This work presents a numerical study of a technically premixed swirling combustor with central air injection at conditions close to flashback using large-eddy simulation with flamelet modelling. The study shows the assumption of perfect premixing is valid during the stable operation of the burner up to flashback conditions. The experimental results are well predicted under inert and reacting conditions by using a perfectly premixed mixture. It is found that the non reacting flow field develops a self-excited oscillation in the form of a precessing vortex core. This oscillation is attenuated by the fuel injection due to the respective increase in axial momentum and it is ultimately suppressed in the reacting flow field. Both experiments and simulations confirm the same trends. The analysis of the flames have shown certain dynamics as the flashback point is approached. The flashback resistance of the burner is minimized due to an increase in the velocity deficit of the incoming mixture. The recirculation region is shifted upstream, the central recirculation is altered and the flame position is displaced towards the reactants. OH-PLIF measurements are compared with the OH predictions by the LES and certain level of disagreement is observed. This modelling approach is found to be valid to predict the hydrodynamic behaviour of the flames in terms of velocity fields and flow oscillations, but it can not predict the OH formation found in the post combustion zone across the reacting layer. Keywords premixed burner • swirl-stabilized flames • flashback safety • precessing vortex core • flamelet Address(es) of author(s) should be given This is a post-peer-review, pre-copyedit version of an article published in Flow, turbulence and combustion.

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In the present study numerical results of simulations, using RANS and LES, of the non-reacting flow in a swirl-stabilized burner are presented. The burner was developed for lean premixed combustion with high fuel flexibility at low emissions. An important challenge for a fuel-flexible, low emission combustor is the prevention of flashback for fuels of high reactivity, such as hydrogen, without compromising on lean blow out safety and mixing quality. Flashback safety can be increased by a sufficiently high and uniform axial velocity at the end of the mixing tube. In the investigated combustor the velocity deficit in the center of the mixing tube, which results from the swirl, is prevented by a non-swirling axial jet. In a parametric study the effect of different amounts of axial injection on the flow field is investigated. The results are validated with experimental data, gained from PIV measurements in a vertical water tunnel. It is shown that the mean flow field can be well captured by steady-state RANS simulations using a realizable k-ε turbulence model. The most suitable geometry is identified and, subsequently, transient LES simulations are conducted. The dynamic flow field characteristics are investigated. It was found that in spite of the high swirl, the flow field is quite stable and no dominating frequency is detected. The flow field of the swirling flow in the combustion chamber can be captured well using LES. Furthermore, the mixing quality is compared to the experiments, which are performed in a water tunnel. In contrast to the RANS simulation, the LES can qualitatively capture the spatial unmixedness observed from experimental data. All simulations were conducted using water as fluid.

It is generally accepted that combustion of hydrogen and natural gas mixtures will become more prevalent in the near future, to allow for a further penetration of renewables in the European power generation system. The current work aims at the demonstration of the advantages of steam dilution, when highly reactive combustible mixtures are used in a swirl-stabilized combustor. To this end, high-pressure experiments have been conducted with a generic swirl-stabilized combustor featuring axial air injection to increase flashback safety. The experiments have been conducted with two fuel mixtures, at various pressure levels up to 9 bar and at four levels of steam dilution up to 25% steam-to-air mass flow ratio. Natural gas has been used as a reference fuel, whereas a mixture of natural gas and hydrogen (10% hydrogen by mass) represented an upper limit of hydrogen concentration in a natural gas network with hydrogen enrichment. The results of the emissions measurements are presented along with a reactor network model. The latter is applied as a means to qualitatively understand the chemical processes responsible for the observed emissions and their trends with increasing pressure and steam injection.

The current study presents a numerical investigation of the flow field of a swirl-stabilized burner featuring a non-swirling axial air jet on the central axis of the mixing tube. The system has been designed and optimized to burn hydrogen at the Technische Universität Berlin over the last 6 years in the context of the EU-funded projects GREENEST and AHEAD. As the burner design was based on experimental work, high-fidelity large-eddy simulations (LES) are used to provide deeper understanding on the non-reacting and reacting flow fields to elucidate the occurrence of flashback under certain operating conditions. The experimental measurements suggest that flashback is produced by a velocity deficit at the mixing tube outlet and these conditions are analyzed here using LES. The work includes code validation for non-reacting and reacting conditions by comparison to water tunnel and combustion test rig data, and aims to evaluate the accuracy of LES with a combustion model based on premixed flamelets to predict the reacting flow field under conditions close to flashback.