Manaswita Bose

Effect of Dune Formation on Pressure Drop in Horizontal Pneumatic Conveying System

Pneumatic conveying is widely used in industries handling large amount of granular materials to transport the solid particles; however, the process is energy intensive as an instability of flow sets in the transportation line even in the dilute regime, causing large fluctuations in the line pressure drop, the reason of which is not clearly understood. Here, we investigate, both by experiments and by using numerical simulations, the instability transition regimes and identify the reasons of the fluctuations observed in the line pressure drop in a horizontal pneumatic transport system operating at near-saltation conditions. It is observed that the increase in the pressure drop (immediately after the saltation) is accompanied by the formation of distinct dunes. It is also observed that the line pressure drop depends on the axial location of the dune and shows large fluctuations in the regime where the dunes are unstable. Results obtained from the numerical simulations suggest that the increase in the line pressure drop in the presence of dunes is essentially due to the shear stresses at the dune surface which are larger than that for the flows in clean pipe.

Dune Formation in Horizontal Pneumatic Conveying System

Pneumatic transport is one of the most widely used methods for the carrying solid particles in process industries; however, the process is energy intensive as the instability in the conveying line sets in at a volume fraction much lower than 0.1. The typical Zenz plot for a pneumatic conveying system shows that there exists a minima in the line pressure drop versus superficial gas velocity plot and the regime immediate to the left of the minima is identified as “unstable”, as a large fluctuation in pressure drop is observed in this operating regime. A recent investigation shows that the fluctuation of the line pressure drop is accompanied by formation and recede of distinct dunes in the transport line. Formation of dunes, though studied extensively in the context of geophysical flows, is less emphasized in the literature of pneumatic conveying. The objective of the present work is to investigate the formation of dunes in the horizontal pneumatic conveying system and to propose a possible phase diagram to demarcate the operating regime where distinct, stable dunes are formed. To that end, a variable length pneumatic transport system is fabricated. The conveying line is made of transparent glass tubes for clear visualization of the test section. High speed images with good resolution are captured to study the dynamics and the morphology of the dunes formed. Single and multiple distinct, equidistant dunes are observed to form in the conveying line. A phase diagram on the ṁ s –ṁ a plane, where, ṁ s and ṁ a are the mass flow rate of the solid and the fluid phases respectively, is proposed to classify the different flow regimes of a horizontal pneumatic transport system.

Transition Criteria for Fluidized Beds Revisited

Stability of the fluidized beds has been a topic of interest for several decades and a number of transition criteria in the empirical form as well as based on hydrodynamic description of the bed are available in literature. In the present work, two criteria derived using analytical arguments (Foscolo and Gibilaro 1984; Batchelor 1988) are revisited and expressed in terms of three non-dimensional independent variables, namely the density ratio, , superficial gas velocity , and particle diameter . The transition criteria appear as a family of curves in between the dimensionless minimum fluidization and single particle terminal velocities for different values of De, on a classical plane. A set of 70 different observations through experiments and numerical simulation is mapped on to the plane to examine the transition criteria. While both the criteria predict the in-homogeneity of the bed at higher De consistently, at lower De, the small scale perturbation is captured in the criterion proposed by Batchelor (1988) but not by the one defined by Foscolo and Gibilaro (1984); nevertheless, the current work presents the mapping of two analytically derived transition criteria on the classical plane which is complimentary with earlier classifications proposed by Kunni and Levespeil (1991).