| 1. |
- Amini, Kasra, et al.
(författare)
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Scaling laws for near-wall flows of thixo-elasto-viscoplastic fluids in a millifluidic channel
- 2024
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Ingår i: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 36:2
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Tidskriftsartikel (refereegranskat)abstract
- Thixo-elasto-viscoplastic (TEVP) fluids are very complex fluids. In addition to elasticity and viscoplasticity, they exhibit thixotropy, i.e., time-dependent rheology due to breakdown and recovery of internal structures at different length- and timescales. General and consistent methods for a priori flow prediction of TEVP fluids based on rheological characteristics are yet to be developed. We report a combined study of the rheology and flow of 18 samples of different TEVP fluids (three yogurts and three concentrations of Laponite and Carbopol, respectively, in water in both the unstirred and a stirred state). The rheology is determined both with standard protocols and with an ex situ protocol aiming at reproducing the shear history of the fluid in the flow. Micrometer resolution flow measurements in a millimeter scale rectangular duct are performed with Doppler Optical Coherence Tomography (D-OCT). As expected, the results show the existence of a plug flow region for samples with sufficiently high yield stress. At low flow rates, the plug extends almost all the way to the wall and the extent of the plug decreases not only with increased flow rate but also with increased thixotropy. The ex situ rheology protocol enables estimation of the shear rate and shear stress close to the wall, making it possible to identify two scaling laws that relates four different non-dimensional groups quantifying the key properties wall-shear stress and slip velocity. The scaling laws are suggested as an ansatz for a priori prediction of the near-wall flow of TEVP fluids based on shear flow-curves obtained with a rheometer.
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| 2. |
- Håkansson, Andreas, et al.
(författare)
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Hydrodynamic difference between inline and batch operation of a rotor-stator mixer head - A CFD approach
- 2017
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Ingår i: Canadian Journal of Chemical Engineering. - : Wiley. - 0008-4034 .- 1939-019X. ; 95:4, s. 806-816
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Tidskriftsartikel (refereegranskat)abstract
- Rotor-stator mixers (RSMs) can be operated in either batch or inline mode. When operating a rotor-stator geometry in batch mode, it typically experiences an order of magnitude higher volumetric flow through the stator than in inline mode. This is expected to cause differences in the flow and turbulence in the rotor-stator region. This study uses computational fluid dynamics (CFD) to study the hydrodynamic differences in and near the stator hole as a function of volumetric flow rates between those experienced in inline and batch modes of operation. It is concluded that both radial flow profiles and turbulent kinetic energy across a range of rotor speeds and flow rates can be described by a velocity ratio: average tangential fluid velocity in the stator hole divided by the rotor tip speed. Moreover, the position where dissipation of turbulent kinetic energy takes place—and hence the effective region of dispersion or mixing—differs between the two modes of operation. The relative importance of the two regions can be described in terms of the velocity ratio and the transition can be predicted based on the relative power input due to rotational and pumping power of the mixer. This study provides a starting point for understanding differences between emulsification efficiency between inline and batch modes of operation with relevance for both equipment design and process scale-up.
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| 3. |
- Innings, Fredrik, et al.
(författare)
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Understanding and analysis of wear in homogenizers for processing liquid food
- 2011
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Ingår i: Wear. - : Elsevier BV. - 0043-1648 .- 1873-2577. ; 271:9-10, s. 2588-2598
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Tidskriftsartikel (refereegranskat)abstract
- The tribological research pertaining to homogenizers in processing liquid food has received much less attention compared to its potential in enhancing the efficiency and durability of the homogenization process equipment. Homogenization is a process used to disrupt fat globules in dairy products to reduce the formation of creamy layer (separation) and also to enhance the viscosity of certain products. This process takes place in a narrow gap in the homogenizer machine and this region is highly prone to wear. The occurrence of wear during processing not only impairs the homogenization effect but also leads to increased downtime of the machine. The aim of this work is to understand the occurrence of wear and wear mechanisms in the homogenization gap using both experimental and analytical approaches. Two experimental test rigs were used to study the differences in wear during operation, i.e. with and without particles. The trajectories of the particles have been simulated without the influence of cavitation using a CFD-code to investigate whether the particles are the cause of wear. The homogenizer gap has been simulated for a worn geometry to see how the occurrence of wear changes the particles trajectories. The results have shown that the presence of particles accelerates wear and most important parameters are the particle hardness and mass. When cavitation and particles are combined they create a synergistic effect on wear. This can be explained by the fact that cavitation can accelerate particles in random directions through the imploding action of cavities. A change in impact angle and increased velocity increases the amount of wear significantly. CFD-simulations and calculations show that the particles do not fully follow the streamlines and therefore create wear on the gap surfaces.
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| 4. |
- Leskovec, Martin, et al.
(författare)
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Pipe flow with large particles and their impact on the transition to turbulence
- 2020
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Ingår i: Physical Review Fluids. - : American Physical Society (APS). - 2469-990X. ; 5:11
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Tidskriftsartikel (refereegranskat)abstract
- The classical transition from laminar to turbulent flow is affected if solid particles are added. The transition behavior is a function of particle size d and solid volume fraction phi and the flow undergoes a smooth transition, as opposed to intermittent, if phi exceeds a certain threshold. In this work we show that, for particle-laden pipe flows with large particle-to-pipe diameter ratios d/D, the phi threshold for altering the transition is much lower than previously reported for smaller particles. Magnetic resonance velocimetry reveals that particles introduce turbulent-like fluid velocity fluctuations in laminar flow. Factors that might control the limits between "classical" and "smooth" transition in the state space spanned by d/D and phi are discussed based on scaling analyses.
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| 5. |
- Ashar, Mohamed, 1979, et al.
(författare)
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Single droplet breakup in a rotor-stator mixer
- 2018
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Ingår i: Chemical Engineering Science. - : Elsevier BV. - 0009-2509. ; 181, s. 186-198
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Tidskriftsartikel (refereegranskat)abstract
- This study presents measurements of droplet transport inside a rotor-stator mixer under turbulent flow conditions and contributes to the understanding of the droplet breakup phenomenon. The measurements contain unique information on droplet breakup down to the limit of equilibrium droplet size. The experimental setup includes a custom-built rotor-stator mixer that provides optical access to the mixing region. High-speed camera imaging is used to study droplet stability and breakup due to turbulent inertial stress at different operating conditions. The analysis reveals when, where, and how the droplets break up inside the rotor-stator mixer. Analysis of the breakup location reveals that droplet breakup mainly occurs in the downstream region of the stator and inside the emanating jets. Prediction of the flow field and turbulent kinetic energy dissipation rate from CFD simulations explains the propensity of droplet breakup at different locations in the rotor-stator. It is found that both the average time for droplet-vortex interaction and the distribution increases with the size of mother droplets, and a model is proposed that explains the results fairly well. Measurement of the breakup probabilities reveals a monotonically increase from zero probability at a low We-number to approximately 50% probability at We > 4. The number of fragments formed upon breakage depend on how far the droplets are from equilibrium size and it is found that the probability for multiple fragmentation dominates even close to the equilibrium size.
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| 6. |
- Bayod, Elena, et al.
(författare)
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Low shear rheology of concentrated tomato products. Effect of particle size and time
- 2007
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Ingår i: Food Biophysics. - : Springer Science and Business Media LLC. - 1557-1866 .- 1557-1858. ; 2:4, s. 146-157
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Tidskriftsartikel (refereegranskat)abstract
- Time-dependent rheological properties of three tomato paste suspensions in the concentration range of 200-1,000 g paste/kg suspension have been investigated by using the vane geometry at shear rates gamma < 10 s(-1). Creep tests were conducted to analyze the influence of the level of stress on the rheological behavior of the samples before and after homogenization. The experimental results indicate that the suspensions exhibit an elastic behavior at long times and relatively low stresses, which proves that this type of material can be characterized by a yield stress (sigma(y)). Applying stresses just beyond the yield stress, an initial rheopectic behavior appeared. This increase in viscosity at low deformations was markedly larger after homogenization, and this difference was attributed to changes in the aspect ratio, shape, and orientation of the particles induced by homogenization. These structural changes were also reflected in the transient viscosity when the samples were subjected to larger stresses (sigma >>sigma y): before homogenization the suspensions exhibited a steady-state viscosity at large deformations, whereas after homogenization, the transient viscosity continuously decreased. That behavior was attributed to flocculation of the particles.
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| 7. |
- Börjesson, Erik, et al.
(författare)
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Entrapment of air during imbibition of agglomerated powder beds
- 2017
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Ingår i: Journal of Food Engineering. - : Elsevier BV. - 0260-8774. ; 201, s. 26-35
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Tidskriftsartikel (refereegranskat)abstract
- Complete wetting is crucial for efficient recombination of powders. On the powder bed scale, wetting is governed solely by capillary forces and the resistance to flow, i.e. spontaneous imbibition. Slow or incomplete imbibition of the powder bed may lead to gelling of the liquid front, which will stop the recombination process and cause the formation of lumps, which is usually undesirable. In this study, the spontaneous imbibition of powder beds consisting of spray-dried dairy powders with diverse morphologies has been investigated. Uniform radial spreading of the imbibition front was seen in all the beds, but a large amount of air was trapped in the inter-particle free space in the imbibed volume. A positive correlation was found between bed porosity and the fraction of air trapped in the bed after imbibition. Since the amount of trapped air was calculated as a fraction of the porosity of the dry bed, this relation was unexpected. The large fraction of air trapped in the bed and the uniform radial spreading of the imbibition front indicate considerable heterogeneity of bed on the microscopic scale, but a homogenous structure on the macroscopic scale. Possible explanations of the large fraction of trapped air are the presence of local dead-end structures in the bed and film flow of the imbibing liquid.
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| 8. |
- Börjesson, Erik, et al.
(författare)
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Evaluation of particle measures relevant for powder bed porosity-A study of spray dried dairy powders
- 2014
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Ingår i: Powder Technology. - : Elsevier BV. - 0032-5910. ; 253, s. 453-463
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Tidskriftsartikel (refereegranskat)abstract
- The formation of lumps during large scale wetting of spray dried powders is a common problem in the industry. The result is a large energy demand if an efficient mixing is to be achieved at the industrial scale. If more knowledge regarding the phenomena governing lump formation is obtained, the unit operation could be optimized and made more efficient. As a part of increasing the understanding of lump formation, the wetting of the porous media that is a powder bed needs to be understood. This wetting relates to the structure and void space (porosity) of the powder bed, which is why a relation between powder bed porosity and readily obtainable powder particle morphology parameters is sought. In this study a thorough investigation of the morphology of five different spray dried milk powders, with arbitrary particle shapes, was performed using manual image analysis of scanning electron microscope (SEM) images as well as automated methods based on light scattering and projected image analysis. The results of the automated methods, when utilized on the complexly shaped samples, were validated by a comparison to the SEM image analysis. This enabled the development of a relation between powder morphology and powder bed porosity based on data from automated methods. The results indicate that automated methods based on light scattering underestimate the particle sizes of arbitrary shaped particles. Methods based on projected image analysis, however, show a better agreement with SEM image analysis. The developed relation between powder bed porosity and powder particle morphology is based on defining the bed as having either an "Agglomerates in fines", "Bicontinuous" or "Fines in agglomerates" structure. The structure of each powder is determined based on calculation of a "BedRatio" which is a function of the phi(agglomerate) and phi(fines) both shown to be readily obtainable by automated methods. The calculated porosities of the powders investigated in this study show a good agreement with experimental measurements and are considered to correspond well to earlier literature. It is, however, clear from the a experiments that care needs to be taken during bulk density measurement of spray dried powders so that no intra particle void space is taken into consideration in the calculations. (C) 2013 Elsevier B.V. All rights reserved.
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| 9. |
- Börjesson, Erik, et al.
(författare)
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Permeability of powder beds formed from spray dried dairy powders in relation to morphology data
- 2016
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Ingår i: Powder Technology. - : Elsevier BV. - 0032-5910. ; 298, s. 9-20
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Tidskriftsartikel (refereegranskat)abstract
- The formation of lumps during industrial recombination of powder is a common problem in many industries. An important step in the recombination process is the wetting of the dry material. Only with an efficient wetting the individual powder particles will disperse and a complete dissolution will occur. In difference, a slow or incomplete wetting will not supply liquid to all portions of the powder, resulting in the formation of lumps.The resistance to wetting can be described by a material's permeability. This parameter is often estimated using the Kozeny-Carman equation, which models the permeability of a porous medium (such as a bed of powder) based on the porosity and specific surface area of the medium. This model has proven valid for a bed of packed spheres, but due to the model assumptions the accuracy of the model when utilized on beds formed from more complex, spray dried powders can be questioned.In this study, the permeability of powder beds consisting of spray-dried diary powders when exposed to an air flux has been characterized, using a method based on air permeametry. The results show that the Kozeny-Carman model is insufficient to estimate the permeability of such beds. Furthermore, the measured porosity of the bed had no influence on the permeability constant.Based on the results of this study a correlation between morphological powder parameters and permeability was developed. This correlation indicates that a small surface weighted average diameter of the particles as well as a narrow width of the particle size distribution have a positive effect on the permeability of a bed. In addition, incorporation of the structural parameter "BedRatio", defining the basis of the bed to consist of agglomerates or primary particles (fines), indicates that a bed consisting of mainly agglomerates has a slightly larger permeability.
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| 10. |
- Börjesson, Erik, et al.
(författare)
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The dissolution behavior of individual powder particles
- 2013
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Ingår i: Dairy Science & Technology. - : Springer Science and Business Media LLC. - 1958-5586 .- 1958-5594. ; 93:4-5, s. 357-371
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Tidskriftsartikel (refereegranskat)abstract
- The formation of lumps is a common problem during powder reconstitution in food industry. If more knowledge regarding the phenomena controlling the process could be obtained, the process of industrial powder recombination could be optimized. This demands investigation of the process at three different levels: the scale of individual powder particles, the scale of powder beds, and the scale of industrial recombination. If knowledge from all three scales could be combined, a model of the entire process could be constructed. In this study, we present a new flow-cell-based method for investigating the dissolution rate of individual powder particles in controlled liquid flow. The flow cell allows for alteration of the flow velocity and only subjects the investigated powder particles to mild stress before the analysis. The dissolution process in the cell is recorded, and data are obtained by image analysis. Sodium caseinate has been used as a model substance in these experiments. The results show that the particle dissolution rate is a function of the flow velocity of the liquid, and the results correspond to earlier established methods. The investigation of individual particles shows a variation in dissolution rate between particles resulting from the diverse particle morphology from an industrial batch of powder. The results indicate that the dissolution interface concentration of dissolving sodium caseinate particles corresponds well to the overlap concentration of the specie.
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