Experimental Analysis of Boiling Enhancement
from Surfactant Addition with Electric Fields
by
Jordan Mizerak
Submitted to the Department of Mechanical Engineering
in partial fulfillment of the requirements for the degree of
MASSAC.sHUS -T
INSTFITUTE
OF TECHN010Gy
Bachelor of Science in Mechanical Engineering
JUL 30 2014
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
LIBRARIES
June 2014
@ Massachusetts Institute of Technology 2014. All rights reserved.
Signature redacted
A uthor ...................................
Department of MechanicafT-ngineering
May 09, 2014
Signature redacted
Certified by...............................
V
Evelyn I
Ag
Associate Professor
Thesis Supervisor
Signature redacted
Accepted by .........................................
Annette Hosoi
Associate Professor of Mechanical Engineering, Undergraduate Officer
Experimental Analysis of Boiling Enhancement from
Surfactant Addition with Electric Fields
by
Jordan Mizerak
Submitted to the Department of Mechanical Engineering
on May 09, 2014, in partial fulfillment of the
requirements for the degree of
Bachelor of Science in Mechanical Engineering
Abstract
This thesis consists of an experimental investigation of the effect of surfactants on
the boiling curve of water. Via adsorbtion to the boiling surface, surfactants alter
the surface energy interaction during bubble formation at nucleation sites. The surfactants initially enhance the heat transfer coefficient at the onset of nucleate boiling
due to higher nucleation density and higher bubble departure frequency. The critical
heat flux, on the other hand, generally dropped by nearly 50% in the presence of
surfactants. As these surfactants are charged molecules, the application of an electric
field was used to increase or decrease adsorption of surfactants on the boiling surface,
thereby tuning the level of boiling enhancement during the onset of nucleate boiling
and further illustrating the role of surfactants in the boiling process.
Thesis Supervisor: Evelyn N. Wang
Title: Associate Professor
2
Acknowledgments
I would like to principally acknowledge Jeremy Cho for guidance and supervision
in boiling experimentation, analysis, and experimental setup. Thanks also to all
members of the MIT Device Research Laboratory for feedback and experimental aid.
3
Contents
1 Introduction
8
2 Background
10
2.1
Water Properties.....
10
2.2
Dynamic Surface Tension.
10
2.3 Boiling ............
11
2.4 Adsorption .........
12
2.5
Critical Heat Flux
.
13
. ..
2.6 Electric Double Layer. .
.
14
3 Experimental Setup and Methods
15
3.1
Experimental Setup ............................
15
3.2
Methods ...................................
16
4 Results and Discussion
19
Surfactant Effect on Nucleate Boiling .....................
19
4.2
Surfactant Effect on Critical Heat Flux . . . . . . . . . . . . . . . .
21
.
4.1
5 Conclusions and Future Work
26
A Boiling Curves
28
4
List of Figures
3-1
Schematic of experimental setup. The copper block receives heat from
the power supply controlled by the PC to boil water on the silver foil.
The PC also collects data from the thermocouples to calculate the heat
flux in real time. The function generator applies a voltage across the silver foil and the titanium counter electrode for desired electrical effects.
The condenser, rope heater, and helium gas supply ensure consistent
and favorable saturated conditions for boiling experimentation.
4-1
. .
.
17
A plot of the full boiling curves up to the CHF for all DTAB tests.
Note the general initial enhancement in the onset of nucleate boiling
region and the reduction in the CHF for the runs with surfactants as
compared to the control experiment of pure water. . . . . . . . . . . .
20
4-2 A plot of the full boiling curves up to the CHF for all SDS tests. The
general trends are comparable to those of Figure 4-1. . . . . . . . . .
20
4-3 The boiling curves in the onset of nucleate boiling regime for the DTAB
tests at various voltages. As a positively charged surfactant, the boiling
curves shifted left as the voltage became more negative, suggesting
higher adsorption from the effect of the electric field.
. . . . . . . . .
22
4-4 The boiling curves in the onset of nucleate boiling regime for the SDS
tests at various voltages. As a negatively charged surfactant, the boiling curves shifted right as the voltage became more negative, opposite
of the effect of DTAB, though consistent given its negative polarity. .
5
22
A-1 Rising and falling curves of pure water control curve for DTAB experiments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
28
A-2 Rising and falling curves with 2.6mM of DTAB added with no applied
voltage.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
29
A-3 Rising and falling curves with 2.6mM of DTAB added with an applied
voltage of -100mV.
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
29
A-4 Rising and falling curves with 2.6mM of DTAB added with an applied
voltage of -1.OV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
30
A-5 Rising and falling curves with 2.6mM of DTAB added with an applied
voltage of -2.OV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
30
A-6 Rising and falling curves with 2.6mM of DTAB added with an applied
voltage of -3.OV. .....................................
31
A-7 Rising and falling curves with 2.6mM of DTAB added with an applied
voltage of -4.OV. .....................................
31
A-8 Rising and falling curves of pure water control curve for SDS experiments. Note the high superheat in the falling curve occurred due to
the sample entering film boiling. . . . . . . . . . . . . . . . . . . . . .
32
A-9 Rising and falling curves with 2.6mM of SDS added with no applied
voltage.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
32
A-10 Rising and falling curves with 2.6mM of SDS added with an applied
voltage of -100mV.
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
33
A-11 Rising and falling curves with 2.6mM of SDS added with an applied
voltage of -1.OV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
33
A-12 Rising and falling curves with 2.6mM of SDS added with an applied
voltage of -2.OV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
34
A-13 Rising and falling curves with 2.6mM of SDS added with an applied
voltage of -3.OV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
34
A-14 Rising and falling curves with 2.6mM of SDS added with an applied
voltage of -4.OV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
35
List of Tables
4.1
Table of the relevant critical heat flux values for the DTAB runs. In
general, it is noteworthy that the CHF occurs at a lower superheat
and a much lower heat flux value in the surfactant tests, though the
amount of adsorption does not show a trend regarding superheat or
CHF value.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
24
4.2 Table of the relevant critical heat flux values for the SDS runs. Note
that as adsorption increases, corresponding to an increasingly negative voltage, the critical heat flux decreases. The less adsorption may
resemble a physical situation more and more similar to the control case. 24
7
Chapter 1
Introduction
Boiling is a mode of heat transfer that is widely used in many industrial processes,
including electricity generation, chemical production, and refrigeration. It is a change
of phase heat transfer mechanism, which allows for a high heat transfer from a relatively small temperature gradient because of the latent heat of vaporization. This is
beneficial from a practical standpoint especially regarding material selection in boiling
equipment. The fundamental physical process of boiling consists of the interaction
between buoyant forces and surface forces of a vapor bubble at a nucleation site. As
soon as the buoyancy force overcomes the surface tension force, the bubble departs
and escapes, allowing for a new vapor bubble to form anew in its place. Therefore,
the rate of bubble growth and departure, as well as the number of available nucleation sites there are for bubbles to form, are what determine the heat transfer rate.
Increasing any of these will enhance the boiling process.
Common ways of enhancing heat transfer in boiling is roughening the surface [11,
which increases number of nucleation sites, or using hydrophilic surfaces [2]. Surfactants, the focus of this study, are known to lower the surface tension forces, thereby
making it easier for bubbles to depart and increase the bubble departure frequency.
Surfactants also tend to increase the number of nucelation sites, thereby increasing
heat transfer by having more bubble growth locations.
Previous works have investigated boiling with surfactants, mostly in the context
of liquid-vapor surface tension, viscosity, and surfactant concentration [3-6]. The
8
effect of surface adsorption is the main enhancement mechanism to be explored in
this work.
9
Chapter 2
Background
2.1
Water Properties
Surfactant solutions are only considered to enhance heat transfer if they are at or
below the critical micelle concentration (CMC) and do not aggregate into micelles [7}.
As the CMC is generally very low (on the order of a few mM), the effect on bulk
properties of the water such as viscosity, thermal conductivity, specific heat, and
saturation temperature are negligible [6,8-10]. The surface tension, however, does
exhibit a noticeable change on account of the surfactants. The target concentrations
of this study are at or near the CMC, so the properties moving forward will be
analyzed accordingly.
2.2
Dynamic Surface Tension
At a liquid-vapor interface in bubble formation, surfactants adsorb to the interface by
way of diffusion. By applying the diffusion equation for a single concentrative agent
with conservation of mass, the surface concentration P12, is as follows:
rl, = 2C,bulk
10
(2.1)
where Cb.1 k is the concentration in the bulk, D, is the diffusivity of the agent, t
is the diffusion time, all assuming that the subsurface concentration is zero. Then,
using the ideal gas type surface equation of state, and combining with Equation 2.1,
the surface tension ,% of the surfactant mixture can be obtained [11]:
?7 = 77,o - RTI',,v
(2.2)
where 9,,o is the surface tension of pure water, R is the ideal gas constant, T is
the temperature. Using Equations 2.1 and 2.2 on approximate boiling time scales
of 50ms, the effect of added surfactants on the liquid-vapor surface tension is small,
changing the value by less than 5%.
2.3
Boiling
Boiling, even without surfactants, is a complex physical process. However, there
are correlations describing the physical parameters of interest, namely the bubble
departure frequency, bubble diameter, and heat flux available for analysis.
The eventual value of interest is the heat flux q", given by the Mikic-Rohsenow
description of boiling [12,13]:
q = 7rDinq.,
(2.3)
The heat transfer coefficient h is an alternative measure of the boiling enhancement
related to the heat flux, defined by:
h =
q(2.4)
ATSH
where ATSH is the superheat temperature.
The bubble diameter
Db
is given by the Cole and Rohsenow correlation [14]:
Db = 1.5 x iO9(
11
(PC
a)I
""
- P v)Puhg(
(2.5)
where p and p, are the densities of liquid and vapor respectively, g is the acceleration
due to gravity, c, is the specific heat, T,t is the saturation temperature, and hf, is
the enthalpy of vaporization. Further, the number of nucleation sites is n, given by
the following expression:
=
(RB
Qe)mAT
(2.6)
2Teatll
where the constant m is empirically determined to be 6 for most surfaces, R, a fitting
parameter related to roughness, 11 a term encompassing the effects of the contact
angle
E and
cone angle 4, and AT being the superheat. To round out the heat flux
expression, q..9 is given by [12]
qavg =
2k1AT
FA
(2.7)
PCP,1
where ki is the liquid thermal conductivity, and fb is the bubble departure frequency,
given by [15]
fA
=
(0.078
)-.
s Db
(2.8)
The notable parameters from these correlations are the following. The bubble
departure frequency fb is a function of the surface tension, which is, although minimally, affected by the surfactants as seen in Section 2.2. The number of nucleation
sites, n, is a function of the surface tension as well as the contact angle
e,
both of
which are affected by surfactants. As can be seen in Equation 2.6 with m=6, even
slight changes in the value of E and 77, propagate into large changes in the number of
nucleation sites, thereby affecting the heat flux significantly. Overall, the number of
nucleation sites in the context of the following experiments tends to be more sensitive
to a change in
2.4
E due to the small change
in surface tension described in Section 2.2.
Adsorption
The initial contact angle
E is given by the Cassie-Baxter
12
equation [16] as follows:
cos(E) = (1 -
f,) cos(E,)
+
f, cos(,)
(2
(2.9)
where f, is the fraction of the surface covered by surfatants, O, is the contact angle
with pure water, and E, is the contact angle over pure surfactant. E, was experimentally determined to be 40*, while in previous work E. was found to be greater
than e,, thereby enhancing nucleation as Q increases with an increase in E [11]. The
area fraction
f.
is given by:
fa = rNaa
with Avogadro's number NA, the surface concentration r.,
(2.10)
and the projected area of
the surfactant agent a,.
2.5
Critical Heat Flux
For pure water, the predicted critical heat flux taken by setting the Helmholtzunstable wavelength to the Taylor wavelength is given by the following expression
[17,18]:
qCHF = 0.149pg
hfg[g(pf - pg)?lv]z
(2.11)
However, the physical process of the critical heat flux with surfactants is not as
well established and could vary greatly from the expression shown in Equation 2.11.
Phenomena such as bubble non-coalescence as well as the altered nucleation site
density change the mechanisms at the critical heat flux. Therefore, the heat flux
expression introduced in Section 2.3 will not be applicable beyond the initial onset
of nucleate boiling stage. In addition, surfactants may not enhance the critical heat
flux.
13
2.6
Electric Double Layer
When applying a potential at an electrode surface, counterions in the solution build up
to balance the excess charge at the electrode surface. The concentration of counterions
decreases moving away from the electrode, eventually reaching the bulk concentration.
This phenomena is known as the electric double layer (EDL).
The relationship between potential and surface concentration was developed by
Guoy and Chapman with the Poisson-Boltzmann formulation. At low potentials,
the Poisson-Boltzmann equation can be linearized to yield the following expression
governing the EDL [19]
d2T
'j,(2.12)
where T is the potential, x is the distance from the surface, and AD is Debye length,
or the length of the EDL. The Debye length AD is given by:
AD
kTe
zeco
VzeFco
=
(2.13)
where k is the Boltzmann constant, e is the permitivity, z is the charge number, e is
the elementary charge, F is the Faraday consant, and coo is the bulk concentration.
By integrating this, the surface charge density
Q is given
by:
Q = -(2.14)
AD
where T, is the surface potential.
Q can
also be expressed in terms of the surface
concentration:
Q = zFr,1
(2.15)
Note that Guoy-Chapman theory has limited application for surfactants as they
are not ideal point charges. However, this theory provides a basic connection between
adsorption of surfactants and their interaction with electric fields.
14
Chapter 3
Experimental Setup and Methods
The goal of this experimentation is to collect data for the boiling curve up to the
critical heat flux for surfactant solutions at various voltages. By varying the elec-
tric field, the adsorption of charged surfactants to the boiling surface is expected to
change and thereby change the heat transfer coefficient. This change in heat transfer
coefficient should be displayed in the boiling curve, shifting it left for a higher heat
flux at a lower superheat temperature as adsorption increases.
As the work presented is principally experimental in nature, a robust experimental setup is required to ensure meaningful and repeatable results. Part of the work
of this thesis was determining the proper experimental process; various boiling surface materials, surfactant concentrations and types, and voltage levels were iterated
through before reaching the setup presented here, leading to the results presented in
this work.
3.1
Experimental Setup
A previously custom designed boiling apparatus is used for all experimentation. Refer
to Figure 3-1 for a visual schematic of the boiling setup. A copper block of cross
sectional area 4cm2 delivers heat to the boiling surface. There are four thermocouples
placed 8mm apart in the copper block near the boiling surface, whose temperatures
are used to calculate the heat flux to be depicted in the boiling curve. Because there
15
are heat losses from the copper block, the following fin equation formulation is solved
in real time by a PC [11]:
2T(x)
Ox 2
hP
kA (T(x) - Too) = 0
with boundary conditions T(x = 0) = T..
and (-)q=L
(3.1)
=
at an arbitrary value
of L, with qg and h used as fitting parameters. A power supply receives inputs from
the PC to control the power being supplied to the heater in the copper block. The
boiling surface is a silver foil soldered onto the copper block, which is roughened with
200-grit sandpaper for enhanced nucleation sites. The heating surface is enclosed
by a glass casing, which has a rope heater wrapped around to ensure saturation
conditions in the bulk. A line of rubber tubing provides an adjustable flow of helium
for degassing purposes. A condenser is used to conserve the volume of water in the
boiling chamber by condensing the steam with chilled water back into the chamber.
A function generator is used to input the desired electrical effects. Leads are
attached to the silver foil and titanium counter electrode, a titanium rod with added
mesh for increased surface area. A multimeter displays the actual output voltage,
allowing for manual tuning of the function generator output to achieve the desired
voltage. The PC collects all of the electrical and temperature data, controls the input
power, and performs heat flux calculations.
3.2
Methods
Upon proper cleaning of all surfaces, 400mL of deionized water is filtered and added
to the boiling chamber. Before every set of experiments, a degassing run is performed.
The sample is heated and allowed to boil for about a fifteen minute period, which
along with a helium flow near 200mL min' rate acts to degas the water. A high air
content shifts the boiling curve significantly, making the degassing process important
for repeatably and accurately characterizing the boiling characteristics of water. The
degassing run also guarantees that the entire sample is saturated, as it gives time for
the rope and surface heaters to input enough energy to bring the entire sample up
16
Helium Flow
Titanium
Counter Electrode
Rope Heater
Function
Electrical Leads
Thermocouples
Thermal Data Eom
Thermocouples
silver Foil
Copper Block
PC
Heat Input
Power Control
P
r
Figure 3-1: Schematic of experimental setup. The copper block receives heat from the
power supply controlled by the PC to boil water on the silver foil. The PC also collects
data from the thermocouples to calculate the heat flux in real time. The function
generator applies a voltage across the silver foil and the titanium counter electrode for
desired electrical effects. The condenser, rope heater, and helium gas supply ensure
consistent and favorable saturated conditions for boiling experimentation.
from room temperature.
Upon finishing any run, the power is shut off and the sample is allowed to cool until all nucleation sites are shut down. This is done between every run, as the opening
of nucleation sites is an important physical process in boiling, and heat flux measurements would be compromised slightly should a few nucleation sites remain active
between runs. However, waiting too long could lead to losing saturation conditions
in the sample.
After the degassing run, a boiling curve of the deionized water is collected, taking
the sample up near the critical heat flux.
This run serves as a baseline between
separate boiling experiments, as small variations such as exact water volume or foil
17
surface finish can vary slightly each time a separate experiment is performed. The
input power is ramped up linearly at a rate of 40W min-
until the experimenter
manually shuts the power off when the boiling curve levels off, carefully trying to
avoid entering the film boiling regime and damaging the experimental setup. Data
is collected for both the heating up to CHF ('rising' curve) and cooling down from
CHF ('falling' curve), as subtle effects such as opening of nucleation sites can be
investigated by comparing the two curves.
This process is repeated with varying voltages and concentrations of surfactant
added. In general the same sequence of different voltage runs is kept between different
experiments, as there may be slight effects based on the history of nucleation sites
opening or water lost throughout the experiment escaping from the chamber.
To prepare the surfactant samples, the mass needed to make 40mL of the desired
concentration was measured out carefully on a scale. The samples were then mixed
and treated with heat and sonics to ensure full dissolution. The two surfactants used
were positively charged dodecyltrimethylammonium bromide (DTAB) and negatively
charged sodium dodecyl sulfate (SDS). The concentration used was 2.6mM, which is
below the CMC at 100'C for both DTAB (14mM) and SDS (10mM).
18
Chapter 4
Results and Discussion
A coarse depiction of the full boiling curves of all runs to be analyzed are shown in
Figures 4-1 and 4-2 for DTAB and SDS respectively. The two major effects to be
discussed in detail will be the onset of nucleation, which is the theorized enhancement
by adding surfactants, as well as the critical heat flux. The varying voltages applied
to the system are meant to tune the enhancement of boiling, as the more negative the
voltage becomes, a decrease in adsorption of SDS and similarly an increase in adsorption of DTAB occurs due to their polarities. As this was principally a preliminary
experimental analysis, potential physical explanations are presented, though would
need further investgation to confirm such effects.
4.1
Surfactant Effect on Nucleate Boiling
The effect of surfactants on the onset of nucleate boiling was consistent across the
trials performed. In general, there was a trend in the DTAB tests that higher negative
voltages corresponded to greater enhancement, as the boiling curve shifted left, as
expected from a positively charged surfactant. This effect is shown in Figure 4-3.
Similary for SDS, the boiling curve shifted right as the voltage became more negative
as seen in Figure 4-4. It was not necessarily monotonically increasing or decreasing
across the voltage changes, but the overall trend is consistent.
A noteworthy qualitative observation regarded the possibility of an electrolysis
19
80-Control
70 -.
60 ---.
-50 -S40 --
.2
oV
V
1.0v
2.OV
3.OV
-4.0V
30120100
0
20
40
60
Superheat (*C)
80
100
Figure 4-1: A plot of the full boiling curves up to the CHF for all DTAB tests.
Note the general initial enhancement in the onset of nucleate boiling region and
the reduction in the CHF for the runs with surfactants as compared to the control
experiment of pure water.
100-Control
-0.0V
80 - _0.1V
-1.0v
E
-2.OV
60
.
-3.OV
-4.OV
40
20
0
0
20
40
60
Superheat (*C)
80
Figure 4-2: A plot of the full boiling curves up to the CHF for all SDS tests. The
general trends are comparable to those of Figure 4-1.
20
of water reaction ococuring at the higher voltage runs. Upon increasing the voltage
up to -4.OV with a steady state current of around 10mA, bubbling started to occur
immediately despite the surface temperature being otherwise too low for any boiling
to occur. This may suggest the production of hydrogen and oxygen gas from splitting
water, arising from the same nucleation sites as the bubbles arose from in boiling.
This brings up two questions.
First, whether or not the surfactants are active in
lowering the activation energy of the electrolysis reaction and thereby enhancing the
chemistry of the system. Second, more relevant in the context of this thesis, is whether
or not the opening of these nucleation sites had an effect on the boiling curve at this
voltage. In the case of DTAB, the -4.OV curve did indeed start above the -3.OV curve,
which follows the trend, but eventually the -3.OV curve rose above the -4.OV curve.
In the case of SDS, the -3.OV curve does not follow the trend and initially started
below the -4.OV curve- which could be explained by enhanced nucleation by -4.OV
due to electrolysis- while later the two curves more or less followed each other. The
trends are not strong enough by any means to attribute these inconsistencies, but it
is an interesting opportunity for boiling enhancement by opening nucleation sites via
different methods.
4.2
Surfactant Effect on Critical Heat Flux
While enhancement in nucleate boiling does improve efficiency in that region, many
engineering applications are targeted to operate in the nucleate boiling regime near
the critical heat flux. Thus, one ultimate goal in boiling enhancement is improving
performance near the CHF.
Referring again to Figures 4-1 and 4-2, the critical heat flux of all surfactant runs is
considerably lower than that of the control run of deionized water. Beyond the initial
enhancement in the nucleate boiling regime, the slope of the boiling curve levels off
and is nearly constant, adopting a value somewhat close to the CHF over a high
range of superheat values of at least 10'C in most cases. In the control case, on the
other hand, the boiling curve remains linear or even increases slightly in slope until
21
3025 --
---
E 20 -15 ---
Control
0.OV
0.1V
1.0V
-2.OV
-
--
3.OV
-4.OV
iC1050
0
5
10
15
Superheat (*C)
20
25
Figure 4-3: The boiling curves in the onset of nucleate boiling regime for the DTAB
tests at various voltages. As a positively charged surfactant, the boiling curves shifted
left as the voltage became more negative, suggesting higher adsorption from the effect
of the electric field.
25-Control
-0.0V
20 -0.1V
-1.0v
15 -- 2.OV
-- 3.OV
10
-4.OV
105
0
0
5
10
15
Superheat (*C)
20
25
Figure 4-4: The boiling curves in the onset of nucleate boiling regime for the SDS
tests at various voltages. As a negatively charged surfactant, the boiling curves shifted
right as the voltage became more negative, opposite of the effect of DTAB, though
consistent given its negative polarity.
22
it reaches CHF, at which point it reaches a narrow maximum and enters film boiling.
A potential qualitative explanation for this phenomenon could be the following. In
a given boiling surface material, there should be a somewhat uniform distribution of
different nucleation site sizes. Thus, in the control case, there is a more gradual and
consistent opening of these new nucleation sites as the input power is increased, as the
higher superheat temperature allows for the effect of buoyancy to overcome that of
surface tension for an increasing amount of supposedly evenly distributed nucleation
site sizes. In the case of adding surfactants, however, as soon as nucleation occurs, the
entire boiling surface almost immediately becomes blanketed by bubbles, displaying
an opening of nucleation sites across the entire surface. Thus, these nucleation sites
cause the immediate enhancement as seen in Section 4.1, but are quicdy exhausted
and reach a saturation in heat flux as no additional nucleation sites can be adopted.
Regardless of why it levels off for such an extended range of superheat values,
what is more important is the near 50% reduction in heat flux seen across most runs.
It was observed qualitatively that much larger bubbles developed in the control experiment due to coalescense as the heat flux approached CHF, while with surfactants
added the bubbles remained smaller as the surfactants prevented coalescence. With
a higher density of nucleation sites with surfactants added, it is a lower energy state
to create small bubbles on these nucleation sites provided by the surfactants lowered
surface energy. Thus, there would be an increase in bubble departure frequency with
additional superheat, but little increase in heat taken away by each bubble if they
remain approximately the same size. With a lower density of available nucleation
sites, not only would the bubble departure frequency increase, but the bubble size
also increases for the larger nucleation sites adopted. Further investigation in this
regard is essential for surfactants to be viable in the boiling process.
As can be seen in the boiling curves presented, the critical heat flux of the surfactant solutions occurred at a much lower superheat than that of the control runs.
This may suggest an alteration in the transition from nucleate boiling to film boiling,
especially considering the vastly different slope behaviors observed in the case of surfactant and control boiling cases. As film boiling arises from the transition between
23
Table 4.1: Table of the relevant critical heat flux values for the DTAB runs. In
general, it is noteworthy that the CHF occurs at a lower superheat and a much lower
heat flux value in the surfactant tests, though the amount of adsorption does not
show a trend regarding superheat or CHF value.
Run
Control
0.OV
-0.1V
-1.0V
-2.0V
-3.OV
-4.0V
Superheat (0C)
81.79
42.75
50.92
50.23
60.25
48.29
47.48
Critical Heat Flux (W cm- 2 )
77.90
37.31
38.53
36.38
35.23
42.95
39.76
%Reduction
0
52.1
50.5
53.3
54.8
44.9
48.9
Table 4.2: Table of the relevant critical heat flux values for the SDS runs. Note that
as adsorption increases, corresponding to an increasingly negative voltage, the critical
heat flux decreases. The less adsorption may resemble a physical situation more and
more similar to the control case.
Run
Control
0.OV
-0.1V
-1.0V
-2.0V
-3.OV
-4.OV
Superheat (0C) Critical Heat Flux (W cm- 2 ) % Reduction
88.25
93.21
0
45.94
38.86
58.3
49.12
39.96
57.1
47.46
46.82
49.8
47.17
47.51
49.0
50.42
49.10
47.3
50.61
50.31
46.0
individual bubbles to a continuous vapor blanket, the size and density of bubble formation on the boiling surface are likely important agents in the transition, of which
appear to vary between surfactant and pure water boiling.
Specific notable experimental data are presented in Tables 4.1 and 4.2 for DTAB
and SDS respectively. In the runs with SDS, there was a negative correlation between
the enhancement in nucleate boiling and the value of the heat flux at CHF. That is,
the more adsorption of surfactants there was, the more reduction in critical heat flux
was observed. Lower and lower adsorption could correspond to a physical situation
more and more like that of the control experiment, adopting the characteristics of a
24
higher CHF and worse performance in the onset of nucleate boiling regime. If this is
indeed the case, a time dependent voltage application based on what stage of boiling
is occurring may be worth exploring. This effect was not seen for DTAB, however.
There also did not seem to be any correlation between the superheat temperature of
the CHF and the adsorption level.
25
Chapter 5
Conclusions and Future Work
Boiling using surfactants shows promise as a potential method for boiling enhancement.
This was primarily an experimental exploration of the theorized effects of
adsorbing surfactants to lower surface energy to enhance nucleation using electric
fields and charged surfactants. This effect was well displayed in the results, as there
was a clear trend of boiling enhancement with higher adsorption of surfactants. This
corresponds to a more efficient process at low superheat values in the initial stages of
the boiling curve.
The effect of surfactants on the critical heat flux, however, was detrimental. While
the superheat temperature of the CHF did decrease, the critical heat flux typically
decreased by about 50%, which is an unacceptable reduction in heat flux. While it
is progressive to enhance efficiency at lower heat flux regimes, improving the performance at the maximum heat flux regions will be the decisive factor in improving
boiling processes on a larger scale.
Thus, there is widespread future work to investigate before boiling with surfactants becomes a viable process. Investigation into understanding the reduction in
critical heat flux that occurs is of paramount importance.
Further, only two po-
tential surfactants were tested in this experimentation; creating a formulation as to
what types of surfactants lend themselves well and their desirable characteristics may
help physically uncover the complex boiling process with surfactants. In addition to
surfactant type, concentration and applied voltage to yield the improvement at the
26
onset of nucleate boiling without sacrificing the critical heat flux would also make the
process more robust. Upon having a grip on these physical issues, the design of a
device or process that takes advantage will be what bridges the gap between theory
and actual boiling enhancement.
27
Appendix A
Boiling Curves
80-Rising
70 -Failing
60S50S4030
$20100
0
20
40
60
Superheat (*C)
80
100
Figure A-1: Rising and falling curves of pure water control curve for DTAB experiments.
28
40
-Rising
35 -Falling
30
E
25-
20LA_
i5,
I
15
10
5
-'
V0
10
20
30
40
Superheat (*C)
50
60
Figure A-2: Rising and falling curves with 2.6mM of DTAB added with no applied
voltage.
40
-Rising
35 -Falling
30
E
25
20
U- 15
I
10
5
n
0
10
20
30
40
50
60
Superheat (*C)
Figure A-3: Rising and falling curves with 2.6mM of DTAB added with an applied
voltage of -100mV.
29
40
-Rising
35
-- Failing
30
E 25
,20
110
5
0
20
10
30
40
Superheat (0 C)
50
60
70
Figure A-4: Rising and falling curves with 2.6mM of DTAB added with an applied
voltage of -1.OV.
40
-Rising
35- -- Failing
30
E
x
25
20
15
ia
cc
I
10
5
0
0
20
40
Superheat (*C)
60
80
Figure A-5: Rising and falling curves with 2.6mM of DTAB added with an applied
voltage of -2.OV.
30
A
.
NI
-- Rising
--- Falling
40
E
-0-30
x
20 |
101
0'
0
10
20
30
40
Superheat (*C)
50
60
70
Figure A-6: Rising and falling curves with 2.6mM of DTAB added with an applied
voltage of -3.OV.
40
-- Rising
-- Falling
35
30
E 25
20
U_ is
CO
10
I
5
. .
0
.
10
-
4
20
-
30
40
Superheat (*C)
50
___
60
____
70
Figure A-7: Rising and falling curves with 2.6mM of DTAB added with an applied
voltage of -4.OV.
31
100
80
E
~z
-- Rusing
T-Falling
60-
Li
40XV
20-
01
0
20
40
60
Superheat (*C)
80
100
Figure A-8: Rising and falling curves of pure water control curve for SDS experiments.
Note the high superheat in the falling curve occurred due to the sample entering film
boiling.
ILI I
35 -
Rising
Falling
3 0E 25
2 01
51
05A
0
10
20
30
40
50
Superheat (*C)
Figure A-9: Rising and falling curves with 2.6mM of SDS added with no applied
voltage.
32
40
-Rising
35 -Falling
30
E 25
20
15
z 10
5
0
0
10
20
30
40
0
Superheat ( C)
50
60
Figure A-10: Rising and falling curves with 2.6mM of SDS added with an applied
voltage of -100mV.
50
40
Using
Failing
E
(30
-
L20
.It
10F
0'
0
10
30
40
Superheat (*C)
20
50
60
Figure A-11: Rising and falling curves with 2.6mM of SDS added with an applied
voltage of -L.OV.
33
Falling
5
-- Rising
-- Failing
-A00
4C
N
3(
x
LL-
2(
4,
UC
C
0
10
20
30
Superheat (*C)
40
so
Figure A-12: Rising and falling curves with 2.6mM of SDS added with an applied
voltage of -2.OV.
50
40
E
Rising
'Falling
30
20
0.,
100'
0
10
20
30
40
Superheat (*C)
50
60
Figure A-13: Rising and falling curves with 2.6mM of SDS added with an applied
voltage of -3.OV.
34
60
-Rising
50 s
Falling
IaIn
E40302010
0
0
10
30
40
0
Superheat ( C)
20
50
60
Figure A-14: Rising and falling curves with 2.6mM of SDS added with an applied
voltage of -4.OV.
35
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