Boundary saltation and minimum pressure velocities in particle-gas systems

The saltation velocity is one of the key design parameters in pneumatic conveying systems. The aim of this work is to experimentally study the mechanism of saltation. Experiments were carried out with various spherical and non-spherical particles in a small wind tunnel with very dilute flow. For each velocity the distribution of halted particles along the tunnel bottom was measured. From this length distribution, the median length was determined and used for further analysis. It was found that the median conveying length approaches infinity at a certain threshold velocity. By testing many materials the boundary saltation velocity followed a simple correlation of the modified Reynolds number as a function of the modified Archimedes number. The conveying length was an accumulation of three lengths: the first flight length, the rebound length, which is affected by the coefficient of restitution, and the rolling/sliding length, which is affected by the coefficient of friction. By analyzing these lengths, the total conveying length and the boundary saltation velocity were easily defined. Furthermore, as the velocities at minimum pressure velocities (referred to in the paper as the minimum pressure velocities) have been found to follow the same trend as the boundary saltation velocity if the solid concentration is taken into account, our simple correlation can describe by ± 30% all the relevant experiments found in the literature.