| 1. |
- Baena-Moreno, Francisco, 1992, et al.
(författare)
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Potential of organic carbonates production for efficient carbon dioxide capture, transport and storage: Reaction performance with sodium hydroxide–ethanol mixtures
- 2023
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Ingår i: Heliyon. - : Elsevier BV. - 2405-8440. ; 9:3
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Tidskriftsartikel (refereegranskat)abstract
- Carbon dioxide storage is one of the main long-term strategies for reducing carbon dioxide emissions in the atmosphere. A clear example is Norway's Longship project. If these projects should succeed, the transport of huge volumes of carbon dioxide from the emissions source to the injection points may become a complex challenge. In this work, we propose the production of sodium-based organic carbonates that could be transported to storage sites and be reconverted to CO2. Solid carbonates can be transported in considerably lower volumes than gases or pressurized liquids. Sodium-based carbonates are insoluble in most of the organic solvents and will therefore precipitate in contrast to in aqueous solutions. Particularly, here we focus on sodium hydroxide-ethanol mixtures as solvents for precipitating sodium ethyl carbonate and sodium bicarbonate. Previous works on this approach used limited sodium hydroxide concentrations, which are insufficient to prove the effectiveness of the proposed process. In this paper, we studied higher sodium hydroxide concentrations in sodium hydroxide-ethanol mixtures than previously reported in the literature. To this end, we use the following strategy: (1) In-line monitoring of the formation of carbonates using an in-line FTIR; (2) In-line measurements of the weight increase, which correspond directly to the captured carbon dioxide and reveal the absorption capacity; (3) Characterization of the solids with X-ray diffraction and scanning electron microscope. Our FTIR results confirmed that both sodium ethyl carbonate and sodium bicarbonate were formed, which agrees with X-ray diffraction and scanning electron microscope. With this reactor design, the absorption capacities reached approximately 80–93% of the theoretical values (4.8–13.3 g/L respectively). We hypothesize that full conversion is hampered because the gas might take preferential paths due to gel formation during the experiments.
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| 2. |
- Djerf, Tove, 1989, et al.
(författare)
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Solids back-mixing in the transport zone of circulating fluidized bed boilers
- 2022
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Ingår i: Chemical Engineering Journal. - : Elsevier BV. - 1385-8947. ; 428
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Tidskriftsartikel (refereegranskat)abstract
- This work investigates the back-mixing of solids in the transport zone of large-scale circulating fluidized bed (CFB) boilers, with the aims of identifying and evaluating the governing mechanisms and providing a mathematical description based on a solid theoretical background rather than on purely empirical correlations. In addition, transient Direct Numerical Simulation (DNS) modeling is used to identify the mechanism that drives migration of the solids from the dilute up-flow in the core region to the down-flow at the furnace walls. Previously published concentration and pressure profiles are collated and analyzed through modeling of the steady-state mass balance of the dispersed solids in the transport zone. The study shows that solids back-mixing at the furnace wall layers is limited (hence governed) by the core-to-wall layer mass transfer transport mechanism rather than by the lateral movement of solids within the core region. The latter is shown by the 3-dimensional (3D) mass balance model, and the transient DNS modeling indicates that this is due to a turbophoresis mechanism. We also show that the use of Pe-numbers to describe the lateral solids dispersion is not straightforward but rather depends on the unit scale, and that Pe-numbers < 26 are needed to yield the solids back-mixing rates measured in large-scale CFB boilers. Finally, we propose a mathematical expression for the core-to-wall layer mass transfer coefficient derived from a Sherwood number (Sh)-correlation fitted to measured values of the characteristic decay constant that result from the solids back-mixing. This expression shows better agreement with the large-scale measurements than do the expressions given in the literature.
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| 3. |
- Fries, Johan, et al.
(författare)
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Key parameters for droplet evaporation and mixing at the cloud edge
- 2021
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Ingår i: Quarterly Journal of the Royal Meteorological Society. - : Wiley. - 0035-9009 .- 1477-870X. ; 147:737, s. 2160-2172
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Tidskriftsartikel (refereegranskat)abstract
- The distribution of liquid water in ice-free clouds determines their radiative properties, a significant source of uncertainty in weather and climate models. Evaporation and turbulent mixing cause a cloud to display large variations in droplet number density, but quite small variations in droplet size (Beals et al., Science, 2015, vol. 350, pp. 87-90). However, direct numerical simulations of the joint effect of evaporation and mixing near the cloud edge predict quite different behaviours, and how to reconcile these results with the experimental findings remains an open question. To infer the history of mixing and evaporation from observational snapshots of droplets in clouds is challenging, because clouds are transient systems. We formulated a statistical model that provides a reliable description of the evaporation-mixing process as seen in direct numerical simulations and allows us to infer important aspects of the history of observed droplet populations, highlighting the key mechanisms at work and explaining the differences between observations and simulations.
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| 4. |
- Fries, Johan, et al.
(författare)
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Lagrangian Supersaturation Fluctuations at the Cloud Edge
- 2023
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Ingår i: Physical Review Letters. - : American Physical Society (APS). - 1079-7114 .- 0031-9007. ; 131:25
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Tidskriftsartikel (refereegranskat)abstract
- Evaporation of cloud droplets accelerates when turbulence mixes dry air into the cloud, affecting droplet-size distributions in atmospheric clouds, combustion sprays, and jets of exhaled droplets. The challenge is to model local correlations between droplet numbers, sizes, and supersaturation, which determine supersaturation fluctuations along droplet paths (Lagrangian fluctuations). We derived a statistical model that accounts for these correlations. Its predictions are in quantitative agreement with results of direct numerical simulations, and explain the key mechanisms at play.
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| 5. |
- Grujić, Anđela, et al.
(författare)
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Collisions among elongated settling particles: The twofold role of turbulence
- 2024
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Ingår i: Physics of Fluids. - : AIP Publishing. - 1089-7666 .- 1070-6631. ; 36:1
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Tidskriftsartikel (refereegranskat)abstract
- We study the collision rates of settling spheres and elongated spheroids in homogeneous, isotropic turbulence by means of direct numerical simulations aiming to understand microscale-particle encounters in oceans and lakes. We explore a range of aspect ratios and sizes relevant to the dynamics of plankton and microplastics in water environments. The results presented here confirm that collision rates between elongated particles in a quiescent fluid are more frequent than those among spherical particles in turbulence due to oblique settling. We also demonstrate that turbulence generally enhances collisions among elongated particles as compared to those expected for a random distribution of the same particles settling in a quiescent fluid, although we also find a decrease in collision rates in turbulence for particles of the highest density and moderate aspect ratios ( A = 5 ) . The increase in the collision rate due to turbulence is found to quickly decrease with aspect ratio, reach a minimum for aspect ratios approximately equal to 5, and then slowly increase again, with an increase up to 50% for the largest aspect ratios investigated. This non-monotonic trend is explained as the result of two competing effects: the increase in the surface area with aspect ratio (beneficial to increase encounter rates) and the alignment of nearby prolate particles in turbulence (reducing the probability of collision). Turbulence mixing is, therefore, partially balanced by rod alignment at high particle aspect ratios.
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| 6. |
- Hidman, Niklas, 1989, et al.
(författare)
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A multiscale methodology for small-scale bubble dynamics in turbulence
- 2022
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Ingår i: International Journal of Multiphase Flow. - : Elsevier BV. - 0301-9322. ; 150
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Tidskriftsartikel (refereegranskat)abstract
- We formulate in this paper a multiscale numerical framework that handles small-scale bubble dynamics in turbulence. Our framework involves bubbles with arbitrary density ratios with the carrier phase. We use a Moving Reference Frame method that follows a bubble to deal with a fast rising of bubbles present at high density ratios between the phases. We use a Proportional Integral Derivative controller to handle an additional acceleration term in the governing equations that stems from the change of a coordinate system from a fixed to a non-inertial one. Our framework accounts for the fact that the dynamics of bubbles are significantly influenced by the unsteadiness of the small-scale turbulent liquid fluctuations that modify the bubble shapes and alter their motion. In addition, we improve and speed up, with at least two orders of magnitude in computational time, the numerical framework recently proposed by Milan et al. (2020). The developed numerical framework can capture processes occurring at time scales even smaller than the Kolmogorov times. It can be applied to droplets, bubbles or particle systems in both laminar and turbulent flows, using any general DNS technique that handles two-phase flows.
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| 7. |
- Hidman, Niklas, 1989, et al.
(författare)
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Assessing passive scalar dynamics in bubble-induced turbulence using direct numerical simulations
- 2023
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Ingår i: Journal of Fluid Mechanics. - 0022-1120 .- 1469-7645. ; 962
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Tidskriftsartikel (refereegranskat)abstract
- By using direct numerical simulations (DNS) of bubbly flows with passive scalars, we show a transition in the scalar spectra from a k-5/3 to a k-3 scaling with the wavenumber k, in contrast with those of single-phase isotropic turbulence. For cases with a mean scalar gradient in the horizontal direction, the scalar spectrum decays faster than at high wavenumbers. While the k-3 scaling is well established in the bubbly flow velocity spectrum, the scalar spectrum behaviour is not fully understood. We find that the transition length scale of the scalar spectra is comparable to or below the bubble diameter and decreases with the molecular diffusivity of the scalar in the liquid. We use DNS to compute the scalar spectrum budget and show that the scalar fluctuations are produced by the mean scalar gradient at length scales above the bubble diameter, contrary to the velocity fluctuations. At length scales below the bubble diameter, the net scalar transfer scales as k-1 inducing the k-3 scaling of the scalar spectra. This finding is consistent with the hypothesis proposed by Dung et al. (J. Fluid Mech., vol. 958, 2023, p. A5) about the physical mechanism behind the k-3 scaling. We also show dependencies of the bubble suspension's convective scalar diffusivity on the gas volume fraction and molecular diffusivity that differ based on the direction of the mean scalar gradient. For a mean scalar gradient in the vertical direction, we find and qualitatively explain a significant effect of the molecular diffusivity in the gas on the convective scalar diffusivity.
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| 8. |
- Hidman, Niklas, 1989, et al.
(författare)
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Laser-induced vapour bubble as a means for crystal nucleation in supersaturated solutions - Formulation of a numerical framework
- 2019
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Ingår i: Experimental and Computational Multiphase Flow. - : Springer Science and Business Media LLC. - 2661-8877 .- 2661-8869. ; 1:4
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Tidskriftsartikel (refereegranskat)abstract
- We use in this work numerical simulations to investigate the evolution of a laser-induced vapour bubble with a special focus on the resolution of a thin layer of liquid around the bubble. The application of interest is laser-induced crystallization, where the bubble acts as a nucleation site for crystals. Experimental results indicate the extreme dynamics of these bubbles where the interface during the period of 200 us, from nucleation to collapse, reaches a maximum radius of roughly 700 µm and attains a velocity of well above 20 m/s. To fully resolve the dynamics of the bubble, the volume of fluid (VOF) numerical framework is used. Inertia, thermal effects, and phase-change phenomena are identified as the governing phenomena for the bubble dynamics. We develop and implement into our numerical framework an interface phase-change model that takes into account both evaporation and condensation. The performed simulations produce qualitatively promising results that are in fair agreement with both experiments and analytical solutions from the literature. The reasons behind the observed differences are discussed and suggestions are made for future improvements of the framework.
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| 9. |
- Hidman, Niklas, 1989, et al.
(författare)
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Numerical Frameworks for Laser-Induced Cavitation: Is Interface Supersaturation a Plausible Primary Nucleation Mechanism?
- 2020
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Ingår i: Crystal Growth & Design. - : American Chemical Society (ACS). - 1528-7505 .- 1528-7483. ; 20:11, s. 7276-7290
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Tidskriftsartikel (refereegranskat)abstract
- Crystallization has been observed in laser-induced cavities in saturated solutions, but the mechanisms behind nucleation of crystals are not entirely clear. A hypothesis is that high solution supersaturation during the bubble growth period triggers the nucleation. Because of small spatiotemporal scales of the cavitation event, the supersaturation is very difficult to measure experimentally. To test the nucleation hypothesis, we perform a two-dimensional axisymmetric direct numerical simulation of an experimentally observed laser-induced cavitation event with crystallization. We demonstrate a significant degree of supersaturation and argue that the nucleation hypothesis is indeed plausible. To analyze factors that lead to a high supersaturation, we develop a comprehensive one-dimensional model for spherical laser-induced cavities. We conduct an extensive investigation on how the solute solubility, solute diffusivity, laser pulse energy, and superheated liquid volume affect the supersaturation. We show that high supersaturation is possible under a range of relevant conditions but not readily obtained for all solutions and laser setups. Guidelines are provided to identify if a specific solution or laser setup may attain high supersaturation. The insights obtained and the numerical methods formulated in this work can be applied to assess and design new laser-induced cavitation setups that allow for precise control of the duration and degree of the supersaturation
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| 10. |
- Hidman, Niklas, 1989, et al.
(författare)
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Numerical simulation of a laser-induced vapour bubble for crystal nucleation at low supersaturation levels
- 2019
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- We use in this work numerical simulations to investigate the dynamics of a laser-induced vapour bubble. The application of interest is laser-induced crystallization, where the bubble acts as a nucleation site for crystals. To fully resolve growth and collapse of the bubble, the Volume of Fluid (VOF) numerical framework is used. Inertia, thermal effects and phase-change phenomena are identified as the governing phenomena for the bubble dynamics. We develop and implement into our numerical framework an interface phase-change model that takes into account both evaporation and condensation. Performed simulations produce qualitatively promising results that are in fair agreement with both experiments and analytical solutions from the literature. The reasons behind the observed differences are discussed and suggestions for future improvements of the framework are described.
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