Figure 1. Schematic diagram of the experimental apparatus.
2. Experimental procedure
The experimental system is schematically shown in Fig 1, it was composed by a 6 m long horizontal portion where the stoppers were introduced into the conveying pipe, a vertical testing section of 6.8 m length and the recovery circuit composed by a small horizontal portion of 1.6 m and a diagonal portion 5.9 m long. The tube was a PVC pipe with 125 mm nominal diameter pipe (122 mm internal diameter, D). Cork stoppers, moving in closed circuit, were introduced into the pneumatic conveying system through the feeding hopper, installed in the horizontal portion of the duct, Fig. 2. Conveying air was introduced into the system through a centrifugal fan controlled by a ABB frequency variator model ACS401000932, with a resolution of 0.1 Hz/50 Hz. Air flow was measured through an orifice plate flow meter, positioned upstream the feeding hopper, and equipped with a differential pressure transducer with an operating range of 0 to 1245 Pa. The orifice plate flow meter was previously calibrated by means of a Pitot tube installed before the feeding hopper.