Large-Eddy Simulation of swirled flame stabilisation using NRP discharges at atmospheric pressure

Abstract

In this work, the use of Nanosecond Repetitively Pulsed (NRP) discharges is investigated numerically to improve the stabilisation of the flame in a swirl burner. The studied configuration is the PACCI burner test rig set up at KAUST, in which it was experimentally demonstrated that NRP discharges can effectively enhance flame stability. Simulations are performed with the reactive compressible Navier–Stokes solver AVBP, using a recently developed phenomenological model for plasma-assisted combustion of methane-air premixed flame. The ability of the numerical model to reproduce the main flow behaviours is first assessed by comparison with PIV measurements in cold flow conditions. Then, a lean atmospheric pressure case (ϕ=0.67) without plasma actuation is simulated and compared with OH* chemiluminescence image from experiment. Both numerical and experimental results reveal an unstable turbulent flame, with intermittent attachment to the burner exit. Finally, two operating conditions with NRP discharges are simulated. The NRP discharges correspond to 10 kV pulses applied at a frequency of 20 and 30 kHz, respectively corresponding to a discharge power 0.72 and 1.17% of the thermal flame power. First, a significant stabilisation effect is observed when applying the discharges, which efficiently mitigate the flame lifting. This effect is quantitatively demonstrated by tracking the flame centre of gravity, which shows that the flame moves upstream under the influence of the plasma, in good agreement with experimental observations. Results also show that a net power gain is obtained by applying NRP discharges to combustion by comparing the thermal flame power to the plasma power. In particular, an increase of plasma actuation efficiency from 4 to 6 as been observed in the cases studied by increasing the plasma power.