Evaluation of Mixing in Three Duct Configurations and Development of a Generic Tee Plenum System (GTPS) for Application to Single Point Aerosol Sampling

Abstract

A Generic Tee Plenum System (GTPS) based on three different duct systems (Configuration-I, II and III) used in nuclear facilities has been developed which creates suitable conditions for single point representative sampling. This single point sampling is an acceptable standard methodology for sampling emissions from stacks and ducts of the nuclear industry (ANSI/HPS N13.1-1999 and US EPA 2000). These air duct systems in nuclear facilities must meet the requirements of ANSI/HPS N13.1-1999 that the exhaust airflow be monitored with continuous sampling in case of an accidental release of airborne radionuclides. Results show that the GTPS system performance is well within the requirements of ANSI/HPS N13.1-1999 - the COVs for velocity, tracer gas concentration and 10 [u]m AD aerosol particle are less than the mixing criteria 20% of ANSI/HPS N13.1-1999. The objective of this work is to provide data on the mixing effectiveness of various duct configurations found in nuclear facilities. These data can be applied in new facilities as well as in upgrades to existing U.S. DOE nuclear facilities. The data should also provide a basis for testing the efficacy of mixing models that may be generated in the future. Experiments were conducted to characterize the degree of mixing at downstream locations as affected by several types of flow disturbances, including a plenum consisting with duct intakes, a rectangular elbow with turning vanes, 90⁰ elbows, tees, commercial static mixing device and GTPS in circular and non-circular ducts. The mixing performance of a plenum duct configuration (Configuration-I) with two-intake sources was studied to develop recommendations for suitable sampling locations less than 4 duct diameters downstream of a plenum. Several mixing element options were considered both upstream and downstream of a security grille. A stack model system with straight circular ducts downstream of rectangular elbow with turning vanes (Configuration-II) were characterized for velocity, tracer gas and particle concentration profiles COVs. The application of a mixing system was designed for achieving maximum conditions in round-elbow and tee element system interfaced to a stack (Configuration-III). Several options were considered when using different types of elbows upstream of the stack. Numerical studies for velocity profile in the GTPS were conducted with a commercial computational fluid dynamics (CFD) software, FLUENT. Good agreement was found between the experimental and numerical results.