Effect of thickness on the unsteady response of turbine submitted to pulsed flow conditions.

Authors

Pierre Bertojo  - ISAE-SUPAERO, Université de Toulouse, 31400 Toulouse, France
Nicolas Binder - ISAE-SUPAERO, Université de Toulouse, 31400 Toulouse, France
Jérémie Gressier - ISAE-SUPAERO, Université de Toulouse, 31400 Toulouse, France

Abstract

A promising way to improve the performance of turbomachinery is to use alternative cycles. Among these, using constant volume combustion (isochoric) instead of the traditional isobaric combustion is one of the best well known option. In aeronautics, prospects of evolution toward detonation engines (PDE for Pulsating Detonation Engine or RDE for Rotating Detonation Engine) are sought to increase thermal efficiency. These particular architectures of propulsion systems require very different power supply conditions for the combustion chamber and the surrounding components compared with classical configurations, as the filling and emptying generates strong flow unsteadinesses. As a consequence, the control of turbine performance in highly unsteady feeding conditions, similar to pulsated flows, is essential to achieve the theoretical benefit of isochoric combustion systems. In this context, the present work is a follow-up to previous studies focused on simulating violent transients and pulsed flows in turbines. These previous works proposed some first recommendations concerning the design of an axial turbine fed by a pulsating flow within the framework of skeletal geometry.In the present research, the thickness effects are now considered in the present research. Based on theoretical elements of unsteady compressible flows physics, the dihedral-shaped blade is considered an interesting candidate to take benefit of the pulsating flow. It promotes the intensification of the pressure difference between the lower and upper surfaces when the shock wave passes. A first round of parametric analysis is performed on a stator 2D shape, based on numerical simulations and seeking the maximization of the instantaneous over-loading. This analysys has led to the development of an optimal dihedral shape (leading edge angle, trailing edge angle, length). Moreover, the influence of the solidity is depicted where a critical value of this parameter is found, which segregates two regimes for the shockwave propagation. The second parametric analysis focuses on some bidimensional cascade arrangements, accounting for a complete stage. The stator and rotor blades are mirrored-shaped (as for a 0.5 degree of reaction). The influence of operating parameters is checked, such as the mass flow, the rotational speed or the cycle’s shape (intensity, frequency, etc.). As a result, some driving non-dimensional parameters for the performance are proposed and the benefit taken from the unsteadiness is quantified with indicators adapted to the framework of unsteady flows. Finally, the relevance of this type of turbines and the perspectives of improvement are also evaluated.