# Numerical Evaluation of Hot Air Recirculation in Server Rack
## 5 Conclusions  
A detailed computational fluid dynamics model for the Dell 2950 2U server was built, calibrated, and validated using the existing experimental test results. Furthermore, the model was used to evaluate the formation of the hot-air recirculation phenomenon at the server level and the methods to eliminate it. The server fan system was reconfigured to a series fan arrangement by adding four more server fans in the series to the original fan system. The following conclusions were drawn:  
1. The server-supply inlet temperature was used to validate the CFD model based on the experimental results. Based on the results, there was an average of 3% between the experimental data and CFD model results under similar environmental and operational conditions. This study represents a significant advancement towards real-life modeling of complex configurations. 2. The use of a compact fan (virtual disk model) to simulate the fans did not affect the accuracy of the simulation results. The model was validated against experimental results, with results within a 3% accuracy. Utilization of this feature shortens the computation time. 3. Increasing the server fan head does not address the issue of hot air recirculation in the server, which aggravates the situation. The server inlet air temperature increased by an average of \(\sim\)5% from \(-\)20 Pa to \(-\)5 Pa back pressure, and by 19% at the free delivery point (0 Pa). The inlet air temperature was further increased by 10% at back pressures of 5-20 Pa. The outlet air temperature decreased by 1%, signifying poor cooling effectiveness owing to high static pressure. Figure 15: Inlet and outlet air temperature comparison between parallel and series fan configurations with variation in back pressure. The server fan system speed is kept at 6800 rpm. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Back-Pressure Pa**} & **Temp, Inlet Air \({}^{\circ}\)C** & **Temp, Outlet Air** & **Temp, Inlet Air \({}^{\circ}\)C** & **Temp, Outlet Air** & **Percentage** \\  & **(Parallel)** & \({}^{\circ}\)**C (Parallel)** & **(Series)** & \({}^{\circ}\)**C (Series)** & **Difference (IAT)** \\ \hline \(-\)20 & 20 & 30.97 & 20.7 & 30.97 & 3\% \\ \(-\)10 & 19.8 & 30.68 & 20.4 & 30.68 & 3\% \\ \(-\)5 & 20.3 & 31.15 & 21.85 & 31.14 & 7\% \\
0 & 19.7 & 31.53 & 24.19 & 31.52 & 19\% \\
5 & 21 & 31.1 & 22.08 & 31.3 & 5\% \\
10 & 21.4 & 30.67 & 25.18 & 30.67 & 15\% \\
20 & 24.2 & 30.71 & 26.98 & 33.06 & 10\% \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Series versus parallel fan system configuration impact on inlet and outlet air temperature results. 4. This also highlights the importance of correctly sizing the server fans, which can significantly impact the overall server thermal performance, even under low-load conditions. It was established that a positive net system pressure does not necessarily result in the elimination of hot-air recirculation. Server fan sizing or fan set configuration should be dictated by the required server component temperature, i.e., CPU. 5. The current work serves as a base for integrating liquid and air-cooling systems to form hybrid cooling systems for high-density racks in legacy data centers, where in-row CDU(L2A) is used in conjunction with CRAH/CRAC as the cooling source. Future work in this area can include rearranging the high thermal components and placing them down towards the discharge section of the server. Different types of cooling fluids should be considered to cool the components. Conceptualization, N.B.M. and T.J.K. ; methodology, N.B.M. ; software, validation, N.B.M. ; formal analysis, T.J.K. ; investigation, N.B.M. ; resources, T.J.K. ; data curation, N.B.M. ; writing--original draft preparation, N.B.M. ; writing--review and editing, T.J.K. ; visualization, supervision, T.J.K. ; project administration, T.J.K. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable.