Electrode Design for Thermal and Nonthermal Plasma Discharge Inside a Constant Volume Combustion Chamber

Abstract

Previous methods of achieving ignition in the Plasma, Combustion and Fluid imaging (PCFi) Laboratory’s Constant Volume Combustion Chamber (CVCC) utilizes either a standard or modified spark plug. The standard spark plug achieves a representation of side wall ignition (similar to a combustion engine), while the modified spark plug has an extended electrode to allow for a center camber ignition used for laminar burning speed (LBS) measurements. The creation of the modified spark plug required welding a stainless-steel (SS) wire to the electrode of the plug. This process is time consuming and requires a large quantity to effectively test a wide range of parameters such as gap size or electrode geometry. Two custom-design electrodes are presented in this paper which extend the experimental range of the PCFi’s CVCC system. Electrode Design A, gives the ability to test thin wire electrode with adjustability of gap size and different diameters through use of a compression fitting. This electrode design (i.e., tip-to-tip) is utilized with a traditional style of automotive ignition system (i.e., capacitive discharge) to study ignition process (i.e., thermal plasma) and spherical flame propagation. Electrode Design B, adds the ability to change tip geometry (i.e., plate-to-plate, tip-to-plate, tip-to-sphere, plate-to-sphere, etc.). In this paper, the plate-to-plate configuration is demonstrated to study uniform low-temperature nanosecond plasma discharge. Both electrode designs reduce structural weakness by removing the welded joint and allow for linear gap size adjustment. The electrode utilizes high-temperature epoxy, ceramic, and grafoil seals to make parameter adjustments easy and precise. The design was analyzed, prior to building and testing, based on the stress induced from the sealant, the total rated voltage, the rated temperature, and the fracture stress of the ceramic material. The stress induced in the electrodes was analyzed with finite element analysis (FEA) and the results were found to be within the limits of the material in terms of the compressive and fracture strengths. The maximum voltage was found to be around 30 kV. Design A is presented with three different electrode diameters of 1.3, 1, and 0.5 mm and Design B which utilizes a threaded connection for adjustable tip geometry. A sample of data, visual and electrical, is presented for the newly created electrode with a 0.5 mm diameter as well as combustion images for up to 10 atm of initial pressure for methane-air mixture. The new electrode design was able to survive several months of experimental use with few issues compared with the previous welded design.