An improved CFD-DEM framework for gas-liquid-solid multiphase free surface flow

Gas-Flüssig-Fest-Mehrphasensysteme sind in technischen Anwendungen allgegenwärtig, z. B. beim direkten Tintenstrahldruck, der Sprühtrocknung und der Sprühbeschichtung. Der direkte Tintenstrahldruck ist eine vielversprechende additive Fertigungstechnologie für die Herstellung temperaturempfindlicher Bauteile. Allerdings leiden tintenstrahlgedruckte Strukturen unter dem so genannten „Kaffeering-Effekt“ oder „Kaffeefleck-Effekt“, der zu einem ringförmigen Abscheidungsmuster führt, bei dem mehr Material um den Rand herum abgeschieden wird, aber viel weniger Material im Inneren des gedruckten Tintenrings übrig bleibt. Das Verständnis der physikalischen Zusammenhänge beim Verdampfen von mit Tintenstrahldruckern gedruckten Tröpfchen mit vielen suspendierten Feststoffpartikeln im Inneren hilft, den Coffee-Ring-Effekt zu unterdrücken und gleichmäßigere Materialabscheidungsmuster zu gewährleisten. Dementsprechend ist die Entwicklung eines numerischen Rahmens für die Modellierung des Mehrphasensystems Gas-Flüssigkeit-Feststoff mit Verdampfung von großer Bedeutung. In der vorliegenden Arbeit wird ein verbessertes CFD-DEM (Computational Fluid Dynamics-Discrete Element Method)-Kopplungssystem vorgeschlagen und entwickelt, um das Mehrphasensystem Gas-Flüssigkeit-Feststoff mit und ohne Verdampfung zu modellieren. Für die Modellierung eines solchen Mehrphasensystems, in dem die Oberflächenspannung dominiert, müssen einige grundlegende wissenschaftliche Probleme angegangen werden: Partikeltransport und-akkumulation, Oberflächenspannung und Erfassen der freien Oberfläche, Tröpfchenbenetzung und Verdampfung, Kontaktlinien-Pinning, Partikel-Fluid-Wechse- lwirkungen usw. Die DEM wird eingesetzt, um die Trajektorie von Feststoffpartikeln zu verfolgen, und CFD wird verwendet, um Oberflächenspannung, Verdunstung, Kontaktlinien-Pinning usw. zu modellieren. Darüber hinaus wird die Kopplung von CFD und DEM eingesetzt, um die komplexen Partikel-Flüssigkeits-Wechselwirkungen zu berechnen. Auf der DEM-Seite wird die konventionelle DEM erweitert, um mikroskopisch kleine Partikel zu modellieren. Die berührungslosen Oberflächenkräfte, z.B. Van-der-Waals-, elektrostatische und Derjaguin-Landau-Verwey-Overbeek-(DLVO)-Kräfte, sowie ein durch Brownsche Bewegung induziertes Zufallskraftmodell werden in den Open-Source DEM-Code LIGGGHTS implementiert. Ausführliche numerische Validierungen zeigen, dass diese neu implementierten Kraftmodelle sowohl berührungslose als auch zufällige Kräfte mit recht guter numerischer Genauigkeit vorhersagen können. Auf der CFD-Seite wird das verbesserte Coupled Level Set and Volume of Fluid (i-CLSVoF)-Framework entwickelt und in die Open-Source-C++-Bibliothek OpenFOAM implementiert, um die oberflächenspannungsdominierte Strömung zu modellieren. Das i-CLSVoF-Framework kann scharfe freie Oberflächen mit wenig Grenzflächendiffusion erfassen. Das in i-CLSVoF implementierte verbesserte Modell der Oberflächenspannungskraft kann diese genauer vorhersagen und bietet eine Unterdrückung unphysikalischer Störgeschwindigkeiten. Drei Verdunstungsmodelle wurden in i-CSLVoF implementiert, um die Verdunstung der flüssigen Phase zu modellieren. Numerische Validierungen zeigen, dass diese Verdunstungsmodelle den Phasenübergang von Flüssigkeit zu Gas genau modellieren können. Darüber hinaus ist ein Kontaktlinien-Pinning-Modell integriert, um die Tröpfchenverdampfung mit einem konstanten Kontaktradius zu beschreiben. Der sogenannte aufgelöste CFD-DEM-Ansatz stellt die Wechselwirkungen zwischen kontinuierlichen flüssigen und diskreten festen Phasen in ortsaufgelöster Form dar. Ein in dieser Arbeit entwickeltes verbessertes aufgelöstes CFD-DEM-Modell ist in der Lage, die mehrphasige freie Oberflächenströmung zwischen Gas, Flüssigkeit und Feststoff mit und ohne Verdampfung der flüssigen Phase zu modellieren. Der i-CLSVoF-Rahmen wird verwendet, um den Oberflächenspannungseffekt zu modellieren und die scharfe freie Oberfläche zu erfassen. Ein verbessertes Kapillarkraftmodell wird entwickelt, um die Kapillarinteraktionen für teilweise schwimmende Feststoffteilchen an einer freien Oberfläche zu berechnen. Zur Validierung des aufgelösten CFD-DEM-Modells werden zwei bekannte Vergleichsfälle durchgeführt, nämlich die Berechnung des Widerstandsbeiwert und das Absinken einer einzelnen Kugel. Es zeigt sich, dass das in dieser Arbeit entwickelte aufgelöste CFD-DEM-Modell die Fluid-Feststoff-Wechselwirkungen genau berechnen und die Trajektorie von Feststoffpartikeln, die mit der flüssigen Phase wechselwirken, vorhersagen kann. Numerische Demonstrationen, z.B. zwei Partikel, die sich entlang einer freien Oberfläche bewegen, wenn die flüssige Phase verdampft, sowie Partikeltransport und -ansammlungen innerhalb eines verdampfenden Tropfens auf einem Substrat zeigen die Leistungsfähigkeit des aufgelösten Berechnungswerkzeugs bei der Modellierung komplexer Partikel-Flüssigkeits-Wechselwirkungen. Der aufgelöste CFD-DEM-Ansatz löst die Strömungsfelder um Feststoffpartikel genau auf, ist aber rechenintensiv. Daher eignet er sich nur für die rechnerische Modellierung eines Mehrphasensystems mit einer begrenzten Anzahl von Partikeln (weniger als 1000). Daher wird in dieser Arbeit ein sogenanntes unaufgelöstes CFD-DEM-Modell weiterentwickelt. In dem unaufgelösten CFD-DEM-Modell enthält jede CFD-Zelle mehrere Feststoffteilchen und kann somit zur Untersuchung des globalen Verhaltens vieler Feststoffteilchen (bis zu $10^{6}$) verwendet werden. Ein neues Widerstandskraftmodell mit einem korrigierten Widerstandsbeiwert, der mit umfangreichen experimentellen Daten gut übereinstimmt, wurde implementiert. Dieses Widerstandskraftmodell ist über einen weiten Bereich der Reynoldszahl ($10^{-4} - 10^{6}$) anwendbar. Zur Validierung des Widerstandskraftmodells und des unaufgelösten CFD-DEM-Ansatzes wurden umfangreiche numerische Validierungen durchgeführt. Ein 3D-Dammbruch-Benchmark-Fall zeigt, dass das unaufgelöste CFD-DEM-Modell die Vier-Wege-Kopplung zwischen der festen und der flüssigen Phase mit etwa $4000$ Feststoffpartikeln realisieren kann. Der Vergleich zwischen numerischen Simulationen und den entsprechenden experimentellen Studien beweist, die Genauigkeit des unaufgelösten CFD-DEM-Modells

Computational modelling of gas-liquid-solid multiphase free surface flow with and without evaporation

Gas-liquid-solid multiphase systems are ubiquitous in engineering applications, e.g. inkjet printing, spray drying and coating. Developing a numerical framework for modelling these multiphase systems is of great significance. An improved, resolved CFD-DEM framework is developed to model the multiphase free surface flow with and without evaporation. An improved capillary force model is developed to compute the capillary interactions for partially floating particles at a free surface. Two well-known benchmark cases, namely drag coefficient calculation and the single sphere settling, are conducted to validate the resolved CFD-DEM model. It turns out that the resolved CFD-DEM model developed in this paper can accurately calculate the fluid-solid interactions and predict the trajectory of solid particles interacting with the liquid phase. Numerical demonstrations, namely two particles moving along a free surface when the liquid phase evaporates, and particle transport and accumulations inside an evaporating sessile droplet show the performance of the resolved model.Comment: 54 pages, 19 figures, 9 table

A review of the Discrete Element Method/Modelling (DEM) in agricultural engineering

With the development of high-performance computing technology, the number of scientific publications regarding computational modelling of applications with the Discrete Element Method/Modelling (DEM) approaches in agricultural engineering has risen in the past decades. Many granular materials, e.g. grains, fruits and soils in agricultural engineering are processed, and thus a better understanding of these granular media with DEM is of great significance in design and optimization of tools and process in agricultural engineering. In this review, the theory and background of DEM have been introduced. Some improved contact models discussed in the literature for accurately predicting the contact force between two interacting particles have been compared. Accurate approximation of irregular particle shapes is of great importance in DEM simulations to model real particles in agricultural engineering. New algorithms to approximate irregular particle shapes, e.g. overlapping multi-sphere approach, ellipsoid, etc., have been summarized. Some remarkable engineering applications of the improved numerical models developed and implemented in DEM are discussed. Finally, potential applications of DEM and some suggested further works are addressed in the last section of this review

A study of particles looseness in screening process of a linear vibrating screen

We investigated the movement of particles in screening process over the sieve plate of a linear vibrating screen using the Discrete Element Method (DEM). The behavior of particles which is affected by a series of vibrational parameters including amplitude, frequency and vibration direction angle determining screening performance. This paper centers on particles looseness by analyzing the looseness coefficient and looseness rate. The relationships between the looseness coefficient, looseness rate and vibration parameters were profoundly discussed. Mathematical models relating looseness coefficient to time were established using the least squares method. An experimental platform which combines high-speed camera system with experimental prototype of vibrating screen was designed. The research made a more in-depth investigation of particles’ movements and analysis of particle looseness. Physical experiments were used to verify the reliability of simulation results. Finally, we would come into the following conclusions: high frequency and large amplitude make particles obtain more energy to be active and the average distances among particles get larger slowly. On the contrary, at low frequency and amplitude, the looseness coefficient and looseness rate were relatively low. When the amplitude approaches 2.7 mm, the frequency is about 34 Hz and the vibration angle is around 42 degrees, the looseness ratio produces better performance. The paper offered insights to the design and manufacturing of vibrating screen

Vibration Parameter Optimization of a Linear Vibrating Banana Screen Using DEM 3D Simulation

In this paper, the effects of vibration parameters of a banana screen, i.e. frequency, amplitude and vibration direction angle, on the screening efficiency per unit time were studied using the discrete element method (DEM). The simulations were validated according to data collected from an experimental prototype screen. Functional relationships between vibration parameters and screening efficiency per unit time are presented. The results showed that the screening efficiency per unit time first displays an increase and later a decrease when the frequency, amplitude or vibration direction angle increased respectively. Vibration parameter optimization was also investigated, using an orthogonal experiment. Based on the banana screen model, it can be concluded that when the frequency is 22 Hz, the amplitude is 2.2 mm and the vibration direction angle is 39°, the screening efficiency of a banana screen is optimal

A review of the Discrete Element Method/Modelling (DEM) in agricultural engineering

With the development of high-performance computing technology, the number of scientific publications regarding computational modelling of applications with the Discrete Element Method/Modelling (DEM) approaches in agricultural engineering has risen in the past decades. Many granular materials, e.g. grains, fruits and soils in agricultural engineering are processed, and thus a better understanding of these granular media with DEM is of great significance in design and optimization of tools and process in agricultural engineering. In this review, the theory and background of DEM have been introduced. Some improved contact models discussed in the literature for accurately predicting the contact force between two interacting particles have been compared. Accurate approximation of irregular particle shapes is of great importance in DEM simulations to model real particles in agricultural engineering. New algorithms to approximate irregular particle shapes, e.g. overlapping multi-sphere approach, ellipsoid, etc. have been summarized. Some remarkable engineering applications of the improved numerical models developed and implemented in DEM are discussed. Finally, potential applications of DEM and some suggested further work are addressed in the last section of this review

Vibration Parameter Optimization of a Linear Vibrating Banana Screen Using DEM 3D Simulation

In this paper, the effects of vibration parameters of a banana screen, i.e. frequency, amplitude and vibration direction angle, on the screening efficiency per unit time were studied using the discrete element method (DEM). The simulations were validated according to data collected from an experimental prototype screen. Functional relationships between vibration parameters and screening efficiency per unit time are presented. The results showed that the screening efficiency per unit time first displays an increase and later a decrease when the frequency, amplitude or vibration direction angle increased respectively. Vibration parameter optimization was also investigated, using an orthogonal experiment. Based on the banana screen model, it can be concluded that when the frequency is 22 Hz, the amplitude is 2.2 mm and the vibration direction angle is 39°, the screening efficiency of a banana screen is optimal

A study of particles looseness in screening process of a linear vibrating screen

We investigated the movement of particles in screening process over the sieve plate of a linear vibrating screen using the Discrete Element Method (DEM). The behavior of particles which is affected by a series of vibrational parameters including amplitude, frequency and vibration direction angle determining screening performance. This paper centers on particles looseness by analyzing the looseness coefficient and looseness rate. The relationships between the looseness coefficient, looseness rate and vibration parameters were profoundly discussed. Mathematical models relating looseness coefficient to time were established using the least squares method. An experimental platform which combines high-speed camera system with experimental prototype of vibrating screen was designed. The research made a more in-depth investigation of particles’ movements and analysis of particle looseness. Physical experiments were used to verify the reliability of simulation results. Finally, we would come into the following conclusions: high frequency and large amplitude make particles obtain more energy to be active and the average distances among particles get larger slowly. On the contrary, at low frequency and amplitude, the looseness coefficient and looseness rate were relatively low. When the amplitude approaches 2.7 mm, the frequency is about 34 Hz and the vibration angle is around 42 degrees, the looseness ratio produces better performance. The paper offered insights to the design and manufacturing of vibrating screen

Overexpression of <i>OsPHT1;4</i> Increases Phosphorus Utilization Efficiency and Improves the Agronomic Traits of Rice cv. Wuyunjing 7

Inorganic phosphate (Pi) is taken up by plant roots and translocated via phosphate transporters. Previously, we showed that phosphate transporter OsPHT1;4 in the PHT1 family participates in phosphate acquisition and mobilization; it facilitates the embryo development of Japonica rice Nipponbare. This study investigated the potential of manipulating the expression of OsPHT1;4 to increase Pi acquisition efficiency and crop productivity in rice cv. Wuyunjing 7 (WYJ 7), a cultivar widely grown in Yangtze River Delta of China. The OsPHT1;4 overexpression lines and wild-type WYJ 7 were treated under different Pi conditions in hydroponic and field experiments. Quantitative real-time RT-PCR analysis and the transgenic plants expressing GUS reporter gene indicate strong expression of OsPHT1;4 in roots and leaf collars of cv. WYJ 7. The total P contents in shoots of the OsPHT1;4-overexpressing plants were significantly higher under Pi-deficient hydroponic conditions than the wild type under Pi sufficiency and deficiency. 33Pi uptake and translocation assays confirmed the results. In the field condition, OsPHT1;4 overexpression lines had a higher P concentration in tissues than the wild type control, and the panicle performance of the overexpression lines including the grain yield was improved as well. Taken together, our results show that OsPHT1;4 plays an important role in the acquisition and mobilization of Pi in WYJ 7, especially under Pi deficiency. The study highlights the importance of OsPHT1;4 in improving the agronomic traits of the widely grown rice cultivar in China

Gut Microbiota and Tumor Immune Escape: A New Perspective for Improving Tumor Immunotherapy

The gut microbiota is a large symbiotic community of anaerobic and facultative aerobic bacteria inhabiting the human intestinal tract, and its activities significantly affect human health. Increasing evidence has suggested that the gut microbiome plays an important role in tumor-related immune regulation. In the tumor microenvironment (TME), the gut microbiome and its metabolites affect the differentiation and function of immune cells regulating the immune evasion of tumors. The gut microbiome can indirectly influence individual responses to various classical tumor immunotherapies, including immune checkpoint inhibitor therapy and adoptive immunotherapy. Microbial regulation through antibiotics, prebiotics, and fecal microbiota transplantation (FMT) optimize the composition of the gut microbiome, improving the efficacy of immunotherapy and bringing a new perspective and hope for tumor treatment