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Control of wake-induced exposure using an interrupted oscillating jet.

Bennett-J; Crouch-K; Shulman-S
American Industrial Hygiene Conference and Exposition, May 20-25, 2000, Orlando, Florida. Fairfax, VA: American Industrial Hygiene Association, 2000 May; :47-48
Ventilation design often involves ideas drawn from potential flow theory. However, in the downstream region of a worker's body, the flow is turbulent, making the concept that a contaminant will flow from high to low pressure problematic. With an air contaminant source in the wake region, reasonable though often ineffective exposure control approaches are to reduce the velocity to deprive the wake of energy needed for eddy formation (i.e., maintain laminar flow) or to increase the flow rate to provide sufficient dilution. In either case, one problem is solved while another is created. From this coupling arose the motivation to simultaneously achieve contaminant removal and eddy reduction in the wake region by introducing a directionally oscillating jet. A 50th percentile male mannequin was placed in a nearly uniform flow of approximately 35 fpm. While the cross-sectional velocity profile varied somewhat, smoke release studies indicated no back flow except near the supply wall. A low-velocity tracer gas source (isobutylene) was held in the standing mannequin's hands with the upper arms vertical and the elbows at 90 degrees. Four ventilation scenarios were compared in terms of time-weighted average (TWA) concentration in the breathing zone as measured by photoionization detectors: 1) uniform flow; 2) addition of a steady jet (approximately 1000 fpm) directed at the mannequin's back, parallel to the main flow; 3) making the jet oscillate to 45 degrees on either side of the centerline with a period of about 12 seconds; and 4) introducing a blockage at the centerline so the oscillating jet never blows directly at the worker. At the 95% confidence level, the interrupted oscillating jet (Case D) achieved at least 90% exposure reduction compared with the uniform flow by itself or with the steady jet (Case A or B), and at least 35% exposure reduction compared with the unblocked oscillating jet (Case C).
Ventilation; Ventilation-systems; Air-contamination; Air-flow; Air-pressure; Control-methods; Control-technology; Equipment-design; Testing-equipment; Air-monitoring; Exposure-methods; Time-weighted-average-exposure; Breathing-zone; Particle-aerodynamics; High-pressure
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American Industrial Hygiene Conference and Exposition, May 20-25, 2000, Orlando, Florida