## 3 Methods
### Tangerine Peel Spatial Position Feature Extraction
#### 3.1.2 Mathematical Model of Drying Chamber  
The mathematical model of the drying chamber was developed using ANSYS Fluent (2022 R1), SpaceClaim (2022 R1), Meshing (2022 R1), and other simulation tools to investigate the heat transfer and airflow mechanisms within the chamber during the drying process. In this model, it is assumed that the flow field in the drying chamber is a steady-state viscous flow [23]. The simulation is based on a standard turbulence model [24], and the energy equation [18] is expressed as Equation (4):  
\[\frac{\partial(\rho\varphi)}{\partial t}+\frac{\partial(\rho u\varphi)}{ \partial}+\frac{\partial(\rho v\varphi)}{\partial}+\frac{\partial(\rho \omega\varphi)}{\partial}=\frac{\partial}{\partial x}(\Gamma\frac{\partial \varphi}{\partial x})+\frac{\partial}{\partial y}(\Gamma\frac{\partial \varphi}{\partial y})+\frac{\partial}{\partial z}(\Gamma\frac{\partial \varphi}{\partial z})+S \tag{4}\]  
Figure 3: Physical model of drying chamber.  
Figure 2: Research framework. (RNN: recurrent neural network; GRU: gated recurrent unit; LSTM: long short-term memory; BiLSTM: bidirectional long short-term memory).  
Here: \(\varphi\)--generalized variable; \(\Gamma\)--diffusion coefficient; \(S\)--source item; \(\rho\)--density; \(t\)--time; \(u\)--\(x\)-direction velocity; \(v\)--y-direction velocity; \(\omega\)--z-direction velocity.  
For the boundary conditions, a velocity inlet was implemented, with the inlet direction set perpendicular to the boundary and the velocity distributed uniformly. The wind speed was fixed at 0.5 m/s, and the turbulence intensity was estimated to be 3.8% using an empirical formula [10]. The inlet temperature was maintained at 313.15 K (40 \({}^{\circ}\)C), and a pressure step of 12 Pa was applied for the circulating fan. The outlet was defined as a free outflow boundary to ensure that all hot air exited the chamber completely. Solid walls were treated with a non-slip condition, and their temperature was set to 25 \({}^{\circ}\)C. The coordinate system was configured with the center of the bottom surface of the drying chamber as the origin. Figure 4 shows the volume rendering and temperature cloud distributions on the planes at y = 0.8, y = 0.535, and y = 0.35 as calculated by Fluent, while the top views of these planes are depicted in Figure 5. The velocity inlet was selected as the boundary condition. The inlet direction is perpendicular to the boundary and evenly distributed. The wind speed was set to 0.5 m/s, and the turbulence value was estimated to be 3.8% by the empirical formula [10]. The inlet temperature was 313.15 K (40 \({}^{\circ}\)C), and the pressure step of the circulating fan was 12 Pa. The outlet condition was set to free outflow, and the hot air at the outlet flows completely from the outlet. The solid wall used a non-slip condition, and the wall temperature was 25 \({}^{\circ}\)C. The coordinate axis was set as shown in the figure, and the center of the bottom surface of the drying chamber was the origin. The volume rendering diagram and the temperature cloud distribution on the surface of y = 0.8, y = 0.535, and y = 0.35 calculated by Fluent are shown in Figure 3. The top view of each plane is shown in Figure 5.  
The temperature cloud maps of the upper, middle, and lower layers clearly reveal noticeable temperature gradients, indicating significant temperature differences across different vertical positions within the drying chamber.