Patent ID: 7644733
Filing Date: 2010-01-12
Classification: F15D,F16L

Abstract:
1. A duct comprising at least one length portion of circular cross-section, wherein said duct further comprises a lobed zone in which its cross-section exhibits at least one lobe, wherein each lobe extends helically around and along the duct, and wherein a transition from a circular duct section having a circular cross-section to the lobed zone having a lobed cross-section is smooth and continuous and takes place over a transition zone occupying a duct length in which each lobe becomes relocated by at least one position, and wherein within a said transition zone, cross-sectional shapes of the lobes at given points along the length of the transition zone are defined by circular arcs drawn on chords defining a series of notional polygons which are at least geometrically similar, and wherein the radius of curvature of said circular arcs progressively decrease from the circular duct radius at a circular end of the transition zone to half a chord length at a lobed end of the transition zone.