US3521705A - Heat exchange structure and electron tube including such heat exchange structure - Google Patents

Description

3,521,705 UDING ET AL ON TUBE INCL UCTURE y 1970 c. A. BEURTHERE HEAT EXCHANGE STRUCTURE AND ELE SUCH HEAT EXCHANGE STR Filed June 12, 196

(ANKLE: 4. 55091751957:

United States Patent Int. Cl. F28d 1 /00; H01j 7/24 US. Cl. 165-74 9 Claims ABSTRACT OF THE DISCLOSURE A heat exchanging separating wall, one surface of which is heated, has the other surface formed with projections (2) separated from each other by channels (3), the dimensions of the channels and the projections being defined by the following relationship: depth (b) of the channels from the heat exchange surface is in the order of one quarter of the thermal conductivity (c) of the material forming the structure, and the smallest dimension (a) between channelsthat is the width of the projectionsis greater than the depth (b) of the channels; all lengths being measured in centimeters and the thermal conductivity being measured in watts/centimeter-degree C. For thermal conductivities greater than 1 w./cm.-degree C., (b) should be between 0/4 and c/8, and the width (d) of the channels (3) less than b/ 2; for 0 less than .5 W./cm. C., b should be between 0/4 and 0/2, and d between d/ 2 and b; eflicient transfer of heat by boiling off of liquid, to obtain maximum cooling due to heat vaporization, is thus obtained.

The present invention relates to improvementsin heat exchangers, and to high-power electronic tubes in which a good deal of heat must be dissipated, including such heat exchange structures; and more particularly to heat exchangers which utilize the heat evaporation of a liquid from a wall of highly heat-conductive material in order to effect cooling of the wall itself.

Heat exchangers, in which one surface of a wall of highly heat conductive material is bathed in a liquid, which evaporates on contact, are known. Such heat exchangers can operate with fluids subject to natural thermal convection, or with forced circulation, and maintained at either atmospheric, or elevated pressures. Liquid vaporized from such a heat exchanger is then condensed in a condenser, or against cooler surfaces.

It is known that heat exchange, utilizing vaporization of a fluid, follows a non-linear relationship with respect to the surface temperature of the heat exchange wall. Maximum efficieucy of heat exchange is obtained at a point of critical heat flux, the value of which depends, in accordance with known relationships, on the nature of the liquid to be boiled, the temperature involved, pressure, and speed of passage of the liquid along the wall of the heat exchanger. This maximal flux, calculated in accordance with theoretical considerations or measured on laboratory samples, cannot always be obtained in actual reality, and articularly not with heat exchange structures of substantial size operating isothermally, in an actual industrial surrounding. In practice, it is found that hot spots appear at random which cause the continued existence of vapor bubbles at points of higher temperature at the wall; at these hot spots, the temperature increases further until melting or failure of the heat exchange wall, occurring even at overall heat flux rates which are much below the critical value.

3,521,705 Patented July 28, 1970 ice Essentially isotherm elements, utilized to cool a surface :bathed within a liquid have been previously constructed, in which the vaporization just below the critical value is controlled and stabilized. Such a mode of operation has previously been referred to as semi-film type transsition. Heat exchange walls in accordance with these structures permit transmission of thermal flux several times greater than, even, the theoretical maximum critical flux. Thus, heat transmission in the order of 500 w./cm. may be obtained with non-circulated water and at atmospheric pressure; a heat flux rate of 2 kw./cm. may even be exceeded in the case when water is forcibly circulated and maintained under a pressure of several bars.

The heat exchange surfaces in accordance with prior proposals have, however, the disadvantage that the shape of these surfaces is quite complex and thus the cost of manufacturing these surfaces was justified only when it was important to reach the limit of maximum heat exchange capacity; such cost may, however, have been excessive for less than maximum performance requirements even though the classical isothermal heat transfer rates should be exceeded.

It is, therefore, an object of the present invention, to provide a heat exchange structure which is simple to manufacture and yet provides for maximum heat transfer rate utilizing the principles of cooling by evaporation of a heat exchange liquid.

SUBJECT MATTER OF THE PRESENT INVENTION Briefly, in accordance with the present invention, one surface, or face of a heat exchange wall is immersed in, or bathed by a liquid which will be evaporated upon contact with the heat exchange wall; the wall itself is formed with projections separated from each other by channels of dimensions which are critical. Measured in centimeters, the depth of the channels should be of a value in the order of one quarter of the thermal conductivity of the heat exchange wall, measured in watts per centimeter-degree centigrade; the smallest dimension of the projections, measured parallel to the wall (or tangentially, if it is curved) should be greater than the depth of the channels. Ranges of dimensions useful in the present invention will be pointed out in connection with the detailed description below.

The structure, organization, and operation of the invention will now be described more specifically with reference to the accompanying drawings, wherein:

FIG. 1 illustrates, in perspective, an anode structure for a power tube (with the tube elements omitted for purposes of clarity) incorporating the present invention;

FIG. 2 is a perspective view of a plane heat exchange structure having transverse channels; and

FIGS. 3, 4 and 5 are enlarged vertical cross-sectional views through heat exchange walls and illustrate various embodiments of the structures, and the channels therein.

The heat exchange anode structure, in form of a tube, of FIG. 1 is heated internally by electronic components within the tube, not shown and not forming part of the present invention. In order to dissipate the heat generated by the tube, the external surface of the anode structure 1 is bathed, or submerged in a cooling liquid maintained within an enclosure, likewise not shown and well known in the art. Forced circulation of the liquid along the wall of the anode 1 constantly supplies new heat exchange liquid at a pressure and temperature such that the heat exchange liquid will boil off upon contact with the heat exchange wall; the resulting vapor is immediately condensed within the remaining pool of the liquid itself, or by contact with an external condenser. The forced circulation of the liquid along the wall is schematically indicated b arrows A-A.

Narrow channels 3, and located as generatrices of the cylinder forming the anode structure of the tube define a series of protuberances or projections 2, arranged parallel to each other similar to the channels 3. The material of the anode itself is a good conductor of heat, such as copper. The dissipating protuberances 2 present, upon contact with the cooling liquid circulated, as indicated by arrows AA approximately isothermal heat exchange surfaces along their median plane. The end portions are cooled by intense heat dissipation occurring at the narrow channels and operating in a complex anisothermal mode of vaporization, which is stabilized.

When the isothermal zone reaches the critical heat flux of vaporization, the temperature thereof is stabilized by the presence of colder end portions subject to stable nucleated boiling. The presence of these relatively cold end regions stabilizes the temperature of intermediate portions by internal conduction of heat within the metal of the heat exchange wall. Additionally, the presence of intense boiling at the edge portions tends to break up vapor films which may form at the hotter parts of the structure so that the entire surface will be capable to transmit heat flux very close to the critical flux.

The heat exchange wall structures, in accordance with the present invention, are different from those known structures which merely have random grooves to increase surface area, but of little significance to cause the currents of the cooling liquid therealong to behave in a certain way, and to support vaporization without permitting hot spots. Turbulence of heat exchange liquid is, of course obtained with any break-up of a smooth heat exchange surface, and increases the heat exchange capability. Shallow grooves do not cause a sufiicient temperature gradient at the edges in order to stabilize the boiling of heat exchange material along the edge walls after the critical temperature has been exceeded, as is the case in accordance With the present invention.

A plane heat exchange structure is illustrated in FIG. 2. Channels 4 and 5 define rectangular projections 2, and separated by different amounts from each other, so that the protuberances will have a smaller dimension a and a larger dimension a The depth b of the channels, measured in centimeters, is in the order of one quarter of the thermal conductivity of the material of the heat exchange structure, measured in watts per centimeter-degree C. In any event, the smaller dimension a of the projections or protuberances 2 must be greater than the depth b of the channels; for example a may be between 2b and 8b. In FIG. 1, where only a single dimension is appropriate to the protuberances, this dimension is indicated by a.

Laboratory experiments enabled determination of optimum value of the location and depth of the channels with greater precision.

If the material of the wall has a thermal conductivity 0 which is greater than w./ cm. C., then the optimum value of b is between 0/ 8 and c/4. The width of the channels, indicated by d in FIG. 2, experimentally, has been determined to be preferably less than b/Z.

For values of thermal conductivity 0 less than w./cm. C., optimum depth of the channels, b, is between 0/4 and 0/2; and the width of the grooves, d, is then preferably between b and b/2.

For values of thermal conductivity 0 less than w./cm. ranges to those indicated above both for values of b and for values of d give good results.

FIG. 3 illustrates a heat exchange structure in which the projections, or protuberances have a trapezoidal cross section, in that the top edges are rounded off as at 7, in order to better prevent break-up of the boiling at the end zones. The width d of the grooves is then measured at the point in which a vertical line (with respect to the cross sectional diagram of FIG. 3) divides the area above and below the intersection of the verticals into two equal zones. As seen in FIG. 3, the width of the protuberance a is measured with respect to a similar reference line,

that is one which, paired with another symmetrical one, divides the area beneath, and above the intersection of the reference lines with the channels into two equal areas. Not only are the channels rounded off at the top, as at 7, but the edge lines 6 are also slanted.

The channels, or grooves 3 may be broken up by a central rib, as illustrated in FIGS. 4 and 5. Rib 9 separates the channels into two subdivisions 8 (FIG. 4) and 8 (FIG. 5). The arrangement of FIGS. 4 and 5 has the advantage that the heat to be dissipated which is produced by the aniso-thermal, active heat exchange regions can be substantially increased, thus permitting increase of heat dissipated from the projections 2. FIG. 4 illustrates a structure in which the sub-divisions 8 of the channels are perpendicular to the surface of the heat exchange structure; whereas FIG. 5 illustrates an arrangement in which diverging channels 8', which can be rounded at the bottom, define a separating rib 9', with the projections 2 therebetween.

As will be obvious from an inspection of the drawing, the heat exchange surfaces in accordance with the present invention may be readily manufactured and machined and thus result in a structure which is less expensive to make. The heat exchange structures in accordance with the present invention are particularly useful to operate with forced-circulation heat exchange fluid under pressure and are particularly applicable to the cooling of high power electron tubes, nuclear reactor rods or elements of thermal machines.

The present invention has been described in connection with a heat exchange structure forming an anode for an electron tube, and in connection with flat heat exchange surfaces. Various changes and modifications may be made, within the inventive concept, as determined by the requirements of specific uses.

We claim:

1. Heat exchange structure comprising a metallic wall section subject to heating at one surface thereof, said wall section being immersed in a heat exchange medium at the other surface thereof and exchanging heat with said medium, said other heat exchange surface being formed with projections (2) separated from each other by channels (3),

wherein the improvement comprises that the dimensions of said channels and projections are defined by the relationship: the depth (b) of the channels is equal c/n where (c) is the thermal conductivity of the wall section and n has a value comprised between 2 and 8, while the smallest dimension (a, a between channels, of said projections, is greater than the depth (b) of the channels, and the average width (d) of the channels is on the order of or less than b.

all lengths being measured in centimeters and the thermal conductivity being measured in watts/ cm. C., to form aniso-thermically acting heat exchange surfaces in the regions of the channels, and isothermically acting heat exchange surfaces in the region of the medium plane of the projections.

2. Heat exchange structure according to claim 1, wherein, for a material having a thermal conductivity greater than 1 w./cm.- C., the depth (b) of the channel is between 0/4 and c/ 8, and the average width (d) of said channels (3) is less than half the depth.

3. Heat exchange structure according to claim 1, wherein, for a material having a thermal conductivity which is less than 0.5 w./cm.- C., the depth (b) of the channel is between 0/4 and 0/2, and the average width (d) of the channels (3) is in the range of between b/Z and b.

4. Heat exchange structure according to claim 1, wherein the material has a thermal conductivity (0) of 1 c 0.5; and the depth (b) of the channels is in the order of 0/4 and the average width (d) of the channels is in the order of b/ 2.

5. Heat exchange structure according to claim 1, angle with respect to a plane perpendicular to the heat wherein the outer edges of the channels (3) are rounded exchange surface.

6. Heat exchange structure according to claim 1, References Clled wherein said heat exchange medium is a fluid subject to 5 UNITED STATES PATENTS forced circulation (A). 7. Heat exchange structure according to claim 1, 523 5:: wherein said channels are formed in pairs, and subdivided e e (FIGS. 4, 5:8) with ridges (9) therebetween.

8. Heat exchange structure according to claim 7, 10 ROBERT OLEARY: Pmnary Exammer wherein said subdivisions are arranged to have their AV 111-, Asslstant Examiner median planes divergent and inclined with respect to the surface of said heat exchange structure.

9. Heat exchange structure according to claim 1, 165 133, 183, 185; 31321, 22, 35, 36 wherein the walls of said channels are inclined at an 15

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