Research On The Stress Distribution Of The Fruit And Vegetable Particles During Dehydration

When drying fruit and vegetable spherical particles by hot air, the phenomenons such as tissues shrinkage of material, the radius decreases, volume decreases and so on will appear along with the water's spreading out and dehydration, it will also cause larger moisture gradient in the direction of water diffusion, which results in a larger radial stress. When the volume of material is reduced, a relative large volume before drying will be reduced to a smaller size and a larger extrusion is generated in the tangential direction which results in a larger shear stress. This paper focuses to the systematical study of shrinkage and stress distribution of the materials during drying fruit and vegetable particles by hot air.First, drying stress model of spherical fruit and vegetable particles is built basing on the equilibrium conditions and viscoelastic theory in the rheological. Let the stress produced during the drying process of materials apply to each discrete units accroding to the nonlinear relationship of the average dry basis moisture content of materials changes. Stress distributions in spherical fruit and vegetable particles were studied respectively on viscoelastic model and elastic model. The results show that:during the drying process, the radial tangential compressive stress and the tensile stress maximums of the elastic model are bigger than those of the viscoelastic model. The maximum of the tangential tensile stress appeares in the exoexine while the maximum of the radial stress appeares in the inner. The maximum of the radial stress is about 0.9 Mpa, while the tangential tensile stress is 1.2 Mpa, which is slightly bigger. The stress reverse phenomenon will be brought out a period of time after the stress of viscoelastic model reaches maximum, the radial stress will be transformed into the radial tensile stress, tensile strength of the the exoexine will be transformed into the tangential compressive stress, and the tangential compressive stress in the inner will turn into the tangential tensile stress. The viscoelastic model is more suitable to describe the fruit and vegetable particles.Second, basing on the above comparisons between the viscoelastic model and the elastic model, calculate by using the viscoelastic model with the hot air of different initial moisture content and the same relative humidity in order to keep the same equilibrium moisture content, then calculate with the hot air of the same initial moisture content and different relative humidity to make different equilibrium moisture contents. The results show that:the drying stress produced from a higher initial moisture content is larger than that produced from a lower initial moisture content. Different relative humidity which limit 6.5% to 10% of the hot air will hardly affect maximum of the stress during the drying process. In the beginning of the drying period, the initial moisture content affects the radial stress and the tangential stress a lot, while the different relative humidity which limit 6.5% to 10% of the hot air make hardly effects. In the end of the drying period, both initial water content and relative humidity of the hot air produce little effects on the radial stress and tangential stress.At last, making the initial temperature of materials respectively to be 30℃,45℃and 60℃before drying, then taking different drying temperatures to make it the same as the initial temperature, which will keep the whole drying process in a constant temperature and without temperature gradient in the materials. Using the viscoelastic model to study the effects of the stress distribution of the material, which are caused by the temperature of the drying hot air. The results turn out to be:the stress curve reaches the maximum earlier than the other stress curves when the temperature is 60℃and the maximum of the stress is also larger than other lower temperature.