The effect of Al2O3, TiO2, and SiO2 concentration on MT performance at different rotating speed

To evaluate the effect of Al₂O₃, TiO₂, and SiO₂ concentration on MT performance, concentrations of 0.05, 0.01, and 0.1 were selected and examined at a constant gas flow rate of 10 L/min with different CO₂ content values (10–50 vol.%) and various solvent flow rates, as shown in Figure 4a–d. The figure demonstrates that k_La was influenced by the kind of nanoparticle as well as nanofluid concentration and flow rate, but it was not changed by varying the CO₂ content. This figure also indicates that NFs had substantially higher MT performance than water, which may be due to the shuttle effect, as well as hydrodynamic and bubble breaking effects in NFs, which can increase the contact area and reduce the boundary layer’s thickness, which, as discussed by Kim et al. [83], can intensify the diffusion of CO₂ molecules [84]. Furthermore, it was found that the concentration of 0.01 wt.% of the NF was more efficient than other concentrations of the suspension. This result confirms Krishnamurthy et al.’s and Kim et al.’s reports [71,85], which demonstrated that an increase in the concentration values of NPs intensifies the diffusion up to a certain value and then decreases. In addition, viscosity does not have a significant effect because of low concentrations, as reported by Samadi [25].

As Figure 3a and Figure 4, reveal, the NF concentration of 0.01 wt.% was tested at different flow rates and a constant gas flow rate (10 L/min) with two CO₂ content values (50 and 10 vol.%) which also included Al₂O₃ NFs (II). The results of these tests are shown in Figure 5.

Figure 5a,b reveal that the TiO₂ and Al₂O₃ NFs (II) were more effective than other NFs and pure water. In addition, as can be seen, the Al₂O₃ (II) NF was more effective than the Al₂O₃ (I) NF, which may have been due to high turbulence that occurs at higher gas and liquid flow rates, and more importantly due to effective surface area values and the microconvection ability of NPs.

Considering the data shown in Figure 5 and using Equation (11), the MT intensification parameter at high rotating speeds was calculated for Al₂O₃ (II), Al₂O₃ (I), TiO₂, and SiO₂. The average amount of k_eff at the high speeds of the RPB were 1.99, 1.47, 1.82, and 1.77 for Al₂O₃ (II), Al₂O₃ (I), TiO₂, and SiO₂, respectively, and the maximum value reached 2.59 for Al₂O₃ (II), which may have been due to its nanoscale mixing abilities in liquid.

In addition, the results shown in Figure 3, Figure 4 and Figure 5 indicate that the performance of metal- oxide NPs was better than that of SiO₂ NF, which could be related to the microconvection, diffusion, and surface-active site ability of NFs, which can adsorb ions. In addition, surface renewal rate or acidic, basic, or amphoteric properties of NF can change the concentrations of H₃O⁺ and OH⁻ [20].

On the other hand, it can be claimed that the Al₂O₃(II) NF performance was better than that of Al₂O₃(I), which may be related to surface area, as indicated in Refs. [23,86]. Based on the study carried out by Ali et al. [87], who showed that a nanofluid’s thermal conductivity coefficient increases with the size of the NPs, and considering the similarity between heat and mass transfer, it may be argued that the better performance of the Al₂O₃(II) NF was due to the increase in the NPs.