Numerical evaluation on the decaying swirling flow in a multi-lobed swirl generator

Accurate predictions of decaying swirling flow behavior in the multi-lobed swirl generator (MLSG) are essential for certain engineering applications. In this study, decaying swirling flow characteristics in an MLSG are examined using the Computational Fluid Dynamics (CFD) technique with a Reynolds stress turbulence model (RSM). The effects of different lobe numbers (n) and pitch length ratios (P/D) on swirl intensity, pressure drop, friction factor coefficient, and decay rate are observed with Reynolds number from 50,000 to 125,000. Combined with the pressure loss, the efficiency of swirl induction is evaluated by introducing a swirl effectiveness (SE) evaluation criterion. The initial swirl intensity is shown to first increases and then decreases as lobe number increases. The maximum value is obtained at lobe numbern = 4. The pitch length ratiosP/Dcorresponding to the optimal SE value obtained from different lobe also differ. The optimal SE value is achieved atn = 4 andP/D = 8. The friction factor ratiof(s)/f(p)decreases as pitch length ratio increases, but the decreasing trend decelerates over time. Correlations based on the numerical predictions of friction factor ratiof(s)/f(p)and decay rate beta are presented accordingly.