A NUMERICAL AND EXPERIMENTAL-STUDY OF LOCAL EXHAUST CAPTURE EFFICIENCY

Direct capture efficiency of a local exhaust system is defined by intr oducing an imaginary control box surrounding the contaminant source an d the exhaust opening. The imaginary box makes it possible to distingu ish between contaminants directly captured and those that escape. Two methods for estimation of direct capture efficiency are given: (1) a n umerical method based on the time-averaged Navier-Stokes equations for turbulent flows, and (2) a field method based on a representative bac kground concentration. Direct capture efficiency is sensitive to the s ize of the control box, whereas its location is less important for the case studied. The choice of sampling strategy to obtain a representat ive background concentration is essential as substantial differences o n direct capture efficiency are Found. Recommendations are given.