Impact of bypass ratio in a double-stage vortex tube on vortex stability and separation performance

The vortex tube separator (VTS) is a critical component of an air filter, with its structure and bypass ratio (BPR) playing pivotal roles in separation performance, further improvements are necessary. Notably, existing literature lacks studies on double-stage series configurations and detailed investigations into the influence of BPR on separation performance. In this study, we employed Large Eddy Simulation (LES) using the Liutex method alongside experimental methods to analyze the effects of different two-stage bypass ratios (BPR₁/BPR₂) on separation performance. The key findings reveal that variations in BPR induce secondary vortices, leading to secondary entrainment through secondary flows, which ultimately affects separation efficiency. For a single double-stage VTS with a suction stream of 12.5 m³·h⁻¹, adjusting the BPR resulted in a maximum pressure drop difference of 590 Pa and a variation in separation efficiency of 0.29 %. Secondary vortices significantly influence the flow field and particle motion. An optimized BPR distribution can maintain centrifugal motion while reducing secondary entrainment in the first-stage separation chamber. The design of the dust discharge blade structure mitigates the impact of BPR on secondary entrainment. As BPR₁ increases, the grade efficiency for 5 μm particles rises by 2.33 %. For particles in the range of 5–35 μm, optimal separation occurs when BPR₁/BPR₂ = 1, with efficiency peaking at 9.05 %, beyond which no further improvement is observed. In the second-stage VTS, an increase in BPR₁ leads to a reduction in the fully separated particle size range from > 8.71 μm to > 5 μm, accompanied by a fishhook effect. The work not only improves the understanding of the vortex structure in double-stage VTS, but also provides innovative ideas for designing and manufacturing future high-performance VTS.