NUMERICAL ANALYSIS OF TURBULENT GAS-SOLID FLOWS IN A ROUGH HORIZONTAL CHANNEL USING THE EULERIAN TWO-FLUID MODEL

Turbulent gas-solid flows are encountered in many industrial processes including pneumatic transport of granular materials such as pulverized coal, circulating fluidized beds and dust and particle-exhaust pollution control systems. Modelling the gas-solid flow is a major challenge since the flow is turbulent which renders the system non-linear. In addition, the presence of particles further complicates the flow. The two-fluid formulation is a popular approach for modelling gas-particle flows that describes the motion of both phases in an Eulerian framework. The current dissertation explores the effects of wall roughness on the particle-phase properties of a turbulent gas-solid flow in a horizontal channel. An in-house numerical code is modified to simulate a fully developed turbulent gas-solid flow; the numerical code is based on the two-fluid formulation adopted from the model of Rao et al. (2011). The gas-solid flow in the horizontal channel is asymmetric due to the gravity acting transverse to the flow. Three different studies were conducted to document the response of the particle-phase properties to different flow conditions. The first study focuses on the effect of hydrodynamic roughness on the gas-solid flow. The hydrodynamic effect of wall roughness was implemented in the model using a two-layer version of the k - ε model based on Durbin et al. (2001). The thesis documents outcomes of the simulations that compare the flow for the rough wall with that for the smooth wall. It was found that the hydrodynamic roughness energized the particles present in the flow via turbulence modulation. Wall roughness alters the particle-wall interactions. The particle-wall interactions were characterized using the boundary conditions of Johnson and Jackson (1987), which defined the specularity coefficient. The second study focuses specifically on the role of the specularity coefficient in characterizing wall roughness. The channel wall is rough from a particle perspective. The outcomes of the simulations were compared to the experimental study of Sommerfeld and Kussin (2004). The experiment explores the effect of different levels of wall roughness on the particle-phase properties. The dissertation documents the comparisons between the simulations and the experimental data for the mean solids velocity and the solids volume fraction profiles. The profiles for properties like turbulence kinetic energy, granular temperature, solids viscosity and solids shear stress for different levels of roughness were also documented and analyzed. It was found that specularity coefficient plays a significant role in characterizing the wall roughness. The predicted profiles for the mean solids velocity and the solids volume fraction deviated from the experimental profile in the near-wall region. The degree of deviation from the experimental data decreased with an increase in the specularity coefficient. This implies that the specularity coefficient is less effective for walls with smaller roughness. The third study focuses on the sensitivity of the particle-phase properties to three different parameters; the specularity coefficient, the mass loading and the Stokes number. Increasing the specularity coefficient increases the number of diffuse particle-wall collisions. It was found that increasing specularity coefficient increased the granular temperature, which resulted in higher predictions for the solids viscosity and the solids shear stress. The increase in the mass loading increased the number of particles present in the flow. It was found that the increase in mass loading increased the granular temperature by increasing the frequency of particle-wall collisions. The effect of particle inertia was investigated by increasing the Stokes number. The solids velocity monotonically decreases with an increase in the Stokes number while the behaviour of the granular temperature and solids shear stress were more complicated

Hydrodynamic electron flow in high-mobility wires

Hydrodynamic electron flow is experimentally observed in the differential resistance of electrostatically defined wires in the two-dimensional electron gas in (Al,Ga)As heterostructures. In these experiments current heating is used to induce a controlled increase in the number of electron-electron collisions in the wire. The interplay between the partly diffusive wire-boundary scattering and the electron-electron scattering leads first to an increase and then to a decrease of the resistance of the wire with increasing current. These effects are the electronic analog of Knudsen and Poiseuille flow in gas transport, respectively. The electron flow is studied theoretically through a Boltzmann transport equation, which includes impurity, electron-electron, and boundary scattering. A solution is obtained for arbitrary scattering parameters. By calculation of flow profiles inside the wire it is demonstrated how normal flow evolves into Poiseuille flow. The boundary-scattering parameters for the gate-defined wires can be deduced from the magnitude of the Knudsen effect. Good agreement between experiment and theory is obtained.Comment: 25 pages, RevTeX, 9 figure

Numerical analysis of effects of specularity coefficient and restitution coefficient on the hydrodynamics of particles in a rotating drum

Various simulations have been conducted to understand the macroscopic behavior of particles in the solid-gas flow in rotating drums in the past. In these studies, the no-slip wall boundary condition and fixed restitution coefficient between particles were usually adopted. The paper presents a numerical study of the gas-solid flow in a rotating drum to understand the effect of the specularity coefficient and restitution coefficient on the hydrodynamic behavior of particles in the segregation process. The volume fraction, granular pressure, granular temperature and their relationships are examined in detail. The boundary conditions of the no-slip and specularity coefficient of 1 are compared. In the simulations, two different sizes of particles with the same density are considered and the Eulerian–Eulerian multiphase model and the kinetic theory of granular flow (KTGF) are used. The results reveal that the hydrodynamical behavior of the particles in the rotating drum is affected by the boundary condition and restitution coefficient. In particular, the increase of specularity coefficient can increase the active region depth, angle repose, granular pressure for both small and large particles and granular temperature for large particles. With increasing restitution coefficient, the angle of repose decreases and granular pressure and temperature increase at the same volume fraction for both small and large particles

3D numerical simulation of Circulating Fluidized Bed: comparison between theoretical results and experimental measurements of hydrodynamic

This work was realized in the frame of the European GAYA project supported by ADEME. This paper presents a description of the hydrodynamic into a CFB according to experimental measurements of gas pressure and solid mass flux. These experimental data are compared to three dimensional numerical simulation with an Eulerian approach. The obtained numerical results show that the applied mathematical models are able to predict the complex gas-solid behavior in the CFB and highlight the large influence of the particle wall boundary condition. Indeed, it is shown that free slip wall boundary condition gives a good prediction a solid mass flux profile in comparison with experimental measurements nevertheless a convex shape. Moreover, the numerical solid hold-up is underestimated compared to the experimental data. On the contrary, a no-slip boundary condition improves the profile shape of solid mass flux but highly overestimates its intensity and the solid hold-up. A compromise appears to be a friction particle-wall boundary condition such as Johnson and Jackson (1) but the model parameters have to be chosen very carefully especially the restitution coefficient

3D Eulerian modeling of thin rectangular gas-solid fluidized beds: Estimation of the specularity coefficient and its effects on bubbling dynamics and circulation times

This study aims at investigating the influence of the wall boundary conditions and specifically the specularity coefficient on the fluidization behavior of a thin rectangular fluidized bed by means of 3D numerical simulation employing an Eulerian description of the gas and the solid phases. Thin rectangular fluidized beds have been extensively used in the research literature since it is assumed that the flow behaves like a simpler two-dimensional flow and hence they offer validation data for 2D simulations. However, the effects of the front and the back walls are significant, influencing the sensitivity of the fluidization hydrodynamics to the third dimension whose consideration is thus necessary. In order to investigate the influence of the specularity coefficient, ϕ (a parameter controlling the momentum transfer from the particles to the wall), on the fluidization hydrodynamics, a parametric analysis is conducted and the response of the bubble dynamics, reflecting the gasmotion, and the circulation fluxes, displaying the solids motion, are examined in detail. The computational results are compared with available experimental data in order to determine the values of ϕ that lead to the accurate description of the fluidization hydrodynamics via a two-fold validation strategy which involves the calculation of the circulation time and the solids concentration maps. It is observed that the appropriate value of the specularity coefficient depends rather strongly on the superficial gas velocity of the bed.BP (Firm

Gas turbulence modulation in a two-fluid model for gas-solid flows

Recent rapid progress in the theoretical and experimental study of turbulence modulation has led to greater understanding of the physics of particle-gas turbulence interactions. A new two-fluid model incorporating these advances for relatively dilute gas-solid flows containing high-inertia particles is established. The effect of aerodynamic forces upon the particulate stresses is considered in this kinetic theory-based model, and the influence of the particles on the turbulent gas is addressed: the work associated with drag forces contributes to the gas turbulent energy, and the space occupied by particles restricts the turbulent length scale. The interparticle length scale, which is usually ignored, has been incorporated into a new model for determining the turbulent length scale. This model also considers the transport effect on the turbulent length scale. Simulation results for fully developed steady flows in vertical pipes are compared with a wide range of published experimental data and, generally, good agreement is shown. This comprehensive and validated model accounts for many of the interphase interactions that have been shown to be important

Numerical Simulation of Catalytic Ozone Decomposition Reaction in a Gas-solids Circulating Fluidized Bed Riser

Computational fluid dynamics (CFD) modeling of catalytic ozone decomposition reaction in a circulating fluidized bed (CFB) riser using iron impregnated FCC particles as catalyst is carried out. The catalytic reaction is defined as a one-step reaction with an empirical coefficient. Eularian-Eularian method with kinetic theory of granular flow is used to solve the gas-solids two-phase flow in the CFB riser. The simulation results are compared with experimental data, with the reaction rate modified using an empirical coefficient to provide better simulation results than the original reaction rate. Moreover, the particle size has great effects on the reaction rate. Studies on solid particle distribution show that the influence of wall boundary condition, determined by specularity coefficient and particle-wall restitution coefficient, plays a major role in the solids lateral velocity that affects the solids distribution in the riser. The generality of the CFD model is further validated under different operating conditions of the riser

3D numerical simulation of a lab-scale pressurized dense fluidized bed focussing on the effect of the particle-particle restitution coefficient and particle–wall boundary conditions

3D numerical simulations of dense pressurized fluidized bed are presented. The numerical prediction of the mean vertical solid velocity are compared with experimental data obtained from Positron Emission Particle Tracking. The results show that in the core of the reactor the numerical simulations are in accordance with the experimental data. The time-averaged particle velocity field exhibits a large-scale toroidal (donut shape) circulation loop. Two families of boundary conditions for the solid phase are used: rough wall boundary conditions (Johnson and Jackson, 1987 and No-slip) and smooth wall boundary conditions (Sakiz and Simonin, 1999 and Free-slip). Rough wall boundary conditions may lead to larger values of bed height with flat smooth wall boundary conditions and are in better agreement with the experimental data in the near-wall region. No-slip or Johnson and Jackson׳s wall boundary conditions, with sufficiently large value of the specularity coefficient (ϕ≥0.1)(ϕ≥0.1), lead to two counter rotating macroscopic toroidal loops whereas with smooth wall boundary conditions only one large macroscopic loop is observed. The effect of the particle-particle restitution coefficient on the dynamic behaviour of fluidized bed is analysed. Decreasing the restitution coefficient tends to increase the formation of bubbles and, consequently, to reduce the bed expansion