Increased demand for computer applications has manifested a rise in data generation, resulting in high Power Density and Heat Generation of servers and their components, requiring efficient thermal management. Due to the low heat carrying capacity of air, air cooling is not an efficient method of data center cooling. Hence, the liquid immersion cooling method has emerged as a prominent method, where the server is directly immersed in a dielectric liquid. The thermal conductivity of the dielectric liquids is drastically increased with the introduction of non-metallic nanoparticles of size between 1 to 150 nm, which has proven to be the best method. To maintain the dielectric feature of the liquid, non-metallic nanoparticles can be added. Alumina nanoparticles with a mean size of 80 nm and a mass concentration of 0 to 5% with mineral oil are used in the present study. The properties of the mixture were calculated based on the theoretical formula and it was a function of temperature. Heat transfer and effect of the nanoparticle concentration on the junction temperature of the processors using CFD techniques were simulated on an open commute server with two processors in a row. The junction temperature was studied for different flow rates of 0.5, 1, 2, and 3 LPM, at inlet temperatures of 25, 35, and 45 degrees Celsius. The chosen heatsink geometries were: Parallel plate, Pin fin, and Plate fin heatsinks.
Transistor density trends till recently have been following Moore's law, doubling every generation resulting in increased power density. The computational performance gains with the breakdown of Moore's law were achieved by using multi-core processors, leading to non-uniform power distribution and localized high temperatures making thermal management even more challenging. Cold plate-based liquid cooling has proven to be one of the most efficient technologies in overcoming these thermal management issues. Traditional liquid-cooled data center deployments provide a constant flow rate to servers irrespective of the workload, leading to excessive consumption of coolant pumping power. Therefore, a further enhancement in the efficiency of implementation of liquid cooling in data centers is possible. The present investigation proposes the implementation of dynamic cooling using an active flow control device to regulate the coolant flow rates at the server level. This device can aid in pumping power savings by controlling the flow rates based on server utilization. The FCD design contains a V-cut ball valve connected to a micro servo motor used for varying the device valve angle. The valve position was varied to change the flow rate through the valve by servo motor actuation based on pre-decided rotational angles. The device operation was characterized by quantifying the flow rates and pressure drop across the device by changing the valve position using both CFD and experiments. The proposed FCD was able to vary the flow rate between 0.09 lpm to 4 lpm at different valve positions.
Over the last decade, several hyper-scale data center companies such as Google, Facebook, and Microsoft have demonstrated the cost-saving capabilities of airside economization with direct/indirect heat exchangers by moving to chiller-less air-cooled data centers. Under pressure from data center owners, IT equipment OEMs like Dell and IBM are developing IT equipment that can withstand peak excursion temperature ratings of up to 45°C, clearly outside the recommended envelope, and into ASHRAE's A4 allowable envelope. As popular and widespread as these cooling technologies are becoming, airside economization comes with its challenges. There is a risk of pre-mature hardware failures or reliability degradation posed by uncontrolled fine particulate and gaseous contaminants in presence of temperature and humidity transients. This paper presents an in-depth review of the particulate and gaseous contamination-related challenges faced by the modern-day data center facilities that use airside economization. This review summarizes specific experimental and computational studies to characterize the airborne contaminants and associated failure modes and mechanisms. In addition, standard lab-based and in-situ test methods for measuring the corrosive effects of the particles and the corrosive gases, as the means of testing the robustness of the equipment against these contaminants, under different temperature and relative humidity conditions are also reviewed. It also outlines the cost-sensitive mitigation techniques like improved filtration strategies and methods that can be utilized for efficient implementation of airside economization.
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.
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