Computer Modeling of Contaminant Jet Flow into Local Exhaust Hoods
Public Deposited- Last Modified
- February 28, 2019
- Creator
-
Smith, Kirby N.
- Affiliation: Gillings School of Global Public Health, Department of Environmental Sciences and Engineering
- Abstract
- A computer model was developed and coded in BASIC topredict the streamline that a jet of gaseous sulfur hexafluoride will follow in the flow field of a flanged circular exhaust hood (FCH). This approximate solution is based on the vector addition of a modified potential flow solution for the FCH, and a jet flow solution. The assumptions underlying the equations describing jet flow are those of the Prandtl mixing length hypothesis. The computer program generates streamlines for the combined flow by means of iterative vector addition. The interactive program prompts the user for the hood and jet diameters and flows, and the distance from the hood at which the jet is placed. A graphic plot of the predicted streamline followed by the gas jet is displayed. The program is used to predict the critical distance [Z/D]50, the distance along the hood centerline (Z), as a fraction of the hood diameter (D), where the jet can be placed such that 50% of the jet contaminant flow is captured. A series of such [Z/D]50 values was generated for twenty-one hood and jet flow combinations. The program was validated in the laboratory. A probe was placed in the duct of a flanged circular exhaust hood and was connected to an electron-capture gas chromatograph, to determine the concentration of SF(6) in the hood. Capture efficiencies (ratios of "captured" gas concentrations at various jet-hood distances to concentrations in the duct when the jet flow is fully captured) were determined for jet positions at intervals along the hood centerline. Five replicate measurements were collected per position, for all combinations of jet and hood flow. Results indicate that the model is quite accurate when crossdrafts are accounted for, except for predicted [Z/D]50 values of less than 0.7, which occur quite close to the hood face. The approximate model errs in this region because it neglects the effects on the jet of the static pressure gradient created by the flow of the exhaust hood, and the shear turbulence of the interacting streamlines of jet and hood flow. The model may be expanded in the future to include definitive crossdraft variations, other jet locations or directions, hoods of other shapes, or heat and gas buoyancy effects. Key Words: Critical distance, flanged circular exhaust hood, capture efficiency, ventilation.
- Date of publication
- August 1989
- DOI
- Resource type
- Rights statement
- In Copyright
- Advisor
- Miller, Cass T.
- Flynn, Michael
- Leith, David
- Degree
- Master of Science in Public Health
- Academic concentration
- Industrial Hygiene
- Degree granting institution
- University of North Carolina at Chapel Hill
- Graduation year
- 1989
- Language
- Deposit record
- ad7e9272-7c4f-43e0-afeb-7a5f2526ad76
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