RADIAL ONE-DIMENSIONAL HEAT AND MOISTURE COUPLING MODEL FOR THE PREHEATING STAGE OF HOT-AIR DRYING OF CYLINDRICAL MEDICINAL MATERIALS

This paper focuses on the preheating equilibrium stage in the hot-air drying process of cylindrical medicinal materials. A radial one-dimensional unsteady heat and moisture coupling model is established under the assumptions of axisymmetric and isotropic material. Heat transfer inside the material follows Fourier’s law, moisture migration follows Fick’s law, and the surface exchanges heat and moisture with the drying environment through convective transfer, which is described by third-kind boundary conditions. The measured drying-room temperature and moisture content are processed by piecewise linear interpolation and adopted as time-varying boundary conditions, and the governing equations are solved by an explicit finite difference scheme. During 0–1800 s, the material is heated as a whole while pre-drying occurs mainly in a thin surface layer. At 1800 s, the surface and center temperatures reach 36.79°C and 33.58°C, respectively. The surface moisture content decreases from 2.55 to 1.45 kg/kg on a dry basis, whereas the center moisture content remains close to its initial value. Convergence tests and parameter sensitivity analysis show that the surface temperature is most sensitive to the convective heat transfer coefficient, while the surface moisture content is most sensitive to the convective mass transfer coefficient and the diffusion pre-exponential factor.