US3397738A - Regenerator matrix systems for low temperature engines - Google Patents

Description

J. G. DAUNT 5 Sheets-Sheet 1 F 2 INVENTOR.

JOHN G. DQUNT BY ATTORN EY Aug. 20, 1968 REGENERATOR MATRIX SYSTEMS FOR LOW 'I'EMPERATURE ENGINES Filed Aug. 19, 1965 Aug. 20, 1968 J. G. DAUNT 3397738 REGENERAOR MATRIX SYSTEMS FOR LW 'TEMPERATURE ENGINES Filecl Aug. 19, 1965 3 Sheets-Sheet 2 FIG. 7

INVENTQR. JOHN G. DAUNT ATTORNEY 1968 J. G. DAUNT 3,397738 RBCENERATOR MATRIX SYSTEMS FOR LOW TEMPERATURE ENGINES Filed Aug. 19, 1965 5 Sheets-Sheet 5 IN VEN'DR.

United States Patent O 3,397,738 REGENERATOR MATRIX SYSTEMS FOR LOW TEMPERATURE ENGINES John G. Daunt, New York, N.Y., assignor to Malaker orporation, High Bridge, NJ., a corporation of New ersey Filed Aug. 19, 1965, Ser. No. 481,051 2 Claims. (Cl. 165-10) ABSTRACT OF THE DISCLOSURE This invention relates to the regenerators and matrices used therein, employed in cryogenic engines, such as those of the modified Stirling cycle type, and through which the working fluid (such as helium) is recycled. These matrix materials are solid masses having open channeling means which, in turn, have an average maximum geometric configuration such that the heat absorbed by the matrix from the flud passing once therethrough is able to diffuse thermally along average paths (in the matrix) having average lengths approximatin g the thermal diiusion depth in the matrix material for the operatin g temperature range and for the fluid single passage periocl through the regenerator.

Such a matrix system can consist of strips coiled around an axially directed plug centrally disposed in the re generator, the coil layers being separated by ribs of thermal insulating material. A series of spaced strips of matrix material, such as lead, are adhered circumierentially onto the bottom surface of the sheet. Another matrix system consists of adjacently-disposed one-piece slices of solid matrix material having a solid peripheral body and an inner integrally-connected screening of the same material, the screening having channelng means through which the operating fluid is cycled.

This invention relates to regenerators employed in engines used to develop low temperatures. More specifically, it deals with a regenerator matrix system made of coiled strips of insulating material on top of which are adhered a circumferentially-directed series of spaced strips of metal, and having an axially-directed series of spaced insulating strips adhered to the bottom thereof.

Increasing use is being made of closed-cycle refrigerator engines using gas, such as helium, as the working fluid, their operation being based on a number of thermodynarnic cycles, and variants thereof, and on a number of difierent mechanical arrangements. Examples of such engines are described in the Malaker -and Daunt Patents 3,074,244, 3147,600 and 3718,288. These particular engines operate on a modified Stirling cycle, and their successfui operation depends, to a great extent, on the utilization of highly eflicient regenerators which have efliciencies in the range of 90% to 99%.

In a regenerator of the type under consideration, the matrix material thereof absorbs heat from the working fluid as it leaves the compression cylinder when the direction is from hot to cold. When the flow of working gas is reversed, the matrix releases its stored heat to the werking gas flowing past it. For high efliciency, the ratio, U, of the heat capacity of the working =gas passing in one period to the heat capacity of the matrix mass in the regenerator must be kept as small as possible. This ratio, U, is called the utlization factor, and it is a measure of the degree to which the matrix mass is utilized for heat storage.

The behavior of regenerators has been studied analyt- 3397,738 Patented Aug. 20, 1968 ically, and workers have shown that their efliciency or therrnal recovery, is defined by the expression:

THTC where T is the inlet temperature of the hot gas, T is the inlet temperature of the cold gas, and AT is the difference in temperature between the inlet hot -gas and the mean exit temperature of the cold gas. The equaton can be expressed as a function of the reduced length, A, of the regenerator and its utlization factor, U, as follows:

2 A+2f( where (U) is a monatonic function of U such that f(U) increases positively as U increases. For a constant A, therefore, the eflicency 1 decreases as U increases.

In refrigeratiug machines using regenerators, the ratio of heat losses, AQ, associated with the regenerator in eflciencies to the heat extracted, Q per cycle at the low temperature, is proportional to (1-1 The minimum temperature to which such machines can reach, therefore, is determined by the fall-oir of the efiiciency of the regenerators at low temperatures, which all-0 is due to the increase in the value of U as the temperature is diminished.

The increase in the value of the utilization factor, U, with diminishing temperature is due to the fact that, for all known solid matrix materials, their heat capacity per unit volume decreases with decreasing temperature. For most solid materials not undergoing phase or other transformations, their heat capacity per unit volume, C, can be closely approximated by the expression:

where d is the density of the material, A is the atomic weight, D(T/) is the molar Debye function, 0 is the Debye characteristic temperature, and 7 is the molar Sommerfeld electronic specific heat term. In general, or T less than about 0/20, D(T/6) can be approximated by 1950(T/9) joules/mole-deg.

For successful operation of regenerators at very low temperatures, matrix materials must be used which have high heat capacities of the matrix per unit volume. T0 achieve this with solid materials which do not undergo phase or other transitions, materials must be chosen which have maximum values of (d/A) and minimum Debye characteristic temperatures, 0. A survey of materials suitable for regenerator matrices shows that, for example, below about 50 K., gold, bismuth and lead show values of C larger than that of copper and bronze. FIGURE 1 depicts -a plot of observed values of C versus T for gold, copper, lead and bismuth. -It will be noted that, below 40 K., lead has the largest C values. For this reason, the use of lead as a matrix material for 10W temperature regenerators has been common for some time (e.g., as in Patent No. 2,966,035), and its use has generally been in the shape of small balls of about 0.01 te 0.03 inch in diameter. However, there are other solid materials than lead which may be used as matrices in regenerators, and this invention encompasses the use of all solid matrix materials which may be formed and shaped.

By means of the present invention the effective heat capacity of the matrix material in the regenerator can be increased sufliciently to permit high regenerator efliciencies to be obtained at low temperature. It is to be noted that, although the specific heat, C, per unit volume of materials such as are used for matrix materials in regenerators, decreases with decreasing temperature, the thermal "conductivity, K; increases with decreasing temperature. This results in an increasing thermal difiusion coeflicient, D=K/C, as the temperature is reduced. In consequence, the thermal dillsin depth, X, defined as the distance through whch a temperature wave ditfuses to 172 of its value in a time, r, and expressed as X2=TD, also increases with decreasing temperature. FIGURE 2 presents a11 evaluation of the thermal dilusion depth, X, in mm. for pure copper, bismuth, lead and gold, as ca-l culated from known values of the specific heat, C, per unit volume (sec FIGURE 1), and from observed values of the thermal conductivity, K, as a function of temperature. The data of FIGURE 2 are calculated for a time period of sec., which is the gas flow period used in a practical miniature regenerator, of the type described in Patent 3074,244. It is noted that, below 30 K., the diiusion depth for these periods become very large in these materials, being of the order of several millimeters. These dilusion depths are many times greater than the diameters of the wires used in gauze-packed regenerators (about a few thousandths of an inch) or than the diameters of the spheres heretofore used to fill regenerators (about 0.005" te 0.030").

According to the present invention, the matrix of the regenerator is given a configuration whch would take advantage of such greater diffusion depths and thus allow larger efiective matrix heat capacities to be realized. In the design of these configurations, it will be observed that two basic requirements of the regenerator are satisficd: (1) The heat transfer surface area and the heat transfer coefiicient are adequate to permit the flow of heat between the gas flow and the matrix material to occur in each period without excessive temperature dilerence, and the void volume is as small as possible consistent with the maintenance of adequate heat transfer coeflicent and sufficiently small pressure drop; and (2) the heat capacity of the matrix material is substantially greater than that of the gas flowing in one period, i.e., the utilization factor is adequately small. It is noteworthy that these two requirements -are independently satisfied by the present invention.

The present invention, broadly, compn'ses the use of a series of strips of matrix material having a thickness sufficiently great to provide adequate total matrix mass in the regenerator, said strips being thermally insulated trom each other. In the preferred embodiment the strips are straight lengths extending over the length of the insulated sheet to whch they are attached in a row. Below the sheet are attached a series of spaced insulating ribs disposed at right angles to the matrix strips. The sheet, then, is coiled around a blank cylindrical core axially and centrally disposed Within the regenerator, with the top of the lead strips being disposed adjacent the wall of the core.

The invention will be more readily understood by reference to the accompanying drawings in which a preferred embodiment is described, and in which FIGURE l, as already mentioned, is a graph on the heat capacity of various metals at very low temperatures, while FIGURE 2, as already outlined, is a graph showing the thermal diffsion depth for a time period of second at various temperasures for the four metals shown in FIGURE 1. FIGURE 3 depicts a top or plan view of a portion of a matrix sheet on an enlarged scale, with the center portion cut away; to be formed into a coil for regenerator use. FIGURE 4 depicts a side view of the strip of FIGURE 3, while FIGURE 5 illustrates an end view thereof. FIG- URE 6 shows a perspective quarter side view section of the inlet rp0rtion, with inlet tube partly cut away, of a regenerator containing thes coiled sheet of FIGURES 3-5 for use in a refrigeration engine. FIGURE 7 depicts a cross-sectional view of another embodiment of the matrix element. FIGURE 8 shows an alternate arrangement of matrix slice elements, of the type depicted in FIGURE 7, spread out at right angles to the direction. of mounting, and as separated by sereen elements, while FIGURE 9 depicts a side view of a regenerator of the present invention, on a reduced scale, and partly cut away to show such alternate slice and sereen arrangement for the matrix system. The same numerals refer to similar parts in the various figures.

Referring again to the drawings, numeral 10 indicates a sheet of thermal insulating material, such as nylon, stainless steel, polyester, phenolic resin, or the like, onto which are cemented square strips of lead 11 laid in parallelrela tion, and spaced away from each other by spaces 12, the spacebeing adequate to provide thermal insulation between each strip. Cemented to the bottom of insulation sheet 10, in spaced relation, and disposed in a direction atright angles to that of strips 11, are ribs 13 made of thermal insulating material, such as nylon, polyester, stainless steel, phenolic resin, or the like. These ribs may be smaller than strips 11, and may be spaced farther apart.

The matrix sheet, indicated generally as 14, and having the lead strip and insulating ribs cemented thereon, is coiled around a centrally-disposed blanked olcylinder or plug 15 (FIGURE 6), in a manner so that the ribs 13 are disposed axially and the lead strips 11 are disposed circumferentially with respect to plug cylinder 15, with the free upper surface of the lead strips wrapped adjacent to and around plug 15. The ends of thelead strips, such as at upper end 18 of the sheet, are chamfered to the contour of the inner regenerator surface, and the free spaces there may be fillcd with epoxycomposition 19 so as to insure against gas by-passing. The workiug gas, such as helium, of the engine, would leave the engine compressor cylinder through tube 20, enter the regenerator in the direction of the arrow, and leave through outlet 42 (FIG. 9). Since it cannot pass through plug 15, it passes through spaces 21 between insulator ribs 13, in the directions of the arrows, and gives off its heat to lead strips 11, after whch it leaves regenerator 16 (and is then recycled).

The lead strips 11 serve, in the present embodiment of FIGURES 3-6, as the matrix material. Here the lead strips, as an example, may be 0.04" wide and 0.04" thick, with spacings 12 therebetween of 0.005". The polyester insulating ribs 13 on the back of the 0.002" thick nsulat ing sheet 10 of Mylar polyester are 0.01" wide, 0.005 thick, and spaced apart 0.07. This composite sheet carrying the lead matrix is coiled as a tght spiral around plug 15, the surface of the plug bei.ug in contact with the lead surface portion of the coil, the lead strips 11 being disposed in a direction at right angles to the axis of the regenerator 16. The insulating ribs 13 form spaces 21 between the coil layers, whch spaces permit the gas to flow therethrough and contact the upper surfaces of lead strips 11 in a drection at right angles thereto. In this embodirnent, these gas channels are 0.07" wide (forming the arc of a circle), and 0.005 thick (radially);

In designing the dimensions to be used in these embodiments, the followingconsiderations must be taken into account. The width of the lead strips determines the number of lead strips or lead spirals along the length of the regenerator. Since each strip at any partcular time will be at uniform temperature throghout its length, it is desirable to have a large nurnber of strips in a given regene rator in order to minimizez the temperature jlimp 'between one strip and the next. For exampl,in a typical regenerator referred to below, using lead strips 0.04" wide and employing a total'regenerator length of 2", there wouldbe 44 lead strips in the matrix. The regenerator would be designed, in this case, to operate between 15 K. and 25 K., and hence, the average temperature jump between one lead strip and the next would be (2515)/ 44=0.23 K., whch would lead to a maximum efliciency The thickness of the strips would be determined by the required total mass of matrix material in the regenerator. In the present design, the thickness clearly can be varied between wide limits without, for example, alteringthe size of the gas passages, i.e., without changing the heat transfer surface area, the heat transfer coeflicient, the pressure drops, or, and most importantly, the void volume in the regenerator, which is an advantageous basic feature of the present invention. There is, of course, one limit to the thickness of the lead strips, namely, that they must be equal to or less (in thickness), than the diusion dept in the material at the temperature of operation. As is apparent trom FIGURE 2, typical diusion depths below, say 30 K., are very large for pure materials, and this limitation therefore is not a serious one for regeneratorg operating at very low temperatures.

The thickness, width, and spacing of the insulating ribs are determined by the design requirements of the gas channels for adequate heat transfer area, heat transfer coeflicient, pressure drop, and void volume.

A typical generator for operation in the temperature range of K. te K., in line with the present embodiment, consists of five layers of composite matrix sheet 14 (in a coil), as dimensioned and defined above, for a length of 2, wound over a plug tube 15 of /s" outside diameter, and within a regenerator casing 16 of 0.855 inside diameter. This results in an open crosssectional area for gas flow of 3.5 10 square inches, and a matrix mass of 108 g. of lead. For comparison, a regenerator of 2" length made of a stack of 200 mesh lead screening, With the same open cross-sectional area for gas flow would have a matrix mass of only 12 g.

It will be noted that one feature of the present inventon is that it makes use of the low temperature phenomenon of the large thermal diusion depth for the difiusion of heat into the matrix. Various geometrie designs are possible for gas channel configuration, satisfying the requirements of surface area, heat transfer, void volume, and pressure drop. FIGURE 7 depicts one cross sectional configuration wherein each thin slice of the cylindrical matrix mounted within regenerator 16 has the shape shown. The thermal ballast material between radii R and R is solid matrix material, such as lead, while the material within radius R consists of a slotted sereen having alternately solid strips 31 of the same material and open slots 32 through which working fluid is cycled. These slices of matrix desgnated as 33, are desirably arranged so that each alternate slice has its strips 31 disposed at right angles to those of the strips of adjacent slices. Also, it is desirable to separate each slice by a strip of conventional sereen 41 of, say, 200 mesh, as in FIGS. 8 and 9. The dimension of the slot channels are chosen to give the required heat transfer coeflicient, heat transfer surface area, void vol ume, and pressure drop in the regenerator. The dimensions R and R are chosen so that the heat absorbed from the gas passing through the center of the regenerator, as for example, a channel near point of FIGURE 7, can difiuse thermally to the perimeter near point 40 during the period of a single gas passage through the regenerator. The requirement for this is that the distance 30-40 be less than the thermal diffusion depth X in the material at the temperature of operation, and that the temperature drop along the matrix material in the region of radius R be small. For the large values of X shown in FIGURE 2 below 20 K., quite large values of R and R may be employed. In the geometrie configuration of the matrix of FIGURE 7, the heat capacity per matrix slice has efiectively an extra component due to the mass of thermal ballast material between radii R and R which material does not form a part of the gas channels withn the generator matrix.

It is to be understood that the examples given are employed for the purpose of illustrating the invention which is not to be limited thereby. For example, although an insulating sheet 10 is depicted in FIGURES 3-6, it is to be understood that, in lieu of such sheet, one may use thin strips of such material disposed over and attached to each insulating rib 13, with its other side attached to lead strips 11. Also, although lead is mentioned as the example of matrix material, other solid materials may be employed, particularly those selected for maximum heat capacity at the werking temperature range of the regenerator.

I claim:

1. A regenerator matrix system for low temperature engines of the type desoribed and operating in a low temperature range and using a werking fluid, comprising:

a cylindrcal casin-g having an inlet at one end and an outlet at the other end,

a cylindrical plug centrally and axially disposed in said casing,

a thermal insulating sheet coiled around said plug, and

filling the free space in said casing,

a series of spaced strips of matrix material adhered cr cumferentially onto the bottom surface of said sheet and having a thickness no greater than the thermal diffusion depth of said material for the operating temperature range for the fluid single passage period through the regenerator, said thermal diffusion depth, X, being defined as the distance through which a temperature wave difiuses to 1/ e of its value in time, 1, and eX-pressed as X2=TD, where D is the thermal coefiicient, and

a series of spaced ribs of thermal insulating material adhered axially onto the top surface of said sheet.

2. A regenerator matrix system for low temperature engines of the type described and operating in a low temperature range and using a werking fluid, comprising:

a cylindrical casing having an inlet at one end and an outlet at the other end,

a cylindrical plug centrally and axially disposed in said casing,

a thermal insulating sheet coiled around said plug, and

filling the free space in said casing,

a series of spaced strips 0 matrix material adhered circumferentially onto the bottom surface of said sheet, and

a series of spaced ribs of thermal insulating material adhered axially onto the top surface of said sheet.

References Cited UNIT ED STATES PATENTS 2,492,788 12/1949 Dennis --10 X 2,833,523 5/1958 Haan 16510 2,958935 1l/1960 Bloem 16510 X 3,216,484 11/1965 Giord 1654 FOREIGN PATENTS 724,983 2/ 1955 Great Britaiu. 1,294,514 4/ 1962 France.

ROBERT A. OLEARY, Prmary Examner.

T. W. STREULE, Assstant Examner.

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