Numerical simulation of unflanged rectangular exhaust openings

Local exhaust hoods are commonly used for controlling airborne contaminants in industry. The transportation of the contaminants is mainly affected by air movements, and therefore it is important to know the flow fields near exhaust openings accurately.
Recent developments in computer technology and in computational fluid dynamics programs have made it possible to simulate turbulent air flows into exhaust openings. However, the accuracy of the simulations is usually unknown. In this study isothermal three-dimensional airflow fields generated by rectangular exhaust openings with aspect ratios 1:1, 4:3, 2:1, and 3:1 were calculated numerically with the FLUENT computer code based on the finite volume method using the standard k-ϵ turbulence model.
The effect of free-stream boundaries on the simulations was studied by solving the airflow fields with different sizes of calculation domains. The accuracy of the predictions was determined by comparing the predicted results with the measured equal velocity contours and centerline velocities.
The agreement between the numerical simulations and the experiments was good for openings with aspect ratios 1:1 and 4:3 and satisfactory for the other when fixed pressure boundary conditions on the free-stream boundaries were used. However, the location of the free-stream boundaries must be properly chosen.
With a mean hood face velocity of 10 m/sec (1970 ft/min), an appropriate distance between the exhaust opening and the free-stream boundary was found to be 4A1/2, where A is the area of the exhaust opening.