Swirler geometry effects (dh/do ratio) on synthetic gas flames. Part 2: dynamic flame behaviour at external y altered acoustic conditions

In a combustion device, unsteady heat release causes acoustic energy to increase when acoustic damping (energy loss) is not that effective, and, as a result, thermo-acoustic flame instabilities occur. In this study, effects of the swirler dh/do ratio (at different swirl numbers) on dynamic flame behaviour of the premixed 20%CNG/30%H2/30%CO/20%CO2 mixture under externally altered acoustic boundary conditions and stability limits (flashback and blowout equivalence ratios) of such mixture were investigated in a laboratory-scale variable geometric swirl number combustor. Therefore, swirl generators with different dh/do ratios (0.3 and 0.5) and geometric swirl numbers (0.4, 0.6, 0.8, 1.0 1.2 and 1.4) were designed and manufactured. Acoustic boundary conditions in the combustion chamber were altered using loudspeakers, and flame response to these conditions was perceived using photodiodes and pressure sensors. Dynamic flame behaviour of respective mixture was evaluated using luminous intensity and pressure profiles. Results showed that the dh/do ratio has a minor impact on dynamic flame behaviour.

Analysis of the Emission Reduction Potential and Combustion Stability Limits of a Hydrogen-Fired Gas Turbine With External Exhaust Gas Recirculation

To mitigate its impact on global climate, the power generation sector must strive towards a transition to net-zero emissions of greenhouse gases. This can be achieved by a massive penetration of renewable power generation. However, a high share of renewable power generation requires dispatchable and flexible power generation technologies such as gas turbines to maintain the stability of power grids. To achieve net-zero green house gas emissions, gas turbines have to be operated exclusively with carbon-neutral fuels. Hydrogen is a promising carbon-neutral fuel, although it comes along with several challenges regarding stable combustion. A possible measure to stabilize hydrogen combustion is the partial external recirculation of exhaust gases (EGR). In a previous study, the authors presented a model-based thermodynamic analysis of an industrial gas turbine featuring EGR. The next step was to answer the question of whether the thermodynamically negative impact of EGR (i.e. lower thermal efficiency) is justified by positive effects, such as reduced NOx emissions or a more controllable combustion of hydrogen. By means of a simple 1-D flame approach, the present study provides further insight into the flame behaviour and stability limits during a fuel switch from natural gas to hydrogen. In a following step, the same approach is used to investigate the flame behaviour in an EGR environment at two recirculation temperatures. The results show that if a hydrogen-fired, diffusion-type combustor is combined with sufficiently high EGR ratios, NOx emissions are potentially in the order of a state-of-the-art diffusion-type combustor fired with natural gas. In addition, based on the calculated laminar flame speeds and extinction strain rates, the higher reactivity of hydrogen could potentially be controlled by employing EGR. However, relevant literature suggests that stronger dilution might be required to compensate for the additional impact of turbulence-chemistry interaction in real application which could lead to flame stabilization issues and higher NOx emissions. Moreover, considering the industry efforts to develop hydrogen-capable premixed-type combustors, the results show that EGR has no significantly positive influence on the reactivity of a premixed pure hydrogen flame. The question regarding the preferred EGR temperature is addressed but cannot be answered conclusively.

Experimental and Numerical Analysis of Gas Premix Turbulent Flames Stabilized in a Swirl Burner with Central Bluff Body

Lean premixed gas turbulent flames stabilized in the flow generated by an industrial swirl burner with a central bluff body are experimentally found to behave bi-stable. This bi-stable behaviour, which can be triggered via a small change in some of the controlling parameters, for example the bulk equivalence ratio, consists in a rather sudden transition of the flame from completely lifted to well attached to the bluff body. While several experimental investigations exist on this topic, numerical analysis is limited. The present work is therefore also of numerical nature, with a two-fold scope: a) simulation and validation with experiments of the bi-stable flame behaviour via Computational Fluid Dynamics (CFD) in the form of Large Eddy Simulation (LES) and b) analysis of CFD results to shed light on the flame stabilization properties. LES results, in case of the lifted flame, show that the vortex core is sharply precessing at a given frequency. Phase averaging these results at the frequency of precession clearly indicates a counter-intuitive and unexpected presence of reverse flow going all the way through the flame apex and the bluff body tip. A simple one-dimensional flame stabilization model is applied to explain the bi-stable flame behaviour.

Experimental and Numerical Analysis of Gas Premix Turbulent Flames Stabilized in a Swirl Burner With Central Bluff Body

Lean premixed gas turbulent flames stabilized in the flow generated by an industrial swirl burner with a central bluff body are experimentally found to behave bi-stable. This bi-stable behaviour, which can be triggered via a small change in some of the controlling parameters, for example the bulk equivalence ratio, consists in a rather sudden transition of the flame from completely lifted to well attached to the bluff body. This has impact on combustion dynamics, emissions and pressure losses. While several experimental investigations exist on this topic, numerical analysis is limited. The present work is therefore also of numerical nature, with a two-fold scope: a) simulation and validation with experiments of the bi-stable flame behaviour via Computational Fluid Dynamics (CFD) in the form of Large Eddy Simulation (LES) and b) analysis of CFD results to shed light on the flame stabilization properties. LES results, in case of the lifted flame, show that the vortex core is sharply precessing at a given frequency. Phase averaging these results at the frequency of precession clearly indicates a counter-intuitive and unexpected presence of reverse flow going all the way through the flame apex and the bluff body tip. The counter-intuitive presence of a lifted flame is explained here in terms of the phase averaged data which show that the flame apex is not placed at the centre of the spinning reverse flow region. It is instead slightly shifted radially outward where the axial velocity recovers to low positive values of the order of the turbulent burning rate. A simple one-dimensional flame stabilization model is applied to explain this peculiar flame behaviour. This model provides first an estimation of the flame radius of curvature in terms of axial velocity and turbulence quantities. This radius is therefore used to determine the total flux of reactants into the flame, given by an axial convection and a radial diffusion contributions. Subsequently the possibility of the flame positioned at the centre of the vortex is excluded based on the balance between this flux and the turbulent burning rate. A clear explanation of the mechanism leading to the sudden flame jump has instead not been identified and only some hypotheses are provided.