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ORNL-TM-3528.txt
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ORNL-TM-3528.txt
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ORNL-TM-13528
Contract No. W-TL05-eng-26
Reactor Division
SOLUTION OF THE EQUATION DESCRIBING THE INTERFACE BETWEEN TWO
FLUIDS FOR THE VOLUME AND PRESSURE WITHIN ATTACHED,
SESSILE SHAPED, BUBBLES AND DROPS
J. W. Cocke
AUGUST 1971
This report was prepared as an accoun
t o
sgonsm_'ed by the United States Government %e‘;(})x‘::-
é e qutgd States nor the United States Atomi.c Energy
th{;l;rrlmlsslon, nor any of their employees, nor any of
makesco:tractors, subcontractors, or their employees,
mmax l-a y warranty, express or implied, or assumes any
eg iability or responsibility for the accuracy, com-
pleteness or usefulness of any information app':n‘atus
product of process disclosed, or represents ’that its use’
would not infringe ptivately owned rights,
OAK RIDGE NATIONAL LABORATORY
Oak Ridge, Tennessee
Operated by
UNION CARBIDE CORPORATION
for the
U. S. ATOMIC ENERGY COMMISSION
iii
CONTENTS
A.bs-tract . . - . * . a - - - . - L] - » - - . . *
Introduction « o« ¢« ¢ & v 4 4 ¢ o o o & o s s e .
Derivation of the Interfacial Equation . . .
Numerical Solution of the Interfacial Equation .
Computer Program . . « « « o & o o o &+ & &
Solution of the Differential Equation . .
Solution for h/a and V/a® versus B and ¢ . .
Solution for h/a and V/a® versus B and r/a .
Solution for h/a and V/a® versus r/a . . . .
Range, Running Time, and Accuracy . . .
Results « ¢ v & ¢ v v v v v v 6 v 0 o 4 o
Discussion of Results . « + 4 ¢ « ¢ & « o
Estimation of the Accuracy . . « . . .
Comparison with Previous Results . . . . . .
ConclusionsS v ¢ ¢ « o o e 2 o o o o o o o s o
References .« v v v ¢ 4 6 o ¢ o 2 s o o « o o
Nomenclature . o o o & o & o o s o s o o o o o 4
AppendiCesS o o ¢« ¢« 4 4 4 s e e s s s s 4 e e e s
A, Parametric Crossplots . . . « . « « . .
B. Tabulated Results « ¢« « ¢ ¢ « o o « « &
C. Computer Program « « « + o « o« o o « « &
10
10
LT
17
21
21
22
25
27
43
SOLUTION OF THE EQUATION DESCRIBING THE INTERFACE BETWEEN TWO
FLUIDS FOR THE VOLUME AND PRESSURE WITHIN ATTACHED,
SESSILE SHAPED, BUBBLES AND DROPS
J. W. Cooke
ABSTRACT
A numerical computer program was written to solve the equa-
tion describing the interface between two immiscible fluids to
obtain the shape, size, volume, and pressure of attached bubbles
and droplets. These relationships are important to the study
of three-phase heat transfer, superheat, critical constants,
and interfacial energies. Previous solutions have been obtained
with limited accuracy for a restricted number and range of vari-
ables. The present results are given in both graphical and
tabular form for a wide range and number of parameters, and the
computer program is included so that an even broader range and
number of variables, as well as specific values, can be obtained
as required. In particular, an expression for the maximum-
bubble-pressure was derived which is considerably more accurate
over a wider range than previous expressions.
Keywords. Surface tension, interfacial tension, bubbles,
drops, contact angle, maximum-bubble-pressure, filuid-vapor in-
terface.
INTRODUCTION
A knowledge of relationships among volume, pressure, shape, and size
of attached bubbles and droplets are important in the study of boiling
and condensing heat transfer, superheat and critical point phenomena,
mass transfer studies, and in the measurement of contact angles and sur-
% These relationships can be obtained from the solution
face tension.'-
of the second-order, nonlinear differential equation describing the inter-
face between two fluids. This equation has been solved by perturbation
analysis for very small bubble sizes by Rayleigh® and Schrddinger,® by
numerical hand calculations for a wide range of discrete drop shapes by
Bashforth and Adams,7 and by numerical computer calculations for droplet
volumes and heights by Baumeister and Hamill.® However, the Rayleigh-
Schrodinger solution is no longer used extensively because of concern
regarding its accuracy; and the Bashforth and Adams (as well as the
1
Baumeister and Hamill) solutions were obtained for a limited number and
range of variables.
In order to check the accuracy and to extend the usefulness of the
above solutions, a numerical computer program was written to solve the
interfacial equation. The subject program solves the interfacial equation
for any value of the dimensionless shape parameter, B, for positive values
of the term 87 (sessile drops or above-attached bufibles)* and extends the
previous sclutions for ¢ > 180 degrees. The solution not only provides
the dimensionless X, 7, ¢ coordinates describing the profiles of the inter-
face, but also gives the bubble volume within and the pressure difference
across the interface as a function of profile shape and, in addition, the
maximim pressure difference across the interface for a given radius of
attachment (which is required for the calculation of the surface tension
by the maximum-bubble-pressure technique).
This report descrites the derivation of the interfacial equation,
its numerical solution, and the computer program employed. The re-
sults and an estimation of their accuracy are presented and compared
with the previous solutions.
DERIVATION OF THE INTERFACIAL EQUATION
At the interface separating two immiscible fluids (usually a liquid
and its vapor), a force imbalance exists which results in an interfacial
tension. The tension is a force per unit length pulling uniformly in all
directions tangent to the interface., Neglecting gravitational forces, a
force balance on an infinitesimal segment of a free surface is shown in
Fig. l1-a. The surface is bowed by a uniform differential pressure, P,
where r; and r, are the principal radii of curvature. The force balance
would thus be:
AZ X
P Az =2y | X — + Az —— , (1)
21‘1 21'2
*
The case for negative values of the term BZ (pendant drops or
below-attached bubbles) will be presented in a later report.
ORNL-DWG 7{- 7609
yAX
PAZAX
LIQUID
BUBBLE (8)
Fig. 1. Schematic View of a Sessile-Type Drop and Bubble
and the Force Balance on an Infinitesimal Surface Element.
or
P = ‘Y(L + }"—) . (2)
If gravitational forces are present, the influence of the dif-
ferential fluid density, p, must be considered. For an interface assumed
To be symmetrical about an axis of revolution, the force balance equation
for the bubble shown in Fig. 1-b will be
1 1 Pg
P_y(——+—-—)——(A—Z)=O s (3)
ry Ta g
c
where p = Pp, — Py At the orgin, ry = rz3 = b and thus
2y g
P=—+A— . (4)
b =
Furthermore, with r; = x/sin ¢, Eq. (3) becomes
2y pgz sin ¢ 1
——+-———y< +-—-—)=o ! (5)
b g X To
c
A similar derivation for the drop shown in Fig. 1-b would also result
in Eq. (5). This equation can now be made dimensionless with respect to
©, and after rearranging:
1 sin ¢
- + =2+ Bz , (6)
R X
where .
gpb
B = (7)
Y&,
and
To X z
R =— " X ==, and Z2 = — .
b b b
Equation (6) can be transposed into Cartesian coordinates by
substituting
1 d*7/dx®
R [1+ (az/ax)*]1¥ =
and
. dz/ax
sin = -
[1 + (az/ax)® ¥~
to obtain
a2z, Az az azel¥ e
— + l+(—-—- — = (2 + 82) 1+<—-—-) , (8)
axe dx Xdx ax
which is a second-order nonlinear differential equation, with boundary
conditions:
X =0 3 2z =0
X =0 ; 4dz/XaX =1
Although Eq. (8) cannot be solved analytically in terms of ordinary
functions, its numerical solution is described in the next section.
NUMERICAL SOLUTICN OF THE INTERFACTAI, EQUATION
Equation (6) can be rearranged to read
1
R = (9)
2 + BZ — (sin ¢)/X
and by definition:
ag
— =1
dg
dX
— =Rcos ¢ =F (X, Z, @) , (10)
dg
dz
— B R Sin¢ = G (X, Z, ¢) - (ll)
d¢
As long as BZ > 0, R will always be finite,.
A numerical technique of fourth-order accuracy developed by Runge-
Kutta” was selected to solve the set of simultaneous Egs. (9), (10),
and (11); the iterative equations decribing this technique are:
1
X . =X +Z (k, + 2k + 2kp + ka) + O(08)° (12)
1
Zas =Bt 7 (mg + 2my + 2mp + my) + 0(ag)® (13)
o
I
e
+
8
“
n+l
where
O n gt
1 1 1
ky = 8 F(X, + =k, 2 +—m,, ¢ +—=09) ,
2 2 2
1 1 1
kg = M@ F(X, +—ky, 2 +—m, ¢ +—208) ,
2 2 2
m =A¢G(Xn+;—ko, zn+;mo, q)+-2-a¢) ,
1 1 1
mp = AP G(X, +—ky, Dyt —m, ¢ +—08)
2 2 D
mg = AP G(Xn + Ka, Zn +my, ¢+ OP)
and where the symbol 0(Ag)° represents a term which is small, of the
order (Ag)®, when Ap is small.
Equations (12) and (13) are of a form that can be readily trans-
formed into a computer program.
COMPUTER PROGRAM
The computer program for the solution of the interfacial equation
and for the calculation of the various output parameters is listed in
Appendix C. The program consisted of four parts which are discussed
below.
Solution of the Differential Equation. Two subroutines in both
single and double precision and consisting of four iteration loops were
written to solve Egs. (12) and (13). The subroutines RHOS and RHOD
calculate the values of R and supply the values of the functions F and
G to the subroutines RUNGKS and RUNGKD. These latter subroutines cal-
culate the values of the coefficients ki and m. and the new values of
X, Z, and ¢ for reintroduction in RHOS and RHOD to continue the
iterative procedure. The iteration procedure is initiated by equating
X, Z, and ¢ to zero and R to one.
Solution for h/a and V/a® versus B and ¢. The pressure and volume
within the attached bubble are calculated from X, Z, and ¢. The pressure
relation is given by Eg. (4) and can be simplified by using Eq. (7) and
the definition of the specific cohesion,
2vg
e = —= (14)
PE
to obtain
h/a = /2/B + Z.J/B/2 (15)
where
Z = Z(X = I‘) = A 3
T
and
h = ch/pg .
The volume relation can be obtained from the integration of
v
d(-—) =X az , (16)
b '
where X and dZ can be cobtained from Eq. (6). Integrating the resulting
eguation by parts gives the relationship:
- -
v TX° 2 sin ¢
- =—|2+pz-—] . (17)
b° B X
Upon substituting Eqs. (6), (14), and (15) into Eq. (17), the final
expression for the dimesionless volume 1s obtained:
Y T r h
~ == (—)(—) — sin ¢ | , (18)
a 8 a a
where
T
- =XV
a
is the dimensionless radius of attachment.
Solution for h/a and V/a® versus § and r/a. To obtain the pres-
gure and volume within attached bubbles for a given radius of attachment
from the numerical solutions X, Z, ¢, of the interfacial equation,
several conditional "IF" statements were required. The interrelation-
ship of h/a, 8, r/a, and ¢ are shown schematically in Fig. 2. The
iterative solution of the interfacial equation proceeds along constant
B lines for given steps of Ap. A conditional check is made to note the
intersection of the given r/a curve (denoted by L), and the values at
the point of crossing are determined by linear interpolation. ©Since
r/a is multi-valued, an additional check is necessary to note when the
increasing values of r/a start to decrease at ¢ = nn/2, n =1, 5 ...
(denoted by O), or when decreasing values start at ¢ = nn/2, n = 3,
7 .... In this manner, solutions for many values of r/a can be obtained
for a given value of 3.
Solutions for h/a and V/a® versus r/a. These solutions were some-
what more difficult to cbtain than those described above since it was
necessary to increment B as well as @¢. Both of the conditional checks
described above for crossing a given value of r/a and a change from in-
creasing to decreasing r/a values were required as well as a check for
the change from increasing to decreasing values of h/a (denoted by A in
Fig. 2).
ORNL-DWG 71- 7610
h/fa —»
B
Fig. 2.
Between h/a,
'rr/2
¢ —»
Schematic Representation of the Relationships
B, r/a, and 3.-
10
For & given value of 8, h/a is a multi-valued function so that the
choice of increasing or decreasing B to approach H/a must be carefully
considered. Furthermore, the direction of approach to E/a along the
given value of r/a must also be carefully considered. To insure the
most trouble-free solution over the entire h/a versus ¢ field, a
decremental approach from right to left (as shown by the arrows in
Fig. 2) was chosen.
To reduce the number of iterations required to locate h/a, an esti-
mate of the value of g is calculated from equations fitted to a few
preliminary results. The initial B value is then decremented along
first a coarse grid, and finally along an extra fine grid to obtain the
final solution using double precision.
Range, Running Time, and Accuracy
The ranges of the computer program as presently written are
0 < ¢ < 360° 0.1 €£r/a £2.0, and 0.02 £ B £ 150; however, these can
be easily extended at some sacrifice of either the running time or
accuracy. The average running time fof the program to obtain a value
of h/a for a given value of r/a is approximately 10 seconds on the
IBM 2360-91 Computer system. The average running time for the other
programs is considerably shorter per solution.
An estimate of the accuracy (to be given later) was obtained by
comparing the values of h/a as a function of ¢ for various values of B,
ANg, and for single and double precision.
The results of the computer solution are discussed in the next
section.
RESULTS
The results in both tabular and graphical form are presented in
this section and in the Appendix.
The profile of a bubble for B8 = 0.8 is shown in Fig. 3. The pro-
file is shown extended to ¢ = 360°, which would be possible if a suitable
11
> ORNL-DWG 74-7641
{.
ol LT TN /T IN
y ) N
\
\
)
/
0.9
0.8
0.6
0.4 N
0.2
0 ——*//
{0 08 06 04 02 0 02 04 06 08 10
x/b
z/b
\
-
/\/
Fig. 3. Profile of a Bubble with B = 0.8 for 0° € ¢ < 360°
12
form of attachment were provided, and the center of bouyancy were to
remain on the z axis.,.
A tabuletion of B, x/b, z/b, h/u, and V/a® for various values of ¢
are presented in Tables B.l through B.6 (in Appendix B). Plots of h/a
and V/a® versus ¢ for various r/a are cshown in Figs. 4 and 5, respectively.
Figure 4 cleaurly shows the attaimment of a maximum value of h/a (h/a) for
a given radius of attachment. (This is the basis of the maximum-bubble-
pressure technique® for the measurement of surface tension.) In addition,
. .o . 3
there is a minimum value of h/a as well as a maximum value for V/a®.
The values of 1i/a and the corresponding values of V/a®, B, ¢, x/b,
E/b for various values of r/a are given in Table 1, and various cross-
plots of these variables are given in Figs. A.]l through A.4 (Appendix A).
These plots show that both Eya and E& approach asymptotic values of
Vfig and 180°,* respectively. Thus, the maximum pressure difference
(h/a) that a large tube can sustain will be very nearly independent of
tube diameter.
A least-squares, polynomial fit of the computer solutions for vari-
ous formulations of h/a and r/a were made. The forumlation that gave
the best fit was
a a a r
- - = T - - ) (19)
h r h a
which is of the same form as the perturbation solutions of Rayleigh-
Schrbdinger. For this reason, Eq. (19) was fitted to the data of Table 1
by the relationship:
£(y) =i + L1y + 1)
F(y') = ; = i3 + i4y + isyz + e » (20)
Y
where io’ i,, and i, are the coefficients of the Rayleigh-Schrddinger
solution and i, i,, 1g, «.. were determined by the least-sguares pro-
cedure and are listed in Table 2.
*
These values can be obtained by the solution of Eg. (3) with
1/ry =0 and 1/b = O,
13
ORNL-DWG 71-T76t2
6
r/a =0.2 3o
3 =
N
4 \
0.3 0.2
.fl—-<
33 0.4 \.\\\\
2 _ \Q\L‘_‘
. — . N\\, 10.0
° 0 40 80 120 160 200 240 280 320 360
¢,
Fig. 4. Variation of the Dimensionless Pressure Difference, h/a,
as a Function of @. for Various Values of 8 and the Dimensionless Radius
of Attachment, r/a.
1h
ORNL-DWG 74-7613
0 50 100 450 200 250 300 350
#:
Fig. 5. Variation of the Dimensionless Volume, V/aa, as a Function
of ¢, for Various Values of the Dimensionless Radius of Attachment, r/a.
Table 1.
15
Radius of Attachment (r/a)
Maximum Values of the Pressure (h/a) and Corresponding
Values of the Size, Shape, and Volume of Bubbles as a
Function of the
r/a ? B x/b 2/b h/a v/a®
0.12 90 .646 0.0290810 0.995156 0.999705 8.413519 0.003651
0.1k 90.795 0.0397216 0.993412 0.998120 7.236L65 0.005806
0.16 91.4ho7 0.0521086 0.991243 1.00397 6.357323 0.008787
0.18 91.906 0.0662774 0.988791 1.00649 5.676507 0.012624
0.20 92. 411 0.082286 0.986012 1.00865 5.134652 0.017439
0.20 92.906 0.100192 0.982929 1.00988 4.693892 0.023404
0.24 93.420 0.120066 0.979526 1.0106k4 L. 328985 0.030656
0.26 93.945 0.141985 0.975814 1.01077 h.opolls 0.0392273
0.28 94,488 0.166037 0.971786 1.01039 3.761790 0.049570
0.30 95 .47k 0.192565 0.966824 1.01574 3.537926 0.062105
0.225 96,472 0.229022 0.960L16 1.01752 3.299450 0.0802973
0.350 97.476 0.269500 0.953463 1.01785 3.097815 0.101911
0.375 98.987 0.315035 0.944859 1.02327 2.,9257k4L 0.128876
0.400 100.225 0.365084 0.936222 1.02319 2.T7TTL3 0.158475
0.4b25 101.985 0.421688 0.925568 1.02746 2.6Lg595 0.197373
0.450 103.483 0.483756 0.914990 1.02640 2.538105 0.239818
0.475 104.821 0.551689 0.904402 1.02173 2. khobes 0.286173
0.50 107.164 0.631617 0.889730 1.02464 2.35527% 0.346254
0.55 111.986 0.822982 0.857398 1.02159 2.214232 0.501889
0.60 117.488 1.072133 0.819487 1.00938 2.104842 0.708170
0.65 123.990 1.407182 0.774913 0.986809 2.01991k 0.987762
0.70 130.993 1.853431 0.727151 0.950582 1.953875 1.347727
0.75 137.489 2.41%075 0.68265L4 0.9002832 1.90210k 1.768822
0.80 1Lko.66k4 2.977900 0.655617 0.853059 1.860k45 2.136163
0.90 152.491 L,841565 0.5784L48 0.742939 1.7986L9 3.270626
1.00 158.hko2 7.100181 0.530738 0.648972 1.753511 L. 356567
1.10 162.989 10.070060 0.490221 0.567348 1.718720 5.521721
1.20 165.994 13.866110 0.L455742 0.4g7922 1.690847 6.726L07
1.30 167.994 18.715220 0.42kg72 0.438376 1.667904 8.005399
1.0 169.991 25.16100 0.394711 0.385323 1.6486L40 9.386485
1.50 171.489 33.48783 0.366575 0.339160 1.632205 10.839000
1.60 172.4904 Lh,16k29 0.340486 0.299033 1.618007 12.356187
1.70 173.488 s58.14618 0.315285 0.263383 1.605608 13.971941
1.80 173.998 75.94175 0.292110 0.232455 1.564682 15.640580
1.90 174.987 99.7358 0.268978 0.204340 1.584k975 17.453834
2.00 175.489 130.1581 0.247919 0.180031 1.576295 19,314144
16
Table 2. Coefficients for the Polynomial Equations
Fitted to the Computer Solution
Coefficients Eq. (19) Eg. (21)
ig 1.00000 -0.00090
iq -0.66667 1.04439
ig -0.66667 -0.47175
ia 0.03230 1.43283
ig -5.52833 -4.59801
ig 61.1913k 5.38228
ig -351.38141 -2.,73720
1 1099.76625 0.51837
ig -1930.93994
ig 1913.36384
110 -1003.22519
111 216.93848
17
The main disadvantage of Eq. (19) is that an iterative procedure is
necessary to calculate a/h. A simpler formulation, but less accurate,
was also fitted to the computer results:
a r
- f(-—) . (21)
h a
The coefficients for the polynomial fit of Egq. (21) are also listed in
Table 2.
The accuracy of these two polynomial fits is discussed in the next
section.
DISCUSSION OF RESULTS
An estimate of the accuracy of the computer results and compari-
sons with previous results are presented in this section.
Estimation of the Accuracy. Results were obtained for h/a versus ¢
for B = 0.1 and 100.0 with three values of Agp = 1°, 1/2°, and 1/4° using
both single and double precision. There was no change in h/a (to the
seventh decimal place) as /Ay was decreased from 1° to 1/4° when double
precision was used for B = 0.1 and only 0.00008% change for B = 100.0.
The single precision results are plotted in Fig. 6, where the percent
difference is with reference to the double precision, A¢ = 1/4°, values.
As can be seen, an interval less than Ap = 1° decreased the accuracy of
the results because of rounding errors when single precision was used.
Except for the maximum pressure results listed in Table 1, all the
tabulated results were computed using single precision and interval
NP = 1°. The values listed in Table 1 were calculated using double pre-
cision and are good to at least the sevnth decimal place. The other
tabulated results are good to at least the fifth decimal place.
Comparison with Previous Results. As anticipated, the careful,
tedious hand calculations of Bashforth and Adams (which required a number
of years to complete) were in good agreement (to the fifth decimal place)
with the present computer sclution. Trouble, however, develops when the
“Bashforth and Adams tables are singly and doubly interpolated to apply
18
ORNL-DWG 74— 7644
0.005
A B
O |r——— _ .00 04
o 0.25 0.
£~
< \\\\\\, ::\\\\\k
= -0.005 \
2 \ \ 100 100
w
a
W —0.010 N N\
| '8
& \
> 0.50 100
-0.015
< \\
Ll
a
-0.020
\ 0.25 100
-0.025
0 50 100 450 200 250 300 350
¢
Fig. 6. Comparison of the Single Precision Results for h/a with the
Double Precision, Agp = 1/4, Values as a Function of ¢, Ap, and 8.
19
their results to practical calculations. Sudgen,lo "oy careful inter-
polation" of Bashforth's tables, constructed a table for calculating
surface tension by the maximum-bubble-pressure method. (This table is
the one most often referred to in current literature on surface tension.)
The percent difference between our solution and Sudgen's as a function of
r/a is shown in Fig. 7. The maximum difference is -0.1%.
Although an error of 0.1% can be neglected for some studies at
elevated temperatures (where other errors are more significant), this
magnitude of error can be significant for many measurements made at room
temperature, where theoretical studies of small changes in the molecular
structure of the interface are being conducted. Futhermore, this error
can be magnified by as much as 20 times when the two tube, differential
technique is used to measure surface tension.
To be particularly noted in Fig. 7 is that the Rayleigh-Schrddinger
solution is in better agreement with the computer solution than Sudgen's
results all the way from O € r/a < 0.45. This range of r/a covers a
large portion of the surface tension studies that have been conducted in
the past. In fact, the Rayleigh-Schrodinger equation is in error by less
than 1.05% all the way to r/a = 1.0. Thus, this much simpler analytic
solution can be used in many cases where precise surface tension values
are not needed.
Also shown in Fig. 7 are the deviations of Egs. (19) and (21) from
the computer solutions. Equation (19) agrees to within x0.05% all the
way to r/a = 1.5. The simpler Eq. (21) agrees to within +0.07% from
0.2 < r/a < 1.5; but Schrddinger's equation is recommended for r/a < 0.2.
The main disadvantages of Egs. (19) and (21) is that double precision
should be used in their solution, especially at the larger values of r/a.
Baumeister and Hamill presented their results as plots of droplet
volumes and heights as functions of droplet radii and contact angles.
In this form, their results could not be conveniently compared with the
present results. In addition, their results were given to only the
third significant figure.
20
ORNL-DWG 74-7645