In the past, most data-center cooling systems were designed based on rules of thumb or trial and error, and large margins of safety were used to compensate for the inaccuracies in those approaches. But the rapid increase in the power density of modern computing, networking, and storage equipment means that energy costs for power and cooling frequently are higher than the cost of the information-technology equipment they support, when amortized over a typical three-year period. It has become critical to analyze data-center design to optimize the local airflow and heat-transfer conditions that play such an important role in cooling-system performance. Computational-fluid-dynamics (CFD) software tools provide the ability to simulate the performance of data centers and predict the effects of air handlers, power-dissipation sources, raised floors, and other features on temperatures throughout a data center. Following is a case of CFD being used to simulate a typical data center and evaluate the effect of design changes on cooling performance and energy costs.

Figure 1. Layout of cabinets around cold aisles.

EVALUATING DATA-CENTER COOLING

The simulations in this article are based on a real data center located in London. A base case was prepared to reflect the original design concept. The data center is 28-m long and 22-m wide and has a ceiling height of 3 m. Cooling is provided by 12 Denco computer-room-air-conditioner (CRAC) units, each of which has a cooling capacity of 92.5 kw and supplies air at a flow rate of 7.6 cu m per second. The supply set point was 14°C. The units were configured to discharge air downward into a 0.7-m floor void. No information was available on the specific distribution of heat sources, so a distributed load of 1,200 w per square meter was specified. All cabinets in the room were 0.6-m wide and 2-m high. The room was set up with four cold aisles, each of which was 1.8-m wide (Figure 1). There was a row of 25 cabinets on each side of each cold aisle. Two depths of cabinets were used — 1.1 m and 0.9 m — but were not mixed within a single row. Various heat loads from 500 w to 6 kw for each cabinet were applied.

The described base case was simulated along with six variants that were designed to assess the impact of changes in design and operating conditions on the data center. The base-case simulation shown in Figure 2 indicates that all of the cold aisles have a supply of cool air ranging from 14°C to 20°C. Warmer patches in the cold aisles match regions without active floor grilles. Warm air can be observed exiting the backs of the cabinets into the hot aisles. Cold air also can be observed moving from the ends of the cold aisles toward the CRACs, indicating the supply air exceeds the amount required by the cabinets. The total supply is approximately 140 percent of the cabinet requirement, based on a temperature rise of 10°C, indicating that the number of CRACs could be reduced to nine (although this would allow no redundancy).