## 3 Results and Discussion  
### Experimental Results  
The reference material salt was tested first and the results are presented in Figure 12. It can be seen that an average value of 0.65 was obtained for the whole time of silo filling. This number represents a constant particle concentration ratio between the wall and the highest particle concentration measured in the experimental silo.  
Repeatability tests were conducted to evaluate the usefulness of the CRF experiments using salt. Results are shown in Figure 13. The CRF values obtained from the three experiments were averaged and a value of 0.65 was obtained. The averaged value showed only small experimental variations and, thus, it can be concluded that the CRF value was suitable to capture the behaviour of spatially distributed particles during the silo filling process.  
Figure 11: Particle information distribution technique used in Fluent.  
Figure 12: Non dimensional Concentration Rank Factor [k] calculated for the period of silo filling.  
The k factor is like a fingerprint of the material i.e., salt and can now be used to evaluate dust concentrations in cylindrical silos filled with the same material.  
The dust concentrations may vary with process parameters such as the material feeding rate and this may causes the CRF value to vary as well. In the following experiments alumina was tested under similar experimental conditions to evaluate the suitability of the CRF approach for materials with different material properties and under different flow conditions i.e., varying the feeding rates. Experimental results are shown in Figures 14 and 15. It can be seen that the CRF values were constant for a significant amount of time after a sudden increase. This increase was related to the finishing of the filling process and only fines circulated as tracer particles in the air. This sudden increase was kept in the figure to show the fast response of the CRF value to changing silo filling conditions. It can also be observed that very similar CRF values were obtained for varying flow rates of alumina powder which is very promising because it shows the independence of the CRF value from the flow rate for the same material.  
Figure 14: CRF value for alumina at flow rates of \(7\times 10^{-3}\) kg/s.  
Figure 13: Repeatability tests with the same experimental set up for the reference material salt.  
The comparison of experiments with the reference material salt and alumina revealed that the k value for the salt experiments was slightly higher for alumina. These differences are partly accounting for instabilities in the flow field and the variation of fines content measured near the wall in the alumina experiments. The variations in the flow field caused fines to segregate more near the central particle jet because the hydrodynamic forces which acted on the alumina particles were less strong than in the salt experiments. At larger scales, such as industry scale, these instabilities could be neglected because stronger and more stable air currents normally occur in industrial sized silos. The results from the alumina experiment with the high feeding rate was used as a benchmark for CFD simulations. The sampling approach in the simulations and results are discussed in the next sections.