title: Gas mixing for achieving suitable conditions for single point aerosol sampling in a straight tube: experimental and numerical results.
abstract: Experimental measurements of velocity and tracer gas concentration are taken in a straight tube to evaluate the effectiveness of mixing in achieving conditions as required by ANSI N13.1-1999 for single point extractive sampling from stacks and ducts of nuclear facilities. Mixing is evaluated for inlet turbulent intensities of 1.5%, 10%, and 20%, achieved by introducing various bi-plane grids, and for conditions generated by a commercial static gas mixer. The data obtained (at Reynolds number = 15,000) highlight the importance of inlet turbulence intensity in the process of turbulent dispersion of a dilute gas. The gas mixer does not introduce significant pressure losses and unlike bi-plane grids, the turbulence downstream of the mixer is not homogenous. A judicious choice of the release location that uses the large scale eddies and inhomogeneity of the turbulence ensures that the specified ANSI N13.1-1999 criteria are attained within 7 diameters downstream of the duct inlet. This is significantly more effective than a bi-plane grid where even with 20% inlet intensity the criteria are met only at 21 diameters downstream. The predictions of a proposed semi-empirical correlation match favorably with data. For example, at 18 diameters downstream with inlet intensities of 1.5% and 10%, the predicted coefficients of variation (COVs) of 150% and 65% are close to the actual values of 154% and 50%; where the COV of a set of measurements is the ratio of the standard deviation of the set to its mean value. The corresponding results obtained using commercially available software are 141% and 12%. Results from a particle-tracking model show good qualitative trends, but they should not be used to determine compliance with the requirements of the ANSI standard.
authors.affiliation: Department of Mechanical Engineering Texas A&M University College Station, TX, USA.
authors.forename: M
authors.initials: M
authors.lastname: Anand
authors.forename: A R
authors.initials: AR
authors.lastname: McFarland
authors.forename: K R
authors.initials: KR
authors.lastname: Rajagopal
mesh_terms: D000336:Aerosols; D004867:Equipment Design; D005740:Gases / Q000032:analysis*; D009688:Nuclear Reactors; D011834:Radiation Monitoring / Q000295:instrumentation / Q000379:methods*
chemical_list: D000336:Aerosols; D005740:Gases