Tabbed transfer fins for air-cooled heat exchanger

Abstract

Heat transfer fins (110), and heat exchangers (100) incorporating such fins (110), enhancing heat transfer with acceptable pressure drop increase. The fins (110) include tabs or secondary fins bent upward and downward from the fin body at a selected bend angle. All or a majority of the tabs are aligned with a simple flow path or with local flow paths for cooling air adjacent the fins (110) to minimize turbulence and pressure drop. The tabs are planar and generally aligned parallel to the simple flow path or local flow paths and are arranged so as to serve as a plurality of sites for starting new boundary layers by offsetting the tabs such that downstream tabs are not shadowed by upstream tabs. The tabs have a height sufficiently large to extend the tabs out into boundary layers on the fin (110). The tabs provide more uniform flow over fins (110) and shrink wake size behind tubes (120).

Description

CROSS REFERENCE TO RELATED APPLICATIONS [0001] This application claims the benefit of U.S. Provisional Application No. 60 / 486,071, filed Jul. 10, 2003, which is incorporated by reference herein in its entirety.CONTRACTURAL ORIGIN OF THE INVENTION [0002] The United States Government has rights in this invention under Contract No. DE-AC36-99GO-10337 between the United States Department of Energy and the National Renewable Energy Laboratory, a Division of the Midwest Research institute.BACKGROUND OF THE INVENTION [0003] 1. Field of the Invention [0004] The present invention relates generally to heat exchangers that utilize fins or plates on or in contact with tubes, pipes, or plates to transfer heat away from the working fluid in the tubes, pipes, or plates, and more particularly, to heat transfer fins, and heat exchangers or condensers that include such fins, that include a plurality of tabs extending from the fins to provide enhanced heat transfer on the air side of the heat exchange...

AI Technical Summary

Benefits of technology

[0015] The present invention addresses the above problems by providing an improved design for heat transfer fins that enhances the heat transfer rate on the gas or air side of heat exchangers with relatively low increase in pressure drop. Briefly, the fins include numerous tabs or secondary fins that are bent upward and downward from the body of the fin at a selected bend angle (such as between about 70 and 110 degrees and more typically, about 90 degrees). In this manner, the material of the fin body is retained for use in heat transfer with the air or gas flowing over the fins. Preferably, all or a majority of the tabs are aligned with the flow path(s) of the cooling gas to minimize the creation of turbulence and pressure drop (i.e., by minimizing creation of flow drag by only “showing” the tab's leading edge to the flowing gas).

[0016] For example, the tabs may be substantially planar and aligned with their surfaces parallel to the main flow path or simple flow path or line (or in some cases, the local flow paths) of the cooling gas relative to the fin. In a first embodiment, the tabs are positioned with their planar surfaces perpendicular to a leading edge of the fin to align the tabs substantially parallel with the main flow path of gas across the fin. In a second embodiment, some or all of the tabs are positioned to be more aligned with local flow paths or with streamlines to guide air flowing in the channel between fins to reduce the size of wakes behind tubes and to reduce pressure drop relative to the first embodiment by producing less turbulent flow. In the second embodiment, the tabs may be positioned substantially parallel to or angled less than about 5 degrees relative to the streamlines.

[0017] This is achieved by positioning the tabs at various, differing offset angles, e.g., 0 degrees (or substantially parallel to the simple flow path), 10 degrees as measured from either side of the simple flow path, and the like. The offset angles are typically less than about 20 degrees and more preferably less than about 10 degrees as measured from either side of the simple flow path with the offset angle, at least in some embodiments, being selected to be substantially parallel (such as within 5 to 10 degrees or less to being parallel) to the local stream line or flow path. In this manner, heat transfer is significantly enhanced by reducing the thickness of the thermal boundary layer on each tab and by placing heat transfer surface area in contact with cooler portions of the flowing gas (for a cooling application), e.g., the surface area of the tabs extends outward into cooler portions of the flow channel between adjacent fins.

[0019] According to one aspect of the invention, a method is provided for fabricating heat transfer fins for heat exchangers. The method comprises providing a plain fin, such as an aluminum fin typically utilized in finned-tube heat exchangers. A tab pattern is selected or provided for the particular fin to define the quantity, size, and location of heat transfer tabs on the fin. The tab pattern selection may comprise performing a variety of flow and heat transfer tests on the fin implementing a number of potential tab patterns to obtain a useful pattern to enhance heat transfer while not unacceptably increasing pressure drop. With a tab pattern selected, a punch mechanism or tool can be fabricated or provided based on the pattern. The punch mechanism can be adapted for punching the tabs in one operation with tabs extending from one or both sides of the fin body. The method continues with forming, such as with the punch mechanism, the heat transfer tabs defined by the tab pattern by creating openings or holes in the fin by removing material from the fin body while retaining a connecting edge between the fin body and the removed material or tab body. The forming comprises bending the removed fin body material along the connector edge to a bend angle, such as 90 degrees, relative to one of the two sides of the fin body. The tab pattern is configured such that all or a majority of the tab bodies are aligned parallel (or within about 10 to 20 degrees) to a simple flow path (i.e., a directional line drawn perpendicular to the leading edge of the fin body) or are aligned parallel (or within about 5 to 10 degrees) of local flow paths. In one embodiment, the tab pattern is configured such that the surface area of the removed material or tab bodies is such that the tabbed fin has porosity of less than about 50 percent and more typically between about 15 and 30 percent. In some embodiments, about half of the tabs extend from one side of the fin body while the remaining tabs extend from the second side of the fin body. Preferably, the tabs on each side of the fin body are arranged in the tab pattern such that adjacent upstream and downstream tabs (or proximal and distal tabs relative to a fin body leading edge) are offset to avoid shadowing of downstream tabs. The tabs are also arranged in such a way that they do not adversely interfere with the tabs on adjacent fins. Further, their pattern encourages uniform flow over the main fin and maximized heat conduction within the fin.

Problems solved by technology

Third, the openings in the main fin surface disrupt the boundary layer on that surface thus enhancing heat transfer.

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