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Testing local exhaust ventilation at controlled turbulence generation by using tracer gas and a 3-D anemometer
University of Gävle, Faculty of Engineering and Sustainable Development, Department of Building, Energy and Environmental Engineering. (Indoor environment)ORCID iD: 0000-0002-0337-8004
2014 (English)Report (Other academic)
Abstract [en]

Local exhaust (LE) ventilation is a ventilation technique where contaminated air is locally extracted close to the contaminant source, usually with the purpose to reduce the exposure of a person doing work which involves the contaminant. There is a need for well-defined and appropriate methods to test the performance of LE constructions. The present study aims at contributing to the establishment of such tests. The study entails full scale experimental measurements that include 3-D air velocity measurements, tracer gas tests and controlled generation of air turbulence through physical movements of a vertical, human-sized plate. The tested exhaust hood (EH) was of circular, flat plate flanged type.

One part of the study concerned the task of determining the 0.4 m/s distance, x0.4, at the EH; i.e. the distance from the EH opening to a point where the air velocity has declined to 0.4 m/s. This is a currently used measure of “safe zone” at an EH. It was found that practicable measurements of good accuracy seem to be attained by using the following fairly simple correction equation:

x0.4=xm*rot(Vm/0.4)

where Vm is a provisionally measured air velocity, preferably within the zone where Vis within 0.35-0.45 m/s in front of the EH, and xm is the measured distance from the EH opening to the measuring point of Vm.

The tracer gas tests implied injection of a neutrally buoyant tracer gas through a perforated sphere placed in front of the EH. The amount of tracer gas that escaped from the suction flow was measured in the room air, thus yielding a sensitive method for measuring the capture efficiency (CE) of the EH. The CE is the percentage of injected tracer gas that is directly captured by the EH. Measurements of CE was performed at several test cases, were exhaust flow rate, gas release distance, turbulence level and EH arrangement were varied. The recorded CE values varied between 75 to 100% and the response to the different test cases appeared trustworthy.

The use of a 3-D sonic anemometer, that yielded both magnitude and direction of the air movement, proved very useful in analyzing the generated air turbulence. Its measurement data was also used to construct another measure of the local exhaust performance: Percentage Negative Velocities, PNV. This measure represents the percentage of the time when the air flow at the measuring point in front of the EH is directed away from the EH nozzle, i.e. when the velocity component in the direction towards the EH opening is negative. The recorded PNV values correlated well with the corresponding CE values, attained at the tracer gas tests. Thus, measuring PNV might be a convenient alternative or complement to tracer gas measurements.

Place, publisher, year, edition, pages
2014. , 37 p.
Keyword [en]
Local exhaust ventilation, Contaminant exposure, Tracer gas, 3-D anemometry, Sonic anemometer, Human movements, Turbulence generation
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:hig:diva-19446OAI: oai:DiVA.org:hig-19446DiVA: diva2:816684
Note

Contract report, produced on request by Swedish Standards Institute and local exhaust ventilation manufacturers. 

Available from: 2015-06-03 Created: 2015-06-03 Last updated: 2015-10-23Bibliographically approved

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