Computational Analysis of Particle Nucleation in Dilution Tunnels: Effects of Flow Configuration and Tunnel Geometry

Measurement of fine particle emission from combustion sources is important to understand their health effects, and to develop emissions regulations. Dilution sampling is the most commonly used technique to measure particle number distribution because it simulates the mixing and cooling of combustion exhaust with atmospheric air, which drives nucleation and condensation of semi volatile material. Experiments suggest that the measured size distribution is dependent on the dilution ratio used and the tunnel design. In the present work, computational analysis using a large-eddy-simulation (LES) based model is performed to investigate the effect of tunnel flow and geometric parameters on H₂SO₄-H₂O binary nucleation inside dilution tunnels. Model predictions suggest that the experimental trends are likely due to differences in the intensity of turbulent mixing inside the tunnels. It is found that the interaction of dilution air and combustion exhaust in the mixing layer greatly impacts the extent of nucleation. In general, a cross-flow interaction with enhanced turbulent mixing leads to greater number of nucleation-mode particles than an axial-flow interaction of combustion sample and dilution air.