Direct Numerical Simulations of Two-Phase Swirling Flows

Swirling jet flows are utilized in a wide range of applications including gas turbine combustors. By imparting swirl to the incoming flow, the structures of both nonreacting and reacting flows can be changed in a dramatic manner. Our current understanding of the effects of swirl on jet mixing and droplet dispersion is largely based on the time-averaged behavior of swirling jets. The transient aspects particularly those associated with large-scale vortex structures have not been examined, although these structures are known to have a dominant effect on flow dynamics in the near jet region.

In this research, we are using direct numerical simulations of two-phase swirling jet to examine the effects of swirl and two-phase momentum coupling on the jet dynamics and structural characteristics. A time-accurate, multidimensional, two-phase algorithm is developed for the simulation. Results for the single-phase axisymmetric jet at a Reynolds number of 800 indicate that the dynamics of large scale structures is strongly affected by the degree of swirl imparted to the incoming flow. For low and intermediate swirl intensities, the vortex rings rollup closer to the nozzle exit, their frequency increases, and pairing interactions become progressively stronger as the swirl number is increased. Thus the addition of swirl to a transitional jet has a synergistic effect on the jet shear layer growth (i.e., jet spreading angle) and entrainment, which are enhanced by both the swirl and vortex structures. For a strongly swirling jet, the presence of a central stagnant zone and recirculation bubble cause a dramatic increases in the jet spreading angle, and this has a very dramatic effect on vortex dynamics. A detailed visualization of the flow dynamics indicates that vortex structures in turn play an important role in determining the location and size of recirculation bubble.

Results for the two-phase swirling jet indicate that for a mass loading ratio of unity, the jet dynamic and time-averaged behavior are strongly affected by both the interphase momentum coupling and swirl intensity. For a nonswirling two-phase jet, the momentum coupling modifies the dynamics of large vortex structures, including their rollup location and frequency, which leads to enhanced mixing and entrainment of colder fluid into the shear layer. In contrast, for weakly and moderately swirling two-phase jets (S < 0.5), the momentum coupling reduces the shear layer growth, as well as mixing and entrainment rate. As the swirl number is increased, the effect becomes progressively stronger, manifested by the reduced rate of decay of gas velocity and temperature along the jet axis. In addition, the relation between rollup frequency and swirl is modified in that the frequency increases with S for a single-phase jet, while it becomes independent of S for the corresponding two-phase jet. Consequently, the vortex pairing interactions, which are responsible for enhanced mixing and entrainment for single-phase swirling jets, are suppressed for two-phase jets. For strongly swirling two-phase jets (S > 0.5), the effect of momentum coupling becomes even more dramatic.