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In this study, the aerodynamic performance of the exhaust system of a two-shaft gas turbine was investigated experimentally and numerically. The investigation focused on the system “Turbine Stage-Diffuser—Collector Box” and aimed to examine the impact of inlet conditions and geometry particularities on the efficiency of the exhaust system. The experiments were conducted on the Test Ring ET4 (Experimental Turbine-4) at the Peter the Great St.Petersburg Polytechnic University, which was equipped with a special diversion channel to examine the non-axisymmetric outlet of the exhaust duct. The collector box was designed to rotate by 180 degrees around the turbine axis to investigate its impact on the system’s performance. Flow traversing parameters were measured with the five-channel pneumatic pressure probes, and numerical simulations were performed with CFX 15.0. The RANS (Reynolds-averaged Navier–Stokes) equations were closed with the SST (k-ω) turbulence model (Shear Stress Transport model). The study concluded that the RANS SST model predicts the flow in the diffuser before the struts accurately. However, downstream the struts, the CFD (Computer fluid dynamic) results over-predicted the exhaust diffuser pressure recovery coefficient by 14% due to the complex vortex structure of the turbulent flow, which the Averaged Navier–Stokes equations did not resolve. The study highlights the importance of considering the last stage of the turbine, diffuser, and collector box as an integrated system when investigating the aerodynamics of exhaust ducts. The study also emphasizes the impact of geometry and inlet conditions on the exhaust diffuser’s performance and efficiency. The results of this study can be used to optimize the design of the exhaust system of two-shaft gas turbines and improve their thermal efficiency. The integrated approach of combining experimental and numerical methods can provide a detailed and reliable flow picture and can be used for future research in this area.

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Currently, thermal insulation of GTU output diffusers uses insulation of their inner surface. This is an expensive and complicated technological procedure. For gas turbines as part of CCGT, in order to reduce the cost of insulation and at the same time increase the useful power of the turbine, cooling the diffuser outer surface with a steam stream of a steam circuit can be an alternative way of internal insulation. Steam and gas parameters of a combined cycle plant with a CCGT-450T, as well as the results of experimental and computational studies of the GTU SGT5-3000E gas turbine exhaust channel model were used. The calculations of the efficiency of the surface cooling of the diffuser with the steam coming from the steam circuit were carried out using the analytical method. A scheme of a superheater located on the outer surface of the GTU outlet diffuser operating in a combined cycle is proposed. Analytic evaluation of its effectiveness showed that the surface area of the GTU diffuser of the type SGT5-3000E is sufficient to provide the necessary overheating of low-pressure steam. Installation of such a heat exchanger using the outer surface of the diffuser provides a decrease of the temperature of its outer wall from 537 to 200 оC. The study validity is confirmed by a patent for an invention. It has been established that the use of the outer surface of the GTU outlet diffuser instead of the heat exchange surface of the low pressure superheater of the utilizer boiler can be applied at CCGT unit to reduce heat and hydraulic losses in the diffuser path and in the utilizer boiler path.

The efficiency of a gas turbine largely depends on the aerodynamics and pressure recovery capacity of the diffuser. For reliable numerical simulation of the flow in the diffuser, the model must be validated on the basis of experimental data on the flow structure. An experimental and numerical study of the stage diffuser system was carried out. The results of this investigation are as follows: the area of applicability of the numerical method for assessing the flow in the stage diffuser system was determined; recommendations for preparing a numerical model and transferring boundary conditions from domain to domain were developed; the importance of profiling the last turbine stage to ensure unseparated flow entry into the diffuser is indicated; the influence of the hub length and the geometry of the struts on the losses in the diffuser and its pressure recovery capacity is determined. It is shown that increasing the hub length to certain limits improves the pressure recovery ratio of the diffuser. The smallest thickness of the struts gives the best results; the tangential and axial slope of the struts does not make a significant contribution in the nominal operating mode of the gas turbine.