This article presents a state-of-the-art review focusing on the current understanding of the multiscale multiphase phenomena inside Bubble Column Reactors (BCRs). Although many reviews are available ...on BCRs, little attention has been devoted to summarizing multiscale multiphase phenomena, which are common fundamental issues encountered in their applications. These issues range from the microscale of single bubble dynamics to the mesoscale of bubble swarms and up to the macroscale of the reactor. Understanding these phenomena in all relevant scales can help the rational design, scale-up and optimization of BCRs. The microscale bubble dynamics including the bubble shape and motion, the relevant forces involved and the single bubble mass transfer, is summarized. At the mesoscale, the hydrodynamics of a bubble swarm is influenced by the bubble-bubble or bubble-liquid interactions and hence the overall transport properties of a bubble swarm are not linearly related to that of a single bubble. The bubble swarm effect and the bubble breakage and coalescence mechanisms are discussed in detail. In the end, the macroscale or reactor scale dynamics is strongly governed by the interplay between microscale and mesoscale phenomena, but more research focusing on mesoscale phenomena will be particularly needed for improving our understanding of BCRs.
•Current knowledge of multiscale multiphase phenomena inside BCRs is reviewed.•Understanding bubble scale is necessary but insufficient for predicting macroscale.•Understanding of mesoscale is key to fill the gap between micro- and macroscales.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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•Solid–liquid mixing of viscous suspensions is investigated using CFD-DEM.•Fraction of suspended particles is measured numerically and compared to experiments.•Only the rolling and ...sliding friction affect the fraction of suspended particles.•For clearances of C=T/4 and C=T/3 a dead zone remains below the impeller.•Decreasing the clearance to C=T/5 removes the dead zone.
In chemical engineering, numerous processes require the suspension of particles in a laminar or transitional regime. For such operations, predicting the fraction of suspended particles as well as their distribution and homogeneity is a major concern. In this work, the unresolved CFD-DEM model introduced by our group for solid–liquid mixing is used to investigate the mixing dynamics of viscous suspensions. The techniques chosen to characterize the degree of suspension, the homogeneity and the distribution of the particles are presented. They are used to assess the efficiency of a pitched blade turbine with a clearance of C=T/4. The impact of solid properties on mixing dynamics is investigated by varying the Young's modulus, the coefficient of restitution and the sliding friction coefficient in the DEM model. Lastly, five alternative configurations of the mixing rig are investigated by varying the clearance of the impeller and introducing baffles.
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GEOZS, IJS, IMTLJ, IZUM, KILJ, KISLJ, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
In borehole heat exchangers (BHE), grouting material plays a significant role in the heat transfer between the ground and the heat carrier fluid in the pipes. To guarantee proper sealing capacity of ...the grouting materials, the grout must also fulfill suitable hydraulic and mechanical properties. This paper evaluates the performance of various grouting materials, through thermal, hydraulic and mechanical laboratory characterizations. In particular, the addition of graphite powder to improve the thermal properties of grouting material is tested. In parallel, the characteristics of two different widely used commercial grouting materials (i.e. bentonite-based and silica sand-based materials) are also investigated. Afterwards, the specific heat exchange rate and the borehole resistance of borehole heat exchangers are assessed experimentally in a 1 × 1 × 1 m3 sandbox under, successively, dry sand and fully water-saturated sand conditions. During the operations, the monitored temperatures in the sandbox are in good agreement with analytical predictions. This study demonstrates that the homemade admixture prepared with 5% natural flake graphite can be considered as an appropriate grout for BHEs regarding to its rheological and thermo-physical properties. Thermally-enhanced grouting can be of significant interest in a high thermal conductivity ground (such as saturated sand) because it minimizes the thermal resistance of the BHE.
•An admixture is developed with natural graphite flakes for grouting material.•Thermo-physical, hydraulic and mechanical characteristics are measured.•The impact on the thermal conductivity is investigated.•The efficiency of the grout thermal conductivity rises for highly conductive ground.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
•We developed an analytical solution for vertical ground heat exchangers.•The model considers multilayer ground and groundwater flow.•Interaction between is considered through a composite model.•The ...model can help to design and to plan ground heat exchangers.
The vertical ground heat exchangers (GHE) are the most common application of the ground source heat pump (GSHP) systems. Due to ground heterogeneity and length of the boreholes, the heat exchangers cross usually several geological layers. However, in most of the current analytical models for GHEs, the restrictive assumption of ground homogeneity is considered. In this paper, a finite line-source model is proposed for GHEs that takes into account not only thermal conduction but also advection and dispersion mechanisms, induced by ground water flow, in a multilayer porous medium. Firstly, the anisotropy is added to the moving finite line-source (MFLS) model, and an existing composite model approach is modified. The temperature comparison with the numerical model results demonstrates the suitability of the approach. The proposed model provides faster solution than typical 3D numerical methods Furthermore, the homogeneous and multilayer assumptions are analyzed in dimensionless form to check the convenience of both of the approaches. The results demonstrate that, in case of high groundwater velocity in one layer, the thermal interaction with the neighboring layers decreases due to strong groundwater flow suppressing the thermal flux interaction. In that case, the prediction of homogeneous assumption is slightly sufficient in the middle of the layer. Otherwise, the multilayer approach is more appropriate in transient conditions, particularly, at the interface of layers.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
In the production of biofuel from scrap tire pieces by pyrolysis in a rotary kiln, particle size segregation is believed to affect the temperature evolution and control within the reactor. To study ...the impact of granular segregation on heat transfer in rotary kilns, a thermal discrete element method-based model incorporating a thermal resistance network was developed for bidisperse particulate blends. It was validated using a system of glass and aluminum beads differing in size. The harmonic mean radius was found to best represent bidisperse contacts. Heat transfer in six different segregated bed structures in a wall-heated drum for two particle size ratios was studied. Granular segregation significantly influenced the temperature profiles in the beds, dragging down the temperature of the center of the bed, where a core of smaller particles had formed. This resulted in an inverse relationship between the effective heat transfer coefficient of the bed and the intensity of segregation. Within the segregated cases investigated, the temperature difference between the large and small particles could be up to 30 times higher than that of a well-mixed configuration.
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•A TDEM model was used to study heat transfer in different segregated bed structures.•The model was validated for a bidisperse bed made of aluminum and glass beads.•The radial temperature distribution is related to the segregation pattern.•The temperature difference between the two types of particles is highlighted.•The effective heat transfer coefficient is related to the intensity of segregation.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•Solid–liquid mixing in early turbulent regime is investigated using CFD-DEM.•Two new approaches to measure the fraction of suspended particles are presented.•The fraction of suspended particles is ...measured numerically and compared to experiments.•The two new methods and the pressure gauge technique agree with the experiments.•Suspending the last few particles requires considerable energy.
Solid–liquid mixing as a unit operation still faces considerable challenges, notably regarding the prediction of the impeller speed required to suspend the particles (Njs), the fraction of suspended solids and the homogeneity of the suspension at a given speed. In this work, we extend to the turbulent regime, by means of large eddy simulation (LES), a CFD-DEM model developed recently in our group for solid–liquid mixing. The resulting model is used to study the mixing of glass particles in a baffled stirred tank equipped with a down-pumping pitched blade turbine. Various characteristics of the liquid dynamics as well as the distribution and motion of the solids are investigated. The fraction of suspended solid particles predicted by the model is validated against experimental data obtained via the pressure gauge technique (PGT). Two new methods to calculate the fraction of suspended particles in a Euler–Lagrange simulation, the so-called Lagrangian suspended fraction analysis (LSFA) and the decorrelated fraction analysis (DFA) techniques are introduced. The results obtained with these two methods, as well as with many others taken from the literature, are compared to the Zwietering correlation and to the results obtained by the PGT. It is found that some techniques proposed in the literature, namely the local concentration, the power consumption and the transient solids concentration analysis techniques, cannot be applied adequately in this case. On the other hand, the LSFA, DFA and PGT techniques are observed to predict accurately the fraction of suspended solids when compared to experimental PGT data.
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GEOZS, IJS, IMTLJ, IZUM, KILJ, KISLJ, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Capillarity in wet granular materials induces cohesion and increases the material strength due to the attractive forces acting on capillary bridges. In the funicular state, water bridges may be not ...only formed between two grains but also binding three or more particles, which breaks the axial symmetry of the liquid bridge. This work presents a fundamental study on capillary forces and rupture behaviours of funicular water bridges between three spherical bodies at equilibrium (or static) configurations. Funicular water clusters are numerically solved by an energy minimization approach. Experimental comparisons are made by measuring capillary forces and these confirm the validity of the numerical solutions. Evolutions of capillary forces and rupture distances are investigated systematically by moving two spheres away from the centre. The fixed water volume condition and the constant mean curvature condition are studied respectively. Comparisons are made between the un-coalesced pendular liquid rings and the coalesced funicular bridge. For a same fixed total water volume, the capillary force is weakened by water bridge coalescence to a funicular bridge when the spheres are packed together, but the situation may vary for different contact angles and inter-particle distances. For the constant mean curvature condition, water bridge coalescence does not alter capillary force significantly when particles are packed closely, but the discrepancy is larger by increasing the gap. Funicular water bridge rupture criteria are also proposed based on the studied configurations. It is observed that in general the transmission from pendular to funicular state extends the rupture distance when it has a relatively high water volume or low air-water pressure difference.
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•Funicular liquid bridges between three spherical particles are numerically solved.•Experimental validation is carried out by measuring capillary force induced by the funicular liquid bridge.•Effects of liquid volume, suction, contact angle and particle separation distance are investigated.•Water bridge coalescence effect (from pendular rings to a funicular cluster) on force and rupture distance is discussed.•Rupture criteria of the studied configuration for liquid volume controlled and constant suction conditions are proposed.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
The vast majority of solid–liquid mixing studies have focused on high Reynolds number applications with configurations and impeller geometries adapted to this type of regime. However, the mixing of ...particles in a viscous fluid is an essential element of many contemporary industries. We used the computational fluid dynamics‐discrete element method model previously developed in our group to investigate solid–liquid mixing with close‐clearance impellers in the laminar regime of operation. We compared different geometries, that is, the double helical ribbon, anchor, Paravisc™, and Maxblend™ impellers. We investigated the impact of fluid viscosity and compared the results with those obtained with the pitched blade turbine, a more commonly used impeller, based on power consumption for equivalent mixing states. This study highlights that the higher the viscosity of the fluid, the more interesting it is to use close‐clearance impellers for their ability to generate a strong shear stress and a strong bulk flow in the entire vessel.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
This study proposed two pedotransfer functions (PTFs) to estimate sandy soil water retention curves. It is based on the van Genuchten's water retention model and from a semiphysical and ...semistatistical approach. Basic gradation parameters of
d60 as particle size at 60% passing and the coefficient of uniformity
Cu are employed in the PTFs with two idealized conditions, the monosized scenario and the extremely polydisperse condition, satisfied. Water retention tests are carried out on eight granular materials with narrow particle size distributions as supplementary data of the UNSODA database. The air entry value is expressed as inversely proportional to
d60 and the parameter
n, which is related to slope of water retention curve, is a function of
Cu. The proposed PTFs, although have fewer parameters, have better fitness than previous PTFs for sandy soils. Furthermore, by incorporating with the suction stress definition, the proposed pedotransfer functions are imbedded in shear strength equations which provide a way to estimate capillary induced tensile strength or cohesion at a certain suction or degree of saturation from basic soil gradation parameters. The estimation shows quantitative agreement with experimental data in literature, and it also explains that the capillary‐induced cohesion is generally higher for materials with finer mean particle size or higher polydispersity.
Key Points
Pedotransfer functions are proposed to estimate WRC from two basic soil gradation parameters (d10 and Cu) through a semiempirical approach
With physical features considered, the model with fewer and more easily obtained parameters shows good performance in WRC prediction
By incorporating with the suction stress definition, it provides a preliminary estimation of strength properties from grain size
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Nanoparticles possess unique, size-driven properties. However, they can be challenging to use as they easily agglomerate - their high surface area-to-volume ratio induces strong interparticle forces, ...generating agglomerates that are difficult to break. This issue prevails in organic particles as well, such as cellulose nanocrystals (CNCs); when in their dried form, strong hydrogen bonding enhances agglomeration. Ultrasonication is widely applied to prepare CNC suspensions, but the methodology employed is non-standardized and typically under-reported, and process efficiency is unknown. This limits the ability to adapt dispersion protocols at industrial scales. Herein, numerical simulations are used in conjunction with validation experiments to define and optimize key parameters for ultrasonic dispersion of CNCs, allowing an operating window to be inferred.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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