Effect of product and process variables in the flow of spherical particles in a carrier fluid through straight tubes

Residence time of particles and fluid-to-particle heat transfer coefficient, two of the major unknowns in the aseptic processing of particulate fluid foods, are intimately related with the linear and rotational velocities of the solid particles. These two velocities were measured by videotaping the flow of individual spherical particles in transparent straight tubes. The effect of fluid velocity and viscosity, particle diameter and density and tube diameter and upward inclination was statistically evaluated using a replicated full factorial design at two levels. Particle linear velocities between around one third of the average fluid velocity and 15% higher were obtained. Rotational velocities were usually lower, but of the same order of magnitude. Particle density and particle diameter had a significant effect on the two particle velocities analysed. The effect of fluid velocity was just noticeable for the linear velocity, with the particle velocity approaching or even exceeding the fluid velocity for the higher flow rates. The fluid viscosity influence was mainly evident on the rotational velocity, with the more viscous solutions slowing down the rotation.