(366a) Use of the Hierarchical Parcel Swapping Method to Simulate Turbulent Subgrid Reacting Flows with Variable Schmidt Numbers

Lignell, D. O., Brigham Young University
Stephens, V. B., Brigham Young University
Wheeler, I., Brigham Young University
Turbulent reacting flows are ubiquitous in Chemical Engineering and a great
deal of work has been performed to understand and quantify them. This is an
extremely challenging problem, however, due to the complexity of turbulent
flows. Such flows exhibit a wide range of length and time scales. Resolving all
continuum scales is not possible for engineering applications due to large
Reynolds numbers. The computational cost of direct simulations that do resolve
all scales increases with the cube of the Reynolds number. Large Eddy
Simulations (LES) overcome this restriction by only resolving the large scale
eddies, while modeling the subgrid scale mixing and reaction processes. The key
challenge for LES is developing accurate subgrid mixing models, and there are a
number of models of varying success that have been published. These include the
Interaction by Exchange with the Mean (EIM) model, the Euclidian Minimum
Spanning Trees (EMST) model, Curl's model, and the Shadow Position Mixing
Model. The central challenge of these models is representing the mixing
processes in a manner that is computationally efficient while retaining as much
physics as possible. We present a new mixing model termed Hierarchical Parcel
Swapping (HiPS). HiPS consists of a collection of notional fluid parcels with
adjacency defined using a binary tree. The levels of the tree correspond to
geometrically-spaced length scales, the number of which are user-defined. The
mixing and pairing and parcels is done at rates consistent with inertial range
turbulent scaling, and this is the key physical processes enabling the
potential improvement of HiPS over other models. The model has aspects in
common with the Linear Eddy and One Dimensional Turbulence Models, but more
efficient and flexible, and does not directly imply diffusive mixing on a
physical domain. We present an overview of the HiPS model and present results
of application to reacting flows. A new formulation of the model allowing
species with widely varying Schmidt (Sc) numbers is presented. Variable Sc
number scalar transport is important in many applications including combustion,
and aerosol formation and transport, including soot formation. Results are
presented for multiple scalar species with Sc greater and less than unity,
including results from scalar energy spectra. Opportunities for use as an
efficient and accurate sub grid scale model are presented, including sub grid
flow structures such as jets or near wall closures.