Many development applications for the
new materials such as textile fabrics used as thermal insulators require a
full study of its thermal insulating properties at different operating
conditions. One of the most important of these studies is the effect of
temperature with thermal conductivity and material density on the response
of the textile fabrics as insulators.
Thermoinsulating properties of perpendicularlaid versus crosslaid lofty
nonwoven fabrics are presented by Oldrich et al^{[1]}. In their
study, the relationship between the thermal conductivity and material
density of samples was studied. They concluded that the thermal conductivity
decreases with increasing material density. Morris^{[2]} presented a
study of thermal properties of textiles and concluded that their thermal
conductivity increases with density, based on his observation that when two
fabrics are of equal thickness, the one with a lower density has the greater
thermal insulation.
However, he reported that there is a critical density of about 60.0 kg/m^{3},
below which the convection effects become dominant and the thermal
insulation falls. Recently, the heat flux sensor was used to measure the
thermoinsulating properties of textiles in an apparatus called the Alambeta^{[3]}.
The thermal properties of fabric insulators are investigated by Ukponmwan^{[4]}.
Heat and mass transfer analysis of textile fabrics are presented in many
researches^{[58]}.
In these researches, the effect of operating parameters such as temperature,
humidity and heat & mass transfer coefficients are examined by
mathematical and experimental studies. A model of heat and water transfer
through layered fabrics was developed by Fohr et al^{[9]}. They
aimed at studying the effect of weather conditions and human activities on
the selection of clothing. Their model takes into account the occurrence of
condensation or evaporation in accordance with the environmental conditions
and their variations.
Thermal expansion behaviour of hotcompacted woven polypropylene and
polyethylene composites was studied by Bozec et al^{[10]}.
Compression and thermal properties of recycled fibre assemblies made from
industrial waste of seawater products are presented by Sukigara et al^{[11]}.
In their study, the effective thermal conductivity of fibre assemblies with
the steadystate, and parallelplates was measured. Their results showed the
lower effective thermal conductivity of recycled fibre assemblies than pure
wool fibre assemblies, which indicate that the effect of heat radiation on
thermal conductivity cannot be ignored.
In this work, the heat transfer through two different fabrics  polyester
and polypropylene is studied. The experiments are carried out using a
special testrig to study the thermal behaviour of the selected textile
fabrics used as thermal insulators in many applications. Temperature,
density, thickness and weight are measured for the selected textile fabrics
used as case study. The thermal insulation properties of the selected
textile fabrics are calculated and studied with respect to the importance of
operating conditions such as; inlet temperature, thickness, weight and
density. The comparison between the selected textile fabrics as thermal
insulators according to certain operating conditions is given. On the basis
of this study, some applications of these materials are considered.
Governing equations
The heat energy can be transferred through the textile fabrics by
conduction, convection and radiation that easily explainable phenomena such
as heat exchange in porous media. Basic concepts of the heat transfer
through fabrics are explained as follows:
Thermal conductivity The heat transfer by conduction depends on their
heat conductivity, ie, their capacity of transferring heat from a warmer
medium to a cooler one. The main characteristics of heat conductivity are:
Conductivity factor ? [W/(mo C)] expresses the heat flow (Q), W, passing in
1 h through area (A) of 1 m2 of the fabric thickness (L) at a temperature
difference (T1  T2) of 1o C, as given in the following equation:
? = Q L / A t (T1  T2) (1)
Heat transfer coefficient K [W/m^{2} ^{o}C] expresses the
heat flow passing during 1 h through 1 m2 of fabric with actual thickness,
(L) and difference temperatures of two media (air and fabric) 1^{o}C,
as the following equation:
K = Q / A t (T1  T2) (2)
Specific heat resistance (r)
The specific heat resistance, r ((m ^{o}C) / W) is a
characteristic inverse to the heat transfer factor, ? ?as the following
equation:
r = 1/ ? ? = A t (T1  T2) / (Q L) (3)
Heat resistance, (R)
The heat resistance, R (m^{2} ^{o}C / W) is a
characteristic inverse to heat transfer coefficient, K as the following
equation:
R = 1/K = A t (T1  T2) / (Q) (4)
The specific heat resistance, ?r and the heat resistance, R characterise the
heat capacity of the fabrics to impede the transfer of heat through them.
Thermal resistance, (Rth)
The thermal resistance, Rth of textile fabrics is a function of the actual
thickness of the material and the thermal conductivity, K. This function is
given by the following relationship:
R_{th} = L / k, ((m^{2} ^{o}C) / W) (5)
Where L is the actual thickness of the sample, m.
Heat flow, (Q)
The heat flow, Q, through the textile fabric is given as the
following:
Q =  k A (T1  T2) / L (6)
Where A is surface area exposed to the hot air, T1 is the initial air
temperature and T2 is the transient air temperature.
The textile fabrics have two thermal functions; they prevent air movement
and provide a shield against radiantheat losses. Within the limit before
heat conducted by fibres becomes dominant, the more densely fibres are
arranged within the fabrics, the better that they will fulfil these two
functions.
Energy equation
The energy equation for textile fabric is simply the transient heat
conduction equation with a heat radiation source term, this equation is
given as:
(7)
Where k, ?, CP, T and t are the thermal conductivity, density which it was
calculated as ? ?= ?/L (? is the basic weight of the sample), specific heat,
temperature and time for the selected fabrics, respectively. X is Xaxis and
is given as: 0 = X = L, 0 = t = 8 (8)
Where, L is fabric thickness. qr is the heat flux by radiation at any point
within the fabric and can be written as,[12]:
qr (x) = 4s To3 (T1  T2) at 0= X = L (9)
Where Sigma is the StephanBoltznan constant and equals 5.67 x 10^{8}
W/m^{2}K^{4 }and T_{o} is the mean temperature in
our experimental (T_{o} = 298 K). Figure (1) shows the schematic
drawing of the fabric insulator model.
Thermal Insulating Value (TIV)
(TIV) represents the efficiency of the textile fabric as an insulator.
It is defined as the percentage reduction in heat loss from a hot surface
maintained at a given temperature. The (TIV) increases to 100% when a
"perfect" insulator is obtained. (TIV) of textile fabric depending
upon the thermal conductivity of the fabric, the thickness of the assembly
and the thermal emission characteristics of the surface fabric. It is
expressed as a percentage, which represents the reduction in the rate of
heat loss due to the insulation, relative to the heat loss from the surface.
Thus, the following relation represents this value:
(TIV) % = 100 [ 1  (Kt /? eo) / (L + (Kt /? e1) ] (10)
Where eo and e1 are emissivity of one surface of the insulator (textile
fabric) and the other surface, respectively.
A typical value of emissivity of textile fabric is 2.06 cal / m^{2}
s^{o} C. The conversion of (TIV) to the tog unit can be written as
the following:
(TIV) % = 100 [1 (Io / I1)] (11)
where Io and I1 are tog values of unclothed and clothed bodies,
respectively, where 1 tog=0.418 m^{2}s ^{o}C/cal.
Table 1 gives the calculated values of the (TIV), % for the samples of the
selected fabrics.
Table 1: Thermal insulating values
(TIV) of the selected fabrics
Selected
fabrics 
Thickness,
m 
(TIV),
% 
Polyester

Sample
1 
3.54x10^{3} 
41.21 
Sample
2 
4.32
x10^{3} 
49.3 
Sample
3 
4.88
x10^{3} 
50.5 
Sample
4 
5.62
x10^{3} 
51.3 
Sample
5 
7.97
x10^{3} 
52.15 
Polypropylene

Sample
1 
3.76
x10^{3} 
41.95 
Sample
2 
4.44
x10^{3} 
49.89 
Sample
3 
4.6
x10^{3} 
50.05 
Sample
4 
5.7
x10^{3} 
51.98 
Experimental work
In order to investigate the heat transfer and thermal behaviour of
textile fabrics as thermal insulators, especially experimental testrig was
designed and constructed to measure the temperature variation with test time
through the selected textile fabrics during the heat exchange process
between the inlet hot air and the fabric sample.
Experiments are carried out on two of nonwoven fabrics. The fabric samples
are prepared by drying rout web formation technique and produced on the
needlepunching machine. A group of samples made from polyester fibres with
different weight per unit area, another group made from polypropylene with
the same weight. The fabric samples are subjected and exposed to different
levels of heat in the emission side (the heat source side) and then the
temperature are measured in the other side of the fabric sample in order to
evaluate its thermal resistance and behaviour as thermal insulator.
Tables 2 and 3 give the numerical values of parameters for the samples of
the selected textile fabrics (polyester and polypropylene, respectively) and
inlet heat exposure levels as temperatures that are used in the presented
study.
Results and discussion
From the results of laboratory experiments and calculation, it is found
that thermal insulating properties of textile fabrics (?, K, CP) affect the
insulation response. Figures 2 to 24 illustrate the thermal response and
behaviour of the selected fabrics (polyester and polypropylene) that used in
this work as thermal insulators.
Temperature variations with time for polyester samples (15) at
different exposure temperatures (40, 80, 120, 160 and 200^{o}C)
through 25 experiments are plotted in Figures (26). From the figures, it is
found that the fabric temperature (TF) variations increase rapidly in the
initial stage of the exposure temperature. This may be, because the
temperature difference between the fabric sample and the exposure hot air is
high in the early stage of the exposure process. Through 20 experiments,
temperature variations with time of polypropylene samples (14) at different
exposure temperatures are shown in Figures 710.
Figures 11 and 12 show the effect of polyester and polypropylene
thickness on fabric temperature (TF), respectively. It is found that higher
fabric thickness means good insulation. Specific heat resistance for the
selected fabrics is shown in Figure 13. It is found that polyester samples
have higher specific heat resistance than polypropylene samples. Also,
thermal resistance of the selected fabrics is shown in Figure 14.
From these figures, the polyester has higher specific heat resistance
and thermal resistance than polypropylene. This may be, because the thermal
conductivity of the polyester is lower than polypropylene. Figures 15 to 22
give the effect of exposure temperatures on the heat flow through the
polyester and polypropylene for the samples (14) each of them. It is found
that the heat flow rises rapidly during the early stage of the hot air
exposure to the fabric. This is due to the high temperature difference
between the fabric surface (cold) and the hot air. Also, high exposure
temperature means high heat flow through fabric.
Figures 23 and 24 show the surface plot and contours of measured fabric
temperatures for 100% of polyester and 100% of polypropylene nonwoven
fabrics, respectively. The figures give the influence of nonwoven fabric
weight and time on the temperature variations when it exposes to different
temperatures (40, 80, 120, 160 and 200^{o}C). It is found that the
temperature variations of the fabric increased with increasing of time and
decreased with fabric weight up to a certain limit, beyond its optimum
level. This may be due to the fibre quantity that increased with increasing
of fabric weight; in other words, the compactness of nonwoven fabrics
increased with increasing basically weight and consequently the fabric
thickness. This is the reason of thermal behaviour of the fabrics. As the
results of ANOVAtwo way, the relations between temperature variation of the
polyester and polypropylene fabrics, respectively, weight and time are given
as the following, (Figures 23 and 24):
Z = 14.68 + 0.035 * X + 0.66 * Y  0.0 X*X  0.0 X*Y  0.003*Y*Y
Z = 21.55 + 0.036 * X + 0.511 * Y  0.0 X*X  0.0 X*Y  0.003*Y*Y
Conclusion
Based on the calculations and experiment results of the selected fabrics
that were used as thermal insulators, the following conclusions can be
drawn:
The laboratory experiments and calculation have shown that the selected
textile fabrics can be used as good thermal insulators in the range of
exposure temperatures of 40200^{o}C.
The study concludes that the selected fabrics have high thermal performance
and thermal response as insulators. The effect of fabric thickness on the
fabric temperature variations has obviously significance that the higher
thickness means good thermal insulation.
Both the thermal conductivity and thermal resistance of all selected fabric
samples increases with increasing of fabric density. Fabric thickness
affects the transient fabric temperatures, and that fabric temperature
variation decreases with increasing fabric thickness.
The exposure temperature affects the heat flow through the selected fabrics,
and the heat flow increases with increasing exposure temperatures. The
temperature variations of the fabric increase with increasing of time and
also decrease with fabric weight up to a certain limit, beyond its optimum
level.
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Note: For the detailed version of
this article please refer the print version of The Indian Textile Journal 
Sept 2006.
Zeinab S AbdelRehim is Associate Prof, Mechanical Engineering Dept; and M M
Saad and M ElShakankery are Professors with the Textile Department,
National Research Centre, Dokki, Giza, Egypt. I Hanafy is Associate Prof,
Helwan University, Cairo, Egypt.
