蔬菜水果流态化速冻的数值分析

Numerical Analysis of Fluidized Quick Freezing of Vegetables and Fruits

蔬菜水果速冻过程是一多物理参数变化(表面对流换热系数,导热系数,比热)并含有冻结潜热的复杂过程。该文建立了球形蔬菜水果流态化速冻过程的偏微分方程组,并用控制容积法推导出有限差分方程组,提出用当量温度计算冻结界面潜热,编制了计算机软件。用所提出的方法模拟速冻过程可以分析任意时刻的温度分布、冻结界面位置,以及任意点的温降曲线和通过最大冰晶生长温区所用时间。模型稍做修改可计算片状、圆柱型果蔬的速冻过程。

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 A' 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 C\ is specific heat in unfrozen zone. In spherical coordinate, the finite differential equation of freezing interface layer was given as:where AS:timc increment, Ar:grid spacing, Tf: freezing point, p:dencity . K1 K2:conductivities in frozen and unfrozen zones, T:equivalent temperature drop. When the cumulated value \T is smaller than or equal to -T, 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. Foods freezing Heat transfer Numerical method