Physical phenomenon occurring inside a SF6 gas circuit breaker involves multi-physics such as magneto-hydrodynamics (MHD), nozzle ablation, radiation, compressible flow and turbulent mixing.

The objective of the present numerical study is to understand the physics occurring inside a breaker during small current interruption. The study is done on a simplified geometry - a convergent-divergent nozzle geometry with two electrodes called as Lewis nozzle - which represents most of the physics in a real SF6 gas circuit breaker.

Numerical simulations are done using general purpose commercial CFD (Computational Fluid Dynamics) software ANSYS FLUENT 13.0. However, in-house C programs – called as user-defined functions (UDF's) – are developed to model physics such as MHD and radiation. The numerical development and its coupling with the software is validated on a transient and 2-D axisymmetric problem - for a low DC current of 300A. Thereafter, the effect of different shapes – semihemispherical, flat, elliptical - of the tip of electrodes for the Lewis nozzle is studied; as previous work is on only semi-hemispherical tips. A detailed CFD analysis is presented here to discuss the reasons for the variation of temperature, arc radius and arc voltage along the axis. This study will help in drawing conclusions applicable to the performance of circuit breaker.