The present invention relates to an elbow of the expansion, normal, or reduction type which is provided with guide vanes, and is used in a joint or an outlet of a bent duct.
A bent duct with a relatively small radius ratio (the ratio of the radius of curvature of the centerline of the bent duct to the breadth of the bent duct) is called an elbow, while a bent duct with a relatively large radius ratio is called a bend. An elbow usually has a right angled configuration, while a bend usually has a concentric circular arc configuration. The loss of flow (hereinafter, called "flow loss") in a bent duct is the sum of the separation loss due to the separation of the flow near the inner side wall of the bent duct and the friction loss due to the surrounding walls of the bent duct. The separation loss decreases as the radius ratio increases. Thus, it is effective for reduction of the separation loss to provide the bent duct with guide vanes for dividing the bent duct into a plurality of sub-channels whose breadths are small and radius ratios are large.
The bend dividing method, wherein the bend is divided into a plurality of sub-channels as described above, can be easily carried out by inserting one or two guide vanes into the bend along the whole length whereof. However, since the increase of the duct wall area owing to the addition of the guide vanes increases the friction loss, this dividing method can be applied only to bends with relatively small radius ratios.
Elbows, which are simply configured and small in size, are widely used in ducts. The radius ratio of an elbow is small and the separation loss of the elbow is rather large, so that the flow loss of the elbow is rather large. The flow loss of an elbow can be reduced by adopting circular arc shaped inner and outer side walls. If the radius ratio of the inner side wall of an elbow is increased enough to reduce the flow loss to a practical level, however, the shape and the size of the elbow approach those of a bend, so that compactness, which is the advantage of an elbow, is lost.
Flow straightening technologies for use in an elbow are described in the technical data booklet "Flow Resistance of Pipes and Ducts" (The Society of Mechanical Engineering of Japan, 1991), especially on page 84. As flow straightening methods which have been developed, this booklet describes one in which many quarter circular arc shaped guide vanes of rather short length and equal size, are appropriately disposed along the flow lines so as to prevent the generation of vortex flow owing to flow separation (1934, Frey, K.). This method did not come into common use, however, because of the difficulty of determining the flow lines and difficulties in production. Another flow straightening method described involves disposing 15-20 quarter circular arc shaped guide vanes of rather short length and equal size at equal radial intervals or at radial intervals which decrease toward the inner side wall (1946, Salter, C.). However, the method is now used only for certain kinds of bent ducts in wind tunnels, etc., and is not commonly used because of the structural complexity.
The development of the elbows mentioned above has not been followed by the development of a new type of elbow provided with guide vanes which can be applied to all types of elbows: expansion, normal, and reduction.