(187f) Filtered Two-Fluid Models for Gas-Particles Flows

It is well known that gas-particle flows exhibit large fluctuations in velocities and local suspension density. In riser flows, these fluctuations are associated with the random motion of the individual particles (typically characterized through the granular temperature) and with the chaotic motion of particle clusters, which are repeatedly formed and broken apart. These clusters occur over a wide range of length scales and their dynamics span a broad range of time scales. This broad range of scales has made it difficult to construct efficient flow models required for practical analysis of flows in large fluidized beds and risers. In this presentation, we will discuss the results of our ongoing study aimed at the development of filtered hydrodynamic models for such systems.

We have extracted filtered drag coefficient and particle phase stresses as functions of the local particle volume fraction and the size of the spatial averaging window (i.e. filter width). This was done through highly resolved simulations of fluidized suspensions in a periodic domain, followed by filtering of the results using different filter widths. These simulations were performed using the MFIX code.

The filtered drag coefficient decreased systematically with increasing filter width, while the filtered particle-phase stresses manifested simple linear relationships with the local particle volume fraction. Both two- and three-dimensional simulations manifest nearly identical scaling.

In this presentation, we will also discuss the sensitivity of the filtered closures to particle and fluid properties. We will also examine what constitutes reasonable filter length scales for these flow problems.