Flow development and turbulence length scales within an annular gas turbine exhaust diffuser

This paper presents the experimental investigation of a scaled down model of a PGT10 gas turbine diffuser by Nuovo Pignone S.p.A. The model was designed to operate in geometric and Reynolds number similarity with the GT diffuser, and 24 guide vanes are mounted at inlet to reproduce typical inlet conditions of an industrial gas turbine diffuser. The PGT10 diffuser has also six high-solidity struts, which support one of the shaft bearings and have lubricating oil supply inside. Two step-motors are used for the circumferential and radial displacement of the probes. Airflow through the test model is provided by an open circuit wind tunnel, which is powered by a 21 kW centrifugal fan. Two hot split-film probes in constant temperature mode were used to measure velocity fluctuations in main-flow, cross-flow and radial-flow directions, thus allowing a 3D characterization of the model’s flow field. Results are presented in terms of mean and fluctuating velocity components at different axial positions. The discussion is focused on the distortion of the flow produced by the struts and on the separation around the hub and the shell. A first set of measurement defines the flow field from the inlet to the exhaust at four axial sections. In three additional axial positions, behind the struts, a more detailed analysis has been made with measuring grid with a double number of nodes. The investigation of the flow field inside the diffuser is also presented by means of the turbulent scale of dissipating eddies, calculated with the Kolmogorov theory. The Kolmogorov length scale seems to be a powerful means to study fluid dynamic phenomena.