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

摘要

One of the critical parameters for the modelling of particle temperature profiles in continuous processing of paniculate foods in a carrier liquid is the fluid-to-panicle heat transfer coefficient between the fluid and the panicles (h_(fp). A laboratory scale apparatus was fabricated to evaluate h_(fp) under conditions imparting particle oscillatory motion while being heated in a model holding tube. Spherical panicles (d = 12.7 x 10~(-3) m) with centrally located fine-wire flexible thermocouples (d = 7.62 x 10~(-5) m) were suspended from the upper mid-section of a curved glass tube (ID = 50.8 x 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 panicle-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 h_(fp) values were calculated from the evaluated heat rate index, f_h, at three panicle velocities (0.09, 0.16 and 0.26 m/s) under three oscillatory amplitudes (90, 180 and 270°). The h_(fp) values increased with amplitude of oscillation and particle velocity, and decreased with the sucrose concentration. The h_(fp) values associated with the fixed particles were higher than those for panicles which were free to move.