The wavelength band of 200-280 nm of UV-C radiation generated by the Ultraviolet Germicidal Irradiation (UVGI) system can destroy the reproduction ability of microorganisms. Several factors related to UV fixtures, HVAC layout, and the resulting airflow flow patterns can affect the performance of upper-room UVGI applications. With the help of Computational Fluid Dynamics (CFD) analyses, this study systematically evaluates the impact of UV-C intensities on the effectiveness of an upper room UVGI system. It shows that the addition of even a small amount of UV-C energy in the upper region of space can significantly reduce the probability of infection as predicted by the Wells-Riley model. Increasing the UV-C output shows a further reduction in the infection probability, although with a diminishing impact. A further investigation is necessary to evaluate the effect of airflow patterns on the performance of UVGI systems. These studies demonstrate that CFD analyses can help optimize the performance of UVGI systems to minimize the probability of infection in indoor spaces.