US3196936A - Heat dissipating structure - Google Patents

Description

y 1965 c. A. E. BEURTHERET 3,196,936

HEAT DISSIPATING STRUCTURE Filed April 23, 1963 2 Sheets- Sheet l Inventor 6% 441mm 6mm y 1965 c. A. E. BEURTHERET 3,196,936

HEAT DISSIPATING STRUCTURE 2 Sheets-Sheet 2 Filed April 23, 1963 I we to 2 n y a Attorneys United States Patent 3,196,936 HEAT DllSSlPATING STRUCTURE Charles Alphonse Emile Beurtheret, Saint Germain en lLaye, France, assignor to Conipagnie Francaise Thomson-Houston, Paris, France, a French hotly corporate Filed Apr. 23, 1963, Ser. No. 275,126 iaims priority, application France, Apr. 27, 1962,

1,334,976 13 Claims. (Cl. 165-47) This invention relates to heat dissipating structures of the kind comprising a heat transfer partition wall having one of its sides exposed to a source of heat to be dissipated, and its opposite side in contact with a vaporizable liquid, and wherein the heat absorbed in vaporizing the liquid assists in dissipating the heat fiux from the surface of the wall.

Heat dissipating structures of the evaporation type as broadly defined above have many advantages owing to the high rate of heat dissipation achievable with them and unattainable with other, more conventional types of heat exchanger. The vaporizable liquid used is generally water, which is desirable owing to the large heat of va porization characterizing that liquid, as Well as its ready availability.

Such evaporation heat dissipating systems have been applied for cooling purposes in many fields of engineering, and do not raise any serious problems so lOng as they operate at a given temperature on the heat transfer wall. In certain applications however, in which the heat source imposes the heat flux rather than the temperature of the Wall extremely troublesome problems are found to arise due to an irreversible overheating of the wall accompanied by hot spot formation liable to cause destructive damage to the partition wall. These effects are ascribable to the so-called spheroidal state of ebullition or film-boiling in the liquid, a form of vaporization which tends to set in whenever the surface temperature of the wall exceeds a certain critical temperature in any area, whereupon a film of vapour develops along the surface in that area between the Wall surface and the body of liquid, enabling the temperature thereafter to increase inordinately to values of 1000 C. or more. The critical temperature at which these effects are initiated can be qiute low, being only about 125 C. in the case of a copper heat dissipating wall with water as the vaporizable liquid.

The applicant has for many years been engaged in the development of evaporation-cooling devices, more especially in connection with the anode cooling of the highpower electron discharge apparatus in which the above noted problems have arisen with great acuteness and had previously gravely limited the maximum power ratings in which such apparatus could be built. In a number of prior patents of the applicant, particularly United States Patents 2,935,305, 2,935,306, 2,969,957 and 3,046,928, means have been disclosed for retarding or preventing the inception of spheroidal state ebullition (or film-boiling) and hot spot formation and thereby greatly enhancing the effectiveness of evaporation-cooling systems over what was previously possible. A common concept underlying the advances taught in those prior patents has been to provide the heat dissipating surface in such a manner, in regard to its geometrical and other characteristics, that said surface becomes the locus of steady stable temperature gradients the range of which encompasses temperatures both below and above the critical or burn-out point. In other words, instead of attempting to maintain the heat-exchange surface so far as possible isothermal at a temperature just below said critical or burn-out point (leading in practice to an inherently unstable situation),

31,1 Qfifiib Patented July 27, 1965 the technique disclosed in the aforementioned patents has involved deliberately creating in the surface a continuous range of temperatures extending from well below to well above said critical point, and it was discovered that this could be done in such a manner that the temperatures then become completely stabilized, that is, there is no longer any tendency for the surface temperature to shoot up locally to inordinately high values wherever temperature exceeded the critical point (e.g. C.) as was previously the case. Briefly, the means for creating the desired continuous temperature gradients according to the earlier patents indicated above, have comprised providing the partition wall, on the side thereof exposed to the vaporizable fluid, with heat dissipating protuberances of prescribed, relatively thick dimensioning preferably projecting a sufficient distance from the surface to penetrate into the liquid region beyond the sheet of vapour adjacent the surface. In a heat dissipating structure according to the above-mentioned earlier patents, the parts of the surface positioned in those regions of the stable temperature gradients, below the critical point were subjected to ordinary, so-called nuclear (or bubbling) ebullition of the liquid, and the parts of the surface lying in the regions above the critical point were subject to the so-called transitional range of vaporization, and both areas including the transitional area, are stable to such a degree that the entire surface participates effectively in transferring heat at a high rate towards the li uid, without any liability to the formation of destructive hot spots as was invariably the case whenever the transitional and spheroidal-state vaporization (i.e. film-boiling) conditions were approached clue to the critical temperature being attained at any point in prior evaporation-cooling systems.

The present invention is a result of continued work in the same field of research and is based on the finding that the stability of the temperature gradient over an evaporation-cooling surface of the non-isothermal character referred to above, can be efiectively improved by the provision of certain geometrical relationships between the opposite surfaces of the partition Wall that are respectively exposed to the source of heat and to the vaporizable fluid.

Objects of this invention therefore are to provide an improved heat dissipating structure of the evaporation type, and more specifically to improve the effectiveness of structures of this general character wherein the heat dissipating surface is non-isothermal, but is the locus of steady temperature gradients encompassing temperature ranges both below and above the critical point of the vaporizable liquid under the conditions involved. An object is to provide a novel way, both simple and effective of creating a tangential temperature gradient along the heat-output surface of an evaporation-cooling partition wall.

According to the invention, an evaporation cooling structure comprising a partition wall having on one side a so-called heat-input surface exposed to a source of heat to be dissipated and having on its opposite side a so-called heatoutput surface contacted by a vaporizable fluid and provided with heat dissipating formations is characterized in that at least one end of the heat output surface is displaced or offset with respect to the corresponding end of the heat-input surface, in a direction generally parallel to both surfaces. Otherwise stated, the two surfaces on the opposite sides of the partition wall, one of which is the heat-input surface effectively exposed to the heat to be dissipated, and the other of which is the heat-output surface provided with the heat dissipating formations and is exposed to the vaporizable fluid, are bounded by respective pairs of parallel end planes which, at least at one end of the partition wall, are non-coincident.

The general result of this offset or displacement between the opposite surfaces of the partition wall which constitute the heat-input and the heat-output surfaces of the cooling structure, is that the paths of heat flow through the surface are oblique and/or curved rather than being normal to the surfaces as would otherwise be the case, and moreover differ in length over the extent of the wall. It has been found at later explained in detail, that with such an arrangement the overall temperature gradient at the heat-output surface can be made considerably more extensive and stable than heretofore, therefore correspondingly increasing the cooling rate achievable.

Preferably according to the invention, the offset or displacement between the two surfaces is in the same direction at both ends of the partition wall, i.e. in the case of a vertical wall for example both the upper and lower ends of the heat-output surface may lie above (or below) the upper and lower ends of the heat-input surface, respectively. "Preferably also, the lengths of the two surfaces are unequal, with the heat-output surface being longer than the heat-input surface. Furthermore, the two surfaces preferably overlap.

It will be understood that in most practical embodiments of heat dissipating structures of the general class considered, as described in the prior patents noted above, the structure has general symmetry of revolution about an axis, with the partition wall being e.g. cylindrical. The heat source, such as theanode surface of an electron discharge tube, is positioned inside the cylindrical wall, while the outer surface of the wall is formed with heat dissipating projections and exposed to the vaporizable liquid. Heretofore, the heat source, such as the anode surface of the tube, was coextensive along the axial dimension with the inner surface of the cylindrical partition wall and the outer heat dissipating formations as well as the body of liquid were substantially coextensive axially with the outer surface of the cylinder. Thus in such prior structures, the heat-input and heat-output surfaces were themselves substantially coextensive rather than being offset or displaced at one end at least of the wall, i.e. the lower or/ and the upper end, as is required according to the main teaching of the present invention. In the prior generally cylindrical form of structure just mentioned, the heat flux paths through the cylindrical wall were substantially radial, or normal to the axis of the cylinder, and were all substantially equal in length.

An exemplary embodiment of the invention will now be described for purposes of illustration but not of limitation with reference to the accompanying drawings, wherein:

FIGURE 1 is a simplified view partly in vertical elevation and partly in axial sect-ion with parts broken away, showing a high-power space charge controlled electron discharge tube with its heat dissipation structure constructed in accordance with this invention, and

FIGURE 2 is a diagrammatic sectional view corresponding to a part of FIGURE 1 and wherein a typical pattern of isotherm curves as obtained in the heat dissipating wall according to the invention is shown.

The electron tube schematically shown in FIGURE 1 as embodying a preferred form of the invention is a highpower tetrode suitable for operation at very high frequencies. The tube per se is relatively conventional and need not be described in detail. The tube includes an anode surface 8 of annular cylindrical shape coaxial with the general axis of circular symmetry YY of the tube structure, and constituting part of the inner cylindrical surface of a heat dissipating wall structure 1 according to the invention in the form of a generally cylindrical casing having a thickened lower part of reduced inner diameter providing an inner cylindrical surface constituting the anode 8 and providing at the same time the heat-input surface of the cooling structure. The parallel boundary planes of this surface spaced along its axial dimension YYare indicated at A and B respectively. The outer, heat-output surface on the partition wall structure or casing 1 comprises a generally cylindrical, though somewhat tapered, upper portion bounded along its axial dimension by the end planes indicated as C and D, and constituting the heat dissipating or heat-output surface proper, provided with the dissipating projections such as 6a, 6b. These projections, in the example shown, are constructed in accordance With one of the embodiments disclosed in the applicants earlier patents, and are in the form of thick axial splines or ribs formed with castellations such as 651, 6b therein. Below the lower boundary plane D of the heat-output surface is an annular flange 1314 projecting from the casing structure 1 for reasons later described, and below this flange the outer surface of the casing structure 1 tapers downwards and inwards to meet with the lower end of the inner surface at the end plane B.

It will be clear from the drawing that the heat-input surface AB and the heat-output surface CD are axially displaced or offset with respect to each other at both the upper and lower ends of the partition wall 1, and moreover, in this embodiment, the output surface CD is substantially longer in axial extent than is the input surface AB.

The outside of the casing structure 1 above the flange 13-14 is surrounded by a hood 16 forming an enclosure for a body of water 17, constituting a so-called boiler type cooler. The bottom of enclosure 16 is desirably welded to the upper surface of the outer part 14 of the aforementioned flange, which part is preferably made from a metal or alloy of poor heat conductivity for reasons later given. Welded to the underside of the outer flange part 14 is an inturned transverse metal flange 15 the inner circumference of which is secured to the top of an annular insulating Wall member 5. Secured to the lower end of insulating ring 5 is a ring-shaped conductor 2 constituting a connector terminal for the screen grid (not shown) of the tube. Supported below this connector ring 2, and insulated therefrom as well as between one another by way of suitable ring members of insulating material, are two further annular conductor members 3 and 4 of progressively decreasing radius and respectively serving as terminal connectors for the control grid and the cathode (not shown) of the tube.

It may be noted that for considerations having to do with electronics and not per se involved with the present invention, it is found desirable to provide the annular anode surface 8 with a relatively small height/radius ratio and, further, to position the anode surface as close as practicable to the terminal conductor rings 2, 3 and 4, so

far as permitted by the indispensable insulating wall 5.-

These considerations are in part responsible for the particular shape selected herein for the heat dissipating wall structure 1, and the electronic conditions just indicated are Well fulfilled by the downward axial displacement imparted to the heat-input surface AB relatively to the heatoutput surface CD in accordance with the present embodiment of the invention.

In the operation of the electron tube, the annular anode surface 8 is heated through electronic bombardment of its inner surface from the cathode, not shown, so that this heat flux is entirely applied to the inner, or heat-input, cylindrical surface AB of the cooling structure 1. Owing to the axial offset present between this surface and the heat-output surface CD provided with its dissipating projections, the heat flux through the partition wall 1 travels over paths which extend outwardly and upwardly, at an oblique angle to the direction of axis YY instead of extending radially and normally to that axis as would be the case were the input and output surfaces AB and CD directly opposite one another, as in prior art devices of similar general type. Referring to FIGURE 2, which shows a cross section of the cooling structure 1 separately, it will be seen that a typical pattern of isothermal surfaces that will be set up through the wall, comprises surfaces that are generally tapered or curved with a downwardoutward curvature. The lines of thermal flux, which are generally orthogonal to the isothermal surfaces, are thus seen to extend obliquely up and out and to increase progressively in length from the lower to the upper end of the wall 1, instead of being radial and all substantially equal in length as would be the case in the earlier type of partition wall wherein the heat-input surface AB and the heat-output surface CD are directly opposite each other and axially coextensive.

The pattern of isotherms shown in FIGURE 2 is a typical pattern observed when the body of fluid 17 surrounding the wall is boiling water at atmospheric pressure, i.e. at 100 C. temperature. Over the heat-output surface CD there is seen to be established a continuous longitudinal temperature gradient, free of any sharp transitions, with the coolest area being at the upper boundary plane C, where the temperature is about 105 C. and the hottest area at the lower boundary plane D, where the temperature is about 300 C. As indicated by the 125 C. isotherm in the figure, the so-called critical temperature, which is about 125 C. for water under the indicated conditions in the case of a copper wall, occurs somewhere along the length of the surface CD nearer the upper end C, but owing to the completely stable character of the temperature gradient above and below this isotherm, encompassing temperatures below and above the critical range, there is no tendency to spheroidal-state ebullition (film-boiling) of the water and accompanying hot spot formation. Over prolonged periods of highpower operation of the electron tube described, steady, stable boiling of the water 16 occurs around the outersurface of the wall casing 1, with the boiling proceeding in the so-called nuclear (or ordinary bubbling) phase over the upper part of the casing where the temperatures are below the critical point 125 C., and in the so-called transitional phase over the remaining part of the casing surface where the temperatures are above the critical point, with the entire surface participating in the heat transferring function.

Comparing once again the improved heat dissipating wall structure 1 described herein with a similar structure according to the teachings of the applicants earlier patents, wherein the heat-input surface AB and the heat-output surface CD would be axially-coextensive cylindrical surfaces, it will be noted that with such an earlier structure, whereinthe lines of heat flux are radial linesnormal to the axis YY and all of equal length each element of the heat dissipating projections provided at the output surface CD would act to dissipate the elementary quantities of heat retransmitted theerto from the elementary area of the heat input surface AB positioned directly opposite to it. Owing to the radial configuration of the heat flux lines thus obtained, the isothermal surfaces along the axial length of the heat-output surface corresponding to CD would be substantially cylindrical, and there would be no material longitudinal temperature gradient occurring along the length of the surface. In such an earlier structure, the temperature gradients would only be present along the sides of the dissipating projections, such as ribs, knobs, or the like, in directions extending radially away from the surface CD. Since as explained above it is the presence of the temperature gradient which achieves the beneficial stabilization of ebullition and prevents hot spot formation in cooling apparatus of the general class to which the invention relates, it will be seen that in the earlier structures only the side surfaces of the dissipating projections participated actively in the stabilizing function. With the axially-offset cooling wall structure of this invention, on the other hand, owing to the oblique and unequal paths of heat transfer and the consequent pattern of isotherms which leads to the establishment of a substantial longitudinal temperature gradient throughout the axial extent of the heat-output surface CD, the entire heatoutput surface including the valleys between the ribs, knobs or other dissipating protuberances used, now participates actively in the stabilizing function, so that considerably larger heat output rates can be effectively dissipated.

As indicated in FIGURE 2, the lower areas of the anode or heat-input surface AB are apt to attain relatively high temperatures, e.g. 600 C. or more, as against 400 C. at thetop of said surface. While steady temperatures of this order are not per se objectionable, substantial differential expansion can occur along the axial length of the anode surface, and to allow for this it is preferred according to the invention to provide narrow radial expansion slots such as 7a, 7b, whereby the anode surface will be able to take up such differential expansion forces Without creating dangerously high mechanical strains in the outer parts of the cooling wall structure 1 towards the heat-ouput surface CD. The slots 7 may be e.g. twelve in number around the anode and each about 1 mm. wide.

Further, because of the comparatively high temperatures, of the order of 300 C., present towards the base D of the heat-output surface, it is advantageou in accordance with a feature of the invention to provide the said base port-ion with the aforementioned radial flange 13, preferably having an outer peripheral part 14 made from a metal having low heat conduction welded thereto. The peripheral area of the flange is thus maintained substantially at the temperature of the liquid 17, as indicated by the progressively decreasing isotherms indicated in FIGURE 2 towards the radially outer end of the flange 13-44. The flange as earlier mentioned convenient-1y serves as a means for supporting the boiler enclosure wall 16 on its upper surface, and the transverse bottom wall 15 connected to its under surface.

It will thus be apparent that the present invention provides an important improvement in evaporation-cooling systems of the non-isothermal type taught by the applicants earlier patents. By the relatively simple and unexpected expedient of making the heat-input and heatoutput surfaces on the opposite sides of the partition wall non-coextensive axially, and thus deflecting the paths of heat transfer through the wall from their otherwise generally radial configuration, a notable axial temperature gradient can be established along the length of the heatoutput surface, in addition to any temperature gradients present along the sides of the heat-dissipating protuberances provided at said surface in accordance with the earlier teachings. This axial or longitudinal temperature gradient along the length of the wall greatly enhances the effectiveness of the ebullition-stabilization effect on which cooling systems of the general type considered rely, ind permits the effective dissipation of larger amounts of eat.

Clearly the invention is applicable to apparatus differing greatly in form and construction from the exemplary embodiment shown herein. The offset or displacement between the heat-input and heat-output surfaces may be provided at only one end of the partition wall. The wall structure is not necessarily of revolution as herein shown, but may be flat. The dissipating projection-s at the heatoutput surface may take any of the various shapes described in the aforementioned patents, or still other forms. The applicability of the invention is of course not restricted to electrical apparatus but extends to all fields of engineering involving the production of heat requiring dissipation, e.g. heat engines, nuclear reactors, etc.

I claim:

1. An evaporation cooling structure comprising a source of heat to be dissipated, a partition wall having on one side a heat-input surface exposed to said source of heat and having on its opposite side a heat-output surface provided with heat-dissipating formations, and means for retaining a body of vaporizable liquid in contact with said heat-output surface and formations thereon, and wherein said heat-output surface has at least one of its ends displaced a substantial distance with respect to the corresponding end of the heat-input surface in a direction generally parallel to both surfaces whereby to establish along the length of said heat-output surface a substantial temperature gradient which will effectively stabilize the vaporization conditions of said fluid over the extent of said heat-output surface.

2. In an evaporation cooling structure comprising a source of heat to be dissipated, a partition wall having symmetry of revolution about an axis and having coaxial generally cylindrical surfaces, the inner one of said surfaces exposed to said source of heat, heat dissipating projections formed on the outer one of said surfaces, and means for retaining a body of vaporizable liquid in contact with said outer surface and projections thereon, that improvement wherein said outer cylindrical surface has at least one of its ends substantially displaced a substantial distance with respect to the corresponding end of the inner cylindrical surface in a direction parallel to said axis whereby to establish along the axial length of said outer cylindrical surface a substantial temperature gradient which will effectively stabilize the vaporization conditions of said fluid over the extent of said surface.

3. Structure as claimed in claim 1, wherein said heatinput surface has at least one of its ends positioned beyond the corresponding end of the heat output surface.

4. Structure as claimed in claim 1, wherein both ends of the heat-output surface are displaced in the same sense with respect to the heat-input surface.

5. Structure as claimed in claim 1, wherein the heatoutput surface is longer than the heat-input surface as measured parallel to said direction.

6. Structure as claimed in claim 2, wherein said outer surface is provided with an outwardly projecting annular flange.

7. Structure as claimed in claim 2, wherein said inner cylindrical surface has one of its ends positioned axially beyond the corresponding end of the outer cylindrical surface, and a radially outwardly projecting annular flange is provided around said outer surface adjacent said corresponding end thereof.

8. Structure as claimed in claim 2, wherein said outer surface is provided with a radially outwardly projecting annular flange, at least a radially outermost circumferential portion of said flange being made of a metallic material having substantially lower heat conductivity than the remainder of said structure.

9. Structure as claimed in claim 1, wherein the heat input surface is formed with expansion slots therein.

10. In a vapor cooling system comprising an electron discharge device having an anode comprising an internal generally cylindrical surface forming a part of the envelope of the device, and having a radiator comprising an external generally cylindrical surface forming a part of the envelope of the device and coaxial with said internal anode surface, heat-dissipating projections on said external radiator surface, a coolant jacket enclosing said radiator and spaced from said external surface and said projections thereon, and means for maintaining a body of vaporizable liquid within the jacket in contact with said external radiator surface and said projections thereon, the improvement wherein said external radiator surface is axially displaced from said, internal anode surface a substantial distance along the common axis of said surfaces so as to establish a substantial temperature gradient along the axial length of said radiator surface which will effectively stabilize the vaporization conditions of said liquid throughout the axial extent of the radiator surface.

11. In a vapor cooling system comprising an electron discharge device having an anode comprising an internal generally cylindrical surface forming a lower end part of the envelope of the device, and having a radiator comprising an external generally cylindrical surface forming another part of the envelope of the device and coaxial with said internal anode surface, heat-dissipating projections on said external radiator surface, a coolant jacket enclosing said radiator and spaced from said external radiator surface and said projections thereon, and means for maintaining a body of vaporizable liquid within said jacket in contact with said external radiator surface and said projections thereon, the improvement wherein said external radiator surface is axially displaced from said internal anode surface a substantial distance in an axially upward direction along the common axis of said surfaces towards an upper end part of the envelope of the device, whereby to establish a substantial temperature gradient along the axial length of said radiator surface which will effectively stabilize the vaporization conditions of said liquid throughout the axial extent of the radiator surface.

12. A vapor cooling system comprising an electron discharge device having an anode comprising an internal generally cylindrical surface of relatively small height diameter ratio forming a lower end part of the envelope of the device, and having a radiator comprising an external generally cylindrical surface forming another part of the envelope of the device and coaxial with said internal anode surface, heat-dissipating projections on said external radiator surface, a coolant jacket enclosing said radiator and spaced from said external radiator surface and said projections thereon, and means for maintaining a body of vaporizable liquid within said jacket in contact with said external radiator surface and projections, said external radiator surface being axially displaced from said internal anode surface a substantial distance in an axially upward direction along the common axis of said surfaces towards an upper part of the envelope of the device, and being substantially longer in axial extent than said internal anode surface, whereby to establish an extensive temperature gradient along the axial length of said external radiator surface which will effectively stabilize the vaporization conditions of said liquid throughout the axial length of said radiator surface.

13. The system defined in claim 11, wherein said outer radiator surface has a radially outwardly projecting annular flange adjacent a lower end thereof, and said coolant jacket has its bottom end attached to an upper surface of said flange.

References Cited by the Examiner UNITED STATES PATENTS 2,829,870 4/58 Poppe 165-80 2,879,977 3/59 Trought 165-47 2,969,957 1/61 Beurtheret l80 ROBERT A. OLEARY, Primary Examiner.

KENNETH W. SPRAGUE, CHARLES SUKALO,

Examiners.

Claims (1)

1. AN EVAPORATION COOLING STRUCTURE COMPRISING A SOURCE OF HEAT TO BE DISSIPATED, A PARTITION WALL HAVING ON ONE SIDE A HEAT-INPUT SURFACE EXPOSED TO SAID SOURCE OF HEAT AND HAVING ON ITS OPPOSITE SIDE A HEAT-OUTPUT SURFACE PROVIDED WITH HEAT-DISSIPATING FORMATIONS, AND MEANS FOR RETAINING A BODY OF VAPORIZABLE LIQUID IN CONTACT WITH SAID HEAD-OUTPUT SURFACE AND FORMATIONS THEREON, AND WHEREIN SAID HEAT-OUTPUT SURFACE HAS AT LEAST ONE OF ITS ENDS DISPLACED A SUBSTANTIAL DISTANCE WITH RESPECT TO THE CORRESPONDING END OF THE HEAT-INPUT SURFACE IN A DIRECTION GENERALLY PARALLEL TO BOTH SURFACES WHEREBY TO ESTABLISH ALONG THE LENGTH OF SAID HEAT-OUTPUT SURFACE A SUBSTANTIAL TEMPERATURE GRADIENT WHICH WILL EFFECTIVELY STABILIZE THE VAPORIZATION CONDITIONS OF SAID FLUID OVER THE EXTENT OF SAID HEAT-OUTPUT SURFACE.

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