Mixing and coherent vortices in turbulent coaxial jets

Abstract

Direct numerical simulations associated with mixing in constant-density round coaxial jets are performed. They are validated by comparison against laboratory experiments. The mixing process is studied by seeding a passive tracer first in the outer annular jet, then in the inner jet. We demonstrate the important role played by coherent vortices in the mixing mechanisms. The turbulent mixing exhibits an intermittent character as a consequence of fluid ejections caused by the counter-rotating streamwise vortices. We quantify also the domination of the outer jet and show that the fluid issuing from the central jet remains confined.

The near field of coaxial jets: A numerical study

“…For r u Ͻ r uc , the outer vortices impose their shedding frequency to the inner vortices and the latter are trapped between two consecutive larger vortex rings. 20 As previously pointed out, the instability develops faster when 01 is decreased and the inner layer vortices are shed at shorter and shorter locations when we go from DNS1_10 to DNS5_10 simulation. Although the value of 02 is identical in the three simulations, it is interesting to note that the outer vortices are also shed at shorter downstream distances for the smaller values of 01 .…”

Numerical study of hydrogen–air mixing in turbulent compressible coaxial jets

“…To overcome this situation, it is then recommended to lock the inner jet by an i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 9 5 3 9 e9 5 5 4 outer jet (replacing the co-flow jet) to avoid the dissipation of H 2 in turbulent zone. This recommendation is inspired from the work of Balarac and Si-Ameur [5] who found that the inner jet remain confined by the outer one in a configuration of incompressible coaxial jets. Fig.…”

“…Pairs of alternate streamwise vortices appear between two consecutive rings, in agreement with the classical phenomena of three-dimensionalization in free-shear layers. The streamwise vortices play a dominant role in the transition processes towards a fully-turbulent state, and their location is very dependent of the inlet conditions [5]. The apparition of streamwise vortices is finally followed by an abrupt increase in the level of small-scale turbulence.…”

“…The fourth order accurate piecewise parabolic method (PPM) [16] together with the approximate Riemann solver of Roe [17] solve the equation (5).…”

Modelling of a multiphase reacting turbulent jet: Application to supersonic carbon injection in siderurgic furnaces

“…The coaxial arrangement of streams has proven to be useful to control mixing of species in a number of industrial applications. Many experiments (see Villermaux and Rehab, 2000;Balarac and Si-Ameur, 2005, among others) have shown that, when the outer high velocity (yet subsonic) jet is dominating, the fluid issuing from the central jet remains confined. A similar effect is expected here.…”