Adding a cover over the cold aisle increased the uniformity of flow (Figure 8). With this change, 85 percent of the grilles are within ±5 percent of the mean flow. Temperature is reduced to the point that no cabinets receive air above 20°C.

ADDING A CEILING VOID

Figure 9 studies the addition of a ceiling void with grilles above the hot aisles for the purpose of removing the hot air and preventing it from being drawn into cabinets. The simulation showed that, for this particular configuration, this change had a detrimental effect. The variation of flow and temperature distribution within the aisles were similar to the base case, but a greater number of cabinets received higher-temperature air intake flow, including one that received air at 25.2°C. The hot cabinets were located at the ends of rows. The reason was readily apparent from the simulation results — the cold air moved up into the ceiling void, where it had no chance to be pulled into the cabinets.

A further simulation was set up to address the possibility of problems caused by a pipe running through the floor void. In this particular room, this change had only a minor impact, with the number of grilles within the 5-percent flow band being reduced to 73 percent and no change in the number of cabinets seeing elevated temperatures at their air intakes.

ACTIVATING ALL OF THE TILES

Activating all of the floor tiles in the cold aisles by setting their dampers fully open reduced the number of cabinets with an air-intake temperature in excess of 20°C from one to zero and reduced the proportion of cabinets with elevated temperatures. Additional simulations investigated the effect of adjusting the damper positions to balance and optimize the volume of cool air entering in each location. Dampers attached to grilles with higher flow rates were reduced to 75-percent open. This achieved more uniform flow rates through the grilles and reduced the number of cabinets suffering from elevated temperatures.

CONCLUSION

Rising power loads are making it more important than ever to optimize data-center cooling to ensure the reliability of computing equipment and reduce cooling costs. CFD simulation software addresses this need by evaluating the performance of alternative data-center designs quickly and inexpensively. In this example, the CFD simulation results showed that the cooling capacity was about 140 percent of that needed for the 1,200-w-per-square-meter load. However, detrimental effects were seen when a single cooling unit failed, primarily because of less-uniform air distribution in the floor void. Reducing the floor-void depth reduced performance for the same reason. Covering the cold aisle improved performance; however, using a ceiling extract system actually made things worse. Increasing the number of active floor grilles provided performance improvements, but for full benefit to be gained, the resistance between the floor void and the room had to be increased in some locations by adjusting dampers. Detailed CFD simulations enable well-informed design changes such as these to be made with complete confidence, thus, substantially improving the energy efficiency of data-center cooling while ensuring the availability of mission-critical resources.

Information and images courtesy of Mentor Graphics (Mechanical Analysis Division) (formerly Flomerics).
Circle 101

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