Effect of Flow Regime on Performance of Rotating Gliding Arc Reactor: Experimental and Simulation Studies

Abstract

Non-thermal plasma (NTP) technology offers a promising approach for pollutant abatement, gas conversion, and plasma-assisted combustion (PAC) by enabling reactions under atmospheric conditions, bypassing thermodynamic constraints. However, scaling up plasma reactors remains challenging due to complex plasma-gas interactions. The rotating gliding arc (RGA) reactor shows potential for high-flow applications like PAC, but accurate characterization of key plasma parameters, such as reduced electric field (E/N), gas temperature 〖(T〗_g), and discharge characteristics, is essential for reactor performance and predictive modelling. This study focuses on the performance of an atmospheric RGA reactor developed by Plasma Lab, CST, IISc, investigating the impact of reducing the swirl-hole diameter from 1.6 mm to 1 mm on different flow regimes i.e., transitional (TRANSFLOW), turbulent (TURBFLOW) and high turbulent flow (HIGH TURBFLOW) and plasma reactivity. The work examines methane 〖(CH〗_4) conversion, toluene 〖(C〗_7 H_8) destruction, biogas (BGCH4|CO2 = BG50|50) conversion to value-added chemicals, and its application in PAC contributing to scalable and efficient plasma technologies for hydrocarbon reforming and chemical processing.

This study: The first objective focuses on understanding how changes in the flow regime (transitional, turbulent and high turbulent) affect the electrical, optical, and chemical characteristics of N_2 in the RGA. This involves enhancing arc rotation by reducing the swirl-hole diameter from 1.6 mm to 1 mm and quantifying changes in parameters such as arc frequency (f_arc), tangential velocity (V_t), Reynolds number (Re), and average reduced electric field (E/N) under these conditions. The transition from turbulent to high-turbulent flow was observed with smaller swirl-hole diameters. For a gas flow rate (Q) of 5 SLPM (TRANSFLOW) with a 1.6 mm diameter, values for f_arc, V_t, and Re are 7 Hz, 0.9 ms-1, and 2528, respectively. Increasing Q to 50 SLPM (TURBFLOW) results in higher values: 138 Hz, 17.3 ms-1, and 43635. Further reduction in swirl-hole diameter to 1 mm at constant Q yields 8 Hz, 1 ms-1, and 3478 for 5 SLPM (TRANSFLOW) and 164 Hz, 21 ms-1, and 55018 for 50 SLPM (HIGH TURBFLOW). The second objective examines the effect of flow regime changes on the energy efficiency (η_e) and conversion or destruction of dilute hydrocarbons (〖CH〗_4 and C_7 H_8) in N_2 - RGA. This is achieved through a combination of experimental investigations and simulations using Chemical Workbench (CWB), enabling a comprehensive understanding of the process. The effect of H_2 formation in ( 〖CH〗_4+N_2 - RGA) with varying Q was studied. H_2 concentration decreases with increasing Q, with the 1 mm swirl hole diameter achieving higher H_2 production (1483 ppm at 5 SLPM to 55 ppm at 50 SLPM) compared to the 1.6 mm swirl hole diameter (1277 ppm to 32 ppm). Compared to 1.6 mm, 1 mm has a higher H_2 production due to higher mixing and higher turbulence. In case of C_7 H_8+N_2 - RGA, the effect of flow regime on the destruction efficiency 〖(η〗_d) is studied, and maximum η_d is at 5 SLPM, which is 21% achieved in this work. The third objective delves into the quantitative influence of flow regimes on H_2 generation via the dry reforming of methane (DRM) in RGA using biogas (BGCH4|CO2 = BG50|50). The study compares product compositions, conversion, and η_e between two swirl-hole diameters at varying Q. Additionally, the research analyses flame speeds (V_f) with and without plasma-assisted DRM under different equivalence ratios (ɸ), providing insights into the interplay between plasma and flame dynamics. Compared to other work, this work reported the highest η_e in grams of H_2 produced per kilowatt-hour (g of H_2 kWh -1), i.e., 64 g of H_2 kWh -1 at 20 SLPM. Compared to the V_f of plasma DRM (H_2 = 5.5 %) and a normal BG50|50 - H_2 mixture (H_2 = 5.5%), plasma DRM exhibits a higher V_f. Finally, the fourth objective explores the application of BG50|50 reformation in PAC using an inverse diffusion flame (IDF). The study assesses the combustion characteristics of biogas in a non-swirling IDF configuration with and without H_2 enrichment, utilizing the RGA reactor to highlight its potential for sustainable energy applications. Combustion studies using reformed BG50|50 with RGA in an IDF burner demonstrated enhanced flame stability and extended lean flammability limits, particularly at lower Q due to higher syngas production in comparison without PAC.

Together, these objectives provide a comprehensive framework for advancing the design and application of RGA reactors in plasma-assisted chemical processes, offering insights into flow regimes, i.e., transitional and turbulent effects and their implications for energy and environmental sustainability. These findings are crucial for scaling up the RGA reactor.