THEORETICAL AND PRACTICAL CONSIDERATIONS IN THE USE OF WIND TUNNELS FOR ODOR EMISSION MEASUREMENT

In 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 used to measure odor 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 reportedly differed by up to 300 times, compared to the conventional flux chamber sampling method. Thus, it is important to provide a review of the system.This paper presents the basic principles and theories used in a conventional wind tunnel, and discusses how these principles and theories pertain to a small-scale wind tunnel system. While basic theories may generally apply to a small wind tunnel system, some limitations exist that may cause errors in odor emission rate measurements, especially under field conditions. Moreover, 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 odor 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 odor emission rates from area sources with low odor emissions.This paper also compares wind tunnel and flux chamber technologies. Compared to a flux chamber, wind tunnel technology is not as commonly used worldwide. Although wind tunnels may be used to simulate emissions under normal ambient conditions with 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 insufficient peer-reviewed papers that support the accuracy or recovery rates of the system.Field measurements have revealed that the results obtained using a wind tunnel were much higher than those obtained with a flux chamber. During the sampling session, observers noted that the inside of the wind tunnel underwent a wave condition. This artificial wave condition may bias the measured odor fluxes and yield results that are upward relative to those measured by a flux chamber.Practical issues are also considered and discussed in this paper. Deployment of wind tunnel sampling systems involves the use of bulky and cumbersome equipment and accessories. Some accessories are also heavy. These conditions may require additional time for system assembly and additional manpower for equipment handling. Therefore, the cost effectiveness of the technology becomes an important issue that must be considered before deploying this technology for odor sampling and emission measurement.