Solid velocity and concentration fluctuations in highly concentrated liquid-solid (slurry) pipe flows

Solid velocity and concentration fluctuations were measured for concentrated sand-water mixtures (20-35% solid by volume) in horizontal pipe flow using Electrical Impedance Tomography (EIT). Narrowly sized sand (d50 = 100 μm) was used to prepare each slurry tested in a 52 mm (i.d.) horizontal pipe loop at mixture velocities (2-5 m/s) that were significantly above the deposition velocity. The EIT measurements were used to obtain solid velocity and concentration fluctuation maps. Results show that the magnitude of the local solid concentration fluctuations is greater near the pipe wall and increases as the mixture velocity increases. Additionally, the concentration fluctuations are greater near the pipe invert, particularly at lower mixture velocities and/or concentrations where the solid concentration profiles are asymmetric. The Fast Fourier Transform (FFT) method was employed to study the power spectral density of these fluctuations. This analysis indicates that concentration fluctuations are produced almost entirely by particle-fluid turbulence interactions, rather than through particle-particle or particle-wall interactions. Comparison of the particle diameter with the characteristic turbulent length scales shows that the particles interact with turbulent eddies in the dissipative range, which is in accordance with the power spectral density analysis. The findings presented here are consistent with previous studies of fluidized beds and gravity-driven flows.