Validation of simplified rack boundary conditions for numerical data center models

As cloud computing and computational needs grow, data centers will continue to become a larger part of our energy load. Proper design and layout is crucial to efficient energy use in data centers. Modeling the rack is one of critical pieces in this design. Often this is done as a black box rather than modeling the rack in detail. Modeling a computer rack as a black box has been done in numerous data center studies, but rarely has it been validated against experimental temperature and velocity data. This study looks at two simplified rack models and compares them against a rack composed of four 10U server simulators. The first model is an open box model that has a heating and fan element and allows air to flow through the rack. The second model is a black box model that allows no flow through the rack and imposes a constant pressure boundary across the inlet and exhaust. The model adds the enthalpy generated by the rack load to the upwind cells at the rack inlet plate to generate the exhaust temperature profile. The models were tested across a range of airflows and rack loads. Average agreements were found to be within 3°C and 0.2 m/s over all experiments. An interesting finding of this study was the importance of correctly capturing the boundary conditions at the perforated floor tile. Modeling the perforated floor tile as a nozzle using the momentum method described in ASHRAE RP-1009 was found to produce acceptable results for airflow at the area between the rack and the perforated floor tile.;Key Words: Computational Fluid Dynamics, Data Centers, Numerical Modeling.