Modeling and Testing Stacks and Chimneys
By Kevin Linfield, Ph.D., P.Eng., P.E.
Flow modeling and testing of smokestacks and chimneys is extremely important. Although they seem like simple tubes, the stack is actually the final part of the gas flow path and contains expensive equipment that is used for the EPA to track gas/particulate matter concentration or emission rates. Computational Fluid Dynamics (CFD) modeling is often used to determine if turbulent, swirling flow exist in the stacks; conditions that can degrade the accuracy of the Continuous Emissions Monitoring Systems (or CEMS).
Physical flow modeling involves the construction of scale models and running air through these models. The results can also provide input values and correlation data for CFD simulations. Physical models are usually constructed with clear walls or with windows so that the flow patterns can be observed via smoke flow or string tufts. Models can include the added complexity of water film, droplet impingement, liquid collection systems, drains, condensation, and evaporation. Constructing models allows engineers to assess performance in a controlled environment.
Using CFD software, liquid/wet stack modeling is done to determine droplet impingement and liquid collection. External analysis, including plume dispersion studies, are used to reduce the potential for problematic interaction with other equipment, or to prevent the ingestion of exhaust gases and particulate into fans or air intakes.
Examples of plume dispersion modeling.
Shown below is an example of CFD modeling of a wet stack, using a combination of Lagrangian particle tracking and a wall film model. Droplet trajectories were tracked from the scrubber exit for four different particle sizes spanning the expected range of droplet sizes. While some particles escaped with flue gas, many impacted the stack walls and were added to the wall film.
Field testing can be performed in multiple stack elevations to measure the volumetric flow according to EPA Methods 2, 2F, 2G, and 2H. ASC's stack testing equipment is extremely accurate, so that plant systems can be correctly calibrated to EPA standards.
Airflow Sciences Corporation (ASC) is a fluid dynamics solutions company, specializing in the design and optimization of equipment and processes involving flow, heat transfer, combustion, and mass transfer. Now celebrating 50 years as an industry leader, ASC has focused on testing and simulation of air, gas, liquid, and particulate flows since 1975.
ASC also manufactures standard and custom test equipment, including probes and wind tunnels, enabling customers to collect data accurately and efficiently. ASC's primary CFD software, Azore®, is also available for customers with in-house CFD personnel. ASC offers comprehensive flow solutions and optimization and serves a wide range of industries including HVAC, power, auto, rail, and food processing.