P. KRACÍK et al.: THE SIZE EFFECT OF HEAT-TRANSFER SURFACES ON BOILING
939–944
THE SIZE EFFECT OF HEAT-TRANSFER SURFACES ON BOILING
VPLIV VELIKOSTI POVR[IN, KI PRENA[AJO TOPLOTO NA
VRENJE
Petr Kracík, Marek Balas, Martin Lisy, Jiøí Pospí{il
Brno University of Technology, Institute of Power Engineering, Faculty of Mechanical Engineering,
Technická 2896/2, 616 69 Brno, Czech Republic
kracik@fme.vutbr.cz
Prejem rokopisa – received: 2015-07-31; sprejem za objavo – accepted for publication: 2015-12-14
doi:10.17222/mit.2015.245
A sprinkled tube bundle is frequently used in technology processes where an increase or decrease of a liquid temperature in a
very low-pressure environment is required. Phase transitions of the liquid very often occur at low temperatures at pressures
ranging in the thousands of pascals, which enhances the heat transfer. This paper focuses on the issue of a heat-transfer
coefficient that is experimentally examined at the surface of a tube bundle. The tube is located in a low-pressure chamber where
the vacuum is generated using an exhauster via an ejector. The tube consists of smooth copper tubes of 12 mm diameter placed
horizontally one above another. Heating water flows in the bundle from the bottom towards the top at an average input tempe-
rature of approximately 40 °C and an average flow rate of approximately 7.2 L min–1. A falling film liquid at an initial tempe-
rature of approximately 15 °C at an initial tested pressure of approximately 97 kPa (atmospheric pressure) is sprinkled onto the
tubes’ surface. Afterwards, the pressure in the chamber is gradually decreased. When reaching the minimum pressure of
approximately 3 kPa (abs) the water partially evaporates at the lower part of the bundle. Consequently, the influence of the
falling film liquid temperature increase is tested. This gradually leads to the boiling of water in a significant part of the bundle
and the residual cooling liquid that drops back to the bottom of the vessel is almost not heated anymore. In this paper we present
the influences of the size of the heat-transfer surfaces.
Keywords: sprinkled tube bundle, water, under pressure, heat transfer
Pr{enje po snopu cevi se pogosto uporablja v tehnolo{kih procesih, kjer se zahteva povi{anje ali zmanj{anje temperature
teko~ine v okolju z nizkim tlakom. Fazni prehod teko~ine se pogosto pojavi pri nizkih temperaturah in tlakih v obmo~ju nekaj
tiso~ paskalov, kar vpliva na prenos toplote. ^lanek se nana{a na koeficient prenosa toplote, ki je eksperimentalno dolo~en na
povr{ini snopa cevi. Cev je name{~ena v nizkotla~ni komori, kjer se vakuum ustvarja s pomo~jo aspiratorja preko ejektorja.
Snop cevi sestavljajo gladke bakrene cevi, premera 12 mm, ki so name{~ene horizontalno ena nad drugo. Voda za ogrevanje te~e
v snop od spodaj proti vrhu s povpre~no temperaturo okrog 40 °C in povpre~no hitrostjo pretoka okrog 7,2 L min–1. Padajo~
vodni film z za~etno temperaturo okrog 15 °C in z za~etnim tlakom okrog 97 kPa (atmosferski tlak) pr{i po povr{ini cevi. Tlak
se v komori postopno zni`uje. Ko dose`e minimalni tlak okrog 3 kPa (absolutni tlak) voda delno izhlapi na spodnjem delu
snopa. Posledi~no je preizku{en vpliv nara{~anja temperature padajo~e vode. To postopno privede do vrenja vode na ve~jem
delu povr{ine snopa in preostala hladilna teko~ina, ki kaplja na dno posode, se skoraj ne segreje ve~. ^lanek predstavlja vpliv
velikosti povr{ine kjer se prena{a toplota.
Klju~ne besede: pr{enje po snopu cevi, podtlak, prenos toplote
1 INTRODUCTION
Due to a decreasing supply of fossil fuels and their
increasing price, the minimization of energy consump-
tion becomes an important priority, followed by saving
the primary fuel entering energy processes that are
supposed to achieve the maximum efficiency possible,
and last but not least, using so-called renewable sources
of energy. Among these renewable sources, a biomass
combustion technology is mainly used to generate ther-
mal energy and electricity in the Czech Republic.
Current research aims to reflect these demands well. For
instance, research is conducted in the field of optimi-
zation of technology for wood mass preparation1,2 before
combustion or further utilization for pellets’ production.
At the Department of Power Engineering long-term re-
search focuses, apart from other things, on the utilization
of waste thermal energy, which is found, for example, in
condensers at large energy units, for the possible gene-
ration of cool in absorption units.
One of the basic elements of an absorption circuit is
an evaporator, inside of which the heat-carrier substance
is sprayed onto a tube bundle. Due to a low pressure
environment inside the container where the bundle is
located the falling film liquid at the tube bundle boils.
The heat necessary for boiling is derived from a cooled
substance flowing inside the tube bundle.
Under ideal conditions water boils at the whole
surface of an exchanger, but in practice it must be con-
sidered that in original spots of contact between the
water and the exchanger wall the water will not boil at
the tubes’ surface, but the cooling liquid will merely be
heated-up. This paper deals with this very situation –
heat transfer behaviour when heating a sprinkled water
film that boils in a low-pressure environment for a real
tube bundle.
Materiali in tehnologije / Materials and technology 50 (2016) 6, 939–944 939
UDK 539.4.016:621.785:620.195 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(6)939(2016)
Research in the area of sprinkled exchangers can be
divided into two major parts. The first part is research on
heat transfer and a determination of the heat-transfer
coefficient with respect to sprinkled tube bundles for
various liquids, whether boiling or not. For water as the
falling film liquid, they were, for example3–7. The second
part is testing of the sprinkle modes for various tube dia-
meters, tube pitches and tube materials and a determi-
nation of individual modes’ interface. This area is mainly
researched in8–10.
All the results published so far for water as the falling
film liquid apply to one to three tubes, for which the
mentioned relations studied are determined in rigid labo-
ratory conditions, defined strictly in advance. The
sprinkled tubes were not viewed from the operational
perspective where there are more tubes and various mo-
des can occur in different parts with various heat-transfer
values.
2 EXPERIMENTAL PART
For the purposes of examination of the heat transfer
for sprinkled tube bundles a test apparatus was con-
structed (Figure 1). A tube bundle at which a heat
transfer from a heated water flowing inside the tubes into
a falling film liquid is studied is placed in a vessel where
the low pressure is created by an exhauster through an
ejector.
The test apparatus chamber is a cylindrical vessel
with a length of 1.2 m with three apertures in which the
tube bundle of an examined length of 940.0 mm is in-
serted. The tube bundle is installed in two fitting metal
sheets which define the sprinkled area. The bundle con-
sists of eight copper tubes of diameter 12.0 mm situated
horizontally, one above another, with a distribution tube
above them, with apertures of the diameter from 1.0 mm
to 9.2 mm. The bundle can be operated using only the
first four or six tubes too.
Two closed loops are connected to the chamber. A
heating one and a sprinkling one. The heating liquid
flowing inside the tubes is intended for an overpressure
up to 1.0 MPa. The second loop contains flowing falling
film liquid. There is a pump, a regulation valve, a flow
meter and plate heat exchangers attached to both loops.
The plate heat exchanger at the heating loop is connected
to a gas boiler, which supplies heat to the heating liquid.
The sprinkling loop uses two plate heat exchangers. In
the first one the falling film liquid is cooled by cold
drinking water from the water mains and the falling film
liquid is cooled in the second exchanger by drinking
water cooled in a cooler, which regulates the temperature
up to 1.0 °C. In order to enable visual control, the heat-
ing loop also includes a manometer and a thermometer.
The thermal status in individual loops is measured by
wrapped unearthed T-type thermocouples on the agents’
input and output from the vessel. All the thermocouples
were calibrated in the CL1000 Series calibration furnace,
which maintains a given temperature with an accuracy
of ±0.15 °C. None of the thermocouples exceeded the
error ±0.5 °C within the studied range from 28 °C to
75 °C. That is why the total error for the temperature
measurement is set uniformly for all the thermocouples
along the whole studied range ±0.65 °C.
There are three vacuum gauges measuring the low
pressure. The first vacuum gauge is designed for visual
control and is a mercury meter, the second one is a
P. KRACÍK et al.: THE SIZE EFFECT OF HEAT-TRANSFER SURFACES ON BOILING
940 Materiali in tehnologije / Materials and technology 50 (2016) 6, 939–944
Figure 1: Measurement apparatus diagram
Slika 1: Shema merilnih naprav
digital vacuum gauge Baumer TED6 and enables
measurements within the whole desired low-pressure
range, but it is less accurate with lower pressure values.
To allow a precise measuring of the low spectrum, a third
digital vacuum gauge in the range 2.0 kPa to 0 Pa is
used. The accuracy of a vacuum gauge, the results of
which have been used for the assessment, is 0.5 % from
the measured range, i.e., ±0.5 kPa.
Electromagnetic flow meters Flomag 3000 attached
to both loops measure the flow rate. The flow meters’
range is 0.0078 to 0.9424 L s–1, where the accuracy is
0.5 % from the measured range, i.e. ±0.00467 L s–1. All
the examined quantities are either directly (thermo-
couples) or via transducers scanned by measuring cards
DAQ 56 with a frequency of 0.703 Hz.
3 METHODOLOGY OF THE DATA
ASSESSMENT
The assessment of the measured data is based on the
thermal balance between the operating liquid circulating
inside the tubes and a sprinkling loop according to the
law of conservation of energy. Heat transfer is realized
by convection, conduction and radiation. At lower tem-
peratures the heat transferred by radiation is negligible;
therefore, it is excluded from further calculations. The
calculation of the studied heat-transfer coefficient is
based on Newton’s heat-transfer law and Fourier’s
heat-conduction law that have been used to form the
following relation in Equation (1):

 

0
0
0
1
2
1 1
2
1
2
=
− − ⋅
⎛
⎝
⎜
⎞
⎠
⎟
⎡
⎣⎢
⎤
⎦⎥
π
π π
r
k r
r
ri i is s
ln
(1)
where o [W m–2K–1] is the heat transfer coefficient at
the sprinkled tubes’ surface
i [W m–2 K–1] is the heat-transfer coefficient at the
inner side of a tube set for a fully developed turbulent
flow according to11,
ro; ri [m] are the outer and inner tube radii

S [W m–1 K–1] is the thermal conductivity
kS [W m–1 K–1] is the heat admittance based on the
above-mentioned laws governing heat transfer, which is
calculated from the heat balance of the heating side of
the loop, which is why the following Equation (2) must
be valid:
  ( )lnQ k L T M c
t t
t ts s p= =
+⎛
⎝
⎜ ⎞
⎠
⎟ ⋅ −Δ 34
3 4
3 42
(2)
where M34 [kg s–1] is the mass flow of heating water
cp [J kg–1 K–1] is the specific heat capacity of water at
constant pressure related to the mean temperature inside
the loop
L [m] is the total length of the bundle
Tln [K] is a logarithmic temperature gradient where a
counter-current exchanger was considered
The final heat-transfer coefficient for various falling
film liquid flow rates is recalculated into the Nusselt
number, due to its more common application for a results
comparison. For sprinkled exchangers the following
form is normally used3,4 for the recalculation of the
heat-transfer coefficient into the Nusselt number:
[ ]Nu
v
g
= 

0
2
3
3 – (3)
where v [m2 s–1] is the kinematic viscosity,
g [m s–2] is the acceleration due to gravity and

 [W m–1 K–1] is the liquid film’s thermal conductivity.
4 RESULTS
Tube bundles comprising four, six and eight copper
tubes were tested and the results are given in this paper.
Constant flow rates of the sprinkling liquid and the
heating liquid of the required temperature were set first;
the pressure in the chamber was then slowly lowered.
The initial pressure in the chamber at that moment
always equaled the atmospheric pressure.
Three temperature gradients of sprinkling water and
heating water 20/40, 15/40 and 20/50 were tested. The
first number designates the temperature of the sprinkling
water in the distribution tube; the other number desig-
nates the temperature of the heating water entering the
bundle. The thermal differences were (20, 25 and 30) °C.
The flow rate of the heating water in all the experiments
was kept at approximately 7.2 L min–1. The flow rates of
the sprinkling liquid were carefully selected and re-
mained "constant" throughout the experiments.
Figure 2 shows a recording of the main quantities
measured in one of the experiments. Only four tubes
were heated in this experiment, which lasted 16 min and
37 s (horizontal axis of the chart). The average tempe-
rature of the heating water entering the bottom tube (T3)
was 40.2 °C ± 0.3 °C, and the average flow rate of the
heating water (V2) was 7.16 ± 0.03 L min–1. The tem-
perature of the heating water leaving the exchanger is
designated as (T4) in the chart. The average temperature
P. KRACÍK et al.: THE SIZE EFFECT OF HEAT-TRANSFER SURFACES ON BOILING
Materiali in tehnologije / Materials and technology 50 (2016) 6, 939–944 941
Figure 2: Record of the main measured quantities
Slika 2: Zapis glavnih izmerjenih koli~in
of the sprinkling water entering the distribution tube was
15.4 °C ± 0.3 °C, and average flow rate (V1) was 4.03 ±
0.02 L min–1. The mass flow of the sprinkling liquid
related to the length of the sprinkled area (which is
0.940 mm for all three exchangers) is more commonly
used for the comparison. The said mass flow was 0.0713
± 0.0004 kg/(s m), i.e., the average Reynolds number
was 304.4 ± 2.1 [-]. Other values in the chart include the
current pressure in the testing chamber (p) (in absolute
values), and the temperature of the sprinkling water (T2)
beyond the tube bundle where the water was heated.
Figure 3 shows the quantities derived from the
measurements presented in the previous figure. The
quantities in the chart depend on the duration of the
experiment. The quantities represented in the chart
include the current heat flow extracted from the heating
water (Q_34), the current heat flow absorbed by the
sprinkling liquid (Q_12), the heat-transfer coefficient on
the inside tube wall (alpha_i) and the analyzed heat-
transfer coefficient on the surface of the tube (alpha_o).
The chart also presents the current pressure in the testing
chamber. The course of the analysed heat-transfer coeffi-
cient clearly shows that decrease in pressure in the
chamber results in a slightly increased coefficient. The
coefficient depends mostly on the temperature of the
heating water and the sprinkling water, and not so much
on their flow rates.
Altogether 21 experiments on tube bundles with four,
six and eight tubes were performed, that is seven experi-
ments for each bundle. Table 1 presents the measured
parameters that helped identify, among others, the mass
flow of the sprinkling liquid. The parameters were rela-
ted to the length of the sprinkled area, the corresponding
Reynolds number, and the heat flow extracted from the
P. KRACÍK et al.: THE SIZE EFFECT OF HEAT-TRANSFER SURFACES ON BOILING
942 Materiali in tehnologije / Materials and technology 50 (2016) 6, 939–944
Table 1: Core values of the experiments
Tabela 1: Temeljne vrednosti eksperimentov
tubes T1 (°C) T2 (°C) T3 (°C) T4 (°C) V1 (L/min) V2 (L/min)  (kg/(s.m)) Re [-] q (kW/m2)
4
19.7 ± 0.1 32.1 ± 0.2 40.1 ± 0.1 32.7 ± 0.1 4 ± 0 7.2 ± 0 0.0711 ± 0.0005 326 ± 2.4 26.03 ± 0.38
20.2 ± 0.1 29.4 ± 0.3 40.3 ± 0.3 31.7 ± 0.2 6 ± 0 7.2 ± 0 0.1065 ± 0.0004 476.9 ± 2.9 30.26 ± 0.64
15.4 ± 0.2 30.2 ± 0.4 40.2 ± 0.3 30.9 ± 0.3 4 ± 0 7.2 ± 0 0.0713 ± 0.0004 304.3 ± 1.5 32.66 ± 0.29
15.7 ± 0.1 28.5 ± 0.3 39.9 ± 0.4 29.9 ± 0.2 5 ± 0 7.2 ± 0 0.0889 ± 0.0003 373.4 ± 1.4 34.97 ± 0.78
15.5 ± 0.2 27.3 ± 0.2 40.6 ± 0.1 29.8 ± 0.1 6 ± 0 7.1 ± 0 0.1065 ± 0.0006 440.3 ± 3.3 37.57 ± 0.4
19.7 ± 0.2 38.4 ± 0.4 50.2 ± 0.2 38.8 ± 0.2 4 ± 0 7.2 ± 0 0.0709 ± 0.0005 348.4 ± 2.7 39.76 ± 0.48
20.1 ± 0.2 33.6 ± 0.2 50.2 ± 0.2 37.1 ± 0.1 6 ± 0 7.2 ± 0 0.1064 ± 0.0005 498.9 ± 2.3 46.08 ± 0.46
6
20.2 ± 0.1 35.6 ± 0.2 40.6 ± 0.1 32.1 ± 0.1 4 ± 0 7.2 ± 0 0.0709 ± 0.0002 339.8 ± 1.6 19.94 ± 0.21
19.6 ± 0.2 32.0 ± 0.2 39.7 ± 0.2 29.6 ± 0.3 6 ± 0 7.2 ± 0 0.1065 ± 0.0003 487.2 ± 2.9 23.8 ± 0.33
15.4 ± 0.1 33.2 ± 0.3 40.4 ± 0.3 30.3 ± 0.2 4 ± 0.1 7.1 ± 0 0.0713 ± 0.0017 309.3 ± 7.9 23.71 ± 0.49
15.5 ± 0.2 32.3 ± 0.2 40.2 ± 0.3 28.2 ± 0.2 5 ± 0 7.2 ± 0 0.0888 ± 0.0002 389.1 ± 1.7 28.2 ± 0.36
15.4 ± 0.3 31.3 ± 0.5 40.5 ± 0.2 28.0 ± 0.3 6 ± 0 7.2 ± 0 0.1065 ± 0.0003 444.4 ± 3.5 29.03 ± 0.38
20.2 ± 0.2 40.8 ± 0.8 50.4 ± 0.3 36.0 ± 0.2 5 ± 0 7.2 ± 0 0.0885 ± 0.0005 448.8 ± 5.2 34.03 ± 0.38
19.9 ± 0.1 38.5 ± 0.5 49.5 ± 0.1 34.2 ± 0.1 6 ± 0 7.2 ± 0 0.1063 ± 0.0003 524.7 ± 3.6 35.64 ± 0.28
8
20.3 ± 0.1 37.2 ± 0.4 40.3 ± 0.2 31.1 ± 0.1 4 ± 0 7.2 ± 0 0.0709 ± 0.0005 346.5 ± 3.2 16.28 ± 0.31
20.2 ± 0.1 34.7 ± 0.2 40.3 ± 0.1 28.9 ± 0.1 6 ± 0 7.2 ± 0 0.1064 ± 0.0004 505 ± 2.3 19.94 ± 0.13
15.3 ± 0.2 36.3 ± 0.3 40.2 ± 0.1 28.7 ± 0.2 4 ± 0 7.2 ± 0 0.071 ± 0.0003 324.7 ± 2.5 20.25 ± 0.22
15.6 ± 0.3 34.1 ± 0.4 39.9 ± 0.4 27.1 ± 0.3 5 ± 0 7.2 ± 0 0.0887 ± 0.0003 397 ± 2.9 22.3 ± 0.53
15.3 ± 0.5 32.8 ± 0.6 39.7 ± 0.3 26.1 ± 0.5 6 ± 0 7.2 ± 0 0.1063 ± 0.0006 467.4 ± 6.5 23.82 ± 0.51
20.0 ± 0.2 45.8 ± 1.2 50.4 ± 0.2 36.3 ± 0.4 4 ± 0 7.2 ± 0 0.0709 ± 0.0003 378 ± 3.5 24.93 ± 0.81
20.2 ± 0.1 41.2 ± 0.7 50.0 ± 0.3 33.0 ± 0.3 6 ± 0 7.2 ± 0 0.1065 ± 0.0003 542.2 ± 4.2 29.73 ± 0.27
Figure 3: Quantities derived from the measurements
Slika 3: Koli~ine pridobljene iz meritev
Figure 4: Dependency of the Nusselt number on the pressure in the
testing chamber and the heat flow extracted from the heating liquid –
four tubes
Slika 4: Odvisnost Nusseltovega {tevila od tlaka v preizkusni komori
in toplotni tok pridobljen z teko~ine za ogrevanje – {tiri cevi
heating water, which was related to the size of the heat-
exchanging surface.
The heat-transfer coefficient on the surface of the
tube bundle was later converted to the Nusselt number.
Values acquired at pressures ranging from 5.0 kPa(abs)
to 95.0 kPa(abs) in 5.0 kPa increments, and values
acquired at atmospheric pressure were selected from the
data measured in each experiment. The values were then
averaged for each pressure. This helped us to develop
matrices of the same sizes for a particular bundle length.
The matrices were interpolated using a cubic curve in
Matlab software. Contour graphs, shown in Figures 4 to
6, were developed using these new matrices. The pre-
sented Nusselt number in the figures is dependent on the
pressure in the chamber (vertical axis) and the extracted
heat flow of the heating water (horizontal axis). The
range of the Nusselt number is identical for all three
charts, i.e., 0.15 to 0.46 [-], so that individual phases can
be compared. However, this procedure was disabled to
observe slight increases in the Nusselt number as the
pressure decreases in particular measurement sessions.
All three analysed sizes of the tube bundle clearly
show three vertical zones that do not connect; these are a
result of a different temperature gradient. The tempera-
ture gradient is 20/40 in the zone 1, 15/40 in the zone 2,
and 20/50 in the zone 3. Temperature gradient 15/40
with the highest Nusselt number seems to be the best
option for all three analysed lengths of the bundle. The
Nusselt number dropped by almost 0.2 [-] when the
temperature of heating water increased. The Nusselt
number also drops at very low pressures. The decrease is
caused by the fact that heat extracted from the heating
liquid is also used for evaporation and not only for
heating the heating water. However, the decrease is not
very significant and the boiling occurs only at the bottom
section of the tube bundle.
Concerning a tube bundle with four tubes, the maxi-
mum Nusselt number was attained at a specific heat flow
of approximately 35 kW m–2 and 38 kW m–2, which
corresponds to a temperature gradient of 15/40 °C. The
maxima ranged at values around 0.44 [-]. The larger the
surface area and the lower the specific loading, the
higher the values. In the case of a tube bundle with six
tubes, the heat flow is approximatelz 28 kW m–2. In the
case of a tube bundle with eight tubes, the maximum
reached almost 0.46 [-] at a thermal load of approxi-
mately 24 kW m–2.
5 CONCLUSIONS
Sprinkled tube bundles are most commonly used as
evaporators since they quickly separate the vapor phase
and the liquid phase thanks to a thin liquid film. How-
ever, the practice proves that there is no boiling on the
first affected tubes, only the liquid is heated. We describe
in this paper how the surface area of the tubes affects the
heat transfer coefficient on the tube surface. This effect
was tested at three thermal differences: (15, 20 and
30) °C. After the flow rate of the heating and sprinkling
liquids and their temperatures were set, only the pressure
in the testing chamber was changed during the
experiment. The initial pressure in the chamber was
atmospheric pressure.
The pressure in the chamber and the exhaustion of
the air-vapour mixture from the chamber (vapour was
formed at the bottom part of the tube bundle at low
pressures) had no major impact on the coefficient. It was
the exhaustion itself which affected the boundary layers
on the tube bundle. A significant change of the coeffi-
cient may come only if the boiling on the tube bundle
increases.
The impact of the heat-exchanging surface area on
the heat-transfer coefficient is evident in zone 3 of parti-
P. KRACÍK et al.: THE SIZE EFFECT OF HEAT-TRANSFER SURFACES ON BOILING
Materiali in tehnologije / Materials and technology 50 (2016) 6, 939–944 943
Figure 6: Dependency of the Nusselt number on the pressure in the
testing chamber and the heat flow extracted from the heating liquid –
eight tubes
Slika 6: Odvisnost Nusseltovega {tevila od tlaka v preizkusni komori
in toplota pridobljena iz teko~ine za ogrevanje – osem cevi
Figure 5: Dependency of the Nusselt number on the pressure in the
testing chamber and heat flow extracted from the heating liquid – six
tubes
Slika 5: Odvisnost Nusseltovega {tevila od tlaka v preizkusni komori
in toplota pridobljena iz teko~ine za ogrevanje – {est cevi
cular bundles where the thermal gradient was 20/50 °C.
The increase in the surface area results in a decrease in
the specific loading of the area but the heat-transfer
coefficient increases.
Acknowledgement
Presented results were obtained in frame of the
project NETME CENTRE PLUS (LO1202), created with
financial support from the Ministry of Education, Youth
and Sports of the Czech Republic under the “National
Sustainability Programme I".
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944 Materiali in tehnologije / Materials and technology 50 (2016) 6, 939–944