Multi-Phase Flow Simulation at NCSU
July 1 , 2006
By Dr. J. R. Edwards, NCSU

The accurate prediction of multi-phase flows remains one of the major challenges in computational fluid dynamics. Processes such as spray atomization, droplet vaporization, pollutant transport, catalytic conversion, and many others involve mass, momentum, and energy transfer between a carrier phase and a dispersed phase. The dispersed phase can often be assumed to be dilute enough so that its occupied volume is negligible relative to that of the carrier phase. In applications such as sheet cavitation, flows of immiscible liquids, foaming, and many others, the phases may occupy similar volumes in some regions, and direct capturing of interfacial behavior may be of interest. Prediction of such processes from first principles within realistic geometries is beginning to become feasible, given advances in computer architectures, physical models such as large-eddy simulation, and advanced algorithms.

Over the past several years, Professor Edwards' group in the Aerospace Engineering program has developed a hierarchy of computer codes for predicting multi-component, multi-phase flows in complex geometries. The most general version, termed REACTMB-MP, describes each phase using generalized equations of state and can include various phase-transition, interfacial transport, and turbulence models. A key element is the use of low-diffusion upwinding schemes to capture both smooth flow features and discontinuous ones (such as phase interfaces) without excessive numerical diffusion. Another feature is the implementation of REACTMB-MP on massively-parallel computer architectures using the MPI standard This enables transient simulations of upwards of 10 million grid nodes to be performed on NC State's 350+ processor IBM Blade Center Linux cluster (http://www.ncsu.edu/itd/hpc*).

Recent applications to dispersed two-phase flows have included predictions of fine particulate re-suspension and transport due to human motion (sponsored by EPA and DARPA). Simulations of circulating fluidized bed absorbers (sponsored by EPA) and projectile penetration into sand (sponsored by ONR) involve the modification of the dispersed-phase module to handle dense fluid-solid interactions, including the use of Mohr-Coulomb stress-strain relationships near maximum compaction. The nucleation and subsequent growth of a dispersed phase has been modeled in projects involving the production of fine particulates from rapid expansion of supercritical solutions (sponsored by ONR) and the analysis of supercritical fuel injection processes (sponsored by Taitech and AFRL). Predictions of water condensation in hypersonic wind tunnels have been performed in support of the Air Force's Scramjet Engine Demonstrator program. Calculations involving two-phase flows within aerated-liquid (or ‘barbotage') fuel injectors (sponsored by Taitech and AFRL), polyurethane foaming (sponsored by Dow Chemical), and cavitation prediction (sponsored by NASA Glenn Research Center ) are examples of those that require sharp phase-interface capturing.

Specific future directions in the development of REACTMB-MP will depend on requirements of sponsoring agencies. However, general improvements are needed in interface capturing for immiscible fluids, better models for phase transitions, and techniques that allow a single algorithm to model the transitions among regimes of two-phase flow (ie. from bubbly to slug to annular). For additional information, please contact Professor Jack R. Edwards at jredward@ncsu.edu or 919-515-5264.

t=2 seconds		t=4 seconds
Snapshots of particulate re-suspension from a carpet layer due to ‘human' walking and stomping motions. Pink iso-surfaces represent fine particles (average diameter of 2.5 microns). Magenta iso-surfaces represent larger particles (average diameter of 7.5 microns)

Snapshots of two-phase flow development in an aerated-liquid injector (gaseous volume fraction contours) . Red contours correspond to regions of mostly gas. Blue contours correspond to regions of mostly liquid. The liquid is pushed toward the walls of the discharge tube by the aerating gas, resulting in the formation of an annular liquid sheet upon exiting the injector