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This study aimed to investigate the effect of titanium dioxide (TiO2) nano additives on the thermal performance of a pilot-scale cross-flow cooling tower. Moreover, it is a continuation of our previous study on the effect of using multi-walled carbon nanotubes (MWCNTs) nanofluid, and the results were compared with the results of TiO2 and previous work. An experimental design by response surface methodology (RSM) based on central composite design (CCD) with two factors (concentration and flow rate) was used to study the effectiveness of the setup, Merkel number, and the cooling range. The nanofluids were prepared by the two-step method. The stability tests were performed considering different surfactants such as Gum Arabic, Triton X-100, and sodium dodecyl sulfate, and Gum Arabic was determined as the optimal surfactant. The visual method, dynamic light scattering (DLS), and Zeta potential analyses were used to ensure the stability of the nanofluids and determine the size distribution of the nanoparticles in the nanofluids. The findings revealed that the heat transfer characteristics of the working fluid were improved with the addition of nanoparticles. Moreover, by comparing the effect of nanoparticles, it was found that MWCNTs could enhance the thermal features better than TiO2. The nanofluid containing 0.085 wt% of the MWCNTs improves the Merkel number, effectiveness, and cooling range by 28, 10.2, and 15.8%, respectively, whereas these values for TiO2 containing nanofluids are 5, 4.1, and 7.4%, respectively. MWCNTs nanofluid with a concentration of 0.069 wt% and a flow rate of 2.092 kg/min was proposed for optimal system setup. Under these conditions, the cooling range, effectiveness, and Merkel number were about 23.5, 55.75%, and 0.64, respectively. [ABSTRACT FROM AUTHOR]

Improving the performance of cooling towers has always been of interest to researchers due to their importance and widespread use in various industries. In the present study, after constructing a cross-flow cooling tower, the performance of the tower was investigated by using nanofluid instead of water as the operating fluid. For this purpose, the effect of concentration and flow rate of nanofluid on the characteristics of tower performance, including tower evaporation rate, effectiveness, and cooling range, were examined using an experimental design by response surface methodology based on the central composite design. The results demonstrated that the use of nanofluids at the lower flow rates in the cross-flow cooling tower is more effective, so that the nanofluid containing 0.085 wt% of nanoparticles with an enhance of 15.8% and 10.2%, respectively, indicates the highest cooling range and effectiveness compared to pure water. Also, based on economic optimization, the optimal concentration and flow rate were obtained 0.07 wt% and 2.249 kg/min, respectively. • MWCNT nanofluid performance in cross-flow cooling tower experimentally investigated. • Design-Expert software used to experimental design & optimization of the conditions. • Various surfactants used to modify surface and increase stability of nanoparticles. • Nanofluid containing 0.085 wt% of nanoparticles has the best cooling performance. • Use of nanofluid at lower flow rates in cross-flow cooling tower is more effective. [ABSTRACT FROM AUTHOR]