The Effects of Inlet Temperature and Turbulence Characteristics on the Flow Development Inside a Gas Turbine Exhaust Diffuser
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Abstract
The overall industrial gas turbine efficiency is known to be influenced by the pressurerecovery in the exhaust system. The design and, subsequently, the performanceof an industrial gas turbine exhaust diffuser largely depend on its inflow conditionsdictated by the turbine last stage exit flow state and the restraints of the diffuserinternal geometry. Recent advances in Computational Fluid Dynamics (CFD) toolsand the availability of computer hardware at an affordable cost made the virtual toola very attractive one for the analysis of fluid flow through devices like a diffuser. Inthis backdrop, CFD analyses of a typical industrial gas turbine hybrid exhaust diffuser,consisting of an annular diffuser followed by a conical portion, have been carriedout with the purpose of improving the performance of these thermal devices usingan open-source CFD code "OpenFOAM". The first phase in the research involvedthe validation of the CFD approach using OpenFOAM by comparing CFD resultsagainst published benchmark experimental data. The numerical results closely capturedthe flow reversal and the separated boundary layer at the shroud wall wherea steep velocity gradient has been observed. The standard k –ε turbulence modelslightly over-predicted the mean velocity profile in the casing boundary layer whileslightly under-predicted it in the reversed flow region. A reliable prediction of flowcharacteristics in this region is very important as the presence of the annular diffuserinclined wall has the most dominant effect on the downstream flow development. The core flow region and the presence of the hub wall have only a minor influence as reported by earlier experimental studies. Additional simulations were carried out in thesecond phase to test the veracity of other turbulence models; these include RNG k–ε,the SST k–ω, and the Spalart-Allmaras turbulence models. It was found that a highresolution case with 47.5 million cells using the SST k–ω turbulence model produceda mean flow velocity profile at the middle of the annular diffuser portion that had thebest overall match with the experiment. The RNG k –ε, however, better predictedthe diffuser performance along the exhaust diffuser length by means of the pressurerecovery coefficient. These results were obtained using uniform inflow conditions andsteady-state simulations. As such, the last phase of our investigations involved varyingthe inflow parameters like the turbulence intensity, the inlet flow temperature, andthe flow angularity, which constitute important characteristics of the turbine bladewake, to investigate their impact on the diffuser design and performance. Theseisothermal CFD simulations revealed that by changing the flow temperature from 15to 427°C, the pressure recovery coefficient significantly increased. However, it hasbeen shown that the increase of temperature had no effects on the size of the reversedflow region and the thickness of the separated casing boundary layer, although theflow appears to be more turbulent. Furthermore, it has been established that an optimum turbulence intensity of about 4% produced comparable diffuser performance as the experiment. We also found that a velocity angle of about 2.5° at the last turbinestage will ensure a better exhaust diffuser performance.