US3490520A - Fixed nucleation site for pool boilers - Google Patents

Description

Jan. Z0, 1970 c. L.. DELANEY ET AL 3,42%052@ FIXED NUCLEATION SITE FOR POOL BOILERS Filed July 18, 1967 3 Sheets-Sheet l u l km Jan. 20, 1970 c. l.. DELANEY ET Ar. 3,490,520

FIXED NUCLEATION SITE FOR POOL BOILERS Filed July 18, 1967 3 Sheets-Sheet 2 0F #inria raaf Jan. 20, 1970 Filed July 18, 1967 c. L. DELANEY- ET AL. 3,490,520

FIXED NUCLEATION SITE FOR POOL BOILERS 3 Sheets-Sheet 3 @gum/@lan United States Patent() Force Filed July 18, 1967, Ser. No. 654,280 Int. Cl. F24h 3/00 U.S. Cl. 165-47 8 Claims ABSTRACT OF THE DISCLOSURE A hollow tube, welded shut at one end, is attached at its open end to the wall near the bottom of a boiler vessel containing liquid metal. A communicating opening in the vessel wall introduces a small portion of the liquid metal into the interior of the tube, and a heater attached to the tube forms vapor bubbles in the liquid metal so introduced. A series of inverted cups, each welded to the tip of a rod, are immersed at various depths in the liquid metal. The lowermost inverted cup is located in the liquid metal in front of and immediately vover the communicating opening in the vessel wall to receive and trap the vapor bubbles formed in the heated tube.

Background of the invention This invention relates generally to the field of nuclear power generating systems and, in particular, to the boiling characteristics of liquid alkali metals being proposed for use in such conversion systems.

During the study of a proposed Rankine cycle power conversion system as an auxiliary nuclear power generating system for use in relatively long space missions of approximately one year duration, it Was observed that the liquid metal suggested for use as the working fluid in such a system had a distinct tendency to boil unstably. This boiling instability was manifested by a non-nucleate boiling period when superheating of the bulk liquid occurred followed by an explosive boiling period and then a gradual subsiding of the nucleate boiling process. Such explosive boiling occurs when some random nucleation site is activated, and results in a system pressure increase and a bulk liquid temperature decrease. The period of the instability cycle and the magnitude of the pressure perturbations are functions of heat iiux and the thermophysical properties of the working iiuid.

In the investigation of this problem of boiling instability, the use of a xed nucleation site introduced into the boiling fluid was considered as a means for counteracting the instability characteristics inherent in the liquid metal during the explosive boiling period. The use of such a xed nucleation site was considered appropriate, since the location of the vapor bubbles formed during the unstable boiling was uncertain. Moreover, the xed site should not only form vapor bubbles at a definitely located position, but it should also trap the bubbles at a xed site within the liquid metal and thus continue to act as a nucleator, or in other words, continually act with stablizing effect on the liquid metal. To this end, several types of articial nucleation sites were tested to determine their effect on the boiling characteristics of liquid metal. These tests were made in a series of low heat flux retluxing test capsules containing a liquid alkali metal alloy of 22% sodium and 78% potassium (NaK78). In one such test, a series of holes were drilled in the boiler section of the reuxing capsule. However, this method, known as the drilled hole nucleation site, proved unsuccessful in stablizing the boiling of the liquid alkali metal.

Patented Jan. 20, 1970 ICC Other tests included the use of unheated and heated tube types of nucleation sites, in which a series of 3-inch long sections of heavy wall 1A inch 316 stainless steel tubing with one end welded shut were inserted in the test capsule wall with their open ends positioned at diiferent depths in the liquid metal. An opening in the wall at each location provided communication between the i11- terior of the capsule and each tube. Although the unheated tube failed to affect boiling instability, the heated tube exhibited considerable success with the tube located nearest the bottom of the capsule proving to be the most effective in controlling or countering the boiling instability. However, the heated tube type nucleation site had one disadvantage in that it required the tube to be heated between F. and 500 F. higher than the bulk fluid temperature. Thus, for fluids that boil above 1600 F., a structural problem would exist with conventional materials, such as 316 stainless steel, if the tube was to be operated continuously.

In addition to the above tests, an inverted dish type nucleation site was examined. In this type of site, an inverted dish was welded to the tip of a rod and was then immersed in the center of the fluid. However, again boiling stability was not achieved, since it was apparent that the inverted dish failed to capture a vapor bubble which could then act as nucleator or fixed nucleation site. Therefore, to overcome the limitations inherent in the above noted previous proposals, additional developments in this area resulted in the improvement of the present invention which will be described hereinafter in detail.

Summary of the invention The principal object of the present invention, therefore, resides in the provision of an improved artificial nucleation site incorporating a unique combined arrangement for eliminating instabilities in both pressure and temperature occurring during the boiling of liquid alkali metals.

A further object of the invention is in the utilization of a novel articial nucleation site having an improved method of initially forming or energizing vapor bubbles, combined with a simplified means for positively trapping and holding the vapor bubbles so formed at a xed nucleation site with the trapped vapor bubbles acting as a nucleator for continually eliminating excessive pressure perturbations and bulk'liquid temperature drift resulting during the unstable boiling of liquid alkali metals.

A still further yobject of the invention is in the use of an artificial nucleation site that combines an inverted cup device for trapping vapor bubbles, with a tube initially heated for forming the Vapor bubbles to be trapped in a unique and yet simple manner that insures both the activation of vapor bubbles at a lixed and positive site, and the continuance of stable boiling even after heat to the tube is turned ofr.

Other objects and advantages of the invention will become apparent from the following description taken in connection with the accompanying drawings in which:

Description ofthe drawings FIGURE 1 is a partly broken away, longitudinal sectional view of the refluxing test capsule used with the combined articial nucleation site of the preferred embodiment of present invention installed.

FIGURE 2 is an enlarged sectional View, illustrating details of the heated tube portion of the combined artiiicial nucleation site of FIGURE l.

FIGURE 3 is a graph illustrating the plot of saturation pressure, heated tube temperature, and bulk fluid temperature versus time during Athe period in which the stable boiling regime involved with the present invention is initiated.

3 FIGURE 4 is a second graph of the plot of the equilibrium bubble size for the metal alloy, NaK78, versus superheat when the saturation temperature is 1600 F.

Description of the preferred embodiment Referring to the drawings and, in particular, to FIG- URE 1 thereof, the reliuxing capsule system used with the present invention is indicated generally at 1 as consisting of a low heat flux refluxing capsule 2, incorporating a condensing section 2a and a boiler section at 2b, a plurality of semi-cylindrical clamshell heaters at 3 for radiantly heating the capsule, and pipe insulation at 4. The liquid alkali metal involved Iwith the present invention consists of an alloy of 22% sodium and 78% potassium (NaK78) and is contained within the boiler section 2b as shown. This alloy is an eutectic which has a melting point of 12 F. and is, therefore, a liquid at room temperature. It was selected mainly because of its availability and for the ease with which it can be handled. It has a normal boiling point of 1455 F. which allows low stress levels in the capsule during operation. The physical properties of the selected alloy, which are representative of most alkali metals, make NaK78 appropriate to test as a Iworking Huid for the determination of a means of stabilizing the boiling thereof and thus obtain data generally applicable to the use of working fluids in the auxiliary nuclear power generating systems under current study.

In the preferred embodiment of the invention, the boiler section 2b of the capsule 2 is equipped with a combined artificial nucleation site consisting of a heated tube indicated generally at 55, and an inverted cup at 6. Although the Original tests on the combined nucleation site were made with a triple inverted cup configuration coupled with the aforesaid heated tube 5 and, therefore, an additional pair of inverted cups are shown mounted respecively at 7 and 8, subsequent tests revealed that only one inverted cup was required to provide stable boiling, if located as illustrated for the cup 6 adjacent to and immediately over the discharge opening of a heated tube positioned, as at 5, in close proximity to the bottom of boiler section 2b. However, in a second modification of the invention, the triple inverted cup configuration, as shown at 6, 7 and 8, respectively, in FIGURE 1, may be ernployed alone with some success in significantly improving the stabilizing of the boiling of the liquid alkali metal. The inverted cups 6, 7 and 8, whether employed singly, as in the preferred embodiment, or together, are each welded to the tip of a stainless steel rod, indicated respectively at 6a, 7a and 8a, which rods .may be retained in position by suitable hardware (not shown) for holding their respective inverted cups in immersed relation with the liquid metal at varying depths, as shown. In arriving at the combined arrangement of the preferred embodiment, initial tests were run on the triple inverted cup configuration combined with the heated tube 5 of the present invention. During these tests, the boiler was first raised to a temperature of 1450 F., and then the tube 5 was heated to a temperature of 1650 F. This initiated stable boiling with typical pressure perturbations below 1A p.s.i.d. After several minutes of stable boiling, the power to the heated tube S was turned off. Despite this power shutoff, stable boiling continued throughout the duration of the test run. This continuance of stable boiling is clearly indicative that, with the inverted cup 6, arranged as described directly over the discharge opening of the tube 5, vapor bubbles formed in the tube were trapped and retained in the inverted cup 6 to thereby act as a nucleator or as a continually operating, fixed nucleation site. Thus, with the combined heated tube-inverted cup configuration of the present invention, a positive means has been established for forming, trapping and retaining a vapor bubble at a fixed site.

With specic reference to FIGURE 2 of the drawings, details of heated tube of the present invention is shown in enlarged form at 5 as incorporating a central passage at 5a, and a 14 gauge nichrome wire heater coil at 5b wrapped therearound, which may be connected to a variac control circuit as shown. One end of the tube 5 is welded shut as indicated at 5c, and the other end thereof is welded at 5d to the wall of the refluxing capsule 2. An opening as at 9 in the capsule Wall 2 provides communication between the liquid metal contained within the boiler section 2b (see FIG. l) and the central passage 5a of the tube 5. In this manner, heating of the tube S by the heater coil 5b to a predeter- -mined value energizes or forms vapor bubbles in passage 5b that are emitted or discharged through opening 9 into the superheated liquid metal. In other words the heat flux imposed upon the outside Wall of the tube 5 causes the small volume of fluid contained in the tube central passage Sa to become quickly superheated, which, together with the actual geometry of the tube, initiates the formation of vapor bubbles. These vapor bubbles are then washed-out into the boiler section 2b of the capsule 2 because of the presence of large convection currents. The vapor so formed is then replaced by the bulk liquid which again quickly superheats sufficiently to produce another vapor bubble. Since the mass of fluid involved in the energy exchange is small, the total energy exchange and thus the overall system pressure perturbations are also small. In the original test configuration, a series of three heated tubes were utilized and positioned in the capsule wall at various positions; however, the heated tube located nearest the bottom of the capsule 2 proved to be the most effective. In this regard, it was shown that if the inside wall temperature of the heated tube 5 at the spot marked X, which represents the thermocouple position in FIG. 2, was from 73 F. to 123 F. hotter than the saturation temperature of the bulk fluid, stable boiling can be maintained with the tube alone; however, to initiate stable boiling a minimum temperature differential of 176 F. between the inside Wall of the heated tube and the saturation temperature of the bulk fluid would be needed. Stable boiling was never initiated until a boiling perturbation occurred after the heated tube had reached operational conditions. From this, it has been concluded that this perturbation was initiated by the entrance of the first bubble, produced by the heated tube 5 into the highly superheated fluid in the capsule 2. A graph plotting bulk fluid temperature, heated tube temperature, and saturation pressure versus time during the period in which the stable boiling regime was initiated is presented in FIGURE 3, which will hereinafter be described in detail.

Referring to the aforesaid FIGURE 3, point al represents the condition whereby the liquid is superheated sufficiently to permit vaporization at some random nucleation site. The vapor produced results in the ullage pressure being raised to point b1. At point b1, the bulk fluid temperature is equal to the saturation temperature. A temperature drop of 23 F. in the liquid was required to raise the ullage pressure from point al to b1. The path between points b1 and c1 represents the condition whereby the condensing rate is greater than the boiling rate. At point c1, the convection current produced by the initial boiling has subsided and boiling ceases. During the quiescent period between points c1 and a2, the liquid superheats, and the explosive cycle is reproduced. This would continue indefinitely at the same heat input conditions. However, with the addition of power to the heated tube 5 of the present invention, a high flux area is created, resulting in a fixed nucleation site and thus providingv stable boiling.

It is noted that pressure perturbations in the Stable boiling regime drops from a pressure of 1/2 p.s.i.d. to A p.s.i.d. when the temperature differential between the heated tube 5 and the bulk fiuid was raised from 150 F. to 500 F. Although the heated tube concept by itself worked well and therefore constitutes a third embodiment, the fact that it must be kept at between 150 and 500 F. higher than the bulk fluid temperature means that, for fluids that boil above 1600 F., a structural problem would exist for conventional materials. This will introduce certain reliabilityv and safety problems unless other materials having higher'design limits are used, and, accordingly, the combined heated tube and inverted cup configuration is preferable for system that are already operating at the design limit of the containment vessel.

In order to understand the mechanism involved in nucleate boiling, reference is made to,one of the main criterions required for this process, which is that some portion of the bulk fluid must be 'sufficiently superheated to create a vapor pressure which is larger than the sum of the hydrostatic head of the fluid, the vapor pressure above the fluid, and the concave surface tension effect of the bubble that the superheated fluid Lis trying to form. The concave surface effect will causelthe magnitude of the difference in equilibrium vapof pressure above a fiat and concave liquid surface to befinvejrsely proportional to the radius of curvature. This can be further demonstrated by reference to the graph Aof FIGURE 4, which represents a plot of the equilibrium b ubble size for the metal alloy NaK78 versus superheat when the saturation temperature is 1600 F. ThusJ the equilibrium curve designated on this graph demonstrates the degree of superheat required to maintain a bubble of a given radius. To make the bubble grow in size and, therefore, accomplish stable boiling, either the hydrostatic head on the bubble must be decreased, or theliquid superheat value increased above a predetermined. Value'of the difference between the vapor and saturation* temperatures present in the system. The only means readily available for decreasing the hydrostatic head on the bubble is for the bubble to become detached from ,the capsule wall to rise upward through the liquid in* the boiler. Unfortunately, a vapor bubble will not detach from a surface until it has grown to a relatively large size. Therefore, the problem is one of making vapor bubbles grow from a very small size to a size of such magnitude that they will detach from the capsule wall and rise through the liquid. The artificial nucleation site of the present invention provides stable boiling by forming vapor bubbles of a size such that they will continue to grow by means of the difference in value between the vapor and saturation temperatures available, in the boiler and subsequently detach and rise through the boiler. This difference in vapor and saturation temperatures is accomplished in the present invention, as previously explained, by adjusting the saturation temperature of the bulk iiuid through use of the clamshell heaters 3 to a first predetermined value, as for example 1450 F., coupled with adjustment of the heated tube 5 to al second higher predetermined value of 1650 F., for example.

Although the preferred embodiment of the invention consists of the combined inverted cup and heated tube configuration, described hereinbefore in detail, the triple inverted cup nucleation site, by itself also proved successful in accomplishing considerable stabilizing of the boiling of the liquid metal under test, as has been previously noted. In this configuration, the liquid metal was heated until a pressure perturbation occurred, after which stable boiling occurred at approximately 1200" F. The bulk liquid saturation temperature was then increased to 1600 F. and stable boiling was maintained throughout the transition from 12010 F. to 1600 F. However, the boiling temperature tended to drift approximately F. to 20 F. It was previously found that the triple inverted cup configuration was necessary when used without the heated tube 5, since initial tests run on a single inverted cup configuration positioned at various depths in the liquid metal failed to stabilize the boiling. On the other hand, with the triple inverted cup configuration nucleation site,

it was concluded that a vapor bubble was trapped by one of the three cups and that this trapped bubble would provide a fixed nucleation site for the fluid, as was previously noted in the description of the preferred embodiment of the invention. Thus, although the use of the triple inverted cup configuration nucleation site alone offers significant improvement over previously developed nucleation sites and, therefore, is considered to be a second modification of the present invention, the uncertain location of the bubbles formed during the explosive boiling period makes the combined arrangement preferable in that the heated tube l5 thereof assists in establishing a positive and definite nucleation site.

Thus, a new and unique method of reducing the instabilities occurring during the boiling of liquid alkali metals has been developed by the present invention, and any one of three forms may be utilized depending on the particular circumstances and prevailing operating conditions. For example, the use of the heated tube configuration alone placed near the bottom of the liquid metal boiler has been found to reduce the alkali metal boiling perturbations and the bulk iiuid boiling temperature drift to an absolute minimum. This configuration, however, requires that the heated tube be operated at a minimum temperature range higher than the bulk fluid temperature of the boiler, thus precluding its use on systems already operating at their design temperature limit. Nevertheless, the heated tube, once it is stabilized thermally, has proven quite effective in operation. In addition, the present invention includes a second modification; namely, the inverted cup configuration used alone, which was successful in achieving stable boiling after the occurrence of' an initial perturbation, although allowing the boiling temperature to drift approximately 15 F. around a mean value.

The preferred embodiment of' the invention, which combines the inverted cup configuration with the heated tube, insures the most effective arrangement in both initiating and maintaining a fixed nucleation site at temperature ranges below the design limit of conventional materials and therefore offers wide applicability to inhouse test loops designed to test the elimination of boiling instabilities, for example, in pool boiler, forced convection boilers and thermal convention loops.

We claim:

1. Apparatus for stabilizing the boiling of a liquid metal that is characterized in part by an explosive boiling period, comprising a boiler vessel containing a main supply of liquid metal adapted to act as the working fluid in a nuclear power conversion-generating system, heater means associated with said boiler vessel adapted to heat the liquid metal contained therein to a predetermined bulk liquid temperature representing the nucleate boiling regime, and an artificial nucleation site combined with said boiler vessel comprising a receptacle having a fluid receiving passage opening into the interior of said boiler vessel and adapted to admit the relatively small volume of liquid metal thereinto, and heater means associated with said receptacle for subsequently raising the temperature of the relatively small volume of liquid metal contained therein a predetermined temperature above the bulk fluid temperature to which the liquid metal was initially raised by said first named heater means.

2. Apparatus for stabilizing the boiling of a liquid metal as in claim 1, wherein said artificial nucleation site further includes means associated with said receptacle and located within the supply of liquid metal contained in said boiler vessel for trapping a vapor bubble ejected from the fiuid receiving passage of said receptacle and retaining said bubble at a definite location to thereby provide a fixed nucleation site continually neutralizing pressure perturbations and boiling temperature drift and thus stabilize boiling of the liquid metal.

3. Apparatus for stabilizing the boiling of a liquid metal as in claim 1, wherein said artificial nucleation site 4 7 comprises the combination of a first means having a passage in communication with said supply of liquid metal adapted to receive the relatively small volume of liquid metal from said boiler vessel and incorporating heater means powered to heat and thereby form a positive source of vapor bubbles from said small volume at a predetermined stabilizing boiling temperature for discharge into the said supply of liquid metal in said boiling vessel at a predetermined temperature above that of the mainv supply, and a second means immersed within the supply of liquid metal Iand located at a predetermined distance above the bottom of said boiler vessel and directly over the discharge from said first means to thereby ensure the capture and retention of the vapor bubbles emitted from said first named means.

4. Apparatus for stabilizing the boiling of a liquid metal as in claim 3, wherein said first means further comprises a heated tube welded shut at one end and in open communication at its other end with the liquid metal in said boiler vessel,

5. Apparatus for stabilizing the boiling of a liquid metal as in claim 3, wherein said second means comprises an inverted cup positioned in relation to the discharge end of said first means to ensure capture and retention of vapor bubbles arising from the discharge of said first means.

6. Apparatus for stabilizing the boiling of a liquid metal -as in claim 1, said artificial nucleation site comprising a heated tube closed at one end and welded to the wall of said boiler vessel at the opposite end and having a passage extending longitudinally thereof and terminating in an opening at said opposite end in communication with the interior of said vessel and vaporizing the relatively small volume of liquid metal admitted thereinto into a plurality of vapor bubbles at a predetermined temperature above the saturation temperature of the liquid metal contained in said boiler vessel, and an inverted cup-welded to a tube and immersed within the liquid metal contained in said boiler vessel and positioned immediately adjacent and directly over the communicating passage opening -formed in said heated tube for capturing vapor bubbles formed therein and emitted therefrom at a fixed nucleation site.

7. Apparatus for stabilizing the boiling of a liquid metal as in claim 1 including a triple inverted cup configuration positioned in immersed relation at various depths within the main supply of liquid metal and terminating in a lowermost inverted cup in proximity to the bottom of said vessel and adapted to receive and hold a vapor bubble therein to thereby establish a fixed nucleation site continuously acting to stabilize the boiling of said liquid metal.

8. Apparatus for stabilizing the boiling of liquid metal as in claim 7, said artificial nucleation site further having positive means for establishing the fixed nucleation site within said inverted cup configuration, said lmeans comprising a heated vapor bubble producing means adapted to be powered to initially produce a series of vapor bubbles and located within the wall of said boiler vessel in communication with the main supply of liquid metal in relatively near proximity to the bottom of said vessel and having an opening for emitting vapor bubbles formed therein into said main supply of liquid metal at a position immediately adjacent the opening of said lowermost inverted cup to thereby ensure its capture at a fixed, artificial nucleation site, the vapor bubbles thereby captured and retained continuing to effect stabilized boiling even after the power to said last-named means is cut-off.

References Cited UNITED STATES PATENTS l/l967 `IGaertner -1 2/1968 Kartluke et al. 165-1 REUBEN EPSTEIN, Primary Examiner

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