Multiphase flows are found in abundant natural phenomena and industrial processes. In particular, pipe flows are widely popular in industries involving chemical processing, dredging, and oil transport to name a few. Particle laden flows are often observed in applications like dredging, sediment, and slurry transport through pipes, etc. Therefore, it is imperative to understand the flow phenomenon in detail. The most important parameter in pipe flow is the pressure drop across a given length. This relates directly to the pumping power which is a significant parameter for industries when viewed from an economic standpoint. This emphasizes understanding the different regimes for particle-laden flows and the impact of particles on transition in particular. Research on transition behavior for particle-laden pipe flows is scarce and the behavior is far from being completely understood.
The literature is replete with the study of transition for single-phase pipe flows. Different perturbation mechanisms were analyzed and the lifetime studies indicated that the puffs are memoryless in nature. The literature provided different formulations to accommodate for the presence of particles with regards to modifying the viscosity of the suspension. The research in multiphase flows has provided inconclusive results in determining the critical Reynolds number for transition as different criteria were provided in different works.
The goal of the current thesis is to perform experiments to understand the transition behavior of particle-laden pipe flows for different particle concentrations. The novelty of this work is the use of an active perturbation mechanism that enabled the study of transition in perturbed and unperturbed flows. The experiments involve varying the particle concentration and keeping the ratio of pipe to particle diameter constant. The study concentrates on understanding the transition behavior using Moody diagrams. The experiments rely on pressure drop measurements to record the average pressure drop across the pipe and study the intermittent structures that drive the transition behavior. The study uses glycerol to make the solution neutrally buoyant when using particles. Single-phase measurements are performed to validate the setup including the pressure sensors and the perturbation mechanism. The Moody chart indicates that the transition is sub-critical with Spatio-temporal intermittency for particle concentrations less than 1.5 %. Interestingly, the particle-induced disturbances are significant and the transition behavior is identical for perturbed and unperturbed flow. This suggests that the disturbance created by the particles is qualitatively similar to that of the perturbation mechanism. However, the transition becomes super-critical for higher particle concentrations as the transition is driven by the fluctuations generated by the particles and the additional friction created by them. The friction factor decreases monotonically for very high particle concentrations (≥ 15 %). The transition behavior is investigated further by analyzing the time series data of pressure drop for different particle concentrations at intermittency of 10 - 20 %. The transition criteria are analyzed based on deviation from the Poiseuille line in the Moody chart and spike in pressure fluctuations in the flow. The latter provides inconclusive evidence for higher particle concentrations. The former holds good for the current study, however, it needs to be revisited for other particle sizes.