Boundaries of the unstable zone in horizontal slug flow pneumatic conveying

Among many modes of flow within pneumatic conveying, slug flow is usually preferred when pipe wear, particle attrition and gas use are constraining factors. However, the operating space of slug flow is bounded by blockage and the so called Unstable Zone. The latter is often less problematic, but it still leads to highly undesirable conditions where extreme pressure fluctuations may damage conveying equipment. These pressure fluctuations are a result of the system alternating between strand flow and unusually long slugs, however, the exact mechanism is poorly understood. This paper provides a theoretical basis for the physics underpinning the emergence and properties of the Unstable Zone in horizontal slug flow. It is shown that the Unstable Zone emerges from a combination of steady flow transitions not being possible due to formation of stationary layers, high stationary layers forming due to saltation, high particle velocities from high superficial gas velocities, and a single dominant slug forming based on gas conservation. The latter allows simplified single slug analysis to be applied and a theoretical upper bound on horizontal slug lengths in the Unstable Zone is derived based on assuming slugs form at the maximum saltation layer fraction. Comparisons with experimental pressure data validates the developed theory and future work is identified to further reduce the upper bound predicted within. Overall, via a novel model predicting both the emergence and properties of the Unstable Zone in horizontal slug flow, this work provides significant progress towards more reliable dense phase design.