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In this study, a novel furnace concept for in-flight particle spheroidization is presented, characterized and evaluated. A natural gas fired burner with a continued staged air principle and internal recirculation (COSTAIR), was used to provide the required temperature for the spheroidization process. Therefore, a numerically inexpensive CFD model for the calculation of combustion and multiphase flow is proposed. Particularly for the calculation of particle trajectories and particle peak temperatures of non-spherical (chiseled and flaky) slag particles, the presented CFD model differs in two modifications from the current state-of-the-art CFD models: first, a numerically efficient combustion model with a detailed chemical reaction mechanism was used in order to calculate the temperature profile of the furnace. While in most current state-of-the-art CFD models the numerically expensive and time consuming eddy dissipation concept (EDC) model or the insufficient eddy dissipation model (EDM) are used. Second, the discrete phase model, which is based on a numerically efficient Euler-Lagrangian approach, is used for multiphase modeling. Non-spherical particles are considered by application of a suitable particle drag model from literature. Although, the assumption of spherical particles is more common in current state-of-the-art CFD models for multiphase modeling. It was concluded that the presented furnace concept is applicable for the semi-industrial scale production of spherical boiler slag particles. The numerical results show that the assumption of non-spherical particles, compared to spherical particles, is more suitable for the calculation of particle trajectories and particle peak temperatures in the presented furnace. • A novel furnace concept for particle spheroidization is presented. • Time saving combustion modelling using a flamelet approach. • Characterization of temperature and flow conditions in the furnace. • Application of a numerically inexpensive Euler-Lagrangian approach. • Comparison between CFD and measurements. [ABSTRACT FROM AUTHOR]

Precise predictions of particle motion behavior after injection into a fluid flow are essential for process design and optimization in industrial applications. The goal of this work is to present a numerically inexpensive CFD model to predict lateral particle distribution after injection into a vertical cross flow. To this end, a specially designed and application-oriented flow channel, a so-called particle cross sifter, was built, which was experimentally and numerically analyzed. In the experiments a type of boiler slag powder, consisting of particles with various sizes and shapes, was injected into the cross sifter. For the numerical simulations different turbulence modeling approaches, such as RANS and LES, in combination with the DPM were applied. The results were compared to experimental data. It is highlighted, that only the LES approach in combination with the DPM is able to predict the lateral particle distribution with sufficient accuracy. Unlabelled Image • Evaluation of lateral particle distribution due vertical to cross flow. • Experiments within a specially designed particle cross sifter. • Investigation of differences between spherical and highly non-spherical particles. • Results of steady-state, transient and LES simulations were evaluated. • LES results on a "coarse" mesh showed good agreement with experimental data. [ABSTRACT FROM AUTHOR]

Abstract In a vast number of industrial applications, particle-laden flows consist of irregularly-shaped particles with various sizes and shapes and thus, the drag coefficient is strongly affected and different for each particle. The key issue is to incorporate these characteristics within numerical calculations. Therefore, a sample-set of irregularly-shaped combustion residues or rather clinker flakes, with sizes between 50 μm and 250 μm was experimentally and numerically investigated. Particle geometry, terminal velocity measurements and drag analysis were carried out within the present work. Areflected-light microscope and an optical particle analyzer were used for geometry analysis. Terminal velocities were measured in a water filled glass column with a commercially available digital camera with a recording rate of 1000 frames per second and a frame-by-frame tracking method. At first a simple particle classification method based on this geometrical analysis and terminal velocity measurements in a static fluid was introduced. As a result, several particle groups were defined based on the obtained data. As a second step, this data were used as input parameters for two simple drag models from literature. The required particle shape parameters were determined by velocity recalculations and data fitting. Furthermore, the applicability of these drag models in combination with the experimentally determined particle shape descriptors was evaluated within a simple test rig, where freely falling particles are exposed to a lateral compressed airflow. The particles would be deflected and settle at different positions at the bottom of the test rig. Numerical calculations of the test rig were performed using a commercial Computational Fluid Dynamics (CFD) code and a numerically efficient Euler-Lagrangian approach in order to predict the accurate motion of particles. The particle mass dispersion at the bottom of the test rig was compared to numerical results. It was shown that the used drag models in combination with the experimentally determined particle shape descriptors improved the prediction of particle drag coefficients and trajectories significantly without an increase of computational cost and time. Graphical abstract Unlabelled Image Highlights • The applicability of existing drag formulations for clinker flakes was tested. • Custom shape factors for variously shaped particles were experimentally determined. • Velocity measurements were carried out in a static fluid. • The established approach was validated by experiments and numerical calculations. • Predicted particle drag coefficients and trajectories could be improved. [ABSTRACT FROM AUTHOR]

Abstract A powder is a composition of many unique particles in almost all cases. The grains have different shapes and sizes which influence drag and heat transfer, so in multiphase flows, each particle also interacts differently with the continuous phase. In this work, a practical approach to calculate the convective in-flight heating of differently shaped, non-spherical particles, is presented. A model from literature, which uses the same parameters for the calculation of particle drag coefficients and Nusselt numbers, was selected. The model parameters were determined by simply measuring the settling velocity of multiple particles in still air. A novel method for particle classification, which has already been proven to be essential for precise drag calculations of powders with differently shaped grains, was extended for heat transfer calculations. The approach was validated by numerical calculations and experimental data of a simple test rig where the particles were heated from room temperature in free fall through hot air. After a drop height of 1.7 m , the mean particle temperature was measured with a specially developed device. The particle temperature in the test rig increased in the order of 300 K. The commercial CFD-code Ansys Fluent 17.0 in combination with an Euler-Lagrangian approach for multiphase flow calculations was used for the simulations. The numerically and experimentally determined particle temperatures agreed very well in the case of spherical and as well for non-spherical particles. In comparison to the standard model of the CFD code, which only considers spherical particles, the error in calculating the mean particle temperature was reduced from −59 K to just 2 K. Moreover, it was shown that the standard deviation of the particle temperature distribution increases significantly when the different particle shapes are included in the calculation. It was concluded that the particle classification improves the calculation of maximum and minimum particle peak temperatures. The proposed approach is fully independent of the particle type and it can be used to determine the heat transfer characteristics of various different powders. Further, the customized model can be added to nearly every CFD-code on the market and improve the results considerably while computational cost remains low due to the numerically efficient Euler-Lagrangian approach. Graphical abstract Unlabelled Image Highlights • A model to calculate convective heat transfer to non-spherical particles is proposed. • The particle shape is included in numerically efficient Euler-Lagrange calculations. • A practical approach to determine the model parameters is shown. • A novel method for particle classification is used. • Precise and efficient numerical calculations of particle temperature are enabled. [ABSTRACT FROM AUTHOR]

One of the key issues with the application of drag models for non-spherical particles is the incorporation of differently shaped particles. Under real world conditions, every single grain in a powder or granule has a unique geometry which strongly affects its drag coefficient. To incorporate the effects of different particle shape factors on their trajectories in the flow field, a drag analysis of crushed coal slag particles with sizes between 400 μm and 850 μm was performed within this work. The particle shapes were examined using brightfield microscopy and an optical particle analyzer. Also, the drop velocities of particles in still air were measured using a “Sony DSC-RX100 IV” standard digital camera at a frame rate of 1000 pictures per second instead of more costly special-purpose high speed camera equipment. A novel particle classification was introduced based on the obtained data. This data was also used as an input for three different drag models from literature. In order to evaluate the practicality of each drag model, a simple test rig in form of an air tunnel separator was built. It basically consisted of a box where freely falling slag particles were deflected by a free stream of air. The particle mass spreading at the bottom of the air flow separator was compared to numerical results. Numerical simulations of the setup were performed using Computational Fluid Dynamics (CFD) and a numerically efficient Euler-Lagrange approach in order to calculate the particle trajectories. It was concluded that the simple particle classification method provided satisfying calculated results. A special drag formulation for crushed coal slag particles was developed on the basis of two drag models from literature. The customized drag model improved the calculated results significantly. It can be added to nearly every commercial CFD code on the market without significant increase on computational cost and the proposed procedure also appeared to be well suited for the determination of drag model characteristics of other particulate materials in many fields of research. [ABSTRACT FROM AUTHOR]

• Specialized drag and particle heat transfer models were used. • The key factors for spheroid formation were investigated. • Numerical methods to predict the product quality have been found. • The impact of particle size and shape was investigated. • Particles were mostly heated due to convection. In order to identify the key factors for successful and efficient production, the formation of spherical particles from highly non-spherical coal slag grains was investigated within this work. The main focus was on precise drag and heat transfer calculations in order to obtain suitable criteria to predict the later product quality. State-of-the-art combustion, radiation and multiphase models in combination with specially adapted closure relations for drag and heat transfer were used for this purpose. The second objective was to investigate the relationship between particle temperature, viscosity, surface tension, shape and time required by the particles to obtain a smooth, round surface while passing through the burner chamber. In the first step, the process was investigated in experimental work. Slag particles were injected into an experimental furnace with a thermal input of 70 kW. The slag particles heated up which caused a reduction of slag viscosity by several orders of magnitude. While passing through the furnace, the sharp-edged powder grains transformed into smooth, highly spherical particles due to their surface tension. Numerical calculations were used to identify the main influence factors of in-flame spheroid formation. The effects of particle temperature, size, initial shape and residence time on the sphericity of the particle were studied using a Volume of Fluid (VOF) multiphase model and single-particle simulations which allowed calculations of the interface between an isothermal particle and the surrounding gas phase, as well as the interface deformation due to surface tension. The calculations have shown, that particles with volume-equivalent diameters in the range 420 µm-840 µm and an initial sphericity of 0.65 transform to spheroids in timescales in the order of 10−1 – 10° s when heated to temperatures above 1373 K. The critical particle temperature value of 1373 K was also confirmed by Euler-Lagrangian calculations of the reactive multiphase flow in the furnace. It has been shown that the particle peak temperature is a sufficient criterion for the formation of spheroids from highly non-spherical particles and that fluid temperature and particle residence time are the most important parameters of the later production process. [ABSTRACT FROM AUTHOR]