Optimizing Laboratory Exhaust Design With Dispersion Studies: Is CFD an Option?

Aimée Smith, RWDI Inc.

Laboratory Buildings discharge various types of hazardous and odorous emissions through their exhaust systems. Among the worst offenders are fume hoods, vivaria, specialty hoods (i.e., radioisotope, acid, etc.), and diesel generators. To provide a healthy and comfortable space, it is important that the design of these exhausts is optimized to ensure that the emissions do not re-enter the building air supply, while balancing other criteria such as fan energy savings and aesthetics. This is commonly achieved through performing a detailed exhaust dispersion study.

The prediction of atmospheric dispersion using physical wind tunnel modeling is a well-established and accepted procedure that is commonly used to study and optimize the design of laboratory exhausts. For many reasons, CFD is becoming an increasingly popular choice in the design process for examination of exhaust dispersion. However, the use of CFD for this application can be computationally intensive, and the results can include unacceptable levels of uncertainty and error. There are many factors that need to be considered in a simulation of atmospheric dispersion including:

  • length scales
  • extent of surrounding model
  • development of the atmospheric boundary layer
  • turbulent flow properties
  • wind velocity profiles
  • similitude parameters
  • buoyancy

The common perception that CFD modeling is quick and inexpensive compared to wind tunnel testing is not accurate for exhaust dispersion studies. Where quick prototyping is required, involving a small model and a few input conditions, CFD can be a powerful tool. It falls down, however when a large number of wind conditions and specific information (i.e., exhaust stack height, exit velocity and flow rate) are required. In these scenarios, CFD can be a reliable, but slow and expensive replacement for the wind tunnel. This can motivate simplification of the modeling to reduce cost and time. However, if over-simplification occurs, results can be highly misleading, leading to potentially unsafe, or overly conservative design decisions.

This session will review the basic principles of fluid flow modeling with reference to established wind tunnel modeling procedures and emerging CFD techniques. Specific case study examples will be used to illustrate scenarios where CFD is currently a useful tool for evaluation of exhaust dispersion, and scenarios where wind tunnel modelling is the preferred method.

Learning Objectives

  • Identify where there is a need to consider exhaust dispersion and evaluate laboratory exhausts for re-entrainment risk.
  • Understand the basics of fluid flow modelling and wind flow around buildings, including established wind tunnel modeling procedures and emerging CFD techniques.
  • Gain an understanding of the potential challenges associated with the use of CFD Modeling to evaluate atmospheric dispersion, particularly for rooftop stacks.
  • Receive in-depth insight (through case study examples)regarding which dispersion modelling tool is the most appropriate for a given exhaust dispersion scenario.

Biography:

Aimée is a Principal of RWDI, and the firm leader for the higher education, laboratory and health care sectors. Aimée is widely recognized among the design community for her deep knowledge of exhaust dispersion and air intake systems and her strong technical understanding of health & well-being of occupants, building energy use, and local microclimate factors including air quality, ventilation, acoustics and wind behavior. Aimee is a licensed Prof. engineer (Ontario), an I2SL and SLCan member.

 

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