Airflow management in a liquid-cooled data center

Electronics densification is continuing at an unrelenting pace at the server, rack, and facility levels. With increasing facility density levels, airflow management has become a major challenge and concern. Hot spots, air short-circuiting, and inadequate tile airflow are a few of the issues that are
complicating airflow management.
This paper focuses on a thermal management approach that simplifies facility airflow management in a cost-effective and efficient manner. Implementation of the technology was undertaken with the DOEs Pacific Northwest National Laboratory.
Under the effort, a single 8.2 kW rack of HP rx2600 servers was converted from air cooling to liquid cooling. The liquidcooling solution employs spray modules that indirectly cool the processors and remove the processor heat load directly to the facility water and not the facility air. An infrared camera was used to measure the temperature distributions over the rear doors of the liquid-cooled rack and several air-cooled racks, as well as the internal areas of a liquid-cooled and an air-cooled
server. A tile hood was also used to measure the airflow rate out of all of the perforated tiles in the data center. The air exiting an 8.2 kW air-cooled rack located in a bestcase facility location reached a maximum of 34C and the air exiting an air-cooled rack located in a worst-case location
reached a maximum of 44C, while the air exiting the liquidcooled rack was 10C to 20C cooler, reaching a maximum of 24C. The air is delivered to the tiles at approximately 14C.
The thermal gradient over the air-cooled racks approximated 10C (hottest servers at the top), while that over the liquidcooled rack was less than half and on the order of 3C-4C.
The image of the internal areas of the air-cooled server showed some significant hot spots on the power pods and memory, while these were significantly diminished for the liquid-cooled server. The tile airflow measurements revealed that the vast majority of the tiles delivered approximately 725 cfm, with five tiles delivering between 1,300 and 1,480 cfm.
This paper provides further details on the study and will analyze the manner in which facility airflow management complexity and cost can be reduced for a liquid-cooled facility.