On the resistance of a round exhaust hood, shaped by outlines of the vortex zones occurring at its inlet

Abstract

This paper works toward a goal of determining outlines of vortex zones occurring at the inlet of an exhaust hood and identifying the effect that a hood shape adjusted to these outlines may have on the local drag coefficient of the hood. A mathematical model and a computer program based on the discrete vortex method have been developed for computing velocity fields and boundaries of the first and second vortex zones occurring upon flow separation at inlet of a cone hood. First vortex zone outlines have been found to be geometrically similar even as exhaust hood flange length and angle vary. A computational experiment using the discrete vortex method and computational fluid dynamics techniques, supported by a laboratory experiment, has been carried out to determine the boundaries of the second vortex zone. It has been found that, even though flange length is not a factor for the second vortex zone, the characteristic dimensions of this zone depend on the hood flange tilt angle. Exhaust hood shaping has been assessed for its efficacy in reducing pressure losses at inlet of a circular exhaust hood. The effect of shaping the hood with the outlines of the first and/or second vortex zones has been found in terms of local drag coefficients at hood inlet expressed. Shaping by the outlines of the vortex zones of the most effective exhaust hood with a flange inclination angle of 90° makes it possible to reduce the local resistance coefficient by more than 10 times.