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Local exhaust ventilation is used to capture contaminants and maintain a comfortable atmosphere in premises. Exhaust hoods with extension units (canopy hoods) are commonly used in open-type local exhaust devices. The goal of this study was to use computational and experimental methods to determine the effect of adding an extension to a local exhaust hood on the separated-flow streamlines, vortex zones, exhaust velocity distribution, and local drag coefficient. We used the discrete vortex method together with computational fluid dynamics to investigate air flows near round and slotted extended exhaust hoods. We then compared the computed airflow velocity field, vortex zone outlines at the inlet of a local exhaust hood, and local drag coefficient factor with data from a field experiment. Outlines of vortex zones resulting from flow separation at the inlet of the hood were considered for a hood length of 5 times gauge (the radius of a round hood or half-width of a slotted exhaust hood), hood tilt angles of 90°, 75°, 60°, 45°, and 30°, and extension lengths of 0; 0.5; 1; and 2 times gauge. We found that longer extensions caused the first vortex zone to enlarge, and the contaminant capture velocity of the local exhaust hood to decrease. We identified the behavior of the local drag coefficient at various hood tilt angles, extension lengths (as listed above), and hood lengths (including 2, 3, and 5 times gauge), and translated them into equations for computing the local drag coefficient. • Algorithm for computing flow separation in extended exhaust hood has been developed. • Vortex zone outlines at inlet of a round extended exhaust hood have been determined. • Calculations of velocity to a hood with varying extension lengths have been performed. • Local drag coefficient has been determined numerically for extended exhaust hoods. • Expressions have been obtained enabling LDC computation for extended exhaust hoods. [ABSTRACT FROM AUTHOR]

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. Highlights • The algorithm for flow separation zones based on the vortex method has been improved. • The first vortex zone have been found to stay geometrically similar. • The second vortex zone and the relation of its dimensions have been determined. • Exhaust hood shaping has been assessed for its efficacy. • Local drag coefficient to decrease with shaped exhaust hoods. [ABSTRACT FROM AUTHOR]

In this paper our objectives are to perform a numerical and experimental study of the velocity field in the area affected by a circular exhaust duct, to define the outline of flow separation area at its inlet, to come up with a reliable mathematical simulation of the separated flow at exhaust duct inlet, and to determine the effect of the duct inclination angle its serviced area. The stationary discrete vortex method has enabled development of an adequate and reliable technique for computation of separated flows at inlet of a local exhaust hood. We have studied airflow patterns around the exhaust duct using the discrete vortex method, CFD and natural experiment. Characteristic dimensions of the flow separation area at exhaust hood inlet have been determined with relation to the length and inclination angle of the hood. Shaping the exhaust duct to fit the resulting separation zone outline will enable savings of energy that would otherwise be wasted on overcoming local drag forces; furthermore, this will prevent escape of contaminants circulating in the separation area to the environment. Determining airflow velocity distribution patterns will facilitate the choice of the most efficient exhaust duct design to minimize the cost of contaminant removal. [ABSTRACT FROM AUTHOR]

Local exhaust ventilation systems are the most effective – yet power-consuming – method of capturing contaminants. Shaping of the ventilation ductwork elements is an approach that is actively pursued now as a means of reducing energy losses. Past research concerned with determining vortex zone outlines at inlet of circular exhaust hoods and shaping the hood along these outlines has shown shaping to be highly efficient as a means of reducing pressure losses at the inlet of the exhaust hood. The goal of this study is to determine the vortex zone outline in a setting where a flanged hood is placed over an impermeable plane, and to determine the distance at which shaping will retain its effectiveness. Using the discrete vortices method, we examined variations in characteristic vortex zones of exhaust hoods with tilt angles α = 0°, 30°, 60°, 90° and lengths of 0.5, 1.5, 2.5, 5.0 times gauge (gauge defined as the air duct radius). We determined distances away from the plane at which the vortex zone dimensions cease to show any difference from the case of an exhaust hood in unlimited space. It follows from natural experiment performed by us that pressure losses at the inlet of a shaped exhaust hood remain the same when the hood is set at a distance up to one gauge away from the plane. The discovered vortex zone outlines can be used for shaping a circular exhaust hoods above an impermeable plane. This will reduce its local resistance factor, eliminate vortex zones. • Developed an algorithm for tracing vortex zone boundaries at the exhaust hood inlets. • Identified laws of vortex zone dimensions from the distance of impermeable plane. • Vortex zone stabilization distances have been determined. • We determine the local drag coefficient for a shaped exhaust hood placed over a plane. • Identified the specific distance of plane where the shaping of boundaries changing. [ABSTRACT FROM AUTHOR]

In this study, we experimentally determined the local drag coefficient (LDC) of a round exhaust hood that was improved with different shaped inlet sections along the boundaries of the vortex zones occurring in the flow at the inlet of an exhaust hood. The LDC measurements were performed using two methods. The first method used a micromanometer and pneumometric tube to determine the LDC with and without considering frictional losses. Meanwhile, the second method determined the LDCs using the pressure and velocity distributions in the boundary layer with specially designed pressure miniprobes. The experimental and numerical results show that it is possible to achieve a drag reduction of more than 90% by shaping the inlet edges of the exhaust hood. This increases the range of contaminant capture, reduces noise, and prevents contaminants from escaping from the exhaust hood by eliminating the vortex zones and reducing the fan power. • Two designs of shaped exhaust hoods have been set up. • Local drag coefficients have been determined experimentally for exhaust hoods. • Numerical computations are compared with experimental measurements. • We show that shaping increases the capture range of an exhaust hood. • Shaped exhaust hoods reduce fan power consumption. [ABSTRACT FROM AUTHOR]