Plasma-Assisted Combustion Above Atmospheric Pressure: Challenges and Opportunities

Over the last decade, combustion enhancement by non-equilibrium plasma discharges has been increasingly investigated. The aim of these studies is to evaluate if electrical discharges can enhance flames by applying an electrical power negligible compared to the thermal power released by the combustion mechanisms. In this context, non-thermal plasmas were successfully used to enhance the flammability limits, increase the burning velocity, improve ignition, facilitate the transition to detonation and control thermo-acoustic instabilities, in various configurations
¹
. However, the vast majority of these studies were carried out at atmospheric pressure, while in real combustion systems, the pressure is usually in the range from 5 to 40 bar. An increase in pressure above atmospheric can impact both the combustion and the discharge processes. The effects of pressure on combustion phenomena are relatively well known, except for the electrical properties of flames. Indeed, while at atmospheric pressure, the concentration of electrons in hydrocarbon flames is known to be about 10
¹⁰
cm
⁻³
, there is no available data at higher pressure. Similarly, the effect of non-equilibrium discharges on combustion properties at pressures above atmospheric are not well documented. In this study, the effect of non-thermal plasmas produced by nanosecond repetitively pulsed (NRP) discharges on the flammability limit of methane-air flames at pressures up to 8 bar is investigated. The NRP discharges are characterized by current and voltage measurements, while the flame stability is assessed by direct visualization performed with a DSLR camera. The results show that NRP discharges can improve the stability of premixed swirl flames at all pressures investigated, even if the ratio of electrical discharge power to flame thermal power is kept as low as 0.7%. It is also observed that the required peak voltage does not increase linearly with increasing the pressure. Based on physical explanations for these results, the challenges and opportunities of plasma-assisted combustion above atmospheric pressure are discussed.