Numerical investigations of turbulent premixed flame ignition by a series of Nanosecond Repetitively Pulsed discharges

Abstract

Nanosecond Repetitively Pulsed (NRP) discharges are an efficient way to promote turbulent flame ignition in lean regimes. The energy released by NRP discharges leads first to an ultra-fast species dissociation and heating phenomena, followed by a slow heating process. A phenomenological plasma model is presented to capture the influence of NRP discharges on the combustion process at low CPU cost. The model is here implemented in a LES flow solver to simulate the ignition sequence of a bluff-body turbulent premixed flame by a series of NRP discharges. Two numerical computations are performed. First, only the thermal effects of the discharge (ultra-fast heating and slow heating due to vibrational energy relaxation) are taken into account. Then both the thermal and chemical effects (mainly O2 dissociation into O) are considered. The results show that in the first simulation the ignition never occur, whereas in the second simulation flame ignition occurs after only 5 pulses. The ignition success or failure results from a competition between the residence time of the reacting gases in the discharge channel and the combustion chemistry time scale. A low-order model based on a perfectly stirred reactor (PSR) is then derived. It confirms that the atomic O produced during the discharge enhances the methane oxidation reactions, reducing the combustion chemistry time scale and leading to a successful ignition. PSR results are used to build-up a plasma-assisted ignition diagram which indicates the number of pulses required to form a turbulent flame kernel.