The present invention pertains to improvements to thermal components of a Stirling cycle heat engine and more particularly to heat transfer surfaces such as the heater head.
Stirling cycle machines, including engines and refrigerators, have a long technological heritage, described in detail in Walker, Stirling Engines, Oxford University Press (1980), incorporated herein by reference. The principle underlying the Stirling cycle engine is the mechanical realization of the Stirling thermodynamic cycle: isovolumetric heating of a gas within a cylinder, isothermal expansion of the gas (during which work is performed by driving a piston), isovolumetric cooling, and isothermal compression.
Additional background regarding aspects of Stirling cycle machines and improvements thereto are discussed in Hargreaves, The Phillips Stirling Engine (Elsevier, Amsterdam, 1991) and in co-pending U.S. patent applications Ser. No. 09/115,383, filed Jul. 14, 1998, and Ser. No. 09/115,381, filed Jul. 14, 1998, which reference and both of which applications are herein incorporated by reference.
The principle of operation of a Stirling engine is readily described with reference to
During the first phase of the engine cycle, the starting condition of which is depicted in
During the third phase (the expansion stroke) of the engine cycle, the volume of compression space 22 increases as heat is drawn in from outside engine 10, thereby converting heat to work. In practice, heat is provided to the fluid by means of a heater head 100 (shown in
Additionally, on passing from the region of hot interface 18 to the region of cold interface 20, the fluid may pass through a regenerator 134 (shown in
Stirling cycle engines have not generally been used in practical applications due to such practical considerations as efficiency, lifetime, and cost which are addressed by the instant invention.
In accordance with preferred embodiments of the present invention, there is provided a method for fabricating heat transfer protuberances, such as for the heater head or cooler of a thermal cycle engine, wherein the heat transfer protuberances conduct heat between an external fluid and a working gas through a cylindrical wall where the working gas is interior to the wall. The method includes casting of the cylindrical wall and the heat transfer protuberances in a single operation. The casting step may include investment casting, sand casting, or die casting. The method may also include steps of fabricating a plurality of negative molds, each mold being of a group of substantially parallel holes corresponding to the heat transfer protuberances in the fabricated part. The plurality of negative molds is assembled to form a negative form for casting the cylindrical wall and heat transfer protuberances.
In accordance with further embodiments of the invention, a method is provided for fabricating heat transfer pins for conducting heat from an external thermal source through a cylindrical wall where the method has the steps of integrally fabricating at least one backing panel and heat transfer pins having axes normal to the backing panel, and then bonding the at least one backing panel to a structure in thermal contact with the cylindrical wall. The step of integrally fabricating the at least one backing panel may include either casting or injection molding the backing panel. The step of bonding may include mechanically attaching the panel to the heater head, brazing the panel of the array of heat transfer pins to the heater head, or transient liquid-phase bonding of the panel of the array of heat transfer pins to the heater head. In accordance with yet further embodiments of the invention, a method is provided for enhancing efficiency of thermal transfer through a heater head to a working gas in a thermal cycle engine, the heater head having an interior surface. The method includes the step of applying a layer of high-thermal-conductivity metal to the at least one of the interior and exterior surfaces of the heater head.
An alternate embodiment of the invention provides an improvement to a heater head for a thermal cycle heat engine that has a substantially cylindrical wall section. The improvement has a plurality of ribs interior to the wall section for providing enhanced hoop strength. Other improvements to a heater head, in accordance with the invention, include a plurality of passages within the wall that extend parallel to a central longitudinal axis and a substantially helical channel within the cylindrical wall section. An additional improvement includes a plurality of ribs interior to the dome for providing enhanced dome strength. A plurality of flow diverters may also be provided, extending transversely from a hot sleeve disposed internally to, and concentrically with, the cylindrical wall section.
In accordance with a further aspect of the present invention, a heat exchanger is provided for transferring thermal energy from a heated external fluid across a cylindrical wall. The heat exchanger has a set of staggered heat transfer protuberances, each heat transfer protuberance having an axis directed substantially away from the cylindrical wall, and a plurality of dividers disposed substantially along the length of the cylindrical wall, for forcing fluid flow through the staggered heat transfer protuberances.
In accordance with yet a further aspects of the present invention, a heat exchanger is provided for transferring thermal energy from a heated external fluid across a cylindrical wall, where the heat exchanger has a set of heat transfer protuberances with axes directed substantially away from the cylindrical wall, and a backer for guiding the heated external fluid in a flow path characterized by a direction substantially along the length of the cylindrical wall past the set of heat transfer protuberances. A gap between the backer and the cylindrical wall may decrease in the direction of the flow path of the external fluid. In other embodiments of the invention, the heat transfer protuberances have a surface area transverse to the flow path that increases in the direction of the flow path. In other embodiments of the invention, the heat transfer pins may have a population density that increases in the direction of the flow path. In yet other embodiments of the invention, at least one of the height and density of the heat transfer pins may vary with distance in the direction of the flow path.
The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which:
a–1e depict the principle of operation of a prior art Stirling cycle machine;
a is a radial view of a group of parallel pins viewed towards the central axis of a cylindrical heater head in accordance with an embodiment of the present invention;
b and 4c are cross sections of the heater head of
d is a perspective view of a heat transfer pin array separately cast for assembly to a heater head in accordance with an embodiment of the present invention;
a shows a perspective top view of a heater head for mounting cast segments of heat transfer pin arrays such as shown in
b shows a perspective top view of a heater head with mounted cast segments of heat transfer pin arrays, with the pin backer removed to show the heat transfer pins;
c is a cross sectional side view of the heater head assembly of
a is a cross sectional side view of a heater head assembly with external heat transfer pin fins shown as well as a pin backer parallel to the wall of the heater head cylinder;
b–6d plot the rate of heat transfer, heat transfer coefficient, and gas temperature, respectively, as a function of distance from the top of the heat exchanger of
e is a cross sectional side view of a heater head assembly with external heat transfer pin fins shown as well as a pin backer parallel to the wall of the heater head cylinder;
f–6h plot the rate of heat transfer, heat transfer coefficient, and gas temperature, respectively, as a function of distance from the top of the heat exchanger of
i is a cross sectional side view of the heater head assembly of
a–7d depict the application of heat transfer pin rings to provide for thermal transfer between fluids and a heater head in accordance with an embodiment of the present invention;
a plots strength curves (left-hand ordinate) and elongation (right-hand ordinate) as a function of temperature for a typical nickel alloy;
b shows plots of creep rate vs. stress for a typical nickel alloy for three temperatures between 1500° F. and 1700° F.;
a and 11b are cross-sectional views of a thermal cycle engine heater head having uninterrupted tubes parallel to the outside walls in accordance with an alternate embodiment of the present invention;
a and 12b are cross-sectional views of a heater head for a thermal engine having interrupted tubes parallel to the outside walls in accordance with an alternate embodiment of the present invention;
a and 13b are cross-sectional views of a heater head for a thermal engine having helical fins along the interior of the outside walls in accordance with an alternate embodiment of the present invention;
a is a side view of a core assembly for casting a heater head for a thermal engine having helical fins along the interior of the outside walls and a ribbed dome in accordance with an alternate embodiment of the present invention;
b is a cross section, viewed downward transverse to the central axis, of the ribbed dome of a core assembly as shown in
Referring to
Expansion volume 98 is surrounded on its sides by expansion cylinder liner 115, disposed, in turn, inside heater head 100 and typically supported by the heater head. The expansion piston 121 travels along the interior of expansion cylinder liner 115. As the expansion piston travels toward closed end 120 of heater head 100, the working fluid within the heater head is displaced and caused to flow through flow channels defined by the outer surface of the expansion cylinder liner 115 and the inner surface of heater head 100.
The overall efficiency of a thermal engine is dependent in part on the efficiency of heat transfer between the combustion gases and the working fluid of the engine. One method known in the art for transferring heat efficiently from the combustion gases in combustion chamber 122 to the working fluid in expansion volume 98 requires a plurality of heating loops (not shown in
In accordance with embodiments of the present invention, protuberances, such as fins or pins, may be used to increase the interfacial area between the hot fluid combustion products and the solid heater head so as to transfer heat, in turn, to the working fluid of the engine. Heater head 100 may have heat transfer pins 124, here shown on the interior surface of heater head 100, in the space between the heater head and expansion cylinder liner 115. Additionally, as shown in
One method for manufacturing heater head 100 with heat transfer pins 124 and 130 includes casting the heater head and pins (or other protuberances) as an integral unit. Casting methods for fabricating the heater head and pins as an integral unit include, for example, investment casting, sand casting, or die casting.
While the use of pin fins is known for improving heat transfer between a surface and a fluid, the integral casting of radial pin fins on the cylindrical heater head of a Stirling engine has not been practiced nor suggested in the art, despite the fact that casting the heater head and it's heat exchange surfaces in a single step is one of the most cost effective methods to produce a heater head. The difficulty encountered in integral casting of radial pin fins is discussed further below. A pin fin that could be cast as part of cylindrical wall would allow the inexpensive fabrication of a highly effective heater head and/or cooler for a Stirling engine.
Castings are made by creating negative forms of the desired part. All forms of production casting (sand, investment and injection) involves forming extended surfaces and details by injecting material into a mold and then removing the mold from the material leaving the desired negative or positive form behind. Removing the mold from the material requires that all the extended surfaces are at least parallel. In fact, good design practice requires slight draft on these extended surfaces so that they release cleanly. Forming radial pins on the outside or inside of a cylinder would require the molds to contain tens or hundreds of parts that pull apart in different directions. Such a mold would be cost prohibitive.
In accordance with the present invention, pins or fins may be cast onto the inside and outside surface of Stirling heat exchangers using production sand, investment or metal injection casting methods. Referring to
In the casting process in accordance with preferred embodiments of the invention, positive or negative molds of each group of parallel fins are formed in a single piece. Several mold pieces are then assembled to form the negative form for a sand casting. In investment mold casting, the wax positive can be formed in an injection mold with only a handful of separate parts that pull apart in different directions. The resulting mold is formed at an acceptable cost, thereby making production of a pin fin heater head economically practical.
Casting of a heater head having protuberances, such as pins, extending to the interior and exterior of a part with cylindrical walls may be achieved, in accordance with embodiments of the present invention, by investment, or lost-wax, casting, as well as by sand casting, die casting, or other casting processes. The interior or exterior protuberances, or both, may be integrally cast, in accordance with the teachings of this invention, as part of the head.
While typically more cheaply accomplished than machining or assembly of the pin arrays, casting pin arrays may still have attendant difficulties and substantial costs. Additionally, the casting process may result in a heater head that is less than fully densely populated with pins, thus increasing the fraction of gases failing to collide with the heater head surface and reducing the efficiency of heat transfer.
An alternate method for populating the surfaces of heater head 100 with heat transfer pins, in accordance with other embodiments of the invention, entails fabrication of heater 100 and arrays of heat transfer pins in separate fabrication processes. An array 150 of heat transfer pins 152 may be cast or injection molded with panel 154 resulting in an integral backing panel structure shown in
Transient liquid-phase (TLP) bonding, as described, for example, in the Aerospace Structural Metals Handbook, Code 4218, p. 6 (1999) is particularly advantageous for brazing the panels to the head, since nickel based superalloys, typically employed for fabrication of the head, is difficult to weld by conventional processes, and operates in a high stress and high temperature environment. Advantages of TLP bonding in this application are that the parts braced by TLP are effectively welded using the parent material and have nearly the same tensile strength properties as integrally cast parts. TLP bonds do not remelt at elevated temperatures, whereas typical brazes will remelt at the brazing temperature. This is of particular significance in the case of continuous operation at elevated temperatures where temperature excursions may occur, as in the present application.
The panels 154 of pins may be attached to the interior or exterior of either the heater head or the cooler by other means. In one alternative embodiment, the panel may be mechanically attached into slots at its lateral edges. The slots are provided in dividers 506 (described in the following discussion). In another embodiment, the panels are attached to the heater head or cooler by brazing. In yet another embodiment, the panels are attached to the heater head or cooler by sintering the panels to the cylindrical walls of the heater head or cooler.
Dividers 506, as shown in
Dividers 566 may serve to improve the thermal effectiveness of the pin fin arrays in the following manner. Referring, once again, to
In certain embodiments of the invention, individual arrays 150, each with its associated panel segment 154, comprise arcuate fractions of the circumferential distance around the heater head. This is apparent in the top view of the heater head assembly shown in perspective in
In one embodiment, flow dividers 506 include structures for mechanically retaining the panel segments 154 during assembly, before brazing, or simply to mechanically retain the panels 154 against heater head 502.
In order to maximize engine power, the hottest part of the heater head is preferably at the highest temperature allowed, considering the metallurgical creep and tensile strength, stress, and appropriate factors of safety. Maintaining the hottest part of the heater head at the highest temperature requires measuring the temperature of the hottest part of the heater head. The dividers provide a convenient location and routing for temperature sensors on the heater had to any axial location along the pin fin arrays. Hot gas flow path 113 (shown also in
Temperature sensing device 138 is preferably disposed within divider 506 as depicted in
The power of the engine is limited, among various factors, by the thermal efficiency of the heater head. This thermal efficiency depends, in turn, on the fin efficiency of the pin fins. Requirements of high creep strength and oxidation resistance at very high operating temperature make the use of high nickel alloys preferable. The efficiency of the interior fins may be advantageously increased by applying a layer of highly thermally conductive metal, such as nickel or copper, of thickness greater than 0.001 in. and preferably about 0.005 in., to interior surface 148 of heater head 120, by deposition or plating, or other application method. Alternatively, a similar coating may be applied to the exterior surface, in accordance with another embodiment of the invention.
In order to keep the size of a Stirling cycle engine small, the heat flux from the combustion gas through the heater head is preferably maximized. Referring now to
At the same time as the top 2200 of the heat exchange surface is getting too hot, the amount of heat transfer at the bottom 2210 of the heat exchanger 2218 is too low. The gas cools rapidly as it gives up thermal energy to the heater head, as plotted in
The most efficient heat exchanger would have high heat transfer over its entire surface. This would maximize the amount of heat transferred to the head for given maximum head temperature. The problem is that heat transfer coefficient 2204, plotted in
A solution, in accordance with preferred embodiments of the present invention, is to vary the heat transfer coefficient to compensate for the change in combustion gas temperature as the gas cools. Therefore, the heat transfer coefficient 2304, plotted in
Referring now to
In other embodiments, the pin backer may have a constant inside diameter and the pin fin geometry may be varied along the length of the heat exchanger. The heat transfer coefficient will vary from a low at the top of the heat exchanger to maximum at the bottom. This can be accomplished in a number of ways. In one embodiment the density of constant size pin fins increases from a low that the top of the heat exchanger to maximum at the bottom. In another embodiment, the pin height increases from the top to the bottom of the heat exchanger. In another embodiment, spacing between the pins decreases from top to bottom. The ideal dimensions of the pin height, diameter and spacing depend on the particular application. The invention teaches that the heat transfer coefficient is to be increased from top to bottom of the heater head heat exchanger to compensate for the decreasing combustion gas temperature, with the objective being a more constant heat transfer rate along the heater head
The mechanical realization of the variable cross-section gas flow channel 113, as described above, is also shown in
Another method for increasing the surface area of the interface between a solid such as heater head 100 and a fluid such as combustion gases as discussed above is now described with reference to
Heat transfer rings 162 may be advantageously applied to the interior of the heater head as well as to both the exterior and interior of the cooler of a thermal cycle engine. In these applications, the rings need not be oxidation resistant. Materials including copper and nickel are preferably used on the interior of the heater head, while the rings for the cooler are preferably made of one of various high thermal conductivity materials including aluminum, copper, zinc, etc.
The total cross sectional area of the heat transfer pins taken in a slice perpendicular to cylinder axis 168 need not be constant, indeed, it is advantageously varied, as discussed in detail above, in reference to
The walls of the heater head must be sufficiently strong, at operating temperatures, to withstand the elevated pressure of the working gas. It is typically desirable to operate Stirling cycle engines at as high a working gas pressure as possible, thus, enabling the head to withstand higher pressures is highly advantageous. In designing the heater head, it must be borne in mind that increasing the pressure at a given operating temperature typically requires increasing the heater head wall thickness in direct proportion. On the other had, thickening the heater head wall results in a longer thermal conduction path between the exterior heat source and the working gas.
Moreover, thermal conduction increases with heat exchanger surface area, thus thermal efficiency is increased by increasing the diameter of the heater head. Stress in the wall, however, is substantially proportional to the diameter of the head, thus increasing the head diameter, at a given temperature and interior gas pressure, requires increasing the wall thickness in direct proportion.
The strength considerations are tantamount at typical Stirling engine head temperatures, in fact, they drive the maximum operating temperature, since, as discussed, efficiency increases with temperature. Both creep and ultimate tensile strengths of materials tend to fall off precipitously when specified elevated temperatures are reached. Referring to
Preferred embodiments of the present invention provide interior ribs (or hoops) 200 that enhance structural support of heater head 100, as shown in cross-section in
Further advantages of providing ribs 200 interior to the heater head include reducing the temperature gradient across the head wall 208 for a given rate of heat transfer, as well as allowing operation at higher hot end working temperatures. Additionally, by reducing the stress requirements on the outer wall, alternative materials to nickel based superalloys may be used, advantageously providing superior conductivity at reduced cost.
A cross section of heater head 100 with ribs 200 is further shown in
Advantages of enhanced hoop strength concurrent with enhanced thermal conductivity, as discussed above with reference to
a shows a further heater head 240, in accordance with other embodiments of the invention, wherein tubular openings 232 run parallel to heater head wall 208 and are interrupted by openings that run out to thinner sections 242 of the heater head wall. As shown in the cross sectional view of
b shows yet another heater head 250, wherein ribs 252 are disposed in a helix within heater head wall 208, thereby providing the wall with enhanced rigidity in both the circumferential and axial directions. The working gas flows through the spiral 254 on a path between the expansion piston and the heater head, on its way to the regenerator.
Heater head 250 of
The devices and methods described herein may be applied in other applications besides the Stirling engine in terms of which the invention has been described. The described embodiments of the invention are intended to be merely exemplary and numerous variations and modifications will be apparent to those skilled in the art. All such variations and modifications are intended to be within the scope of the present invention as defined in the appended claims.
The present application is a divisional application of U.S. application Ser. No. 09/884,436, now U.S. Pat. No. 6,694,731 filed Jun. 19, 2001, itself a continuation-in-part of U.S. application Ser. No. 09/517,245, now U.S. Pat. No. 6,381,958 filed Mar. 2, 2000, itself a continuation-in-part application of U.S. application Ser. No. 09/115,383, filed Jul. 14, 1998, and issued May 16, 2000 as U.S. Pat. No. 6,062,023, and a continuation-in-part also of Ser. No. 09/115,381, filed Jul. 14, 1998 and now abandoned, claiming priority from U.S. provisional application No. 60/052,535, filed Jul. 15, 1997, all of which applications are herein incorporated by reference.
Number | Name | Date | Kind |
---|---|---|---|
646406 | Anderson et al. | Apr 1900 | A |
2585457 | Holm et al. | May 1952 | A |
2688228 | De Brey et al. | Sep 1954 | A |
4069670 | Bratt et al. | Jan 1978 | A |
4387568 | Dineen | Jun 1983 | A |
4527394 | Corey | Jul 1985 | A |
5743091 | Penswick et al. | Apr 1998 | A |
5878570 | Reithofer | Mar 1999 | A |
6062023 | Kerwin et al. | May 2000 | A |
6161381 | Lohrmann | Dec 2000 | A |
6293101 | Conrad | Sep 2001 | B1 |
6381958 | Kamen et al. | May 2002 | B1 |
Number | Date | Country |
---|---|---|
8411960 | Apr 1984 | DE |
3734009 | Oct 1987 | DE |
4219583 | Jun 1992 | DE |
1063612 | Sep 1952 | FR |
306274 | Feb 1929 | GB |
395374 | Jul 1933 | GB |
422823 | Jan 1935 | GB |
689484 | Aug 1950 | GB |
917278 | Jan 1963 | GB |
500313 | May 1953 | IT |
55 037540 | Mar 1980 | JP |
58 25556 | Aug 1981 | JP |
56 133597 | Oct 1981 | JP |
Number | Date | Country | |
---|---|---|---|
20040144089 A1 | Jul 2004 | US |
Number | Date | Country | |
---|---|---|---|
60052535 | Jul 1997 | US |
Number | Date | Country | |
---|---|---|---|
Parent | 09884436 | Jun 2001 | US |
Child | 10753918 | US |
Number | Date | Country | |
---|---|---|---|
Parent | 09517245 | Mar 2000 | US |
Child | 09884436 | US | |
Parent | 09115383 | Jul 1998 | US |
Child | 09517245 | US | |
Parent | 09115381 | Jul 1998 | US |
Child | 09115383 | US |