This paper presents a numerical investigation of the operating performance of an autonomous air cooling system based on water–air heat exchange. The variation of air temperature along the length of a tubular heat exchanger was analyzed as a function of the outdoor air temperature and inlet air velocity. The study was conducted using a physical model developed in COMSOL Multiphysics 6.1. The simulation results show that the cooling efficiency of the system varies significantly when the outdoor air temperature ranges from 30 to 40℃ and the inlet air velocity changes from 2.5 to 4.0 m/s. The findings confirm that an increase in inlet air velocity reduces the residence time for heat transfer, leading to higher outlet air temperatures, and demonstrate the necessity of employing a two-loop cooling configuration under high outdoor temperature conditions.