In the field of convective heat transfer, there is in general a tradeoff between heat transfer and pumping power. Power to operate a pump or fan to move a fluid involved in heat transfer is often an expense associated with achieving heat transfer. This is especially of concern in heat exchangers in which the fluid on at least one side is gas such as atmospheric air. Also this is especially of concern when, as is usually the case, there are limitations on the overall space which can be occupied by the heat exchanger. Designs, tradeoffs and calculational methods for heat exchangers are given in “Compact Heat Exchangers” by Kays and London. There is a continuing need for improvement in regard to the tradeoff between heat transfer and pumping power. Such improvement would increase the efficiency of any of the various devices employing forced convection heat transfer or even natural convection heat transfer.
Issued U.S. Pat. No. 6,669,815 discloses a geometry of fins designed to provide an improved ratio of heat transfer to pressure drop or pumping power, by using fin-to-fin spacings which are different in different regions of a fin array. The fin geometry of that patent is shown in FIG. 1. The geometry illustrated in U.S. Pat. No. 6,668,915 accomplishes that intended goal, but in that geometry the flow may be subject to certain geometry-related flow losses at the changes of cross-sectional area. In U.S. Pat. No. 6,668,915, when the flow at transition region 175 of the second channel expands from a smaller flow cross-sectional area in region 170 to a larger flow cross-sectional area in region 180, the flow on the right side of the narrow region 170 of the channel essentially may not have to shift at all, while the flow on the left side of the narrow region 170 of the channel may have to shift considerably more. Such flow shifting and associated possible separation of flow from its adjacent solid boundary are possible sources of loss of pressure or head, and so it is desirable for the flow to have to shift as little as possible. In order to avoid such separation of flow from solid boundaries, it has typically been necessary to maintain the divergence angle of the flow at a sufficiently small value, which in turn has required a considerable length of transition region in order to achieve a desired expansion of cross-sectional area.
Accordingly, it is desirable to provide designs of the type disclosed in U.S. Pat. No. 6,668,915 but having improved flow patterns in the transitions between regions, such as to provide for smoother flow and hence smaller pressure losses associated with the expansion or contraction. It also is desirable for the transition region to occupy as little of the overall flow length of the heat exchanger as possible.