共查询到20条相似文献,搜索用时 15 毫秒
1.
Q. T. PHAM 《Journal of food science》1987,52(3):795-800
In food freezing situations where heat flows at different rates from the two surfaces of a slab, a Plank-type converging-front model enabled the position of the thermal center and hence the freezing time, to be calculated. The thermal center was defined as the point where the freezing fronts meet, i.e. the last point to cross the “mean freezing temperature”. Finite-difference calculations showed that this model successfully accounted for the effect of asymmetry in heat transfer coefficient or coolant temperature and enabled previously developed simplified prediction methods to be applied to asymmetric situations. 相似文献
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Since there was no procedure available for estimating freezing time of rectangular or finitely cylindrical food through manual calculations, the present study was initiated to develop such a procedure. For this development, our computerized model was used together with statistical techniques. A design for screening tests was used to identify eight dimensionless groups, which influences significantly the freezing rate, out of 22 independent groups. Among the eight groups, only two were related to the thermophysical properties of food. Through the joint application of a central composite design of experiments and of the computerized model, algebraic formulae containing the eight significant groups were derived for the freezing time estimation. Their validity was verified through experimental freezing processes. 相似文献
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Two simple methods for predicting the freezing times of rectangular bricks, slabs, cylinders, and spheres in situations where boundary conditions change with time are proposed. These methods are based on numerical integration of a simple differential equation derived from a previously proposed modification to Plank's equation. The methods were tested against a three time-level finite difference scheme for varying cooling medium temperatures and surface heat transfer coefficients. Agreement was generally good (difference < 10%) between the two methods and the corresponding finite difference solution. 相似文献
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Existing food freezing time prediction methods yield widely differing results at high Biot numbers (internal resistance controlling), a situation that could not be previously resolved due to lack of data. Freezing tests were carried out on slabs of Tylose gel in a plate freezer to obtain more data, which alone and in combination with previous data, were used to test the methods of Cleland and Earle, Hung and Thompson, Pham, and finite differences. Good agreement was obtained with Pham's methods and with finite differences. Biot number, freezing rate and supercooling had negligible effect on the accuracy of these methods. The accuracy of the finite difference method has been significantly improved by our new thermal property data for Tylose gel. 相似文献
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Parametric analyses were performed to examine heat transfer in freezing of spheroidal (including prolates and oblates) and finitely cylindrical foods through computerized simulation which considered volumetric changes. The boundary conditions included convective surface heat and moisture transfer and radiative surface heat transfer. Regression equations for freezing time estimation were developed through screening and central composite analyses and verified experimentally using a food simulator with physical properties similar to lean beef. Sensitivity analysis showed the convective surface heat transfer coefficient was the most significant factor affecting accuracy of calculated freezing time followed by operational temperatures, ther-mophysical properties and food dimensions. Effect of volumetric expansion on freezing time estimation was not significant. 相似文献
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简单形状食品冻结时间计算方法的分析研究 总被引:1,自引:0,他引:1
冻结时间对食品的冻结质量和能耗有重要意义.通过对几种简单形状食品冻结时间计算模型和相应公式的分析和计算,对它们在特定工况下应用的准确性和可靠性进行了比较,并对相关计算参数的选取给出了说明和建议. 相似文献
8.
Freezing Time Predictions for Brick and Cylindrical-Shaped Foods 总被引:1,自引:0,他引:1
A simplified model previously developed for freezing time calculations in plate freezers is extended to systems with two or three dimensional heat flow. The model combines Plank's equation with the unsteady heat transfer solutions for bodies with constant properties, through the addition of pre-cooling, change of phase and tempering times. Average thermal properties, different for each period are used in order to take into account their change with the ice content along the freezing process. Freezing time predictions show a maximum difference of 10% with respect to freezing experiments performed with meat blocks shaped as cylinders or rectangular bricks. Processing times from 0.7–5 hr were compared with satisfactory agreement. 相似文献
9.
A modification to an empirical food freezing time prediction formula is proposed that allows the formula to be used for a range of final product temperatures. Over a large data set (275 runs) the percentage difference between experimentally measured times and predictions has a mean of 0.2% and a standard deviation of 6.8% for the improved formula compared to -2.4% and 8.5%, respectively, prior to modification. Ninety percent of the predicted freezing times by the new method were within ± 11% of the experimentally measured times and ± 9% of predictions by an accurate finite difference scheme. This performance compares favorably with other published freezing time prediction methods. The same type of modification for varying final product temperature may be suitable for other empirical formulae. 相似文献
10.
Assessment of Freezing Time Prediction Methods 总被引:2,自引:0,他引:2
A systematic procedure for testing the accuracy of freezing time prediction methods is discussed. It involves carrying out freezing time predictions by an accurate numerical method as well as the method under test, and careful cross-checking of prediction errors for correlation with a number of physical parameters. Criteria are given for selection of an appropriate numerical method. The need for systematic cross-checking is illustrated by comparisons between four recent prediction methods. The comparisons show the strengths and weaknesses of the four methods. They also indicate general shortcomings in available methods and data, and so highlight those areas in which future research might be most profitable. 相似文献
11.
A model was developed consisting of a modified Plank's equation to estimate phase change time and two unsteady state cross-product heat transfer equations for estimating precooling and tempering times. It accurately predicted total time to proceed from an initial temperature above the freezing point to a final temperature of ?18°C. A correction factor was developed and incorporated in the P term in Plank's equation to correct for the effect of initial and freezer medium temperatures and heat transfer coefficients. The model, tested over a broad range of freezing conditions, had a mean absolute error of 5.9% in predicted values relative to experimental values. 相似文献
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A Simplified Model for Freezing Time Calculations in Foods 总被引:1,自引:0,他引:1
A model is proposed for freezing time calculations which combines Plank's equation with the unsteady heat transfer solutions for the cooling of a slab of constant properties through the addition of pre-cooling, change of phase and tempering periods. The change of thermal properties with the ice content is taken into account by proposing average values for the different periods. No adjustable parameters are used in developing the model. Results are compared for the case of beef freezing with those obtained numerically by using a heat transfer model with simultaneous change of phase and with experimental measurements showing good agreement. 相似文献
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食品挤压过程的分析方法——RTD分析、流变分析 总被引:1,自引:1,他引:0
挤压过程的分析方法分为6种,着重介绍RTD分析和流变分析2种方法.首先介绍了停留时间分布(RTD)的测定方法、参数求解以及模拟分析,然后论述了流变特性的测定方法以及流变模型的建立.最后指出了今后的研究方向. 相似文献
14.
SUDHIR K. SASTRY 《Journal of food science》1984,49(4):1121-1127
Equations were developed for prediction of freezing times of foods of slab geometry, for three different boundary conditions, using a method of solving for enthalpy per unit volume instead of temperature. Experimental verification was performed for the operation with a convective heat transfer boundary condition, using ice cream and green peas in a granular packed bed as test materials. Results indicate good prediction of freezing times over a range of conditions. Results of thawing tests are less satisfactory if the ambient temperature lies in the zone of rapidly changing food properties. A comparison with literature predictions is presented. 相似文献
15.
Any prediction of freezing time for a food product depends on accurate heat transfer coefficients. The purpose of this investigation was to examine experimental procedures for accurate determination of convective transfer coefficients during freezing of food products. Measurements of heat transfer coefficients during cooling of any acrylic transducer and ground beef with similar size and shape were conducted at air temperatures below initial freezing point of the product. The results indicate that accurate measurements of the coefficients can be achieved through nonlinear regression analysis of temperature histories within an acrylic transducer. 相似文献
16.
An improved analytical method for predicting the freezing time with one dimensional heat transfer for slabs was developed. Tylose- MH-1000 was used as a model test material. The new model is similar to Plank's equation, but has a more theoretical basis. Total enthalpy difference instead of latent heat and weighted average temperature difference instead of the temperature difference between initial freezing point and freezer temperature were used in the improved prediction method. Linear regression was used to estimate shape parameters. Four different foods were used to test the model. Predicted times for foods were within 6% of the measured times. 相似文献
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The thermal behavior of a conductive canned food during retorting was represented using an input-output linear system, with a finite order N, and a time-delay. The input of the system was the retort temperature, the output was the temperature at the thermal center of the can. This model was applied for a typical sterilization process composed of two isothermal steps. The can internal temperature was expressed as a sum of N exponential terms in which appeared the constant holding retort temperatures, the come-up and come-down times and N pairs of heat penetration parameters identified in a preliminary experiment. The model was tested on a potato mash packed in metal cans processed in a vertical still retort. Temperatures estimated by the model agreed closely with those measured during thermal processing. 相似文献
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《Journal of food science》2015,80(8):iii-iii
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C. S. CHEN 《Journal of food science》1987,52(2):433-435
A simplified freezing point depression (FPD) equation was derived for calculating water activity (aw) of food systems. The aw values as calculated by FPD data agreed with literature data for a variety of foods to within ± 0.01 aw units. The FPD equation was found particularly useful for calculating aw values of frozen foods at temperatures between 273.1.5–233.15 °K (0 to ?40°C). 相似文献