Small-scale data centers suffering from low cooling efficiency consume an intense power for reliable operation of IT equipment. In this study, thermal distribution in an air-cooled data center is simulated using an open source Computational Fluid Dynamics (CFD) model to examine the underlying mechanism that reduces thermal and cooling efficiencies. The numerical model is validated with the temperature measurements conducted in the data center. Numerical simulations have revealed that recirculating hot flows observed at the top of the racks increased the air temperature beyond the allowable maximum temperature. To this end, the data center has been retrofitted by creating a hot aisle with the implementation of a moving baffle at the rear of the rack. Numerical simulations conducted for two working scenarios have demonstrated that such a minor modification could result in remarkable enhancement in the cooling efficiency. Efficiency of the data center has improved by 47.2% and 22.7% with respect to the RCI (Rack Cooling Index) and RHI (Return Heat Index), respectively. The present numerical model can capture distributions of the efficiency metrics over the racks. Present methodology can be used to reduce power consumption by the cooling and ventilation systems in existing data centers.