This research was conducted to better understand the mechanisms of heat and moisture transfer of walnuts during drying in a fixed-bed column dryer with heated air, and to develop a mathematical model to simulate this process. Drying experiments were performed under constant temperature and step-down temperature conditions (43, 55, 65 and 75 °C) with fixed air velocity of 0.7 m/s. The nonequilibrium transfers of liquid moisture, water vapor and heat within the single walnuts (both shell and kernel), as well as between the walnuts and drying air in the column were simulated and studied using a finite element method. The numerical results showed that the distribution of temperature and moisture content in the single walnuts played an important role in the heat and moisture transfer in the deep-bed column. The drying characteristics of walnuts were significantly affected by the non-uniformity of drying conditions surrounding the walnuts (particularly for air temperature and humidity) along the column heights. The predicted values of walnut moisture contents, temperatures, and air relative humidity at different column heights agreed well with the experimental values (R_adj² > 0.972) under different drying conditions. This study established the theoretical basis for improving and optimizing the walnut drying process in deep-bed dryers. The developed mathematical model can be used to describe the drying characteristics and understand the mechanisms of walnuts drying in deep bed dryers.

进行这项研究是为了更好地了解在固定床柱式干燥机中使用加热空气干燥期间核桃的热量和水分传递机制，并开发一个数学模型来模拟该过程。干燥实验在恒定温度和降压温度条件（43、55、65 和 75°C）下进行，固定风速为 0.7 m/s。使用有限元方法模拟和研究了单个核桃（壳和仁）内以及核桃与柱内干燥空气之间的液体水分、水蒸气和热量的非平衡传递。数值结果表明，单个核桃的温度和水分分布对深床柱内的热量和水分传递起着重要作用。核桃的干燥特性受到核桃周围干燥条件（特别是空气温度和湿度）沿柱高的不均匀性的显着影响。不同柱高核桃水分含量、温度和空气相对湿度的预测值与实验值吻合较好(R_adj ²  > 0.972) 在不同的干燥条件下。本研究为改进和优化深床烘干机核桃烘干工艺奠定了理论基础。开发的数学模型可用于描述核桃在深床干燥机中的干燥特性并了解核桃的干燥机理。