Figure 4. – Definition of the acceleration length.
To determine the acceleration length for each experimental run in an objective way, the pressure drop per unit length for each run was plotted versus the pipe length for each tested situation; two fitting lines were also obtained, one logarithmic concerning the acceleration region and another linear and constant for the steady state conveying region. The intersection of both lines defined the acceleration length. Figure 4 presents an example concerning a given experiment for 38×22 stoppers. This procedure was adopted for all tested situations. The obtained acceleration lengths are of the same order of magnitude of those found in horizontal pneumatic conveying of cork stoppers, Pinho (1999).
After obtaining the acceleration lengths, the fitting of these values according to the formulation of Eq. (3) gave the following result,