In the horizontal pneumatic conveying of cork stoppers, fine cork dust is released and acts as a lubricant reducing friction effects, Neto and Pinho (1998). The deposition of this fine dust is still enhanced by electrostatic generation, Smeltzer et al. (1982). The conveying air pressure drop is then reduced, with the increase of the solids mass loading. This situation has already been found on solid-liquid systems; Lee et al. (1974) studied polymeric solutions containing suspended fibres and Radin et al. (1975) studied the drag reduction in several dilute solid-liquid suspensions. Szikszay (1988) argued that, as the measured experimental data referred to both the solids and the conveying air pressure drop, it was unreasonable to separate them. Weber (1991) also suggested that a single friction factor for the gas-solid mixture should be used, instead of two separate friction factors, for the air and solids flow. This question of drag reduction with the addition of particles into a flow stream has been recently reviewed by Fan and Zhu (1998), who presented a phenomenological model to account for the drag reduction.
Although drag reduction was detected in horizontal conveying of cork stoppers, in vertical pneumatic conveying the action of the gravity forces was such that the fines felt against the main gas flow and no preferential fines deposition occurred on the inner walls of the conveying pipe. Thus no drag reduction took place and in vertical pneumatic conveying, the overall gas plus solid pressure drop was higher than the corresponding value for the single gas flow, Barbosa and Pinho (2004). Classical theories like those of Barth (1960a, 1960 b) or of Yang (1973; 1974; 1975; 1978), could then be adapted to the vertical pneumatic conveying of cork stoppers. However, to follow the same approach used in the studies of horizontal pneumatic conveying of cork stoppers and to obtain comparable final correlations, a single friction factor for the gas solid mixture was adopted also for the vertical pneumatic conveying.