Modeling of Streamer-to-Spark Transition during Plasma Assisted Ammonia Ignition

Despite being considered as a promising carbon-free, renewable fuel alternative, green ammonia ignition and combustion is plagued with several challenges that can partially be addressed using non-equilibrium plasmas. This work focusses on modeling the evolution of streamers and the transition to spark formation in atmospheric pressure plasmas in premixed mixtures of ammonia and air. The electron avalanche, streamer formation and transition to spark are all modeled using a 1D self-consistent fluid-plasma solver which uses implicit time integration with geometric multigrid method based preconditioning. All simulations are performed between parallel plate electrodes. The production of charged species and neutral radicals, and the change in the reduced electric field due to gas heating by the plasma are quantified, and their effect on the plasma conductivity and the overall time for streamer to spark transition has been discussed in this work. Spatiotemporal budgets of electron energy used towards different processes at different phases of the streamer and spark evolution are explained by performing parametric investigations by varying the mixture equivalence ratio, inter-electrode gap distance and the applied voltage. Conditions which result in failed transition to sparks are emphasized and their efficacy towards ignition success has been discussed.