THE ROLE OF DENSITY GRADIENT IN SPECIES MIGRATION AND SECONDARY FLOW STRUCTURES IN AXIAL FLOW TURBINES

Physical quantities at combustor exit, hence turbine inlet, are highly non-uniform not just due to spatially varying combustion but also due to the dilution air and film-cooling air used in the combustor design. The effects of inlet total pressure and temperature ("hot streak") non-uniformity on the unsteady flow and heat transfer of turbine stages have been widely studied. However, few studies have considered the effects of inlet density non-uniformity derived from spatially varying species concentration ("species streak"). This "species streak" results in density gradients which, if not aligned with pressure gradients, will lead to the generation of baroelinie torque which will influence the generation, migration and evolution of voracity within the turbine passages and hence the secondary flow structure and mixing within the blade row. This paper examines the "species streak" effects on the unsteady flow and heat transfer within a high-pressure axial flow turbine stage focusing on the flow through the rotor. First, a validation study was carried out to check the capability of the selected CFD in modeling unsteady turbine stage flows. Time-accurate solutions were achieved and the results agreed well with the available experimental measurements. Based on the validation, two expanded case studies were carried out to investigate the "species streak" effects on the secondary flow and heat transfer in turbine rotor passages. It was found that the "species streak" could generate "hot streak enhancement structure" as well as "baroclinic torque structure" in rotor passages, which would work with the inherent secondary flow structures in rotor to determine its heat transfer. It was also found that the contributions of the baroclinic torque source term could have magnitudes comparable to other effects, such as vortex stretching, in creating secondary flows. In some circumstances, however, the overall effects of the baroclinic torque could be partially reduced by opposite vortex stretching effects generated by the same density gradients.