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One of the most important parameters in the optimal ventilation of a raised air cooled data center is the amount of cool air that is delivered to the front of a server. It is very common in data centers for a significant fraction of the conditioned cool air to bypass the desired locations, i.e. the perforated tiles in front of the IT racks, and leak out through cable cutouts and other openings. This represents energy inefficiencies of two kinds: firstly, the wastage of blower pumping power at the air conditioning units, and secondly, the misdirection of the refrigeration work to create sub-ambient air which performs limited cooling function. The characterization of this leakage flow is the focus of this study. A representative CFD numerical model of a 12000 square foot facility, comprising of 100 server racks, at a heat flux of 100 W/ft2, was utilized for the thermal characterization to provide data to concretize the trends. For a fixed total volumetric air flow rate for a given data center, the amount of leakage air flow was varied and its impact on the inlet air temperature to the servers was quantified. The results showed a decrease in average rack inlet temperature of 0.7-0.9degC for every 10% reallocation of leakage flow to a desired rack inlet location. Observed trends for rack air inlet temperatures for different rack positions within a row, and for different groups of rack or aisles located in different parts of the data center, are also presented. In addition to studying the effect of leakage flow, two perforated tile layouts were characterized; a traditional one and a novel guard tile design, and the new design was shown to yield better data center thermal designs.