  Multiphase flows are commonly found in chemical engineering processes such as
distillation columns, bubble columns, fluidized beds and heat exchangers.
Physical boundaries in numerical simulations of multiphase flows are generally
defined by a mesh that conforms to the physical boundaries of the system.
Depending on the complexity of the physical system, generating the conformal
mesh can be time-consuming and the resulting mesh could potentially contain a
large number of skewed elements, which is undesirable. The diffuse-interface
approach allows for a structured mesh to be used while still capturing the
desired solid-fluid boundaries. In this work, a diffuse-interface method for
the imposition of physical boundaries is developed for two-fluid incompressible
flow systems. The diffuse-interface is used to define the physical boundaries
and the boundary conditions are imposed by blending the conservation equations
from the two-fluid model with that of the solid. The results from the
diffuse-interface method and mesh-defined boundaries are found to be in good
agreement at small diffuse-interface widths.
