In order to enhance the efficiency and work output of gas turbines in power generation systems it is beneficial to minimize the turbine backpressure. To achieve this goal, an axial diffuser is installed downstream of the turbine thus increasing the enthalpy difference over the last stage by recovering the dynamic pressure head in the exhaust flow.

Due to the current transformation of the energy market, the operation of gas turbines varies from extreme part-load to extreme over-load conditions, resulting in a wider range of diffuser inlet conditions. The resulting flow phenomena, such as large-scale endwall separations, the extension of the hub wake or strut separations, do not only affect the performance of the whole engine but can also have a negative impact on the mechanical integrity of rotating or stationary turbine and exhaust duct components. Therefore, it is important to analyze these aerodynamic phenomena within the diffuser and optimize diffuser geometries to achieve an optimum with respect to both performance and stability over the complete operating range.

In the current work, a scaled model of a generic axial diffuser is studied in detail. The geometry of the generic model is representative for a typical industrial diffuser and consists of an annular diffuser including a row of six profiled struts, followed by a cylindrical and a conical section. Experimental and numerical investigations are performed to study the flow behavior at different load points. To match the exhaust flow conditions of real turbines, a wire screen is installed in the inlet section of the test rig to generate a total pressure profile which corresponds to the turbine load conditions. Moreover, a swirler and an additional device to model the tip leakage flow across the last stage of rotor blades are placed between wire screen and diffuser inlet. Experimental data of wall pressure measurements at the hub, casing and one strut passage as well as radial probe traverse measurements at several axial locations along the test rig are used to validate steady-state CFD results obtained from simulations using the flow solver ANSYS CFX. The static pressure recovery is used as a global performance diffuser parameter.

In the current paper, experimental and numerical data obtained for the design point, a part-load and an over-load operating point are analyzed. The investigated operating points vary in terms of swirl angle, total pressure profile and Mach number according to typical real engine conditions. Furthermore, the impact of the tip leakage strength on diffuser flow field and performance is assessed. As expected, the analysis of the experimental and numerical data shows a strong dependence of diffuser performance on the inlet conditions. However, when comparing the impact of the three main parameters governing the diffuser flow field – inlet pressure profile, swirl and tip leakage – the strut separations caused by the inlet swirl as well as a superimposed tip leakage flow can be identified as the dominant factors that characterize the overall flow field and thus determine the pressure recovery.


Investigation of the Flow Field and the Pressure Recovery in a Gas Turbine Exhaust Diffuser at Design, Part-Load and Over-Load Condition

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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