## 4 Conclusions  
In this paper, a heat sink combining the confined jet impingement with metallic phase change material (PCM) was proposed for the thermal management of millisecond-pulsed heat sources. A transient model was established to simulate the conjugated heat transfer and the main outcomes from the present work are summarized as follows.  
1. Under the same volumetric flow rate of the cooling medium, the heat transfer of jet impingement can be enhanced with the decrease in jet diameter and jet height, but the pressure drop increases dramatically. For a jet diameter of 2 mm and impingement height of 2 mm, the maximum surface temperature on a 3 \(\times\) 3.5 mm\({}^{2}\) surface of a constant heat flux of 150 W/cm\({}^{2}\) can be cooled to 62.2 \({}^{\circ}\)C with a coolant inlet temperature of 35 \({}^{\circ}\)C.
2. Under the heat pulses with a peak heat flux of 600 W/cm\({}^{2}\) and duration of 1 ms per 4 ms, the temperature fluctuates in the same period as the heat pulses, and the maximum temperature is increased to 66.9 \({}^{\circ}\)C for jet impingement cooling. After adding a thin layer of metallic PCM, the maximum temperature can be reduced to 61.5 \({}^{\circ}\)C.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
\begin{tabular}{c} PCM melting under forced \\ convection was modeled using \\ the enthalpy–porosity approach, \\ and the heat transfer coefficient \\ on the air-cooled channel surfaces \\ was calculated by correlation. \\ Only the heat transfer and PCM \\ melting processes were involved, \\ and the apparent heat capacity \\ method was used to model phase \\ transition processes. \\ method were used to model phase \\ enthalpy method were used to model phase \\ method were used to \\ model phase \\ transition processes. \\ modeled using the \\ enthalpy–porosity method, and \\ the natural convection heat \\ transfer coefficient of the heat sink \\ outer wall was set to constant. \\ Only the heat transfer and PCM \\ melting processes were involved, \\ and the enthalpy–porosity \\ method was used to model phase \\ transition processes. \\ The \(k\)-\(\omega\) turbulence model and \\ the apparent heat capacity \\ method were used to model the \\ conjugated heat transfer of \\ confined jet impingement cooling \\ and PCM melting. \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table}
Table 3: Comparison of the current study with other existing numerical studies.  
3. The PCM thickness has a significant influence on the cooling performance. Results show that the maximum temperature can be maintained around the phase change temperature only if the melted PCM during the heating period can be completely solidified within heat pulse intervals, suggesting that a thickness smaller than 0.1 mm is appropriate.
4. The heat pluses' peak heat flux and duty cycle also have effects on the buffer performance of PCM. Results show that low peak heat flux and large duty cycle pulse heating can take full advantage of metallic PCMs.
5. Compared to the thermal silicon grease, metallic PCM can achieve a lower temperature and a smaller amplitude of fluctuations under pulses, showing a better thermal contact performance.  
Conceptualization, L.C. and Y.H.; methodology, L.C. and Q.W.; software, L.C. and Q.W.; validation, L.C. and Q.W.; formal analysis, L.C. and Q.W.; investigation, Q.W. and Y.S.; resources, L.C., Q.W. and Y.H.; data curation, Q.W.; writing--original draft preparation, L.C. and Q.W.; writing--review and editing, L.C. and Y.H.; visualization, L.C. and Q.W.; supervision, L.C. and Y.H.; project administration, L.C. and Y.H.; funding acquisition, L.C. and Y.H. All authors have read and agreed to the published version of the manuscript.  
This study was supported by the National Natural Science Foundation of China (52176020) and the Youth Innovation Team of Shaanxi Universities.  
The authors declare