Improving the Aerodynamic Performance of Wide-Angle Flat Diffusers

Abstract

In designing the transition pipes from the gas turbine output diffuser to the heat-recovery steam generator used in modern combined-cycle power plants, the compactness requirements are in some cases the governing ones. As a result, channels having non-optimal aerodynamics are used, including those in the form of wide-angle diffusers, the current in which may contain zones with flow separation and a sharply non-uniform field of outlet velocities. Such a flow pattern downstream of the transition pipe at the heat-recovery steam generator inlet significantly impairs the heat transfer in the steam generator’s tube bundles located first along the flow. Among the aerodynamic methods used to modify the flow pattern in diffusers, methods that allow their performance to be improved significantly without additional energy expenditures are of great interest. A flow separation delaying method involving the use of so-called vortex generators as received wide use. These vortex generators are made in the form of flat or profiled plates installed directly on the diffuser channel wall perpendicular to it. As the current flows over the plates, near-wall vortices are generated, which intensify turbulent heat transfer in the boundary layer and make this layer more resistant to separation. The article presents the results from an experimental study of the method for reducing energy losses and equalizing the field of outlet velocities in wide-angle diffuser channels by installing a short plate in the channel inlet section in parallel to the deflected wall. With such an arrangement, the plate is not only a generator of vortices that transfer additional energy to the boundary layer, but also transmits an additional momentum to the wall due to flow deflection. The investigations were carried out in an open-type wind tunnel with the dimensionless velocity values at the diffuser inlet not exceeding 0.3. The values of total energy loss coefficients were obtained in moving the plate both along and perpendicular to the diffuser channel deflecting wall. The plate optimal location in the channel inlet section in terms of the greatest reduction of energy losses has been determined. The velocity distribution patterns in the diffuser outlet section for regimes with and without flow separation have been obtained. It is shown that in the case of separation flow modes in the diffuser, the use of a plate installed in it can significantly reduce the total energy loss, increase the diffuser effect, and equalize the velocity field in the channel outlet section of due to partial or complete elimination of flow separation. During the experiments, static pressure pulsations on the deflected wall were measured in five sections along its length. It is shown that in case of separation flow modes, the plate installed in the inlet section decreases the intensity of the near-wall pulsating motion and, hence, the dynamic loads on the channel walls. The obtained results allow us to recommend the investigated flow control method in a wide-angle diffuser for being used in the transition pipes from the gas turbine to the combined cycle plant’s heat-recovery steam generator in the course of their modernizations and repairs to equalize the velocity field at the heat-recovery steam generator inlet, to decrease the energy losses, and to reduce the dynamic loads.