H. Shalaby

Numerical Calculation of Particle-Laden Cyclone Separator Flow Using Les

Abstract Numerical flow calculations were carried out at various axial positions of a gas cyclone separator for industrial applications. Due to the nature of cyclone flows, which exhibit highly curved streamlines and anisotropic turbulence, we used the advanced turbulence model of Large Eddy Simulation (LES). The application of LES reveals better agreement with the experimental data, however, it requires higher computer capacity and longer running times when compared to standard turbulence models. These calculations of the continuous phase flow were the basis for modeling the behavior of the solid particles in the cyclone. Particle trajectories, pressure drop and the cyclone separation efficiency have been studied in some details. The paper is organized into five sections. The first section consists of an introduction and a summary of previous work. Section 2 deals with the LES turbulence calculations of the continuous phase flow. The third section treats modeling of the dispersed phase behavior. In section 4, computational issues are presented and discussed as applied grids, boundary conditions and the solution algorithm. In section 5, prediction profiles of the gas flow at axial positions are presented and discussed in some details. Moreover, pressure drop, particle trajectories and cyclone efficiency are discussed. Section 6 summarizes and concludes the paper.

Turbulent Flame Visualization Using Direct Numerical Simulation

Combustion phenomena are of high scientific and technological interest, in particular for energy generation and transportation systems. Direct Numerical Simulations (DNS) have become an essential and well established research tool to investigate the structure of turbulent flames, since they do not rely on any approximate turbulence models. In this work two complementary DNS codes are employed to investigate different types of fuels and flame configurations. The code is π3 is a 3-dimensional DNS code using a low-Mach number approximation. Chemistry is described through a tabulation, using two coordinates to enter a database constructed for example with 29 species and 141 reactions for methane combustion. It is used here to investigate the growth of a turbulent premixed flame in a methane-air mixture (Case 1). The second code,Sider is an explicit three-dimensional DNS code solving the fully compressible reactive Navier-Stokes equations, where the chemical processes are computed using a complete reaction scheme, taking into account accurate diffusion properties. It is used here to compute a hydrogen/air turbulent diffusion flame (Case 2), considering 9 chemical species and 38 chemical reactions.For Case 1, a perfectly spherical laminar flame kernel is initialized at the center of a cubic domain at zero velocity. A field of synthetic homogeneous isotropic turbulence is then superposed and the turbulent flow and the flame can begin to interact. Various species can be used as an indicator for the flame front in a combustion process. Among them, the isosurface of species CO2 at a mass fraction of 0.03 is retained here, since this value corresponds to the steepest temperature gradient in the associated one-dimensional laminar premixed flame. The results obtained have been post processed in order to study the interesting aspects of the coupling between flame kernel evolution and turbulence, such as straining and curvature impact on the flame surface area and local thickness.For Case 2, the instantaneous structure of a non-premixed hydrogen/air flame evolving in a turbulent flow and starting from an initially planar structure is investigated. Here again, the properties of the resulting turbulent flame are of high interest and will be visualized, defining the flame front in a classical manner for non-premixed combustion using a mixture fraction isosurface. Considering the context of this publication, the emphasis is clearly set on the post-processing and visualization of the DNS data, not on the fundamental issues associated with turbulent combustion.

Direct Numerical Simulations of turbulent flames to analyze flame/acoustic interactions

Direct Numerical Simulations (DNS) are becoming increasingly important as a source of quantitative information to understand turbulent reacting flows. For the present project DNS have been mainly used to investigate in a well-defined manner the interaction between turbulent flames and isolated acoustic waves. This is a problem of fundamental interest with practical applications, for example for a better understanding of combustion instabilities. After developing a specific version of the well-known Rayleigh’s criterion, allowing to investigate local amplification or damping of an acoustic pulse interacting with a reaction front, extensive investigations have been carried out. The present publication summarizes the main findings of all these studies and describes in detail the underlying numerical and physical models, in particular those used to describe chemical reactions. Post-processing of DNS data in the light of turbulent combustion modeling is also discussed. The results illustrate the complexity of the coupling between reaction fronts and acoustics, since amplification and damping appear mostly side by side, as alternating layers. The influence of individual reactions and species on the damping process can also be quantified in this manner. This publications concludes with perspectives towards higher turbulence levels and effects of differential diffusion.

Potential of Direct Numerical Simulations to Investigate Flame/Acoustic Interactions

The interaction between turbulent flames and isolated acoustic waves is a problem of fundamental interest with practical applications, for example for a better understanding of combustion instabilities. After developing a specific version of the well-known Rayleighs criterion, allowing to investigate local amplification or damping of an acoustic pulse interacting with a reaction front, extensive investigations have been carried out on this subject. The present publication summarizes the main conclusions of all these studies. Premixed as well as non-premixed flames have been considered, using different fuels. All these investigations rely on high-fidelity models and high-accuracy methods, in order to reproduce quantitatively all important physical processes controlling this configuration. Direct Numerical Simulations (DNS) employing detailed physical models are best suited for this purpose, but lead to considerable requirements in terms of computing time. Due to these very high computational costs, first studies considered only two-dimensional flows. It is then questionable how general the obtained results can be for real turbulence. Therefore, the investigations have been later on extended to three-dimensional flows. A dedicated compressible DNS code has been developed for this purpose. For all DNS computations the developed Rayleighs criterion then allows to quantify amplification or damping and to examine the reason for it. Strong focusing effects, highly local amplification and damping phenomena are observed. The influence of individual reactions and species on the damping process can also be quantified in this manner.

Direct Numerical Simulation of Flame/Acoustic Interactions

Combustion phenomena are of high scientific and technological interest, in particular for energy generation and transportation systems. Direct Numerical Simulations (DNS) have become an essential and well-established research tool to investigate the structure of turbulent flames, since they do not rely on any approximate turbulence models. In this project the DNS code π 3C is employed to investigate different flame configurations. This DNS code is an explicit, three-dimensional code solving the fully compressible, reactive Navier-Stokes equations. Chemistry is described through tabulation, using two coordinates to enter a database constructed for example with 29 species and 141 reactions for methane combustion. The tabulation procedure has been first validated using a laminar household burner configuration computed with the in-house laminar combustion code \(\mbox{\textit{\textsf{UGC}}}^{+}\) . DNS is used here to investigate the growth of a turbulent premixed flame in a methane-air mixture. For this purpose a perfectly spherical laminar flame kernel is initialized at the center of a cubic domain at zero velocity. A field of synthetic, homogeneous isotropic turbulence is then superposed and the turbulent flow and the flame can begin to interact. Various species can be used as an indicator for the flame front in a combustion process. Among them, the isosurface of carbon dioxide (CO2) at a mass fraction of 0.03 is retained here, since this value corresponds to the steepest temperature gradient in the associated, one-dimensional laminar premixed flame. The obtained results have been post-processed in order to study some interesting aspects of the coupling between flame kernel evolution and turbulence, such as straining and curvature, flame surface area and local thickness.