PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Airflow resistance measurements between room temperature and
800℃
Thamasha SAMARASINGHE1; Carl HOPKINS1; Gary SEIFFERT1, Jilly KNOX2
1

Acoustics Research Unit, School of Architecture, University of Liverpool, United Kingdom
2Morgan Advanced Materials Plc, Thermal Ceramics

Tebay Road, Bromborough, Wirral, CH62 3PH, United Kingdom

ABSTRACT
Acoustic properties of porous materials are usually measured at room temperature but in many industries,
they are required to provide sound absorption in high temperature applications such as in a Heat Recovery
Steam Generator (HRSG), automobile silencers and engine liners. In this paper, the airflow resistance has
been measured for Alkaline Earth Silicate (AES) fibrous materials that are intended for high temperatureapplications. Measurements were made at room temperature to ISO 9053-1 and in a bespoke test rig attemperatures up to 800℃ inside a kiln. Above 600℃ the thickness of the AES material decreased; hence themeasurements reported the specific airflow resistance rather than airflow resistivity. The high temperature
tests indicated that it might be possible to assume no significant effect of temperature on specific airflow
resistance between 20 and 100 ℃ but not at higher temperatures. The specific airflow resistance depends onthe mass of the sample up to 800℃. Up to 600℃, the dependence is similar but as the AES material begins
to crystallize between 600 and 800℃ it changes significantly. Regression analysis was also used to establishthat up to 600℃ the specific airflow resistance is proportional to the absolute temperature to the power of
1.6.
Keywords: airflow resistance, porous materials, high temperature

1. INTRODUCTION
In the building industry, porous materials are often used to provide sound absorption at room
temperature. However, other industries have high temperature applications which require soundabsorption such as gas turbine exhaust silencers in a Heat Recovery Steam Generator (HRSG), aeroengine liners, combustion chambers, automotive silencers. In an HRSG, Alkaline Earth Silicate (AES)
porous material is used to reduce sound levels inside the silencers and near to the exhaust area which
operate at temperatures between 800 and 1300℃ (1). Measurement of the properties of acoustic
materials at room temperature is well-established; however there is not usually any information
available on how the properties change with increasing temperature. Hence to provide data andfacilitate product development on absorbent materials for high temperature applications, the aim wasto develop experimental procedures to measure the acoustic properties of AES material at hightemperatures. As sound propagation in porous materials is dependent on their resistance to airflow,
measurements of this resistance can be used to predict both sound absorption and sound attenuation.
The ratio of the differential pressure, οܲǡ (Pa) across a layer of porous material and the volumetricairflow rate,‫ݍ‬௩ , (m3 /s) passing through the layer gives the airflow resistance, R, (Pa.s/m3 ) as
ܴൌ

οܲ
‫ݍ‬௩

(1)

The specific airflow resistance, R s, (Pa.s/m) applies to a specific thickness of material and is givenby1carl.hopkins@liverpool.ac.uk, thamasha.hambange-dona@liverpool.ac.uk

1387ܴୱ ൌ ܴܵ(2)where S, (m2 ) is the area of the porous material.
For homogeneous materials of a specified thickness it is common to refer to the airflow resistivity,r, (Pa.s/m2 ) given by
‫ݎ‬ൌ

ܴୱ
݀

(3)

where d,(m) is the thickness of the material.
For fibrous porous materials, the resistance to airflow depends on the fibre diameter, fibre
shape/type, fibre density, number of fibres per unit volume and fibre orientation (note that the airflow
resistivity in the lateral direction can be significantly lower than in the longitudinal direction (2)).
This work aimed to carry out measurements up to 800℃ for HRSG applications, using
measurements on three different densities of AES fibrous materials, 64, 96 and 128 kg/m 3 with anominal thickness of 50 mm. The differential pressure across porous materials at high temperatures islower than at room temperature; hence it is expected that instead of a single sample, two samples
might need to be stacked on top of each other (i.e. to form a double sample) to give a measurable
differential pressure.
The only previous published airflow measurements at similarly high temperatures appears to havebeen by Christie (3) who measured one density of rock wool (80 kg/m3 ) at 500℃. In Christie’s
research (3), the flow velocity through the sample was in the range 1×10 -2 to 4×10 -2 m/s. For thismaterial, Christie showed that (a) the airflow resistivity was proportional to the 0.6 th power of theabsolute temperature in Kelvin and (b) above 400℃, the airflow resistivity increases at a lower rate,and (c) airflow resistivity at a particular temperature did not depend on flow velocity within the range
of velocities used. Since, AES material is used at temperatures above 500℃, this research investigatedthe measurements of airflow resistance at room temperature (20℃) according to ISO 9053-1 (4) andin a bespoke test rig inside a kiln at temperatures from room temperature up to 800℃.
In this paper, section 2 describes the experimental apparatus design for both room and hightemperatures. The results for room temperature and high temperature are discussed in section 3.
Section 4 gives conclusions on the main findings at room and high temperatures.

2. EXPERIMENTAL APPARATUS
The experimental apparatus for both room and high temperature consists of a specific test rig,
differential pressure, flow meter and a pressure regulator. A Furness Control FCS 523 instrument isused to measure differential pressure and airflow rate; this consists of two main parts, a laminar flow
meter and differential pressure meter. The laminar flow element has the ability to measure volumetricgas flow rates from 0 to 2 l/min and measures volumetric gas flow rate based on the Poiseuille equation.
According to the manufacturer, the device generates a very low differential pressure, while offering
little restriction to the flow, typically a pressure drop of 100 Pa at full flow rate at 2l/min; however
the maximum temperature that the measuring instrument can withstand is 34℃ such that this needed
consideration in the experimental design. The differential pressure device can measure to two decimal
places with an accuracy of <0.25%. However, since the airflow meter can only withstand pressure൑300 Pa, it is necessary to reduce the supply air pressure which is supplied at 1200 kPa and to
dehumidify the supply air before it enters the meter. This was achieved with a pressure regulator andair filter.
For the high temperature test rig it was necessary to correct the differential pressure that was being
measured at room temperature using temperature measurements inside the test rig and at thedifferential pressure meter.

οܲᇱᇱ ൌ ܲଵᇱᇱ െ ܲଶᇱᇱ ൌԢԢܶ ᇱᇱ ᇱሺܲ െ ܲଶᇱ ሻ
ܶᇱ ଵ

(4)
Ԣ

where ܶ indicates at the high temperature inside the test rig and ܶ indicates room temperature,
T is temperature in Kelvin.
Airflow resistance measurements according to ISO 9053-1 (4) and ASTM C522 (5) require a1388

minimum flow velocity of 0.5×10 -3 m/s (corresponding to a sound pressure level of 80 dB re 2×10 -5 Pa
in a plane wave) and with the valve it is possible to control the flow to ± 0.01×10-3 m/s. As the crosssection of the sample holders are all 100 mm diameter, this gives the range of volumetric flow rates
as between 0.231 and 0.240 l/min.
2.1 Room temperature test rig
The room temperature test rig was designed to satisfy the requirements of ISO 9053-1 (4) and
ASTM C522 (5) using the direct airflow method with controlled unidirectional airflow through thetest specimen. In order to visually check the position of the test sample, a transparent material(Perspex) was used to fabricate the cylindrical specimen holder (internal diameter of 100 mm and aheight of 200 mm). The differential pressure is measured between the volume of air underneath thesample and atmospheric pressure.
2.2 High temperature test Rig
The test rig for high temperature airflow resistance measurements was designed to give nominallyidentical results at room temperature to the ISO test rig described in ISO 9053 -1 (4) and ASTM
C522 (5). The initial aim was to design a test rig to withstand temperatures up to 1000 ℃. A similar
test rig design to Christie (3) was used as a starting point with the aim of extending its use above
500 ℃. This test rig consists of a preheater with a packing material to ensure that the air enters the
sample at the same temperature as the internal kiln temperature and to provide uniform airflow acrossthe surface of the sample (3). After performing several experiments with different packing material,
50 mm Superwool ® Plus Blanket was selected as the preheater material. This material has a highmelting point, is mechanically stable with very low shrinkage at high temperatures and is mechanicallyneedled for added tensile strength and surface integrity. In contrast to the room temperature test rig
where the differential pressure was measured with reference to the atmospheric pressure outside thetest rig, the high temperature test rig needs pressure taps to determine the differential pressure acrossthe sample within the sample holder inside the kiln.
The high temperature test rig is shown in Figure 1 and consists of a cylindrical specimen holder
and cylindrical preheating chamber. There is one air inlet, one air outlet, two outlets for the differential
pressure measurement and one tapping point for the temperature probe. The specimen holde r has two
perforated meshes to hold the sample in place and a preheating chamber consist ing of an air inlet anda differential pressure meter probe inlet. The specimen holder has a height of 150 mm which allowed
vertical orientation of the test rig within the kiln. As the high temperature test rig needed to work up
to 1000℃, it was necessary to check whether the length of the preheating chamber might need to besignificantly longer than that used by Christie (3) to ensure that the air reached the same temperatureas the air in the kiln. The process was modelled using Matlab Simulink software to identify a suitable
length for the preheating chamber to achieve the required temperature output. By considering the heattransfer occurs from the kiln to preheating chamber it was found that the required length for thepreheater was 250 mm.

Figure 1 - High temperature test rig assembly.
By considering the material strength and corrosion resistance, Grade 310 aus tenitic stainless steel
was initially selected to fabricate the test rig. However, due to lack of availability of hollow tubes of1389

Grade 310 austenitic stainless steel and difficulty in machining it, Grade 316 was used instead. As
Grade 316 is not designed to withstand such high temperatures, the maximum temperature feasible
for measurements was expected to decrease to approximately 900℃. In order to assess the thermal
stability and the thermal stress at 900℃, Finite Element Analysis (FEA) was carried out for the hightemperature test rig design using Autodesk Mechanical Simulation software. Thermal stress issimulated by coupling transient heat transfer and structural analysis. However, the results showed that
Grade 316 austenitic stainless steel design would fail at temperatures near 900℃; hence the upper
temperature for all measurements with the high temperature test rig was reduced to 800℃.
The next step was to avoid heat damaging the measurement instruments which cannot withstandthe air temperatures generated inside the kiln and to avoid heat conduction via the pipes melting theconnections to the measuring instruments. Fortunately, pre-tests showed that the air temperature at
the measuring instrument was below 34℃. Hence, there was no need to use a heat exchanger.

3. RESULTS
3.1

Room temperature airflow resistance results

3.1.1 Comparison of the ISO and high temperature test rigs
At room temperature (20℃), a comparison is made to check that the high temperature test rig
gave nominally identical results to the ISO 9053-1 test rig. The reason to use different density
materials is that the test sample holders are different and therefore ‘soft’ (low density) and ‘stiff’ (highdensity) samples might be fitted differently in the two test rigs. This is partly due to the ISO rig beingtransparent which allows the experimenter to see the fitted sample (which is not possible with the hightemperature rig) and partly due to the different length of the sample holder tubes.
The average results for ten single and double samples at room temperature are shown in Table 1
indicating the increase in airflow resistivity with increasing density. The coefficient of variation (CoV)is highest for the low density material and could be due to distortion of these soft samples when
positioned in the test holder or due to larger variation in the physical properties between samples. For
single samples, ANOVA tests indicate significant differences (p≤0.05) for the 64 and 96 kg/m3
materials and non-significant differences (p>0.05) for the 128 kg/m3 material. The significant
differences could be due to different fixing and/or compression of the ‘softer’ samples in the test rigs.
The ISO test rig is transparent which allows a clear check on the sample position and therefore thefitting for air tightness is expected to be better with this test rig than with the opaque high temperature
test rig.
The CoV values for all double samples are lower than the corresponding single samples,particularly for the low density material due to the mixing of two different samples of this morevariable material. The lower CoV is beneficial as it allows for a more reliable comparison of the ISO
and high temperature test rigs. In theory, the airflow resistivity for single and double samples shouldbe the same if the fitting and compression of the samples inside the sample holder is identical.
However, double samples were found to be more compressed than single samples. Therefore,
statistical comparisons were carried out using the average airflow resistivity from single and double
samples (same batch of material) and the results indicate that there is a statistically significant
difference (p≤0.05) between single and double samples. This is attributed to the different fitting of
the samples in test rigs.

1390

Table 1 – Airflow resistivity measurements at room temperature in the ISO and high temperature test rigs.
ISO test rig
SingleordoublesamplesBulkdensity(kg/m3)

Average

Averagethickness

airflow(mm)

resistivity(Pa.s/m2)

High temperature test rig

Standard
deviation(Pa.s/m2)

Average

Average

CoV

thickness

airflow(-)(mm)

resistivity(Pa.s/m2)

Standard
deviation(Pa.s/m2)

CoV(-)

Single64

44.2

10,373

983

0.094

47.4

7,940

1158

0.146

Single96

49.6

36,479

706

0.019

38.5

37,448624

0.017

Single128

46.2

102,016

6710

0.066

47.8

107,102

2349

0.022

Double64

95.5

10,106

331

0.033

88.3

9,961

581

0.059

Double96

80.8

37,312

605

0.016

78.1

37,766394

0.010

Double128

90.4

103,145

4879

0.047

90.7

105,284

1948

0.019

3.1.2 Regression analysis
A linear relationships between airflow resistivity and bulk density is established by considering
the logarithm of both parameters as carried out by Nichols (7). Regression curves were determinedfrom the two test rigs using the three material densities and single and double samples as shown in
the Figure 2. One-way ANOVA for the two regression models show that there is a statistically non significant difference (p>0.05) between the intercepts and gradients for the two different types of rigs.
For the combined dataset for single and double samples it is concluded that the measured airflow
resistivity does not differ between the two test rigs.

Figure 2 - Relationship between airflow resistivity of single and double samples and bulk density at room
temperature: Regression plots for the ISO test rig (blue) and the high temperature test rig (red).
3.2 High temperature measurements
All the high temperature measurements used double samples to ensure a measurable differential
pressure. During initial experiments it was observed that the measured thickness of the material wasdifferent before and after exposure to high temperatures. According to (6), AES is almost amorphousat room temperature but undergoes crystallization and shrinkage near 900℃ although the properties
start to change between 750 and 800℃. When there is an unknown reduction in thickness at high1391

temperature it is not possible to calculate the airflow resistivity as this requires knowledge of the
thickness. Therefore it is more appropriate to calculate the specific airflow resistance to assesschanges over a wide range of temperatures. In addition this reduction in thickness means that it is nolonger possible to attribute a bulk density to the material above 600℃ such that regression analysis
needs to use sample mass rather than density.
3.2.1 Regression analysis to relate specific airflow resistance to sample mass for different
density materials
Empirical relationships are sought using the mass of each double sample at different temperatures.
The regression lines are plotted along with the individual data points in Figure 3 confirming that the
specific airflow resistance depends on the mass of the sample. Up to 600℃, the gradient of theregression lines is similar but it changes significantly at 700℃ and 800℃ as crystallization occurs.

Figure 3 - Relationship between specific airflow resistance and mass of the double samples at temperatures
between 20 and 800℃.
3.2.2 Power law relationship between airflow resistance and temperature
From the Poiseuille law, airflow resistance is proportional to air viscosity and Sutherland (8) has
shown that the viscosity for ideal gases is proportional to T n where the exponent n ranged from 0.7
for hydrogen to 1.0 for other gases. Hence, when relationships are sought between the airflow
resistance and temperature it is reasonable to assume that airflow resistance will be proport ional to T nbecause airflow resistance is proportional to air viscosity.
From experimental work, Christie (3) stated that air viscosity varied with T 0.7 and whilst Christie
did not prove this it can be shown to be a reasonable estimate by using regression analysis of data
calculated with Sutherland’s equation (8). For rock wool at temperatures up to 400℃, Christie’s
measurements indicated that airflow resistance was proportional to T 0.6. However, Miglietta et al (10)concluded that it varied with T 1.2 based on measurements between 0 and 30℃ on a range of materials(including generic materials such as polyethylene, rubber, glass wool, rock wool, cotton waste, andpolyester).In order to identify the power law relationship between specific airflow resistance andtemperature for AES material, regression analysis was carried out from 20 to 600℃ (higher
temperatures were not considered because the material changed significantly above 600℃). Powerlaw regression for each material density gave a close fit to the measured data (coefficient ofdetermination R 2>0.99). However, the average exponent n was 1.6 which is larger than those publishedin the literature mentioned above; hence it would be worthwhile testing different materials in future
projects to see whether similarly high values occur with other materials.

4. CONCLUSIONS
A “high temperature” test rig has been designed for measurements of airflow resistance attemperatures up to 800℃ by modifying a design used by Christie (3). This has allowed measurements
on Alkaline Earth Silicate (AES) fibrous materials of three different densities. A room temperature1392

comparison was made of the high temperature test rig and an “ISO” test rig built according to ISO
9053-1:2018. These measurements were carried out using single samples and double samples (i.e. two
samples on top of each other) because it was expected that double samples would eventually be neededto achieve measurable differential pressure drops at high temperatures. The statistical tests were morereliable for the combination of single and double samples and confirmed that there was no significant
difference between the two test rigs.
High temperature experiments were conducted up to 800℃ using double samples to ensure a measureable
differential pressure. However, after exposure to temperatures above 600℃ the thickness of the AES material
decreased due to crystallization and due to the opaque test rig this decrease in thickness could not be
quantified at individual temperatures; hence all measurements reported the specific airflow resistance rather
than airflow resistivity. The high temperature tests on AES materials indicated that it might be possible toassume no significant effect of temperature on specific airflow resistance between 20 and 100 ℃ but not at
higher temperatures. The specific airflow resistance depends on the mass of the sample up to 800℃. Up to
600℃, the dependence is similar but as the AES material crystallizes at 700℃ and 800℃ it changes
significantly.
Regression analysis was also used to establish that the specific airflow resistance is proportional
T 1.6 for this AES material.
In this paper the measurements were all carried out using an airflow velocity of 0.5 u 10 -3 m/s.
However, additional measurements (not reported in this paper) show that at room temperature there
is little change in the specific airflow resistance when measured with flow velocities between 0.5 u10-3 and 4 u 10 -3 m/s but there can be significant differences at 800℃. Hence for high temperatureapplications the airflow velocity will need to be chosen to correspond to the sound pressure level forthe specific industrial application.

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International Organization for Standardization.
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