| Present work studies the mixing in turbulent flow behind a backward-facing step, which is a classical example of mixing of jet in cross flow. In this study the cross flow was an unconfined cross flow flowing through a duct, which has geometry of backward facing step with varying cross section. Three jet geometries, a slot jet, a single circular jet and three circular jets with different injection angles were studied. Spatial unmixedness based on helium mass concentration was used as an indicator for determining the degree of mixing. Velocity ratio, defined as ratio of jet velocity to the cross flow velocity, was varied during the analysis. Three different values of velocity ratio were considered during the study. Effect of variation of turbulence model, grid size and physical time scale on mixing was also studied. Computational fluid dynamics analysis was performed using commercial CFD software CFX 5.6. Results were extracted in the form of helium mass concentration fields, concentration profiles, concentration contour plots and velocity vector plots.; From the results obtained it can be concluded that, angle of jet inclination is the important controlling parameter in mixing in flow behind a backward-facing step. Single jet geometry gives better mixing for the given conditions. A stream wise rib vortex is a mixing mechanism in the single circular jet geometry. |
