GPPF-2017-32 INVESTIGATION OF THE FLOW FIELD IN A GAS TURBINE EXHAUST DIFFUSER AT DESIGN AND PART LOAD

This paper presents an experimental and computational study of the flow in a laboratory model of a gas turbine exhaust diffuser. The exhaust diffuser, placed downstream of the turbine, has a significant impact on the performance of the whole power system. The experimental campaign was carried out in the Laboratorio di Fluidodinamica delle Macchine of Politecnico di Milano (Italy). The diffuser model, designed by Ansaldo Sviluppo Energia, includes an annular section equipped with struts, preceded by inlet guide vanes (IGVs) and followed by a conical divergent part. To reproduce two different loading conditions, two interchangeable IGVs were applied. Pressure and flow measurements in several positions along the diffuser were carried out. Numerical simulations were performed by Ansaldo Sviluppo Energia, applying a commercial CFD package and using high-fidelity numerical schemes and physical models. The reliability of the computational technique was assessed by virtue of a detailed comparison with the comprehensive set of experimental data. The analysis of the streamwise evolution of the flow along the duct highlights regions of flow detachment on the struts and at the hub, which highly depend on the level of loading. CFD simulations and hot wire measurements show that these regions are affected by significant back-flows, which have a relevant impact on the diffuser performance. Unsteady flow measurements and transient numerical simulations indicate, for full load condition, a time-periodic flow detachment at a frequency in the range 55–60 Hz. INTRODUCTION In view of increasing the flexibility of operation of gas turbine (GT) power systems, all the devices that compose the plant layout have to be designed and characterized in both full load (FL) and part load (PL) conditions. From this perspective, the exhaust diffuser is among the most critical components. Interposed between the last turbine stage and the stack (or the heat recovery steam generator in a combined gas-steam cycle), the diffuser has the purpose to recover the largest possible kinetic energy out of the turbine, thus increasing its power output end eventually the overall system efficiency. The flow in exhaust diffusers is highly complex, and the diffuser ducts are prone to flow detachment, which might trigger aerodynamic instabilities in some operating conditions. These phenomena prevent the diffuser from a proper operation, inducing a significant drop in performance at PL conditions, and they may trigger significant time-dependent aerodynamic forcing. As a result, the investigation of the diffuser performance is a challenging as well as a crucial task for the assessment of the overall GT performance. Several authors have published research studies on the performance characteristics of annular as well as conical diffusers. Kline et al. [1] pioneered the development of optimum design methodology for conical, while Reneau et al. [2] focused on two-dimensional plane diffusers. Afterwards, Sovran and Klomp [3] gave a remarkable contribution for annular diffusers through an extensive database of systematic experiments. The available investigations show that an optimal diffuser design allows keeping the flow at the edge to separation. The aforementioned guidelines, commonly used by companies, lead to the design of a “reasonable” geometry, which under specific conditions can be close to the optimum. However, the diffuser aerodynamics and performance strongly rely on the adjacent components, which provide inflow-outflow conditions largely depending on the specific application. Steven and Williams [4] investigated the effect of inlet conditions (including different turbulence levels) on the performance of annular diffusers. Sultanian and Sakamoto [5] studied experimentally and numerically the aerodynamics of a 1/10 scaled model of a gas turbine exhaust diffuser, at both FL and PL conditions. At PL, in particular, the flow enters the diffuser with a significant swirl component, due to the offdesign operation of the turbine, causing flow separation even within the inter-strut passage.