Continuous flow of solid-liquid food mixtures during ohmic heating: Fluid interstitial velocities, solid area fraction, orientation and rotation.

摘要

The flow behavior of solid-liquid mixtures through tubes is an important consideration for proper design of continuous sterilization processes. Little work has been done on flows of the type found in aseptic processing systems, in which the density of the solids is similar to the carrier fluid (non-Newtonian) and the solid size is of the same order of magnitude as the tube diameter. Solid area fraction, rotation, and orientation of solids in continuous flow of solid-liquid mixtures are important parameters affecting the uniformity of ohmic heating. In this study, Particle Tracking Velocimetry (PTV) was used to measure instantaneous velocity vectors of the two phases in three dimensions. Digital image processing of tracer and solid pieces recorded on stereoscopic motion picture films were used to identify solid pieces and tracers, and to obtain data on location and velocity. The flowing mixture was rapidly immobilized by cooling-induced gelation and samples were then sliced to determine solids area fractions. Rotation and orientation of solids were investigated experimentally using the principle of Eulerian angles and direction cosine matrices.;The results showed that the velocity distribution is asymmetric and that the typically presumed laminar regime does not apply even at low flow rates. Fluid to solid relative velocities existed in all cases studied and decreased with increasing solid concentration, and increased with increasing aspect ratio and particle size. Increasing solid concentration complicated the liquid continuity, and created more tortuous paths. Considerable mixing occurred during continuous flow, which decreased with increasing solid fraction and solid to tube size ratio. Results of the solid fraction distribution with respect to the tube section showed a normal distribution. Increasing solid concentration flattened the distribution, and also significantly affected both the area and flowing solid volume fraction. Solid volume fraction was always lower than solid area fraction. The orientation study showed that 50% of particle orientation is aligned with the flow. Increasing aspect ratio resulted in an increase of the particle alignment parallel to the tube axis by 69% for cylindrical particles and 25% for cubic particles.