CFD study for aerosol deposition in turbulent isokinetic sampling probe Conference Paper uri icon

abstract

  • CFD was used in this study to simulate the flow and particle transportation behavior in an isokinetic sampling probe and to analyze affecting factors on the wall deposition including particle size, gravity, Saffman lift force etc. using FLUENT 6.2. Reynolds Averaged N-S (RANS) and Large Eddy Simulation (LES) turbulent models were used to calculate the flow field to check the flow pattern near the inlet of the nozzle. An unsteady simulation for the probe facing incoming air shows a pair of different velocity zones at the edge of the probe where inside of the probe has higher velocity and outer side has lower one and fluctuation is generated by the pair of different velocity zones. Near the inlet, the boundary layer is developing and very thin. So a possible reason for the heavy particle deposition near the edge is that, the particles tend to move toward outside to the lower velocity zone near the edge, and hit the wall of the probe which has almost no-boundary covering. Discrete Phase Model (DPM) was used to calculate particle trajectory after the turbulent flow field is calculated. It was found that κ-ω can obtain both right trend and good prediction for particle loss in the probe while κ-ε is not suitable since it can not predict even the right trend of the particle loss. For each particle size, about 2970 particles are released to calculate its deposition. Three particle sizes, 5 um, 10um and 15um are calculated at three different free stream velocities 8.9 m/s, 15.6 m/s and 21.3 m/s to check the wall loss of the probe. Prediction of k - w is close to experimental data for lower wind velocity 8.9 and 15.6 m/s. For 21.3 m/s free stream, predicted wall loss is lower than experimental result. It was found that the Saffman lift force has little influence on the total wall loss of the probe for the relatively large particle sizes in this study. However, gravity is important for the wall loss since the particle is large. When the probe is horizontal, the wall loss at the lower half is almost 2-3 times to the upper part because of the gravity settling effect. A special deposition phenomenon of the isokinetic probe is that majority of the deposition inside the probe is close to the inhaling inlet of the probe. Beyond 5 times of the inlet diameter of the probe, there is very small amount of the deposition. LES was used to analyze the flow pattern near the inhaling inlet of the probe and special velocity and pressure distributions were found. When the flow was inhaled into the tube, the inside velocity is high and outside is low, the particles tend to move toward outside and hit the wall.

author list (cited authors)

  • Hu, S., McFarland, A. R., & Hassan, Y. A.

publication date

  • September 2008