Experimental performance analysis of an annular diffuser with and without struts

In this paper, the performance analysis of an annular diffuser is presented. In a typical industrial gas turbine diffuser, a certain number of structural members, called struts, serve both as load bearings support and as passages for cooling air and lubricant oil.

Measurements were made in a 35% scaled down model of a PGT10 gas turbine exhaust diffuser with and without struts in order to determine the total and static pressure development and the effect of struts on both the local phenomena and the overall performance. More realistic flow conditions are made available by a ring of 24 axial guide vanes at inlet, which represent the last turbine rotor. The model has been tested on a wind tunnel facility developed at the University of Perugia with inlet speed around 80 m/s, allowing satisfactory accuracy for flow measurements and similarity with the PGT10 diffuser in terms of Reynolds number. Static pressure taps located at various streamwise positions on the hub and the casing allowed the estimation of pressure recovery development. A Pitot tube and a hot split-film anemometer were used to determine static and total pressure inside the diffuser at different axial positions. The comparison between the two cases, with and without the struts, was made also by the use of global parameters, which correlate static and total pressure.

In a previous paper, a detailed three-dimensional analysis of the flow path inside the diffuser was presented and the detrimental effect of the struts, in terms of flow separation and unsteadiness, was discussed. The stationary flow measurements and the investigation of the diffuser without the struts are presented in this paper. The whole research project represent a complete diffuser investigation available to develop an optimal design and to advance the computational and design tools for gas turbine exhaust diffusers.