Direct Numerical Simulation (DNS) of multiple passive scalars with varyingmolecular diffusivity in a spatially evolving axisymmetric vortex breakdown fl ow is performed atReynolds number of 1500. Vortex breakdown is peculiar to swirling fl ows, and generally occurswhen the ratio of azimuthal to axial velocity exceeds a certain level (Billant et al, 1998). Vortexbreakdown is known to enhance mixing of scalars. However, little about the mixing characteristicand mechanism behind the transport of passive scalars in such fl ow is known. Recent studies ofscalar transport in turbulent fl ows have shown that the evolution of scalar fi eld in the fl ow dependson the turbulence transport as well as the molecular diffusivity of the scalar (Saylor & Sreenivasan,1998). This process, called differential diffusion, is a potential complication of the mixing process.The present study investigates the role of scalar molecular diffusivity in passive scalar transportin an axisymmetric vortex breakdown fl ow in a swirling jet. Both instantaneous and mean scalarand velocity fi elds are analysed. The instantaneous radial profi les of velocity and passive scalar arealso examined. The convective and diffusive budgets in the passive scalar transport equation areanalysed, and diffusion-dominated regions are identifi ed.DNS data reveals that differential diffusion has a signifi cant effect on spatial evolution of apassive scalar fi eld depending on its Schmidt number. A scalar with lower molecular diffusivitypredominantly responds to transport by momentum of the fl ow. In contrast, molecular diffusion isthe predominant transport mechanism for a scalar with higher molecular diffusivity. It is shownthat turbulence preferentially transport scalars with lower molecular diffusivity relative to one withhigher molecular diffusivity even in the presence of high turbulent stirring, e.g. vortex breakdown.Furthermore, the instantaneous scalar radial profi les are shown to deviate from the instantaneousvelocity profi le depending on the scalar Schmidt number. The result has direct implication onlimitations of dye-fl ow visualisation and on transfer of heat in vortex breakdown fl ows.
|Journal||Australian Journal of Mechanical Engineering|
|Publication status||Published - 2007|