Numerous practical applications exist where dispersed solid particles are transported within a turbulent accelerating or decelerating gaseous flow. The large density variation between phases creates the potential for significant differences in velocity known as velocity 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 know 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 (PDA) 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 4μm were used to measure the gas phase velocity. At least three sizes in the range of (38 – 212 μm) for four different particles 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 PMT (photo multiplier tubes) gain to isolate the particle size of interest for each size measured. The same technique was also used to measure terminal velocities of the particles in quiescent air. The measured gas phase velocity and spherical solid phase particles were in good agreement with previous measurements in the literature. The results showed relative velocities between the particles and gas phase to be in the range of 0 – 3 m/s which is in transition between stokes flow and fully developed turbulent flow. Drag coefficients were an order magnitude higher for non-spherical particles in turbulent flows in comparison to stokes flow which agreed reasonably well with quiescent terminal velocity drag. This information is valuable for modeling turbulent two-phase flows since most assumptions of the drag are currently based on correlations from empirical data with particles moving through a still fluid.



College and Department

Ira A. Fulton College of Engineering and Technology; Mechanical Engineering



Date Submitted


Document Type





non-spherical particles, backward facing step, particle dispersion, coefficient of drag