AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
Abstract View
A Novel In-stack Pre-cutter for Separating Droplets in Gas Streams Saturated with Water Vapor
Chih-Hsiang Chien, Joshua Udvardy, CHANG-YU WU, Zachery Emerson, Derek Sain, Leland Carlson, Vipin Varma, Cathe Kalisz, University of Florida
Abstract Number: 641 Working Group: Control and Mitigation Technology
Abstract Current EPA guidance states that facilities required to measure filterable PM2.5 in saturated or droplet-laden stacks should use EPA Method 5/5B and use the total filterable PM as a surrogate for filterable PM2.5. However, these two methods have no size classifier and therefore a gravimetric assessment of the downstream filter yields a higher-than-true emission rate of PM2.5. The limitations of the available test methods warrant the development of a new method. A novel in-stack pre-cutter comprised of an exchangeable nozzle, an impactor and an absorption pad, has been designed to isokinetically separate droplets > 10 µm. Computational fluid dynamics was applied to evaluate and optimize the prototype. An SST K-Omega turbulence model was adopted to solve for gas transport, while the aerosol was simulated using a discrete-phase model. Our study reveals that there are three factors to be considered. First, to isokinetically extract, the exchangeable nozzle is designed as a diffuser to reduce particle inertia and achieve a cut size of 10 µm in the downstream impactor. Our study shows that flow separation could occur in the diffuser and lead to ineffective velocity reduction, causing higher particle inertia than expected and thus a smaller cut size. An optimal combination of the nozzle’s half-angle at 7 degrees and extended throat length of 3 cm was identified to reform the boundary layer and slow down the free stream, leading to a desired cut size. Second, while the flow direction in a conventional impactor for ambient sampling is along that of gravity, the flow direction in the stack is against gravity. A previous study reported a smaller cut size due to gravity while this present study shows that the opposing flow direction can lead to a slightly larger cut size. Third, the pre-cutter dimension was enlarged to prevent secondary impaction of particles.