Figure 2. Images of the feeding hopper and of the data acquisition system.
For each tested situation characterized by a given air flow rate and solids loading factor θ(ratio between the solids mass flow rate and the conveying air mass flow rate), pressure drop measurements were made throughout the straight vertical transporting pipe, using all the available pressure taps. Through the adequate grouping of the pressure drop measurements, like for example the sequential combination between the first and subsequent pressure taps, it was possible to get an overall idea of the evolution of the pressure drop, as a function of particle entrance effects in the vertical portion of the pipe and also of the particle acceleration.
To calculate the mass flow rate of transported cork stoppers a basket was used as collection device for conveyed particles. The collected batch of particles, for a definite time interval, was weighted and the solids mass flow rate could then be calculated. The measurement of the voidage fraction during pneumatic conveying was carried out through the knowledge of the amount of cork stoppers that in a given instant remain inside the pipe. Having the experimental installation working in steady state conditions, the feeding hopper was suddenly stopped and all the cork stoppers being conveyed were immediately collected and weighted. This was, for the stoppage instant, the amount of particles remaining inside the conveying system. Comparisons between the overall corks volume and the inside volume of conveying pipe gave the voidage fraction under normal conveying conditions.
Table 1 presents some physical characteristics of the tested cork stoppers; l, d and ψ are respectively the length, diameter and sphericity of the stoppers, deq is the diameter of a sphere with identical volume and ρp is the density of the particles.