NUMERICAL ANALYSIS OF TURBULENT GAS-SOLID FLOWS IN A ROUGH HORIZONTAL CHANNEL USING THE EULERIAN TWO-FLUID MODEL
Date
2018-01-24
Authors
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ORCID
Type
Thesis
Degree Level
Masters
Abstract
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.
Description
Keywords
gas-solid, roughness, horizontal channel
Citation
Degree
Master of Science (M.Sc.)
Department
Mechanical Engineering
Program
Mechanical Engineering