The Vuilleumier Cycle was originally described by Rudolph Vuilleumier in U.S. Pat. No. 1,275,507 issued Aug. 13, 1918. In the patent he describes a method and apparatus for inducing heat changes using this particular cycle. The Vuilleumier cycle is capable of producing either a positive or a negative work upon expansion or contraction of a fluid under absolute pressure to secure secondary heating or cooling effects, respectively, in a second body of fluid that is in pressure interchanging relation therewith. A relatively cool fluid when brought into contact with a lower temperature fluid will raise the temperature of the latter fluid by primary heating and can also induce secondary heating in a fluid at another higher temperature. Conversely, a relatively warm fluid when brought into contact with a body of fluid at a higher temperature will lower the temperature of the latter fluid by primary cooling and can also induce secondary cooling in a fluid at a lower temperature. This secondary heating or cooling effect is produced by moving a portion of a fluid between several constant temperature and pressure zones and including the secondary effect in the remaining portion of the fluid. The apparatus generally comprises a pair of cylinders and a pair of piston-like displacer elements moving through adjacent halves of the two cylinders. However, the Vuilleumier Cycle apparatus differs from conventional heat pumps in that variations in the gas pressure are not produced by exerting an external force on a piston, but are caused by forcing the gas to flow back and forth between two ends of the fixed volume cylinder which ends are maintained at different constant temperatures to cause the pressure to change accordingly. The gas pressure variations which result from the displacer motion produce the desired heating or cooling effect.
The primary energy input to a Vuilleumier cycle unit is in the form of heat which is employed to elevate the temperature of one end of the cylinder. Referring to Vuilleumier's patented device, the gas or working fluid is forced to flow through the cylinders by means of a pair of reciprocating displacers or pistons 5 and 6 which are powered by a small drive means such as a small electric motor. As the displacers move back and forth in the cylinders, the air is forced to flow through heat regenerators 7, 8 in each of the displacers 5, 6, respectively, which comprises a central boring in the displacers having a large number of heat absorptive walls or elements. Since the pressure on opposing ends of the displacers 5, 6 is assumed to be essentially the same at any instant in time, the motor power required to move those displacers is small in comparison to the rate of heat input. As described in Vuilleumeir's patent a simple apparatus for employing the Vuilleumier cycle, shown in FIG. 2 of the Vuilleumier patent, includes a cylinder 4 having three distinct areas: a heated area at one end of cylinder 4 encompassed by a heating jacket 9, a centrally located cooling area cooled by jacket 15, and the refrigeration area cooled by jacket 12. Two pistons 5 and 6 are disposed to move between these areas for the purpose of effecting the heat exchange. For purposes of illustration the Vuilleumier cycle can be described as including 4 distinct phases. During phase 1, one of the pistons 5, which is disposed adjacent the heating jacket 9 moves outwardly towards the area encompassed by cooling jacket 15. As piston 5 moves, the air contained in cooling jacket 15 is displaced through regenerator 7 in piston 5 and directed into the now empty space encompassed by heating jacket 9. As the air moves through regenerator 7 it is subjected to a sudden increase in temperature. This primary heating of the air results in a secondary inductive heating of the air remaining in the cooling space when the heated air expands upon heating and a portion of it returns via regenerator 7 to the cooled area. The heating effect is drawn away by the cooling medium of jacket 15. During phase 2, the other piston 6, moves outwardly from the area encompassed by refrigeration jacket 12 into the remaining space that is cooled by cooling jacket 15 which maintains the cooled area at a constant temperature. During this movement the remaining air in cooled space 15 is displaced through the regenerator 8 in piston 6. Since the air embraced by the cooled space is assumed by Vuilleumier to initially be at the same temperature as that embraced by jacket 12, no change of temperature occurs and no secondary cooled effect is induced. During phase 3, piston 5 returns to its original position and the air is thereby returned from the heated space encompassed by jacket 9 through regenerator 7 to the cooling space encompassed by jacket 15 and arrives at the cooling space at a much higher temperature. As the air is moving through regenerator 7 it becomes cooler and this cooling causes a secondary cooling affect to take place throughout the cylinder due to a net reduction in pressure throughout the cylinder. This pressure reduction causes a slight flow of air from the refrigerating space encompassed by jacket 12, and as this air flows through regenerator 8 it is heated slightly partially cancelling out the secondary cooling effect. During the fourth phase, piston 6 returns to its original position and the remaining air in the refrigeration space is displaced through regenerator 8 to the cooling space and, as it passes through generator 8, it also absorbs a small amount of heat. This heating of the air as it moves through regenerator 8 causes a slight secondary heating to occur in the cylinder. The net effect, under normal working conditions, is that displaces 5 and 6 move air between different constant temperature areas and produce primary heat changes of equal but opposite value alternatively and also induce secondary heating and cooling effects. Heat absorbed from the heated space is imparted at a lower temperature to the cooling space, while heat drawn from the refrigerating space is imparted at a higher temperature to the cooling space. By applying a heat balance equation it can be shown that under the assumed temperature conditions for every BTU of heat transferred from the heated to the cooled space, two BTU's of heat will be transferred from the refrigerated space to the cooled space. This ratio can, as noted in the Vuilleumier patent, be increased or decreased by changing the temperatures and heat differentials.
This inherent characteristic of a Vuilleumier Cycle unit to use heat directly as an input power, makes it attractive for many uses. Vuilleumier cycle devices could easily be used with solar or waste heat powered heating and air conditioning systems. However, the currently available Vuilleumier Cycle units such as heat pumps have certain inherent disadvantages. For example, displacer cylinders of Vuilleumier cycle machines, for various applications, are necessarily larger than those of conventional compressors having the same capacity, and the operating pressure tends to be relatively high. Also, it is more difficult to transfer heat to the gas in the cylinders of a Vuilleumier Cycle Heat Pump, than to an evaporating liquid in the coils of a conventional refrigerator. This increases the cost of the Vuilleumier cycle unit. As a result, in recent years work has been geared toward identifying more efficient heat exchangers for use with the Vuilleumier Cycle units in order to reduce the size and complexity and the associated higher costs and to improve the efficiency of the heat exchange process.
For a Vuilleumier cycle unit to operate at its potential high efficiency it is necessary that the heat exchange be conducted with minimal temperature gradient from an external source to the working fluid in the cylinders. The conventional designs for such heat exchange call for enveloping the particular constant temperature area with means for maintaining that temperature and effecting the heat exchange through the walls of the cylinder. With larger sized cylinders for the larger capacity requirements, it becomes increasingly difficult to bring the working fluid into contact with the exterior walls of the cylinder of the Vuilleumier unit for a sufficient time to effect the proper heat exchange without causing undesirable pressure losses. In addition the larger the size of the particular cylinder, the more difficult it will become to uniformly effect the heat exchange with the result that there will be a temperature gradient through the working fluid generally corresponding to the size of the cylinder.
One approach to solving this difficulty is shown in Australian patent application No. 126969 published Apr. 2, 1947 in the name of the applicant N. V. Philips' Gloeilampenfaberieken, but withdrawn before being accepted for patent. The application disclosed improvements relating to refrigeration machines employing the reverse hot-gas motor principle (stirling cycle). The unit described therein, employed a series of complementary partitions 22, 23, 24 and 25 intended to make the expansion and compression of the gas more isothermal. However the means for heating, via inlet 18 and outlet 19, and cooling, refrigerating box 10, are still positioned to simply envelope their respective heated area 15 and cooled area 12.
Another approach is disclosed in U.S. Pat. No. 3,508,393 issued to D. A. Kelly on Apr. 28, 1970. Kelly discloses a chemical heating means for use with low friction Sterling Engines. The heating means comprises in addition to the conventional wraparound type a series of heating conduction rods placed within the hot side of the displacer volume. The rods are uniformly spaced and are intended to increase the heat transfer area and the rate of heat transfer.
None of the above-described systems adequately solves the problem of effecting a highly efficient, isothermal heat transfer, especially as relating to a Vuilleumier cycle unit. The heat transfer in all cases is highly dependent on the thermal conductivity of the materials of construction and nowhere is there disclosed a heating or cooling means that is virtually in intimate contact with the working fluid.
It is therefore an object of the present invention to provide a mechanism for achieving near isothermal heat transfer between the external fluid medium in the jackets of the Vuilleumier heat pump and the internal gas working fluid.
It is another object of the invention to provide a means for causing the external fluid to flow through finned passages that protrude into the working fluid area of the cylinder. It is another object to shape the displacer so that it's motion causes the internal gas to flow uniformly over the external surfaces of the finned passages.
It is another object to design the heat exchanger such that the gas is moving over the finned surface at the time that it expands or is compressed. The steady and uniform gas velocity provides forced convective heat transfer at the time needed for efficient operation of the heat pump.
It is another object to provide flow passages that are designed so that a minimum power input is required to cause the gas and external fluid to flow through the heat exchanger.
Finally it is an object to provide for external fluid passages that are machined into the cylinder head, and where the internal gas passages are integrated with the displacer regenerator such that the entire unit is easily manufactured and has minimum dead volume.