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A thermistor based method for measurement of thermal conductivity and thermal diffusivity of moist food materials at high temperatures.

机译：一种基于热敏电阻的方法，用于在高温下测量潮湿食品的热导率和热扩散率。

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摘要

The purpose of this research was to assess the suitability of the thermistor based method for measuring thermal conductivity and diffusivity of moist food materials at high temperatures. Research focused on aspects of calibration, thermal contact in solid food materials, natural convection in liquid media and the performance in moist food materials at high temperatures.; Thermistor probes were constructed in house and calibrated in three materials of known thermal conductivity and diffusivity, water, glycerol, and a heat transfer fluid, HTF 500. With few exceptions, the calibrated probe estimated thermal properties with an error of less than 5%, over the range of thermal properties spanned by the those of the calibration media. An alternate calibration using two media was also investigated. It was found to give better accuracy over a more limited range.; Thermal contact in potato and lean beef was investigated through a comparative study that used a miniature line heat source probe as a reference method. The food materials were measured at 25, 50 and 100°C. Good agreement was found between the measurements with the line heat source probe and the bead thermistor probe, indicating adequate thermal contact at the thermistor probe.; The effect of fluid viscosity and the magnitude of the temperature step on the occurrence of natural convection was studied for aqueous solutions of a thickening agent. During a sample time of 30 seconds, convection was absent in solutions with a viscosity of 25 cp or greater, when measured with a temperature step of 1.5 and 2.5°C, and in solutions with a viscosity of 50 cp or greater, when measured with a temperature step of 5.0°C. A Rayleigh number was defined to study the notion of a critical Rayleigh number at the onset of convection. This study found that when the Rayleigh number was below 25, convection could not be demonstrated. For a Rayleigh number of 55 and higher, convection was observed.; The performance at high temperatures in food materials was studied through tests in tomato concentrate and in a liquid food supplement. Tomato puree and tomato paste were sampled at 100, 130 and 150°C. The thermal conductivity of tomato puree at 100, 130 and 150°C was measured as 0.638, 0.645 and 0.647 W/m°C respectively. The thermal diffusivity was 1.63, 1.64 and 1.62 10--7m2/s respectively. For tomato paste at 100, 130 and 150°C, a thermal conductivity was obtained of 0.590, 0.597 and 0.534 W/m°C respectively. The thermal diffusivity was 1.63, 1.84 and 2.36 10--7 m2/s respectively. With some notable exceptions the results of this study agreed well with Choi and Okos (1983). A liquid food supplement was also studied at 95 and 150°C. The thermal conductivity of the food supplement decreased with increasing solids content from 0.62 W/m°C at a solids level of 15% to 0.41 W/m°C at a solids level of 50%.; The results of this study indicate that the thermistor based method was suitable for measuring thermal conductivity and diffusivity of moist food materials at high temperatures. However, the type of thermistor used in the research, a glass encapsulated thermistor, was too fragile for routine work. In particular the high temperature use of the glass thermistor was impacted by its susceptibility to fracture.
机译：这项研究的目的是评估基于热敏电阻的方法在高温下测量潮湿食品材料的导热系数和扩散系数的适用性。研究侧重于校准，固体食物材料中的热接触，液体介质中的自然对流以及高温下潮湿食物材料中的性能。热敏电阻探头是在室内构造的，并用三种已知的导热率和扩散率的材料，水，甘油和传热流体HTF 500进行校准。除少数例外，校准后的探头估计的热特性误差小于5％，在校准介质的热性能范围内。还研究了使用两种介质进行的替代校准。发现在更有限的范围内可以提供更好的精度。通过使用微型线热源探针作为参考方法的比较研究，研究了马铃薯和瘦牛肉的热接触。在25、50和100°C下测量食品原料。在管线热源探头和磁珠热敏电阻探头的测量结果之间找到了很好的一致性，表明在热敏电阻探头上有足够的热接触。对于增稠剂的水溶液，研究了流体粘度和温度步幅对自然对流发生的影响。在30秒的采样时间内，当温度步长为1.5和2.5°C时，粘度为25 cp或更大的溶液中不存在对流；当粘度为50 cps或更高时，在对流中不存在对流。温度阶跃为5.0°C。定义瑞利数以研究对流开始时临界瑞利数的概念。这项研究发现，当瑞利数低于25时，无法证明对流。对于瑞利数为55或更高的数值，观察到对流。通过在番茄浓缩汁和液态食品补充剂中进行测试，研究了食品材料在高温下的性能。番茄泥和番茄酱在100、130和150°C下采样。番茄泥在100、130和150°C的热导率分别为0.638、0.645和0.647 W / m°C。 10--7m2 / s的热扩散率分别为1.63、1.64和1.62。对于100、130和150℃的番茄酱，分别获得0.590、0.597和0.534W / m℃的热导率。热扩散率分别为1.63、1.84和2.36 10--7 m2 / s。除了一些显着的例外，这项研究的结果与Choi and Okos（1983）一致。还在95和150°C下研究了液态食品补充剂。随着固体含量的增加，食品补充剂的热导率从固体含量为15％时的0.62 W / m°C降低到固体含量为50％时的0.41 W / m°C。这项研究的结果表明，基于热敏电阻的方法适用于在高温下测量潮湿食品的热导率和扩散率。但是，研究中使用的热敏电阻类型为玻璃封装的热敏电阻，对于日常工作而言太脆弱了。尤其是玻璃热敏电阻的高温使用受到其断裂敏感性的影响。

著录项

  • 作者

    van Gelder, Maarten Frans.;

  • 作者单位

    Virginia Polytechnic Institute and State University.;

  • 授予单位 Virginia Polytechnic Institute and State University.;
  • 学科 Agriculture Food Science and Technology.; Engineering Agricultural.
  • 学位 Ph.D.
  • 年度 1998
  • 页码 160 p.
  • 总页数 160
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类 农产品收获、加工及贮藏;农业工程;
  • 关键词

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