In a non-transferred plasma torch, the working gas becomes ionized and forms plasma as it interacts with the electric arc at the cathode tip. However, in certain cathode shapes, particularly flat ones, and under specific conditions, the gas flow can separate at the cathode tip, forming a vortex region. While this flow separation is influenced by geometric factors, it occurs in the critical zone where plasma is generated. Understanding the causes of this separation is essential, as it may significantly impact torch performance. If the separation proves detrimental, it is important to identify ways to mitigate it. This paper presents a computational analysis of a non-transferred plasma torch to investigate the physics behind flow separation. The results highlight the location and causes of the separation, as well as its potential advantages and disadvantages. Finally, the paper explores theoretical approaches to address flow separation in plasma torches, offering practical insights for enhancing their design and efficiency.

The plasma jet produced by a non-transferred plasma torch may initially appear steady and laminar, but it undergoes significant turbulence as it interacts with the surrounding atmosphere. Within the plasma torch, the jet begins as laminar; however, upon exiting, it transitions into a turbulent flow, extending into a long, wavy structure as it develops. This paper explores the complexities of computational modeling for non-transferred plasma torches, focusing on the challenges of simulating the multiphysics and multiphase interactions at the outlet and tracing the evolution of the plasma jet. The computational analysis uses COMSOL Multiphysics software on a 2D axisymmetric geometry, with steady-state simulations incorporating various turbulence models. A comparative assessment of the results from each turbulence model is provided, highlighting their respective strengths and limitations. Although the diffusion of the turbulent jet at the outlet is presented, the turbulence models employed in this study only offer time-averaged values, rather than a detailed breakdown of the complete jet structure. The paper concludes by validating the computationally obtained velocity magnitudes against experimental data, ensuring the accuracy and reliability of the simulation results.

A non-transferred plasma torch is a device used to generate a steady thermal plasma jet. Plasma torches have the potential to replace fossil fuel burners used as heat sources in the process industry. Today, however, the available plasma torches are of small scale compared to the power used in the burners in the process industry. In order to understand the effects of large scales on the plasma flow dynamics, it is essential to understand the operation of the plasma torch under different operating conditions and for different geometries. In this study, the analysis of a non-transferred plasma torch has been carried out using both computational and experimental methods. Computationally, the magnetohydrodynamic (MHD) equations are solved using a single-fluid model on a 2D axisymmetric torch geometry. The experiments are performed using emission spectroscopy to measure the plasma jet temperature at the outlet. This paper explains the changes in the arc formation, temperature, and velocity for different working gases and power inputs. Furthermore, the possibilities and disadvantages of the MHD approach, considering a local thermal equilibrium, are discussed. It was found that in general, the computational temperature obtained is supported by the experimental and equilibrium data. The computational temperatures agree by within 10% with the experimental ones at the center of the plasma torch. The paper concludes by explaining the significant impact of input properties like working gas and power input on the output properties like velocity and temperature of plasma jet. 

Plasma, a complex fluid consisting of electrons, ions, neutrals, and excited species, exhibits both fluid-like behavior and electrical conductivity due to the presence of charge carriers. Consequently, computational modeling of plasma requires the integration of fluid and electrical models. This research paper presents a study on the steady-state computational modeling of a plasma torch with a 2D axisymmetric geometry using single-fluid and two-fluid modeling approaches in the COMSOL Multiphysics® software. The single-fluid modeling (SFM) approach combines the individual equations governing the behavior of different particles into a unified equation. Specifically, the SFM approach utilized in this study focuses on a fully ionized plasma and employs the Magnetohydrodynamic equations whose adaptation is equilibrium discharge interface (EDI) model available in COMSOL Multiphysics®. The EDI model solves the magnetohydrodynamic (MHD) equations, encompassing electric and magnetic fields, heat transfer in solids and fluids, and laminar models. By employing this approach, the researchers simulated and analyzed the behavior of the plasma torch. In contrast, the two-fluid modeling (TFM) approach separates the fluid equations for electrons and ions, considering a weakly ionized plasma. The TFM model is developed by deriving fluid equations based on kinetic theory for neutrals, ions, and electrons. These equations are then implemented in COMSOL Multiphysics®, utilizing models for the transport of diluted species, laminar flow, heat transfer in solids and fluids, and electric and magnetic fields. By adopting the TFM approach, the researchers aimed to gain insights into the behavior of the plasma torch. Throughout the study, various properties such as temperature, velocity, current density, and particle concentrations are analyzed within the plasma torch. Results obtained from both the single-fluid and two-fluid modeling approaches are compared and evaluated. This comparative analysis allows the researchers to highlight the advantages and challenges associated with each modeling approach. In conclusion, this study contributes to understanding plasma behavior by employing computational modeling techniques. The research presents and compares the outcomes of single-fluid and two-fluid modeling approaches applied to a plasma torch. By examining the advantages and challenges of each approach, the study offers valuable insights for future plasma modeling endeavors.

Greenhouse gases and their negative effects on climate is one of the most discussed topics around the world. Globally, fossil fuel-related emissions from process industries, transportation, and electricity generation are one of the biggest contributors to greenhouse gases. One of the prime questions asked globally is how to reduce these emissions. Plasma burners can be an answer to the question. They are entirely electric-driven burners and operate at high temperatures. Presently, the available burners are small scale due to which they are not applicable in industries. So a substantial amount of interest lies in up-scaling them. However, to begin the up-scaling process, it is fundamental to clearly understand the working of the plasma burner and the various factors that affect its operation. The present thesis explains the working of a plasma burner under different operating conditions is studied experimentally, computationally, and the obtained results are validated with theoretical data. Experimentally, the temperature measurements at the plasma torch outlet were carried out using optical spectroscopy. The velocity and structure of the plasma jet coming from the outlet were studied using a high-speed camera. The experimental measurements were carried out for varied input working gases, velocities, and powers. The computational analysis was perfomed using COMSOL multiphysics software. The primary modeling was done using the equilibrium discharge interface model (EDI) in which plasma is considered to be fully ionized and at local thermal equilibrium. But considering the drawbacks of the EDI model, further computational analysis was initiated by modeling weakly ionized plasma. Different geometries of the plasma torch, working gases, velocities, and power are analyzed computationally. Further, the experimental and computational results are validated with each other and thermodynamic equilibrium data obtained using the TEC program. Finally, this thesis promises to give an overview of the plasma torches, their working under different operating conditions, and a brief discussion about the future focusing on up-scaling the plasma burners.

Over the decades, computational methods have been used to model and describe the flow andionization dynamics in plasma torches. However, the impact of the operational parameters such as gas flow rate, swirl number and input current density on flow is still inexplicit. In this study, the flow in a non-transferred plasma torch is modelled using COMSOL Multiphysics, and the influence of these parameters is studied. The analysis is carried out on an axisymmetric geometry with the conical-shaped cathode, nozzle-shaped anode, and Argon is used as the plasma gas. A thermal plasma (equilibrium discharges) is considered, i.e., the plasma is underpartial to complete local thermodynamic equilibrium in which the magnetohydrodynamic (MHD) equations are solved. This is treated in the Equilibrium Discharge Interface in COMSOL’s plasma module that has been used in the present study. The laminar flow analysisis performed for low-velocity cases and turbulent flow analysis for higher velocities. It was found that the velocity increase across the plasma arc due to ionization and gas expansion, couldbe observed only for sufficiently high plasma inflow velocities. The position of the plasma arcis determined for different operating conditions. It was further found that the velocity has anegligible effect on the length of the plasma arc, whereas the dependency of the arc length andattachment point on the anode wall, to the input current density and cathode tip temperature iswell explained. The paper concludes by presenting the variations in temperature and velocityof plasma arc due to swirling inflow.