Study of gas turbine exhaust diffuser flow and performance characteristics at off-design conditions

Authors

Simon Hummel - Institute of Thermal Turbomachinery and Machinery Laboratory University Stuttgart
Maximilian Bauer - Institute of Thermal Turbomachinery and Machinery Laboratory University Stuttgart
Damian Vogt - Institute of Thermal Turbomachinery and Machinery Laboratory University Stuttgart

Abstract

Axial exhaust diffusers significantly enhance the available power output and efficiency of gas turbines by allowing lower turbine exit pressures. The residual dynamic pressure of the turbine outflow is converted into static pressure, which is referred to as pressure recovery. In today’s electricity market with a strong mix of renewables and traditional energy sources, heavy-duty gas turbines often have to operate at extreme part-load and over-load resulting in a wide range of different inflow conditions. The associated flow phenomena, which include, amongst other, 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 analyse these aerodynamic phenomena within the diffuser and to optimize diffuser geometries such as to achieve an optimum with respect to both performance and stability over the complete operating range. In the current work, an investigation of experimental data and numerical results at off-design operating points for a scaled model of a generic axial diffuser is presented. The geometry is representative for a modern gas turbine exhaust diffuser and consists of an annular diffuser including a row of six profiled struts, followed by a cylindrical and a conical diffuser. To reproduce realistic turbine exhaust flow conditions, a wire screen is installed in the inlet section of the test rig to generate a total pressure profile, which varies with 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 the wire screen and the diffuser inlet. The investigated operating points vary in terms of swirl angle, total pressure profile and Mach number according to typical real engine conditions. 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 assess the validity of the underlying numerical model for diffuser flow at off-design conditions. The static pressure recovery is used as a global performance diffuser parameter. Finally the results are discussed in a wider context of loading conditions in order to correlate the main flow characteristics with the diffuser performance.