## 3 Materials and Methods
### Numerical Analysis  
A numerical simulation of the cold storage was taken up by employing a 3D Experience simulation software (version: 2024x.D31.R426 rel.202403212040) to analyze its thermal behavior. A general outline of the process undertaken is displayed in Figure 2.  
Figure 1: Experimental setup (**a**) front view (**b**) side view. Figure 2: Procedure undertaken for numerical simulation. Using 3D Experience software, a 3D cold storage unit model was created as the first step for initialization, with all the essential elements such as a storage space, external PUF insulation, VCR system, and stainless steel rack incorporated. Figure 3 illustrates the CAD model of the cold storage unit. The cooling effect provided by the evaporator is displayed as an inlet in the model to allow for air flow inside the cold storage for simulation purposes, with the provision of an outlet opening below the evaporator slot to ensure proper air circulation. Low-temperature air (inlet condition), stainless steel (SS), and copper material properties were assigned, as specified in Table 2, and the system's initial conditions, as well as external factors that affected the simulation, such as ambient temperature or heat sources, were defined along with boundary conditions for steady state with 3 m/s inlet velocity. Further, the geometry is discretized for accurate simulation to generate fine mesh. A mesh-independent study was undertaken by carrying out simulations by changing the mesh size. The maximum mesh sizing refers to the largest allowable element size in the generated mesh, thus controlling its coarseness and enabling the capture of the necessary details of the model with adequate resolution. A larger maximum mesh size will reduce the computational time at the cost of accuracy, and the converse is the outcome in the case of a smaller maximum mesh size. Absolute sag is another important parameter that controls the allowable deviation of the mesh elements from the true geometry of the model. Higher values permit greater deviation, which would simplify the mesh and reduce the simulation \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Properties** & **Units** & **Air (Inlet)** &
\begin{tabular}{c} **Stainless** \\ **Steel** \\ \end{tabular} & **Copper** \\ \hline Density (\(\rho\)) & kg/m\({}^{3}\) & 1.176 & 8000 & 8196 \\ Specific Heat (Cp) & J/kgK & 1005 & 500 & 385 \\ Thermal Conductivity (k) & W/mK & 0.0275 & 19 & 400 \\ Thermal Expansion & /\({}^{\circ}\)C & 0.00367 & \(16\times 10^{-6}\) & \(17\times 10^{-6}\) \\ Coefficient (\(\beta\)) & Pa.s & \(18.4\times 10^{-6}\) & - & - \\ Dynamic Viscosity (\(\
u\)) & m\({}^{2}\)/s & \(15.7\times 10^{-6}\) & - & - \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Material properties assigned. Figure 3: 3D Model of the cold storage system. time, but again, accuracy is compromised. Table 3 details the variation in these parameters of maximum mesh size and absolute sag through three different meshing solutions along with the required simulation time, while Figure 4 represents the images generated for this meshing. It has been observed that changing the mesh sizes impacted the simulation by affecting the display resolution with negligible change in temperature; however, the time taken for the simulation was almost 1.5 times with a higher mesh count. The simulation was carried out with Case I as the optimum mesh design through 3D Experience software, striking a balance between computational time and consistency of results. The simulation parameters, such as time step, simulation duration, and solver settings, were configured, and the cooling system within the unit was modeled, with temperature, flow rates, and control mechanisms taken into account. The initial temperature of the cold room was set to 26 \({}^{\circ}\)C, which was in line with the internal cold room temperature during the experimentation. The simulation was then run with continuous error monitoring, and the results were analyzed using post-processing tools.