Three-dimensional measurements of particle clusters at the exit of a turbulent pipe jet

Abstract

An investigation into the phenomenon of particle clustering at the exit of a turbulent two-phase pipe flow was conducted utilising detailed time-resolved tomographic-PIV measurements in conjunction with Voronoi analysis. The flow consisted of an air jet laden with solid, spherical particles, issuing from a long round pipe into a weak co-flow. The pipe diameter was D = 6.22mm, while the pipe length-to-diameter ratio was L=D=160. The jet Reynolds number, based on the pipe diameter, was fixed at ReD = 10; 000. Two different particle diameters were investigated, namely dp = 0.3 and 10μm. The corresponding Stokes numbers, based on the large-eddy time-scale, was SkD = 0.004 and 1.4, respectively. The particle bulk concentration was ≈190particles/mm³, which was sufficiently large to result in significant particle-fluid interaction (that is, the flow was in the two-way coupling regime). The results show that for the SkD = 1.4 case, particle clustering is significant, with the clusters characterised by their filament or rope-like morphology and their coherence in time. Importantly, they are also detected right from the exit plane, which implies that these clusters were generated inside the pipe. For the case SkD = 0.004 particle clustering is negligible, and where detected, clusters tended to be small and incoherent in time, suggesting that they most likely arise out of random motion of individual particles.