Model analytical and verification of heat and moisture characteristics in litchi drying

Abstract: The dynamic characteristics of temperature and moisture distribution within litchi, which is the key influence on drying quality formation and energy-consumption, is the important technical foundation problem for litchi postpartum processing. In order to reveal the dynamic characteristics of temperature and moisture distribution within litchi in unsteady drying process, guide the drying process design, improve the drying quality, and reduce drying energy, based on the diffusion model, the characteristic parameter of litchi is studied. Meanwhile, the drying characteristics representation of litchi is investigated on the basis of the drying characteristics test on litchi peel, pulp, and kernel. When the relative humidity is 10 percent and temperatures range from 35℃ to 50℃, the drying constant and dry basis equilibrium moisture content is researched, and the results are 0.5965 h-1-0.6717 h-1 and 7.55 percent -60.92 percent, respectively. While the relative humidity is 60 percent, and the temperature is 40℃ and 50℃, the researchers have determined that the drying constant is 0.076 h-1 and 0.098 h-1, respectively, and that the dry basis equilibrium moisture contents is 211.35 percent and 141.06 percent, respectively. The results show that the drying conditions have great influence on the drying characteristic parameters of litchi. The water vapor pressure inside litchi exists at an extreme value point during litchi drying, and after that, the drying process is closed to isothermal processes. At the beginning of litchi drying (within 20 min), the internal temperature difference of litchi is the greatest. The thermal conductivity coefficients of peel, pulp, and kernel have some differences, but in the same drying time, they have less effect on the internal temperature distribution of litchi. By using the Lars transformation, the heat conductivity and thermal balance equations are solved simultaneously, and the internal temperature, water vapor partial pressure distribution and its analytic formula of variation law of litchi are achieved. The validation test is investigated when the drying medium temperature is 50 ℃, the relative humidity is 10 percent, the litchi initial temperature is 10 ℃, and the dry basis moisture content is 388 percent. The results show that the measured values of the internal temperature change of litchi is consistent with the analytical values, which confirm the reliability of the analytical model. The research results are of great significance for providing reference to reveal the drying quality mechanism of litchi, enriching analytical theory of the litchi drying thermal and mass transfer, as well as improving the drying quality of litchi, guiding drying design, achieving high efficiency, and saving energy.