Granular Heat Transfer Research Articles

Numerical simulation of granular heat transfer within lifter rotating drum using smoothed particle hydrodynamics (SPH)

Abstract The rotating drums are typical industrial thermal processing equipment, which is widely used to heat or cool material particles in the circle fluid bed (CFB) systems. The traditional cylindrical rotating drum cannot significantly promote the heat transfer process of granules in operation, largely limiting further application. In this research, using smoothed particle hydrodynamics (SPH), two different types of lifters are employed to investigate the effects of shape, size, and number of lifters on the coefficient of heat transfer within the rotating drums by combining the flow and mixing characteristics of particles, the stress distribution, the transient velocity and temperature distribution, and the trend of the Peclet number.

Bridging micro nature with macro behaviors for granular thermal mechanics

The connection between micro-level characteristics and macroscopic properties in granular heat transfer and mechanics is fundamental and crucial. This study proposes a novel discrete element approach incorporating granular heat transfer, contact bonding, and granular stress tensor models to investigate the mechanical and thermal responses of continuum media composed of constituent spheres. Eight benchmark tests were devised to bridge the long-standing gap between micro and macro properties in granular materials. Through these tests, the numerical solutions obtained from discrete element modeling match well with existing analytical or finite element solutions derived from continuum-based theory. This validation underscores the rationality and reliability of the granular heat transfer model, contact bonding model, and granular stress tensor model. Moreover, the study highlights the consistency between continuum-based theory and discontinuum-based theory. A minor distinction between continuum-based models and discrete element models emerges near the boundaries due to variations in the specification of boundary conditions. This discrepancy can be clarified by Saint-Venant's Principle, thus validating the accuracy of the microscale heat transfer and mechanics theory for granular materials. Five mono-disperse packing structures, including simple cubic (SC), body-centered cubic (BCC), face-centered cubic (FCC), hexagonal close packing (HCP), and random packing (Random), were further analyzed to examine their influence on heat transfer performance. Numerical results reveal that higher coordination numbers and solid volume fractions correspond to higher apparent thermal conductivity of granular assemblies, thus elucidating the connection between micro packing configurations and macroscopic heat transfer properties. The apparent thermal conductivity for different crystal configurations follows the sequence: HCP ≒ FCC > BCC ≒ Random > SC. To improve the accuracy and physical relevance of the proposed model, the effect of particle contact area needs to be further incorporated into the granular heat transfer model.

Assessing the Effects of Material Properties on Load Behavior in Dry Ball Mills Using DEM

Understanding how the mill load behaves is crucial for enhancing ball mill effectiveness. This study aimed to create a discrete element method (DEM) model to simulate the motion charge in ball mills and to analyze how the simulation material properties affected the load behavior. The steel balls were modelled as a collection of distinct particles, each of which was subject to Newton’s laws of motion and tracked in a Lagrangian manner. Hertzian contact law was used to describe inter-particle collisions. Then, this numerical model was coded using the open-source C++ program LAMMPS Improved for General Granular and Granular Heat Transfer Simulation (LIGGGHTS) to mimic laboratory and pilot-scale ball mills. The load positions measured from the DEM simulations were compared to the published experimental data and empirical models of comparable laboratory and pilot-scale experiments to validate the findings. The angular shoulder position ranged between 137° to 154° for the range of Young’s modulus of 0.5 to 1000 MN/mm2. Angular shoulder and toe positions had a variation of less than 10% from the laboratory and pilot-scale experimental data. The outcomes demonstrated a significant relationship between load position and material characteristics such as Young's modulus in DEM simulation. This preliminary model can be used for choosing the appropriate material parameters for ball mills both with DEM and coupled CFD–DEM multiphase simulations. This assessment concluded that material properties affect the load behavior in computer simulations of ball mills.

Influence of vibration characteristics on temperature uniformity of particles during heating processes in a vibrationally fluidized bed

Granular heat transfer is encountered in many industries, including food production, solar energy harvesting and nuclear engineering. The present study investigates influence of vibration characteristics on granular heat transfer in a vibrationally fluidized bed of particles using a thermal discrete element method. The bed is heated by transferring a constant heat flux from its bottom wall. Vibrations generally had two influences: first, particles reached a higher average temperature, and second, they attained more uniform temperature distribution. The particles average temperature generally increased by increasing vibration amplitude and frequency. However, the best temperature uniformity occurred in an optimal set of amplitudes and frequencies. Contrary to claims in previous studies, the effective thermal conductivity (ETC) of the vibrated bed was not the only determining parameter for quality of heat transfer. Other parameters such as flow patterns, convective and diffusive motions must be also analyzed to evaluate the heating process in vibrating beds.

Numerical and experimental investigation of the effect of interstitial gases on conductive heat transfer for dense particulate systems

Many industrial processes involve granular heat transfer. Within these processes are a sub-set of applications where the particles are densely packed and the surrounding gas is predominantly stagnant. For materials with low thermal conductivity much of the heat transfer will occur through the interstitial gases so capturing this effect is critical to represent these systems numerically. Resolving the gas phase explicitly using a fluid solver and coupling to a particle method is possible but computationally expensive. Higher fidelity models often make simulation of applications at the industrial scale intractable. With this in mind we seek a numerical method that captures the important heat transfer phenomena in dense granular systems with minimal additional computational effort. In this work we first characterize one common particle based approach proposed by Rong and Horio that calculates the contribution through a stagnant gas film surrounding each particle. We compare this approach to fluid simulations of a particle-wall and a particle-particle collision to evaluate the validity of the underlying assumptions of the Rong and Horio model. We then use a multi-scale approach to examine the relationship between the local packing structure and the contribution of the interstitial gas. From this we propose a new dimensionless parameter that permits the inclusion of the gas effect in a particle based simulation method with minimal additional computational effort and without explicitly having to include the gas phase. The model is then validated against a static experiment and compared with the Rong and Horio model.

Granular flow around the horizontal tubes of a particle heat exchanger: DEM-simulation and experimental validation

Moving bed heat exchangers are a central element in solar thermal power plants using granular heat transfer fluids. Their performance is determined by a complex granular flow field in the device. This calls for accurate simulation models forming the basis for novel design tools.In this study we propose a discrete element model (DEM) to calculate the dense granular flow field around the horizontal tubes of a moving bed heat exchanger.The simulation results are compared to measurement data using particle image velocimetry (PIV). In the upper part of the tube the model agrees well with the experiment, capturing the relevant flow phenomena like the stagnant zone on top of the tube.Below the tube, a void area is observed both in the experiment and the simulation. However, the PIV analysis of the experimental data doesn’t resolve the void area and the associated flow phenomena due to the highly dynamic flow state, the inherent nature of PIV technique and additional disturbing effects in the setup. Therefore, the model can’t be validated in the corresponding regions.Still, the deviations between simulation and experiment are confined to a very narrow region directly at the tube surface in the lower part of the tube. In the rest of the flow field, the model proves to be a reliable tool for further investigations and will be used in future studies.

Modeling of granular heat transfer in cryogenic grinding system: Black pepper seeds

In the case of grinding of spices, the cryogenic counterpart is commonly used for obtaining a high-quality spice powder retaining the maximum thermo-sensitive flavoring compounds; however, the operational cost restricts its industrial application. Therefore, in a cryogenic grinding system, an insight into the micro-mechanical behavior and heat transfer phenomena of the granules is obligatory in improving the design aspects. The present study aims to represent the three-dimensional thermal transport process for black pepper seeds in a cryogenic grinding system (precooler and mill). The explicit numerical method based on microstructural approaches such as Discrete Element Method (DEM): utilizing a Hertz–Mindlin with conduction contact model was used to corroborate the granular flow and thermal profile of the seeds. The adequacy of the proposed granular heat transfer model was validated by the experimental data such as granular temperature as well as the flow pattern. Concurrently, it confirms the applicability of DEM as a capable tool to model the heat transfer along with a better understanding of particle mechanics in the cryogenic grinding system.

Pore-scale simulation of flow and turbulence characteristics in three-dimensional randomly packed beds

Fluid flow and turbulence characteristics were numerically simulated at the pore scale in actual packed beds under different operating conditions. Real geometrical structures of randomly packed beds were modeled using the discrete element software LIGGGHTS (LAMMPS Improved for General Granular and Granular Heat Transfer Simulations). The numerical model was validated against experimental data. Good agreements were achieved between the predicted results and measurements. The influences of different parameters, such as the gas velocity and particle, size were studied and discussed in detail. The results of a macroscopic analysis of the mean parameters show that inside the packed beds, both the mean porosity and flow parameters had fluctuate variations. As the pellet radius increased, the fluctuations in the mean velocity and pressure increased. The results of a local detail analysis demonstrate that complicated turbulent eddy structures exist in the pores of the packed beds. Eddies with large sizes were primarily distributed at the entrance of the main channel and near the spherical wall, and eddies with high intensities were mainly distributed near the above spherical wall and on the right side of the main channel.

Calibration of Discrete Element Heat Transfer Parameters by Central Composite Design

The efficiency and precision of parameter calibration in discrete element method (DEM) are not satisfactory, and parameter calibration for granular heat transfer is rarely involved. Accordingly, parameter calibration for granular heat transfer with the DEM is studied. The heat transfer in granular assemblies is simulated with DEM, and the effective thermal conductivity (ETC) of these granular assemblies is measured with the transient method in simulations. The measurement testbed is designed to test the ETC of the granular assemblies under normal pressure and a vacuum based on the steady method. Central composite design (CCD) is used to simulate the impact of the DEM parameters on the ETC of granular assemblies, and the heat transfer parameters are calibrated and compared with experimental data. The results show that, within the scope of the considered parameters, the ETC of the granular assemblies increases with an increasing particle thermal conductivity and decreases with an increasing particle shear modulus and particle diameter. The particle thermal conductivity has the greatest impact on the ETC of granular assemblies followed by the particle shear modulus and then the particle diameter. The calibration results show good agreement with the experimental results. The error is less than 4%, which is within a reasonable range for the scope of the CCD parameters. The proposed research provides high efficiency and high accuracy parameter calibration for granular heat transfer in DEM.

Simulations with Granular Material Motion for Extraterrestrial Applications

A simulation study utilizing Hooke style particle models was conducted to better understand the characteristic modes, translational/rotational energy distribution, the complete energy cascade and applications of granular interactions occurring within a horizontal drum tumbler. This study was conducted in conjunction with a physical study performed by Nathaniel Helminiak, entitled: “Experiments with Granular Material Motion for Extraterrestrial Applications.” These simulations were created through the discrete element modeling software: LIGGGHTS (LAMMPS Improved for General Granular and Granular Heat Transfer Simulations) and the program it is based upon, LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator). The resulting flows were visualized within VisIt visualization software and analyzed using the high-level computer language, GNU Octave. Simulations predicted areas of interest to study and extend tests beyond the physical test bed’s material limitations. The study suggests the existing idea of force chains as applied to rotational forces, namely “bro-chains,” and also that the industry mass finishing process may benefit from investigation into a complete energy characterization of granular flow.

Numerical study of mixing and thermal conduction of granular particles in rotating tumblers

A discrete element method (DEM) study is conducted to investigate the mixing and heat‐transfer characteristics of steel spherical particles under various rotation speeds and flow regimes of a rotating tumbler. The mixing degree, weighted temperature, temperature discrepancy at the mixing interface, temperature radial distribution, and information entropy are used to analyze the effect of mixing structure and evolution duration on the heat‐transfer characteristics. The results under the same revolution and the same evolution time are compared to show the effects of evolution time and mixing structure on thermal conduction. After a detailed analysis, the joint contribution of mixing degree and duration to granular heat transfer is explained, and the different approaches in static thermal conduction and dynamic mixing are shown. Moreover, a new method is proposed using the mean increase rate of temperature information entropy to determine the most effective operating condition for thermal conduction in granular particles. © 2013 American Institute of Chemical Engineers AIChE J, 59: 1906–1918, 2013