Computation of the initially unknown boundaries of flow fields generated by local exhaust hoods

Local exhaust hoods are important in controlling contaminants in the workplace. To predict hood effectiveness, it is important to have knowledge of the airflow field that it generates. Currently, there are theoretical models adequate for predicting the flow fields of hoods with flanged openings. These models are solutions of Laplace's equation in terms of the velocity potential. Comparison of experimental and theoretical values of air velocities show good agreement. With the exception of the plain slot, no such models are available for plain hoods or other hoods with complex geometries. This paper explores the feasibility of approximating the equal air velocity contours for any local exhaust hood by assuming that these contours are also equipotential contours. A slot configuration, for which an analytical model is available, was used to evaluate the accuracy of the assumption. Starting with a good approximation for the 15% velocity contour, three other boundaries were generated. The procedure used in generating boundaries after the initial one involved solution of Laplace's equation, assuming constant potential along the boundary and adjustment of boundary location on the basis of differences between the calculated value of the normal derivative of the velocity potential at a point on the boundary and the specified value (15%). The next-to-last boundary generated by the procedure exhibited an oscillation in the values of the normal derivative, which was detrimental to the desired solution. Possible causes for this oscillation and possible refinements in the procedure are discussed.