Being free from carbon content, hydrogen has been considered as a promising candidate to reduce pollutant emissions in Gas Turbine Combustion Systems. Due to hydrogen’s significantly different burning characteristics, its implementation requires adjustments to the design philosophies of traditional combustion chambers. The micromix concept offers an alternative diffusive combustion injection system, improving the mixing characteristics without the risk associated with pre-mixing, thereby reducing the likelihood of hotspots forming. The importance of turbulence-chemistry interaction modelling, particularly for highly diffusive flames such as hydrogen, has been widely addressed. A turbulence-chemistry interaction study on such a micromix injector was performed investigating the coupling between the Flamelet Generated Manifold (FGM) combustion model and different hydrogen reaction mechanisms. This methodology correctly reproduces the typical micromix micro-flame behaviour and the analysed mechanisms are shown to be in good agreement in terms of flow characteristics prediction. A comparative study between two reduced order emissions prediction models was then carried out: a CFD post-processing technique for NOx emissions calculations and a hybrid CFD-CRN approach were explored. Due to the coupling between accurate turbulence-chemistry interaction modelling and the ability to handle detailed chemistry, the hybrid CFD-CRN approach gives valuable results with a modest computational cost and it could be used as an optimising tool during the injector geometry design process.
Hydrogen has been proposed as an alternative fuel to meet long term emissions and sustainability targets, however due to the characteristics of hydrogen significant modifications to the combustion system are required. The micromix concept utilises a large number of miniaturised diffusion flames to improve mixing, removing the potential for local stoichiometric pockets, flash-back and autoignition. No publicly available studies have yet investigated the thermoacoustic stability of these combustion systems, however due to similarities with lean-premixed combustors which have suffered significant thermoacoustic issues, this risk should not be neglected. Two approaches have been investigated for estimating flame response to acoustic excitations of a single hydrogen micromix injector element. The first uses analytical expressions for the flame transfer function with constants obtained from RANS CFD while the second determines the flame transfer function directly using unsteady LES CFD. Results show the typical form of the flame transfer function but suggest micromix combustors may be more susceptible to higher frequency instabilities than conventional combustion systems. Additionally, the flame transfer function estimated using RANS CFD is broadly similar to that of the LES approach, therefore this may be suitable for use as a preliminary design tool due to its relatively low computational expense.
Combustor-exit conditions in modern gas turbines are generally characterized by significant temperature distortions and swirl degree, which in turn is responsible for very high turbulence intensities. For this reasons combustor-turbine interaction studies have gained a lot of importance. Past studies have focused on the description of the effects of turbulence, swirl degree and temperature distortions on the behavior of the high pressure stages of the turbine, both considering them as separated aspects, and accounting for their combined impact. Concerning the external heat transfer coefficient, swirl and temperature distortions represent a severe challenge for the commonly adopted measurement techniques. The work presented in this paper was carried out on a non-reactive, three-sector test rig made by a non reactive combustor simulator and a nozzle guide vane cascade; it is able to create a representative combustor outflow, characterized by all the flow characteristics described before. A novel experimental approach, that was developed in a previous work, was exploited to experimentally retrieve the heat transfer coefficient and the adiabatic wall temperature distributions on a noncooled nozzle guide vane. The results allowed to evidence the effect of the inlet swirl on the heat transfer coefficient distribution, as well as the evolution of the temperature distribution on the vane surface moving through the cascade, constituting the first attempt to evaluate these aspects from a purely experimental point of view.
The pronounced non uniform temperature distribution in the core engine flow path is a recurring problem of gas turbine engine design process, as it directly affects turbine performance and lifetime. Specifically, turbine entry conditions are usually characterised by severe temperature distortions, often referred to as hot and cold streaks. Temperature distortions remain an issue even at the exit section of the NGV, with additional cold streaks coming from the vane cooling system. Various studies focus on the description of streaks migration through a direct investigation of the thermal field, providing a global evaluation of the phenomenon. As a deeper understanding is often required, experimental techniques based on the detection of tracer gases can be successfully adopted. In this study, a realistic combustor outlet swirl profile was imposed on a fully cooled NGV cascade to analyse both film-cooling behaviour and cold streaks migration. A concentration probe based on the fluorescence behaviour of an oxygen sensor was here employed to track the position of the film cooling flows at the NGV cascade exit plane, while the adiabatic film-cooling effectiveness was evaluated on the NGV surfaces employing the PSP technique. Overall, the swirling structure strongly affect both the film-cooling behaviour and cold streaks migration through and downstream the vane. The importance of examining the unsteady aspect is also highlighted. A global understanding of the occurring phenomena is therefore provided, as well as significant pieces of information useful for the design phases of both the NGV and the following rotor cascade.
Combustor-exit conditions in modern gas turbines are generally characterized by significant temperature distortions and swirl degree, which in turn is responsible for very high turbulence intensities. These distortions have become particularly important with the introduction of lean combustion, as a mean to control NOx pollutant emissions. For this reasons combustorturbine interaction studies have recently gained a lot of importance. Past studies have focused on the description of the effects of turbulence, swirl degree and temperature distortions on the behavior of the high pressure stages of the turbine, both considering them as separated aspects, and accounting for their combined impact. Aspects like pressure losses, hot streaks migration and film-cooling behavior have been widely investigated. Even if some studies have focused on the characterization of the heat transfer coefficient (HTC) on the nozzle guide vane external surface, none of them have addressed this aspect from a purely experimental point of view. Indeed, when inlet conditions are characterized by both swirl and temperature distortions they represent a severe challenge for the commonly adopted measurement techniques. The work presented in this paper was carried out on a non-reactive, annular, three-sector test rig made by a non-reactive combustor simulator and a nozzle guide vane cascade; it is able to create a representative combustor outflow, characterized by all the flow characteristics described before. A novel experimental approach, that was developed in a previous work, was exploited to experimentally retrieve the heat transfer coefficient and the adiabatic wall temperature distributions on a non-cooled nozzle guide vane. Temperature measurements on the cascade inlet and outlet planes were also used to provide boundary conditions and achieve a better understanding of the investigated phenomena. The results allowed to evidence the effect of the inlet swirl on the heat transfer coefficient distribution, as well as the evolution of the temperature distribution on the vane surface moving through the cascade, constituting the first attempt to evaluate these aspects from a purely experimental point of view.
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