## 3 Results and Discussion  
### The Performance of Jet Impingement under a Constant Heat Load  
Figure 3 shows the velocity contour and streamlines of confined jet impingement when \(H\) is 2 mm and \(d\) is 2 mm. Taking this working condition as an example, the flow and heat transfer characteristics of confined jet impingement are described. According to the velocity characteristics, the confined jet area can be divided into two parts: the stagnation area and the wall jet area. In the stagnation area, the velocity changes sharply from the axial direction to the radial direction, and the momentum and energy change significantly, showing a strong heat transfer capability. In the wall jet area, the working fluid shows a decelerated flow and its velocity decreases with the increase in radial distance. Vorticities are generated by violent impact and interference with other jets. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Mesh**} & **Number of** & \(\mathbf{T_{max}}\) & \multirow{2}{*}{\(\left|\left(T_{max}\right)^{j}-T_{max}\mathbf{{}^{j+1}}\right)\)/\(T_{max}\)\({}^{j}\)} & \(\Delta P\) & \multirow{2}{*}{\(\left|\left(\Delta P^{j}-\Delta P^{j+1}\right)/\Delta P^{j}\right|\)} \\  & **Cells** & \(f^{c}\mathbf{C}\) & & & \\ \hline Mesh1 & 26,718 & 63.83 & 6.75 \(\times\) 10\({}^{-3}\) & 196.11 & 3.68 \(\times\) 10\({}^{-1}\) \\ Mesh2 & 68,546 & 63.39 & 1.27 \(\times\) 10\({}^{-2}\) & 268.35 & 1.70 \(\times\) 10\({}^{-1}\) \\ Mesh3 & 127,091 & 62.59 & 5.67 \(\times\) 10\({}^{-3}\) & 222.79 & 2.75 \(\times\) 10\({}^{-2}\) \\ Mesh4 & 323,075 & 62.24 & 6.11 \(\times\) 10\({}^{-4}\) & 228.93 & 2.81 \(\times\) 10\({}^{-4}\) \\ Mesh5 & 998,199 & 62.20 & — & 229.00 & — \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Results of the mesh independence test. Figure 2: Comparison of \(Nu_{0}\) determined by the numerical calculation and the correlation. The distribution characteristics of the flow field affect the heat transfer of the impinged surface. Figure 4 shows the heat transfer coefficient (\(h\)) distribution on the impinged surface, the temperature (\(T\)) distribution on the heat sink bottom, and the variations of \(h\) and \(T\) along the symmetric line. As seen in the figure, \(h\) at the stagnation area is much higher than that at other areas due to the strong disturbance caused by a sharp change in velocity. With the increase in radial distance, \(h\) decreases, and its distribution is approximately bell-shaped. The large \(h\) in the stagnation area exactly meets the requirement of high heat flux chip heat dissipation, and the lower \(h\) in the wall jet area is sufficient to satisfy the low power heat dissipation of areas around the chip. The temperature at the center of the chip is the highest and gradually decreases outwards. The maximum temperature of the heat sink bottom (\(T_{\text{max}}\)) is 62.2 \({}^{\circ}\)C, meaning the jet impingement can meet the heat dissipation requirements of the heating surface under the constant heat load (150 W/cm\({}^{2}\)). The heat sinks with different jet heights (\(H\)) were simulated with a specified jet diameter (\(d\)) of 2 mm. Considering the device's strict requirement on the size of heat dissipation space, the jet height studied is between 0.5 to 2.5 mm. Figure 5 shows variations of \(T_{\text{max}}\) and pressure drop (\(\Delta P\)) with \(H\). It can be seen that \(T_{\text{max}}\) increases with the increase in \(H\), and the increase rate gradually decreases as \(H\) is larger than 1.5 mm, while the pressure drop \(\Delta P\) has an opposite trend with the maximum temperature. As the coolant impinges into a confined space, the smaller the jet space, the stronger the disturbance near the impinged surface with the same jet velocity, leading to a better heat transfer performance but a higher pressure drop. However, the area influenced by jet impingement is limited, and the weakening effect on disturbance due to the increasing jet space decreases with the increasing jet height.