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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