Effect of the conical diffuser angle on the confined swirling flow induced Precessing Vortex Core

This paper presents the effect of the conical diffuser angle (θ=0°,2°,4°) on the Precessing Vortex Core (PVC) induced in confined swirling airflows generated by axial swirlers with two swirl number (Sw=1.1,0.7). Planar Particle Image Velocimetry (2D-PIV) synchronized with unsteady wall pressure fluctuation measurements are performed to identify the Strouhal number of the precession frequency and pressure fluctuations amplitude induced by the PVC, as well as the PVC parameters related to its phase-averaged vortex structure. Furthermore, the distribution of pressure recovery rate based on the mass flow averaged pressure, and the relationship between the local axial pressure gradient and the local PVC structure is also investigated to understand the relationship between the PVC parameters and pipe geometries. These relationships are derived from transient simulations with the Shear Stress Transport Curvature Correction model, since the pressure field data cannot be obtained from the experiments conducted in this paper. Experimental and numerical results in pipe configurations 1 (diffuser followed by a straight pipe) and 2 (diffuser followed by an open exit) clarify the diffuser angle effect. As the diffuser angle increases, the Strouhal number decreases, whereas the pressure fluctuations amplitude increases. Moreover, the PVC parameters are also affected by increased diffuser angle: the vortex trajectory radius is enlarged, and the helical vortex pitch is shortened. In addition, the geometry change (from diffuser to straight pipe) leads to a sudden change in the vortex pitch and a modification of the pressure recovery rate evolution. The vortex structure in the conical diffuser is affected by a difference in the diffuser angle, but the latter does not affect the vortex structure in the downstream straight pipe. This highlights that the local vortex pitch depends only on the local pipe geometry. Finally, a theoretical model of the relationship between the pressure fluctuation amplitude and PVC parameters is proposed, and the relationship between the local pressure gradient and the local vortex trajectory radius gradient is clarified. The amplitude factor Cp∗ is a function of the dimensionless vortex trajectory radius r∗. Positive or negative local pressure gradient leads to the mitigation or promotion of the PVC development.