Dune Formation in Horizontal Pneumatic Conveying System

Abstract

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.