Numerical Analysis of Fluidized Quick Freezing of Vegetables and Fruits

Fluidized quick freezing of vegetable and fruit is a complicated heat transfer problem due to the differences in thermal properties between frozen and unfrozen zones. In this paper, a numerical method was employed. The partial differential equations were discretized by a finite-difference scheme, in which the heat conductivity K and the specific heat C were taken different values in frozen and in unfrozen zones. The latent heat energy released in freezing interface is considered by equivalent temperature Te,Te= L/C₁, where L is freezing latent heat and C₁ is specific heat in unfrozen zone. In spherical coordinate, the finite differential equation of freezing interface layer was given as:(?)where \S:time increment, \r:grid spacing, T_f: freezing point, p:dencity . K₁ K₂:conductivities in frozen and unfrozen zones, \T:equivalent temperature drop. When the cumulated value Σ\T is smaller than or equal to -T_e, then this layer is fully frozen.By this method, the quick freezing of strawberry from 5 C to -18 C was simulated. The result was coincidental well with that of empirical equation given in literature. By this method, the transient temperature distribution and the freezing interface regime can be also determined which is unfeasible by empirical equation. This method can be also used to simulate the defrost course of vegetables and fruits.