The gas-liquid separator is a key component in the gas removal system of the Thorium Molten Salt Reactor. Phase separation is driven by a swirling flow, and the fundamental principle is that dispersed bubbles are accumulated and coalesced into an air core to realize separation from the liquid phase. In this paper, simultaneous particle image velocimetry (PIV) and pulsed shadowgraphy techniques are applied to characterize the two-phase-flow patterns in the evolutionary process of the air core. The PIV technique utilizes fluorescent particles as tracers in the liquid flow field, and a charge coupled device (CCD) camera records the planar laser-induced fluorescence signal of the particles. Another camera simultaneously detects the shadow and motion of the air core via backlighting from an array of infrared light-emitting diodes. The signals originating from the different phases are separated by a beam splitter with a dichroic filter and optical filters, and only undisturbed signals from the shadow of the air core and fluorescence tracer particles of the fluid are effectively captured by the two CCD cameras, respectively. Experimental data are carried out for three Reynolds numbers Re for a range of outlet pressures Pout. The morphology of the air core tail periodically transforms from a linear type to a single-helix type to a double-helix type before reaching a stable state at the critical outlet pressures Pcout. The analysis of gas-liquid flow patterns indeed indicates that axial velocity has a strong influence on the air core evolution. The periodic fluctuation results from the magnitude and direction of axial velocity.