Study Of Thermal Conductivity Measurement System And Temperature Field Simulation Of Postharvest Fruits And Vegetables

With the rapid development of life sciences, there have been so many problems that are difficult to be solved from the biological point of view. So do the explorations of hot problem and the hot phenomenon of plant with the life characteristic. For the study of the plant life sciences, people have had better mastering to the hot response law of plant in terms of biology, but mostly overweight qualitative and experience analysis. There are much more research on the quantitative research of the plant heat transfer at abroad, but less report at home. Not only can a lot of complicated biological phenomenas be explained succeedly with the aid of the thermal physics model, but also it becomes possible to make phenomenological plant life sciences towards quantitative study.Knowledge of the thermal properties of the biological tissues is important for investigation the thermal models. It is a fundamental research to determine the thermal conductivity of biomaterials, with respect to the modeling of bioheat transfer, predicting the the biological body's changing routine of tempreture field, investing the ability of the heat transport of these samples, and further studying bio-heat transfer. The first objective of this thesis is to set up an experimental apparatus for determining thermal conductivities of fruits and vegetables. Then the transient model for investigating the temperature field inside the samples is established. This is an important foundation for further study the heat transfer law of the plant systems. Based on the idea mentioned, the issues investigated in this thesis are listed as following:1 .We will first study the principles of the hot probe for thermal conductivity measurement. Then, some factors will be analyzed, and the measurement uncertainty can be compensated through the calibration using the standard samples. Experimental results show that the measurement precision can be improved after the probe is calibrated by the Ultrapure water. Therefore, the practicability of the hot probe for measuring thermal conductivity of the fruits and vegetables is validated.2.Based on the characteristic of fruits and vegetables, an experimental apparatus is set up for measurement of the thermal conductivity of them. The advantages are that it is stable, and easy to control and adjust. Considering the high water content of fruits and vegetables, and metabolism of the tissues, the hot probe with small scale is adopted, in order to shorten the measurement duration, minimize the affected area and the damage to the tissues, restraint the distribution of the environment, and operate on-line conveniently. The routine of the measurement are all performed by the micro-computer, including the test, the data collection, the data processing, and the result output. Experimental results show that the measurement is convenient and the reproductivity of the results is fine.3.We have tested the standard samples, such as the glycerin, glycol, and alcohol, and the fruits and vegetables, such as the banana, tomato, and apple. Compared with the values in literatures, the measurement uncertainty can be controlled within 3%. These results validate the proposed probe is suitable for engineering and scientific research. The application of the proposed technology can be expected in future.4.Using the proposed measurement system, we have investigated the thermal conductivities of fruits and vegetables at different ripening stage and of juices having different concentrations. The results show that the thermal conductivities of fruits and vegetables with different variety, and that of the same variety fruits and vegetables and that of fruit juices having different concentrations are different, but lies between 0.3 and 0.6 W/m'k, which are all less than that of the water.5.The key indexes reflecting physiological biochemistry and quality of fruits and vegetable were simultaneously tested, including: density, water content, free water content, so did juice. Then draw the inherent relatinship with the thermal conductivities. The results show that the thermal conductivities of fruits and vegetables with different variety have obvious relativities with the density, water contend free water content > solube solids content and rigidity. The thermal conductivities of the same variety tomatoe fruits gradually decline from green ripening to pink ripening and to red ripening, but rise to over ripening. The thermal conductivities of tomato fruits decline with the solube solids contents, change with the water contents free water content and the density but no obvious rules.The apple juice concentration has an important influence on the thermal conductivity. And an empirical expression model of predicting the thermal conductivity of apple juice with concentration is obtained. The results...