• Energy Research

Abstract This paper presents a numerical study based on the discrete element method (DEM) to investigate the transverse mixing of wet particles in a rotating drum. The effects of the liquid surface tension, the drum rotation speed and the filling level on particle mixing were investigated. The results showed that particles had quick mixing in the transverse plane and the well mixed states were achieved within a few revolutions. The Lacey mixing index showed an exponential increase with mixing time. The presence of the capillary force in general reduced mixing performance. However, the mixing of dry particles was poorest at 64% filling level compared with other filling levels, and increasing cohesion at that level actually improved particle mixing. The analysis of particle movements indicated that particle mixing was dominated by the particle circulation period, which is the time required for a particle to complete one circulation in the drum, and its standard deviation. A model was proposed to estimate the circulation periods at different streamlines which were comparable with the simulation results, thus providing a general method to predict mixing performance in the transverse plane.

Abstract Particle shape has a significant effect on the characteristics of fluidized beds due to the anisotropy of non-spherical particles. In particular, particle shape can affect the bubble dynamics such as bubble size, shape and velocity. However, the relationship between bubble dynamics and the orientation of non-spherical particles is not clear. This paper presents a detailed analysis on orientation of spheroids in a gas-solid fluidized bed operated with a continuous central jet. Spheroids with aspect ratios at 0.5 (oblate) and 2 (prolate) are employed to represent disc-like and rod-like particles, respectively. The results show that the orientation of spheroids is directly related to bubble motion. The orientation evolves with bubble progression and prefers to align in the bed horizontally or vertically depending upon the bubble position. Further, frequency distributions of spheroids orienting parallel/perpendicular with fluid flow direction were analysed in terms of several hydrodynamic parameters such as particle position and velocity, and fluid drag force. The results indicate that spheroids have different orientations depending upon the hydrodynamic parameters. The findings obtained from this study can aid to better understand fluidization dynamics associated with non-spherical particles.

AbstractAn investigation was carried out of the transformation between the number, length, surface and volume size distributions expressed by Johnson's SB distribution function – the bounded log‐normal distribution function. As is well known, if any of the number, length, surface and volume distributions is log‐normal, all the others will also be log‐normal. Theoretical analysis suggests that the SB function may have a similar property. This was confirmed by a computer‐aided numerical simulation, in which emphasis was given to the transformation between successive order size distributions, i.e. ƒi(x) → ƒi + 1(x) or ƒi(x) → ƒi − 1(x). The numerical results can be applied to the particle size distribution transformation because this transformation can generally be made step by step, for example, ƒi → ƒi−1 (x) → ƒi − 2(x) → … → ƒj(x) for ƒi(x) → ƒj(x) ( i > j).

AbstractIn order to understand the complicated phenomena of pulverized coal injection (PCI) process in blast furnace (BF), several mathematical models have been developed by the UNSW and BSR cooperation. These models are featuring from coal combustion in a pilot‐scale test rig, to coal combustion in a real BF, and then to coal/coke combustion in a real BF, respectively. This paper reviews these PCI models in aspects of model developments and model applicability. The model development is firstly discussed in terms of model formulation, their new features and geometry/regions considered. The model applicability is then discussed in terms of main findings followed by the model evaluation on their advantages and limitations. It is indicated that the three PCI models are all able to describe PCI operation qualitatively. The model of coal/coke combustion in a real BF is more reliable for simulating in‐furnace phenomena of PCI operation qualitatively and quantitatively. Such model gives a more reliable burnout prediction over the raceway surface, which could better represent the amount of unburnt char entering the coke bed. These models are useful for understanding the flow‐thermo‐chemical behaviours and then optimising the PCI operation in practice.

Abstract A method for optimizing the combustion in an ultra-supercritical boiler is developed and evaluated in a 660 MWe ultra-supercritical coal fired power plant. In this method, Artificial Neural Networks (ANN) models are established for predicting the boiler operating and emission properties. To enhance the generalization of the ANN models, Computational Fluid Dynamics (CFD) simulation is performed to generate some data as training samples for ANN modeling, together with the historical operating data. The inputs of the ANN models are unit load, coal properties, excess air and air distribution scheme, and the outputs are thermal efficiency and NOx emission. Based on the ANN models, Genetic Algorithm (GA) is used to optimize the air distribution scheme to achieve a higher thermal efficiency and lower NOx emission simultaneously. The predictions of the thermal efficiency and NOx emissions show a good agreement with the plant data, with mean errors of 0.04% and 3.56 mg/Nm3, respectively. The results indicate that the use of CFD data can help generalize the ANN models. The application to a practical plant demonstrates that the proposed approach provides an effective tool for multi-objective optimization of pulverized-coal boiler performance with improved thermal efficiency and NOx emission control.

Abstract This paper presents a numerical study on the particle dispersion in a horizontally vibrating vessel with round corners under zero gravity. Such a vessel is specifically designed for particle handling in the outer space. The numerical model is validated by good agreement between the simulated and experimental results. The effects of key variables, including overall particle number concentration and vibration amplitude and frequency, are studied by a series of controlled numerical experiments. The particle flow in the vessel is analyzed by the detailed particle scale information obtained from the simulations. The results are used to reveal the mechanisms and clarify some speculations of the particle flow observed in the experiments. In particular, it is found that the conveying velocity generated by the round corner can be correlated to the velocity amplitude, and so is the overall kinetic energy of the particles inside the vessel. The findings are useful for the optimum design of an effective technique for particle transportation under microgravity.

Abstract In this work, the combined effects of vibration parameters on the circularly screening processes are analysed based on the Taguchi orthogonal experiment method. First, numerical investigations of the screening process including the average particle velocity and screening efficiency of different particle size fractions are presented. The variations of the average velocity and the screening efficiency of different particle size fractions for both spherical and non-spherical particle models are examined. The effect of vibration intensity on the screening process is discussed and the potential error induced by the analysis of a single factor on the screening performance is demonstrated. Then, the orthogonal test data of average particle velocity and screening efficiency for 16 cases are analysed by using the direct analysis method. The combined effects of amplitude, frequency and the inclination angle of the screen deck on the screening performance are discussed based on the calculated average values of each level of the factors. The corresponding optimal combinations of the three factors for two screening performance indexes are obtained, respectively. The effect weights of the vibration parameters on the screening performance indexes are also obtained by comparing the ranges of the factors. This work should be helpful for the understanding of the combined effects of operation parameters and provides essential references for efficient operation and optimal design of a vibrating screen.

AbstractThis paper presents a numerical study of the gas–powder flow in a typical Lapple cyclone. The turbulence of gas flow is obtained by the use of the Reynolds stress model. The resulting pressure and flow fields are verified by comparing with those measured and then used in the determination of powder flow that is simulated by the use of a stochastic Lagrangian model. The separation efficiency and trajectory of particles from simulation are shown to be comparable to those observed experimentally. The effects of particle size and gas velocity on separation efficiency are quantified and the results agree well with experiments. Some factors which affect the performance of cyclone were identified. It is shown that the collision between gas streams after running about a circle and that just entering occurred around the junction of the inlet duct and the cylinder of the cyclone, resulting in a short-circuiting flow. The combination of flow source and sink was distributed near the axis of cyclone, forming a flow dipole at axial section. Particles entering at different positions gave different separation efficiency. A particle with size exceeding a critical diameter, which was condition-dependant, would stagnate on the wall of cyclone cone. This was regarded as one of the main reasons for the deposition on the inner conical surface in such cyclones used in the cement industry.