中文摘要：

预冻是食品冷冻干燥的重要阶段，在预冻阶段，食品必须冻结至共晶点温度以下。为了弥补实验温度测量耗时耗材且获得信息有限的缺陷，本文在建立预冻过程数学模型的基础上，采用计算流体力学的的方法，用FLUENT6．3软件对胡萝卜预冻过程的温度场和冻结相变界面的动态推进过程进行了三维非稳态数值模拟。模拟结果表明，随着时间的推移，冻结界面从胡萝卜底部逐渐推进到胡萝卜表面，但由于胡萝卜外表面与周围冷空气存在自然对流作用，因此最后冻结凝固的位置位于略低于胡萝卜上表面的区域。对于10mm厚度的胡萝卜而言，大约25min时，内部冻结完成。在40min以后，内部的温度就已经低于共晶点温度．10℃。对胡萝卜冷冻干燥预冻过程进行了实验验证，数值模拟结果与验证实验结果较为吻合，并且准确的预测了预冻过程。

英文摘要：

Pre-freezing is a significant stage during freeze-drying of foods in which the freezing temperature must be under the eutectic point. In order to compensate the defects in terms of consuming time and materials as well as getting limited information for experimental temperature measurement, the mathematical model of freezing was established and three dimensional unsteady numerical simulation of temperature field and dynamic propulsion of freezing interface were analyzed using FLUENT 6.3 based on computational fluid dynamics. The simulation results indicated that the freezing interface gradually moved from bottom to surface of carrot as the time increased. Due to the natural convection of cold air surrounding carrot surface, the last solidification zone was slightly below the upper surface area. The solidification time was about 25 minutes for carrot with thickness of 10 mm. After 40 minutes, the internal temperature was -10 ℃ lower than the eutectic point temperature of carrot. Numerical simulation verified by experiments showed thatthe result was in good agreement with the experiment, and could be accurately prediction of the freezing process.