Investigation of CRAH Bypass for Air-Cooled Data Centers using Computational Fluid Dynamics

Computer room air handling (CRAH) bypass (BP) method utilizes fan-equipped perforated tiles to force or induce room air through relatively lower flow resistance than that in CRAH units, as CRAH fans operate at lower speed. Increasing the BP airflow rate reduces the total fan power and increases the chiller power since the CRAH unit needs to provide colder air when a certain fraction of warm room air is allowed to mix with cold CRAH airflow. Simplified airflow network and thermodynamic modeling tools addressed this optimization problem and indicated cooling energy savings potential via CRAH BP method for enclosed aisle (EA) data centers. These tools assume uniform air temperature at the server inlets, even though recent experimental and computational studies indicate temperature non-uniformities at higher fractions of BP flow. This study addresses energy implications of the uniform temperature assumption of the reduced order modeling tools on the cooling energy saving potential of EA data centers by using computational fluid dynamics (CFD) modeling. Moreover, existing literature does not thoroughly investigate the application of CRAH BP in the open aisle (OA) data centers, which requires CFD models to resolve complex airflow patterns due to recirculation of hot air into the cold aisle and leakage of cold air into the hot aisle. This study utilizes CFD to investigate both induced and forced CRAH BP methods in a representative quadrant of a 1MW data center with and without aisle containment.