The Paris climate agreement calls for a complete halt to greenhouse gas emissions by 2050, making it a top priority for the 21st century. Many sectors can shift from combustion of fossil fuels to cleaner energy sources like wind and solar power, but in areas like aviation and peak-load power generation it is hard to replace traditional combustion engines. These sectors have unique demands on compact, reliable, and efficient solutions. For gas turbines and jet engines, the key lies in sustainable alternative fuels. Hydrogen, ammonia, and HVO are potential options for gas turbines, while jet engines can run on bio-kerosene. But here's the catch: these new fuels have different properties than their fossil counterparts, making stable combustion a challenge.

Plasma-Assisted Combustion (PAC) is a promising approach to tackle this critical issue. It involves applying a small amount of electrical energy to the combustion process, resulting in higher efficiency and/or reduced emissions. By coupling electrical energy to the fuel, either slightly before the flame or directly into the flame, it boosts chemical reactivity, reduces ignition delays, and increases flame speeds. This can change the emissions, stabilize leaner flames that would otherwise have blown out, and make a wider range of bio-fuels viable. Researchers are exploring various PAC techniques such as microwave plasma, dielectric barrier discharges, gliding arc discharges and plasma jet torches. Most of the research so far has been experimental, but simulations are becoming more important.

In microwave-enhanced combustion, an external electric field (E-field) is applied through the flame, causing ionization and transforming part of the gas into a plasma. The plasma is non-thermal, meaning that electrons will experience significantly higher temperatures than the surrounding gas. Consequently, the elevated electron temperature leads to excitation within the plasma, resulting in changes to the gas composition and enhancing its reactivity.

The gliding arc discharge has some similarities with microwave-enhanced combustion, but there are also differences. In this method, the plasma is confined to a narrow arc that forms between two electrodes. From a modelling point of view this is more challenging and additional equations are needed for electron energy and electric potential since these are not constant and must be calculated as part of the simulation. Additional complexity is associated with the electric conductivity of the gas and with the boundary layer near the electrodes.

In our group we focus on modeling and simulation of PAC to understand how PAC can enhance fuel flexibility and extend the stability of turbulent flames, particularly in gas turbines and jet engines. This has been done as part of the projects EFFECT and EFFECT II, funded by the Swedish Energy Agency, and the ELECTRONICS project, funded by the Swedish Research Council.