Experimental investigation of triboelectric charge on particles in helical gas/solids pipe flow systems
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In gas/solids pipe flow systems, particles often get triboelectrically charged during their transport, due to frequent collisions with the pipe wall. Helical shaped pipes, due to their mixing effect owing to the curvature induced secondary flow, have been considered advantageous for processes such as coal liquefaction and gasification and also for achieving a desired pressure drop for a compact pipe design. However, this enhanced mixing also causes the helical pipe to generate a significantly high amount of particle charge. For a detailed understanding of triboelectrification in helical gas/solids pipe flow systems, it is hypothesised that a variation in the critical parameters governing such flow systems will also affect the triboelectric charge generated on particles. Hence, the present study focuses on experimentally investigating the triboelectrification of particles with an isolated variation to the critical parameters, namely: non-dimensional Curvature ratio δ, non-dimensional Torsion parameter β, particle Stokes number St and flow Reynolds number Re. Curvature ratio is varied within the limits of 0.053 to 0.19, by changing the radius of curvature of the helical coil at negligible torsion. Torsion parameter is varied within the limits of 0.038 to 0.26, by changing the pitch of the coil at a constant curvature ratio. The respective curvature and torsion variation is performed at two St 0.82 and 1.67, at fixed Re. The Reynolds number effect is investigated at Re 1875 and 3750 at a fixed St. Under a particular experimental condition, known mass of particles is injected in batches in the dilute particle limit into the helical pipe air flow wherein the particles acquire charge, which is later quantified using Faraday cup and an electrometer. At both St conditions, it was observed that charge magnitude on particles increased exponentially and in a similar trend with the increase in the curvature ratio. With torsion parameter, the particle charge effectively remained unchanged at lower values of torsion, followed by a gradual increase at higher values of torsion and here also the variation was similar at both St. For the St variation, it was seen that for all the cases of respective curvature ratio and torsion variation, doubling the St doubled the charge generated on particles. For Re variation at a fixed St, the particle charge was higher for the case of high Re flow.