Mixture quality of a vortex generator premixer and alternative premixer designs in the auto-ignition regime of hydrogen air flames

The application of vortex generator premixers (VGPs) is particularly challenging for highly reactive fuels in recuperated gas turbines, because high combustor inlet temperature leads to a potential risk of premature self-ignition and flame flashback. As current knowledge does not extend to the temperature range far above the self-ignition temperature, an experimental investigation of the operational limits of VGPs is conducted at the Thermodynamics Institute of the Technical University of Munich. The study is particularly focused on highly reactive fuels and the thermodynamic conditions present in recuperated gas turbines with pressure ratios of 4-5. The present study is focuses on fuel-air mixing at the corresponding high air temperatures. A fuel-air mixing device is required to achieve sufficient mixing quality without excessive premixer length. Vortex generators are known to be effective in augmenting the distribution of fuel injected from the tube wall over the cross section of the tube. In the range of typical gas turbine combustor inlet temperatures, the performance of VGPs has already been investigated for methane as well as for hydrogen-methane blends. The limits of operating a VGP under auto-ignition relevant conditions were presented in a previous study. In this study, the VGP's mixture quality under these conditions is experimentally investigated. For this purpose, the existing test rig has been modified to conduct high speed PIV and MixPIV measurements. Measurements at different positions inside and downstream of the injector have been performed. Two other mixer types in addition to the VGP are investigated to determine the influence of mixture quality on auto-ignition behavior in a future study and to validate MixPIV measurements. The influence of the momentum flux ratio on mixture quality is presented for the three mixer types. Comparison shows that the VGP exhibits significantly better mixture homogeneity at the mixer exit than do the two other mixer types.