Capture Efficiency of Flanged Slot Hoods under the Influence of a Uniform Cross Draft: Model Development and Validation

Abstract Capture velocity is the primary design parameter now used for slot hoods. However, capture velocity has an uncertain relationship with the true variable of interest, capture efficiency, the fraction of contaminant generated which is captured directly by the hood. Using the three-dimensional velocity field developed from potential flow theory, and including an arbitrary cross draft term, a model was developed that predicts the dividing velocity streamline in front of a slot hood. The location of this streamline, together with an empirical factor accounting for turbulent diffusion of contaminant around streamlines, are the parameters needed to determine capture efficiency. Wind tunnel experiments to validate the model consisted of measuring the fraction of a tracer gas captured by six different hoods under various face and cross draft velocity conditions. The measured capture efficiencies are compared directly with predictions, and errors less than 20 percent are observed for most values of capture...