Turbulent Mixing in a Tubular Reactor: Assessment of an FDF/LES Approach

The aim of the present research was to investigate the feasibility of a probability density function (PDF) methodology combined with a large eddy simulation (LES) for turbulent reactive mixing in industrial geometries. In order to incorporate the PDF transport equation into a LES context, a filtered version was employed, denoted as filtered density function (FDF) equation. Using FDF/LES, the yield of mixing-sensitive, parallel competitive reactants was numerically studied in a tubular reactor with perpendicular inlet operated at Re 4,000 for varying Damköhler numbers and feed-stream concentrations. The “interaction by exchange with the mean” (IEM) model was used to close the filtered conditional scalar energy dissipation rate (SED) appearing in the FDF equation. The modeling assumptions were assessed by means of laser induced fluorescence (LIF) experimental data, providing the mean conserved concentration field in a horizontal and vertical center plane downstream the injector, and concentration PDF’s at several downstream positions. In the vicinity of the injector, the FDF/LES model slightly overpredicts both the spreading of jet and the dissipation of small scale fluctuations, which was attributed to the overprediction of the turbulent diffusivity in the transition region of the laminar jet to the the turbulent wake of the injector. Further downstream, however, the global macro transport of the reactants was qualitatively well predicted, and the sensitivity of the yield to the Damköhler number and feedstream concentration showed consistent behavior. © 2005 American Institute of Chemical Engineers AIChE J, 51: 725–739, 2005