Fluidization of irregular particles - Part II: A particle-resolved simulation method to model hydrodynamic interactions

Irregular particle shapes are ubiquitous in many real-life systems and in particular in the chemical engineering industry. Most of the corresponding numerical simulations are carried out using spherical particles due to the lack of appropriate numerical methods at the particle level or appropriate closure laws for hydrodynamic and collisional interactions in Euler-Lagrange and Euler-Euler models. Since in Part I, we presented a numerical technique implemented in our granular code Grains3D to treat the collisional behaviour of particles of (almost) arbitrary shape, we are now in a favourable position to suggest a corresponding Particle-Resolved Simulation (PRS) method to which Grains3D is coupled to (1,2,3). It is based on a Distributed Lagrange Multiplier/Fictitious Domain technique combined with Finite Volume/Staggered Grid discretization (2,3), that supplies solutions of satisfactory accuracy. This study aims to go one step further and to extend our numerical method to non-convex particles. This extension is implemented in our parallel numerical platform PeliGRIFF (4) . We illustrate the novel simulation capabilities of PeliGRIFF on the problem of the fluidization of trilobic/quadrilobic particles encountered in Oil & Gas catalytic reactors. First, we assess the space convergence and overall accuracy of the computed solution in the case of the flow past an infinite array of trilobes/quadrilobes. Then, we show results of the flow through a fixed bed made of trilobes/quadrilobes at random loose packing. Finally, we present preliminary results relevant to an actual fluidization. In conclusion, we discuss the computing challenges of these simulations and the integration of their results in a comprehensive multi-scale approach. REFERENCES A. Wachs. A DEM-DLM/FD method for direct numerical simulation of particulate flows: Sedimentation of polygonal isometric particles in a Newtonian fluid with collisions. Computers & Fluids, 38, 1608-1628, 2009. A. Wachs, A. Hammouti, G. Vinay, M. Rahmani. Accuracy of Finite Volume/Staggered Grid Distributed Lagrange Multiplier/Fictitious Domain simulations of particulate flows. Computers & Fluids, 115, 154-172, 2015. F. Dorai, C. Moura Teixeira, M. Rolland, E. Climent, M. Marcoux, A. Wachs. Fully resolved simulations of the flow through a packed bed of cylinders: Effect of size distribution. Chemical Engineering Science, 129, 180-192, 2015. PeliGRIFF, A multi-scale numerical modeling tool for fluid/particles flows. http://www.peligriff.co

Direct numerical simulation of reactive flow through a fixed bed of catalyst particles

Many catalytic refining and petrochemical processes involve two-phase reactive systems in which the continuous phase is a fluid and the porous phase consists of rigid particles randomly stacked. Improving both the design and the operating conditions of these processes represents a major scientific and industrial challenge in a context of sustainable development. Thus, it is above all important to better understand all the intricate couplings at stake in these flows: hydrodynamic, chemical and thermal contributions. The objective of our work is to build up a multi-scale modelling approach of reactive particulate flows and at first to focus on the development of a microscopic-scale including heat and mass transfers and chemical reactions for the prediction of reactive flows through a dense or dilute fixed bed of catalyst particles. Please download the full abstract below

Hybrid Model For Word Prediction Using Naive Bayes and Latent Information

Historically, the Natural Language Processing area has been given too much attention by many researchers. One of the main motivation beyond this interest is related to the word prediction problem, which states that given a set words in a sentence, one can recommend the next word. In literature, this problem is solved by methods based on syntactic or semantic analysis. Solely, each of these analysis cannot achieve practical results for end-user applications. For instance, the Latent Semantic Analysis can handle semantic features of text, but cannot suggest words considering syntactical rules. On the other hand, there are models that treat both methods together and achieve state-of-the-art results, e.g. Deep Learning. These models can demand high computational effort, which can make the model infeasible for certain types of applications. With the advance of the technology and mathematical models, it is possible to develop faster systems with more accuracy. This work proposes a hybrid word suggestion model, based on Naive Bayes and Latent Semantic Analysis, considering neighbouring words around unfilled gaps. Results show that this model could achieve 44.2% of accuracy in the MSR Sentence Completion Challenge

A study of thermal stratification in the cold legs during the subcooled blowdown phase of a loss of coolant accident in the OSU APEX Thermal Hydraulic Testing Facility

The phenomena of interest in this work is the thermal stratification which occurs during the early stages of a loss of coolant accident (LOCA) in the OSU APEX Thermal Hydraulic Test Facility, which is a scaled model of the Westinghouse AP600 nuclear power plant. Thermal stratification has been linked to the occurrence of pressurized thermal shock (PTS). Analysis of the OSU APEX facility data has allowed the determination of an onset criteria and support for the postulated mechanisms leading to thermal stratification. CFX 4.1, a computational fluid dynamics code, was used to generate a model of the cold legs and the downcomer and the phenomena occurring within them. The following are the accomplishments of the work contained within this report; Determined the causes of thermal stratification in the cold legs of the Westinghouse Advanced Passive 600 MW (AP600) nuclear power plant. Predicted the onset of thermal stratification in the cold legs of the Westinghouse Advanced Passive 600 MW (AP600) nuclear power plant. Modeled the phenomena associated with thermal stratification in the cold legs of the Westinghouse Advanced Passive 600 MW (AP600) nuclear power plant

Fluidization of irregular particles - Part I: A discrete element method to model collisions between non-convex particles

The flow dynamics of a fluidized bed can be very complicated. As the solid volume fraction is generally high, particle-particle collisions cannot be ignored. Many studies in the literature deal with perfectly spherical particles while very few deal with non-spherical ones and even less with angular or non-convex particles. However, these irregularly shaped particles are not uncommon in chemical engineering. Among others, Escudié et al (1) showed that the particle shape influences markedly the dynamics of such a system. We suggest an accurate and efficient way to model collisions between particles of (almost) arbitrary shape, that can be integrated into a comprehensive modeling of a fluidized bed. For that purpose, we develop a Discrete Element Method (DEM) combined with a soft particle contact model that treats the contact between bodies of various shape and size (2). In particular, for non-convex bodies, our strategy is based on decomposing a non-convex body into a set of convex ones (3). Therefore, our novel method can be called “glued convex method”, as an extension of the popular “glued-spheres” method (4). It hence uses all the features involved in DEM simulations of convex bodies, such as the contact detection strategy based on a Gilbert-Johnson-Keerthi algorithm (5) and the linked-cell spatial sorting which accelerates the contact resolution (6). The problem of multiple contact requires a particular attention (4,7). The method is implemented in our granular dynamics code Grains3D. As an illustration of the powerful modelling capabilities of Grains3D, we show results of simulation of settling non-convex catalytic pellets in a cylindrical chemical reactor. REFERENCES R. Escudié, N. Epstein, J.R. Grace, H.T. Bi, Effect of particle shape on liquid-fluidized beds of binary (and ternary) solids mixtures: segregation vs. mixing. Chemical Engineering Science, 61(5): 1528, 2006. A. Wachs, L. Girolami, G. Vinay, and G. Ferrer. Grains3D, a flexible DEM approach for particles of arbitrary convex shape - Part I: Numerical model and validations. Powder Technology, 224:374-389, 2012. A. D. Rakotonirina, A. Wachs, J.-Y. Delenne, F. Radjai. Grains3D, a flexible DEM approach for particles of arbitrary convex shape - Part III: extension to non convex particles, submitted to Powder Technology, 2015. D. Höhner, S. Wirtz, H. Kruggel-Emden, and V. Scherer. Comparison of the multi-sphere and polyhedral approach to simulate non-spherical particles within the discrete element method: Influence on temporal force evolution for multiple contacts. Powder Technology, 208(3):643-656, 2011. Elmer G. Gilbert, Daniel W. Johnson, and S. Sathiya Keerthi. A fast procedure for computing the distance between complex objects in three-dimensional space. Robotics and Automation, IEEE Journal of Robotics and Automation, 4(2):193-203, 1988. Gary S. Grest, Burkhard Dünweg, and Kurt Kremer. Vectorized link cell Fortran code for molecular dynamics simulations for a large number of particles. Computer Physics Communications, 55(3):269-285, 1989. H. Kruggel-Emden, S. Rickelt, S. Wirtz, and V. Scherer. A study on the validity of the multi-sphere discrete element method. Powder Technology, 188(2):153-165, 2008

Formulation and Testing of Paraffin-Based Solid Fuels Containing Energetic Additives for Hybrid Rockets

Many approaches have been considered in an effort to improve the regression rate of solid fuels for hybrid rocket applications. One promising method is to use a fuel with a fast burning rate such as paraffin wax; however, additional performance increases to the fuel regression rate are necessary to make the fuel a viable candidate to replace current launch propulsion systems. The addition of energetic and/or nano-sized particles is one way to increase mass-burning rates of the solid fuels and increase the overall performance of the hybrid rocket motor.1,2 Several paraffin-based fuel grains with various energetic additives (e.g., lithium aluminum hydride (LiAlH4) have been cast in an attempt to improve regression rates. There are two major advantages to introducing LiAlH4 additive into the solid fuel matrix: 1) the increased characteristic velocity, 2) decreased dependency of Isp on oxidizer-to-fuel ratio. The testing and characterization of these solid-fuel grains have shown that continued work is necessary to eliminate unburned/unreacted fuel in downstream sections of the test apparatus.3 Changes to the fuel matrix include higher melting point wax and smaller energetic additive particles. The reduction in particle size through various methods can result in more homogeneous grain structure. The higher melting point wax can serve to reduce the melt-layer thickness, allowing the LiAlH4 particles to react closer to the burning surface, thus increasing the heat feedback rate and fuel regression rate. In addition to the formulation of LiAlH4 and paraffin wax solid-fuel grains, liquid additives of triethylaluminum and diisobutylaluminum hydride will be included in this study. Another promising fuel formulation consideration is to incorporate a small percentage of RDX as an additive to paraffin. A novel casting technique will be used by dissolving RDX in a solvent to crystallize the energetic additive. After dissolving the RDX in a solvent chosen for its compatibility with both paraffin and RDX, the mixture will be combined with the melted paraffin. With the melting point of the paraffin far below the decomposition temperature of the RDX, the solvent will be boiled off, leaving the crystallized RDX embedded in the paraffin. At low percentages of RDX additive and with crystallized RDX surrounded by paraffin, the fuel grains will remain inert, maintaining a key benefit of hybrids in the safety of the solid fuel

Mechanical and Combustion Performance of Multi-Walled Carbon Nanotubes as an Additive to Paraffin-Based Solid Fuels for Hybrid Rockets

Paraffin-based solid fuels for hybrid rocket motor applications are recognized as a fastburning alternative to other fuel binders such as HTPB, but efforts to further improve the burning rate and mechanical properties of paraffin are still necessary. One approach that is considered in this study is to use multi-walled carbon nanotubes (MWNT) as an additive to paraffin wax. Carbon nanotubes provide increased electrical and thermal conductivity to the solid-fuel grains to which they are added, which can improve the mass burning rate. Furthermore, the addition of ultra-fine aluminum particles to the paraffin/MWNT fuel grains can enhance regression rate of the solid fuel and the density impulse of the hybrid rocket. The multi-walled carbon nanotubes also present the possibility of greatly improving the mechanical properties (e.g., tensile strength) of the paraffin-based solid-fuel grains. For casting these solid-fuel grains, various percentages of MWNT and aluminum particles will be added to the paraffin wax. Previous work has been published about the dispersion and mixing of carbon nanotubes.1 Another manufacturing method has been used for mixing the MWNT with a phenolic resin for ablative applications, and the manufacturing and mixing processes are well-documented in the literature.2 The cost of MWNT is a small fraction of single-walled nanotubes. This is a scale-up advantage as future applications and projects will require low cost additives to maintain cost effectiveness. Testing of the solid-fuel grains will be conducted in several steps. Dog bone samples will be cast and prepared for tensile testing. The fuel samples will also be analyzed using thermogravimetric analysis and a high-resolution scanning electron microscope (SEM). The SEM will allow for examination of the solid fuel grain for uniformity and consistency. The paraffin-based fuel grains will also be tested using two hybrid rocket test motors located at the Pennsylvania State University s High Pressure Combustion Lab