Development and loss mechanism of turbine secondary flows at a low Reynolds number: A synergy analysis

Abstract

To further explore the development of turbine secondary flows and associated losses at a low Reynolds number, a synergy analysis was developed and implemented. Loss is defined in terms of entropy generation in the present study. Inspired by the field synergy principle in convective heat transfer, the synergy between velocity and pressure gradient was derived from the three-dimensional mechanical energy conservation in a rotating frame. The loss of mechanical energy is not only related to the viscous dissipation but also the included angle (i.e., the synergy angle) between the velocity vector and the pressure gradient vector. A larger synergy angle (i.e., a worse synergy) is found to result in a higher loss for a fixed flow rate and pressure difference. This has been verified by both time-averaged and time-resolved numerical results. It is demonstrated that a worse synergy could be observed in high-loss regions, such as the turbine end wall, the suction-side separation and the wake. The velocity vector is not aligned with the pressure gradient vector in the vicinity of the reverse flow or the adverse pressure gradient, and the synergy angle could be employed as an indicator of these flow deteriorations. It is hoped the synergy could offer the potential method of future turbomachinery aerodynamic optimizations.