NUMERICAL PREDICTION OF DILUTE PARTICULATE FLOWS IN HORIZONTAL AND VERTICAL DUCTS

Two-phase turbulent flow calculations are performed using a commercial Computational Fluid Dynamics (CFD) software CFX-4.4. With the aid of available experimental data, the work is aimed at exploring a range of issues concerning the application of CFD to model gas-solid flows having different cross-section geometry and flow orientation. In order of increasing flow complexity, the present study first looks at the importance of drag coefficient and inlet conditions to particle track calculations in a vertical duct. The same analysis is then performed for two-phase horizontal flows, where gravitational settling starts to exert a stronger influence over the distribution of particles within the flow domain. All computations carried out take into account the effect of Saffman force and particle-wall collisions. Although both flows considered could be reasonably regarded as two dimensional, knowledge gained from such a study is expected to be also applicable to three-dimensional gassolid flow problems. NOMENCLATURE e restitution coefficient d particle diameter F force vectors acting on particle surface H channel height k turbulence kinetic energy Lp particle mass loading m particle mass u velocity vector U horizontal velocity V vertical velocity W channel width α particle impact angle ε turbulence dissipation rate μ gas dynamic viscosity ω specific turbulence dissipation rate INTRODUCTION Dilute gas-solid flows prevail in a wide range of engineering applications, one of which is pneumatic conveying of pulverised fuel (pf) in coal-fired power stations. Before coals are burnt at the boilers, they are first pulverised into particles of various sizes by the mill and then delivered to downstream furnaces by flue gases under different, sometimes unknown, flow conditions. In such an industrial flow, a thorough on-line measurement is usually very difficult and expensive to perform. In laboratories, numerous experimental studies have been carried out by various researchers in the past on pneumatic conveying flow phenomena (Fan et al., 1997), however the majority of them tend to focus on cases with mass loading ratios Lp above 1.0 and particle sizes greater than 100 μm. Such conditions are distinctively different from the operating environment that normally prevails in a power plant mill-duct system, ie. Lp = 0.1 and particle sizes between 45 μm and 80 μm (Manickam et al., 2001). In view of this apparent lack of published studies on dilute particulate flows involving fine suspended particles, design of the mill-duct network for coal-fired power plants has so far been largely based on empiricism. With the advent of high-speed digital computers, engineers nowadays could resort to Computational Fluid Dynamics (CFD) for a better understanding of the pf delivery process before any future plant optimisation is carried out. Unfortunately, for many complex flows, CFD analysis can only offer a qualitative, rather than a quantitative, prediction of the flow behaviour. This is because the majority of the numerical models used in CFD were created using flow data that are sometimes only remotely representative of the real industrial flow. Model validation is therefore essential to ensure the numerical models are capable of solving industrial flow problems that have not been extensively studied by the scientific community. The present investigation focuses on an evaluation of dilute gas-solid flows in straight ducts, which are extensively used in mill-duct networks. The two flow conditions considered are vertical upward flow through a circular-sectioned pipe (Maeda et al., 1980) and horizontal channel flow with a width-to-height ratio (W/H) of 10 (Kussin and Sommerfeld, 2002). Unlike in pure-gas flows where the influence of gravity on gas dynamics is negligible, motion of the dispersed solids is strongly affected by gravitational settling. Consequently, one should expect the solid distributions in the two flows to vary drastically. In the process of obtaining the final numerical prediction, low Reynolds number two-equation models are applied for solving gas phase turbulence. For the solid phase, particle tracks are calculated through a Lagrangian approach taking into account only one-way coupling effect. Particle velocity profiles are compared against the measured data. Where possible, particle number distributions are also presented. Copyright © 2003 CSIRO Australia 135 FLOW CONDITIONS CONSIDERED Vertical upward pipe flow Maeda et al. (1980) experimentally investigated gas-solid flow in a vertical pipe. In their experiment, an upward flow system had been set up to facilitate Laser-Doppler Anemometry (LDA) measurements of solid velocity as well as hot wire anemometer measurement of gas-phase velocity and turbulence quantities. A 4 m straight circular section pipe of unknown material (presumably plexiglass) with internal diameter of 56 mm was fitted downstream of an upward bend. Spherical glass powder with a mean diameter of 136 μm was introduced into the gas flow at a solid mass loading ratio Lp of 30%. At pipe Reynolds number of 2.0×10, the experimenters confirmed that the mean axial velocities for the single-phase flow have the same profile as that in a fully-developed turbulent flow. The corresponding mean air velocity was 5.7 m/s. Horizontal channel flow Calculations are also performed for a gas-solid flow through a horizontal channel. The flow configuration studied corresponds to a flow experiment conducted by Kussin and Sommerfeld (2002) who tested a 6 m horizontal channel with a width (W) of 350 mm and a height (H) of 35 mm. Owing to its large width-to-height ratio (W/H), the experimenters were satisfied that measurements made at the channel’s centre plane were reasonably free from wall effects, and hence the results represent two-dimensional flow behaviours. The upper and bottom walls were made from stainless steel plates that could be replaced with plates of different surface finish to facilitate a more in-depth study on the effect of wall roughness. Spherical glass beads of 100 μm were injected into the channel to attain a solid mass loading ratio of 10%. The average air velocity (Ub) was 19.7 m/s.