A NEW TECHNIQUE FOR EVALUATING FLUID‐TO‐PARTICLE HEAT TRANSFER COEFFICIENTS UNDER TUBE‐FLOW CONDITIONS INVOLVING PARTICLE OSCILLATORY MOTION

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Abstract
One of the critical parameters for the modelling of particle temperature profiles in continuous processing of particulate foods in a carrier liquid is the fluid‐to‐particle heat transfer coefficient between the fluid and the particles (hfp). A laboratory scale apparatus was fabricated to evaluate hfp under conditions imparting particle oscillatory motion while being heated in a model holding tube. Spherical particles (d = 12.7 × 10−3m) with centrally located fine‐wire flexible thermocouples (d = 7.62 × 10−5 m) were suspended from the upper mid‐section of a curved glass tube (ID = 50.8 × 10−3 m) to provide lateral movement of the particle as the tube was subjected to an oscillatory motion. A variable speed reversible motor, in conjunction with an electronic circuit board, was used to control the frequency and amplitude of the tube thereby keeping the particle in continuous motion. Time‐temperature data were gathered continuously from the time the particle‐mounted oscillatory unit, set at room temperature for the desired amplitude and frequency, was transferred to a heated sucrose solution (0, 30 and 50% w/w) at 70 or 90C. The hfp values were calculated from the evaluated heat rate index, fh, at three particle velocities (0.09, 0.16 and 0.26 m/s) under three oscillatory amplitudes (90, 180 and 270°). The hfp values increased with amplitude of oscillation and particle velocity, and decreased with the sucrose concentration. The hfp values associated with the fixed particles were higher than those for particles which were free to move.