Mixing in turbulent flows: Influence of particle injection

When two co-axially fed turbulent gas streams are allowed to mix, literature [1] shows that complete mixing of the gases takes place at a distance of around 35 times the pipe diameter from the plane of mixing. In this work, the nature of such mixing process when seeding particles are introduced along with the flow is studied. Air and carbon-dioxide are supplied through core and annular portions of two coaxially placed pipes. The length of the core (inner) pipe is smaller than that of outer pipe. The gas streams, along with alumina particles (having a diameter of around 12 microns) are allowed to mix at the end of the core pipe. Mass flow rates of both gases are kept the same and Re is varied from 5000 to 12000. Results show that, with alumina particles complete mixing (assessed by measuring carbon-dioxide concentration at several locations) occurs at a distance of around 30 times the pipe diameter, less than that without particles. This opens up a question of how dynamically inert are the seeding particles, and on a positive note, this concept can be used to facilitate rapid mixing of co-axial streams in combustion applications by deliberately injecting inert particles.