Multiphase flow - the volume of fluid method (VOF)

Process intensification is a term that is on everyone's lips. The aim of process intensification in process engineering is to increase the space-time yields, improve selectivity and reduce overall production costs. The optimization of a process is often limited by mass and heat transfer. One way of increasing the mass and heat transfer is to increase the surface to volume ratio of a device like in microstructured devices.

Especially multiphase processes in microstructured components are considered to have great potential in terms of process intensification. The reason for this are the self-adjusting large phase interfaces, thin liquid films and short diffusion paths in microchannels. Combining all these points will result in apparatuses with large mass and heat transfer coefficients.

Another goal of process intensification is to create optimal conditions for the processes within the apparatus. For this purpose it is often necessary to study the physical and chemical processes with location and time resolved measurements in more detail. But it is much more difficult in the micro process technology to get the informations needed due to the small dimensions of the microchannels. A possible alternative to the measurement technique is the location and time resolved simulation using CFD.

Most CFD-codes include the VOF method which was originally developed by Hirt and Nichols (1981). The basic concept of the VOF method is the definition of a non-dimensional scalar quantity f, which represents the fraction of the mesh cell volume occupied by the continuous phase, which is for example the liquid phase. Thus, for f =1, the mesh cell is entirely filled with liquid while for f =0 it is entirely filled with gas (dispersed phase). In a mesh cell which instantaneously contains a part of the interface, both phases coexist and it is 0< f <1.

To demonstrate interface capturing between gas and liquid faces and mass transfer across faces we present two simulations visualized in video clips. In video 1 the inflow of gas through a small opening into a microchannel filled with liquid is shown. Video 2 shows the rise of a gas bubble in a microchannel. Across the interface gas is discharged to the liquid and therefore the volume of the bladder is decreasing during its rise.