Laser Doppler Anemometry Measurements of Dilute Pneumatic Transport in a Vertical Lifter

Vertical pneumatic transport in a lab-scale vertical lifter has been studied using Laser Doppler Anemometry (LDA). The experimental system consists of a lower fluidized silo, an upper receiving tank and a connecting vertical transport pipe made of clear glass. The experimental study has been performed in order to get detailed information of the complex gas-particle flow behavior in a dilute vertical conveying system. Axial and tangential particle velocities, as well as root mean square velocities are measured simultanously for different flow conditions. The particulate material transported is Zirconium Oxide (ZrO2) solids. Measurements is obtained using transport air flow rate of 24, 36, 48 and 60 m/h. The air is feeded into the tranport air through a nozzle. The length this nozzle is sticking into the transport pipe is varied in order to study the nozzle position influence on the flow behavior. The receiving tank is equipped with weighing cells, so that solids mass fluxes also can be measured for different flow conditions. INTRODUCTION Transport of particulate materials is encountered in numerous industrial processes, and successful design of the transport system is often critical to a process. Vertical transport of gas/solids mixtures can be handled using a vertical lifter. Design of such systems can be extremely complex due to insufficient understanding of the flow behavior. Vertical pneumatic transport may be classified into dilute and dense phase flow regimes. In traditional design of industrial gas/particle pneumatic conveying systems, the main consideration has normally been to choose a correct transport gas velocity, e.g. Knowlton (1986). This is to avoid unstable flow conditions that occurs at a too low gas velocity, or to avoid the excessive gas velocity requirements and too high pressure drop that occurs at a too high velocity. Hence, the design criterion has normally been based on a general relationship between velocity and pressure drop per unit length. Little emphasis has been put on gaining fundamental knowledge of the complex flow behavior in pneumatic conveying systems like vertical lifters. Laser Doppler Anemometry (LDA) and Phase Doppler Anemometry (PDA) measurements have previously been applied to dilute gas/solids flow systems, as circulating fluidized beds by Mathiesen et al. (1999a, 2000), and vertical gassolids pipe flow by Maeda et al. (1980), Lee and Durst (1982) and Tsuji et al. (1984). In order to study dilute vertical transport more extensively, Mathiesen et al. (1999b) built a lab-scale vertical lifter designed for LDA/PDA measurements. Some preliminary measurements were also presented in their paper. The transport pipe was made of clear glass in order to gain detailed information about the complex flow behavior using the optical and non-invasive LDA measurement techniques. Particle diameter, and axial and radial mean and turbulent particle velocities were measured simultaneously for different dilute phase transport conditions. The particle mass transported was monitored on-line by three weighing cells, which are mounted on the receiving tank. LDA/PDA measurements and the solids flux measurements may give complete information about the flow behavior in the vertical pneumatic transport system, and may increase the depth of understanding of the underlying mechanisms in such gas/solids flow systems. The work performed by Mathiesen et al. (1999b) is continued in the present study. LDA measurements are conducted for different flow rates of transport air and different positions of the transport air inlet nozzle relative to the transport pipe. MEASUREMENTS TECHNIQUES LDA has, during the last decades, become a commonly used experimental technique in dilute gas/particle flow systems. The major reasons are that LDA is a non-invasive optical technique, which does not disturb the flow and has a high spatial resolution with a fast dynamic response and range. LDA together with PDA is able to measure mean and fluctuating velocity, diameter and in some cases even concentration of the dispersed phase simultaneously. Particle Velocity Measurement When two coherent Gaussian laser beams are intersecting, the intersection will cause a pattern of plane interference fringes. The fringe spacing δf can be shown to be proportional to the wavelength λ, and inversely proportional to half of the angle ν between two incident laser beams: ) 2 ν 2sin( λ = δf [1] When a particle is passing through the intersection area, it will scatter light and the intensity will change according to the interference fringes. The intensity variation of the scattered light, or the frequency, can be obtained by a photo detector. The velocity of the passing particle will be proportional to the Doppler frequency fD and the fringe spacing: i D f D v = f = f 2 ( / 2) δ λ θ sin [2] Where vi is the absolute velocity of the particle in the direction that is perpendicular to the bisector of the two incident laser beams. To detect the direction of the flow, a frequency shift for one of the beams is introduced. The Doppler frequency is the frequency obtained by the photo detector minus the frequency shift. The mean velocity, V is obtained by: V = 1 N vi N