I k . | OF | ORNLP 1659 - - . . . - 1 4 5 SO IR Il 3.2 13.6 엘엘 ​94.0 il- 125 LLE MICROCOPY RESOLUTION TEST CHART NATIONAL BUREAU OF STANDARDS - 1963 . .. 1. ... ... .... .!. . " O .. . ! " . 4-6.509 460 1965 HOV'S SUPERHEAT WITH BOILING ALKALI METALS J. A. Edwards and H. W. Hoffman Reactor Division RELEASED FOR ANNOUNCEMENT IN NUCLEAR SCIENCE ABSTRACTS Submitted for presentation at the Fourth High-Temperature · Liquid-Metal Heat Transfer Technology Conference to be held September 28-29, 1965, at Argonne National Laboratory, Argonne, Illinois. LEGAL NOTICE This report mo prepared u an account of Goverament sponsored work. Neither the United Statos, por the Commission, nor any person acting on behalf of the Commission: A. Makes ay warranty or representation, expressod or implied, with respect to the accu- racy, completenos), or usefulness of the information contained in this report, or that the uso of lay information, apparatus, method, or process disclosed in this report may not Infringo printoly owned rigata; or B. Asmumos any liabilides with respect to the use of, or for damagos resulting from the use of any information, apparatus, method, or procesu disclosed in this report. As used in the above, "person acting on beball of the Commission's includes any om- ployee or contractor of the Commission, or employee of such contractor, to the extent that much employs or contractor of the Commission, or employee of such contractor preparos, dienominato, or provides access to, any Information pursuant to his employment or contract with the Commission, or ho employment with such contractor. CAK RIDGE NATIONAL LABORATORY Oak Ridge, Tennessee Operated by UNION CARBIDE CORPORATION for the U. 8. ATOMIC ENERGY COMMISSION : Errea, reumatomo Yrev ATITIS Tratx SUPERHEAT WITH BOILING ALKALI METALS J. A. Edwards and H. W. Hoffman Oak Ridge National Laboratory Oak Ridge, Tennessee Nut loro banco bois l e inimeste ABSTRACT The results of experiments in a natural-circulation loop. to establish the magnitude of the superheat associated with alkali-metal boiling are reported. For potassium boiling on an as-received (commercially smooth) sur- face, the superheat measured ranged from 500°F at a saturation temperature of 1500 °F down to 230°F at Tost = 1770°F. Wall-temperature oscillations, which occur in the presence of these large superheats because of the unusual thermal properties of the liquid metals, were of the order of 170 to 300°F at a fre- quency of 1.5 cycles/min. Because of maximum surface temperature limitations, sodium could not be boiled on this surface. Significant reductions in the magnitude or the superheat and, hence, in the amplitude of the wall temperature oscillations were effected by modifying the boiler surface. Thus, on a surfacr containing eight 0.006-in. diameter x 0.040-in.-deep cylindrical holes, the superheat with potassium was reduced about fortyfold below the value obtained on the as-received surface. A porous surface, formed by sintering of stainless steel particles on the inside tube surface, was less effective than the surface with discrete holes but still showed appreciable reductions in the superheat. Similar results were obtained with sodium. Over the limited range of thene experiments, the superheat data for both potassium and sodium correlate reasonably with predictions based on a model involving a spherical vapor bubble in thermal equilibrium within a liquid pool of uniform temperature. L ! : WAS Research sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Corporiztion. SORINS Summer Research Participant; Professor of Engineering Mechanics, North Carolina State University, Raleigh, North Carolina. , * THE ** * . cos 2 MY LILL + 1 INTRODUCTION Experiments with potassium boiling under pool, natural-circulation, or forced-convection conditions have indicated that large superheats are associated with the boiling process in Alkali liquid metals. An initial calculation by Krakoviak,' reported at the preceding conference on liquid-metal technology, suggested that for potassium the superleat for bubble initiation should lie in the range 125 to 375 °F and for sodium, between 250 and 775°F. Since the first prezentation by Krakoviak in 1963, we have quantitatively measured the superheat in experiments with pool and natural-circulation systems and have demonstrated agreement with predictions. At the same time, we have refined and extended our analyses as our understanding of the mechanisms of bubble formetion and growth in a liquid-metal environment and of the role of the surface in boiling has developed. While some of these studies have been reported previously, 2-4 a brief review of the theory and a description of the experimental apparatus 18 ! appropriate. 1A. I. Krakoviak, "Superheat Requirements with Boiling Liquid Metals," pp. 310–333, Proceedings of 1963 High-Temperature Liquid-Metal Heat Transfer Technology Meeting, Vol. I, USAEC Report ORNL-3605, Oak Ridge National. Lab- oratory, November 1964. 2H. W. Hoffman and A. I. Krakoviak, "Convective Boiling with Iii quid Potas- sium," pp. 19-37, Proceedings oi the 1964 Heat Transfer and Fluid Mechanics Institute, W. H. Great and S. Levy, editors; Stanford University Press, Stanford, California, 1964. PA. I. Krakoviak, "Boiling Potassium Heat Transfer: Superheat Phenomena," 26, Studies in Heat Transfer and Fluid Mechanics, Progress Report for Period January 1 - September 30, 1963, H. W. Hoffman and J. J. Keyes, Jr., editors, USAEC Report ORNL-TM-915, Oak Ridge National Laboratory, October 1964. "A. I. Krakoviak, "Boiling Potassium Heat Transfer: Superheat Phenomena," pp. 15-28, Studies in Heat Transfer and Fluid Mechanics, Progress Report for Period October 1, 1963 - June 30, 1964, H. W. Holfwan and J. J. Keyes, Jr., editors, USAEC Report ORNL-TM-1148, Oak Ridge National Laboratory, August 1965. ... ! 1 . TER SA? . thi - - er oni bila kita bertan bentuk terus berkenaan met handler om . .... . . . me...... en wann -.-.--.--. SUPERHEAT PREDICTION A force balance on a spherical bubble in thermal equilibrium within & liquid of uniform temperature yields: :. . 20 (2) 1 . . .... . . . .... .. where P, is the vapor pressure within the bubble; Pes the liquid pressure out- side the bubble; o, the surface tension at the interface; and r, the bubble radius. Thermal equilibrium requires that the vapor temperature equal the temperature of the surrounding liquid. The vapor temperature must also cor- respond to the saturation temperature associated with the vapor pressure in the bubble. Hence, since the pressure in the bubble exceeds the liquid pres- sure (by 2 0/r at equilibrium), the liquid must be superheated - 1.e., T, must:. exceed the saturation temperature 28Rociated with Po The magnitude of this superheat can be estimated directly from Eq. (1), following the above reasoning, by assuming values for T, and r. Alternatively, the superheat can be expressed explicitly as a temperačure difference by com- bining Eq. (1) with the integrated Claus ius-Clapeyron;* thus : 20 i . ... ..... .. . - .. - -- ... - - ...... . - .... .... - - -- Apat = .- Epat vset on (2+2), (2) - heg r Post .. where T, is the vapor temperature at the pressure, Py; Teat, the liquid temper- ature at the pressure Pp; Ry, the gas constant; and hoped the latent heat of · "The Clausius-Clapeyron relation applies correctly only along the satura- tion line where Py = Pe; a more precise thermodynamic formulation requires that (Op/ar)p = A8/Vy but significantly complicates the computation of the superheat. 3 vaporization at the temperature, T, = T. In this derivation, we assume that the perfect gas approximation (relating T and P along the saturation line) is valid and that the liquid specific volume may be neglected in comparison with the vapor, volume.* For small values of the superheat, Eq. (2) may be written in the approximate form: < R12 sat o 2 Sat 3 e pgr Poat sat In applying Eqs. (2) and (3), T, (and hence T,) is taken to be the meas- ured wall temperature and r, the effective radius of the surface cavities (nucleation sites). Thus, Eq. (1) describes the maximum superheat (sin B = 1)* and, hence, possibly overpredicts the superheat. Measurements of the contact angle with the alkali liquid metals are needed before compensation for this Pactor can be included in the analysis. On the other hand, temperature gradients _ + V in the liquid near the heater surface and adjacent to the bubble interface lead to an average liquid temperature which is less than the measured wall tempera- ture and, hence, to experimental values for the superheat which are greater than those predicted for cavities of a given radius. Data obtained in this study I with a cavity of known size demonstrate this effect. If Eq. (1) when coupled with reliable data on the vapor pressure is accepted as correctly predicting the superheat, the errors introduced by the assumptions : *Since V/V, ~ 1300 at the atmospheric boiling temperature (1394 °F), there is ample justification for the latter assumption. Further, as seen in footnote on p. 2, only V, enters in the precise thermodynamic presentation. . *For bubbles nucleating at a surface cavity, AP = (2 o/r) sin B, where B is the angle of contact between the bubble and the surface measured with respect to a normal to the surface. . . : .. . . . * necessary to the development of Eqs. (2) and (3) can be assessed. The results of a comparison of superheats calculated using these three equations are given in Table 1 for three cavity sizes at two saturation temperatures. We note * Table 1. Comparison of Calculated Superheats for Potassium -. - . ... - - - - * Method of Calculation Isat (°R) - r (in.) . Superheat, or at 1860 10-4 10- 5 10-3 80 390 2.2 80 : 385 | 2.2 132 1320 2.4 10-3 9.50 9.5 13.2 Eq. (1)&, b Eq. (2) Eq. (3) 2260 10-4 21.6 21.3 24.1 10-5 179 178. 243. ------- ------- evapor pressure: log p = 6.12758 – 8128.77/T - 0.53299 log T, where p is the absolute pressure (atm) and 1, the absolute temperature (°R). [C. T. Ewing et al., "High Temperature Properties of Sodium and Potassium, Twelfth Prog. Rep. Period 1 July to 30 Sept. 1963," U. S. Naval Research Laboratory Report NRL-6094, June 1964.) Surface tension: 0 = 0.0091 - 0.00246 x 10-3 T, where o is the sur- face tension (16p/it) and T, the absolute temperature (°R); for T > 2200°R, o is assumed constant at 0.00370 lbe/ft. (J. W. Cooke, "Thermophysical Properties: Alkali Liquid Metals, "p. 114, Studies in Heat Transfer and Fluid Mechanics, Prog. Rep. Oct. 1, 1963 – June 30, 1964, H. W. Hoffman and J. J. Keyes, Jr., editors, USAEC Report ORNL-TM-1148, Oak Ridge National Laboratory, August 1965.] Superheats shown according to Eq. (1) were obtained by interpolation between machine-calculated values; accuracy of estimate -2%. : ------------- ---- first that in the temperature range examined Eq. (2) predicts superheats which agree substantially with the values calculated from Eq. (1), while Eq. (3) seriously overestimates the superheat. At lower saturation temperatures, Eq. (2) also generates higher values ; 'e.g., at Isat - 1460°R, Eq. (2) predicts superheats about 5% greater than those calculated by Eq. (1). Further, the dis- crepancy between the superheats calculated according to Ego. (2) or (3) and Eq. (1) decreases with increasing saturation temperature and increases with decreasing cavity radius. Equation (2) has been found by several investigators to be valid for water even at extremely high superheats. In particular, Griffith and Wallis, 5 studying nucleation from cavities of various shapes, obtained favorable agreement between experimental and predicted values. Since the surface tensions for the alkali metals .differ from that for water by at most a factor of 2, it 18 reasonable to anticipate that Eq. (2) will also satifactorily predict the superheat nec- essary for boiling with this class of fluids. Based on the above arguments, Eq. (2) has been selected (in preference to Eq. (1)) as being adequate for the prediction of the superheat and is used sub- sequently for comparison with the experimental data obtained in this study. . Thus, a primary objective of our experiments is the verification of Eq. (2) with respect to the alkali liquid metals. Associated with the high superheat , required for bubble mucieation are surface-temperature oscillations of large amplitude. These oscillations, which arise because the high thermal conduc- tivities and diffusivities of the alkali liquid metals effect a rapid release of the energy stored in the superheat once a bubble forms, can cause signifi- cant damage to the physical system. Hence, en attendent purpose of this in- vestigation is the characterizing of various surface treatments as to their effectiveness in reducing the superheat. .. i 5p. Griffith and J. D. Wallis, The Role of Surface Conditions in Nucleate Boiling," Chemical Engineering Sympostum Series, Vol. 56, pp. 49-63, 1960. PA - ar 2 . . .. . . . . . . . .. . .. . V A .. 14. . - . . -. .. EXPERDENTAL APPARATUS The small, natural-circulation loop used in this study of boiling with the alkali liquid me cals 18 illustrated schematically in Fig. 2. This loop, of 20-in. x 20-in. overall dimensions, was constructed of 1/2-in.; schedule 40, type 347 stainless steel pipe and contained three test surfaces (A, B, and C la Fig. 1) having the characteristics listed in Table 2. By properly orienting Table 2. Bolling Surface Characteristics Region Surface Treatment .A. Perous suriace coating of sintered stainless steel particles; average pore size: 44 H (min = 12.7 m; max = 111.8 u) Two axial, diametrically opposed rows of four 0.006-in.-dian, 20.040-in.-deep holes arranged on 2-in. centers; rows offset &xially by l in. with respect to each other As received the loop and controlling the heaters, the boiling performance of each of these surfaces could be examined in turn. Outside tube-surface temperatures were measured with 0.010-in. Chromel-Alunel couples; the loop pressure was deter- mined using a Bourdon tube gage (0 to 30 psia range) maintained of the calibration temperature. TEMPERATURE PATTERNS Boiling of both potassium and sodium on the three test surfaces was studied at heat fluxes between 10,000 and 37,000 Btu/hr.ft2. Outside-surface temperature patterns at several saturation temperatures are shown in Figs. 2 and 3; these traces are random selections from the continuous recordings of the wall temperatures. . - Pressure Gage ORNL DWG. 64-8743 Vacuum - Clamshell. i Heater Chromel-Alumal Thermocouples [0.010-in.) wwwwwwy biquid Lovel . young) a hitrstwyth mummy * 118., I. Natural-Circulation Superheat loop. - .. -- + - - - - - - - - - - - - SS U . TON . **** * Y Y TE 21 Teat = 1074 °F 25.0 الللللسلام. و sat:= 1044 °F DIII - Bat & Tube Outer Wall Temperature (millivolts) AM. 28.0 Tisat = 1235°F . ... . i - 28.5 ... . sat14600m 33.5 1468°F 34.0 (A) Sintered (B) 0.006-1n. Holes **, * .....de - - - Time (min) Fig. 2. Outside Tube Wall Temperatures for Potassium Boiling on Treated: Surfaces. : ...... ...... .................... .... : Avond. c y .. .. ..... ..... ..... .... www.ma .................... .. ... . - ► 11 - 1:9) . UNCLASSIFIED ORNL - DWG 63-7539A 1700 _ __ 0. 1680 HEATER 1660 1640 1620 .. 1600 1580 TUBE OUTER WALL TEMPERATURE (°F) 0 1520 . . Isat ~ 1460°F . - . . 4500 CONDENSER... 1480 1460 1440 2 . TIME (min) As Received i Fig. 3. Outside Tube Wall Temperatures for Potassium Boiling on an As-Received Surface. .. .... . ......... .... ., i oni .. made tomars e botomiend........... seli. ....... solaithe . . ..... ... While these data were obtained with potassium, the behavior exhibited is also representative of that observed with sodium. The salient features deduced from these temperature profiles may be sum- marized as follows: : : 1. Boiling of potassium on an as-received (commercially smooth) surface is accompanied by wall-temperature variations of extremely large amplitude (of the order of 170 to 300°F), which recur cyclically at a frequency of w1.5 cycles/ min. Because of maximum-allowable-temperature limitations, we were unable to achieve boiling with sodium on this surface. 2. On all surfaces, the magnitude of the temperature fluctuations de- creases with increasing saturation temperature, while the frequency of the oscillations increases. 3. In general, the surface with discrete holes (surface B) is more effec- tive than the sintered surface (surface A) in reducing the amplitude of the temperature oscillations. The sintered surface is also more erratic in per- formance. A comparison of these two. surfaces in these aspects with boiling potassium is made in Fig. 4; the same general trends were noted with sodium. · SUPERHEAT RESULTS The results obtained for the superheat with sodium and potassium on the . various surfaces in the natural-circulation loop are summarized in Figs. 5 through 9 in relation to the saturation temperature. The superheat has been taken to be the difference in wall temperatures - corrected for the temperature drop through the wall – between the maxime in the heated region and the minima in the condenser region (see Fig. 3). The bol.iº curves shown in these figures are the superheats predicted by Eq. (2) as a function of the saturation temperature us . 4 S.'.. + d . o . 25 IS . 11 ORNL DWG. 64-8755 * * ** . Sintered Surface Holes in Surface, -0.006-in. diameter . . Wall Temperature Fluctuation, °F 1 1000 1600 1100 | 2200 2300 2400 1500 Saturation Temperature, opre Fig. 4o Boller Wall Temperature fluctuations with Potassium. · HARIAN l'II! AH OHHH HAMME millill WITHAIHIIHII WIWIKI E. WHIHAT AIMI WITHINAHRH HIIHINAMA ARTE ORNI. DWG. 64-8745 VI ROWTHIMIT TOM HIT! VARNIHIMHINIHINNAHIHI WITHIHHILL Hulluu llllllllllll IT LOTNIHIIHIIHIIIII U UTUUDIIDIINI 0111MM HINTHNUTIMULIIVAHINI DITULINITIVAMU ILUMIIHIILIHI HINDI ILI Hull USD lllll 1AMIID Hilmir V UT MIT Meri M THNI Illllllllllllllll TMV II Munt path AUT WIIIIIIIII (UL LATIMU MINI Ulm LIN 111111 III WIMURUNIID MUUTUU Milli EURIM AMTM IIIIIIIHIIIII: MTIIIIIIIMIII IIIIIIIIIIIIIIIIIHIIIIIIMII' MMMIMI MINIUMMUM:21 TELUT MINIMITUS:11 AM LINI WllllllUIVUMI IlllllllllllllllIll IINO M UI II WWW III MUNDU1Uill! 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HUWIIIIIIIIIIIIIITWAUMINIUM JUNIRII A illlllllllll PUNIHIHI Will AllIIlIlIllIIlIII UlumuLWlllllII HMUDDIN IUL UUTIIVID DU L I Ulumul IVULUI HARJUTUHITINIMUMINIUM HUU NIHHUUMINIMUI JAMIHAN Mull JilliAllu Allllli MUTIAUIU SHINIIIIMV HHNI SHUNIIHII HIIHILABILA ODINAMI UIMIWI MiniT 11THUNIMIT'JUTNI IMUNITIH ANNIHIU HUHITATUM HHHii UNIDAU Wllnili WITH MUUTUVAD WHIL JILIITTI HUHUV AID! WHHAIN A TT HHAIHI MINUT UUUUU IIIIIIIIII MMID AHITIMUL BUIT Allul WANIIIIIII BAHINIIlIlIIIIIIMIII JAMULUIWlllllll W ull WNUMILI INITIU III 16 - HIHI 900 1000 80 288 do '(*** I -. "I) (7Bəqsədns jo səa1824 . 2 : 01 17 : and the cavity radius. Since these data were obtained under "steady-state" conditions, the superheats determined may be assumed to represent boiling from activated sites. Thus, presumably, the initial activation of a cavity requires a larger superheat. However, the possibility that these sites are quenched at the end of each cycle and, hence, that the data give maximum superheats mist : be admitted until more detailed experiments are performed. As-Received Surface. Results for potassium boiling on the as-received surface are shown in Fig. 5; superheats as high as 500°F (at Tost = 1560°F) were observed. For saturation temperatures above 1400°F, the data lie slightly above the curve for an apparent cavity radius of 10-5 in. Since water at atmospheric pressure boils readily on commercial finish surfaces at a super- heat of ~30°F and with some care at superheats as high as 90°F, we can deduce from Eq. (2) tħat nucleation sites of the order of 10-4 in. to 3 x 10-5 in. exist. On very smooth surfaces, even higher superheats can be achieved with water. Thus, Kenrick et al. measured a superheat of 306°F for atmospheric water boiling on the inside surface of a glass capillary; according to Eq. (2), this superheat corresponds to a bubble radius of 0.6 x 10-4 in. In view of the extremely good wetting characteristics of the alkali liquid metals, the in- dication of effective cavity sizes equivalent to those for water boiling on smooth surfaces is not unreasonable. For saturation temperatures below 1400°F, the data for potassium boiling on the as-received surface, show a progressive decrease in the superheat with . decreasing saturation temperature. This behavior was not observed in earlier . A .. . . .. - BF. B. Kenrick, C. 8. Gilbert, and K. L. Wismer, "The Super:heating of Liquids," Journal of Physical Chemistry,. 28: 1297–1307 (1924). ' 5 1: --- 18 data obtained with a slightly altered as-received surface and will be re- examined in later experiments. Sodium could not be boiled on this surface, since the superheat required for boiling could not be attained without exceeding the maximum allowable tem- perature in the test section. "Liquid temperatures ranging up to 1850°F were reached without evidence of boiling. For this maximum condition, the satura- tion temperature was 900°F (measured system pressure was 0.78 psia); hence, the superheat required to initiate boiling was greater than 950°F in this cir- cumstance. Surface with Holes. Figures 6 and 7 show, respectively, the results with potassium and sodium boiling on the surface containing 0.006-in.-diam holes. The superheats observed are reduced significantly below those found for boiling on the as-received surface; e.g., with potassium, the superheat measured at 1400°F on this surface was -20°F as opposed to a value of ad100°F on the as- received surface. With sodium at its atmospheric boiling temperature (1618.8°F), the measured superheat was about 65°F. The cause for the large scatter in the potassium data is unresolved. Over the saturation temperature range of 900 to 1300°F, the potassium data correlate reasonably with superheats predicted by Eq. (2) for cavities of 2 x 10-3 in. This result agrees well with the actual cavity radius of 3 x 10-3 in. In contrast, Eq. (2) indicates a cavity size slightly smaller than 10-3 in. (in the range 1200 < Teet < 1400) for sodium boiling on this same surface. How- ever, this discrepancy may derive from an uncertainty in the value for the sur- face tension of sodium. The data for both potassium and sodium indicate that a minimum superheat is reached at a saturation temperature about 150°F below the normal boiling point. May For potassium, this minimum value is between 7 and 12°F; and for sodium, be- tween 60 and 80°F. The data with sodium preclude our earlier supposition that the minimum observed with potassium derived from sensitivity limitations in the measurement instrumentation. A possible explanation for this behavior, involving the time-dependent variation in the liquid temperature, 18 in a too. early stage for presentation now. Sintered Surface. Superheats measured for potassium and sodium boiling at : 011ing at in oder various saturation temperatures on a surface treated to create a porous coating · are displayed in Figs. 8 and 9, respectively. The reduction in the superheat, while not as great as observed with the surface having discrete, small-diameter holes, is still substantial. For example, the superheat for potassium at 1400°F was about 18 + 5°F and for sodium at 1620 °F, 50°F.. The scatter in the potassium data (Fig. 8), most noticeable in the temper- i ature span 1300 to 1450°F, has been discussed in detail previously2,4 and shown. to relate to changes in the boiling mode on this surface. For the "stable mode" in which the wall temperature fluctuations are similar in character to those observed on the surface with holes (see Fig. 2), the superheats correlate with values predicted by Eq. (2) for a cavity radius of w5 10-4. The average pore radius for this sintered surface was determined to be between 1 x 10-4 in. and 6 x 10-4 in. Again; the sodium data suggest cavity sizes soinewhat smaller than those indicated by the potassium results; over the temperature range 930 to 1300°F the apparent cavity size 18 ~2 x 10-4 in. The uncertainty in the sodium surface tension may account for most of this discrepancy. Minimum superheats (w22°F for potassium, 50°F for sodium) do not differ sig- nificantly from the values observed on the surface with holes. This suggests H . further that this phenomenon 18 dependent on liquid, rather than surface, conditions. .**. • ..- mardit . . 20 DISCUSSION These experiments have shown that extremely large superheats exist for boiling with the alkali liquid metals on normal, as-received surfaces. For potassium, these superheats range from 2500°F at a saturation temperature of 1500°F down to about 230°F at Teat = 1770°F. Along with the high superheats, wall-temperature oscillations of large. magnitude occur; these oscillations are a source (through therma). fatigue) of potential damage to the boiler walls. Hence, in considering boil ing alkali metals as coolants for nuclear power sys- tems, means must be found for reducing the superheat. The data suggest two methods: 1. The operating temperature can be increased. Thus, for potassium on . the as-received surface, the superheat can be reduced from about 500°F at Tost = 1400°F to ~200°F at 1800°F. On the surface with holes, the wall- temperature fluctuations, which were of ^65°F amplitude and ~1.2 cycle/min .! frequency at Teet = 820 °F, decreased to ~1°F amplitude and ~50 cycle/min fre- quency as the saturation temperature increased to 1260°F. However, with in- creasing saturation temperature, the operating pressure also increases - from . 1 atm at 1400°F to slightly more than 5.5 atm at 1800°F. 2. The surface can be modified. Thus, the presence of discrete 0.006- , in.-diam holes resulted in a nearly fortyfold decrease in the superheat with potassium at 1400°F. The sintered metal surface, while not as effective as the one with holes, also reduced the superheat required for boiling significantly. However, with this latter surface, the performance was more erratic and fluctu- ated between several modes. The studies described constitute a beginning in our understanding of super- heat phenomena in the alkali liquid metals. While we have obtained a quantitative 21 . . . measure of the superheat for boiling with potassium and sodium on several sur- faces, we have not defined an optimum surface treatment or established the effects of heat flux and cavity shape. A second-generation natural-circulation loop has been constructed that incorporates a number of instrumentation and design improvements as well as cylindrical and re-entrant cavities and a wider. heat flux capability. With this system, we hope to Increase the precision and accuracy of our measurements and thus resolve some of the uncertainty in the data trends. The theory thus far developed (Eq. (2) ] appears adequate for predicting the superheat for nucleation from cavities of known radius at moderate heat fluxes; testing of Eq. (2) at higher fluxes and under controlled conditions with cavities of other than cylindrical shapes and different L/d ratios remains. Problems relating to bubble growth rates, the effect of inert gases on bubble nucleation, and the influence of temperature gradients in the liquid are also being considered in our attempt to understand and quantitatively describe the boiling process with the alkali liquid metals.: ITALY 323 UN IS ti SA 11 1.IR . ! . . . END .. : , DATE FILMED 11/ 24 /65 . )