Investigation of Velocity and Drag With Spherical and Non-Spherical Particles

Abstract

Abstract Numerous practical applications exist where dispersed solid particles are transported within a turbulent accelerating or deceleration gaseous flow. The large density variation between phases creates the potential for significant differences in velocity known as slip. Flow over a backward facing step provides a well characterized, turbulent, decelerating flow useful for measuring the relative velocities of the solid and gaseous phases in order to determine velocity slip and particle drag. Numerous investigations have been conducted to determine the gas phase velocity in a backward facing step for both laminar and turbulent flows and therefore the gas phase flow is well known and documented. Furthermore, some studies have also been conducted to determine the velocity of various sizes of spherical particles in a backward facing step and compared with their corresponding gas phase velocities. Few, if any, velocity measurements have been made for non-spherical particles in a backward facing step. In this work, a Phase Doppler Particle Analyzer (PDPA) was used to measure gas and particle phase velocities in a backward facing step. The step produced a 2:1 increase in cross sectional area with a Reynolds number of 22,000 (based on step height) upstream of the step. Spherical particles of 1–10 μm with an average diameter of 4 μm were used to measure the gas phase velocity. At least three sizes in the range of 38–212 μm for four different particle shapes were studied. The shapes included spheres, flakes, gravel, and cylinders. Since the PDPA is not able to measure the size of the non-spherical particles, the particles were first separated into size bins and a technique was developed using the Photo Multiplier Tubes (PMT) gain to isolate the particles size of interest for each size measured. The same technique was also used to measure terminal velocities of particles in quiescent air. This paper will discuss the results of the measurement of the particles and show that for the gas phase velocity and spherical solid phase particles that the measurements were in good agreement with previous measurements in the literature. However, for the non-spherical particles it will be shown that the drag coefficients were an order of magnitude higher in turbulent flows when compared to the literature values which are based on particles moving through a still fluid. This information is valuable for modeling turbulent two-phase flows since most assumptions of the drag are based on correlations from empirical data with particles moving through still fluid.