Mixing of two variable-density jets, one of which is essentially a negatively buoyant jet (He) and the other a nonbuoyant jet $(O2)$ is studied experimentally using digital particle image velocimetry technique for separated nozzle geometry. Two jets are separated from each other, i.e., He jet exit plane is located upstream of the $O2$ jet exit plane. Experiments were carried out keeping the nonbuoyant $O2$ jet at constant Reynolds number $(Re2=245)$ and varying the Richardson number $(Ri1=1.8,4.7,16.4)$ of the buoyant He jet. The interaction between two jets as a function of buoyancy strength of helium jet is investigated. The flow visualization images clearly demonstrate the growth and shape of buoyant jet as a function of Richardson number. Mean velocity and vorticity field results provide quantitative picture about the mixing and interaction between the two jets. The stream trace results show the flow structures, i.e., recirculation zone and foci of vortex structures as a function of Richardson number. The mixing between two jets takes place at far downstream region for low Richardson number $(Ri1=1.8)$. At high Richardson number $(Ri1=16.4)$, the buoyant He jet is located near its exit plane without any direct interaction with the $O2$ jet. At the intermediate Richardson number $(Ri1=4.7)$, the buoyant jet encroaches the nonbuoyant $O2$ jet at a favorable penetration distance and a good amount of mixing between the two jets take place. The jet growth results based on the $y0.5$-location development in the streamwise direction clearly demonstrate the interaction between the He and $O2$ jets in the interjet region. The entrainment coefficient, vorticity magnitude, and turbulent kinetic energy magnitude are maximum at the intermediate Richardson number $(Ri1=4.7)$ demonstrating direct relationship among each other. The mixing between buoyant He jet and nonbuoyant $O2$ jet is a function of both shear between the two jets and the strength of buoyant plume.

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