Breakdown of the Entropy Transport Terms in a High-Speed Low-Pressure Turbine

Abstract

In this work, we investigate the mechanisms of entropy generation in the SPLEEN C1 high-speed low-pressure turbine cascade using high-fidelity large-eddy simulations. The analysis covers four exit Mach numbers representative of off-design operation. We compute the full entropy transport equation and separate all contributions into mean and turbulent terms. The method is applied to the pressure side, suction side, passage, and wake. Mean viscous dissipation is the dominant source of loss in the boundary layers, while turbulent dissipation governs the wake. Although the simulations are adiabatic, irreversible heat-transfer terms remain non-negligible near the blade surfaces, which highlights the need for a complete entropy-based framework in compressible cascades. The wake loss depends strongly on the vortex shedding regime. At low Mach number, coherent roll-ups generate high turbulent dissipation. At high Mach numbers, detached shedding reduces wake loss and shifts it downstream. At the intermediate Mach number, both mechanisms coexist and increase the wake contribution. Two turbulent terms usually neglected in turbine loss studies (the turbulent transport and the turbulent pressure diffusion) play a key role in shaping the turbulent entropy budget and must be retained for compressible flows. Off-design operation reduces total loss by weakening suction side separation and lowering turbulent activity in the wake.