Download - Indian Society for Heat and Mass Transfer

Proceedings of the 22nd National and 11th International
ISHMT-ASME Heat and Mass Transfer Conference
December 28-31, 2013, IIT Kharagpur, India
HMTC1300232
EXPERIMENTAL STUDY OF EFFECT OF ADDITIVES IN POOL BOILING HEAT TRANSFER
Sameer S. Gajghate
Dr. Daulatrao Aher College of
Engineering
Karad, Maharashtra, 415124
India
[email protected]
Anil R. Acharya
Government College of
Engineering
Karad, Maharashtra, 415124
India
[email protected]
ABSTRACT
The addition of additives to the water is known to enhance
boiling heat transfer. In the present investigation, boiling heat
transfer coefficients are measured for Nichrome wire,
immersed in saturated water with & without additive. An
additive used is ethyl alcohol with varying concentrations in
the range of 0-700 ppm. Extensive experimentation of pool
boiling is carried out above the critical heat flux. Boiling
behavior i.e. bubble dynamics are observed at higher heat
flux for nucleate boiling of water over wide ranges of
concentration of additive in water. Results are encouraging
and show that a small amount of surface active additive makes
the nucleate boiling heat transfer coefficient considerably
higher, and that there is an optimum additive concentration
for higher heat fluxes. An optimum level of enhancement is
observed up to a certain amount of additive 400 ppm in the
tested range. Thereafter significant enhancement is not
observed. This enhancement may be due to change in thermophysical properties i.e. mainly due to a reduction in surface
tension of water in the presence of additive.
The pool boiling phenomenon is recorded with camera to
study behavior bubble as well as bubble size and its velocity.
Here it is found that as Ethyl alcohol concentration increases
in water, CHF (critical heat flux) is shifting towards the left in
the pool boiling curve. Its maximum shifted at 400 ppm which
means that as the concentration increases up to 0 -700 ppm in
distilled water with additives Ethyl alcohol enhancement of
heat transfer takes place at a low excess temperature.
Additive promotes activation of nucleation sites; the
bubbles appeared in a cluster mode; the lifetime of each
bubble in the cluster is shorter than that of a single water
bubble. The diameter of water bubble increases with
increasing heat flux, whereas analysis of bubble growth in
water with additive reveals the opposite effect: the detachment
diameter of the bubble decreases with increasing heat flux.
The kinetics of boiling was investigated by high-speed video
recording. Boiling curves bubble behavior and the heat
transfer mechanism for the surfactant solution are quite
different from those of water with & without additive.
Keywords- Surfactant, Pool boiling, additive, Surface
Tension, Bubble behavior.
NOMENCLATURE
A
∆T
σ
q
h
hfg
α
Ra
ρWire
TWire
Ashok T. Pise
Government College of
Engineering
Karad, Maharashtra, 415124
India
[email protected]
Cross Sectional area of wire, m2
Excess Temperature, K
Surface tension, N/m
Heat flux, kW/m2
Boiling heat transfer coefficient, kW/ m2 K
Latent heat, J/g
Void Fraction
Surface roughness, µ
Resistivity, ohm-cm
Wire Temperate, K
INTRODUCTION
There is a general need to increase the heat transfer rate in
pool boiling applications for saving the energy required to
phase change. Energy crises and global warming are forcing to
exert more to save the energy from different applications.
Hence the main motive here is to reduce the energy i.e.
increases the heat transfer rate in pool boiling. Researchers are
trying to find enhancement techniques in heat transfer for the
boiling. The addition of surfactant or additive in the solution
can be the cost effective technique and simple to handle. Some
of the researchers had conducted the experiments on pool
boiling enhancement-using additives. Author measured the
surface tension of ethanol/water mixtures over the whole
fractional range with and without the surface active agent. The
boiling heat transfer coefficient, the onset of boiling and the
critical heat flux were measured for water and ethanol/water
mixtures, with and without the surface active agent, over a
horizontal fine heated wire which is kept at a pressure of 0.1
MPa1. The experimentation was carried out in the whole range
of the ethanol fraction and in a surfactant concentration
varying from 0-5000 ppm for investigation the Bubble growth
rate and boiling heat transfer is carried for different of
concentration of ethyl alcohol–water mixture2. An experiment
carried was carried out on nucleate boiling of aqueous
solutions of Ethyl alcohol- water over relatively wide ranges
of concentration and heat flux in pool boiling apparatus. The
experimental results show that a small amount of surface
active additive makes the nucleate boiling heat transfer
coefficient h considerably higher, and that there is an optimum
additive concentration for higher heat fluxes. Beyond the
optimum point, further increase in additive concentration
makes h lower3. The kinetics of boiling was investigated by
high-speed video recording. Boiling curves for various
concentrations were obtained and compared. The results show
that the bubble behavior and the heat transfer mechanism for
the surfactant solution are quite different from those of pure
water. Specific features of boiling of non-ionic surfactant
solutions were revealed4. Development of experimental
technique for precise & systematic measurement of boiling
curves under steady-state and transient conditions. Pool
boiling experiments for good wetting fluids and fluids with a
larger contact angle (FC-72, iso-propanol, water) yield single
and reproducible boiling curves if the system is clean5. In
saturated nucleate pool boiling of water, with constant wall
heat flux q = 10 and 50 kW/m2, the captured images showed
that the bubble shape is close to axially symmetric and
vertically non-symmetric6. In nucleate pool boiling of dilute
aqueous polymer solutions and compared with results for pure
water. Solutes where Hydroxyethyl cellulose (HEC) of three
molecular weights, poly acrylamide (PA) of two molecular
weights, and acrylamide; solute concentrations ranged from
62- 500 ppm. Liquids were boiled at atmospheric pressure on
horizontal nichrome plate. Results showed distinct differences
in bubble size and dynamics, between polymeric and
nonpolymeric liquids. The results produced by variations of
concentrations and molecular weight appeared to be
correlatable with the solution viscosity. The HEC is a
surfactant but PA is not, so surface tension is believed to be
only a minor variable7.
The characteristics of deterioration in the heat transfer of
mixtures have recently been clarified, but there are few reports
on the heat transfer enhancement of binary mixtures. Here, the
effects of surface-active agents in the boiling heat transfer
were reported as a means of the heat transfer enhancement.
This is the reason that surface tension is one of the important
factors governing the boiling heat transfer. In this study, the
effect of the surfactant concentration on surface tension was
clarified in water and ethyl alcohol with a cationic surfactant
as the fundamental study on boiling heat transfer enhancement
in water. We clarified experimentally the effect of the
surfactant on surface tension, the heat transfer coefficient,
bubble velocity, bubble diameter, Leidenfrost point and the
CHF. Here, the present surfactant was employed, because its
surface tension is very small and its adsorption to a metal
surface is excellent.
EXPERIMENTAL METHOD
Experimental Set Up
The apparatus for experimental studies on pool boiling is
shown in Fig. 1. It consists of cylindrical glass container
housing (250mm X 110mm), the test heater and the heating
coil for the initial heating of the water. The heater coil is
directly connected to the mains (Auxiliary Heater R1, 1Kw)
and the test heater (Nichrome wire, 0.3mm diameter & 10cm
long) is connected to the mains via a dimmerstat (10 A, 230
V). An ammeter (range 0-10A) is connected in series while the
voltmeter across it to read the current and voltage. The voltage
selector switch is used to select the voltage range 100/200V.
These controls are placed inside the control panel. K-type
thermocouple is used to measure the temperature of nichrome
wire (0-900ºC) which connected to Digital temperature
indicator of 0-9999 ºC.
To study the kinetics of vapor bubbles in pool boiling
phenomena for water with and without surfactant a camera is
fixed near to apparatus in such a way that boiling phenomenon
can be recorded by camera to make observations in terms of
bubble nucleation, growth and its departure. Electronic
balance was used for the measurement of the mass of
surfactants has least count of 1mg.
Figure 1. EXPERIMENTAL SET UP
1. Glass Container 2. Wooden Platform 3. Auxiliary Heater
(R1) 4. Test Heater (R2) 5. Thermometer 6. Thermocouple 7.
Clay Lid 8. Nichrome Wire 9. Heater Connecting Cable 10.
Digital Temperature Indicator 11.Control Panel 12.Ammeter
13.Voltage
Range
Selector
Switch
14.Voltmeter
15.Dimmerstat 16.Electric Power Switch
Test Procedure
Various researchers have investigated a number of surfaceactive agents for the mass and heat transfer enhancement. The
additive used here is Ethyl Alcohol. Ethyl Alcohol is an
inorganic compound with the formula C2H6O. Ethyl Alcohol
is a volatile, colorless liquid that has a slight odor. It burns
with a smokeless blue flame that is not always visible in
normal light. A water based solution for different ppm (parts
per million) is prepared with Ethyl Alcohol viz 50, 100, 200,
300, 400, 500, 600 & 700 ppm. As the boiling heat transfer
rate is very sensitive to the state of the heating surface, boiling
of pure water was carried out until the reproducibility of the
boiling curve became very good before beginning of each sets
experiments with the addition of various amounts of additive.
In the beginning, the glass container is cleaned with
distilled water and then filled 2.5 litre distilled water in the
container. One end of thermocouple is connected to test heater
nichrome wire and is insulated properly and the other end is
connected to the digital temperature indicator. Heater coil and
test heater wire are connected across the studs and necessary
electrical connections are made. It is confirmed that both the
heaters are completely submerged in water. The water is
heated by bulk heat to its Saturation temperature. Then the
supply to nichrome wire heating is started and heat flux is
adjusted by dimmerstat. Every moment is recorded by
advanced Canon Camera with frame rate about 120fps. The
rise in wire temperature, current and voltage at each position
are measured carefully. The electric power is supplied
gradually up to the point where nichrome wires break. As
heating element is thin and long axial heat transfer due to
conduction can be neglected. The heat flux released from the
heating element to the liquid is controlled by adjusting the
current supplied. A supplementary heater is installed for the
purpose of bringing up the temperature of the liquid at the
beginning of the experiment and maintains it at the boiling
point of the test fluid during the period of operation.
The procedure from the beginning is repeated by emptying
the pool of distilled water and filling with aqueous surfactant
of different concentrations. Every time nichrome wire is
changed and thermocouple is attached. The pool temperature
is measured by a calibrated thermometer. Boiling on the
surface of the heating element can be illuminated, observed
and photographed at the pool is of high class standard Borocil
glass.
3.10 278.81
1
Ω. ∆Texcess is the difference between
en the wire temperature and the
bulk fluid temperature. The error on the boiling curve results
from imprecision in the ammeter and voltmeter, it is too small
to be seen the graph. It shows that the there is a sudden
decrease in the current during the work; it means that at
certain point heat flux decrease at a particular volt which can
compare with the Nukiyama curve to study the boiling
regimes8. A similar graph can be prepared for the different
solution of water with & without surfactants.
Figure 2. RAW EXPERIMENTAL DATA
Comparative studies of results of surfactant were broadly
discussed into two categories as boiling behavior and boiling
curves. The boiling phenomenon mainly depends on physical
properties of fluid that are density, surface tension and
kinematic viscosity. The addition of a small amount of
surfactant does not affect the density of the solution but it
slightly increases the viscosity if surfactant
factant is polymeric.
Researchers
esearchers measured the surface tension data with and
a
without surfactants in water, [See fig. 3.] This shows with an
increase in surfactant concentration surface tension decreases.
The enhancement of heat transfer is might be due to the
depression of surface tension.
The data collected during experimentation are
tabulated in the form of graphs. The graphs are plotted for the
heat transfer coefficient for water with and without varying
concentration of surfactant and compare the results. The
results are described as follows.
Effects of Surfactants on Surface Tension
The surface tension data with and without surfactant in the
water is taken from the literature. The effect of surface tension
of the water σ against temperature T is shown in the literature.
The trend shows an increase in temperature surface tension
decreases. The measurements of surface tension are carried
out for different concentrations of Ethyl alcohol as a surfactant
in pure water over a range of temperature 00–1200C. They
found that the surface tension is decreased as the temperature
increases with varying concentration10; it is found that at 400700 ppm concentration surface tension values fall nearer to
zero, N/m which is shown in Fig. 3.
0.025
Surface tension (σ), N/m
EXPERIMENTAL RESULTS
As the work is in pool boiling it should be a comparative study
of Nukiyama curve8, because it gives the extensive knowledge
regarding the bubble regimes with varying heat flux & excess
temperature. The extensive experimentation of pool boiling
was carried for pure water with and without surfactant of
varying concentrations of Ethyl Alcohol. From the obtained
experimental data, results are plotted in terms of boiling curve
as a heat flux vs. heater excess temperature. The raw data
obtained from experiments is plotted in Fig. 2, the voltage
drop across the nichrome wire was measured with a voltmeter,
and the current was obtained from an ammeter (for deoinized
water) were recorded after the circuit reached equilibrium.
equilibrium
The resistance across the nichrome
rome wire was found using
Ohm’s Law, and then the wire geometry was divided out to
determine the resistivity of the wire in Ω-cm9. The nichrome
n
resistivity is linearly related to wire temperature
erature by Eqn.
E
(1).
50ppm
200ppm
400ppm
600ppm
0.02
100ppm
300ppm
500ppm
700ppm
0.015
0.01
0.005
0
0
20
40
60
80
Temperature, °C
100
120
140
Figure 3. EFFECT OF TEMPERATURE AND
CONCENTRATION OF ETHYL ALCOHOL ON
SURFACE TENSION
ON IN WATER10
Boiling Behavior
Studies of the evolution of vapor bubbles in the pool
boiling phenomena of water with and without surfactants were
observed.. The growth of bubbles is one of the parameters
determining the intensity of the heat transfer from a heated
surface. The growth of the bubble in the liquid containing
surfactants is affected by a number of specific factors. The
pool boiling experiments are carried out under atmospheric
pressure.
In the present study, the height of the liquid phase over tthe
heater is not less than 35 mm throughout all experiments. The
bubble behavior is recorded at 120fps by the video camera.
The typical stages of bubble growth analyzed for this study are
shown in Fig. 4(a-k) & 5.
Fig. 4(a-k)
k) show typical pictures of pure water boiling on
the Nichrome
rome wire at heat fluxes 0.0186- 1.66 MW/m2
respectively. A population of bubbles was observed in the
vicinity of the heated wire. The bubble dynamics for water are
seen to depend on heat flux, similar to well
well-known boiling
visualization data.
a) 0.0186
e) 0.255
b) 0.0186
c) 0.0651
d) 0.145
f) 0.392
g) 0.548
h) 0.743
i) 0.907
j) 1.280
k) 1.662
Figure 4. (a- k) PHOTO IMAGES OF BUBBLE OF PURE
2
WATER FOR DIFFERENT HEAT FLUXES (MW/m )
After the onset of nucleate boiling, the regime of single
bubbles occurs close to the heated wall (Fig. 4.a). As the heat
flux increases, bubble coalescence take place (Fig. 4.d). This
phenomenon is more pronounced at a heat flux 0.392 MW/m2
(Fig. 4.f). For pure water, the average bubble size was
observed to slightly increase with increasing heat flux. The
bubbles have an irregular shape at all values of heat flux.The
result obtained can be explained: for very low wall superheat
the bubble departure is solely a function of the buoyancy and
of the surface tension.
For 50 ppm
a) 6.34
For 200 ppm
a) 6.37
For 300 ppm
b) 38.1
b) 74.3
c) 74.0
c) 64.4
d) 93.4
d) 134.1
Velocity, m/s
a) 1.74
For 100 ppm
between detaching bubbles from wire and reaching the bubble
up to free surface is measured in Windows movie maker and
the distance between wire and free surface is measured in
Adobe software. In Fig. 6 it is found that at various
concentration with different heat flux the velocity decreases
and after 400 ppm concentration velocity is slightly becomes
constant, their no effect seems beyond 400 ppm. The diameter
of bubble measured with the help of adobe photoshop (Fig.
7). The same procedure is repeated in three times for every
concentration and average velocity & diameter is calculated.
b) 93.4
c) 80.9
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
d) 134.4
Ethyl alcohol
0
100
200
300
400
500
600
700
Concentration, ppm
Figure 6. EFFECT OF CONCENTRATION OF
SURFACTANTS ON VELOCITY OF BUBBLES
b) 95.9
c) 81.5
2.5
d) 11.8
Diameter, mm
a) 6.68
For 400 ppm
2
Ethyl Alcohol
1.5
1
0.5
a) 6.79
For 500 ppm
b) 75.8
c) 65.0
d) 134.8
0
0
100
200
300
400
Concentration, ppm
500
600
700
Figure 7. EFFECT OF CONCENTRATION OF
SURFACTANTS ON BUBBLES DIAMETER
b) 89.1
a) 6.37
For 700 ppm
b) 74.3
c) 77.2
c) 94.2
d) 134.9
d) 79.8
a) 5.92
b) 89.8
c) 81.7
d) 100.3
Figure 5. PHOTO IMAGES OF BUBBLE DYNAMICS AT
DIFFERENT HEAT FLUXES (kW/m2)
Fig. 5. Shows the photographs of boiling of 50–700 ppm Ethyl
Alcohol in pure water. As the surface temperature increases,
the surface tension decreases (for most fluids), which should
result in the decrease of the departure diameter. However, as
the superheat increases, dynamic forces (i.e. Inertial forces)
start dominating the bubble growth process and hence the
bubble diameter increases with the surface temperature. A
different trend is however observed in Ethyl alcohol which
presents a sharp increase of the bubble departure diameter at
low heat fluxes, but starts to decrease at higher heat fluxes.As
compared to pure water boiling behaviour are different for
aqueous ethyl alcohol for low heat flux. The approximate
bubble velocity is measured using the software Windows
movie maker and Adobe Photoshop 7.0 (Fig. 6). The time
Fig. 7 show as the bubble diameter decreases with increasing
concentration for higher heat flux, similarly its velocity also
decreases it is due effect of surface tension of additive
concentration in base fluid. Results show the difference of
bubble diameter & velocity for the different additives in base
fluid for higher heat flux.
Boiling Curves
Presence of Ethyl alcohol in the water, boiling phenomena
shows violent incipient with larger and less distributed
bubbles. Furthermore, the ethyl alcohol requires a higher
superheat to reach the incipient boiling region, as depicted in
Fig. 8.
q, MW/m^2
a) 6.32
For 600 ppm
1.65
1.5
1.35
1.2
1.05
0.9
0.75
0.6
0.45
0.3
0.15
0
50ppm
100ppm
200ppm
300ppm
400ppm
500ppm
600ppm
700ppm
water
0
25
50
75 100 125 150 175 200 225 250 275 300 325
∆Texcess, ºC
Figure 8. BOILING CURVES FOR VARYING
CONENTRATION OF ETHYL ALCOHOL IN PURE
WATER
Conc. ppm
700
600
500
400
300
200
100
0
CHF
Leidenfrost point
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
q, MW/m^2
Figure 9. EFFECT OF DIFFERENT CONCENTRATION
ON CHF & LEIDENFROST POINT IN WATER WITH
ETHYL ALCOHOL
110
100
∆Texcess,°k
90
80
70
Ethyl
Alcohol
60
50
0
100
200
300
400
500
600
700
Conc., ppm
Figure 10. EFFECT OF CHF FOR DIFFERENT
CONCENTRATION ON ∆TEXCESS IN WATER WITH
ETHYL ALCOHOL
However, once hfg is overcome, an “explosive boiling”
occurs. This non-uniform boiling behavior seems to
deteriorate the efficiency of the pool boiling heat transfer for
Ethyl alcohol. The power required to reach the critical heat
flux is low as compared to other surfactant solutions means
heat required to reach the maximum heat flux is low at 400
ppm beyond this concentration heat flux increases with
increasing excess temperature. This might be due to the
solubility limit of surfactant in the solution is reached i.e.
400ppm. Fig. 11(a,b) shows the heat transfer co-efficient
increases with increase of heat flux at low excess temperature
for varying concentration of ethyl alcohol mixed with the
water.
0.0035
0.003
h, MW/m^2.K
0.0025
0.002
50ppm
200ppm
400ppm
0.0015
0.001
0.0005
100ppm
300ppm
0
0
0.25
0.5
0.75
1
q, MW/m^2
a)
1.25
1.5
0.0035
0.003
h, MW/m^2K
It is observed in figure that upto the solubility limit of
concentration i.e. 400 ppm an increased in heat flux found
lower excess temperature; beyond this concentration excess
temperature goes on increasing. The Figures 9-10, are
described in details of the concentration effect on CHF &
Leidenfrost point with changing ∆Texcess, for ethyl alcohol.
The figure shown optimum quantity of Ethyl alcohol can be
selected for the boiling process in base fluid. At 400ppm the
result was better for boiling process at low excess temperature
with comparative water. Lower ∆Texcess was found at 400 ppm
in fig. 10 which shows clearly heat transfer enhancement takes
place in mixture of water with ethyl alcohol.
0.0025
0.002
500ppm
600ppm
700ppm
water
0.0015
0.001
0.0005
0
0
0.25
0.5
0.75
1
1.25
1.5
1.75
q, MW/m^2
b)
Figure 11. (a-b) HEAT TRANSFER COEFFICIENT VS.
HEAT FLUX FOR VARIOUS CONCENTRATION OF
ETHYL ALCOHOL IN WATER
The pool boiling modes observed during the experimentation
with and without surfactant are natural, nucleate, transition
and film boiling is discussed.
Boiling Curves, Natural Convection Boiling: In Natural
convection there is insufficient vapor in contact with liquid
phase to cause boiling at the saturation temperature; this
condition is observed in Ethyl alcohol. As the excess
temperature increases, bubble inceptions occur Ethyl alcohol
which is due to free convection effects on fluid motion. As the
water with different surfactants whose boiling point is
different the heat flux varies as ∆Texcess to the 5/4 or 4/3 power
fig. 11(a-b). At low temperature and with an increase in
concentration at a varying flux range diameter of bubble
decreases, also heat transfer co-efficient increases with
increasing heat flux with a minimum temperature difference.
Boiling Curves, Nucleate Boiling: In this regime, isolated
bubbles are formed on nichrome wire with different sizes as
the surface tension varies with different surfactants in the
boiling with water; isolated bubbles separate from the surface
with different bubble velocity. But these bubbles are
dissipated in the liquid shortly after they separate from the
surface. It happens due to increase in ∆Texcess of ethyl alcohol
with different concentration in water. Bubble form in clusters
on heater surface and the different CHF occur for different
water with surfactants, the heat flux varies from 0.381- 1.28
MW/m2. With increasing heat flux Bubble velocity decreases
and bubble diameter decreases with increasing concentration
of different additives.
Boiling Curves, Transition Boiling: In the transition
boiling a continuous layer of vapor cover the heating surface
and keeps the liquid from contacting the surface. The
insulating effect of the vapor reduces the rate of heat transfer
and the coefficient. As the temperature difference increases,
the vapor film becomes thicker and eventually reaches a
maximum thickness somewhere near Leidenfrost point and
also the heat transfer coefficient slowly decreases. The
transition from nucleate to film boiling involves a zone where
the rapid vapor evolution blankets occur on the heated surface.
At this stage of boiling, heat flux decreases due increased
resistivity9 and hence low thermal conductivity (vapor act as
insulation); therefore it observed that the heat flux value is low
as compare to nucleate boiling stage i.e. 0.542– 0.908
MW/m2. The bubble velocity decreases as the bubble diameter
increases and for a while heat transfer coefficient decreases
with decreasing heat flux due low thermal conductivity with
an increasing temperature difference.
1.75
Boiling Curves, Film Boiling: In this region the heater
surface is completely covered by a continuous stable vapor
film beginning from a Leidenfrost point till the end. The heat
transfer rate increases with increasing excess temperature as a
result of heat transfer from the heated surface to the liquid
through the vapor film by radiation, which becomes
significant at high temperatures. Due to high heat flux the
bubble diameter is larger compared to other boiling
phenomenon, also the velocity decreases for all different
concentration of additives in the water.
CONCLUSIONS
The detailed studies of bubble dynamics in presence of
surfactant in the water the following conclusions can be
drawn.
i) Bubble action is seen to be extremely chaotic, with
extensive coalescence in the beginning for pure water.
ii) Presence of the surfactant in the water quite different the
boiling behavior.
iii) The bubbles appeared in clusters form and its life-time is
shorter than that of a single water bubble. They covered the
surface of wire faster.
iv) The detachment diameter of water bubbles increase with
increasing heat flux, while the presence of surfactant the
opposite effect was observed.
v) At CHF point the boiling excess temperature ∆T becomes
smaller upto surfactant concentration 400 ppm & the vapor
bubbles are formed more easily. Also it promotes activation
of nucleation sites in a clustered mode.
vi) Presence of surfactant showed the enhancement in heat
transfer coefficient increased up to the solubility limit. The
addition of the surfactant beyond the solubility limit is not
encouraging and on the other hand slightly decreases the heat
transfer coefficient.
vii) As additives in liquid evaporate on the heating surface, the
vapor contains more amounts of the light component. This
results in mass diffusion of the light component from the bulk.
viii) The boiling curve shifted to the left side as the surfactant
concentration increased.
ix) It is observed beyond CHF the nichrome wire works even
for radiation heat transfer; CHF and Leidenfrost point are at
different heat flux for that of Platinum wire.
Also the efforts are needed to find the boiling behavior in
presence of surfactant in the different surface textures.
UNCERTAINTY ANALYSIS
Experimental measurements always involve uncertainties
and error analysis is useful in assessing the scatter in data and
identifying the source of any abnormal error. Here maximum
uncertainties for 95% confidence level for heat flux at 100
ppm, 400 ppm and 600 ppm concentration of Ethyl alcohol in
aqueous solution are obtained within limits. The voltmeter and
ammeter used for experimentation are within ±1% accuracy
and for area of wire uncertainty found within 0.11%. Method
used here to evaluate uncertainty is proposed by J. P.
Holman11. Error contributed due to Ethyl alcohol
concentration is small and mentioned in the Table 1.
TABLE . 1. ERROR ANALYSIS FOR HEAT FLUX AT
VARIOUS CONCENTRATIONS OF AQUEOUS ETHYL
ALCOHOL
Ethyl Alcohol + water
Heat Flux MW/m2 (max.
error)
Uncertainty
100 ppm
400 ppm 600 ppm
0.925
1.53
0.934
±1.95%
±1.95%
±1.95%
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