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LEGAL NOTICE
This report was prepared as an account of
Government sponsored work. Neither the
United States, nor the Commission, nor any.
person acting on behalf of the Commission:
A. Makes any warranty or representa-
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or that the use of any information, appa-
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B. Assumes any liabilities with respect
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method, or process disclosed in this report.
As used in the above, "person acting on
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prepares, disseminates, or provides access
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MASTER
2.8
STUDIES IN BOILING WITH LIQUID POTASSIUM
vailable from the
Office of Technical Services
Department of Commerce
Washington 25, D. C.
H. W. Hoffman and A. I. Krakoviak u
Oak Ridge National Laboratory
Oak Ridge, Tennessee
The experimental program at the Oak Ridge National Laboratory into the
thermal and hydrodynamic characteristics of boiling alkali liquid metals has
been continued without essential change in either scope or direction.
(A
sumunary of this program was presented at the Eurator-USAEC Conference? on
Two-Phase Flow Problems in 1962.) While all aspects of the work are being
pursued, this report is concerned only with those portions of these studies
involving convective boiling with potassium.
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Forced-Convection Experiments
The study of convective boiling with liquid potassium flowing inside or a
..
vertical, circular tube has been continued with emphasis on the determination
of critical heat flux values. Following the deterioration of parts of the
copper-stainless steel bond and the loss of a significant number of thermo-
couples, the previously described? high-flux boiler (~500,000 Btu/hr.ft?) was
removed from the system. Details of this boiler are indicated in Fig. 1. Con-
siderable difficulty has been experienced in fabricating a replacement boiler
of the same design; Pt-Pt 10% Rh was substituted for the original Chromel-Al. umel
Ett
couples, a pressure tap was added at the test-section midpoint, and wall thermo-
ST
couples were attached at several locations.
A
.
.
"Research sponsored by the U. S. Atomic Energy Commission under contract
with the Union Carbide Corporation.
11. W. Hoffman, "Studies in Two-Phase Flow and Boiling Heat Transfer at
the Oak Ridge National Laboratory," pp. 18-23 in Two-Phase Flow Problems,
European Atomic Energy Community Report EUR 352.e, 1963.
- LEGAL NOTICE

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Pending satisfactory solution of the construction problems, a medium
flux level boiler (~100,000 Btu/hr.ft2) was prepared and has been recently
installed. This unit (Fig. 2) was formed from a 0.84-in.-OD, 0.27-in.-ID,
type 347 stainless steel tube approximately 6 ft long. Heat was supplied
radiantly to this tube by a set of six 11.6-in.-long cylindrical electric
resistance heaters; these primary heaters can in turn be guarded by a second !
set of surrounding heaters. The heaters were separated by 1.5-in. gaps; in
these regions the tube wall was thinned fron the originai 0.285 in. to 0.073
in. Thermocouples (Pt-Pt 10% Rh) were placed on the wall in these regions as
well as in radial wells (~0.25-in. deep) at the midpoints of the heaters. Pres-
sure taps were located in the final four gaps and at the boiler exit. Iwo small,
diametrically opposed holes (L/d ~10 to 15) on the inside of the tube about 6 in.
upstream of the boiler inlet functioned as sites for bubble nucleation; this
region was separately heated. With these sites, the loop has operated stably.
Analysis of the data thus far obtained with this boiler for heat transfer
below the critical is incomplete. Two series of runs have led to Murnout";
the results are shown in Fig. 3 in comparison with the prediction based on the
Lowdermilk, Lanzo, and Siegel correlation for water. As with the results ob-
tained previously, ? the agreement with the estimated values is excellent.
(Note that the version of Fig. 3 given in ref. 1 is incorrectly drawn. )
Superheat Studies
A consideration of the previously discussed results,? coupled with observa-
tions in loops designed for materials compatibility studies, has led to the
SW. H. Lowdermilk, C. D. Lanzo, and B. L. Siegel, "Investigation of Roil-
ing Burnout and Flow Stability for Water Flowing in Tubes," Nat. Adv. Comm.
Aero. Technical Note 4382, September 1958.
3
conclusion that the extreme wall-temperature fluctuations found had occurred
in the presence of unexpectedly high liquid superheats. To verify this de-
duction, an experiment providing a quantitative measure of the liquid superheat
associated with boiling potassium was devised.
The apparatus constructed (shown in Fig. 4) provided for natural-convection
boiling within a small (20-in. X 20-in.) loop of 1/2-in. schedule-40, type 347
stainless steel pipe. Three test sections, indicated by the brackets A, B, and
C in Fig. 4, were included; the structure of the surfaces studied is given in
Table 1. Chromel-Alumel thermocouples (0.010-in. wire) were welded to the
Table 1. Boiling Surface Characteristics
Region
Surface Treatment
Porous surface coating
Two opposed rows of 6 small diameter holes
spaced at 1 1/2-in. separation
As received
.
:-
-
outside tube wall at the indicated positions. Each test region was heated by
*
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-
*
.
a pair of electrical resistance heaters; the power input to the heaters was
used to calculate both the heat flux and the temperature drop through the tube
wall. The loop pressure was measured with a calibrated bronze Bourdon-tube
gauge (o to 30 psia range) maintained at a constant temperature of 160°F. In
general, the saturation temperature, as determined from the loop pressure,
averaged 3°F above the value indicated by the thermocouples in the condenser
region; the maximum deviation was 10°F.
The loop was operated by boiling potassium in region A while region B re-
mained unheated; the vapor was condensed by cooling the tube wall at the top
of the loop. Alternatively, surface B could be studied by heating this region
while section A remained unheated. Finally, region C could be examined by
rotating the loop clockwise by 90 deg.
Typical of the temperature oscillations recorded are those given for sur-
faces A (labeled treated) and B (smooth) in Figs. 5 and 6, respectively. (As
the result of fabrication errors, surface B was found subsequent to the tests
to be in an essentially as-received condition.) The heat flux in these experi-
ments ranged between 16,000 and 37,000 Btu/hr.fte. All temperatures given
(except as specifically noted) are outside tube wall temperatures; the calcu-
lated temperature drop through the wall was generally less than 13°F.
With the smooth surface, temperatures in both the heater and condenser
region are shown in Fig. 5. For each cycle, the saturation temperature was
taken to be the minimum in the condenser curve. The maximum superheat was
then obtained as the temperature difference between the maxima in the heater
curve and the minima in the condenser curve; uncorrected superreat values were
210 to 240°F for the specific data pictured. The tube wall temperature fluc-
tuated with an average amplitude of vi40°F at a frequency of 0.025 cps. This
pattern of wall temperature oscillations is very similar to that which had been
previously observed in the low-flux boiler.
Contrasting with this behavior was the temperature fluctuation character-
istic found with the treated surface (Fig. 6).
Three distinct operational
features were observed:
1. Temperature spikes of large magnitude (95 to 125 °F) and irregular
occurrence were noted. The thermal pattern and the sounds associated with
these "umps" were similar to those observed with the smooth surface; uncor-
rected superheats ranged between 140 and 180°F.
2. Separating these peaks were periods of various length during whicii
"quiet" operation existed. The liquid superheat associated with this voil-
ing mode was of the order of 25 to 30°F; the thermal oscillations were of low
magnitude (2 to 3°F) and relatively high frequency (~1.3 cps). It is believed
that these regions correspond to periods of stable boiling.
3. Finally, as shown by the right portion of the curve in Fig. 6, some
periods of fluctuations of intermediate magnitude and frequency were present.
These oscillations were -20°F in magnitude at a frequency of 0.13 cps; the
superheat was 60°F.
It should be noted that the temperature trace of Fig. 6 is not a composite
but a continuous four-minute portion of the recorded temperature profile chosen
to display the three features described above. The saturation temperature
associated with the data of Fig. 6 was 1420 to 1450°F.
These results are sumarized and compared in Fig. 7; the liquid superheat
(corrected for the temperature drop through the tube wall) is plotted against
the saturation temperature at each operating condition. Comparison is also
made with the superheat values estimated by the equation,
AT sat = TW, i - Tsat -
2 R Tato
AHP, r
(1)
where Tw, 1 is the inside wall temperature; Tsat, the fluid saturation temper-
ature at pressure, Pe; R, the gas constant; o, the surface tension of the
liquid; AH, the latent heat of vaporization; and r, the radius of the bubble
nucleation cavity. The derivation of Eq. (1) is given in the Appendix to this
paper. In Fig. 7, r was introduced as an independent parameter to generate
the multiple curves shown.
-
-
.
The upper set of data (triangles) were obtained with surface B and show
remarkably little scatter along with good reproducibility. The measured
superheat ranged from 701 °F at Teet of 950°F to a low of 269°F at 1480°F;
the saturation temperature level was controlled by adjusting the loop pressure.
Corresponding results with surface A are given by the solid circles. Over
the same temperature span as with the smooth surface (590 to 1480°F), the super-
heat varied from 107°F to 21°F, respectively. Thus, the addition of "artifical"
bubble-nucleation sites over the whole boiling surface has resulted in a six- to
thirteenfold decrease in the superheat required for bubble formation. Over
most of the temperature range examined, data scatter, while greater than ob-
served with the smooth surface, was still reasonable. The deviations found in
the limited temperature span between 1300 and 1450°F can be related to the boil-
ing modes discussed in reference to the temperature trace in Fig. 6; the upper-
most superheat values in this band derive from the thermal spikes, the inter-
mediate data points correspond to the lower amplitude peaks, and the main body
of data, to the stable boiling regions. A consistent explanation for this
anomalous behavior near the atmospheric boiling point is not now available.
A fourth group of points in Fig. 7 cluster at a saturation temperature
near 1300°F with the lowest superheat values measured in these experiments
(13 to 17°F) and correspond to a temperature pattern not observed in the time
span of Fig. 6. During the period in which these data were obtained, the tube
wall temperature remained constant without fluctuations for times as long as
one minute.
Discussion
-
-
With the demonstration of the high superheat required for bubble nucleation
with potassium flowing in a smooth tube, it is possible to develop a reasonably
.
ri..
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th,
coherent explanation for the phenomena observed during operation of both the
low-flux and the high-flux boilers. Under equivalent conditions of geometry,
heat input and mass flow, the radial temperature profile for potassium moving
through a tube will be essentially flat in contrast to that for water where a
significant temperature drop occurs across a narrow region close to the wall.
(The thermal conductivity of potassium is nearly 50 times that of water; and
the thermal diffusivity ratio, K to H20, is of the order of 355). Thus, with
water, a bubble nucleating at the wall grows rapidly in the thin film of super-
heated liquid near the surface and either collapses after leaving the wall, if
the bulk fluid is subcooled, or remains as a stable or growing entity, if the
bulk fluid is saturated. In contrast, with potassium, the bubble once formed
remains in a highly superheated environment even at the tube center?ine and can
be expected to grow rapidly (even explosively) until the total energy available
from the superheat is consumed. In the low-flux boiler, the energy per unit
volume in such a situation has been estimated to be sufficient to vaporize the
potassium to a quality of about 7.5% (void fraction of ~0.99 assuming zero slip).
Thus, a periodic mechanism is pictured in which the wall temperature first rises
along with the creation of significant liquid superheat and then falls as the
buöble-nucleation threshold is exceeded and the heat is released through the
formation of vapor. This pattern is consistent with observation in botir the
forced-convection low-flux boiler and the natural-circulation boiler.
Stable performance (1.e., no oscillations in temperature, pressure, or flow)
has been observed in the forced-convection boiling-potassium loops only when a
quality inlet condition existed. It is probable that stability resulted from
the presence of the bubbles in the liquid so that the high superheat required
for bubble initiation was circumvented. Alternatively, it has been estimated
from data on the flow of air-water mixtures in a vertical tube that annular
f.low will exist for potessium liquid-vapor mixtures at the quality present at
the boiler inlet. If the liquid remaining exists as a thin film on the wall
(this has been calculated to be 0.040-in. thick in a l-in.-ID tube), then,
Sa
because of the high threshold for bubble nucleation, conduction alone will
sustain a substantial therral flux; e.g., for AT at = 125°F, conduction can
account for a heat flux of 0.7 x 106 Btu/hr.fta. Since the heat flux in all
tests to date have been below this limit, the heat transfer can be explained
entirely by conduction through a thin liquid film at the wall followed by
evaporation at an inner liquid-vapor interface.
It is also conceivable that
the liquid remaining exists in the form of small droplets entrained in the
vapor flow. Again, stabie flow accrues with the liquid dropleta diffusing
along the radial velocity gradient and evaporating at the wall. In either cir-
cumstance, Murnout" is reached when the total mass of liquid available is no
longer sufficient for cooling the wall.
APPENDIX
Derivation of Superheat Equation
The condition for equilibrium of a spherical bubble in a liquid pool can
be expressed as:
20
Pr - P
=
(2)
for bubble growth, Pn, the pressure within the bubble must exceed that in the
PN. A. Radovcich and R. Moissis, "The Transition from Two-Phase Bubble Flow
to Slug Flow," MET Report No. 7-7673-22, Department of Mechanical Engineering,
Massachusetts Institute of Technology, Cambridge, Massachusetts, June 1962.
V
Y
.
..
external liquid, Po; when evaporation into the bubble also exists, the tem-
perature of the liquid exceeds that corresponding to the saturation tempera-
ture of the liquid at pressure, Py. This excess of temperature, over the
saturation value of P, is termed the liquid superheat. By combining Eq. (2)
with the Clausius-Clapeyron relation and including the assumptions that the
specific volume of the lqiuid can be neglected in comparison to that of the
vapor ard that of the gas law relation, PV = RT, describes the specific volume
of the vapor, an equation for predicting the superheat in terms of the radius
SL
-
of the bubble-nucleation site, r, is obtained. Thus,
2 RT 20
"sat
At sat = TW, 1 - Isat -
(1)
TH P, I
This equation has been tested and found adequate for water and a number of
organic fluids by several investigators. 4,5
If it is then presumed that Eq. (1) can also be applied with the liquid
metals, the superheat required for bubble nucleation with potassium and the
other alkali metals can be estimated; results are given in Table 2. These
Table 2. Estimated Superheats Required to Sustain
Boiling with the Alkali Metals

Fluid
Superheat, ord
Fluid
Superheat, °F
Sodium
Rubidium
258
125
101
67
Potassium
Cesium
4p. Griffith and J. D. Wallis, "The Role of Surface Conditions in Nucleate
Boiling," Chemical Engineering Symposium Series, Vol.56, pp. 49-63, 1960.
Sp. Berenson, "Transition Boiling Heat Transfer from a Horizontal Surface,"
MIT Report No. 17, Heat Transfer Laboratory, Massachusetts Institute of Technology,
Cambridge, Massachusetts, March 1960.
10
liquid-metal superheats were evaluated by forming the ratio (ar set.w/ar.sst. )
from Eq. (1); the superheat for water at atmospheric pressure was taken as
30°F.
While this calculational procedure avoided the issue of the cavity radius
involved in liquid-metal boiling, some discussion of this point is warranted.
Equation (1) shows that as the surface temperature exceeds saturation, bubble
formation will begin first in those sites (vapor-filled cavities) of largest
radlus. As the wall temperature continues to rise, smaller and smaller cavities
will be brought into action. For a natural surface (such as an as-received pipe
wall), these cavities should show a normal distribution in sizes. Presumably,
ܙܟܫ
with ordinary ?luids, these cavities are not wet by the fluid; i.e., the liquid
does not displace trapped or adsorbed gases. On the other hand, the liquid
alkali metals at their boiling temperatures wet most container metals readily;
and it is suspected that the nucleation sites are either entirely eliminated or
at best predominate in small sizes. Comparison on the basis of Eq. (1) should
then be with very pure water against "clean" surfaces; for this circumstance,
superheats of the order of 90°F have been measured with water. Thus, calculated
alkali metal superheats may be as much as a factor of three greater than the
values listed in Table 2.
-
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UNCLASSIFIED
ORNL.LR-DWG. 761138


HIGH FLUX BOILER
og god
-BOILER TUBE
CLAMSHELL
HEATERS
HEATER-
COIL
LEADS
COPPER
DISKS
TEST SECTION BOILING K EXPERIMENT
THERMOCOUPLES-
os
LOW FLUX BOILER
12"
UNCLASSIFIED
ORNL DWG. 63-7537
PRESSURE
GAGE
VACUUM
1/2"- SCHED 40-3475S
(20" X 20")

tlemiselle
Human right
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-
OHEATER
00.010" C/A THERMOCOUPLE AC/A CONTROL
THERMOCOUPLE
NATURAL CONVECTION SUPERHEAT LOOP
UNCLASSIFIED
ORNL OWO. 63-7539
1700

Y.. :
1680
HEATER
1660
1640
LII
1620
OUTSIDE WALL TEMPERATURE, °F
o.L
1500

CONDENSER
1480
1460
1440
2
TIME, min.
TEMPERATURE OSCILLATIONS-SMOOTH SURFACE
ORNL DWG. 63-7538
1600

4 1580
F 1560
1540
OUTSIDE WALL TEMPERATUR
0 1480
1460
14601
2
3
TIME, min.
TEMPERATURE OSCILLATIONS-TREATED SURFACE
UNCLASSIFIED
ORNL DWG. 63-7540
1000r
800,
600
A SMOOTH SURFACE
O TREATED SURFACE
CALCULATED

P: 3 X 10-6 in
1- 400
10-5
10-5
TX 10-4
DEGREES OF SUPERHEAT, (Twisor) °F
Oo3
X 10-4 +
.
1x 10-3
40
13 X 10-3
0.030 VX10-2
900
1600
1000 1100 1200 1300 1400 1500
SATURATION TEMPERATURE, tsat. °F
LIQUID SUPERHEAT WITH POTASSIUM
UNCLASSIFIED
ORNL DWG. 64-1689R
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C_0

0
Lid : 45
__
L/d : 71
HEAT FLUX, $crit, Btu/hr.ft
Lid = 285
Lid : 834
s
104L
104 2 4 6 8 1052 4 x 105
MAȘS FLOW, G, 1bm/hr.ft2
CRITICAL HEAT FLUX WITH BOILING POTASSIUM

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- 12