Patent Application
20170298865
This invention is directed to improvements in Stirling engines and can be used in other Stirling machines, such as Stirling coolers and may have utility as a heat exchanger in other specialty machines. More particularly the invention is directed to improvements in heat exchangers used in these machines.
Working gas flows in alternating directions between the expansion space 18 and the compression space 26 through the series-connected compression space heat exchanger 24, the regenerator 30 and the expansion space heat exchanger 22. The purpose of Stirling machine heat exchangers is to transfer heat to or from the working gas. For a typical Stirling engine, a heat source, such as a gas flame, is applied to the head 20 for supplying the heat energy that drives the engine. The purpose of the expansion space heat exchanger 22 is to transfer heat from that heat source into the working gas within the engine as the working gas flows in alternating directions through the expansion space heat exchanger 22.
In order to have a high heat transfer rate from the heat exchanger to the working gas flowing through the heat exchanger, it is desirable to have a large number of gas passages that are small in cross section in a plane that is perpendicular to the gas flow direction through the passages. That configuration provides a larger total surface area in contact with the gas for facilitating heat transfer. However, heat exchangers of the prior art that have sufficiently small passages are very costly if those passages are machined by conventional machining tools and techniques because of the difficulty of machining so many passages that are as small as needed. Folded fin heat exchangers have also been used but the passages cannot be made sufficiently small. Sometimes folded fin heat exchangers have been partially crushed to reduce the passage size. But this crushing makes the passages non-uniform in their cross sectional area and therefore non-uniform in flow resistance and heat transfer rate.
One purpose of the present invention is to provide an improved heat exchanger that has thin, narrow gas passages through the heat conducting metal of the heat exchanger without having to machine thin narrow passages. Embodiments of the invention have sufficiently small gas passages but do not require the machining of passages that are as small as the gas passages in the completed heat exchanger.
Another purpose of the invention is to provide a heat exchanger that has a simple structure, is relatively easy to machine and to assemble and therefore has a lower manufacturing cost.
Yet another purpose of the invention is to provide a heat exchanger with very few parts and very few part connections and joints which results in a lower cost heat exchanger that has improved reliability and durability.
Unlike some prior art heat exchangers that require a separately manufactured manifold for interconnecting the heat exchanger to the regenerator, the heat exchanger of the invention can be cast or machined with annular shoulders or other interfacing edges at an end that can fit directly against the regenerator and consequently eliminate the need for a separate manifold.
The invention is a free piston Stirling engine and particularly the heat exchanger at its heat accepting end. The heat exchanger has an inner component part that is assembled within an outer component part. The outer component part has a tubular outer wall and circumferentially spaced ridges that extend inward from the tubular outer wall and also extend longitudinally along the tubular outer wall. The inward extending ridges are separated from each other by inward opening slots. The inner component part has a tubular inner wall and circumferentially spaced ridges that extend outward from the inner tubular wall and also extend longitudinally along the tubular inner wall. The outward extending ridges are separated from each other by outward opening slots. The ridge widths of the outer component part are less than the slot widths on the inner component part and the ridge widths of the inner component part are less than the slot widths of the outer component part so that the ridges can fit into the slots. The two component parts are assembled with the ridges of each component part extending into the slots of the opposite component part to form gas passages between interfacing sidewall surfaces of the ridges.
In describing the preferred embodiment of the invention which is illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, it is not intended that the invention be limited to the specific term so selected and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose.
In describing the preferred embodiment of the invention, reference will be made to various terms that are used for describing their characteristics. The terms “inward” and “outward” are used and denote a direction generally along radials toward or away from a central axis of a pair of concentric tubular walls. The term “longitudinal” is used principally to refer to a direction that is parallel to the central axis which is also the gas flow direction through the preferred embodiment of the invention. The term “gas passage width” is used to refer to the distance in a circumferential direction between the interfacing sidewall surfaces of the ridges that bound the gas passages and are subsequently described. The term “ridge height” refers to the distance in a radial direction from the base of a ridge to the crest of a ridge. The terms “inward opening slot” and “outward opening slot” mean that the open end of the slot faces inward or outward respectively.
The principal and most advantageous application of the invention is for a heat exchanger that is positioned at the heat accepting end of a Stirling engine where the heat flux is greatest. However, it can also be used at the heat rejecting end. It can also be used at the heat accepting and/or the heat rejecting end of a Stirling cooler, cryocooler or heat pump and its most advantageous application to a heat pumping Stirling machine is at the heat rejecting end of a cryocooler where the heat flow is greater.
The most common positioning of a heat exchanger 22 embodying the present invention is illustrated in
The outer component part 34 has a tubular outer wall 36 which preferably is circularly cylindrical. A series of circumferentially spaced ridges 38 extend inward from the outer wall 36 and form a unitary body with the tubular outer wall 36. The ridges 38 also extend longitudinally along the tubular outer wall 36. The ridges 38 of the outer component part 34 are separated from each other by inward opening slots 40. The sidewall surfaces of the ridges 38 are the same as the sidewall surfaces of the slots 40 because the sidewall surfaces of the ridges define the sidewalls of the slots. Preferably, all those sidewall surfaces are planar surfaces.
The inner component part 32 has a tubular inner wall 42 which preferably is also circularly cylindrical. A series of circumferentially spaced ridges 44 extend outward from the inner tubular wall 42. The ridges 44 also extend longitudinally along the tubular inner wall 42. The outward extending ridges 44 of the inner component part 32 are separated from each other by outward opening slots 46. Preferably, the centerlines of the ridges on both component parts and the centerlines of the slots on both component parts are along radials from the central axis. But, as subsequently will be seen, preferably the sidewall surfaces of the ridges and the slots do not fall precisely along radials, although they could be constructed in that manner.
As illustrated in
Importantly, the ridge widths of the outer component part are less than the slot widths on the inner component part and the ridge widths of the inner component part are less than the slot widths of the outer component part so that the ridges can fit into the slots. However, it is not necessary that the two component parts have either the same ridge width or the same slot width, as long as the ridges of each component part can fit into the slots of the other component part. The component parts are assembled so that the ridges of each component part extend into the slots of the opposite component part. Because the slots are wider than the ridges, gas passages 47 are formed between the interfacing sidewall surfaces of the ridges. If, as desired, the ridges are centered in the slots, the gas passages 47 on the two opposite sides of each ridge have the same width. Consequently, the width of the gas passages 47 are one half of the difference between the width of a slot and the width of the ridge in the slot. The preferred width of the gas flow passages is a function of the particular machine design, which can vary over a range of power outputs. For the most common applications, the preferred gas passage width is in the range of 0.25 mm to 1.5 mm.
The above-described dimensional relationship between the respective widths of the ridges and the slots has important consequences. The ridges are formed by machining the slots into the inner and outer component parts. The width of the slots is determined by the width to which the slot is machined. The width of the ridges is determined by the spacing of the slots between the ridges. Because the width of the gas passages 47 is one half of the difference between the slot width and the ridge width, the slots and the ridges can be machined much wider than the desired width of the gas passages. The ridges and slots are made to have a width difference that is much less than the width of the slots that are machined into the inner and outer component parts. The ultimate result is that the machining operations are much less expensive with the invention because it is much less expensive to machine wide slots than it is to machine thin, narrow slots or other passages. For example, the wide slots can be machined by broaching, a machining process in which a multiple tooth cutting tool is moved linearly relative to the work in the direction of the tool axis.
Another advantage of the invention is that the relatively wide ridges that separate the gas passages 47 and form the walls of the gas passages 47 extend radially so that heat conducting metal extends along a wide conductive path continuously and directly from the outer surface of the heat exchanger to side walls of the gas passages 47. That configuration maximizes heat conduction from the outer surface of the heat exchanger, where heat is input to the Stirling engine, to the walls of the gas passages 47 where heat is transferred to the gas in the passages 47.
Ridge Gap.
Embodiments of the invention could be manufactured with the height of all the ridges equal to the height of all the slots so that the crests of all the ridges are in contact with the bottoms of all the slots when the two component parts are assembled. However, that would require machining the component parts with more precision and closer tolerances which would needlessly increase the cost of manufacture. It is also possible to custom machine each slot and its received ridge to a height that differs from the heights of other slots and ridges. However, that would make embodiments of the invention even more expensive to machine and assemble.
Referring to
The gap 48 provides a high thermal resistance to heat flow from the outer component part 34 to the inner component part 32. However, the crests of the outward extending ridges 44 are in direct contact with the tubular outer wall 36 of the outer component part 34 to provide a low thermal resistance to heat flow.
The general purpose of the heat exchanger is to conduct heat from the flame or other driving heat source to the walls of the gas passages 47 so that heat will be transferred to the gas in the gas passages 47 of the heat exchanger. The reason for the ridge gap 48 is to reduce machining cost by not requiring the close tolerance machining that would be required if all the ridge crests of both component parts were to contact the bottom of all their respective slots .
An additional advantage of the gap 48 is that it adds additional gas passage cross sectional area for gas flow and additional surface area for transferring heat to the working gas. Preferably, the inner ridge gaps 48 should be equal to or less than the width of the gas passages 47. If the gaps 48 are larger in width than gas passages 47, they would have less resistance to gas flow, more gas would flow in gaps 48 and less would flow in the preferred gas passages 47. If gaps 48 and gas passages 47 are equal in width, that is useful because gaps 48 provide additional effective heat exchange passages. For that reason, it is undesirable to have large gaps 48 because that would allow more gas to flow in this less effective heat exchange region.
The desirable heat flow in the metal of the heat exchanger is to transfer heat, which is received by the outer component part 34 from the engine's heat source, to the gas in the gas passages 47. To do that, heat is conducted from the tubular outer wall 36 of the outer component part 34 to the walls of the ridges 38 and 44 because those walls are in contact with the working gas in gas passages 47 of the heat exchanger. Consequently, it is desirable to have highly thermally conductive contact between the crests of the outward extending ridges 44 of the inner component part 32 and the tubular outer wall 36 of the outer component part 34.
Uniform Gas Passage Width.
From the drawings and the above description it is apparent that the sidewall surfaces of the ridges and slots could lie along radials from the axis of the heat exchanger component parts 32 and 34. Although that configuration is acceptable for some applications, the gas passages would be slightly wedge shaped with the gas passages becoming progressively wider as they extend further in the outward direction.
However, as illustrated in
One way of accomplishing this is to machine the slots of one component part with parallel sidewall surfaces and machine the ridges of the other component part with parallel sidewall surfaces. As seen in
In
Rotational Alignment.
Preferably, all of the ridges and slots on the outer component part 34 are the same size and shape and are uniformly distributed around its tubular outer wall 36. Similarly and preferably, all of the ridges and slots on the inner component part 32 are the same size and shape and are uniformly distributed around its tubular inner wall 42. It is desirable to have the ridges centered in the slots when the inner component part 32 is assembled into the outer component part 34. The reason for centering is to make the gas passages 47, that are on opposite sides of a ridge, have the same width. Therefore, the two component parts 32 and 34 should be rotationally aligned when they are assembled.
Rotational alignment of the inner component part 32 relative to the outer component part 34 in a manner that centers the ridges in the slots can be conveniently accomplished by forming a surface contour on at least one crest of the outward extending ridges 44, and preferably on all of them, and a mating surface contour on the bottom of at least one slot in the outer component part 34, and preferably into all of them. These mating surface contours are centered on the crest and on the bottom of the slot for centering the ridges in the slots. They are similar to the formation of a key and keyway on the inter-fitting component parts. An example is illustrated in
Tapered Outer Wall.
Referring to
Although the taper that is described above is not necessary for use with embodiments of the invention, it has multiple desirable consequences when applied to embodiments of the invention.
An advantageous consequence of the above described taper is that the taper makes it easier to assemble the two component parts together. As a result of the taper, the bottom surfaces of the inward opening slots 40 of the outer component part 34 lie along a cone. Similarly, the crests of the outward extending ridges 44 of the inner component part 32 also lie along a cone. Those cones are preferably identical. When the two component parts 32 and 34 are aligned coaxially and moved together along the common axis, the two component parts 32 and 34 do not contact each other or slide in frictional contact against each other until the conically arranged crests of the outward extending ridges 44 seat against the conically arranged bottoms of the inward opening slots 40 of the outer component part 34. Upon that seating, the two component parts can slide no further with respect to each other. At that point of contact, the two component parts are brazed or otherwise bonded together.
Of course the crests of the outward extending 44 ridges and the bottoms of the inward opening slots 40 can be machined to lie along a circular cylinder instead of a cone. It would then be necessary to slide the two parts together in frictional contact when the inner part is inserted into the outer part. The relative axial positioning of the inner part and outer part would be indefinite and require insertion to a measured distance followed by brazing or other bonding together. Alternatively, the two component parts can be attached together by heating the outer part to expand it, sliding it over the inner part and then allowing the parts to cool so they are connected together by thermal shrinking.
Another advantage of the above-described taper is obtained by placing the thicker part of the tubular outer wall 36 at the expansion space end 64 of the heat exchanger where the heat flux is the greatest in a Stirling engine. The thicker part provides a greater circumferential cross sectional area for heat conduction through the metal of the heat exchanger in this region of high heat flux in turn promoting more uniform circumferential head temperature. The reason for the high heat flux near the expansion space is that the working gas has expanded and cooled significantly in the expansion space before gas flows from the expansion space into the heat exchanger. So there is a large temperature difference between the heat exchanger at this region and the gas that has been cooled in the expansion space.
Another advantage of the taper is that, because of the taper, the gas passages have a smaller dimension in the radial direction at the expansion space end 64 of the heat exchanger than at the regenerator end 62. This allows the manifold that connects the end of the heat exchanger to the expansion space to be made smaller. As a consequence, the effective volume of the expansion space is less so more power output is produced.
The effective volume of the expansion space can also be reduced by forming projections 70 on the ends of the ridges at the expansion space end 64 of the heat exchanger. In the drawings, the projections 70 are triangular.
Regenerator End Finishing.
Typically it is undesirable to have a regenerator directly engaging right up against the heat exchanger gas passages because gas flow would be restricted. In the prior art a separate manifold is sometimes interposed between the heat exchanger and the regenerator. The manifold is a spacer that provides an open space, for example 1 mm, between the regenerator and the gas passages of the heat exchanger. Referring to
Additionally, the ends of the ridges of both the inner and the outer component parts are optionally chamfered at the regenerator end 62 for providing a smoother transition of the gas flow path between the regenerator 30 and the heat exchanger 22. The chamfered surfaces 68 are surfaces that are inclined to the gas flow direction to provide a less abrupt, smoother transitions between the regenerator and the heat exchanger. The gas passing between the regenerator and the heat exchanger is directed more smoothly between them and can more smoothly change its velocity. This helps avoid creating additional turbulence which is not desirable because it causes more pressure drop through the heat exchanger and non-uniform flow in the regenerator.
As stated above, the principal and most advantageous application of the invention is for a heat exchanger that is positioned at the heat accepting end of a Stirling engine where the heat flux is greatest. However, it can also be used at the heat rejecting end and can be used at the heat accepting and/or the heat rejecting end of a Stirling cooler, cryocooler or heat pump. However, if it is used at the heat rejecting end of the regenerator, it should not have an axial taper or, if an axial taper is used, the taper should be small.
This detailed description in connection with the drawings is intended principally as a description of the presently preferred embodiments of the invention, and is not intended to represent the only form in which the present invention may be constructed or utilized. The description sets forth the designs, functions, means, and methods of implementing the invention in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions and features may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention and that various modifications may be adopted without departing from the invention or scope of the following claims.
