A three-dimensional numerical study on the effect of geometric asymmetry on arcjet thruster performance

In an arcjet thruster, the cathode and constrictor degrade with time, and the electrical arc discharge may become unsymmetrical. In this work, a three-dimensional numerical model of a hydrogen plasma arcjet is developed and validated to study the effect of unsymmetrical electric arc discharge on thruster performance. The unsymmetrical arc discharge is realized by introducing a radial shift of the cathode so that the cathode tip offset is 80 μm (25% of the constrictor radius). Simulations are conducted for both axially centered cathode (coaxial) and off-centered cathode (non-coaxial) configurations with identical propellant flow rates and input current. Simulations show asymmetrical arc discharge in the non-coaxial cathode configuration, resulting in azimuthally asymmetric Joule heating, species concentrations, and velocity field. This asymmetry continues as the plasma expands in the divergent section of the nozzle. Temperature, species concentrations, and axial velocity exhibit asymmetric radial distribution at the nozzle exit. The computed Joule heating was found to reduce with cathode shift, and consequently, the thrust and specific impulse of the thruster was decreased by about 6.6%. In the case of the non-coaxial cathode, geometric asymmetry also induces a small side thrust.