Basics of Plume Dispersion

Kishor Khankari, AnSight LLC
Brad Cochran, CPP Wind Engineering & Air Quality Consultants

Exhaust gases from laboratory facilities should be diluted and dispersed effectively into the atmosphere without significant entrainment into the adjacent air intakes. Several inter related factors can affect the performance of the lab exhaust systems including the discharge velocity, stack height, stack diameter, wind speed, wind direction, building shape and size, surrounding structures, and resulting airflow patterns around the building. As an example, high wind speeds cause a plume to grow rapidly, increasing the downwind dilution. However, the high winds also knock down the plume, and potentially cause stack-tip downwash which can create low dilution (high concentration) values on the roof, at or near the base of the stack. Similarly, corner vortices can create high turbulence and downwash at the top of the stack. Therefore, a thorough investigation of the plume behavior under a wide range of wind speeds and wind directions is necessary to design a safe laboratory exhaust system. The theoretical equations available for estimating the contaminant dilution and plume rise are in some cases too complex and in other, insufficient. This presentation will try to describe the dynamics of plume dispersion and will show what is involved in the calculation of plume height and related dilution. The impact of important parameters on the dilution and dispersion of plumes will be demonstrated with the help of examples. In addition, this presentation will demonstrate how Computational Fluid Dynamics (CFD), analytical numerical models, and wind tunnel simulations can help in the design and optimization of the lab exhaust systems. This lecture will also present insightful animations showing the nature of plume dispersion and resulting dilution at the air intakes.

Learning Objectives

  • Understand the basic theory of plume rise
  • Understand the impact of wind speed on the plume rise
  • Understand the basic steps in the calculation exhaust-to-intake dilution procedure
  • Understand how Computational Fluid Dynamics (CFD) and wind tunnel can be employed in design and optimization of lab exhaust systems


Dr. Kishor Khankari, Ph.D. is noted expert in Computational Fluid Dynamics (CFD) with several years of experience in providing engineering insights and optimized HVAC design solutions using analytical techniques. He has developed patented technology of exhaust fan assembly. He has published several technical papers and trade magazine articles. Dr. Khankari is ASHRAE Fellow member, Distinguished Lecturer, and recipient of the ASHRAE Exceptional and Distinguished Service Awards.

Mr. Brad Cochran, PE, is a Principal and Director of Air Quality Services for CPP Wind Engineering. He has over 25 years of experience conducting dispersion modeling studies for laboratory, hospital, and data center ventilation design. Over the past decade, Brad has developed new design techniques to minimize the energy requirements for laboratory exhaust stacks through VAV technologies. He has successfully designed and employed VAV exhaust systems for more than 100 laboratory systems throughout the US, Canada, and the United Kingdom. He has authored and presented several papers on laboratory exhaust design.


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