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A special issue of Processes (ISSN 2227-9717). This special issue belongs to the section "Chemical Processes and Systems".

Deadline for manuscript submissions: closed (20 February 2021) | Viewed by 37013

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Prof. Dr. Václav Uruba

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1. Institute of Thermomechanics of the Czech Academy of Sciences, 182 00 Prague, Czech Republic
2. Department of Power System Engineering, University of West Bohemia, 306 14 Pilsen, Czech Republic
Interests: fluid mechanics; turbulence; experimental techniques in FM
Special Issues, Collections and Topics in MDPI journals

Prof. Dr. Witold Elsner

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Department of Thermal Machinery, Faculty of Mechanical and Computer Engineering, Czestochowa University of Technology, Armii Krajowej 21, 42-200 Czestochowa, Poland
Interests: Prof. Witold Elsner is a professor of fluid mechanics and fluid-flow machinery. His fields of expertise are: unsteady aerodynamics of turbomachinery, boundary layers, transitional flows, experimental analysis of turbulent flows, CFD modeling (RANS and LES), modeling and optimization of thermal cycles, combustion and pyrolitic gasification of biomass, municipal waste and sewage, diagnostics of rotating machinery. Prof. W. Elsner teaches fluid flow machinery, energy conversion technologies, machine maintenance and diagnostics

Special Issue Information

Dear Colleagues,

Nowadays, intensification of chemical processes is a rapidly growing research field, bringing better, simpler, and ultimately cheaper technological means. This could include reduction of the number of devices in the whole system and/or improving heat and mass transfer by advanced mixing technologies, which enables shorter diffusion pathways. Novel approaches enable miniaturization and integrated optimization and control strategies. Those achievements could not be possible without addressing the mixing process itself, as it is involved in several levels of the process.

The mixing process can be subdivided into a cascade of macromixing, mesomixing, and micromixing subprocesses, respectively, in order to more easily address the effects on different spatial scales. These mechanisms are still subjected to intensive research.

Studies clarifying the mixing process itself are welcome, as a better understanding of the physics behind this process enables proper modification of the procedures and choice of the optimal strategy in a given case. Single-phase blending of liquids with component properties that are close to each other is taken into account as a basic case; however, combination of substances with different physical properties could be of great practical interest, including multiphase cases. Practical cases could differ in physical properties of the mixing medium. Detailed knowledge on the physics of the mixing process will be a good starting point for the design of new mixing optimization and control strategies.

The topics could include, but are not limited to, the following:

Macro- and micromixing interactions;
The role of turbulence in mixing process (physical mechanisms);
Mixing intensification strategies (time, space, and energy aspects);
Interactions of the mixing process with other processes (heat transfer, chemical process); and
Mixing process modeling, simulation, and control applications.

Prof. Dr. Václav Uruba
Prof. Dr. Witold Elsner
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Processes is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2400 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

chemical process intensification
chemical reactor
mixing
mixing length scales
Batchelor scale
Kolmogorov scale
micromixing
macromixing
turbulence
diffusion

Published Papers (11 papers)

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Research

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25 pages, 5177 KiB

Open AccessFeature PaperArticle

Investigation of Low-Frequency Phenomena within Flow Pattern in Standard Mixing Vessel Induced by Pitched Blade Impeller

by Tomáš Brůha, Pavel Procházka and Václav Uruba

Processes 2021, 9(3), 545; https://doi.org/10.3390/pr9030545 - 19 Mar 2021

Cited by 2 | Viewed by 1872

Abstract

An experimental study on the flow pattern dynamics in a standard mixing vessel with radial baffles filled with water and induced by a pitched blade impeller pumping downward is presented. Investigation is mainly focused on detection and analysis of quasi-periodical or periodical low-frequency phenomenon connected with time- and length-scales considerably exceeding the Blade Passage Frequency (hereinafter BPF) and common turbulent eddies. This phenomenon, which is expressed as large-scale mean-flow variations, is generally known as flow Macro-Instability (hereinafter MI). It could break-down just below the liquid surface, or it crashes to the liquid surface and causes its Macro-Swelling (hereinafter MS). Our investigation was based on classical two-dimensional (2D) Particle Image Velocimetry (hereinafter PIV) measurement within 3 selected vertical planes in the vessel and subsequent analysis of the velocity field. The dominant frequencies evaluated in the selected points and overall analysis of the quasi-periodical macro-flow pattern behavior is to be shown. Identification of the quasi-periodical substructures appeared within the flow pattern was performed using the Oscillation Pattern Decomposition (hereinafter OPD) method. Observation of the macro-flow patterns confirmed presence of the macro-flow structures detected within flow pattern at the identical mixing pilot plant setup by previous investigations of the MIs phenomenon, i.e., the primary circulation loop and strong impeller discharge jet located in the lower vessel segment and the strong ascending wall current at the baffle, which break-down below the surface very often. A further important contribution of the presented work is the investigation of both flow pattern within the baffles vicinity and in the middle of the sector far from the baffle, showing a significant difference. Low-frequency periodical (or quasi-periodical) behavior of the investigated macro-structures was qualitatively confirmed by the presented results and it was quantified using the velocity dominant frequencies evaluation, as noted below. This frequency analysis brings insight into the supposed and detected interconnections between dynamics of the adjacent flow structures. Detected different flow patterns within the main plane near the baffles and in the inclined plane reveal a strong influence of the baffle presence to the local vertical flow, especially within the upper part of the vessel. Quite a different flow pattern appears tangentially in front of and behind the baffle, where a wake is revealed, indicating significant influence of the baffle on the local tangential flow. The new findings represent a contribution to better understanding the physical phenomena behind the standard mixing process. Full article

(This article belongs to the Special Issue Advances in the Chemical Mixing Process)

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16 pages, 2869 KiB

Open AccessArticle

Numerical Analysis of a Flow over Spheres Embedded on a Flat Wall

by Ewa Szymanek and Artur Tyliszczak

Processes 2021, 9(2), 277; https://doi.org/10.3390/pr9020277 - 01 Feb 2021

Cited by 2 | Viewed by 1798

Abstract

This paper presents the results of numerical simulations of flow in a periodic channel with the walls covered in the central part by spherical elements that have the same overall surface areas but different radii. Two distributions of the sphere are considered, with the subsequent rows placed one after another or shifted. The computations are performed using the high-order code, whereas the solid elements are modelled with the help of the immersed boundary method. For selected cases, the results are validated by comparison with the solutions obtained using the ANSYS Fluent code on a very dense body-fitted mesh. It was found that the increase in the sphere diameter slows down the flow, which is attributed to the larger blockage of the channel cross-section caused by larger spheres and the occurrence of intense mixing (recirculation) between the spheres. The velocity profiles in the vicinity of the sphere are largely dependent on sphere diameter and rise when it increases. It was found that the distribution of the spheres plays an important role only when the spheres are large. In the part of the channel far from the sphere, the velocity profiles are significantly influenced by the sphere diameter but seem to be independent of the sphere distributions. Full article

(This article belongs to the Special Issue Advances in the Chemical Mixing Process)

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15 pages, 2679 KiB

Open AccessArticle

The Dynamics of Globally Unstable Air-Helium Jets and Its Impact on Jet Mixing Intensity

by Agnieszka Pawlowska and Andrzej Boguslawski

Processes 2020, 8(12), 1667; https://doi.org/10.3390/pr8121667 - 17 Dec 2020

Cited by 4 | Viewed by 1531

Abstract

The paper presents experimental investigations of the low-density air-helium jets. The paper is aimed at the analysis of the flow conditions promoting the local absolute instability leading to global flow oscillations. A number of the test cases are analysed with a wide range of the shear layer thickness showing conditions favorable for the global modes and also mixing intensity triggered by such a regime. It is shown that high mixing intensity is determined not only by the global regime but also by the vortex pairing process. The results are compared with a recently proposed universal scaling law for an onset into the global mode. The results turn out to be far from this scaling law and the reasons for such discrepancies are discussed. The measurements show also that if the shear layer at the nozzle exit is thin enough the global modes are suppressed. The mechanism leading to the global mode suppression under such conditions is carefully analysed. Full article

(This article belongs to the Special Issue Advances in the Chemical Mixing Process)

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17 pages, 4260 KiB

Open AccessFeature PaperArticle

An Experimental Study of Turbulent Mixing in Channel Flow Past a Grid

by Daniel Duda, Vitalii Yanovych and Václav Uruba

Processes 2020, 8(11), 1355; https://doi.org/10.3390/pr8111355 - 27 Oct 2020

Cited by 14 | Viewed by 1795

Abstract

Grid turbulence is considered to be a canonical case of turbulent flow. In the presented paper, the flow structure is analyzed from the point of view of mixing properties, where vortical structures and their properties play a significant role. That is why the effect of various length-scales in turbulence is studied separately. The experimental study uses the Particle Image Velocimetry (PIV) method. The original method for spatial spectrum evaluation is applied. Results on vortex spatial spectrum and isotropy are presented. The scaling of turbulent kinetic energy (TKE) is measured; furthermore, the TKE is decomposed according to the length-scales of the fluctuations. By this method, we found that the decay of TKE associated with the smallest length-scales is more sensitive to the Reynolds number than that at larger length-scales. The TKE at the largest investigated length-scales decays more slowly. The turbulence decay-law is studied for various Reynolds numbers. The second and fourth statistical moments of vorticity are evaluated at various Reynolds numbers and distances from the grid. The isotropy is investigated in the sense of ratio of fluctuations in stream-wise to span-wise directions as the used data are captured using the planar PIV method. The full 3D fluctuation invariants were investigated in a representative position by means of the Stereo-PIV method. Full article

(This article belongs to the Special Issue Advances in the Chemical Mixing Process)

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14 pages, 2106 KiB

Open AccessFeature PaperArticle

Large Eddy Simulations of Reactive Mixing in Jet Reactors of Varied Geometry and Size

by Krzysztof Wojtas, Wojciech Orciuch and Łukasz Makowski

Processes 2020, 8(9), 1101; https://doi.org/10.3390/pr8091101 - 04 Sep 2020

Cited by 6 | Viewed by 1897

Abstract

We applied large eddy simulation (LES) to predict the course of reactive mixing carried out in confined impinging jet reactors (CIJR). The reactive mixing process was studied in a wide range of flow rates both experimentally and numerically using computational fluid dynamics (CFD). We compared several different reactor geometries made in different sizes in terms of both reaction yields and mixing efficiency. Our LES model predictions were validated using experimental data for the tracer concentration distribution and fast parallel chemical test reactions, and compared with the k-ε model supplemented with the turbulent mixer model. We found that the mixing efficiency was not affected by the flow rate only at the highest tested Reynolds numbers. The experimental results and LES predictions were found to be in good agreement for all reactor geometries and operating conditions, while the k-ε model well predicted the trend of changes. The CFD method used, i.e., the modeling approach using closure hypothesis, was positively validated as a useful tool in reactor design. This method allowed us to distinguish the best reactors in terms of mixing efficiency (T-mixer III and V-mixer III) and could provide insights for scale-up and application in different processes. Full article

(This article belongs to the Special Issue Advances in the Chemical Mixing Process)

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12 pages, 5168 KiB

Open AccessArticle

The Liquid–Liquid Dispersion Homogeneity in a Vessel Agitated by a High-Shear Sawtooth Impeller

by Roman Formánek and Radek Šulc

Processes 2020, 8(9), 1012; https://doi.org/10.3390/pr8091012 - 19 Aug 2020

Cited by 6 | Viewed by 2650

Abstract

The agitation of immiscible liquids or solid suspensions is a frequent operation in chemical and metallurgical industries. The product quality yield and economy of the processes are significantly affected by mixing conditions. Prediction of mean drop size distribution (DSD) during agitation is fundamental for processes in many branches of industry where the mass transfer is crucial. This contribution aims to analyze the homogeneity of a dispersed system in a vessel agitated by a high-shear sawtooth impeller. The homogeneity of liquid–liquid dispersion is determined by comparison of Sauter mean diameters and drop size distribution (DSD) from different measured regions and for various dispersion times. The experiments were carried out in a baffled vessel for various impeller speeds. The sizes of droplets were obtained by the in-situ measurement technique and by the image analysis (IA) method. Full article

(This article belongs to the Special Issue Advances in the Chemical Mixing Process)

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15 pages, 1573 KiB

Open AccessFeature PaperArticle

Analysis of Power Input of an In-Line Rotor-Stator Mixer for Viscoplastic Fluids

by Mehmet Ayas, Jan Skocilas and Tomas Jirout

Processes 2020, 8(8), 916; https://doi.org/10.3390/pr8080916 - 01 Aug 2020

Cited by 1 | Viewed by 2351

Abstract

In this work, the power draw and shear profile of a novel in-line rotor-stator mixer were studied experimentally and the laminar flow regime was simulated. The power draw of the rotor-stator mixer was investigated experimentally using viscoplastic shear-thinning fluid and the results of the obtained power consumptions were verified through simulations. The power draw constant and Otto-Metzner coefficient were determined from the result of experimental data and through simulations. A new method is suggested for the determination of the Otto-Metzner coefficient for the Herschel–Bulkley model and the term efficiency is introduced. It was shown that the proposed method can be applied successfully for the prediction of the Otto-Metzner coefficient for the mixing of viscoplastic shear-thinning fluids. The effect of geometry and rotor speed on power consumption and shear rate profile in the investigated mixer is discussed from the results of the simulations. It was found that numerical methods are a convenient tool and can predict the power draw of the in-line rotor-stator mixer successfully. Full article

(This article belongs to the Special Issue Advances in the Chemical Mixing Process)

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10 pages, 3167 KiB

Open AccessFeature PaperArticle

Scale-Up of Mixing Equipment for Suspensions

by Tomáš Jirout, František Rieger and Dorin Ceres

Processes 2020, 8(8), 909; https://doi.org/10.3390/pr8080909 - 01 Aug 2020

Cited by 2 | Viewed by 2487

Abstract

This paper deals with the scale-up of equipment for the mixing of suspensions. The measurement of just-suspended agitator speeds was carried out with standard, pitched, four-blade turbines and folded, four-blade turbines in three vessels (290 mm, 600 mm, and 800 mm in diameter) for several particle sizes and concentrations. The results of measurements confirmed that scale-up based on dimensionless Froude number dependence, on the relative particle size and concentration, can be used. On the basis of the results, a scale-up rule for agitator speeds in a given suspension and equipment geometry was recommended, and various conclusions reported by different investigators were discussed. Full article

(This article belongs to the Special Issue Advances in the Chemical Mixing Process)

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Review

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27 pages, 3898 KiB

Open AccessFeature PaperReview

Mixing in Turbulent Flows: An Overview of Physics and Modelling

by Jacek Pozorski and Marta Wacławczyk

Processes 2020, 8(11), 1379; https://doi.org/10.3390/pr8111379 - 30 Oct 2020

Cited by 3 | Viewed by 4681

Abstract

Turbulent flows featuring additional scalar fields, such as chemical species or temperature, are common in environmental and industrial applications. Their physics is complex because of a broad range of scales involved; hence, efficient computational approaches remain a challenge. In this paper, we present an overview of such flows (with no particular emphasis on combustion, however) and we recall the major types of micro-mixing models developed within the statistical approaches to turbulence (the probability density function approach) as well as in the large-eddy simulation context (the filtered density function). We also report on some trends in algorithm development with respect to the recent progress in computing technology. Full article

(This article belongs to the Special Issue Advances in the Chemical Mixing Process)

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38 pages, 1485 KiB

Open AccessReview

A General Review of the Current Development of Mechanically Agitated Vessels

by Marek Jaszczur and Anna Młynarczykowska

Processes 2020, 8(8), 982; https://doi.org/10.3390/pr8080982 - 13 Aug 2020

Cited by 31 | Viewed by 8751

Abstract

The mixing process in a mechanically agitated vessel is a widespread phenomenon which plays an important role among industrial processes. In that process, one of the crucial parameters, the mixing efficiency, depends on a large number of geometrical factors, as well as process parameters and complex interactions between the phases which are still not well understood. In the last decade, large progress has been made in optimisation, construction and numerical and experimental analysis of mechanically agitated vessels. In this review, the current state in this field has been presented. It shows that advanced computational fluid dynamic techniques for multiphase flow analysis with reactions and modern experimental techniques can be used with success to analyse in detail mixing features in liquid-liquid, gas-liquid, solid-liquid and in more than two-phase flows. The objective is to show the most important research recently carried out. Full article

(This article belongs to the Special Issue Advances in the Chemical Mixing Process)

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26 pages, 6185 KiB

Open AccessFeature PaperReview

Application of Theoretical and Experimental Findings for Optimization of Mixing Processes and Equipment

by Tomáš Jirout and Dita Jiroutová

Processes 2020, 8(8), 955; https://doi.org/10.3390/pr8080955 - 08 Aug 2020

Cited by 6 | Viewed by 6178

Abstract

The homogenization of the agitated batch and ensuring the suspension of particles are the most frequently encountered requirements in terms of mixing applications. These operations are affected by the flow of the agitated batch. The geometrical parameters of the mixing system, especially the shape of the agitator blade, affect flow and circulation in the agitated batch. The present work provides a general description of the most common processes in the agitated batch (blending and particle suspension), hydrodynamic parameters (flow in agitated batches, pumping and circulation capacity of impellers) and the geometrical configurations of the mixing equipment (shape of vessel, baffle and impeller, and their mutual arrangement) that influence the process. The dimensionless process characteristics of the agitator were derived by theoretical analysis. These characteristics were applied to evaluate an extensive set of experimental data with various geometric configurations of the mixing equipment. This study shows how the flow in the agitated batch, caused by the pumping and circulating effects of the agitators, affects the parameters and energy efficiency of these processes, depending on the geometric configuration of the mixing equipment. Moreover, the benefits of the hydrofoil impellers used for these mixing processes are presented. Full article

(This article belongs to the Special Issue Advances in the Chemical Mixing Process)

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