Study of single-particle residence time in impulse, symmetric and asymmetric coaxial impinging streams

Abstract

Short particle residence time in the active area limits the engineering applications of impinging stream reactors. The single-particle residence time in the active area (ttot) is measured and compared among impulse impinging streams (IIS) in which both inlet velocities exhibit step variations during one period, asymmetric impinging streams (AIS) and symmetric impinging streams (SIS). In addition, the effects of the impulse inlet velocity conditions, the reactor geometry and the particle properties on the single-particle motion behaviors and trajectories in a coaxial impinging stream reactor are investigated and discussed in this study. T denotes the variation period of the impulse inlet flows. The results show that single particles with different particle relaxation time (τ*) exhibit different motion behaviors in an impinging stream reactor. A single particle with a short τ* is accelerated directly by the original fluid and leaves the active area without oscillatory motion in SIS,AIS and IIS. A single particle with a long τ* exhibits at most three oscillatory motions in SIS,AIS and IIS. However, a single particle with an even longer τ* exhibits only three oscillatory motions in SIS and AIS but at least three oscillatory motions in IIS. The ttot value in a coaxial impinging stream reactor generally varies with the inlet velocity conditions, the properties of the particles themselves and the size of the active area. In particular, at long τ*, the ttot value reaches a maximum in the IIS case when T = Tb. Tb is determined by τ*, Rem and L/D. By contrast, at short τ*, the ttot values in SIS,IIS and AIS are all the same. However, at long τ*, the value of ttot is greater in the case of IIS at T = Tb than it is in the AIS case, increases with a decreasing inlet velocity ratio in the AIS case, and is the shortest in the SIS case under identical simulation conditions. On the whole, at long τ*, ttot and the particle motion distance in the active area can be significantly enhanced in the IIS case in comparison with the SIS and AIS cases at the same mean inlet velocity by reasonably designing the inlet velocity conditions.