| 摘要: |
| 本文采用液氮汽化后的低温氮气与食品接触进行热交换，搭建了低温液氮实验装置，研究了液氮冻结传热过程中热流量和冷却速度的变化规律。在﹣170~﹣50 ℃之间以﹣20 ℃为间距设置七个温区进行冻结实验，将马铃薯从初始温度18 ℃降至冻结点﹣18 ℃。采用拟合公式法对采集的数据进行计算，得到换热过程的平均热流量和温度分布；分析热流量变化规律及温度变化率得到最佳氮气温度。结果表明：当氮气温度为﹣122.87 ℃时，热流量增长速率达到最大值，继续降低温度，热流量增长幅度减小，此时有部分热量聚于内部，造成冷量浪费；通过对食品中心﹣3 ℃时不同界面的温度变化率计算，得到最佳氮气温度为﹣133.11 ℃，与前者仅相差6.71%。因此，﹣128 ℃左右的氮气温度为最佳温度，既可以保证食品实现快速冻结又可以提高氮气的有效利用率。 |
| 关键词: 冻结食品 冷却速度 冻结点 热流量 拟合公式法 |
| DOI： |
| 投稿时间：2016-11-24
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| 基金项目:上海市教育委员会重点学科资助项目(J50502)；上海市科委建设项目(13DZ2260900)；上海市部分地方院校能力建设专项计划（16060502600）。 |
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| Heat Transfer Research on Food Frozen by Cryogenic Liquid Nitrogen |
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Fang Jinlin1, Liu Jianhua1,2, Liang Yaying1, Zhou Xiaoqing1
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| (1.School of Energy and Power Engineering, University of Shanghai for Science and Technology;2.Shanghai Key Laboratory of Multiphase Flow and Heat Transfer in Power Engineering) |
| Abstract: |
| To study the heat-flux change and cooling rate in the freezing and heat-transfer process of liquid nitrogen, a low-temperature liquid-nitrogen experimental device was made. It was based on the idea of the direct contact heat exchange with the food after the liquid nitrogen vaporized. The experiment was conducted at seven temperature regions ranging from ﹣50 ℃ to ﹣170 ℃ with ﹣20 ℃ interval. A potato was frozen from the initial temperature 18 ℃ to the freezing point ﹣18 ℃. The average heat flux and the temperature distribution of the heat-transfer process were calculated using a curve-fitting formula. The heat-flux change and temperature change rate were analyzed to obtain the most appropriate nitrogen temperature. The results show that when the nitrogen temperature is ﹣122.87 ℃, the heat-flux growth rate reaches the maximum. As the temperature continues to reduce, the heat-flux growth rate decreases; at this point, part of the heat gathered internally from the food results in wasted energy. The temperature change rate for different interfaces was calculated when the food-center temperature was ﹣3 ℃, and the optimum nitrogen temperature was found to be ﹣133.11 ℃. This was only 6.71% different from the previous result. Thus, the optimum nitrogen-freezing temperature is about ﹣128 ℃, which can ensure the rapid freezing of the food and improve the effective utilization of the nitrogen. |
| Key words: frozen food cooling rate freezing point heat flux fitting formula method |
