The focus of this study is the combustion of hydrogen in air as it relates to typical gas turbine engines. Hydrogen–air combustion occurs in the absence of any carbon-based emissions and the only combustion products are water vapor and oxides of nitrogen (NOx). However, due to the very low flammability limit of hydrogen, it can be burned at much lower equivalence ratios than typical hydrocarbon fuels, resulting in excellent low NOx potential. Lean premixed combustion of low reactivity fuels, such as natural gas, is nowadays state of the art in stationary gas turbines. In the long term, it is also a promising approach for aero engines. For lean premixed combustion, with increasing fuel reactivity lean blow out limits are extended but the disposition for flashback, an undesired event of upstream flame propagation, is increased. Therefore, combustor design strategies that are applied for conventional fuels have to be revisited in case of hydrogen, which represents the upper end of the scale of high reactivity fuels. The current thesis aims at developing a combustor design that is capable of safely operating on hydrogen–air mixtures up to stoichiometric conditions while meeting strict emission regulations. To this end, several measures affecting the flashback resistance of a hydrogen–air combustor are investigated. In addition to their effect on flashback resistance, all measures are evaluated with respect to their impact on fuel–air mixing which directly affects NOx emissions. Unlike most previous investigations on hydrogen–air combustion, the current investigations are conducted at partially premixed instead of perfectly premixed conditions. This poses a challenging task with respect to achieving flashback resistance as well as low \mathrm{NO_{x}} emissions with limited premixing space and time. Experimental investigation of non-reacting and reacting combustor flow fields of a partially premixed model combustor were conducted using particle image velocimetry in an atmospheric combustor tests rig. Results reveal a strong influence of geometric modifications and fuel momentum on the combustor flow field. Stability maps were recorded that allow for comparison of the operational range of different combustor geometries with respect to flashback and lean blow out. It was shown that already moderate flow rates of a central non-swirling air jet significantly extend the flashback limits, while the lean blow out limits remained unaffected. Moreover, recordings of planar laser-induced fluorescence of the hydroxyl radical (OH-PLIF) within the flame revealed that, the axial location of the upstream flame front, x_{f}, constitutes a telling estimator for flashback resistance. At the investigated conditions, x_{f} is shifted downstream with increasing equivalence ratio due to the added momentum of the fuel flow. Thereby, the local gain in axial velocity due to fuel momentum supersedes any parallel augmentation in the turbulent flame speed. This has been identified as a driving mechanism affecting the combustor stability limit.Performance and emissions data facilitate the conclusion that the desired flashback-safe operation at very low NOx emissions at ambient pressure and relevant combustor inlet temperatures is feasible.


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    Title :

    Flashback prevention in lean-premixed hydrogen combustion


    Additional title:

    Vermeidung von Flammenrückschlag in mager vorgemischter Verbrennung von Wasserstoff


    Contributors:

    Publication date :

    2017



    Type of media :

    Miscellaneous


    Type of material :

    Electronic Resource


    Language :

    English



    Classification :

    DDC:    629



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