Improved CFD modeling of a small data center test cell

An improved CFD model is presented to predict the thermal field in a small data center test cell. CFD analysis reported in an earlier paper tended to exaggerate hot and cold spots in a small data center test cell as a result of the inability of the CFD model to predict the correct level of mixing of the cold air emanating from the perforated floor tiles and the warmer room air. The same can be said about the mixing of the hot air stream exiting the rack and the surrounding room air. The overall RMS error in the earlier study was 4°C between numerical results and test data. Another indication of low mixing in the CFD results is illustrated in the comparison of the temperature profile along a vertical line in front of the rack between CFD results and test data. While the temperature of the cold flow from the under floor plenum entering the data center was measured to be 12.8°C, temperature measurement at the same location via a temperature mapping tool showed that it is 15°C at 6" above the floor and rising quickly to 17°C at a height of 4 ft. The present investigations yield several improvements to the CFD model, including modeling of the perforated tile flow and rack exhaust flow to conserve both mass and momentum, inclusion of the effects of buoyancy, and an improved thermal boundary condition for the floor. With these improvements, the overall RMS error was reduced to 2°C between numerical results and the same test data reported in.