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CFD Analysis of Data Center Cooling

An increased demand for computational resources in nearly every industry and the availability of affordable hardware have led to a significant growth in data center facilities all across the world. With the recent surge in Cloud computing, this demand has been pushed even further. According to Moore’s law, the computing speed for CPUs doubles every two years. Advancements in chip design, such as multiple core CPUs, has resulted in denser computers with increased power consumption and heat generation. Reduced space and increased heat output of the server units poses a significant challenge for the cooling system design of these centers.

State-of-the-art cooling centers use different types of systems (air or liquid cooled) and designs (raised floor or hot/cold aisles).  One of the major concerns in designing an efficient air conditioning system is to eliminate possible “hot-spots” (regions of high temperature) in the room. CFD analysis can play an integral role in determining these hot-spots. In addition, it also provides a detailed analysis of the heat dissipation rate and flow patterns in the room. It can also provide insight into vent placement (both inlet and outlet), flow rate and temperature required of the cooling fluid and other design constraints for these systems.

Figure 1 - Temperature distribution on racks and floor surfaces without any cooling

The engineers at CAE Associates are very familiar with the analytical CFD techniques which can be used to ensure successful cooling schemes in data centers. For example, CAE Associates recently conducted a CFD analysis of an air-cooled small sized data center design using the ANSYS CFX suite of simulation tools. The 32 x 20 x 10 feet server room consists of 20 server racks, each generating the same amount of heat. Following an alternate hot-cold aisle floor cooling design, four rows of racks are placed equidistant from each other with inlet vents on the floor in the cold aisle and outlet vents on the ceiling in the hot aisle. In the absence of any cooling, the ambient temperature of the room reaches nearly 35°C, with several surfaces of racks showing hot-spots above 40°C (Figure 1).  In order to cool the room and the racks, cold air was injected at a constant rate from the floor of the cold aisles at a temperature of 21°C.

Figure 2- Temperature distribution on racks and floor surfaces with floor cooling
Figure 3 - Isovolume of temperature exceeding 25 deg. C in the room

As the cold air passes through the rack units, it absorbs heat from the hot surfaces, thereby raising the air temperature. As this hot air reaches the hot aisle, it is rises towards the ceiling and is eventually pushed out through the outlet vents. The air temperature now drops to about 26°C while the surface temperature of the rack drops below 30°C (Figures 2,3).  Streamlines (Figure 4) show that the some portion of the heated air is also recirculated back to the rack units, further enhancing heat dissipation from the racks.

Figure 4 - Streamlines of the fluid flow colored by temperature

The three-dimensional flowfield, temperature distribution, and heat transfer rates computed by this CFD analysis provided confidence that the cooling system was robust and efficient.

For more about CAE Associates’ CFD consulting services or to learn about modeling of data centers from our team of CFD consultants, please contact our office at 203-758-2914.