Non-isokinetic aerosol sampling

 

This tutorial case considers the problem of non-isokinetic sampling of an aerosol. The dispersed aerosol droplets may not completely follow streamlines in the flow and ‘drift away’ from the flow paths due to their inertia. This leads to a slight redistribution of particles of different sizes over the flow domain in response to curved flow patterns that may arise. In case one wishes to sample the aerosol size distribution using a sampling tube in which flow is drawn into at a specified speed, the measured aerosol size distribution entering the sampling tube will differ somewhat from the actual size distribution. Understanding the relation between the sampled aerosol size distribution and the actual size distribution is important, in order to correctly interpret the findings of measurements. In Figure 1, we sketched the geometry that is considered in two spatial dimensions.

 

 

sampler-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 observe the type of quantities and the format of their specification in the case files provided. To clarify this:

  • 0.org: the initial vapor (Y) and liquid (Z) mass fractions 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'.
  • system: numerical parameters that govern the discretization 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 interfacefor later postprocessing with ParaView.

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 computa tional grid and the decomposition of the problem for later parallel computtions. 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 also exploit other methods of post-processing to study the results of a simulation. This aspect will not be elaborated on here.

sampler-fig2

After running the case of non-isokinetic aerosol sampling, 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. We consider a situation of non-isokinetic sampling in which the flow inside the sampling tube is set to a lower value compared to the oncoming flow velocity in the far field. A first impression of the flow is summarized in Figure 2. One clearly observes the flow patterns into and around the sampler tube, expressed by characteristic high and low values in the u, v velocity components and the pressure distrtibution. 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 characteristic section of the size distribution. The aerosol droplets are seen to ac- cumulate in a thin boundary layer. This illustrates some of the capabilities of the AeroSolved code in analyzing the effect a sampler tube operating at non-isokinetic conditions has on the aerosol distribution.

sampler-fig3