Evaluation of oscillating heat pipes performance using nanofluids: A review

Oscillating heat pipes (OHPs) and pulsating heat pipes (PHPs) have emerged as highly efficient thermal management devices due to their compact structure, high heat transfer capability and passive operating characteristics. In recent years, nanofluids have attracted significant attention as working fluids in heat pipes because of their enhanced thermophysical properties, including higher thermal conductivity and improved convective heat transfer performance. This review presents a comprehensive analysis of recent experimental and numerical investigations on the thermal performance of heat pipes employing various nanofluids and hybrid nanofluids. The effects of important operating and geometrical parameters, such as filling ratio, heat input, inclination angle, nanoparticle concentration, working fluid type, and channel geometry, are critically discussed. Furthermore, the underlying mechanisms responsible for thermal enhancement, including Brownian motion, thin film evaporation, capillary action and oscillatory flow behaviour, are systematically analyzed. The review also highlights the practical challenges associated with nanofluid applications, such as nanoparticle agglomeration, stability issues, increased viscosity, and scalability limitations. In addition, the study identifies current research gaps and future directions for improving the thermal efficiency and reliability of heat pipe systems in advanced thermal management applications. The findings indicate that the thermal performance of heat pipes is strongly dependent on the combined influence of thermophysical properties, operating conditions and geometric configurations, emphasizing the need for integrated optimization approaches in future studies.