Distributed Electrical Discharge to Improve the Ignition of Premixed Quiescent and Turbulent Mixtures 2016-01-0706
The present work investigates the efficacy of distributed electrical discharge to increase the ignition volume by means of multipole spark discharge and radio frequency (RF) corona discharge. A range of ignition strategies are implemented to evaluate the efficacy of distributed ignition. The multipole spark igniter design has multiple high-voltage electrodes in close proximity to each other. This distributed spark ignition concept has the ability to generate multiple flame kernels either simultaneously or in a staggered mode. A novel elastic breakdown ignition strategy in responsive distribution (eBIRD) high frequency discharge is also implemented via the multipole igniter. The RF corona discharge is generated through an in-house developed ignition system. A form of distributed ignition is initiated along the streamer filaments. The igniter efficacy is evaluated visually at low pressure conditions in an optically-accessible constant volume chamber to assess the flame kernel growth rate and the mutual interaction of the flame kernels. Pressure-based burn-rate measurements are conducted at moderate pressure conditions in a non-optical chamber. The test charges used are methane-air and propane-air mixtures ranging from stoichiometric conditions to the lean ignitability limits of each igniter configuration. The results indicate that in the inductive ignition system, increasing the discharge energy has only a limited ability to enhance the initial flame growth and extend the stoichiometry ignitability limits. When an equivalent amount of energy is distributed into several ignition kernels, however, the effect is remarkably increased early flame growth and in some configurations extended lean ignition limits. The coupling of a small peaking capacitor into the secondary circuit of each electrode further extends the efficacy of the distributed spark ignition method without any increase in primary energy consumption. The corona discharge shows the ability to initiate multiple ignition spots in the igniter proximity and continuously impact the ignition flame propagation when a long energizing period is employed. The corona discharge duration plays an important role for igniting the mixtures under both quiescent and turbulence mixture conditions. Longer corona discharge duration results in both accelerated ignition flame propagation and extended lean ignitable boundary.