MW CH " UP UNCLASSIFIED ORNL . . monde HI om ΡΙ 18 TOFI . . . " - - -v-v.co v - .- - this in LEGAL NOTICE 12 - This report me propared us an account of Govonament sponsored work. Neither the Valled Statos, nor thow Commission, vor my pornon noting on behalf of the Commission: A. Makes uy warranty or roprosoata uon, expressed or implied, with respect to the accu- racy, complotanono, or uw humous of the information contained in two report, or that the wa. of any information, apparatus, mothod, or procos, disclosed us whilo roport may not intring. privately owned righus; or B. Assumos may ilabilities with respect to the use of, or lor damucos resulting from the une of any information, apparatus, method, or process discloud in wo roport. As used in the abovo, "person acting on behall of the Commission" includes way •a. ployw or contractor of the Commission, or employm of such contraotor, to the extent that such employ. or contractor of the Commission, or employs of such contractor properos, diomminator, or provides accos. to, way tutormation pursuant to his employment ur contract with the Commission, or wo employmeat with sucha contractor. Y . 2 " EL. PIE u . . . IS TM Nii . L . ! ! L 2 + . . L M . . - "YA . 22 AN 2 O . .. SMO M. PL DTIE MICROCARD ISSUANCE DATE . . 8 / 26 , 2 . 2 1964 ( f/if CONF-531-3 TWO rouri wa wogard un accord a Corona mume word work. Nellor dhe Unit mate, mo the Cowana, www porno suing ou model of Coundatos: A. Weka wyvurmay or rowoscute, wussed or implied, mu reagiert w we accu. iwy, owns, or total of the toto wados contained to worsport, or whers who were My wiwatham, murow, mohd a macmi dostawa w rogor my not worlage Windows or *. As .. way Itu .. mind respect to this was of, or for tomos reswung true whe to ay wlorati, nu, med, a menu decloud u wa roport. M an Who whore, "No nya lalu Ho Custoa" lncle My M. moyo w www of the weaton, a plasma wa causw, he enco What well mainan and Cavalentine, wapoyo of such cawachor popru. Identem, mert nem tu, wywanoo pwman to No pogut or caract with Cauciu, or be p une bouch cool actor. RECENT STUDIES OF BOILING BURNOUT -LEGAL NOTICE Facsimile Price S_L 60 Microfilm Prico S_ 80 W. R. Gambill Oak Ridge National Laboratory Avoiloble from the Oak Ridge, Tennessee Office of Technical Services Department of Commerce Woshington 25, D. C. Five recent investigations of the critical heat flux in water systems are described. These studies were conducted in the interval from early 1963 through the present. A. Pool Studies 1. Inherent Uncertainty in the Critical Heat Flux The inherent uncertainty of the critical heat flux in saturated pool . . boiling has been inconclusively debated for some time. In an effort to as- . certain this uncertainty, a series of 234 tests was conducteu at atmospheric . . . pressure with saturated water outside horizontal, ac heated, 0.234-in.-OD, .. A-nickel tubes in an open 6 x 6 x 9-in.-deep pool. Approximately fifty tests were conducted with each of four test sections which were protected from physical burnout by an interna).-thermopile detector circuit which interrupted ...... the applied heating current before the wall temperature exceeded ~450°F. A schematic of the experimental system is shown as Fig. 1. A typical plot of the critical heat flux and of the critical outside wall ... temperature versus test number is shown as Fig. 2. Comparison of the observed variation of the critical heat flux with the maximum relative uncertainty in the experimental critical fluxes of +3% indicates that there is an inherent uncertainty in $. (of ~+9%. in the present instance), under conditions of mini- mum surface variability. Early tests during which inadvertent electrodeposition "Research sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Corporation. NOT FOR PUBLIC RELEASE -FICIAL DISTRICTION MAY BE WIDE. OFFICIAL REQUESTS MAY BE FILLED. REPORT CONTKINS NONING OF PATEM UNTEREST. PROCEDURES ON ME IN RECEING SECTION. ---......................po of copper onto the test sections took place showed that can vary signifi. cantly with the nature and condition of the surface, as also observed by Costello. During these tests, buildup of the deposit consistently gave in- creasing critical fluxes, and removal of the deposits by thermal shocking always resulted in a sharp decrease of the critical flux. Four auxiliary tests were conducted to determine the variation of $. with liquid level when the liquid is even with or below the top surface o.. a horizon- tal tube. A heat flux was preset, and the water allowed to vaporize until the internal burnout detector interrupted the power, at which time the liquid level was immediately measured. The results are shown in Fig. 3, wherein the heat flux was computed using the total external surface area of the tube. This program has been described in detail elsewhere by the writer.2 2. Heat Transfer Inside of Horizontal, Open Ended Heated Tubes Immersed in a Pool This interesting, and previously untreated, case of pool boiling is of possible relevance to the storage of spent tubular fuel elements. Thirty- four series of tests were made with externally vacuum-jacketed tubes of 1/8-, 1/4-, and 3/8-in. diameters, and heated lengths of 3, 7, and 11 in. Pool sub- coolings of 0, 60, and 120°F were used. The flow at the ends of a tube pulsed as pool liquid entering was converted to a two-phase mixture at exit; pulse rates varied from 0.01 to 0.67 cps. At small ratios of L/D, a critical heat flux could be attained in the sense that a very rapid excursion of the tube-wall temperature eventually occurred as the heat flux was gradually increased. At large L/D ratios, how- ever, a critical state was never reached, and the tube-wall temperatures merely leveled off at a higher plateau shortly after each increase of t:eat flux. Reduction of the data obtained during these tests has not been completed. 3. Natural-Convection Burnout of closed Vertical Channels The meager critical-flux data for a vertical heated channel open at the top to a liquid supply and closed at the bottom have been collected and correlated in two ways. The data for water coolant3-5 were used to develop the empirical. correlation: (telmin. - (2.0 X 309) ( ) (1) C'min. 0 .8 in which the units of heat flux, length, and pressure are Btu/tır.fta, in., and psia, respectively. The other correlation is based on Wallis' proposal for prediction of the limit of gas flow at which no liquid will run down the inner wall of an unheated vertical empty tube and may be expressed in dimensionless form for the case of heat addition by: ($c'min. In 1 D.3/2 ( 840)2/ 2 (2) Each equation satisfactorily predicts a minimum above which the critical flux data for the subject situation fall, but the greater generality of Eq. (2) recommends it for predictive purposes. B. Forced-Convection Investigations 1. Subcooled Boiling and Burnout with Heated Twisted Tapes In past RNL swirl-flow tests with twisted tapes,7,8 ~99% of the heat generation took place in the metallic tube walls. In the present program, all the heat was generated in the twisted tape. As shown in Fig. 4, water flowed by gravity from a constant-head tank at 5 to 8 fps through a vertical 0.27-in.-ID glass tube ~13 in. long, in which was positioned a full-width, resistance-heated, 16-mil-thick A-nickel tape. Tape twist ratios were varied from 2.7 to w inside tube diameters per 180-deg twist, inlet water tempera- tures from 63 to 173°F, and heat fluxes from 0.2 x 106 to 1.2 x 106 Btu/hr.ft2. Figure 5 is a photograph of a typical test section with water flowing but no heat applied. The mean surface temperatures of the tapes for 64°F inlet water are plotted versus the heat flux in Fig. 6. The shape of the curves is typical for water in subcooled forced-convection boiling. In Fig. 7, the ratio of the twisted-tape wall superheat at burnout to the average straigüit-tape critical wall superheat is plotted versus the tape twist ratio for each inlet water temperature. The critical superheat in- creases continuously with decrease of 'y', and the increment is smallest for the lowest inlet, temperature. A similar plot of the burnout heat flux ratio is shown as Fig. 8; the burnout fluxes for the twisted tapes are between 93% and 122% of those for flat tapes, maximizing in all cases at a tape-twist ratio of 7 to 10. It is postulated that the deleterious effect of centripetal acceleration with this geometry, which tends to hold the vapor on the heated surface, is compensated by inertial impingement of the liquid onto the tape surface, which is inclined to the axial flow direction when the tape is twisted. This impingement tends to remove vapor bubbles from the tape surface, and the opposite influences of centripetal acceleration and impingement act to form a coalesced vapor cloud displaced almost equally between the inside tube surface and the tape surface. The power density of an assembly of swirl. flow tubular fuel elements could therefore be increased significantly by generating heat ir. the twisted tape as well as in the tube wall. 2. Swirl-Flow Forced-Convection Bulk-Boiling Loop This system, shown schematically in Fig. 9, will be used to measure critical heat fluxes of two-phase water-steam mixtures in swirl pl.ow induced by full-length twisted tapes. The loop is open, and a large pressure drop can be taken across the valve immediately upstream of the test-section entrance in order to stabilize the ilow. It is planned to investigate vertical upflow for exit pressures of 15 to 45 psia, inlet liquid velocities of 1 to 6 fps, inlet subcoolings of 0 to 30°F, and tape twist ratios of 2.5 to 8 inside diameters per 180-deg twist. Empty tubes will be used to obtain axial-flow burnout data for comparison with the swirl data. The present heat.-fluy capability of the system is ~2.5 x 106 Btu/hr •ft2 with 18 in. long A-nickel tubula: test sections. In the tubes fitted with twisted tapes, 93.8% of the total heat generation wiil take place in the tube wall.. At $ = 106 Btu/hr •fts, the temperature difference across the wall will be ~85°F, and the adiabatic rise rate of tube-wall temperature ~840°F/sec. Turbine Plowmeters with a calibrated reproducibility of £1/2% maximum devia- tion are used for measuring flow rates, and energy balances to date have checked within 5.6%. A closeup of an empty test section is shown as Fig. 10; pressure tapes may be seen at the entrence, at 3/4 the heated length, and at the exit. Figure 11 shows the same test section following installation of thermocouples, a current transformer, insulation, and a shroud tube concentric with the test section. On the interior surface of the nonconducting Lavite shroud tube are mounted 32 subminiature light-activated switches which will act as the sensing elements of the burnout protection system. These solid-state switches of the silicon PNFN type are normally open, but switch into conduction in microseconds when the radiant intensity rises to ~3 mw/cm2; they are sensitive in the near- infrared and visible portions of the spectrum. The switches are wired in parallel and the closing of any one will activate a relay and circuit breaker to interrupt the test-section heating current. The total response time of the entire circuit is ~80 msec. This system of detecting incipient burnouü allows continuous monitoring of the entire external surface area of the test section. Literature Cited 1. C. P. Costello and W. J. Frea, "A Salient Non-Hydrodynamic Effect on Pocil Boiling Burnout," University of Washington Technical Report, Merch 1963. 2. W. R. Gambill, "An Experimental Investigation of the Inherent Uncertainty in Pool-Boiling Critical Heat Fluxes to Saturated Water," presented at the A.I.Ch.E. 51st National Meeting, San Juan, Puerto Rico, October 1963, Pre- print No. 18; scheduled for publication in the A.I.Ch.E. Journal, July 1964. 3. M. Troy, "Zero Net Flow Burnout Tests at 2000 Psia," WAPD-TH-251, 1956; WAPD-TH-304, 1957. 4. W. R. Gambill and R. D. Bundy, "Burnout Heat Fluxes for Low-Pressure Water in Natural Circulation," USAEC Report RNL-3026, pp. 11-21, Oak Ridge National Laboratory, December 20, 1960. 5. P. Griffith, W. A. Schumann, and A. D. Neustal, "Flooding and Burn-out in Closed-End Vertical Tubes," Paper No. 5, Symposium on Two-Phase Fluid Flow, Institution of Mechanical Engineers, London, February 7, 1962. 6. G. B. Wallis, "The Transition from Flooding to Cocurrent Annular Flow in a Vertical Pipe," British Atomic Energy Authority Report AELW-R-).\2, 1932. 7. W. R. Daunbill, R. D. Bundy, and R. W. Wanobrough, "Heat Transfer, Burnout, and Pressure Drop for Water in Swirl Flow Through Tubes with Internal Twisted Tapes," Chem. Eng. Progr. Symposium Series, Vol. 57, No. 32, 127- 137 (1961); also USAEC Report CRNL-2911, Oak Ridge National Laboratory, March 28, 1960. 8. W. R. Gambill and R. D. Bundy, "High-Flux Heat Transfer Characteristics of Pure Ethylene Glycol in Axial and Swirl Flow," A.I.Ch.E. Journal, 9(1): 55-59 (January 1963). Notation outside diumeter of tube inside diameter of tube equivalent diameter of flow channel gravitational acceleration heated length of channel length between voltage taps absolute system pressure temperature stsat wall superheat, (tw - tsat) tape twist ratio, inside tube diameters per 180-deg twist latent heat of vaporization saturated liquid density saturated vapor density phase density difference, ( 40 pv) surface heat flux Subscripts average of surface avg. bo burnout sat saturation critical st of straight tape min. minimum of twisted tape mn. mean wall outside ILLUSTRATIONS AND CAPTIONS Fig. No. Title Schematic Pool Boiler Configuration Critical Heat Fluxes and Outside Wall. Temperatures for Test Section No. 3 Effect of Liquid Level on the Critical Heat Flux for 2 Horizontal Tube Schematic of Heated-Tape Experimental System Typical Test Section, Water Flowing, Zero Heat Flux Mean Surface Temperature Versus Heat Flux for 64°F Inlet Water Ratio of Critical Wall Superheats versus Tape Twist Ratio for Three Inlet Temperatures Ratio of Burnout Heat Fluxes versus Tape Twist Ratio for Three Inlet Temperatures Swirl-Flow Two-Phase Boiling System Photo of 0.47-in.-ID Test Section Photo of Test Section After Installation of Thermo- couples, Shroud Tube, Current Transformer, and In- sulation UNCLASSIFIED ORNL OWO. 64-1761 1.0 Slo: 171°F LIITTTTT soy. 121°F bot To do'st, avg. 4 6 8 10 12 14 16 18 20 22 24 26 28 TUBE DIAMETERS 180 deg. TWIST UNCLASSIFIED ORAL-LR-DWG 79598R BOTTOM OF TUBE LIQUID LEVEL AT POWER TRIP (in. BELOW TOP OF TUBE) TUBE NO.3. SATURATED POOL BOILING OF WATER P = 14.3 psia TOP OF REDUCED SECTION 0.1 0.1 0.4 0.5 0.2 0.3 PRESET $ (106 Btu/hr.f72) Fig. 15. Effect of Liquid Level on the Critical Heat Flux for a Horizontal Tube. UNCLASSIFIED ORNL-LR-OWG 79587R 1000 L SANBORN RECORDING WATTMETER SWITCH AC POWER LEAD AC POWER LEAD warm water . TEFLON GASKET -- 375 kva TRANSFORMER AND SATURABLE REACTOR CONTROL 'CURRENT TRANSFORMER 15000:5) . 00 POOL SURFACE STAINLESS STEEL POOL FLANGE Yehoren WK -COPPER ELECTRODE : 0.250 in. : 0.234 in. 2 in. TYP a UZTIELIZ12212 were og m 5 OF 6 TC'S AT EACH END CONNECTED IN SERIES TO MICROSWITCH AND CIRCUIT BREAKER S o 2 in. --- 4 lain. .. STAINLESS STEEL POOL BASE 1 -------- 6% in. ------------ Ni LINER (ADDED LATER) TUBE FIRST 6 1,2,3,4,40 dolin. 0.250 0.234 0; (in.) 0.180 0.180 Lolin. 6.5 6.5 Lylin.) 4.0 - 2.5 NOTES: PRELIMINARY TESTING, TUBE O.O. NOT REDUCED FOR FINAL TESTS AS SHOWN IN FIGURE Fig. 1. Schematic Pool Boiler Configuration. UNCLASSIFIED ORNL-LR-DWG 79592R MEAN 10-34c (Btu/hr.ft2) - (twole (°F) 5 10 15 20 12 THERMOCOUPLE AVG 25 30 35 40 TEST NUMBER 45 50 55 Fig. 6. Critical Heat Fluxes and Outside Wall Temperatures for Tube No. 3. UNCLASSIFIED ORNL OWO. 04-1764 botthhhh - - (Algerido, o (Ai sordbo, si, avg. 2 4 6 8 10 12 16 18 20 22 24 26 28 30 14 TUBE DIAMETERS) 180 deg. TWIST UNCLASSIFIED ORNL OWO. 64-1756 320 ܘܘܕ 64°F ' ܙo'_ ܘ28 266 (*ܢ uu (.) SYMBOL ܘ20 21.4 ܘ.12 ܘܘܠܐ ܘ ܗ ܘ.3 160 440 4.| 2.ܙ . ܘ. ܙ 8.o_ 6 . 6_ 4.ܘ_ 2.ܘ _ ܘ_ ܝ2;; iu / mr ) ܤ 6-no UNCLASSIFIED ORNL OWO. 64.1763 POTENTIOMETER (INLET WATER TC) FLOAT VALVE UNCLASSIFKD ORNL OVI 64-3407 -CONSTANT-HEAD TANK COOLING WATER OUT CALCINED SILICA INSULATION VENT VALVES - STEAM COIL WATER IN STEAM CONDENSER- ENTRANCE ELECTRODE COOLING WATER IN PRESSURES RELIEF VALVE Voo CURRENT TRANSFORMER STEAM SEPARATOR MKARTA BLOCKS - WATER INT $2.50 LB. 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