Heat transfer enhancement mechanism of supercritical pressure fluids in circular pipes under transverse vibration

Although vibrational excitation holds significant research value in influencing the flow and heat transfer characteristics of supercritical pressure fluids, the underlying mechanisms remain poorly understood. In this study, large eddy simulation (LES) is employed to investigate the effects of overall vibrational excitation on the flow and heat transfer characteristics of supercritical pressure fluids within pipes. The research primarily focuses on analyzing its suppressive effect on typical heat transfer deterioration events, aiming to elucidate the fundamental mechanisms by which the coupling of physical property variations and vibrational excitation induces heat transfer enhancement. The results show that near-wall streamwise vortex sheet structures (SVS), generated by vibrational inertial forces, and Prandtl’s third-kind secondary flows, resulting from vortex tilting, are key mechanisms contributing to heat transfer enhancement. Transport equations for streamwise vorticity and turbulent kinetic energy, outline the conditions needed for the occurrence of these two types of flows: the former is periodically generated within the high radial density gradient layer near the pseudocritical temperature through the vibrational inertial generation. The latter arises from the enhancement of the lift-up mechanism, which facilitates the self-sustaining recovery of turbulence. The streamwise vortices experience increased tilting and deformation due to the entrainment action of the SVS, leading to the formation of turbulent secondary flows. The results demonstrate that turbulent secondary flows can be induced by applying transverse vibrational excitation in pipe flows with strong density stratification. This constitutes a novel mechanism for maintaining turbulent secondary flows and provides a new strategy for enhancing turbulent heat transfer.