The influence of geometry on particle capture efficiency in trap impactors

In this study, trap impactors are used to collect polydisperse liquid droplets and are compared to the efficiency predicted by a conventional inertial impactor with excellent agreement. Polydisperse droplets are atomized and vary from $0.3\mu m$ to $10.5\mu m$ in diameter. Droplets are characterized after the impactor nozzle with an optical particle sizer to determine the size distribution and the corresponding distribution of Stokes numbers (St) at the tested flow rates. The trap ratio is defined as the difference between the trap and nozzle diameters divided by the total depth of the trap. To characterize the trap geometry, multiple traps are tested with varying trap ratios and demonstrate that decreasing the trap ratio results in a reduction in trap efficiency and an increase in wall losses. Specifically, a trap ratio of 1.00 resulted in a maximum trap efficiency of 94%, whereas a trap ratio of 0.27 had a maximum trap efficiency of 31%. Trap impactor design recommendations are made to maximize droplet collection inside the trap by increasing the trap ratio.