Technical Publications

Theoretical and Practical Considerations in the Use of Wind Tunnel for Odour Emission Measurement

Lawrence Koe, Yih-Ming Koh, Chuck Schmidt, Jay Witherspoon, Tom Card, Yan Wu

EXECUTIVE SUMMARY

In the past decades, wind tunnel systems have been commonly used for aerodynamic research including simulation of wind loads on buildings, automobiles and airplanes. In recent years, this methodology has been deployed for measurement of odour emission rates from area sources such as primary sedimentation tanks at sewage treatment plants. The size of the wind tunnel used for this purpose is much smaller than conventional wind tunnels. Emission rates obtained from such measurements have been reported to differ by up to 300 times, compared to the conventional flux chamber sampling method. Thus, it is important to provide a review of the system.

In this paper, the basic principles and theories used in a conventional wind tunnel system is presented and the pertinency of these theories to a small-scale wind tunnel system is discussed. It has been found that while the theories are generally applicable to a small wind tunnel system, there exist some limitations that may give rise to errors in odour emission rate measurement, especially under field conditions. Deployment of wind tunnel system onto a moving area surface may upset the development of boundary layers in the system, leading to erroneous measurement of odour emission rates. A lack of understanding of the effect of upward rising air bubbles on the dynamics of flow in wind tunnels limits the application of wind tunnels for area sources with outward flows such as aerated area sources. High detection limits of wind tunnel systems also pose a problem in measuring odour emission rates from area sources with low odour emissions.

Comparisons between wind tunnel and flux chamber technology is presented. Compared to a flux chamber, wind tunnel technology is not as commonly used worldwide. Though wind tunnels may be used to simulate emissions under normal ambient conditions by having results that are scaleable, no clear documentation has been provided for these adjustments. There is also a general lack of information and documentation on issues regarding the sampling protocol for a wind tunnel system and peer reviewed papers that support the accuracy or recovery rates of the system. Whereas the flux chamber technology is a commonly accepted direct measurement technique available for various odour sources including open wastewater, solids surfaces as well as aerated surfaces.

Field measurements have revealed that the results obtained using a wind tunnel were much higher than those obtained with a flux chamber. It has been observed that during the sampling session, the inside of the wind tunnel was undergoing a wave condition. This artificial wave condition may bias the measured odour fluxes and yield results that are upward relative to those measured by a flux chamber.

Practical issues have also been considered and discussed. Deployment of wind tunnel sampling systems involves the use of bulky and cumbersome equipment and accessories. Some accessories are also found to be heavy. This may result in additional time for system assembly and manpower for equipment handling. Due to these problems, cost effectiveness of the technology becomes an important issue that must be considered before deploying the technology for odour sampling and emission measurement.