Effect of Velocity-Ratio on the Mixing of Non-Isothermal Counter-Flowing Jets in Channel

Abstract

Good mixing in a short time and smaller mixing chambers is an important step for any unit operation to yield desirable products with high quality and low cost. Mixers in which opposing-jet streams impinge with each other, causing mixing of mass, momentum and energy to take place without the use of moving parts or internal baffles, are preferred. So a 2-D numerical investigation of two non-isothermal opposing jets/streams of different fluids (same phase, miscible) impinging in an adiabatic channel is carried out in the mixed convection regime. The two buoyancy forces, thermal as well as intrinsic, have been considered through the Boussinesq approximation. Apart from other dimensionless parameters, the two buoyancy forces give rise to thermal and intrinsic Richardson numbers, RiT and RiC, respectively and ratio of two jet velocities (VR = U1 / U2) are the important parameters of the problem. The upper stream is of heavier fluid at lower temperature and lower stream is of lighter fluid at higher temperature. The properties of the two streams are chosen such that the two buoyancy forces always aid each other. Numerical experiments have been performed at three pairs of finite RiT and RiC, namely (0.5, 0.5), (1, 1) and (1.5, 1.5). Forced flow case (RiT = RiC = 0) is taken up to assess the effects of thermal and intrinsic buoyancy forces on the flow dynamics. Three different values of velocity ratio (VR) is chosen viz., 0.5, 1 and 2. The values of Pr, Sc and Re is fixed at 0.7, 0.8 and 200 respectively. Two distinct flow regimes are obtained: long term steady flow regime and an unsteady, oscillatory periodic flow with vortex shedding, within the parametric space of dimensionless numbers for the present study. The qualitative nature of instability, which is causing unsteadiness in the flow, is different for VR = 1 and VR = 2. Three scalar measures of mixing are used to examine mixing process namely, Mixing Index, Mixing Length and Mixing Effectiveness. In steady flow regime, increase in buoyancy has little effect on flow dynamics as well as mixing of two streams. For the unsteady periodic flow at VR = 1, the mixing process is significantly enhanced due to the formation of a sinuous flow pattern with the fluid particles following a wavy path. The effect of buoyancy for unsteady cases at VR = 2, is less as compared for unsteady cases at VR = 1. This can be ascribed to lesser mechanical mixing / churning which is associated with formation of sinuous streamlines of lesser gradients for VR = 2 as compared to VR = 1.