Axial gas-liquid separators have been adopted in fission gas removal systems for the development of thorium molten salt reactors.In our previous study,we observed an unsteady flow phenomenon in which the flow patte is directly dependent on the backpressure in a gas-liquid separator;however,the underlying flow mechanism is still unknown.In order to move a step further in clarifying how the flow patte evolves with a variation in backpressure,a large eddy simulation (LES) was adopted to study the flow field evolution.In the simulation,an artificial boundary was applied at the separator outlet under the assumption that the backpressure increases linearly.The numerical results indicate that the unsteady flow feature is captured by the LES approach,and the flow transition is mainly due to the axial velocity profile redistribution induced by the backpressure variation.With the increase in backpressure,the axial velocity near the downstream orifice transits from negative to positive.This change in the axial velocity sign forces the unstable spiral vortex to become a stable rectilinear vortex.