ON THE DYNAMICS OF A TWO-PHASE, NONEVAPORATING SWIRLING JET

Abstract

In this paper, we present direct numerical simulation of a droplet-laden swirling jet, and examine the effects of swirl and two-phase momentum coupling on the jet dynamics and structural characteristics. A time-dependent, multi-dimensional, two-phase algorithm is developed for the simulation. Results for the single-phase swirling jet at a Reynolds number of 800 indicate that the dynamics of large-scale structures are strongly affected by the degree of swirl imparted to the incoming flow. For low and intermediate swirl intensities, the vortex rings roll up closer to the nozzle exit, their frequency increases, and pairing interactions become progressively stronger as the swirl number ( S) is increased. Thus, the addition of swirl to a transitional jet appears to modify its vortex dynamics in a way that enhances the beneficial effects of both swirl and vortex structures on the shear layer growth and entrainment. For a strongly swirling jet, the presence of a central stagnant zone and recirculation bubble causes a dramatic increases in the jet spreading angle, and this has a very dramatic effect on vortex dynamics. Based on a detailed visualization of the dynamic structure, we speculate 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 roll-up 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 roll-up 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. Results for S=0.75 indicate a drastic reduction in the size of the recirculation bubble for the two-phase jet.