Thermal Behavior of Aerogel-Embedded Nonwovens in Cross Airflow

Abstract

Thermal performance of aerogel-embedded polyester/polyethylene nonwoven fabrics in cross airflow was experimentally studied by using a laboratory-built dynamic heat transfer measuring device in which the fabric could be applied on a heating rod. Experiments were performed with different airflow velocities and heating conditions. The temperature–time histories of different materials were collected and compared. The temperature difference and convective heat transfer coefficient under continuous heating were analyzed and discussed. Results showed that under preheated conditions, the aerogel-embedded nonwoven fabrics had very small decrease in temperature and good ability to prevent against heat loss in cross flow. As for the continuous heating conditions, the heat transfer rate of each material showed an increasing trend with increase in the Reynolds number. The aerogel-treated nonwoven fabric with the least fabric thickness and aerogel content delivered a significantly increased heat transfer rate at higher Reynolds number. Thicker fabrics with higher aerogel content could provide better insulation ability in cross flow.

Full text

THERMAL BEHAVIOR OF AEROGEL-EMBEDDED NONWOVENS IN CROSS AIRFLOW
Xiaoman Xiong1,3,*, Mohanapriya Venkataraman1, Darina Jašíková2, Tao Yang1,3, Rajesh Mishra1, Jiří Militký1,
Michal Petrů3
1 Technical University of Liberec, Faculty of Textile Engineering, Department of Material Engineering, Liberec 46117, Czech Republic
2 Institute for Nanomaterials, Advanced Technologies and Innovations, Department of Physical Measurements, Liberec 46117, Czech Republic
3 Institute for Nanomaterials, Advanced Technologies and Innovation, Department of Machinery Construction, Technical University of Liberec,
Liberec, 461 17, Czech Republic
*Corresponding author. E-mail: xiaoman.xiong@tul.cz
1. Introduction
High-loft  brous materials are widely used in various  elds
for thermal insulating applications due to the large amount
of void space in the  brous structure. The thermal properties
of a  brous material are affected by the physical parameters
of the  ber component and the structural parameters of the
 brous structure. Silica aerogel, as a coherent with a rigid
three-dimensional network of contiguous particles of colloidal
silica, prepared by the polymerization of silicic acid or by
the aggregation of particles of colloidal silica, demonstrates
superior thermal insulation performance with extremely low
thermal conductivity and thus has been well acknowledged as
one of the most attractive thermal insulating materials [1-2].
However, since silica aerogels generally have poor mechanical
stability, they are usually incorporated with a lightweight  brous
material to deal with various heat transfer problems [3]. Various
aerogel-embedded  brous materials, developed by the sol–gel
method or by incorporating aerogel beads into a nonwoven
 brous web by using low-melting  bers or additive binding
materials, have been well studied [4-7]. These materials
exhibit improved thermal insulation and thus have a potential
to be used in buildings, industry facilities, and protective textile
applications such as winter jacket, sleeping bed, and gloves in
extremely cold weather.
Aerogel-embedded nonwoven fabric is basically a highly
porous,  brous material. It is well known that heat transfer in a
 brous material is a complex property because the three basic
mechanisms of heat transfer, namely, conduction, convection,
and radiation, play a part [8]. The insulating property, resulting
from the fact that the gas contained in the pores is at rest,
has been well understood and documented. Convective heat
transfer through a  brous material, resulting from a  uid moving
across a surface that carries heat away, involves complex and
diverse  ow patterns around the solid particles or  bers [9].
Generally, the rate of convective heat transfer is a function of
the  uid and surface temperatures, the surface area, and the
speed of the  ow across the surface. Due to the inherently air-
permeable characteristic of a highly porous material, air intrusion
through the void space usually occurs in the  brous structure,
which strongly affects the convective thermal behavior of a
porous material. In practical use, the in uence of air movement
depends on the structural parameters of the  brous material
and the nature of the air ow as well [10]. Forced convection
usually generates highly turbulent, multidirectional  ows as a
function of venting characteristics, material geometries, and
wind speed [11]. Due to the complexity of the geometry and
 ow pattern, it is dif cult to obtain a clear relationship between
the individual fabric parameters and the thermal behavior,
as most of them are interrelated to each other and therefore
impossible to separate. In order to observe the convective
heat transfer through textiles, approximate solutions based
on computational  uid dynamics and well-established
dimensionless numbers are usually used [12]. The effect of
 brous structures on  uid  ow has been numerically studied
Abstract:
Thermal performance of aerogel-embedded polyester/polyethylene nonwoven fabrics in cross airflow was
experimentally studied by using a laboratory-built dynamic heat transfer measuring device in which the fabric could
beappliedonaheating rod.Experimentswere performedwithdifferentairflowvelocities andheatingconditions.
The temperature–time histories of different materials were collected and compared. The temperature difference and
convectiveheattransfer coefficientundercontinuous heatingwereanalyzedanddiscussed.Resultsshowedthat
under preheated conditions, the aerogel-embedded nonwoven fabrics had very small decrease in temperature and
goodabilitytopreventagainstheatlossincrossflow.Asforthecontinuousheatingconditions,theheattransferrate
ofeachmaterialshowedanincreasingtrendwithincreaseintheReynoldsnumber.Theaerogel-treatednonwoven
fabricwith theleastfabricthicknessand aerogelcontent delivereda significantlyincreasedheattransferrate at
higherReynoldsnumber.Thickerfabricswithhigheraerogelcontentcouldprovidebetterinsulationabilityincross
fl o w .
Keywords:
Nonwovenfabrics,silicaaerogel,crossairflow,heattransfercoefficient
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[13]. A study on the modeling and simulation of convective heat
transfer in aerogel-embedded nonwoven fabric has also been
carried out [14]. Experimental investigations of convective
thermal behavior of aerogel-embedded brous materials in
a controlled climate chamber have been reported [15-16].
However, all these works deal with a simple airow moving
over an absolutely at fabric placed on a heating plate. In some
actual cases, the airow and the state of the fabric are more
complicated, especially in applications involving buildings and
industrial facilities, for example, brous insulators applied on
domestic hot water plumbing lines to improve energy efciency,
on process equipment, piping, steam distribution systems, and
boilers for process control, energy efciency and safety. Such
applications involve a cross airow around a hot cylinder that is
insulated with a brous material.
The aim of the present work is to perform an experimental
study on the thermal behavior of aerogel-embedded polyester/
polyethylene nonwovens when subjected to wind-induced
cross airow. Aerogel-embedded polyester/polyethylene
nonwovens with different structural parameters and aerogel
content were selected to carry out the measurements. A
laboratory-built dynamic heat transfer device was used to gure
out the convective thermal behavior of the selected materials
under different airow velocities and heating conditions. Real-
time temperatures of the fabrics were collected and compared.
The temperature difference and heat transfer coefcient under
continuous heating condition were calculated and investigated.
2. Experimental
2.1. Materials
Three types of polyester/polyethylene nonwoven fabrics
with at 50:50 composition ratios and embedded with aerogel
were selected to assess their thermal performances under
convective heat transfer with air. The aerogel used was
hydrophobic amorphous silica aerogel, which is mesoporous
and has nearly 98% of air and 2% solid. Its specications are
presented in Table 1. Due to the interconnected nanoporous
characteristic, the aerogel can hold air within its structure and
does not allow free ow of air, which enables it to be a superior
thermal insulation material. The aerogel particles were added
during thermal bonding of the nonwoven web. A high-resolution
image of the aerogel/polymer nonwoven fabric B is shown in
Figure 1. It is clear that aerogel particles are dispersed both
on the surface of a single ber and in the void spaces between
bers. The physical properties of the aerogel-embedded
nonwovens are given in Table 2. All these fabrics have high
porosity of >90%.
2.2. Measurement of thermal behavior in cross ow
2.2.1. Experimental setup
The tests were carried out in a laboratory-built device; the
experimental section is presented in Figure 2. The main part of
the measured section consists of a subsonic wind tunnel with
a square cross section of 20 cm x 20 cm, an electric heating
rod with a diameter of 2 cm placed perpendicular to the airow
direction in the wind tunnel, and a fan ventilator connected in
the right corner of the lower duct. The heating rod, composed of
a Ni-Cr 80-20 heating wire with a stainless steel shell, having a
rated power of 500 W and voltage of 230 V, was connected to
a control unit and powered by a Sefram DC power supply. The
input power was adjusted by the control unit. The fan ventilator,
acting as a suction fan, supplies airow at room temperature.
Different airow velocities can be achieved by adjusting the fan
speed.
The experiments involve measurements of real-time
temperatures of the heating rod, the fabric insulators, and the
upstream airow, as well as determination of the velocity of the
upstream airow. Temperatures of the heating rod and the fabric
were measured with T-type copper–constantan thermocouples.
Two thermocouples were respectively attached on the heating
rod and the fabric surface with the help of high-temperature
RTV silicone. A thermoanemometer sensor FVAD35TH5 was
mounted in the upstream region to monitor the velocity and
temperature of the free stream. All these thermocouples and
sensor were connected through an ALMEMO 2590-2A - 2 data
logger to a personal computer device.
Figure 1. Scanning electron microscope (SEM) image of aerogel/
polymer nonwoven.
Table 1. Amorphous silica aerogel specications
Properties Value range
Particle size range 0.1–0.7 mm
Pore diameter ~20 nm
Density 135±15 kg/m3
Surface chemistry Fully hydrophobic
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2.2.2. Experimental procedures
The selected aerogel-embedded nonwoven fabric was cut into
20 cmx8 cm pieces and wrapped on the heating rod; the lateral
gap was sealed with insulating rubber tape. Any gaps between
the measured fabric and the heating rod were eliminated.
The tests were carried out with two types of heating conditions,
i.e., preheated and continuous heating conditions, to investigate
how the materials prevent against heat loss from the heating rod.
The preheated condition refers to the preheating of the heating
rod to a speci c temperature and subsequently switching off
the heating power to let the system cool down at a selected
air ow velocity. The continuous heating condition involves
nonstop heating during the whole measurement process.
The voltage and current supplied from the direct current (DC)
power for heating were 37 V and 0.33 A, respectively, for each
measurement. Five air ow velocity levels, i.e., 0, 1, 5, 10, and
15 m/s were used to study the effect of air ow velocity on the
heat loss rate of different fabrics during cross  ow.
In this study, the preheated temperature was chosen to be
around 60°C, and the ambient temperature was maintained
at 23°C±2°C. At the beginning of a measurement at the
preheated condition, the heating rod was heated to 60°C, the
power supply was cut off, and the fan ventilator was switched
on to let in air ow with a selected velocity. The sensed data on
temperatures and air ow velocities were collected at intervals
of 3.2 seconds. During the continuous heating measurement,
the heating rod was maintained being powered, and the
air ow was allowed to be delivered when the temperature of
the heating rod reached 60°C. Data were taken as soon as
the inlet air ow reached a selected velocity. The experimental
work includes two parts: the  rst dealt with the  ow around a
heating rod without fabric, and the second with the  ow around
the porous fabric wrapped on the heating rod. For each part,
measurements were performed under both preheated and
continuous heating conditions at different air velocity levels.
Each measurement was conducted three times, and the results
were averaged.
3. Results and discussion
3.1. Fluid  ow around the heating rod without fabric
For the circular heating rod in cross air ow, as the air ow
approaches the front side of the rod, the  uid pressure rises and
the free stream  uid is brought to rest at the forward stagnation
point [17]. The high pressure forces the  uid to move along the
surface, and boundary layers develop on both sides. The free
stream velocity depends on the angle from the stagnation point.
The pressure force is counteracted by viscous forces, and the
 uid separates from both sides of the rod and forms two shear
layers. The innermost part of the shear layers is in contact with
the rod surface and moves slower than the outermost part. As
a result, a highly irregular wake is formed in the downstream
region. The  ow pattern is dependent on the Reynolds number
Re, which is given by the following expression:
VD
Re
v
= (1)
where V is the velocity of air (in meter/second [m/s]), D is the
diameter (in meters), v is the kinematic viscosity of air at the
 lm temperature (in square meter per second [m2/s]), which is
15.89
×
10-6 m2/s at 25°C.
The Reynolds number range for the heating rod was 1259–
18880. The temperature–time histories of the heating rod
are presented in Figure 3. Apparently, under the preheated
condition, there is a rapid decrease in temperature and a high
heat loss rate; this rate subsequently slows down until the
heating rod cools to ambient temperature. The overall heat loss
rate is determined by the air ow velocity. At air velocity 0 m/s,
the rod temperature slowly decreases. As the air ow velocity
increases, the temperature values dramatically decrease,
and the overall heat loss rate signi cantly increases. As for
the continuous heating condition, the same trend is observed.
Remarkably, at air ow velocity 1 m/s, the heating rod quickly
reaches a stable temperature value. The probable reason is
that at this air ow velocity, heat gain due to the heat  ow inlet
into the heating rod is exactly equal to the heat loss, leading
Table 2. Characteristics of aerogel-embedded nonwoven fabrics
Samples Fabric density,
kg/m3
Areal density,
g/m2
Thickness,
mm
Aerogel content,
%
A79.6 278.6 3.5 1.5
B 66.7 440.2 6.6 2.0
C 80.4 498.5 6.2 2.5
Figure 2. Schematic diagram of the measurement device.
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to a steady state without any change in temperature of the
heating rod. As expected, an increase in air ow velocity gives
rise to a dramatic rise in the rate of heat transfer and thus a
rapid decrease in heating rod temperature.
3.2. Thermal behavior of the heating rod and fabrics under
preheated condition
In the preheated condition, the heating rod has an initial
temperature around 60°C. It loses heat by conduction due to
the temperature difference between the heating rod and the
 brous insulator, and the fabric gains heat. Some of this heat
energy is turned into heat capacity if the fabric temperature
increases, and the remainder is transferred to the fabric
surface and lost as the air  ows over it. The net heat loss from
the system results in the changes of the rod temperature and
the fabric temperature.
The temperature–time histories of different fabrics are
compared in Figure 4. Apparently, a low air ow velocity gives a
gentle downslope of the heating rod and fabric temperatures,
whereas a high velocity causes rapid temperature decline
and higher heat loss rates of the system. Comparing with the
temperature of the naked rod, the heating rod wrapped with
an aerogel-embedded nonwoven fabric has much lower heat
loss rate, indicating an effective insulating function of the
fabrics during cross  ow. Meanwhile, very small temperature
decreases are observed for all the three fabrics, further
improving their good thermal performance during cross  ow.
With the increase in air ow velocity, the heating rod temperature
lowers down; the decrease in each temperature–time curve is
more obvious. Generally, the heating rod with aerogel-treated
nonwoven samples B and C demonstrates higher temperature
in comparison with sample A, and the temperature gradient
is more signi cant at higher air ow velocities. This could be
explained by the difference of fabric structural parameters and
aerogel content. Nonwoven fabrics have very close porosity
values > 90%, with pore size ranging from 2 µ to 4.5 µ, which
allows the bulk of air molecules to  ow through [18]. However,
the aerogel particles, with an average pore size around 20 nm,
will retard – or even prevent – air ow stream. As a result,
greater aerogel content gives less permeability to air ow and
thus lowers the heat transfer rate. Meanwhile, thinner fabrics
are easier to be penetrated by the air stream, causing more
heat loss from the heating rod. As a result, aerogel-treated
nonwovens with higher aerogel content and fabric thickness
demonstrate better ability to prevent against heat loss under
air ow-induced convection. Moreover, at higher air ow velocity,
fabrics with more mesopores (diameter: 2 –50 nm) tend to be
less affected by the air ow. This is further proved by the higher
temperature values observed for fabrics with more aerogel
content when the air ow velocity is >5 m/s.
3.3. Thermal behavior under continuous heating
In the continuous heating condition, a speci c electric power
is supplied to the heating rod. For the system composed of
the heating rod and the fabric, the heat  ow generated by the
heating rod was partly converted into its heat capacity, and
the remainder was conducted to the fabric and  nally turned
into heat capacity of the fabric or dissipated by air-induced
convection. The whole dynamic process is in uenced by power
supply, air ow nature, physical parameters of the heating rod,
as well as the physical and structural parameters of the fabric.
The real-time temperature curves of the heating rod and
different fabrics under continuous heating are presented
in Figure 5. Generally, the fabrics maintain quite stable
temperature values under continuous heating, while the
heating rod temperature curves have slight  uctuations at
low air ow speed (1 m/s and 5 m/s), which gently increase at
a higher air ow speed (10 m/s). These  uctuations are well
consistent with the current switch. It is notable that the heating
rod with Fabric A has rapid temperature decrease from 58°C to
48.8°C when the air ow velocity is 15 m/s. This implies that the
aerogel content and fabric thickness are very important factors
in protecting against heat loss at high air ow velocity. Similar
to the results from the preheated condition, samples B and C,
with higher fabric thickness and aerogel content, demonstrate
better ability to prevent against heat loss from the heating rod.
Especially at higher air ow speed (10 m/s and 15 m/s), the
heating rod is able to achieve quite high temperature values,
indicating better thermal protection given by these fabrics. For
Figure 3. Temperature-time histories of the heating rod under preheated
condition and continuous heating condition.
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Figure 4. Temperature-time histories of the heating rod and fabric under preheated condition.
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Figure 4 continued. Temperature-time histories of the heating rod and fabric under preheated condition.
the reason given with reference to the preheated condition,
the real-time temperature curves of different fabrics vary with
air ow velocity.
3.4. Heat transfer coeffi cient under continuous heating
The continuous heating of the system under air ow-induced
convection is a dynamic process involving complex  ow
patterns around solid particles or  bers. Due to the random
orientations of the solid phase, exact solutions to the detailed
local  ow  eld are impossible. In order to compare the thermal
performance of different  brous materials, the system can
be considered to reach steady state if the temperature of the
heating rod and the fabric are  uctuating within a narrow range
or showing a very gentle upslope. In this case, the heat gain
by the system is roughly equal to the heat loss from the  brous
material by convection, as shown in Figure 6.
Heat  ow was measured by noting the current and voltage
input to the heating rod. The average heat transfer coef cient
is determined by the following expression:
(2)
where Qinlet is the heat  ow into the heating rod (in Watts); Tf
is the temperature of the fabric surface (in Kelvin); Tair is the
temperature of ambient air (in Kelvin); and S is the surface area
of the fabric exposed to the air ow (in square meters [m2]).
Since the Reynolds number is important in predicting  ow
patterns in different  uid  ow situations for convective heat
transfer problems, the heat transfer coef cient is plotted
for different Reynolds number in Figure 7. The heat transfer
coef cient increases with the increase in Reynolds number.
The data fall in distinct groups depending on the fabric density
and permeability of the material. For lower-density materials,
dispersion dominates any molecular conduction, resulting
in heat transfer independent of solid conductivity but more
affected by the air ow. The heat transfer coef cient of each
material shows an increasing trend with the increasing of
Reynolds number. A  at upstream trend is observed for aerogel-
treated nonwoven fabrics B and C. Aerogel-treated nonwoven
fabric A, having the least fabric thickness and aerogel content,
delivers slightly lower heat transfer rate at small Reynolds
number. However, the coef cients signi cantly increase with
the increase in air ow velocity and Reynolds number. For
Reynolds number <10000 (air ow velocity <10 m/s), the heat
transfer rates of different fabrics are quite close because of free
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Figure 5. Temperature–time histories under continuous heating.
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 brous materials for thermal insulation. Additional fundamental
and numerical works are necessary for better understanding
of convective heat transfer through aerogel-embedded  brous
materials.
Acknowledgments
This work is supported by the Ministry of Education, Youth
and Sports of the Czech Republic and the European Union
(European Structural and Investment Funds-Operational
Programme Research, Development and Education) in the
frames of the project “Modular platform for autonomous chassis
of specialized electric vehicles for freight and equipment
transportation”, Reg. No. CZ.02.1.01/0.0/0.0/16_025/0007293.
convection and radiation [19]. With the increase in Reynolds
number (air ow velocity >10 m/s), the forced convective heat
transfer – which strongly depends on the fabric permeable
property – overwhelms free convection and radiation [19]. As a
result, the heat transfer rate for Fabric A remarkably increases,
while fabrics with higher thickness and aerogel content
(samples B and C) could maintain relatively stable values.
3.5. Difference in temperature between heating rod and
air under continuous heating
Figure 8 shows the difference in temperature between heating
rod and air ow under continuous heating. The temperature
difference is fairly invariant over a range of Reynolds numbers.
For the heating rod without fabric, temperature difference lies
in the range of 32°Cto 8°C, showing considerable decrease
with increasing Reynolds number, as expected. The present
fabric maintains the heating rod at a higher temperature
because of its ability to prevent against heat loss. Comparing
the temperature differences of different fabrics, it is seen that
the heat losses from fabrics B and C are very close because
of the close proximity of their thickness and greater aerogel
content. Meanwhile, Fabric A gives much greater heat loss.
It can be concluded that fabrics with higher thickness and
aerogel content provide better insulation ability during cross
 ow, especially for high Reynolds numbers.
4. Conclusions
In this work, three aerogel-embedded nonwoven fabrics were
selected to investigate thermal behavior in cross air ow by using
a laboratory-made device. It was found that under preheated
conditions, all the three fabrics showed very small temperature
decrease, indicating that these fabrics are effective in preventing
against heat loss during cross  ow. The fabric with higher
thickness and aerogel content had better ability to retain heat
during cross  ow. The results from continuous heating showed
that the thinner fabric with lower aerogel content had limited
ability to protect against heat loss at high air ow velocity, while
the thicker fabrics with higher aerogel content demonstrated
lower heat transfer rate, especially at high Reynolds numbers
or air ow velocity >10 m/s. The  ndings can be used for further
study in the areas of aerogel-embedded high-performance
Figure 6. Heat transfer through the system in the steady state.
Figure 7. Heat transfer coef cients of the materials.
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