Abstract: Closed wet cooling tower (CWCT) can be considered as a conventional open wet cooling tower in which the filler is replaced by a bank of tubes carrying the process water. In particular, in a CWCT the process water goes inside the tubes, while the ambient air and spray water flow over the external surface of the tubes. As a result, there is no direct contact between the process water and air flows, and the mass and heat transfer primarily occurs only on the tube surface through convection between the spray water and air. This indirect contacting mode ensures that the process water keeps clean and will not be polluted by environment. Currently, closed wet cooling towers (CWCTs) have received substantial research interests and have gained growing industrial concerns due to their clean operating mode and good thermal performances. In this paper, the enhancement effect of Al₂O₃-water nanofluid used as the spray fluid on the thermal performance of counter-flow CWCTs was investigated numerically. A three-dimensional model of CWCTs was built where VOF and DPM models were employed to calculate vapor-phase flow and the behavior of nanofluids. The influence of the mass fraction of nanofluids and spray mass flow rates on thermodynamic characteristics of CWCTs was analyzed by CFD. The results showed that the humidity difference and spray water mass in CWCTs first increased and then decreased along with the increasing of the concentration of Al₂O₃ nanofluid. Compared to the pure water, the humidity difference increased by nearly 18.3% when the mass fraction of Al₂O₃ nanofluid was 0.5%. The heat transfer coefficient increased with the increase of spray water mass rate. The maximum mass transfer coefficient was 0.18 kg/(m²·s) when the spray mass flow rate was 0.28 kg/s.