US3684007A

US3684007A - Composite structure for boiling liquids and its formation

Info

Publication number: US3684007A
Application number: US102387A
Authority: US
Inventors: Elias George Ragi
Original assignee: Union Carbide Corp
Current assignee: Katalistiks International Inc; Honeywell UOP LLC
Priority date: 1970-12-29
Filing date: 1970-12-29
Publication date: 1972-08-15
Anticipated expiration: 1989-08-15
Also published as: IT945701B; FR2120091B1; GB1328919A; CA958160A; DE2165114A1; FR2120091A1; JPS513581B1

Abstract

A composite metal structure for boiling liquids comprising a smooth surface metal substrate and a cover sheet bonded to the substrate with sub-surface cavities and spaced restricted openings extending through the cover sheet being joined to the cavities.

Description

United States Patent [151 3,684,007 Ragi [4 Aug. 15, 1972 1541 COMPOSITE STRUCTURE FOR [56] References Cited gggifig gg AND ITS UNITED STATES PATENTS 1,228,816 6/1917 Peterson et a1. ..126/387X [m Invenmr' Gmge s Amherst 3,384,154 5/1968 Milton ..165/133 x Assignee: Union Carbide Corporation, New

York, NY.

Filed: Dec. 29, 1970 App1.No.: 102,387

US. Cl ..165/133, 29/1573 R, 62/527,

165/180 1111. c1. .1281 13/00 Field of Search ..165/l33, 105, 180, 181;

Primary Examiner-Albert W. Davis, Jr.

Attorney-Paul A. Rose, Harrie M. Humphreys, John C. LeFever and Lawrence G. Kastriner [5 7] ABSTRACT A composite metal structure for boiling liquids comprising a smooth surface metal substrate and a cover sheet bonded to the substrate with sub-surface cavities and spaced restricted openings extending through the cover sheet being joined to the cavities.

16 clams, 10 Drawing Figures PATENTEDAums I372 I 3. 684' 007 sum 1 0r 6 I F/GZZ.

INVENTOR ELIAS G. RAGI ATTORNEY PATENTEDMJB 1 m2 3.684; 007 sum 2 OF 6 INVENTOR ELIAS (G RAGI ATTORNEY BACKGROUND OF THE INVENTION This invention relates to composite metal structures for improving heat transfer from heated surfaces to boiling liquids and in particular structures which enhance nucleate boiling. The invention also relates to a method for forming such a composite structure.

' It is known that structures for improved nucleate boiling may be prepared by bonding metal powders onto metal substrates to form interconnected pores with restricted openings to the outer surface having equivalent pore radius less than about 6 mils, as for example described in US. Pat. No. 3,384,154 to R. M. Milton. It is also known that improved nucleate boiling structures may be prepared by cutting closely spaced grooves in a metal wall with the outer ridges partly deformed into the grooves so that sub-surface cavities result with restricted openings through the outer ridges, as described in US. Pat. No. 3,454,081 to L. C. Kun and A. M. Czikk. Disadvantages of such prior art nucleate boiling structures are either the relatively high cost of machining equipment or the inability to form relatively large restricted openings (preferred for boiling liquids characterized by high Kelvin parameters) from metal powder.

An object of this invention is to provide a highly efiicient but inexpensive nucleate boiling heat transfer structure.

Another object is to provide an inexpensive method for preparing a highly efficient nucleate boiling heat transfer structure. Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.

SUMMARY This invention relates to an improved composite metal structure for nucleate boiling heat transfer to liquid from a heated surface and to a method for preparing such a composite metal structure.

According to this invention a composite metal structure comprises an impervious smooth surface metal substrate and an impervious metal cover sheet bonded to the metal substrate and having at least 25 cavities per inch of substrate surface with each cavity having an effective diameter of at least 0.003 inches, and a multiplicity of spaced restricted openings extending through the cover sheet in fluid communication with the cavities and having effective diameter such that the effective diameter ratio of restricted openings to cavities is less than 0.8. As used herein, the effective diameter of the cavity is the diameter of the largest sphere which can be fitted within the confines of the cavity. The effective diameter of the restricted opening represents the largest diameter vapor bubble which may emerge from the cavity to the outer surface of the cover sheet. For example, the effective diameter may be the minor dimension of an ovoid or elliptically shaped restricted opening.

Another aspect of this invention relates to a method for forming one composite metal structure of this invention. According to this method, a die having spaced pyramid shaped projections is pressed against a thin metal foil backed by a resilient surface to initially form discrete pyramid shaped sections in the foil. The

pyramid tip ends are then pierced to form holes of irregular non-circular cross section. The foil with the soforrned discrete pyramid sections is thereafter bonded to the metal substrate to form a composite metal structure, either with the tip end of each pyramid pointing downwardly toward the substrate surface or with the tip end pointing upwardly away from the substrate surface. In the latter arrangement, the pierced hole through the tip end of each pyramid comprises the restricted opening, the pyramid forms the cavity and the bottom side of the unraised section of the foil surrounding each pyramid section is bonded to the substrate. When the pyramid tip ends are arranged to point downwardly toward the substrate, the tip edges are bonded to the substrate with unbonded gaps between the bonds forming the aforementioned restricted openings and the enclosed space bounded by the substrate, shaped foil and the pyramid tip end, form the cavities.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view looking downwardly on an up wardly turned pyramid-shaped embodiment of the composite metal structure having pierced holes in the pyramid tips.

FIG. 2 is an elevation view taken in cross-section of the FIG. 1 structure.

FIG. 3 is a plan view looking downwardly on a downwardly turned pyramid-shaped embodiment of the composite metal structure having pierced holes in the pyramid tips.

FIG. 4 is an elevation view taken in cross-section of the FIG. 3 structure.

FIG. 5 is an isometric view of a corrugated-serrated sheet composite metal structure comprising another embodiment of this invention.

FIG. 6 is an elevation view taken in cross-section of a dimpled sheet pierced hole embodiment.

FIG. 7 is an elevation view taken in cross-section of a flat cover sheet-venturi type restricted opening embodiment.

FIG. 8 is a graph showing water boiling performance data for the FIG. 1-4 pierced hole pyramid, the FIG. 5 corrugated-serrated sheet, and the FIG. 6 dimpled sheet embodiments.

FIG. 9 is a graph showing water boiling performance data for the FIG. 7 flat cover sheet-venturi type restricted opening embodiment, and

FIG. 10 is a graph showing fluorot'richloromethane boiling performance data for the upwardly turned pyramid-shaped cover sheet and flat cover-venturi type restricted opening embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the FIG. l-4 embodiments, cover sheet 11 com prises a thin impervious metal layer of for example 0.002-0.030 inch thickness which may be readily deformed into a multiplicity of discrete raised sections. These sections may be in the general shape of pyramids 12 having locally sloping sides 13. Alternatively, they may be conically shaped with a circular base rather than the generally square (or rectangular) base of the pyramid type configuration. As illustrated in FIGS. 1-2, pyramids 12 are arranged in parallel rows some of which are oriented at 90 to each other. Alternatively, the individual discrete sections may be arranged in a staggered pattern. The individual discrete raised sections 12 are preferably separated and surrounded by valley sections 14. At least one restricted opening 15 is provided in each discrete section 12 and may be circular but is preferably irregular and non-circular. Restricted opening 15 is preferably in the tip end of the raised section 12.

Cover sheet 11 may for example be formed by the use of a metal die having spaced projections of the desired configuration and in the desired density per square inch of surface area. The die is pressed against the undeformed cover sheet which is preferably backed by a resilient layer, as for example rubber foam, to initially form the desired discrete sections in the sheet by deformation against the mating surfaces of the die. The restricted openings 15 may be formed in the tip end of each raised section 12 by pressing the die against the now deformed cover sheet with additional force so that the tip end of each die projection ruptures or pierces the tip end of the mating deformed section of the cover sheet so as to form irregular non-circular openings 15 with jagged edges 16. The openings usually have an xshape or star-shape in the plan view as illustrated in FIG. 1. The torn edges 16 of the openings 15 may for example extend about half-way down the sloping sides 13 of the raised sections 12 (see FIG. 2). The size of the openings may be varied relative to the size of the cavity formed, by controlling how far the projections on the forming die deform the cover sheet 11. Also, the size of the restricted opening 15 may be controlled to some extent by selecting the thickness and/or physical properties of the cover sheet 12. For example, if the cover sheet is formed of metal foil, more ductile or annealed foil materials will generally permit the formation of smaller openings than less ductile or work hardened foil materials.

The resulting cover sheet 1 1 having a multiplicity of discrete raised sections 12 separated by valley sections 14 is bonded to the metal substrate 17 to form a composite metal structure, either with the tip ends pointing upwardly (FIGS. 1-2) or downwardly (FIGS. 3-4). The bonding may for example be accomplished by sprinkling a layer of brazing metal powder of desired thickness, e.g. 0.0020.003 inch, onto the substrate, positioning the deformed cover sheet 11 thereon and thereafter heating the assembly in a brazing furnace. In the FIG. 1-2 embodiment, the bond is formed between the valley sections 14 surrounding the raised sections 12 and the smooth top surface substrate 17.

The FIG. 3-4 embodiment of the composite metal structure may be formed in the same manner as the FIG. 1-2 embodiment except that the tip ends are positioned against the substrate surface and the bond is between the latter and the jagged edges 16 of openings 15.

There are however important differences between the composite metal structures of FIGS. 1-2 and 3-4 with respect to their performance for improved heat transfer from heated surfaces to boiling liquids. In the tip-up embodiment, the cavities 18 where the vapor bubbles are formed and retained are bounded by the sloping sides 13 of each raised section 12 and the top surface of substrate 17. There is preferably at least a minor degree of communication, i.e. fluid communication passages, between at least some of the adjacent cavities 18 by virtue of gaps in the bond between the valley bottom surface and the substrate 17 top surface.

In the tip-down embodiment of FIGS. 3-4, the restricted openings are formed by the unbonded gaps 15 between the tip edges 16 and the substrate 17 top surface. For example the piercing operation usually produces four comer tears 19 extending a considerable longitudinal distance along the sloping sides 13, and only the lower portion of these tears is filled with bonding material 20 during the brazing operation. The upper portion is open and comprises the restricted opening of the tip-down embodiment. The cavities 18 comprise the space bounded by the substrate 17 top surface, the shaped cover sheet 11, and the tip ends of adjacent raised sections 12a and 12b. It should also be noted that the tie-up cavities 18 of FIGS. l-2 are directly beneath the restricted openings 15 whereas the tip-down cavities of FIGS. 3-4 are located between the restricted openings of adjacent raised sections 12a and 12b. FIGS. 3-4 also reveal that unlike the tip-up" embodiment, there are numerous fluid communication passages between adjacent cavities.

As previously indicated, the composite metal structures of this invention have cavities with effective diameters of at least 0.003 inch, and a multiplicity of spaced restricted openings extending through the cover sheet in fluid communication with the cavities, such that the effective diameter ratio of restricted openings to cavities is less than 0.8. The cavities act as nuclei for the growth of many bubbles of the boiling liquid. As the bubbles grow, vapor emerges from the cavities through the restricted openings due to continued generation of vapor therein, breaks away from the outer surface of the cover sheet, and rises through the liquid. The liquid continues to flow into the cavities to replenish a thin liquid layer which is maintained between a trapped vapor bubble and the adjacent metal surface defining the cavity. The extreme thinness of the liquid film within the cavity is believed primarily responsible for the strikingly high boiling heat transfer coefiicients achieved with this invention.

The cavities must have effective diameters of at least 0.003 inch and preferably at least 0.006 inch to permit appreciable bubble growth therein, and the effective diameter ratio of restricted openings to cavities is less than 0.8 and preferably less than 0.7 to retain the hub bles in the cavities for sufficient duration for such growth to occur. The composite metal structure has at least 25 cavities per inch of substrate surface to provide an adequate number of nucleation sites for high rate heat transfer.

In general, larger cavity and restrictive opening effective diameters provide optimum performance (maximum boiling coeflicients) for liquids characterized by relatively high Kelvin parameters. The latter is defined as 2Co- T IA P in units of mils x F where 0'= Surface tension, lbs. force/ft.

T Saturation temperature of boiling liquid corresponding to the vapor pressure of the liquid, R

P Density of vapor, lbs. mass/ft.

A Latent heat of boiling liquid, Btu/lb.

C= Conversion factor, 15.48 Btu mil/ftFX lb. force Conversely, smaller cavity and restrictive opening effective diameters provide optimum performance for liquids characterized by relatively small Kelvin parameters. However the cavity effective diameter of the composite metal structure of this invention is preferably at least 0.006 inch to minimize possible plugging during the metal substrate-cover sheet metal bonding. These relationships are illustrated by Table A:

TABLE A Kelvin Pressure Optimum Cavity Effective Fluid Parameter -psia Diameter 1X10")in.

Water 18 1.5 18

In a preferred embodiment of the composite metal structure, the cover sheet is shaped to provide fluid communication passage between at least some of the cavities adjacently positioned to each other. This is to .assure continuous flow of liquid into active cavities from which bubbles are continuously emerging even though liquid may not be simultaneously entering the restricting openings of such cavities due to vapor obstruction. Under these circumstances liquid may enter the active cavities from inactive cavities through the fluid communication passages. As previously indicated the tip-up embodiment of FIGS. 1-2 is characterized by a limited number of such passages whereas the tip down embodiment of FIGS. 3-4 has a large number of vapor communication passages 21.

In another preferred embodiment the restricted openings of the composite metal structure have irregular non-circular cross-sections. The latter will assure continuous flow of liquid into the restricted opening along at least part of its perimeter even when vapor is emerging therefrom. This is because the vapor bubbles generally assume a circular configuration and do not fill the entire cross-section of an irregular non-circular configuration. Continuous flow of liquid into a cavity having an active vapor nucleation site will of course insure continuous vapor generation therein. Accordingly, if the restricted openings have irregular non-circular cross-sections the importance of also providing fluid communication passages between at least some adjacent cavities is diminished and the converse is also true. However, for continuous vapor nucleation in a large number of cavities per unit surface area to attain maximum boiling heat transfer coefficients, restricted openings having irregular non-circular cross-sections and cavityto-cavity fluid communication passages are both employed in the composite metal structure.

Although the FIGS. 1-4 embodiments are formed with a continuous single cover sheet positioned over the entire smooth surface substrate, it is contemplated that the composite metal structures may be formed using a discontinuous cover sheet. For example, sheets may be folded into a U-shaped configuration and positioned with the open ends against the substrate for bonding thereto. Spaced channels could then extend the entire length (or width) of the substrate, and restricted openings to the channels could be formed in the channel top surfaces. As another alternative, the channels may be formed by spaced parallel folds in a single cover sheet.

In the FIG. 5 embodiment, cover sheet 11 has a series of corrugations 22 arranged in longitudinal rows parallel to each other with transversely separating valleys 23 between adjacent rows and bonded to the substrate. Each corrugation 22 has transverse slits or serrations 24 to form corrugation sections with longitudinally

alternate sections

25 and 26 being transversely displaced in the same direction. That is, alternate sections 25 are displaced to one side of the corrugation 22 centerline and alternate section 26' are displaced to the opposite side of the centerline. Moreover, the transversely aligned sections of adjacent corrugations, e.g. 22a and 22b, are displaced in the same direction, so that alternate sections 25 of adjacent corrugations are transversely aligned and alternate sections 26 are transversely aligned. The cavities 18 providing the boiling nucleation sites comprise the space covered by the

corrugation sections

25 and 26 so that the partially enclosing walls are substantially vertical. The restricted openings 15 of this composite metal structure are formed by transverse slitting and displacement of contiguous corrugation sections, so extend substantially the full height of each cavity with four such openings per cavity.

The FIG. 6 embodiment differs from the FIGS. l-4 embodiment in that the discrete raised sections 12 are not separated by unraised sections in the form of parallel rows. Instead the raised sections 12 are dimples in cover sheet 11, and may for example be formed by deforming sections of the flat sheet into perforation openings of an undersheet in any desired configuration. The restricted openings 15 may be provided in dimples 12 by piercing with a sharp pro-jection as for example a pin or punch. As illustrated, the openings 15 have been formed by downward piercing so as to outwardly and downwardly displace the hole edges into the cavity 18 formed by dimples 12. Although only one restricted opening per cavity is shown, it should be understood that multiple openings may be provided if desired. Moreover the openings may be provided in the slope or base portion of dimples instead of the tip (as illustrated).

The FIG. 7 embodiment differs from the FIGS. 1-6 embodiments in that cover sheet 11 is not deformed to provide discrete sections raised from the smooth surface metal substrate 17. lnstead, cavities 18 are formed in the flat cover sheet itself, as by chemically milling therein from opposite sides of the sheet to provide a restricted opening 15 intermediate the sides. It should be noted that chemical milling normally produces a substantially circular opening. Also, since the bottom side of cover sheet 11 bears directly against the top side of substrate 17 and is bonded thereto, only a relatively small number of fluid communication passages between adjacent cavities are likely to be formed. This means that the boiling in a particular cavity is likely to be intermittent rather than continuous, as occurs for the embodiments having either or both irregular noncircular restricted openings and a relatively large number of cavity-to-cavity fluid communication passages. In the FIG. 7 embodiment, liquid flows into a particular cavity through its restricted opening, is boiled to form vapor during which period very little (if any) additional liquid enters the cavity, and the resulting vapor bubbles are ejected through the restricted opening whereupon the sequence is repeated. This embodiment may be broadly described in terms of the cover sheet being bonded to the substrate and provided with a multiplicity of spaced venturi-type openings extending through the cover sheet cross-section with the larger cross-sectional portion adjacent to the substrate and forming the cavities, and the smaller cross-sectional portion communicating with the cover sheet outer surface and forming the restricted openings.

The invention and its unexpected advantages will be more clearly understood by the following examples.

EXAMPLE 1 A metal die having twenty uniformly spaced pyramid-shaped projections per lineal inch in each direction (400 projections per square inch) was pressed against 0.002 inch thick brass foil backed by a resilient rubber surface to initially form the discrete pyramid sections in the foil with walls sloping about 45, and then pierce the pyramid tip ends to form non circular openings with cracks or tears at the edges in an x-shaped or star-like configuration. The torn edges of these openings extended about one-half the distance down the angled walls of the pyramids from the tip toward the base and the adjoining valleys. The pitch P (valley-to valley) was about 0.050 inch and the effective diameter R of the pierced holes was about 0.008 inch. The width and height of the pyramid cavities were about 0.039 and 0.012 inches respectively. The ratio R/C was about 0.008/0.012 0.67 and the cavity density was 400 per square inch. The projections on the die were in the configuration of parallel rows with one group oriented at 90 degrees to another group, so the resulting deformed foil assumed the FIGS. 1-2 configuration of metal cover sheet 11.

The deformed and pierced foil was then brazed to a 0.035 inch thick smooth copper substrate sheet with the pierced tip ends of the pyramids pointing upwardly. This was accomplished by sprinkling a layer of brazing metal powder of about 0.002-0.003 inch thickness onto the substrate, positioning the deformed metal foil thereon and then heating the assembly in a brazing furnace. Accordingly, the valleys between the pyramids were the only areas where metal bonding to the sub strate occurred. Microscopic examination of the composite metal structure revealed that there was only a minor degree of interconnection between adjacent pyramid cavities due to the proximity of the intervening brazed valleys and the smooth substrate surface. That is, interconnection only existed in the areas where brazing was incomplete.

EXAMPLE 2 A composite metal structure was prepared from the same materials as Example 1 and using the same procedure, except that the pierced tip ends of the pyramids were positioned adjacent to the smooth metal substrate. It will however be apparent from FIGS. 3 and 4 that the effective diameter ofthe resulting pyramid cavities and the degree of fluid intercommunication are quite different from the Example 1 structure. The pierced tip ends were brazed to the smooth copper substrate and the braze metal substantially filled the central portion openings. The effective diameter R of the restricted openings is the width of the relatively long and narrow torn sections extending upwardly from the brazed central portion as illustrated in FIGS. 3-4, and averaged about 0.0036 inch. The cavity effective diameter C was about 0.016 inch so that the ratio R/C was about 0.22. The pitch P (center-to-center distance of adjacent brazed central portions) was 0.050 inch. The width of the pyramid cavities was about 0.035 inch and height was about 0.016 inch. There was free fluid interconnection between adjacent cavities. The cavity density was about 1,200 per square inch.

EXAMPLE 3 A composite metal structure was prepared from 0.005 inch thick copper foil using the same metal die as employed in Example 1 so that the cavity density was also 400 per square inch. However, this relatively thick foil was softer so that the die failed to punch the same size openings as in Example 1. Whereas the effective diameter R of the latters restricted openings was about 0.008 inch, the openings in Example 3 were only about 0.005 inch. Since the cavity effective diameter C was about 0.012 inch the ratio R/C was about 0.42. The resulting deformed and pierced foil was brazed to a 0.250 inch thick smooth copper substrate plate with the pierced tip ends pointing upwardly in the manner of FIGS 1 and 2 and Example 1. The thickness of the brazing powder layer prior to heating was about 0.003 inch. Microscopic examination revealed that many of the narrow comer tears and some of the crest tears were plugged with filler during brazing. This was due to the relatively small diameter of the pierced openings combined with an excess of braze metal.

EXAMPLE 4 A composite metal structure was prepared from 0.002 inch thick copper foil following the same procedure as Example 1, but a thicker layer of brazing metal powder was used on the substrate, i.e., about 0.003-0.004 inch instead of 0.002 inch. Microscopic examination of the composite metal structure revealed that too much braze powder had been used because a majority of pierced openings were completely filled with the brazed metal.

EXAMPLE 5 A composite metal structure was prepared from an 0.006 inch thick corrugated aluminum sheet and a 0.250 inch thick smooth, flat aluminum plate. The pitch P of the corrugations was about 0.165 inch (valley-to-valley) and their height was about 0.065 inch. At

0.125 inch intervals in the longitudinal direction, the corrugations were transversely notch-slit and alternate sections pushed inwardly. The resulting openings were about 0.036 inch wide and this dimension represents the effective diameter R of the spaced restricted openings according to this invention. The cavities were about 0.063 inch effective diameter so that the ratio R/C was about 0.57. The cavity density was about 48 per square inch. The corrugated and serrated aluminum sheet was then brazed to the flat impervious aluminum plate as the substrate using a brazing powder layer about 0.003 inch thick, and the structure closely resembled the FIG. embodiment. It should be noted that the side walls of the cavities were substantially vertical and the serrated slot openings thereto extended substantially the full height of the cavities.

EXAMPLE 6 A composite metal structure of the pierced hole dimpled sheet type (illustrated in FIG. 6) was prepared from 0.004 inch thick copper foil and 0.250 inch thick smooth flat copper plate. The dimples were formed by first placing the copper foil over a perforated undersheet, superimposing a rubber sheet on the copper foil and pressing downwardly on the rubber sheet, thereby deforming sections of the foil into the perforation openings of the undersheet in staggered rows. The pitch P (center-to-center) of the dimples was about 0.125 inch, the width of the dimple base was about 0.10 inch, and the dimple height was 0010-0015 inch. The dimpled foil was bonded to the smooth flat copper sheet by soft soldering at the valley sections. A 0.005 inch diameter needle was then used to downwardly pierce 1-3 holes per dimple through the dimple top. These restricted openings R were about 0.005 inch effective diameter with the outer edge of the surrounding wall downwardly indented as illustrated in FIG. 6. The cavity effective diameter C was about 0010-0015 inch so that the ratio RIC was 0.3-0.5. The cavity density was about 60 per square inch.

EXAMPLE 7 A composite metal structure of the flat cover sheet venturi-shaped restricted opening type (illustrated in FIG. 7) was prepared from a 0.015 inch thick copper cover sheet and a 0.250 inch thick smooth flat copper substrate. The venturi-type restricted openings were formed in the cover sheet by chemical milling in parallel rows at spacing of 33 openings per inch in both the longitudinal and transverse directions, so that the density was 1089 holes per inch surface area. The pitch P (center-to-center) was about 0.030 inch. In this particular experiment, the venturi configuration was formed by chemical milling from both the top and bottom and joining in intermediate section of the cover sheet. The opening in the top plane was about 0.013 inch diameter and in the bottom plane about 0.019 inch diameter, with an intermediate restricted section R of about 0.005-0.008 inch diameter. The cavity effective diameter was about 0.010 inch so that the ratio R/C was about 05-08. In contrast to the composite metal structures of Examples 1-6, the cross-section of the restricted openings was substantially circular due to the method of formation. The flat cover sheet having the venturi-shaped restricted openings was bonded to the copper substrate by coating a thin layer of metal brazing paste on the latter, superimposing the cover sheet and pressing it down with a small weight, then heating the assembly in a furnace.

Microscopic examination of cross-sections revealed that the resulting composite metal structure was unevenly bonded together. There were numerous areas where there was no metal bond between the cover sheet and the substrate and some of these gaps were 0001-0002 inch wide. This indicates the presence of many fluid communication passages between at least some of the cavities adjacently positioned to each other.

EXAMPLE 8 Another composite metal structure was prepared from the same materials and using the same procedure as Example 7, except that a thin layer of brazing powder and a porous fibrous layer between the cover sheet and the weight were used in an effort to eliminate the uneven bonding and fluid communication passages between adjacent cavities. Microscopic examination of cross-sections revealed that bonding was more uniform although there were a few gap areas where there was no metal bond. It was therefore concluded that a small number of .fluid communication passages between adjacent cavities may have existed.

EXAMPLE 9 Still another composite metal structure was prepared from the same materials and using the same procedure as Example 7 and 8, except that a. more uniform pressure holddown arrangement for the cover sheet was used. Microscopic examination of cross-sections indicated virtually complete and uniform brazing with only about 1.2 percent of the possible joint area unfilled with braze metal.

The Example 8 and 9 composite metal structures were studied to determine whether liquid could freely enter the isolated cavities. This was important to determine because the liquid reentry path employed by prior art nucleate boiling surfaces, i.e., fluid communication passages from adjacent inactive cavities, had been virtually eliminated. Also, the circular cross-section of the restricted openings to these cavities in the Example 8 and 9 composites are substantially filled by the vapor bubbles emerging therefrom, so that very little area is open for liquid ingress (in contrast to the irregular noncircular cross-sections of the Example l-6 openings prepared by metal piercing).

In this investigation, a reagent was used which reacted with the copper surfaces of the Example 7 and 8 composite structures to produce a coating of black CuO. The reagent, consisting of sodium hypochlorite in a basic solution, was added in concentrated solution to the liquid pool in which the composite structure had been immersed for boiling tests. The reagent was added after the stable boiling conditions had been attained. About 15 minutes was required to blacken the copper surfaces, whereupon the pool was dumped and the composite washed immediately in fresh water. Since the reagent could only act on surfaces in contact with the liquid phase, any areas which remain dry during boiling should not show the presence of the oxide layer.

When the Example 8 composite was cross-sectioned, oxide was discovered inside all of the cavities including those in areas where the brazed joint was good so as to isolate a given cavity from adjacent cavities. The oxide layer appeared to be as thick inside these isolated cavities as in the unisolated cavities, indicating that liquid had easily penetrated the former through the circular restricted openings.

It was also observed that the restricted openings of the Example 8 composite were not perfectly circular and were not smooth-edged. To determine whether a smooth-edged perfectly circular opening would result in a dry cavity which would be sealed from liquid by the vapor bubble, a block of 36 restricted openings of the Example 9 composite were reamed by insertion of a 0.008 inch diameter wire. After the boiling test in the CuO-containing liquid, the Example 9 composite was cross-sectioned and the oxide coating was found in equal thickness in the cavities joined to the reamed and unreamed openings. These tests demonstrate that (l) interconnections between adjacent cavities are not essential, and (2) smooth-edge perfectly circular openings do not prevent liquid ingress to the sub-surface cavities.

EXAMPLE 10 The Example l6 composites were each tested to determine the water boiling performance at various heat fluxes and 1 atmosphere pressure, and the results, of these tests are plotted in the FIG. 8 graph. The numerals identifying the various curves correspond to the aforelisted examples, and the dotted line indicates the performance of a smooth surface for comparison. It will be readily apparent from FIG. 8 that each of the Example 1-6 composites is greatly superior to a smooth surface. That is, the temperature difference (A T) required for a given heat flux using the Example l-6 composites is only a small fraction of that required using a smooth surface. The performance of these composites may be compared at the same heat flux, as for example 20,000 Btu/hr.- F., and is summarized in' EXAMPLE 11 The Example 7-9 composites were also tested to determine the water boiling performance of the flat cover sheet-venturi type restricted opening embodiment at various heat fluxes and 1 atmosphere pressure. The results of these tests are plotted in FIG. 9 graph along with the smooth surface (dotted line) for comparison. It is also apparent that each of the Example 7-9 composites is greatly superior to a smooth surface as indicated by the 20,000 Btu/hr. F. heat flux boiling coefficients in Table C.

TABLEC cavity effec- Boiling inter tive Cav- Co- Comcom- Cavdiameters(l l0) ities efficient posmuniity in. restricted per Btu/hrite cation (C) opening(R) R/C in ft"-F 7 Many 10 5-8 0.5-.8 1,089 5,500 8 Small 10 5-8 05-8 1,089 7,100

number 9 None 10 5-8 0.5-.8 1,089 5,300 Smooth None None None None None 620 EXAMPLE 12 The outstanding performance of the composite metal structures for boiling liquids at room temperature was demonstrated using Examples 4, 7 and 9 to boil fluorotrichloromethane at one atmosphere (boiling point of 738 F.) and various heat fluxes. The performances are plotted in FIG. 10 and a smooth surface is included for comparison. A comparison at heat flux of 20,000 Btu/hr. F. is summarized in Table D.

Comparison of the boiling coefficients in Tables B-D with the corresponding values for the prior art high performance nucleate boiling surfaces reveals that the performances are similar. However, based on equivalent surface area, the cost to manufacture the pyramidshaped composite metal structures of this invention is only about one-third that of the deformed groove type described in Kun et al. US. Pat. No. 3,454,081.-

Although preferred embodiments of this invention have been described in detail, it is contemplated that modifications of the structures and the forming method may be made and that some features may be employed without others, all within the spirit and scope of the invention.

What is claimed is:

l. A boiling to heat transfer surface formed from a composite metal structure comprising an impervious smooth surface metal substrate and an impervious metal cover sheet bonded to said metal substrate and having at least 25 cavities per inch of substrate surface with each cavity having an effective diameter of at least 0.003 inch, and a multiplicity of spaced restricted openings extending through said cover sheet in fluid communication with said cavities and having effective diameter such that the effective diameter ratio of restricted openings to cavities is less than 0.8.

2. A boiling to heat transfer surface formed from a composite metal structure according to claim 1 wherein said cover sheet is shaped to form a multiplicity of discrete sections raised from said metal substrate as said cavities.

3. A boiling to heat transfer surface formed from a composite metal structure according to claim 1 wherein said cover sheet is shaped to form a multiplicity of discrete sections raised from said metal substrate as said cavities, with each raised section surrounded by unraised sections bonded to said cover sheet.

4. A boiling to heat transfer surface formed from a composite metal structure according to claim 1 wherein said cover sheet is shaped to form a multiplicity of discrete sections raised from said metal substrate as said cavities, and said restricted openings have irregular non-circular cross-sections.

5. A boiling to heat transfer surface formed from a composite metal structure according to claim 1 wherein each cavity has an effective diameter of at least 0.006 inch.

6. A boiling to heat transfer surface formed from a composite metal structure according to claim 4 wherein said discrete sections are pyramid-shaped with the tip end of each pyramid pointing downwardly toward the substrate surface, and at least one of said restricted openings extends through each tip end.

7. A boiling to heat transfer surface formed from a composite metal structure according to claim 4 wherein said discrete sections are pyramid-shaped with the tip end of each pyramid pointing upwardly away from the substrate surface and at least one of said restricted openings extends through each tip end.

8. A boiling to heat transfer surface formed from a composite metal structure according to claim 2 wherein said cover sheet is shaped to provide fluid communication passages between at least some of said cavities adjacently positioned to each other.

9. A boiling to heat transfer surface formed from a composite metal structure according to claim 1 wherein said cover sheet is metal foil shaped to form a multiplicity of discrete pyramid sections raised from said metal substrate as said cavities with the tip end of each pyramid pointing upwardly away from the substrate surface, holes of irregular non-circular cross-section pierced through said tip end of each pyramid comprise said restricted openings, and the unraised sections of said foil surround said pyramid sections with the foil bottom side bonded to the substrate.

10. A boiling to heat transfer surface formed from a composite metal structure according to claim 1 wherein said cover sheet is metal foil shaped to form a multiplicity of discrete pyramid sections raised from ii'is i i81l $ieilf8ifh th fi$ %2 l ii pyramid, being positioned with said tip ends pointing downwardly toward said substrate and the tip end bonded thereto with unbonded gaps between said tip end and said substrate forming said restricted openings, and the space bounded by said substrate, the shaped foil and the pyramid tip ends forming said cavities.

1 1. A boiling to heat transfer surface formed from a composite metal structure according to claim 1 wherein said cover sheet comprises a sheet having a series of corrugations arranged in longitudinal rows parallel to each other with separating valleys between adjacent corrugations bonded to said substrate, each corrugation having transverse slits to form corrugation sections with longitudinally alternate sections in each corrugation being transversely displaced in the same direction and the transversely aligned sections of adjacent corrugations being displaced in the same direction.

12. A boiling to heat transfer surface formed from a composite metal structure according to claim 1 wherein said cover sheet is bonded to said substrate and is provided with a multiplicity of spaced venturitype openings extending through the cover sheet crosssection with the larger cross-sectional portion adjacent to said substrate and forming said cavities, and the smaller cross-sectional portion communicating with the cover sheet outer surface and forming said restricted openings.

13. A boiling to heat transfer surface formed from a composite metal structure according to claim 1 wherein the effective diameter ratio of restricted openings to cavities is less than 0.7.

14. A method for forming a boiling to heat transfer surface formed from a composite metal structure comprising: providing a die having at least 25 spaced pyramid shaped projections per inch of die surface area and pressing same against a thin metal foil backed by a resilient surface to initially form discrete pyramid shaped sections in the foil; piercing the tip ends to form holes of irregular non-circular cross-section; and bonding the shaped foil to a smooth surface metal substrate.

15. A method according to claim 14 wherein the pierced pyramid tip ends are positioned upwardly away from said substrate, and the unraised sections of the foil bottom side are bonded to said substrate with said holes forming restricted openings to cavities formed by said pyramid shaped sections such that. the cavities have an effective diameter of at least 0.003 inch and the effective diameter ratio of said restricted openings to said cavities is less than 0.8.

16. A method according to claim 14 wherein the pierced pyramid tip ends are positioned downwardly toward said substrate and bonded thereto with unbonded gaps between said tip ends and said substrate forming restricted openings to cavities formed by said pyramid shaped sections such that. the cavities have an effective diameter of at least 0.003 inch and the effective diameter ratio of said restricted openings to said cavities is less than 0.8.

Claims

2. A boiling to heat transfer surface formed from a composite metal structure according to claim 1 wherein said cover sheet is shaped to form a multiplicity of discrete sections raised from said metal substrate as said cavities.
3. A boiling to heat transfer surface formed from a composite metal structure according to claim 1 wherein said cover sheet is shaped to form a multiplicity of discrete sections raised from said metal substrate as said cavities, with each raised section surrounded by unraised sections bonded to said cover sheet.
4. A boiling to heat transfer surface formed from a composite metal structure according to claim 1 wherein said cover sheet is shaped to form a multiplicity of discrete sections raised from said metal substrate as said cavities, and said restricted openings have irregular non-circular cross-sections.
5. A boiling to heat transfer surface formed from a composite metal structure according to claim 1 wherein each cavity has an effective diameter of at least 0.006 inch.
6. A boiling to heat transfer surface formed from a composite metal structure according to claim 4 wherein said discrete sections are pyramid-shaped with the tip end of each pyramid pointing downwardly toward the substrate surface, and at least one of said restricted openings extends through each tip end.
7. A boiling to heat transfer surface formed from a composite metal structure according to claim 4 wherein said discrete sections are pyramid-shaped with the tip end of each pyramid pointing upwardly away from the substrate surface and at least one of said restricted openings extends through each tip end.
8. A boiling to heat transfer surface formed from a composite metal structure according to claim 2 wherein said cover sheet is shaped to provide fluid communication passages between at least some of said cavities adjacently positioned to each other.
9. A boiling to heat transfer surface formed from a composite metal structure according to claim 1 wherein said cover sheet is metal foil shaped to form a multiplicity of discrete pyramid sections raised from said metal substrate as said cavities with the tip end of each pyramid pointing upwardly away from the substrate surface, holes of irregular non-circular cross-section pierced through said tip end of each pyramid comprise said restricted openings, and the unraised sections of said foil surround said pyramid sections with the foil bottom side bonded to the substrate.
10. A boiling to heat transfer surface formed from a composite metal structure according to claim 1 wherein said cover sheet is metal foil shaped to form a multiplicity of discrete pyramid sections raised from said metal substrate with holes of irregular non-circular cross-section pierced through the tip end of each pyramid, being positioned with said tip ends pointing downwardly toward said substrate and the tip end bonded thereto with unbonded gaps between said tip end and said substrate forming said restricted openings, and the space bounded by said substrate, the shaped foil and the pyramid tip ends forming said cavities.
11. A boiling to heat transfer surface formed from a composite metal structure according to claim 1 wherein said cover sheet comprises a sheet having a series of corrugations arranged in longitudinal rows parallel to each other with separating valleys between adjacent corrugations bonded to said substrate, each corrugation having transverse slits to form corrugation sections with longitudinally alternate sections in each corrugation being transversely displaced in the same direction and the transversely aligned sections of adjacent corrugations being displaced in the same direction.
12. A boiling to heat transfer surface formed from a composite metal structure according to claim 1 wherein said cover sheet is bonded to said substrate and is provided with a multiplicity of spaced venturi-type openings extending through the cover sheet cross-section with the larger cross-sectional portion adjacent to said substrate and forming said cavities, and the smaller cross-sectional portion communicating with the cover sheet outer surface and forming said restricted openings.
13. A boiling to heat transfer surface formed from a composite metal structure according to claim 1 wherein the effective diameter ratio of restricted openings to cavities is less than 0.7.
14. A method for forming a boiling to heat transfer surface formed from a composite metal structure comprising: providing a die having at least 25 spaced pyramid shaped projections per inch2 of die surface area and pressing same against a thin metal foil backed by a resilient surface to initially form discrete pyramid shaped sections in the foil; piercing the tip ends to form holes of irregular non-circular cross-section; and bonding the shaped foil to a smooth surface metal substrate.
15. A method according to claim 14 wherein the pierced pyramid tip ends are positioned upwardly away from said substrate, and the unraised sections of the foil bottom side are bonded to said substrate with said holes forming restricted openings to cavities formed by said pyramid shaped sections such that the cavities have an effective diameter of at least 0.003 inch and the effective diameter ratio of said restricted openings to said cavities is less than 0.8.
16. A method according to claim 14 wherein the pierced pyramid tip ends are positioned downwardly toward said substrate and bonded thereto with unbonded gaps between said tip ends and said substrate forming restricted openings to cavities formed by said pyramid shaped sections such that the cavities have an effective diameter of at least 0.003 inch and the effective diameter ratio of said restricted openings to said cavities is less than 0.8.