Flow recirculation and restriction in a CO₂ microwave plasma, inferred from in-situ Raman scattering

The performance of plasma reactors used for chemical conversion depends on bypass fractions and residence time distributions. This study investigates the underlying flow dynamics in a vortex-stabilized CO2 microwave plasma, demonstrating how flow recirculation upstream of the plasma and central flow restriction downstream of the plasma can be inferred from in-situ diagnostics. We used in-situ Raman scattering data of the local temperature and gas composition in a 1 kW CO2 plasma to test flow profiles for the conservation of mass and heat. We found that the most commonly assumed flow profiles (plug flow, homogeneous mass flux, Poiseuille flow) violate heat conservation by 250-1500 W. The high enthalpy measured upstream of the plasma center requires 10%-30% recirculating mass flow for heat conservation, resulting in extended residence time distributions. Downstream of the plasma center, the high enthalpy requires a change in central flow restriction from 50% to 75% for 10 slm vs 20 slm experiments, causing a significant change in flow bypass. This work underlines that insight into transport in this type of plasma reactor could be inferred from in-situ measurements of temperature and composition, and cannot be based on global parameters such as flow rate and power.