# Numerical Evaluation of Hot Air Recirculation in Server Rack
## 1 Introduction
The study was conducted experimentally, where three 2U servers from various manufacturers and generations were utilized to analyze the internal thermal performance of the server. The experimental results confirmed the interdependency between the ITE internal design, back pressure (hot aisle pressure), and hotspot formation owing to hot air recirculation. [PERSON] et al. [13] numerically studied the impact of changing  
Figure 1: Formation of hotspots in air-cooled data centers; permissions republished with permission from [PERSON] et al. Elsevier (2023) published a critical review of the thermal management of data centers for local hotspot elimination was published by Elsevier (2023) [11]. the chassis fan location with regard to the server hot air recirculation. The CFD model revealed that chassis fan relocation significantly improves the server's overall cooling efficiency, but it brought in the new phenomenon of \"reverse flow through the power supply units\" due to back pressure (hot aisle pressure). To improve the air-cooled data center cooling system capacity, a novel hybrid cooling system (air and indirect single-phase liquid) has been experimentally evaluated [14]. The hybrid system was composed of two L2A (liquid to air) in-row CDUs, three racks, and fully containerized cold and hot aisles. Hot air recirculation was observed at the inlet of the servers owing to the open rack-discharge side. Blanking panels were added to the back of the racks, the inlet air mixing subsided, and visible hotspots remained in the intake. This may have been due to the hot aisle back pressure pushing against server chassis fans. Most previous studies have focused on hot air recirculation as a data center-level issue instead of server-level inefficiency, as noted by [PERSON] et al. [11]. This study focuses on evaluating the hot air recirculation phenomenon inside a server unit resulting from the hot aisle back pressure. An air-cooled server is selected for this study so that the hot air recirculation evaluation can be narrowed down to the airflow impact on the hot recirculation formation. A detailed computational fluid dynamics model for the Dell 2950 2U server is calibrated and validated using the existing experimental test results from [PERSON] et al. [10]. To accurately forecast fan performance at lower costs, simplified models, such as the extended actuator disk model (EADM) and reverse-engineered empirical actuator disk model (REEADM), have been developed because of the high computational cost of solving full three-dimensional numerical models of axial flow fans [15]. [PERSON] et al. [16] used momentum theory in conjunction with a pressure jump and a virtual blade model to investigate the effect of rotors on the aerodynamics of a helicopter fuselage. [PERSON] et al. [17] used a pressure-sensitive paint rotor [17] to simulate rotor flow wakes using a virtual disk (AD) model, and the outcomes were compared to those of fully resolved rotor blade simulations. They proved that compared to the completely resolved models, the CFD findings for the AD models accurately depicted the primary vortical structures surrounding the rotor disk. The central focus has been on the main cooling components of the server, that is, the \"fan set\". The server thermal design requires knowledge of the impact of the internal configuration of the server on cooling efficiency. Hence, this study attempts to understand the impact of the fan set configuration on the server inlet temperature at multiple back-pressure readings. The use of computational fluid dynamics to study server thermal performance under various operational conditions has risen sharply over the past decade owing to the introduction of high-density server racks [8; 18; 19]. Complex CFD models have high computational requirements, and convergence of the solution is time-consuming. Several experimentally validated CFD compact server models have been developed in multiple studies to minimize the computation time and resources required for the solution [19; 20; 21].