Researches Of Water Loss Mechanism At The Micro-level During The Tea Drying Process

The traditional study of drying technology emphasis on the macroscopic level,ignoring the mechanism of water loss at the micro-level.The mechanism of water loss at the micro-level is studied in order to infer the macroscopic characteristics of the tea drying.This idea has opened up a new way for the study of the drying mechanism of tea.The influence of pressure,temperature,wind speed and humidity of the air during the drying process of tea are explored by the coupling of theoretical analysis,experiment and numerical simulation methods.The main work and results are as follows:(1)Tea's tissue fluid flow equations: mass transfer equation,continuity equation,momentum equation and energy equation are deduced based on the existing theories of the micro-fluid flow,considering the water vapor diffusion effect on the fluid flow in porous media.(2)The porosity of tea is determined about 0.46 with the experiment.The internal diffusion coefficient of water molecules is determined about 2×10-13m2/s with experiment and simulation.The moisture gradient and temperature gradient within the tea during the drying process are simulated with COMSOL Multiphysics and the coefficient of water diffusion.Results show that during the drying process tea form inside to outside moisture gradient,hot air drying form outside to inside temperature gradient,while microwave drying form opposite temperature gradient.(3)Tea-fluid's velocity and temperature rise under different pressure differentials are simulated with COMSOL Multiphysics.Results show that the presence of pressure differential leads to a significant increase in speed and temperature.So think about the resultsdynamic pressure method can be put forward during the drying process of tea.(4)The water vapor concentration distribution and temperature distribution on the surface under different wind speed are simulated by COMSOL Multiphysics.Results show that if wind speed less than 6.1m/s,it will influence water vapor diffusion;if wind speed greater than 10.1m/s,it will lead to nonuniform drying.(5)The orthogonal experiment was carried out for drying process involves factors.Multiple regression analysis was carried out on the results of orthogonal test.The analysis results show that the temperature and wind speed are most important influence factors during the drying process.Tea's drying theory provides the basis for the actual production,has important significance to save energy and improve production efficiency.