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。     

液氮冻结食品冻结时间的计算方法及实验验证

冻结时间是评价冻结食品质量的重要指标,正确计算冻结时间是食品冻结装置优化设计及运行的关键。本文通过建立液氮速冻实验平台,测试了食品的冻结时间,介绍了现有的理论模型,选择了几种典型的计算方法应用于食品冻结,并将各模型的理论结果与实验值进行比较,分析各偏差原因,获得计算食品冻结时间最准确的方法。以有限长圆柱状食品为例,通过研究发现在整个氮气温度场中国际制冷学会模型计算结果与实验值最相近,平均偏差为8.86%,分段计算法次之,平均偏差小于15%。     

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。     

空气压力对食品冷冻速度的影响

本文介绍了在食品的冷却和冻结过程中,提高冷却介质的压力对食品冷冻速度、冻结时间的影响。实验研究结果表明,加压冷冻将能够大大提高食品的冻结速度,缩短冻结时间,减少由于冻结过程所引起的干耗。     

食品热物性的多项式数学模型

食品中含有大量的水,食品在冻结阶段水分发生相变,食品中的水分大部分结成冰晶,而水的热物性和冰的热物性有很大的差别,这就导致了食品热物性在冻结过程中发生较大变化,本文阐述了食品热物性模型的重要性,并根据已有的食品热物性经验公式进行编程计算,利用统计分析软件回归分析出食品热物性随温度变化的分段多项式数学模型.     

食品冻结过程若干影响因素的实验研究

利用自行研制的多功能低温实验台,对猪肉、芋艿、香蕉和糯米小圆子等进行了冷冻实验研究,实验测得猪肉、芋艿、香蕉和糯米小圆子的初始冻结温度分别为:-1.62℃、-1.4℃、-1.24℃和-0.9℃;分析了这些食品的形状、堆积方式、传热方式、冷气流方向和食品纤维方向等因素对食品冻结时间的影响.并研究了采用不同包装材料包装食品后,在冷冻过程中对食品质量和冻结时间的影响.     

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。     

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.     

冻结过程中流场分布改变对冻结特性的影响

为考察鼓风冻结过程中流场分布对食品冻结的影响,针对两排圆柱形食品模型(马铃薯泥)建立了鼓风冻结的三维数值模型,进行了三维非稳态模拟,实验验证表明该模型与实际情况吻合较好,冻结时间相对误差为3.2%。以冻结曲线、冻结速率、温度变异系数、冻结时间不均匀度和能耗为指标,研究马铃薯泥冻结过程中的位置改变对其冻结特性的影响,即在冻结了80 min,100 min,120 min时将两排马铃薯泥对调。结果表明,在预冷段进行位置改变的影响效果小于相变段;在相变段的3个时刻中,t=100 min时改变马铃薯泥位置效果最好,能够缩短冻结时间6.7%,降低能耗9.7%以及减小冻结的不均匀性。冻结过程中食品的位置改变(实际中也可以改变风向)能够有效缩短冻结时间,减小冻结不均匀度,在保证冻品质量的前提下,达到节能目的。     

基于分形描述食品材料的收缩特性

食品材料在干燥过程中被分为湿相和非湿相,大量实验证明水分分布具有分形特性.建立了湿相分布的分形模型,用记盒法计算出此模型中的局部含水率和分形维数.构造了用局部含水率和分形维数表示食品材料收缩率的本构关系,真实地反映了水分的内部分布形态,并计算出正确的体积收缩率.结果表明,用局部含水率表示线性与非线性体积的变化都是成立的,局部含水率可用于计算各种类型的收缩模型.通过收缩实验数据与模拟结果的对比得到很高的相关系数,证明了数学模型的正确性.     

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     

Thawing and freezing of selected meat products in household refrigerators

Recently, several manufacturers of domestic refrigerators have introduced models with “quick thaw” and “quick freeze” capabilities. In this study, the time required for freezing and thawing different meat products was determined for five different models of household refrigerators. Two refrigerators had “quick thaw” compartments and three refrigerators had “quick freeze” capabilities. It was found that some refrigerator models froze and thawed foods significantly faster than others (P<0.05). The refrigerators with the fastest freezing and thawing times were found to be those with “quick thaw” and “quick freeze” capabilities. Heat transfer coefficients ranged from 8 to 15 Wm⁻²K⁻¹ during freezing, and the overall heat transfer coefficients ranged from 5 to 7 Wm⁻² K⁻¹ during thawing. Mathematical predictions for freezing and thawing time in the refrigerators gave results similar to those obtained in experiments. With the results described, manufacturers can improve their design of refrigerators with quick thawing and freezing functions.     

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%.     

Numerical simulation of individual quick freezing of spherical foods

A one-dimensional three time level implicit finite difference computer program was developed to predict temperature profiles during the individual quick freezing of spherically shaped foods. Temperature varying thermal properties necessary for these predictions were calculated based upon properties of the unfrozen product. The centre temperatures of individual peas within a bed of peas being frozen under various conditions of fluidization were recorded. Comparison of experimental results and published data with predicted freezing curves showed good agreement.     

Freezing of foods under surface boiling boundary conditions

The surface boiling boundary condition is encountered in the freezing of foods when foods are immersed in boiling freezants such as R12. This phenomenon was incorporated in a mathematical model of the freezing process as a surface temperature dependent convective boundary condition. A finite difference numerical scheme was formulated to solve the model for one- and two-dimensional geometries. The pool boiling characteristic for R12 was obtained by inverse heat transfer analysis of the experimental quenching curves of a transient calorimeter. The model was used to simulate the experimental freezing processes with reasonable agreement.     

Freezing of strawberry pulp in large containers: experimental determination and prediction of freezing times

Strawberry pulp packed in bulk in containers of various shapes and of large sizes (10–2001) was frozen in a tunnel at −35°C. Time-temperature curves and freezing times for heat transfer in one, two and three dimensions were obtained. Experimental freezing times were compared with predicted values given by approximate methods based on two different equations for the calculation of freezing times in slabs, which in turn used three different methods for the calculation of shape factors. The predicted results obtained in this work confirmed that both the simplified prediction methods of freezing times and the shape factors used were sufficiently accurate for the design of freezing equipments for these types of product and working conditions.     

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.     

Heat transfer coefficients for forced-air cooling and freezing of selected foods

To maximize the efficiency of cooling and freezing operations for foods, it is necessary to optimally design the refrigeration equipment to fit the specific requirements of the particular cooling or freezing application. The design of food refrigeration equipment requires estimation of the cooling and freezing times of foods, as well as the corresponding refrigeration loads. The accuracy of these estimates, in turn, depends upon accurate estimates of the surface heat transfer coefficient for the cooling or freezing operation. This project reviewed heat transfer data for the cooling and/or freezing of foods. A total of 777 cooling curves for 295 food items were obtained from an industrial survey and a unique iterative algorithm, utilizing the concept of ‘equivalent heat transfer dimensionality’, was developed to obtain heat transfer coefficients from these cooling curves. Nine Nusselt–Reynolds–Prandtl correlations were developed from a selection of the 777 heat transfer coefficients resulting from this algorithm, as well as 144 heat transfer coefficients for 13 food items, collected from the literature. The data and correlations resulting from this project will be used by designers of cooling and freezing systems for foods. This information will make possible a more accurate determination of cooling and freezing times and corresponding refrigeration loads. Such information is important in the design and operation of cooling and freezing facilities and will be of immediate usefulness to engineers involved in the design and operation of such systems.