An aerodynamic study of industrial gas turbine exhaust systems
A combined expenmental and computational study has been carried out on a scale model of an industrial gas turbine exhaust system to improve understanding of its complex flow field and to validate CFD predictions. The model consists of a set of OGVs which guide flow into a strutted annular diffuser followed by a volute box and an exit duct. Turbulent flow diffusion and turning processes occurring inside a typical industrial gas turbine exhaust system are complex and three-dimensional in nature. With a growing trend towards high-efficiency/low-noise gas turbine power plants, both aerodynamic and acoustic management of gas turbine exhaust systems are receiving attention in more recent designs The aerodynamic and acoustic performance of such systems is particularly influenced by off-design conditions (power turbine operatmg at part load) when the incidence angle onto the OGV s increases considerably. This aspect is given particular attention in the present work. Detailed 3D velocity measurements were carried out inside the annular diffuser and in the exit duct using five-hole pneumatic probes and hotwires. The performance was shown to be particularly sensitive to the inlet OGV wake conditions Measurements carried out downstream of the diffuser struts indicated that there was no evidence of dominant vortex shedding from the struts, which was initially thought to be a potential source of noise generation in exhaust systems. Numerical analysis was performed using a multi-block 3D RANS solver utilising a pressure-correction method and a k-s turbulence model. When the inlet conditions for the CFD predictions were matched to the measured wake structure, the flow within the annular diffuser and the system total pressure loss coefficient were predicted adequately. The calculations were analysed to investigate the distribution of loss between individual components. This indicated that 50% of the loss was due to flow turning and mixing in the volute, and this allowed possible geometric modifications to reduce system loss to be suggested. Based on the overall comparison between the measurements and predictions, this study concludes that the applied CFD method is capable of predicting complex gas turbine exhaust system flow sufficiently and accurately for design applications.