Study On Three-field (Thermo-moisture-mechanics) Coupled Numerical Simulation And Drying Characteristic Of Heat Pump For Shiitake Mushroom

Shiitake mushroom is one of the important agricultural products in China known as �precious food produced in the mountain forest�.Considering the high initial moisture content and easy to mildew of the shiitake mushroom,the process of drying and dehydration is deemed to be the important operation of production and processing.The heat pump drying has been widely applied in many industries.After the adequate investigation and analysis for the current situation of heat pump drying of shiitake mushroom,it is found that there are still some problems in the relevant drying theory,such as unsound mathematical model and insufficient understanding of drying characteristics.Therefore,the study is carried out from the three aspects of drying experiment,numerical simulation technology and heat pump drying characteristics of shiitake mushroom.The specific contents and results of study are as follows.Design and completion of drying experiment study of shiitake mushroom.The scheme to determine the moisture content and shrinkage rate of shiitake mushroom materials is designed;and the preferred process and operating temperature and humidity range is selected according to the existing drying experience of and producing shiitake mushroom to complete the drying experiment under the condition of constant temperature and humidity.According to the experimental data,the main factor affecting the moisture content of shiitake mushroom in the drying process is the temperature of hot air.Increasing temperature is beneficial to shorten the drying time.The main factor affecting the shrinkage of shiitake mushroom in the drying process is the humidity of hot air.Increasing the relative humidity properly is beneficial to the drying quality of shiitake mushroom.For the shortcoming of not considering volume shrinkage in the current heat pump drying mathematical model of shiitake mushroom,the shiitake mushroom is deemed as linear elastic porous media,and a three-field(thermo-moisture-mechanics)coupled drying mathematical model for the drying process of shiitake mushroom is built considering the influence of thermal strain and hygroscopic strain in the drying process.Meanwhile,it is to derive the key physical parametric expressions such as mass transfer coefficient and evaporation rate of three-phase structure of substrate,liquid water and water vapor in the shiitake mushroom in the drying process,and to achieve the closure of mathematical equations;and the mathematical model built is solved efficiently based on the COMSOL software.The numerical simulation results of the proposed method are in line with the experimental data,which verifies the accuracy of the proposed mathematical model and solution method.The drying characteristics under the condition of heat pump are studied based on the three-field(thermo-moisture-mechanics)coupled mathematical model for the drying process of shiitake mushroom:(1)the evolution process of drying characteristics of shiitake mushroom is studied under the condition of constant temperature and humidity,and it is found that the heat is transferred from the surface to the interior and the moisture is transferred from the interior to the surface during the 12-hour heat pump drying process of shiitake mushroom,while the porosity and shrinkage of the whole material shows more uniform change.(2)The effect of drying parameters on drying process is studied and the results show that the drying temperature,humidity and wind speed have a great effect on the shrinkage rate and moisture content of mushroom,however,the minor changes of temperature,humidity and wind speed have little effect on the drying process.It is beneficial for energy-saving to adopt the drying process with increasing temperature and decreasing humidity and wind speed.