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Research - Multi-Phase Flows


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Multi-Phase Flows

Multiphase flows are characterized by flows with a mixture of fluids and solids that have unique velocity fields. Common examples are rain falling in air, the transport of coal in a liquid in a pipe, the initial mixing of milk in coffee. Interpenetration of the different phases pose significant problems with traditional methods for handling flows with single phases or multiple phases with well-behaved interfaces (e.g., air about water in a cup). These challenging interpenetration problems are just now being simulated and understood by specially developed codes for multiphase flows.

The numerical origins of the CFDLIB collection of codes began with a thesis study [Kashiwa, 1987] in T-3 to find a stable, well-coupled, Finite Volume integration scheme for incompressible flow with co-located primitive variables. Two possibilities emerged [Kashiwa, 1986] [Kashiwa, 1986], neither of which seemed fully satisfactory. This was because both schemes required solving two Poisson equations at each time step, rather than one Poisson solution, as in the original MAC scheme. Nevertheless, these schemes both represented cell-centered Finite Volume schemes for incompressible flow.

On another front was an internally supported project to develop a computer code, CAVEAT, for two and three dimensional, compressible flows with resolved material interfaces and large deformation that used co-located primitive variables and an ALE split computational cycle. The CAVEAT code [Addessio et al, 1992] in its final incarnation included both a Godunovs method and the so-called Total Variation Diminishing (TVD) Finite Volume method. Because of CAVEAT's versatile block structure and highly efficient computational approach, it furnished the basic data structure that was ultimately to become CFDLIB.

One of the goals of the research project was to develop a capability for integrating the compressible multiphase flow equations, and this effort focused on the TVD schemes, because Godunov's method, an explicit scheme, had not yet been applied to multiphase flows. One of the fundamental features of TVD schemes is the use of space-time centered fluxes for advancing a cell-centered state vector. When these fluxes are exactly centered in space and time, the method is second-order accurate, and is known as the Lax-Wendroff scheme. The TVD approach was to devise a 'limiter' to sense when the state is tending toward new extrema, and to use the limiter to introduce a first order fluxing in such localities. During this development it became clear that classical staggered meshes and TVD space-time centered fluxes were two different ways of accomplishing the same coupling of the momentum and pressure fields. Hence, the next step was to examine a space-time centered fluxing scheme for incompressible flow. What emerged is what is now called CCMAC; a cell-centered generalization of the MAC method, which requires a single Poisson pressure solution each time step [Kashiwa et al, 1994].

The CCMAC scheme was the key development that provided a common numerical treatment for CFDLIB: a collection of hydrocodes that are suitable for compressible flow, incompressible flow, multiphase flow of all kinds, magnetohydrodynamic forces and multi-fluid solutions, each with their own set of conservation equations. The design of each code volume in the library is modular, making the development of codes for specialized applications exceptionally fast. For example, a k-e model of the Reynolds stress, developed for one code volume, is easily inserted into another because of the common data structure among the codes.

The FLIP approach is now being installed into CFDLIB as an option, so one can take full advantage of the nondiffusive Lagrangian approach (see section 3.3 on Particle Methods below). With the FLIP option, one can simulate the motion of a Lagrangian projectile, passing through an Eulerian gas, penetrating a Lagrangian wall, and into an Eulerian liquid.

A current area of application of CFDLIB is the modeling of a reactive flow in multiphase, multi-field problems, such as encountered in oil refining, chemicals manufacturing, metals production, and fiber processing [Kashiwa, 1996]. Fig. 3.2-1 illustrates one time of a full simulation of the startup and operation of a recirculating fast-fluidized bed (FFB) reactor. Here, the goal is to model the interpenetration of gases and liquids, relative to a field of solid catalyst grains. Solid catalyst grains circulate in a flow loop consisting of a cyclone separator with a gas exit. A realistic simulation of the FFB reactor is dependent upon the physical models used to represent the effects of chemical species conversion, physical kinetics of phase change, granular flow, and multiphase fluid turbulence. The equations that embody these physical models are developed using a combination of detailed mathematics, definitive laboratory experiments, and physical intuition. The large-scale simulation is a means of bringing together these diverse sets of information in order to test the validity of the theories, and to provide important guidance to the design and operation of modern equipment.

There also exist many applications [Apen et al, 1996] that are a subset of the FFB application, such as the two-phase flow in human cardiovascular systems, or the dynamics associated with a lifeboat dropped onto the sea from a search-and-rescue aircraft [Lewis et al, 1994]. These and many other contemporary applications in modern technology are addressable by the Los Alamos code library CFDLIB. Some current applications include the smelting of iron ore, alumina precipitation, combined granular and fiber flow in manufacturing, and the effects of a near-miss in the performance of defensive missiles.

Los Alamos Multi-Phase Fluids Projects
All of the filled circles are linkable projects.
All of the empty circles are navigational guides.

  • Interfacial phenomena
  • Loss of Circulation in 3-Phase Draft-Tube
  • Interface reconstruction methods
  • Thermomechanical Response of Solids
  • Slurries
  • Multiphase interpenetration
  • Loss of Circulation in 3-Phase Draft-Tube

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This is from "The Legacy and Future of CFD at Los Alamos" (LAUR#LA-UR-1426)(365Kb pdf file)






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