Dense-phase pneumatic transport of cohesionless solids

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Virginia Polytechnic Institute and State University

An experimental program has been undertaken to gain a more fundamental understanding of dense-phase pneumatic transport of cohesionless solids. A 50.8 mm internal diameter circulating unit with both horizontal and vertical sections has been constructed . The pipe material is transparent lexan which allows for visual observation of the flow pattern. The particles used were a mixture of 95% white and 5% black polyethylene granules (particle diameter approximately 3 mm). The black particles were used to aid the visual observation of the flow pattern. The flow patterns ranged from dilute-phase flow to dense-phase plug flow. High-speed photographic techniques have been used to document the flow patterns in both the horizontal and vertical sections. Pressure drop measurements across a 70 cm test section have been coordinated with the film work.

At the higher superficial air velocities (approximately 15 m/sec), the particles flow in a dilute suspension within the air stream. The pressure drop across the 70 cm section fluctuates very rapidly. For the horizontal dilute-phase flow, the mean pressure drop is approximately 0.12 kPa with fluctuations ranging from 0 to 0.3 kPa. For the vertical dilute-phase flow, the mean pressure drop is approximately 0.25 kPa with fluctuations ranging from 0 to 0.5 kPa. Upon reducing the superficial air velocity to 6.8 m/sec, the flow pattern in the horizontal section changes to a type of strand flow. The particles are conveyed in a dilute phase above a stationary layer. Large peaks in the pressure drop data (approximately 1 to 2 kPa) correspond to increases in the dilute-phase solids concentration.

At the lower superficial air velocities (below 5 m/sec) , the solids flow pattern changes to dense-phase flow. The particles move in the form of plugs that occupy the entire pipe cross-section. For the horizontal flow, the plug length ranged from 0.17 to 0.60 m and the pressure drop across the plugs ranged from 1 to 5.2 kPa. The pressure gradient range can be predicted from the equations of Konrad et al. (1980). The analysis of the vertical dense-phase flow films is not as straightforward as the horizontal films. However, the flow pattern resembles that described by Konrad (1987) and there is qualitative agreement with the concepts outlined by Konrad (1987).