Rising power densities at the server level due to increasing performance demands are being met by using efficient thermal management methods such as direct-to-chip liquid cooling. The use of cold plates that are directly installed yields a lower thermal resistance path from the chip to the ambient. In a hybrid-cooled server arrangement, high-heat-generating components are cooled with water or a water-based fluid, while the rest of the components are cooled with air using server-level fans. It is imperative to characterize the heat capture ratio for various server boundary conditions to ascertain the best possible liquid and airflow rates and temperatures. These parameters serve as inputs in defining the Total Cost of Ownership (TCO). The present investigation numerically evaluates the heat capture ratio in a hybrid cooled server for peak server load and varying inlet temperature for air and liquid. The CFD model of a Cisco Series C220 server with direct-to-chip liquid-cooled CPUs was developed. The cold plate for the CPU was experimentally characterized for pressure drop and thermal resistance characteristics and a black-box model was used for CFD simulations using 25% propylene glycol as the coolant. The heat capture ratio value was obtained under the varied temperature and flow rate boundary conditions of air and liquid. Based on the heat capture ratio values obtained, optimum values of inlet temperatures and flow rates are recommended for air and liquid for the server being investigated.
In recent years, there has been a significant increase in cloud computing, networking, virtualization, and storage applications, leading to an increase in demand for high-performance servers. The increase in performance demands is currently being met by increasing CPU and GPU power densities that require more efficient cooling technologies as compared to air traditional cooling methods. Cold plate-based liquid cooling in air-cooled servers enables efficient thermal management with minimal changes to existing air-cooling infrastructure. In a hybrid cooled server, the demand for air cooling is reduced as the primary heat-generating components are indirectly cooled by cold plates. In this study, experiments are performed with optimized chassis of a hybrid cooled Cisco C220 server. The chassis design is optimized to improve the airflow by providing additional vents on the chassis to allow more low-temperature airflow rather than the heated airflow approaching from the drive bay. Also, the design of the heat sink baffle is improved which allows a more streamlined flow to approach the heat sinks. This is done by designing and manufacturing a new 3-D printed baffle. This optimized baffle design helps in reducing the pressure drop across the system hence helping in the reduction of fan speeds and reducing the fan power consumption. Results are generated by iterating the fan speed and inlet temperature of air and comparing them with the baseline design of the server. Conclusions are made on the reduction in fan power due to the improved chassis design and any reduction in temperatures of air-cooled components.
Submerging a cluster of servers inside a large tank is the customary way of employing single-phase immersion cooling. But this approach requires a complete renovation of existing air-cooled infrastructure. A practical approach to convert an air-cooled data center to immersion cooled data center can be retaining the rack and server arrangements and supplying each server with immersion liquid in sled configuration – retaining horizontal position. The present study aims at characterizing the thermal performance of a 2-socket server in sled and tank configurations using CFD. In the tank configuration model, the server is immersed vertically with the coolant supply from bottom to top as in the case of a typical single-phase immersion deployments. In the sled configuration, the server orientation is retained (horizontally) and the fluid supply is modeled as an inlet and outlet manifold connected to the same side of the server. The CFD modeling approach is aimed to determine the heat transfer behavior of the server in two configurations being looked at was done for a commercially available dielectric immersion liquid, EC 110. A detailed baseline geometry of the server was first simplified, considering only the components that are significant source of heat and/or impact the server flow characteristics. Some of the components considered for analysis include CPU, storage drives and memory modules. The performance of the server in two configurations is compared to determine the efficiency of both the server configurations while ensuring the components do not exceed their respective thermal threshold. Component temperatures are obtained by varying the coolant flow rates and dielectric temperatures.
Due to increased use of high-performance computing in datacenters to cater to huge workloads, old low-performance compute servers must be replaced endlessly with high-performance compute servers. Traditional air-cooling systems are insufficient to provision and run the servers in optimal conditions as the datacenter thermal footprint or rack density grows, resulting in thermal throttling. To sustain the growing needs, Rear Door Heat Exchangers (RDHx) are deployed in existing datacenters along with peripheral Computer Room Air Handling/Conditioning (CRAH/CRAC) units. RDHx transfers heat from the rear end of the racks and rejects it into the facility’s chilled water. This study will demonstrate the suitability of RDHx for low density as well as high density rack applications. A baseline CFD model had a generic datacenter layout with peripheral CRAH/CRAC units and RDHx. Several case studies were conducted by varying the air and liquid inlet temperatures for rack and RDHx, respectively. We also compared active and passive modes of operating RDHx while server fans provide flowrate based on the IT inlet temperature. The paper will also discuss the feasibility of designing a datacenter with only RDHx and no peripheral CRAC/CRAH units while maintaining the thermal envelop. The research will also provide a guideline in implementing RDHx based on the heat load and server design.
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