Numerous industrial processes such as conveying, fluidizing and handling involve two-phase (fluid-solids) flows. Investigation of the two-phase flow mechanisms have importance in both industrial manufacturing processes and natural phenomenon such as dust storms, erosion, air pollution and soil deposition in rivers and ocean flows. One of the most significant parameters for designing two-phase (fluid-solids) flow conveying systems is the fluid velocity magnitude. Inaccurate determination of the fluid velocity can lead to high energy consumption, particle attrition, pipe erosion and in some cases pipe blockage. This work reviews numerous studies presented in the literature concerning various threshold velocities in horizontal two-phase (fluid-solids) flow systems. The threshold velocities include: incipient motion, pickup from a layer of particles, pickup from hill-shape particle deposits, boundary saltation and minimum pressure velocity. Most of the studies were conducted in pneumatic and hydraulic conveying systems and large-scale wind tunnels systems. As each threshold velocity characterizes a transition between two or more flow regimes and mode of particle movement, all possible flow regimes of the particle-fluid horizontal conveying systems are present first. Then the threshold velocity models and correlations are analyzed and compared. The theoretical models are compared for the forces and torque vectors taken into account for various threshold conditions and particle package geometry. In addition, the main parameters influencing the pickup and saltation mechanisms and the dimensionless numbers are reviewed. Finally, four threshold velocities are used to characterize a generalized flow regime diagram for horizontal conveying systems. This diagram makes a significant contribution to researching and designing two-phase flow systems. For example, the diagram may assist in the transition conditions between dilute and stratified flow regimes. However, more theoretical and empirical research is required to discover all the possible states of horizontal systems.
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