Recent experiments \[Prieur, K., Vignat, G., Durox, D., Schuller, T., and Candel, S. (January 9, 2019). "Flame and Spray Dynamics During the Light-Round Process in an Annular System Equipped With Multiple Swirl Spray Injectors." ASME. *J. Eng. Gas Turbines Power*. June 2019; 141(6)\] indicate that the flame initiated during the ignition of a swirling injector features different configurations before being stabilized at a distance from the injector exhaust. During an initial period the flame penetrates the injection unit and the flame foot is located in the inner part of this device. During a second stage the flame is expelled from this unit and after a delay it reaches a new equilibrium where it is stabilized by the swirling flow and recirculating hot gases at a distance from the injector exhaust. These important dynamical phenomena have been characterized with experiments on a single injector configuration consisting of a swirling unit and a simplex pressure atomizer fed with liquid n-heptane and air. The combustion chamber is operated at ambient pressure and is confined by a cylindrical quartz glass to allow for optical access. The flame configuration was recorded with a high-speed camera while the heat release rate was estimated from the free radical emission detected by a photomultiplier. The temperature at the inner chamber wall and the combustor backplane was tracked with thermochromic paint whereas thermocouples were used at the outer chamber wall. The flow on the upstream side of the injector was measured with a hot wire.

The characterization and understanding of such flame dynamics during ignition is vital for the operation of such a combustor as the flame transient stabilization inside the injection unit could lead to detrimental effects.

In this study, the observed phenomena are investigated by combining simulation results and theoretical analysis. Calculations are carried out in the same configuration using large eddy simulations including an Euler-Lagrange representation of the liquid spray. Results of calculations are used to interpret the observed phenomena. It is shown for example that the initial penetration of the flame in the injector unit is linked with the positive pressure excursion that is induced by the volumetric expansion of burnt gases associated with the fast increase in heat release rate corresponding to the initiation of the process. Theoretical estimates of the pressure increase agree with the values observed in the simulation. These positive and negative pressure disturbances induce a rapid reduction of the mass flow rate through the injector followed by a rapid acceleration of the flow and a return to the nominal value. It is also shown that the flame foot disappears after another delay, which results in the flame foot lifting, an increase of the flame surface and a flame stabilization at a distance from the injector exhaust corresponding to steady operation of the device. This detachment of the flame foot is only observed when wall heat losses are accounted for in the calculation. The relatively long delay time before this phenomenon takes place is seen to correspond to the residence time of the cooled burnt gases in the vicinity of the chamber walls which are ultimately entrained by the internal recirculation zone and quench the upstream flame foot.


Large-Eddy Simulation of Flame Dynamics During the Ignition of a Swirling Injector Unit and Comparison With Experiments

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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