Aerosol deposition in a bent pipe

 

This tutorial case considers the deposition of aerosol droplets on the wall of a bent pipe configuration. Due to the finite size of aerosol droplets, they have a small inertia which causes their paths in a fluid flow to deviate somewhat from the streamlines of the flow itself. As a result, curved streamlines as may arise in curved flow domains may not be tracked exactly by the aerosol droplets and collision of the droplets with the domain walls may occur. Such collision of droplets can be the cause of actual deposition of the droplet on the walls, thereby effectively being taken out of the flow. In Figure 1, we show the specific geometry that is used in this tutorial. The precise prediction of aerosol deposition in a bent pipe geometry is a crucial generic example with which differences between an inflow and an outflow aerosol size distribution can be understood. Depending on the flow conditions and the size distribution at the inflow of the bent pipe section a degree of ‘filtration’ of the aerosol will take place, altering the outflow size distribution relative to the inflow distribution. This alteration of the size distribution can be simulated with the AeroSolved code, resolving the size distribution with a number of discrete size classes, referred to as sections. Moreover, the precise locations, where aerosol droplets of different sizes deposit can be inferred from such simulations.

bent-fig1

 

The simulation of the problem employs the AeroSolved code, closely adopting the framework of the OpenFOAM platform. The user is required to provide an initial condition in a directory ‘0.org’, specify physical properties in a directory ‘constant’ and specify aspects of the numerical method in a directory ‘system’. The reader may readily observe the type of quantities and the format of their specification in the case files provided:

  • 0.org: the initial vapor (Y) and liquid (Z) mass fractions for air and water are specified as well as the sections (M) in which the aerosol size distribution is approximated. Similarly, pressure, temperature and fluid velocity components are initialized and boundary conditions specified.
  • constant: a large number of physical properties of the flow and species contained in the cavity are specified in detail. These properties relate to clearly identifiable classes of physical quantities and are grouped accordingly. The format for their specification is directly following OpenFOAM conventions. Finally, details of the computational grid are contained in the directory 'polyMesh. In this case the grid for a bent pipe is obtained by suitably deforming a grid for a straight pipe.
  • system: numerical parameters that govern the discreti'zation and methods of solution are specified in a number of files. In addition, a linked file ‘controlDict.foam’ is created from the controlDict file in order to create an interface for later post-processing with ParaView.

bent-fig2

To facilitate managing, setting up, running and evaluating a case, four shell scripts are provided. With ‘clean.sh’ one can restore a default setting, creating a similar point of reference for the simulations. Using ‘prep.sh’ the user can issue for a number of required preparations to be made, such as a computational grid and the decomposition of the problem for later parallel computing. The actual execution of a case can then be started by issuing the ‘run.sh’ script. Finally, gathering data in a manner suitable for further post-processing by the user is made easy using the ‘post.sh’ script in which the decomposed representation for parallel computing is used as basis to reconstruct the solution in the total domain, after which it could be sampled in specific ways for later evaluation. In the tutorial, we use only ‘ParaView’ for simple post-processing. The user may of course also exploit other methods of post-processing to study the results of a simulation. This aspect will not be elaborated on here.

bent-fig3

After running the case of aerosol deposition in a bent pipe, a number of properties is available for further study. Before the actual simulation results are investigated, it is worth checking the log files and make sure that the solution process has been executed properly. To illustrate some of the capabilities of the AeroSolved code we consider a few physical quantities as found on the selected simulation grid. A first impression of the flow is summarized in Figure 2. One clearly observes the characteristic patterns of high and low values in the u, v velocity components and the pressure. Note that in each figure the colormap differs, but in each case it is such that red refers to high values and blue to low values. In Figure 3, we visualized the liquid water droplet mass fraction in (a) an impression of the M0 section of the size distribution near the outflow. This illustrates some of the capabilities of the AeroSolved code in approximating the deposition of an aerosol with droplets of various sizes in a bent pipe.