Nanocatalytic Enhancement of Local Heat Transfer in Continuous-Flow Thermal Reactors

An experimental investigation was conducted to evaluate the thermal conductivity (TC) and local heat-transfer coefficients (LHTCs) of nanofluids containing alumina (Al2O3), hematite (Fe2O3), and copper oxide (CuO) nanoparticles dispersed in deionized water. A newly developed non-invasive LHTC probe was integrated into the inner wall of the test section to enable direct quantification of interfacial heat-transfer performance. The measurements were conducted under laminar and turbulent flow conditons across Reynolds numbers ranging from 1000 to 10,000. The selected nanoparticles were chosen based on their high intrinsic thermal conductivity, cost effectiveness, and, in the case of Fe2O3, magnetic recoverability. The nanoparticles enhanced both TC and LHTCs through improved thermophysical propoerties and possible interfacial effects. Maximum TC enhancements of 19%, 21%, and 25% were achieved for Al2O3/distilled water (DW), Fe2O3/DW, and CuO/DW nanofluids, respectively, at 0.05 vol% and 55 °C, while the corresponding LHTC enhancements reached 44%, 50%, and 53%. Under turbulent flow, CuO/DW exhibited the highest heat-transfer performance, attributed to a 25% increase in TC and corresponding improvement in connective heat transfer. Since the boundary-layer thickness exceeded the nanoparticle diameter (30 nm), nanoparticles penetrated the interfacial film, inducing localized micro-convection and catalytic micro-mixing, which intensified interfacial heat transport. The experimentally determined Nusselt numbers showed strong agreement with the Xuan–Qiang correlation at 55 °C, suggesting that the nanoparticle volume fraction governs the catalytic interfacial heat-transfer mechanism.