Numerical model of a pulsed subcritical streamer microwave discharge for problems of plasma ignition of fuel mixtures in the gas phase

An approximate model for estimating plasma heating and conversion caused by a subcritical streamer microwave discharge has been considered and verified. Ignition occurs in an environment with a pressure of 13 kPa, a temperature of 150 K, there is an external air flow at a speed of up to 500 m/s, an initiator antenna and a flat mirror are used to focus electromagnetic radiation, a stoichiometric mixture of propane-air and pure propane is supplied through the cavity in the antenna. Radiation power is 3 kW. The model is three-stage and semi-empirical. The plasma region and its conductivity are set based on experimental statistics; this is a key feature that reduces the consumption of computing resources. The finite element method is used. At the first stage, the Boltzmann equation for the electron gas in the medium is solved in the zero-dimensional formulation for the given parameters of the external electric field. The distribution functions of the electronic energy are obtained as well as the functions of the reaction coefficients. At the second stage, the Helmholtz equations are solved to obtain the distribution of electromagnetic fields near the antenna-initiator, taking into account the given conducting region “plasma”. Based on the obtained distributions of the electric field, the Joule heating powers and the values of the reaction coefficients are calculated. At the third stage, the Navier-Stokes and transfer equations of various types of particles for a compressible medium are solved; taking into account combustion processes for given local heating and plasma reactions. The results are compared with the data of a physical experiment. The distributions of temperature, composition of the medium, velocity of the medium are considered for given local heating power and additional reactions in the plasma region. A stoichiometric mixture of propane with air and pure propane supplied through the antenna are ignited by plasma under the conditions under consideration: the mixture burns in a small area, propane is oxidized in a thin layer of mixing with air. The temperature fields and composition of the medium are compared with photographs of the flame from the experiment. The numerical study shows that under all the conditions considered the model gives results close to the experiment, but there is an overestimation of the power required for ignition by up to two times. The study of the processes of ignition of gaseous mixtures by a subcritical microwave discharge is of interest for the design of propulsion systems with increased reliability, with the possibility of using hardly flammable mixtures. The proposed model gives approximate estimates, while the requirements for resources and time are significantly reduced compared to classical models.

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