Numerical study of optical concentration measurements for a turbulent slurry pipe flow

Scope and method of study. The present study involves numerically studying the effect of turbulent fluctuations on the measurements made by optical sensors, prediction of concentration profiles in a developed slurry flow, and computation of the optimal sample thickness, beyond which the diffuse reflectance will be an asymptote. In the present work, modeling and grid generation of 3-D pipe is done using ANSYS software and Computational Fluid Dynamics analysis is done using commercial FLUENT software. A discrete random walk model is used to numerically simulate the particles in flow (only when the particle concentration is less than 12%). A mixture model is used when the particle concentration is 35%. The simulations are performed for a pipe of 0.0508m diameter and mixture: (1) water-calcium carbonate (density ratio of 2.8) and (2) Xylene-Oustextra (density ratio of 1.5). The limiting sample thickness is calculated using Representative Layer Theory. A study of interaction of near infrared light with particles inside the sample is done using various theories such as Mie-theory, Kubelka-Munk theory, Discrete Model Theory, Representative Layer Theory.;Findings and conclusions. From the numerical simulations, radial and axial concentration profiles are found for water-calcium carbonate and Xylene-Oustextra mixtures in a fully developed slurry pipe flow. Discrete phase modeling is conducted for plotting sample particle trajectories. It is observed that the turbulence plays a key role in the motion of particle inside the fluid. Wall y+ at constant y is plotted at different particle diameters for discrete phase flow and multiphase flow. For a fully developed discrete phase flow condition, there is no change in the wall y+ and its value is 0.185 for particle diameters of 1,2,5,10,20,100 µm. Therefore, there is no change in wall shear stress and the sub-layer provides a measure of a low turbulence region for optical measurements. The same phenomenon has been observed in multiphase flows. Limiting sample thickness is computed using the Benford equations. Thus, it can be inferred that the limiting sample thickness depends on initial sample thickness, concentration of particles inside the slurry and the material properties of the mixture.