Study on the novel suppression of heat transfer deterioration of supercritical water flowing in vertical tube through the suspension of alumina nanoparticles

One of the novel strategies of improving the heat transfer characteristics of heat exchanger systems is the application of nanofluids as the heat transfer media. The purpose of the current study is to evaluate the effectiveness of utilizing the suspension of nanoparticles in water at supercritical pressures in the mitigation of heat transfer deterioration (HTD), an adverse phenomenon in supercritical fluids heat exchanger systems. The thermohydraulic performances of the supercritical water-based nanofluids are analysed using the dimensionless parameters based on normalized Nusselt numbers and Fanning friction factors, Nu_nf/Nu_bf, f_nf/f_bf and PEC = (Nu_nf/Nu_bf)/(f_nf/f_bf)^1/3. The results reveal that the suspension of Al₂O₃ nanoparticles increase the mitigation of HTD, with 1.5% of Al₂O₃ nanoparticles volume fraction achieving up to 20.22% suppression of wall temperature peaks. In accordance with the Reynold’s analogy, the results show a positive correlation between heat transfer performance and pressure loss. Furthermore, the effect of nanoparticles on buoyancy flow are investigated based on Jackson's buoyancy criterion (Gr/Re^2.7). It is obtained that (Gr/Re^2.7) decreases at the pseudo-critical region as the nanoparticles volume fraction increases because the increase in the volume fraction leads to the reduction in difference between the bulk and near-wall fluid densities, thus leading to weakening of buoyancy flow and subsequent mitigation of HTD. Also, the increasing nanoparticles volume fraction causes the reduction of kinematic viscosity but enhances the velocity and turbulent kinetic energy close to the wall, which also, contributes to the mitigation of HTD.