食品冻结时间的数值计算

本文对常见的平板状食品冻结时间进行了计算机模拟,建立描述食品冻结过程传热特性的偏微分方程（温度模型、焓模型）,采用有限差分法进行数值求解,并与实测值,简易公式计算值对比,结果表明,数值法计算食品冻结时间具有较高的精度,与实测值吻合得较好。     

有限长圆柱状食品冻结时间的计算方法及实验验证

食品冻结时间的准确计算关系到食品冻结装置的优化设计及运行.文中提出了一种新的食品冻结时间计算方法,将冻结时间划分为三个阶段:冷却阶段、冻结和再降温阶段,并对各个阶段建立了动态计算模型.通过对有限长圆柱状食品的实验研究,将普朗克计算式与通过该模型计算得到的结果和实验数据进行比较,结果证明建立的计算模型是可靠的.     

有限长圆柱状食品冻结时间的计算方法及实验验证

食品冻结时间的准确计算关系到食品冻结装置的优化设计及运行。本文提出了一种新的食品冻结时间计算方法，将冻结时间划分为三个阶段：冷却阶段、冻结和再降温阶段，并对各个阶段建立了动态计算模型。通过对有限长圆柱状食品的实验研究，将普朗克计算式与通过该模型计算得到的结果和实验数据进行比较，结果证明建立的计算模型是可靠的。     

带喷嘴的连续冻结隧道装置的设计及其冻结效果分析

本文以一个冻结能力为800 kg/h的网带速冻装置为原型,对现有连续冻结隧道装置增设喷射结构,经过静压箱均混的气流再经喷嘴射流后,吹风速度加快,食品冻结时间缩短.理论计算表明:本设计的冻结时间减少到原来的65.3%,单位时间内冻结量提高了53%.     

连续冻结隧道的冻结时间分析及其对能耗的影响

冻结时间影响速冻食品的质量和能耗 ,本文通过对冻结时间的编程计算 ,并针对不同的隧道冷却介质温度、隧道吹风速度和不同的食品厚度情况下的冻结时间进行分析比较 ,得出根据冻结时间来选择设备的运行工况 ,既可以节能 ,又可以使装置得到最合理使用的结论     

冷冻食品与现代冻结装置

本文介绍目前国内外冷冻食品的发展动态,影响食品冻结时间主要因素的实验研究,以及现代食品冻结装置国内外的进展状况.     

食品在低温箱和连续式冻结隧道中的冻结过程的比较分析

本文通过食品在低温箱中的冻结过程和在连续冻结隧道装置中的冻结过程的实验研究结果的进行对比分析，得出食品在低温中冻结时，冻品表面温度在冻结中心温度处于—1℃～—5℃的区域时会出现温度回升的现象，而连续式冻结隧道中不存在这个现象，并从理论上对其原因进行了分析。另外，冻结曲线的结果还表明，在相同冻品和品质要求的前提下，采用连续冻结隧道比较节能。     

水产品冻结时间的试验研究

冻结时间的确定是设计和评价冻结装置性能优劣的主要依据之一，作者综合比较多种冻结时间的计算模型，对于平板鼓风式冻结装置装置（水产品常用冻结装置）中冻品冻结时间进行了试验研究，并用焓模式数学模型编制了计算机仿真程序，经用鳕鱼为例进行仿真研究，得出了有关此类冻结装置设计，使用的一些有实用价值的结论。     

复杂形状食品冻结时间的几何因子预测法

本文阐述了用于计算复杂形状食品冻结晶解冻时间的几何因子法，给出了大平板冻结和解冻时间及多种复杂状食品形状因子的计算公式。经验证，利用形状因子法计算各种形状食品冻结和解冻时间的精度较高。     

低温液氮冻结食品传热研究

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7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。     

Freezing of a parallelepiped food product. Part 1. Experimental determination

A number of procedures are used to predict the freezing time of food. The objective of the project reported here was to test the adequacy and applicability of the various mathematical models and methods used to predict the freezing time of a small, parallelepiped food product. The approach was to compare experimental and predicted freezing times. In this paper the experimental methods and results are described; the freezing time and the thermophysical properties of both the frozen and non-frozen food were determined. Comparison between experimental and predicted freezing times will be given in a subsequent paper. The experimental conditions consisted of individually freezing the product in an air blast; the food product was french fries. The time required to lower the temperature of a french fry from 31 to −18°C in the −29°C air blast freezer used was approximately 1200 s. The mean moisture content of the fries was 73.7%. The average densities of the non-frozen and frozen fries were 1069 and 1012 kg m⁻³, respectively; the average thermal conductivities were 0.50 and 1.0 W m⁻¹ °C⁻¹; and the heat capaciti were 3420 and 1870 J kg⁻¹ °C⁻¹. The overall surface heat transfer coefficient of a parallelepiped object in the air blast freezer used for these experiments was 21.0 W m⁻² °C⁻¹. The low surface heat transfer coefficient resulted in a flat temperature profile within the fries.     

Measurement of thermophysical properties and analysis of heat transfer during freezing of slab-shaped food commodities

Experiments have been designed to measure water content, packing density and transient temperatures during cooling and freezing of fresh green peas packed in rectangular containers. Specific heat capacity in unfrozen and frozen states and latent heats of fusion are calculated from the measured water content. Experimental temperature records are used to calculate bulk thermal diffusivity, both in the frozen and unfrozen states, on the basis of transient heat transfer analysis. In the frozen state, the measured thermal diffusivity was found to vary almost linearly with temperature. A calculation scheme has been presented and applied to compute temperature variations during immersion and air-freezing of peas, using the measured thermal properties. The proposed scheme yielded the calculated freezing time lying within ±8% of its measured values. The calculated temperatures, with both first and second (Von Neumann) type of boundary conditions, were in good agreement with the measurements of present and earlier investigations.     

Measurement of thermophysical properties and analysis of heat transfer during freezing of slab-shaped food commoditiesMesure des propriétés thermophysiques et analyse du transfert de chaleur au cours de la congélation d'aliments en forme de plaques

Experiments have been designed to measure water content, packing density and transient temperatures during cooling and freezing of fresh green peas packed in rectangular containers. Specific heat capacity in unfrozen and frozen states and latent heats of fusion are calculated from the measured water content. Experimental temperature records are used to calculate bulk thermal diffusivity, both in the frozen and unfrozen states, on the basis of transient heat transfer analysis. In the frozen state, the measured thermal diffusivity was found to vary almost linearly with temperature. A calculation scheme has been presented and applied to compute temperature variations during immersion and air-freezing of peas, using the measured thermal properties. The proposed scheme yielded the calculated freezing time lying within ±8% of its measured values. The calculated temperatures, with both first and second (Von Neumann) type of boundary conditions, were in good agreement with the measurements of present and earlier investigations.     

Prediction of the freezing characteristics of spherical-shaped food productsPrévision des caractéristiques de congélation des aliments de forme sphérique

An analysis of the freezing characteristics of spherical shaped food products with time dependent surface-temperature variation is presented. The one dimensional heat conduction equations in spherical coordinates for the frozen and unfrozen regions are solved simultaneously by a suitable transformation to obtain an equivalent slab. Goodman's integral method is applied to obtain the solution in terms of four dimensionless parameters. The results are presented for an exponential variation of the surface temperature for parameter values covering the ranges of the thermophysical properties and processing conditions encountered in food freezing practice. A correlation for facilitating the estimation of the freezing time is also given.     

Freezing of a parallelepiped food product. Part 2. Comparison of experimental and calculated results

The objective of the project reported here was to test the adequacy and applicability of existing mathematical models when used to predict the freezing time of a small parallelepiped food product. The freezing time of french fries was determined experimentally by individually freezing the samples in an air blast; this work was presented in a previous paper. In this paper the experimental results are compared with estimates from 19 mathematical models. Three empirical models and one approximate model yielded estimates within 10% of the experimental freezing time and most of the other empirical and approximate models overestimated by more than 10%. Neither of the two exact models examined was applicable to the freezing of a small food product when the surface heat transfer coefficient was finite.     

A survey of commercial meat block freezing in the United Kingdom and Fire

The results of a survey of meat block freezing in the UK and Eire have been compared with experimental and calculated data. This comparison showed that only 58% of the throughput is being frozen within ±20% of the actual time required. There was little standardization of block size, type of wrapping and carton, freezing and storage conditions. The weight of the majority of blocks was between 25 and 30 kg and 80% of the meat was frozen in cartons between 14.6 and 15.9 cm thick. The survey concludes that refrigeration and meat companies must aim for more realistic freezing times and conditions in order to avoid either incomplete freezing or wasting energy by freezing for longer than necessary.     

The effect of supercooling on ice structure in tuna meat observed by using X-ray computed tomography

Several studies have reported that freezing a homogeneous food such as soy bean curd with deep supercooling (supercooled freezing) results in the formation of many particle ice and homogenous ice structure. However, ice crystal morphology may be affected by the cellular structure of the food. In this study, the ice crystal structure in tuna meat, a cellular food, frozen by the supercooled freezing method was investigated by X-ray computed tomography and compared with ice structures in tuna meat frozen by conventional freezing methods. The results showed that rod-like ice crystals grew parallel with the myofibers, and inhomogeneous ice structures formed in tuna meat frozen by the supercooled freezing method regardless of the degree of supercooling, in contrast to the ice structure in frozen soy bean curds. These ice crystals linked with each other to form rod-like ice structures due to mobility limitations imposed by the cellular structure.     

深厚表土层冻结壁厚度计算方法研究

针对冻结法凿井工程面临的表土层厚度不断增加,而现有冻结壁的设计理论滞后于工程实践的问题,目前冻结壁的设计和计算多数是将冻结壁视为均质和各向同性的厚壁圆筒,基于弹性和弹塑性理论进行计算的结果已经不能适用于未来深厚表土层的冻结法凿井工程,文章综述了冻结壁厚度设计理论的研究现状和存在的问题,并提出了考虑冻结壁径向非均匀的粘弹性分层计算模型,考虑径向非均质条件,井帮的径向变形较均质冻结壁小25%～30%,可以充分发挥冻结壁的承载和变形性能,有效减小冻结壁的厚度,具有一定的的工程应用价值。