the broad combustion chamber and axial injection of the feedstock through it, as well as the relatively low combustion temperature of an air-fuel mixture and the low gas velocity that provides enough time for the mild heating of the powder particles. The current work discusses the inventive thermal spray procedure used for SAE 1008 carbon steel, a cost-effective substrate material. All of the compositions that were treated have undergone microstructure investigations. A scratch test is conducted in accordance with ASTM guidelines. Assessment of surface morphology clearly demonstrates the relationship between the evaluated parameters. According to the occurrence, scratch methods such as delamination, cracking, plastic deformation, and elastic deformation are highlighted. However, the findings of the scratch test showed that the samples' scratch resistance increased as the coating thickness rose. In comparison to samples with thinner coating, those with thicker coating demonstrated a stronger resistance to scratching. This is explained by the fact that coatings with a higher thickness and density can support the subsurface more effectively and stop cracks from scattering. This can retain the coating's integrity and stop more damage from occurring, improving scratch resistance. Better scratch resistance was displayed by the samples with denser microstructures and smoother surface morphologies. The outcome is greater scratch resistance because a higher density covering can withstand deformation and fracture better than a lower density layer. This is due to the mechanism of deformation and fracture in the coating material. This improvement in scratch resistance can be due to the composites' increased HVAF coating's hardness and adherence. The findings imply that using an HVAF coating to increase the scratch resistance of new nanocomposites may constitute a successful strategy.

Multi-objective optimization of two-phase flow in the proton exchange membrane fuel cells based on a data driven surrogate model

Flooding is detrimental to Proton Exchange Membrane Fuel Cells (PEMFCs). Reducing two-phase pressure drop and flooding relief time alleviates the devastating effect of flooding. To this end, an innovative surrogate model based on Artificial Intelligence is developed for flooding conditions. Volume of Fluid is employed to capture two-phase flow during the flooding in the simulations. A dataset is generated by 3D two-phase CFD simulation. An Artificial Neural Network is developed based on the dataset to predict the dynamic behavior of two-phase during flooding in time series. Flooding criteria are formulated based on the time series. The results show that the model predicts two-phase flow behavior accurately. Low gas velocity compels small parasitic power, but flooding conditions are exacerbated by the low flow rate, especially in the geometries with corners and virtual bottlenecks. Two distinct methods are used to develop multi-objective optimization, fed by the presented formula. Features are cathode channel characteristics, and objectives are relief time and stabled two-phase pressure. The optimum case brings a time of 9.11 ms and a pressure drop of 27.44 Pa. Also, it reduces the stable pressure drop and the time by 83% and 90%, compared with the worst conditions.

Dust-removal performance of an improved spherical cylindrical ECP under magnetic confinement effect

In order to further improve the trapping effect of fine particles, an improved electrostatic cyclone precipitator (ECP) was proposed. The electromagnetic dust-removal mechanism of spherical cylindrical ECP was revealed, and the influences of flue gas velocity on the dust-removal effect of fine particles with and without magnetic confinement effect were discussed. The results show that the overall efficiency curve of fine particles shows a ' hump ' type with the change of flue gas velocity, and the increase of magnetic induction intensity promotes the hump to move to the low flue gas velocity area. Increasing magnetic induction intensity can improve the trapping performance of spherical cylindrical magnetically constrained ECP, and the improvement effect weakens gradually when the same amplitude increases.

A numerical analysis of ash fouling characteristics in elliptical tube bundles

Ash fouling in flue gas heat exchangers (FGHE) detrimentally affects waste heat recovery systems' performance and efficiency by impeding heat transfer, increasing operational resistance, and escalating maintenance costs. The main purpose of this study is to develop an improved particle deposition model for predicting ash fouling characteristics and to explore the fouling reduction performance of elliptical tube bundles (ETB) at various system parameters. The findings reveal that ETB can efficiently reduce operation resistance and ash fouling in FGHE with an insignificant loss in heat exchanger efficiency, especially at large eccentricities (E) and/or low flue gas velocity (vinlet). The most cost-effective E is 0.78, which decreases ash deposit and pressure drop by 66.4% and 48.4%, respectively, at the expense of a 14.2% Nusselt number reduction at vinlet = 10 m/s. Ash fouling reveals three distinctive hotspots: Hotspot 1 located approximately at ±30°, Hotspot 2 at the flow stagnation area, and Hotspot 3 roughly at ±150°. The deposit layer thickness at these hotspots varies between rows and with the E, vinlet and particle diameter (dp). The layer thickness at the flow stagnation regions increases with any of the E, vinlet, and dp. The study also found that medium-sized particles (dp = 5 μm) are the easiest to deposit on tube surfaces, followed by large particles (dp = 10 μm). Small particles (dp = 1 μm) are the least likely to deposit due to their small inertia. Medium-sized and large particles deposit primarily at the windward side of the tubes, while small particles tend to deposit more on the leeward side as they can be easily drawn into the recirculation zones behind those tubes. Increasing the E would significantly alleviate fouling by medium-sized and large particles but does not significantly change the ash fouling situation of small particles.

Experimental investigation on interface characteristics of gas-liquid two-phase flow in a kilometer-scale pipeline

It is greatly essential for the construction of multiphase flow models and the flow assurance of oil and gas pipelines to study the fully developed interface characteristics at different flow patterns in the industrial-grade long-distance pipelines. In this paper, the phase interface structures of dispersed bubble flow and three slug sub-flow patterns are captured by visualization method in a 46 mm ID, 1657 m long pipeline. It is found that the interface characteristics between elongated bubbles and liquid film can effectively distinguish different slug sub-flow patterns, and a quantitative division criterion between different slug sub-flow patterns is proposed. The transition boundary prediction model for dispersed bubble flow and slug flow in long pipeline is established, and the flow pattern map at high liquid velocities and low gas velocities is plotted. With the increase of two-phase mixture velocity, the radial position of elongated bubble head approaches the middle of the pipeline, and the liquid film thickness is stably distributed around 0.37 D. The correlations that predict the radial position of elongated bubble head, liquid film thickness, and elongated bubble velocity for long pipeline are established, with maximum error less than 15 %.

Patterned keyhole porosity formation in laser powder bed fusion caused by local disturbances in the shielding gas flow

Porosity is a major challenge in laser powder bed fusion systems (PBF) and a crucial contributor to fatigue life. The current approach to remedy this challenge is thermal management, which involves optimizing process parameters to minimize porosity formation from repeated stacking of layers to produce a fully dense part. However, auxiliary mechanisms other than laser power, scan speed, and hatch spacing simultaneously affect the repeatability of the process. For example, the irregular flow of shielding gas over a powder bed disrupts the melt’s pool convective heat transfer resulting in keyhole porosity. This study aims to explain the defect formation associated with the print process caused by local disturbances of the shielding gas flow pattern and the interaction of adjacent rasters. Additively manufactured 316 stainless steel cuboids were investigated by x-ray computed tomography. Results indicated that pores were spatially aligned in a pattern and uniformly distributed. A comparison of the CT scans and the print file showed that the pore pattern aligned with the layout of print cells during the melting of odd layers. During odd layers, the position of the recoater disrupted the shielding gas flow. The disrupted flow created regions with low gas velocity, which led to heat retention at the boundary of the print cells. The heat retained generated a concentrated occurrence of keyhole defects since the melt pools were deeper.

Role of Droplet Dynamics on Contact Line Depinning in Shearing Gas Flow.

Sessile droplets exposed to shearing gas flows resist depinning owing to surface tension and contact angle hysteresis. It is known that contact line depinning occurs when the shearing gas flow is large enough to deform the droplet beyond its contact angle hysteresis. This work explores the contact line depinning process by visualizing growing droplets on a porous layer in laminar shear gas flows. High-speed imaging of droplets revealed an oscillatory motion in droplets, which is speculated to originate from an interaction between the drag force and surface tension effects. This oscillatory motion creates an inertial force within the droplet which combines with the drag force when droplet acceleration is in the stream-wise direction. The combined effect competes against the droplet adhesion force, setting the depinning criteria. Analyzing droplet images revealed that droplet local velocity and acceleration (i.e., sessile droplet dynamics prior to detachment from the substrate) increase with the superficial gas velocity. At the same time, the contact line depinning occurs at a smaller droplet size for higher superficial gas velocities. This results in a "hill-like" variation of the inertial force as a function of the convective Weber number, Weconv, causing a local maximum in the inertial force data (Weconv scales the inertia effects of the shear flow to surface tension effects). For the experimental condition tested in the current study, the inertial force created in the droplet could reach up to half of the adhesion force, making the drag force only responsible for the other half to depin the droplet contact line. Even at low superficial gas velocities, which featured lower droplet oscillations, the inertial force created in the droplet was considerable with respect to the adhesion force, reaching around one-third of the adhesion force.

Enhancement of electrostatic suppression in bubbling fluidized bed through carbon fiber addition

Electrostatics is known to cause significant disturbance to particles in gas-solid fluidized beds, necessitating the deployment of an efficient electrostatic suppressor. As such, this study delves into the electrostatic suppression effects of carbon fiber, carbon fiber powder, and their hybrids in a fluidized bed, including their fluidization characteristics, using electrostatic measurements and CFD-DEM simulations. The results show that the additives had noteworthy electrostatic inhibition effects, albeit with distinct limitations. Carbon fiber was effective at high air velocities, but its performance was subpar at low gas velocities. In contrast, carbon fiber powder was suitable for low gas velocities, but turbulence diluted its effectiveness. Such limitations can, however, be mitigated through optimization of a specific mixing ratio between the two additives. The proposed carbon fiber (powder) composite reveals its potential as an effective electrostatic suppressor.

Effects of pressure and particle size on bubble behaviors in a pseudo 2D pressured fluidized bed with Geldart A/B, B and D particles

Bubble behaviors of three types of particles from Geldart A/B, B and D groups were comprehensively investigated in a pseudo 2D pressurized fluidized bed via digital image analysis. Based on the analysis of a large quantity of experimental data, the effects of pressure and particle size on the spatial distribution of bubbles, bubble diameter, rise velocity, bubble fraction and flow regimes were obtained. Results show that bubbles are smaller and uniformly dispersed throughout the bed of Geldart A/B particles, while they become densely populated and grow into much larger sizes in the bed of Geldart B particles. A single peak of large bubbles in the center and emptying of bubble near the walls is formed in the bed of Geldart D particles due to the preferable rising path of bubbles towards the center and the significant coalescence of bubbles. Bubble diameter decreases with increasing pressure, and the effect of pressure is more pronounced on Geldart D particles. Bubble rise velocity increases with the increasing particle size and decreases as pressure rises. Large bubbles/slugs hardly appear in the bed of Geldart A/B particles, while they are clearly visible in the bed of Geldart B and D particles. The flow regime transition from slugging to fast fluidization shifts to the lower gas velocity for Geldart D particles compared with Geldart B particles.

Filtration for nano/micron-sized particulates with a moving bed filter

Particulate Matter (PM) pollution has attracted much attention as it represents a serious threat to public health. Granular bed filtration is a highly efficient gas cleaning technique at high temperatures. A counter-current moving bed granular filter with novel internal components was proposed to capture fine particulates. The effects of the moving bed filter on removing the fine dust particulates with various sizes (micron and nanometer scale) were investigated. The experimental results indicate that dust is easily captured under a low superficial gas velocity or a high capturing particle moving velocity. In a moving bed filter with the counter-current flow, diffusion is the dominant collection mechanism for the nanometer dust; on the other hand, the micron dust capturing is a combined effect with multiple mechanisms including diffusion, sedimentation, interception, and impaction. An efficiency calculation model was constructed based on the experimental results and the single-particle filtration mechanism.

Configuration-dependent dynamics of non-spherical particles in a gas–solid fluidized bed

Configuration-dependent dynamics of non-spherical particles are investigated experimentally in a quasi-two-dimensional gas–solid fluidized bed. We show that the vertical velocity profiles are sensitive to the aspect ratio at low gas velocity, but not sensitive at high gas velocity. The velocity distribution, especially the rotational velocity distribution, deviates from the Maxwellian distribution. The joint probability density distribution of vertical and rotational velocity is asymmetric. We identify two high-probability particle configurations, two particles in a parallel or vertical arrangement, by calculating the radial distribution function, nematic correlation function and coordination number. We further explain the cause of this order appearance through entropy theory. By comparing the drag force of the particle configurations at various angles and Re, we demonstrate that it is necessary to put forward the configuration-dependent drag force for non-spherical particle fluidization.

Porosity formation from disrupted gas flow in laser powder bed fusion of 316 stainless steel

Eliminating porosity remains a major challenge in metal additive manufacturing (AM). The current approach is thermal management, which involves optimizing process parameters to minimize porosity formation from repeated stacking of layers. However, auxiliary mechanisms other than laser power, scan speed, and hatch spacing simultaneously affect the repeatability of the process. For example, the flow of shielding gas over a powder bed causes convective heat transfer that results in keyhole porosity. Although conduction is widely attributed as the main mode of heat transfer during printing, in this study, the importance of convection was highlighted when keyhole porosity was observed to be spatially aligned in a pattern and uniformly distributed based on the recoater position. The pore pattern aligned with the layout of print cells when the recoater position was on the left (odd layers). Local disturbances caused by the printing process on 316 stainless steel were investigated by x-ray computed tomography. Results indicated that porosity formation emanated from an irregular nitrogen flow across the build-plate. The nitrogen flow was periodically disrupted throughout the print as the position of the recoater during odd layers obscured gas movements close to the build-plate. These observations were validated with a simulation of the gas flow on a 2D cross-section of the build-chamber using ANSYS Fluent. The disrupted flow created regions with low gas velocity, which led to a heat retention at the boundary of the print cells. The heat retained generated a concentrated occurrence of keyhole defects since the melt pools were deeper. The effect of the shielding gas, which is usually neglected, was highlighted as an important factor to ensure part quality.

Rapid establishment of algal-bacterial granular sludge system by applying mycelial pellets in a lab-scale photo-reactor under low aeration conditions: Performance and mechanism analysis

Light-driven algal-bacterial granular sludge (ABGS) is an innovative low-carbon technology with significant merits in treating municipal wastewater, but how to shorten the photogranulation process, especially under low aeration conditions, is largely unknown. Herein, two strategies were proposed to accelerate the start-up of the ABGS system in photo-sequencing batch reactors (PSBRs) with a low superficial gas velocity of 0.5 cm/s. Compared to directly dosing mycelial pellets (MPs), applying MPs to flocculate algae and using the formed algal-mycelial pellets (AMPs) as carriers enhanced the establishment of the algal-bacterial symbiosis. The ABGS system developed rapidly within 20 days, with a large particle diameter (mean diameter of 321 μm) and excellent settleability (SVI30 of 55.4 mL/g). More importantly, this system could be stably operated for at least 100 days, mainly attributed to the reinforced secretion of protein with unique secondary structure and elevated hydrophobic functional groups. As for the reactor performance, the average removal efficiencies of the ABGS system were 97.8% for organic matter, 80.0% for total nitrogen, and 84.4% for phosphorus. The enrichment of functional bacteria and algae, and the up-regulation of functional genes and enzymes involved in electron production and transport processes likely drove the transformation of the pollutants, underlining the inherent mechanism for the excellent nutrient removal performance. This study provides a promising approach to solve the problem of a long ABGS start-up period and unstable granular structure under low aeration conditions, which is significant for achieving effective wastewater treatment without energy intensive aeration.

A mesoscale bubble-induced turbulence model and simulation of gas–liquid flows

In gas–liquid two-phase flows, bubble motion significantly affects liquid phase turbulence, and adding bubble-induced turbulence (BIT) source term is widely used to improve the simulation accuracy. This paper presents a new BIT model based on the energy-minimization multi-scale (EMMS) methodology. The model was constructed by considering two mesoscale factors, i.e., the sub-grid structures through analyzing the slip velocity and the gas holdup gradient, and the equivalent diameter of turbulent eddies calculated by the EMMS-based turbulence model. In order to verify its performance, the model was incorporated to the Eulerian–Lagrangian simulating framework and applied to two typical experimental systems. Both mean flow characteristics and turbulence quantities were well predicted, and the new model showed advantages over traditional BIT models, especially at higher gas velocities. Moreover, a strategy for counting energy dissipation in the simulation was devised and performed whereby the dual effects of promotion and suppression on liquid phase turbulence by bubbles can be reflected. The simulations demonstrated that BIT dominated the energy dissipation and turbulence was enhanced by BIT at higher gas velocities, while shear-induced turbulence dominated the energy dissipation and turbulence is reduced due to the suppression by bubbles at lower gas velocities.

Numerical study of particles removal from dusty gas in separation device with straight arc-shaped elements

The paper presents an original design of the separator with straight rows of arc-shaped elements, which allows efficient separation of solid particles without significant abrasive wear of the device’s surfaces. A three-dimensional model of the device is developed, and the principle of its operation is described. In this paper, a numerical study of the capture of solid particles from a gas flow in the separation device is performed. It is found that at a relatively low inlet gas velocity (1 m/s), the separation efficiency of particles with a diameter of 20 to 100 μm is on average 78.9%. An increase in the inlet velocity of the dusty gas flow results in a decrease in the efficiency of the device for particles ranging in size from 25 to 80 μm. The pressure drop of the separator at a velocity of 1 m/s ranges from 45.5 to 95.4 Pa, and at a gas velocity of 4 and 7 m/s – from 740.8 to 4652.1 Pa. An increase in the number of straight arc-shaped elements from 4 to 12 and an increase in particle density from 3400 to 10000 kg/m3 leads to improved separation efficiency.

Investigation of the hydrodynamics of packed-fluidized beds: Characterization of solids flux

This work investigates the impact on solid flux of applying Aluminum Silicate Balls (ASB) random packing in packed-fluidized beds. The interaction of different packing sizes, bed particle sizes and superficial gas velocity was studied. Experiments were conducted in a cylindrical cold-flow reactor with an inner diameter of 12 cm and a height of 1 m. The ASB packings were applied on a net grid in the middle of the reactor. Bed material was added from the top and temporarily formed a fluidized bed above the packed section. Bed particles then trickled downwards through the packed section during confined fluidization. The bed material was silica sand, with mean particle sizes of 119,181, and 303 μm. ASB packings of two sizes (12.7 and 6.3 mm) were evaluated. Air was used as fluidizing gas, applied at the superficial gas velocities of 0.1 m/s, 0.2 m/s and 0.3 m/s. The measured solid flux was 1.2–13.6 kg.m−2.s−1. Large particle size, large packing size and low superficial gas velocity resulted in high flux, and vice versa. It was concluded that ASB packings constitutes a significant hindrance for solids throughflow in packed-fluidized bed applications. However, it could be interesting for counter-current flow applications.

Evaporative cooling for polymer electrolyte fuel cells - An operando analysis at technical single cell level

Evaporative cooling has the potential to reduce stack and system volume of polymer electrolyte fuel cells by up to 30% and to cut costs significantly by simplifying the multi-layered structure of bipolar plates and eliminating the need for external humidification.This study provides an experimental proof of concept by analyzing the evaporation behavior, cooling power, internal humidification, electrochemical performance and operational stability of evaporative cooling under technical cell boundary conditions. Isothermal in situ as well as operando studies are carried out in an evaporatively cooled fuel cell (15 cm2 active area, 80 °C, ambient pressure). Relative humidity boundary conditions imitate the inlet, center and outlet of a technical cell.Main findings show that the evaporation rate is saturation limited at low gas velocities, whereas it is transport limited at higher gas speed. Operando measurements prove that evaporative cooling works at a technical cell level with multiple water supply lines. The entire waste heat is removed and sufficient membrane humidification is achieved when dry inlet gases are used. Furthermore, it is shown that optimal performance is achieved in a counter-flow arrangement. The observed electrochemical performance is comparable to conventional cooling with humidified gases at inlet, center and outlet of the cell.

Dynamic densification of wet granular materials in continuous dense flows under gravity

AbstractThe continuous granular flow under gravity extensively exists in chemical processes, and the dynamic behaviors of granular flow under different gas–liquid environments are investigated by synchronous slide‐type high‐speed photography. Results show that a hexagonal arrangement is observed after three‐stage densification in wet granular flow, while these structures are almost absent in the dry system. Further analysis shows that under the induction of liquid, the special cavity collapse in stage І is the pre‐condition of granular densification, and the subsequent densification in stage ІІ for the first time gives the experimental proof for the hypothesis that energy barriers exist and depend on the friction characteristics, and the cage effect in stage ІІІ provides a guarantee for the formation of a hexagonal contact network. Besides, three‐stage densification in the three‐phase system mainly occurs in the region with low gas and liquid velocities (G* &lt; 0.716 and L* &lt; 0.023 in this work).