Study on lifted flame stabilization under different background pressures

A numerical experimental investigation is presented for a steady methane lifted-flame and a non-reaction jet flow in a co-flow of hot combustion products from lean premixed air/hydrogen combustion. A pressurized vitiated co-flow burner has been employed to study the methane lifted flame and non-reaction jet flow under different background pressures. The lift-off height has been measured with a high-speed camera, and the central jet flow velocity has been measured by means of a Schlieren imaging system. The experimental results show that the lift-off height decreases for an increment in the background pressure and in the co-flow temperature. As far as the experimental tests on the non-reaction jet flow is concerned, the jet velocity becomes extinct faster as the background pressure rises. The evolution of the jet velocity has been proved to be another important factor that affects the lift-off height under different background pressures, in addition to the fuel autoignition delay. The simulation data led with a RANS/PDF model show that an increment in the background pressure makes the temperatures increase and induces a brighter yellow part of lifted flame, which leads to more soot production. This proves that the flame is not completely premixed. On the other hand, the Schlieren images of a non-reaction jet flow highlight that the flame is partially premixed, since the edge of the jet is not well defined, as the jet penetration increases with time.

Numerical Modeling of Gaseous Partially Premixed Low-Swirl Lifted Flame at Elevated Pressure

Lifted flames have been investigated in the past years for their benefits in terms of NOx emissions reduction for gas turbine applications. In a lifted flame, the flame front stabilized on a position that is significantly detached from the nozzle exit, improving the premixing process before the reaction zone. The distance between the flame front and the nozzle exit is called lift-off height and it represents the main parameter that characterize this type of flame. In the present work, a partially premixed lifted flame employing air-methane mixture is investigated through numerical simulation. Indeed, even if lifted jet flames have been widely studied in the literature, there are only a few examples of lifted partially premixed flames. Nevertheless, this kind of flames assumes an important role considering the current gas turbine applications, since their benefits in terms of stability and low pollutant emissions. This study has been performed with LES calculations using a commercial software suite and the numerical results are compared with experimental data coming from a dedicated campaign held at Karlsruher Institute für Technologie (KIT) on a novel low-swirl injector nozzle. Quenching effects due to strain, curvature and heat loss have been introduced into the combustion model thanks to a correction of the source term in the progress variable equation within the FGM model. The comparison between numerical results and experimental data have been performed in terms of lift-off height and OH* chemiluminescence maps, showing the capability to properly predict the overall flow and to catch flame lift-off even if with an underpredicted height. This points out promising capability of the numerical model in the representation of lifted flames, allowing further investigations of the flame structure otherwise not available from experimental techniques.

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.