# Vortex Breakdown Control by the Plasma Swirl Injector
## 3 Results and Discussion  
The flowfield, obtained by 2D PIV, provides information about the presence and location of vortex breakdown. The leading edge of the VB could be identified by a stagnation point along the centerline. Contours of axial velocity in the centerline plane (the laser plane in Figure 2) were captured for each case to provide a global quantitative and qualitative depiction of the flowfield. Figure 3a-e, from 2D PIV measurements, show the axial velocity magnitude contours and vectors in the XZ plane for the five conditions tested, with a bulk velocity of 3 m/s and a corresponding Reynolds number of 7171 based on the injector diameter. The approximate exit of the injector was at Z = -2 mm. Location of a breakdown zone can be identified by the negative axial velocity and recirculating flow. Generally, the axial velocity distribution of a low swirl flowfield accords with the normal distribution of this type of jet flow, but the maximum axial velocity moves away from the axis due to the effect of the mesh plate of the low swirl injector, forming a double peak velocity profile as shown in Figure 3a. The velocity in the annular region is higher because of the significant downward momentum of the flow in this region, compared with the flow in the central region. The central low velocity region, as well as the shear layer between the non-swirling core and swirling annulus, are critical for flame stabilization. When the swirl jet flows out of the low swirl injector, the low pressure in the center region of the swirl flow recovers gradually, which leads to the generation of the reverse pressure gradient in the axial direction. Without plasma actuation, the axial pressure gradient is not large enough to cause reverse flow. At 12 kV, the decrease of axial velocity near the central region can be observed, where the central low velocity region swells slightly and the degree of flow expansion increases as well, but the actuation is not strong enough to cause flow reversal and no vortex breakdown is observed, as shown in Figure 3b. Increasing the strength of actuation to 15 kV resulted in the generation of a stagnation point as a result of the reverse pressure gradient increase and the decrease of the axial velocity. Although the  
Figure 2: Schematic of the experimental setup. low velocity region and the degree of flow expansion remained almost unchanged, flow reversal occurred in a small region and the onset of the breakdown bubble was observed in regions close to the centerline at around \(Z=15\) mm, as shown in Figure 3c. Bubble-type breakdown is characterized by a nearly axisymmetric region of reversed flow, with a stagnation point at the forward end. As the actuation voltage was further increased to 18 kV, the stable bubble-type vortex breakdown was observed to migrate upstream and was fully established with a forward stagnation point at about \(Z=1\) mm and a backward stagnation point at \(Z=36\) mm, as shown in Figure 3d. Its outer shape was nearly axisymmetric and can be visualized as a bubble. Due to the existence of the vortex, the outer boundary of the jet expanded immediately after it flowed out of the injector. As the strength of actuation was increased to 21 kV, the breakdown zone grew further due to the enhanced backflow, and its nose becomes less pointed and wider, as shown in Figure 3e. The breakdown zone shifted upstream with the forward stagnation point penetrating into the injector, but the position of the backward stagnation point changed very slightly compared to that of 18 kV. This shows that the actuation does not significantly affect the overall location of the bubble, although it enhances its size. The interior structures of the bubble were nearly the same for the two cases. Based on these results, it can be concluded that the 15 kV actuation is strong enough to trigger the onset of vortex breakdown with a relatively low power input.