# Numerical Analysis of the Cooling Performance in a 7.2 kW Integrated Bidirectional OBC/LDC Module
## 2 Development of the Numerical Model
### Properties and Boundary Conditions  
To determine the heat flow characteristics inside the integrated module when mounted in a vehicle, the model used physical properties and boundary conditions that were identical to those of the actual device. First, the solid properties of the external charger (aluminum), inductor (copper), diode (plastic), transformer (steel + copper), and printed circuit board (PCB) of the charger were defined. Then, the physical properties of a fluid were assigned to the internal air and cooling water. The values of the physical properties are listed in Table 1. In the case of the inductors and diodes, although these devices contain various materials, the physical properties were selected based on the representative element that occupied the largest part. Furthermore, although the actual physical properties of the various components varied with temperature, these factors were ignored because they were assumed to be small.  
The boundary conditions of the integrated module are listed in Table 2. The inlet velocity applies to the cooling water inlet used for system cooling. The cooling water flow rate of 6 LPM in an actual operating charger was converted to velocity for use in the analysis. The outlet pressure refers to the pressure at the cooling water outlet to ensure that the circulating water can be discharged without encountering resistance. Most of the heat generated in the integrated module is removed via the circulating cooling water, and the remainder is discharged to the atmosp  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Description**} & \multirow{2}{*}{
\begin{tabular}{c} **Density** \\ **[kg/m\({}^{3}\)]** \\ \end{tabular} } & **Viscosity** & **Specific Heat** & **Thermal Conductivity** \\  & & **[kg/m\({}^{3}\)]** & **[Pa+s]** & **[J/kg-K]** & **[W/m-K]** \\ \hline \multirow{2}{*}{Fluid} & Water & 998.2 & \(1.003\times 10^{-3}\) & 4182 & 0.600 \\  & Air & variable & \(1.789\times 10^{-5}\) & 1006 & 0.024 \\ \hline \multirow{4}{*}{Solid} & Aluminum & 2719 & - & 871 & 202.4 \\  & Copper & 8978 & - & 381 & 387.6 \\ \cline{1-1}  & Diode & 1190 & - & 1465 & 0.190 \\ \cline{1-1}  & PCB & 1890 & - & 600 & 0.810 \\ \cline{1-1}  & Steel & 8030 & - & 502 & 16.3 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Physical properties of the components and materials in the integrated module.  
external surface is exposed to air. Therefore, a convection condition was allocated to the external surface (300 K, h = 10 W/m\({}^{2}\)-K) in the model to represent the heat being discharged to the outside air. Furthermore, the internal air is circulated via natural convection based on the temperature difference (density difference), which results in heat diffusion. To simulate this, the air density (assuming an incompressible ideal gas) was calculated based on the temperature, which was used to specify the heat transfer by natural convection considering the effects of gravity. The results of the previous circuit simulations were used to determine the heating value of each part and device in this model.