On the influence of the wall friction coefficient on void fraction gradients in horizontal pneumatic plug conveying measured by electrical capacitance tomography

This paper focuses on the experimental investigation of horizontal pneumatic dense phase conveying using high-speed twin-plane electrical capacitance tomography (ECT). This allows an insight in the inner structure of plugs. It was used to monitor the conveying behavior of two types of plastic pellets and to reveal the inner structure of plugs and their propagation velocity. For one of the test materials the latter was compared with particle motion at the wall, measured by particle tracking. Both test materials were characterized with regards to density, packing porosity, particle size distribution, fluidization behavior and the coefficient of wall friction. Whereas the former three are similar for both materials, the coefficient of wall friction is significantly different. According to ECT measurements the porosity of plugs is close to minimum fluidization. The cross-sectional averaged porosity of the plugs was found to be only slightly higher than the porosity at minimum fluidization. The amount of material in the stationary layer varies strongly in between plugs. It depends on the carrier gas velocity, covering up to 50% of the pipe's cross section at low gas velocities. The influence of the wall friction coefficient on the material distribution in the cross section showed to be high. Whereas for a low wall friction coefficient the porosity in a plug cross section is almost homogeneous, plugs formed from the material with a higher wall friction coefficient show decreasing porosities towards the pipe wall. This observation clearly casts doubt on the common assumption of plug flow as a hopper flow problem. In the study, the particle velocity at the wall determined by a high speed camera was almost constant at different carrier gas velocities. Plugs determined by ECT moved faster than the single particles itself though, taking particles up at the front and loosing particles at the rear. The experimental findings presented in this study may further be used to verify DEM simulations or two-phase continuum simulations. (C) 2017 Elsevier B.V. All rights reserved.