[6-1130-P] Postharvest/Food Technology and Process Engineering (6th)

[6-1130-P-23] Inverse Method Using Heat Transfer Simulation to Estimate Thermal Diffusivity of Agricultural Products

*Yoshiki Muramatsu¹, Masanori Hashiguchi², Eiichiro Sakaguchi¹, Shotaro Kawakami¹ (1. Tokyo University of Agriculture(Japan), 2. Keisoku Engineering System Co., Ltd.(Japan))

Keywords:Thermal diffusivity, Heat transfer simulation, Finite element method, Agricultural products, Inverse problem

The thermal diffusivity is an important thermophysical property needed in modeling and computations of transient heat transfer in basic food processing. In addition, the prediction of nutritional and microbial changes occurring in food during thermal processing requires knowledge of thermal diffusivity of foods. The measurement methods of thermal diffusivity are classified into direct measurement, to which the present work belongs, and indirect measurement. The thermal diffusivity can be obtained from the experimentally determined values of thermal conductivity, specific heat, and density in the indirect measurement. This indirect measurement requires much time and experimentation. Some direct measurement methods need expensive and/or special devices. In addition, it is frequently necessary to do the complicated calculation procedures to determine thermal diffusivity under direct measurement methods. Therefore, it would be useful to easily determine thermal diffusivity with simple and inexpensive devices. The thermal diffusivity of some agricultural products and foods have been measured by Dickerson method. The calculation in the Dickerson method is based on the analytical solution of the heat conduction equation. Several kinds of the analytical solution of the heat conduction equation have been used to estimate the thermal diffusivity of the agricultural product based on the temperature profiles of the material. However, the geometry and/or the boundary conditions are strictly limited for those methods. The objectives of this study were to propose a new determination method of thermal diffusivity and to estimate the thermal diffusivity of some agricultural products using that new method. Thermal diffusivities of three kinds of vegetable (burdock, carrot, and radish) were estimated using an inverse technique. The burdock and radish were cut into a cylinder (diameter (D) 20 and height (H) 100 mm). The carrot was created three kinds of geometry: cylinder (D = 20, H = 100 mm), cylinder (D = 20, H = 20 mm), and disk (D = 40, H = 10 mm). Each sample was fitted with a needle-type thermocouple to measure the center temperature. The samples were heated in a water bath at 90°C. The rotational axisymmetric 2-dimensional transient heat conduction problem for radial coordinates and the 3-dimensional transient heat conduction problem for cartesian coordinates were numerically solved by a finite element method using the commercial finite element software: COMSOL Multiphysics^®. The thermal diffusivity of each sample was determined by an ordinary nonlinear least squares method using the MATLAB^® which is a programming platform designed specifically for engineers and scientists, and a numerical optimization technique using COMSOL Multiphysics^®, respectively. The thermal diffusivity values of the samples ranged from 1.1 × 10^-7 to 1.5 × 10^-7 m²/s by the ordinary least squares method. A significant difference was not statistically recognized among the values of thermal diffusivity of all sample sizes and shapes for the carrot. Also, between the rotational axisymmetric 2-dimensional analysis and the 3-dimensional analysis, there was no significant difference for all samples. The advantages of this method are that the device and the estimation method are simple, inexpensive, rapid, and can apply to various shapes of a sample and the dimension. The results obtained in this study will be useful in the design of equipment and in calculations for the thermal processing of vegetables.