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Proceedings of the 23rd National Heat and Mass Transfer Conference and 1st International ISHMT-ASTFE Heat and Mass Transfer Conference
IHMTC2015 17-20 December, 2015, Thiruvananthapuram, India
IHMTC2015-371
THERMO-OPTICAL CHARACTERIZATION OF MATERIALS USED IN BASE REGION OF LIQUID ENGINES
Ram Prabhu. M AHTD/AERO/VSSC/ISRO
Thiruvananthapuram, Kerala-695022, India
email: [email protected]
Radhakrishnan. T. V AHTD/AERO/VSSC/ISRO
Thiruvananthapuram, Kerala- 695022, India
Chacko. M. J AHTD/AERO/VSSC/ISRO
Thiruvananthapuram, Kerala-695022, India
ABSTRACT
Hot nozzle divergents of liquid engines in a
satellite launch vehicle are prominent radiative heat
sources to adjacent base structures. Thermo-optical
properties namely: absorptivity, reflectivity,
transmissivity and emissivity determine the thermal
response of a material exposed to such intense
radiative environments. Hence thermal design of the
base region requires good knowledge on thermo-
optical properties of materials at their service
conditions. Even though standard methods exist for
measurement of thermo-optical properties, it is
difficult to carry out measurement at operating
conditions as in base region of launch vehicles. In this
paper, an approach using temperature
measurements by IR pyrometer and thermocouple is
evolved to determine emissivity at high temperature
operating conditions. Spectroscopic methods are
used to measure the spectral reflective and
absorptive characteristics. The emissivity, reflectivity
and absorptivity characteristics of typical materials
used in base region of launch vehicles are measured.
These measured values are critical inputs for detailed
thermal analysis and TPS design.
Keywords: Emissivity, Absorptivity and Reflectivity.
NOMENCLATURE
ε Emissivity of the surface
α Absorptivity of the surface
τ Transmissivity of the surface
λ Wavelength of radiation
θ Observer angle
σ Stefan Boltzmann constant
F View factor
Ts Surface temperature
q Heat flux
INTRODUCTION The thermal behavior of a material exposed to thermal
radiation is influenced by its thermo-optical properties
which include absorptivity, reflectivity, transmissivity and
emissivity. These properties depend on wavelength of
radiation, angle of incidence and/or departure, nature of
radiation (specular/diffuse), operating temperature and
surface characteristics. The variation of these properties at
different temperatures are critical inputs for radiative load
estimation, Thermal Protection System (TPS) design, non-
2
contact type temperature measurement, thermal
management of electronic packages and satellites in orbit.
The nozzle divergent of liquid engines in satellite
launch vehicles is cooled by radiative and film cooling. A
high emissivity exterior surface for the nozzle divergent is
necessary for efficient radiative cooling. This emitted
thermal energy is incident up on neighboring structures in
the vicinity of nozzle divergent resulting in its heating.
Figure 1 illustrates the different modes of radiation
interaction between nozzle and neighboring structures in
the base region of a launch vehicle.
FIGURE 1. RADIATION EXCHANGE AT BASE
REGION OF A LAUNCH VEHICLE
Emitted thermal energy from the nozzle divergent is
(1)
The emitted energy from nozzle divergent is absorbed
by adjacent surfaces, which is given as
(2)
The energy absorbed by adjacent surfaces depends on
its surface absorptivity and view factor with the nozzle
divergent. Further radiation interactions include re-
radiation and reflection from the adjacent surface to nozzle
throat region. The incident radiation on the nozzle throat
region is
(3)
where ρ is the reflectivity of surface. Hence it is
essential to characterize the thermo-optical properties of
nozzle and adjacent surfaces for service conditions [1], to
estimate radiative heat load and design TPS in the base
region. The nozzle divergent is made of stellite. A
combination of glass fabric and silica fabric forms the
neighboring surfaces in the vicinity of nozzle divergent.
The emissivity, reflectivity and absorptivity at different
wavelengths and temperatures for these materials were
characterized. The measured values were useful as an input
for detailed thermal analysis, TPS design and testing.
BACKGROUND
Thermal radiation falling on a body will be partially
reflected, transmitted and absorbed depending on its
thermo-optical properties. The principle of conservation of
energy states that
(4)
A black body emits maximum amount of heat for a
given absolute temperature. Moreover, all radiant energy
incident on a black body is fully absorbed hence
and [2]. Emissivity characterizes emissive
power of a non-black body with that of a black body.
Emissivity of non-black bodies varies from 0 to 1.
A body in thermodynamic equilibrium emits as much
energy as it absorbs at each direction and wavelength.
Kirchoff’s law relates monochromatic, directional
emittance with monochromatic, directional absorptance for
a surface that is in thermodynamic equilibrium with its
surroundings
(5)
This indicates that the emissivity of a surface can be
indirectly obtained by measuring its absorptivity [3]. For
opaque bodies,
(6)
For diffuse and gray body radiation, Equation (6) can
be rewritten as
(7)
The diffuse and gray body form of Kirchoff’s law may
yield misleading results as many surfaces are not even
approximately gray. Moreover the surface whose thermo-
optical properties are to be measured need not always be in
thermal equilibrium with its surrounding especially at
elevated temperatures.
The total emittance depends on its physical state and
temperature of the surface. The total absorptance of a
surface depends on source from which radiation is
absorbed and its own surface characteristics. A surface may
absorb better at some wavelength regimes than others.
Thus total absorptance depends on the distribution of
incoming radiation energy over the wavelength range,
which is determined by temperature and surface properties
of the source. Hence it is crucial to evaluate thermo-optical
properties at service conditions.
The different methods of measuring thermo-optical
properties include calorimetric, optical reflectivity, multi-
3
spectral radiation thermometry and radiation energy
methods. In calorimetric method, heat power delivered to
the test specimen, temperature of emitting surface and
surroundings in vacuum over time are the measured
quantities. Then energy balance equations are used to
evaluate emissivity. The calorimetric method demands a
high degree of insulation to avoid parasitic heat losses. It is
not suited for measurement at elevated temperatures [4].
The reflectance from a heated test specimen is measured in
optical reflectivity method and used to compute
absorptance and emittance. The radiant energy is measured
at three or more wavelengths in multispectral radiation
thermometry.
Analytical approach involves estimation of emissivity
using an emissivity model. The emissivity model is not
universal for all materials and hence it lacks accuracy [5].
In radiation energy method, the radiation power of a
sample surface is measured in comparison to that of a
black body at same temperature. The selection of black
body plays determines the accuracy of this method [6].
MEASUREMENT OF THERMO-OPTICAL PROPERTIES
EMISSIVITY
Figure 2 shows the algorithm used for determining
emissivity of a surface using IR pyrometer and
thermocouple. This methodology holds good for materials,
which are opaque in nature. The test specimen is heated
using Infra-Red (IR) radiative heaters. Temperature
measurement on the heated surface is error prone due to
the effect of direct radiation from the heat source. Hence
all measurements were proposed to be taken at the back
wall. The temperature at back wall of the specimen is
measured using K-type thermocouple and IR pyrometer
simultaneously. A single color IR pyrometer is used for this
study. For temperature measurement of a surface, its
emissivity value should be fed as an input to the single
color IR pyrometer. The following procedure was adopted
to determine emissivity:
a) Initialize emissivity input of IR pyrometer to unity
and point it towards the back wall.
b) Connect the K-type thermocouple welded at back
wall to a digital temperature reader.
c) Heat the test specimen using IR heaters until its back
wall attains a desired steady state temperature at
which emissivity is to be measured.
d) At this steady state condition, monitor the
temperatures read by both K-type thermocouple and
IR pyrometer.
e) Correct the emissivity input of IR pyrometer so that
the updated temperature reading of IR pyrometer
matches with that measured by the thermocouple.
f) This corrected emissivity fed into the IR pyrometer
will be the effective emissivity of the surface at that
temperature.
g) Repeat the above steps at different steady state
temperatures to determine emissivity at
corresponding surface temperatures.
REFLECTIVITY AND ABSORPTIVITY
The effective absorptivity of different materials was
computed for a wavelength range of 0.25m - 10m at
different source temperatures. The absorptivity of a surface
does not change as long as the surface quality remains
same. However emissivity changes with change in surface
temperature. The computations of absorptivity/reflectivity
have been carried out based on measured spectral variation
of reflectance using spectroscopy methods.
FIGURE 2. ALGORITHM FOR MEASURING
EMISSIVITY USING IR PYROMETER
Effective absorptivity, 𝑒𝑓𝑓is computed as
𝑒𝑓𝑓 ∫𝛼 𝐸
∫𝐸 (8)
where is spectral absorptivity of the material. 𝐸 is
spectral emissive power of the black body at a particular
temperature which is defined as
𝐸 2 𝜋 𝐶
(𝑒 − 1)
(9)
where 𝐶 and 𝐶2 are radiation constants; λ is
wavelength and T is the temperature [7]. The main input
for computing effective absorptivity is its measured
spectral variation which is obtained from spectral
reflectivity, .
(10)
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EMISSIVITY CHARACTERIZATION
SILICA FABRIC
Silica fabric was directly exposed to IR heating to
attain steady state temperature. The temperature at the back
wall of silica fabric was measured using K-type
thermocouples and IR pyrometer simultaneously. The
emissivity input of pyrometer was adjusted until the
temperature reading of the pyrometer matched with that of
thermocouple. Figure 3 shows the measured emissivity
variation, which varies from 0.80 at 150°C to 0.66 at
630°C. It was observed that silica fabric being porous in
nature was having higher transparency to higher glow
intensity. Hence IR pyrometers are sensitive to the
transmitted radiation from IR lamps and other surfaces
through silica fabric. To minimize the influence of
transparency, a mild steel plate with buffed surface was
placed in contact with silica fabric. The steel plate alone
was subjected to IR heating. Figure 4 shows the measured
emissivity of silica fabric for temperatures up to 400°C.
The emissivity varies between 0.76 and 0.79 for
temperatures ranging from 100° to 400°C.
FIGURE 3. MEASURED EMISSIVITY OF SILICA
CLOTH EXPOSED DIRECTLY TO IR HEATING
FIGURE 4. MEASURED EMISSIVITY OF SILICA
CLOTH HEATED USING METALLIC PLATE
COMBINATION OF GLASS FABRIC AND
SILICA FABRIC
Glass fabric is highly transparent in nature and hence
its reflectivity is affected by the back-up materials. The
emissivity of glass fabric with two layers of silica fabric
followed by aluminized silica fabric as backup materials
was measured. Silica side of aluminized silica fabric was
exposed to IR heating. Two trials were made to measure
emissivity for temperatures up to 270°C. Figure 5 shows
the measured emissivity of glass fabric with silica fabric
combination. The measured emissivity reduces from 0.89
to 0.56.
REFLECTIVITY AND ABSORPTIVITY
CHARACTERIZATION
The effective absorptivity of a material is an averaged
spectral absorptivity combined with its spectral emissive
power. The dependence of absorptivity on the source
temperature is brought in by its spectral emissive power.
Hence in order to obtain absorptivity at different source
temperatures, the spectral emissive power is computed at
those particular temperatures [4]. Figure 6 shows the
measured spectral reflectivity characteristics for different
materials. Aluminium side of aluminized silica cloth is
having higher reflectivity, followed by stellite. Silica fabric
and glass fabric being semi-transparent/porous are having
low reflectivity.
FIGURE 5. MEASURED EMISSIVITY OF GLASS
FABRIC WITH SILICA FABRIC COMBINATION
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
90 150 210 270 330 390 450 510 570 630
Temperature (oC)
Em
iss
ivit
y,
Silica Cloth
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
50 100 150 200 250 300 350 400
Temperature (oC)
Em
iss
ivit
y,
Silica Cloth
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
70 95 120 145 170 195 220 245 270
Trial 1Trial 2
Temperature (oC)
Em
iss
ivit
y,
Heat Lab + 2 Silica Cloth + Glass Cloth
5
FIGURE 6. MEASURED SPECTRAL REFLECTIVITY
VARIATION FOR DIFFERENT MATERIALS
SILICA FABRIC
Figure 7 shows the spectral energy absorbed by silica
fabric from a radiant source at 1273K, which forms
integrand of numerator in Equation (8). Figure 7 also
shows its spectral emissive power for a radiant source
temperature of 1273K, which forms the integrand of
denominator in Equation (8). After integrating over the
entire wavelength, Equation (8) shows that the effective
absorptivity of silica fabric is 0.6. The spectral energy
distribution of silica fabric for a radiant source of 1773K is
shown in Fig. 8. The effective absorptivity of silica fabric
at a source of 1773K is 0.46. Figure 9 shows that even
though silica fabric exhibits very large absorptivity at large
wavelengths, its absorptivity is pretty low at high
temperatures because energy level attributed at larger
wavelengths are relatively low.
FIGURE 7. ENERGY CHARACTERISTICS OF SILICA
FABRIC FOR A SOURCE TEMPERATURE OF 1273K
FIGURE 8. ENERGY CHARACTERISTICS OF SILICA
FABRIC FOR A SOURCE TEMPERATURE OF 1773K
FIGURE 9. ABSORPTIVITY VARIATION OF SILICA
FABRIC WITH WAVELENGTH
STELLITE
Figures 10, 11 and 12 show the spectral absorptivity
characteristics of stellite material for source temperatures
of 1073K, 1273K and 2273K respectively. The absorptivity
of stellite is high at lower wavelengths compared to that at
higher wavelengths. The estimated effective absorptivity of
stellite varies between 0.343 and 0.455 for source
temperature of 1073K to 2273K. These values give a
higher estimate of absorptivity at higher wavelength.
0
0.2
0.4
0.6
0.8
1.0
0 2 4 6 8 10
Wave length, m
Ab
so
rpti
vit
y
6
FIGURE 10. ABSORPTIVITY OF STELLITE FOR
SOURCE TEMPERATURE OF 1073K
FIGURE 11. ABSORPTIVITY OF STELLITE FOR
SOURCE TEMPERATURE OF 1273K
FIGURE 12. ABSORPTIVITY OF STELLITE FOR
SOURCE TEMPERATURE OF 2273K
ALUMINIZED SILICA FABRIC
The absorptivity variation of aluminized silica fabric
(aluminium side) for a source temperature of 1073K is
shown in Fig. 13. The effective absorptivity of aluminized
silica fabric (silica fabric) is 0.48.The computed effective
absorptivity and reflectivity characteristics at different
source temperatures for different materials are given
Tab. 1.
FIGURE 13. ABSORPTIVITY CHARACTERISTICS OF
ALUMINIZED SILICA FABRIC (ALUMINIUM SIDE)
TABLE 1: ABSORPTIVITY AND REFLECTIVITY OF
DIFFERENT MATERIALS
CONCLUSIONS The thermo-optical properties of different materials
used in base region of liquid engines are critical inputs for
radiative load estimations and TPS design. The emissivity,
7
reflectivity and absorptivity characteristics of stellite, silica
fabric, aluminized silica fabric and glass fabric were
measured. The variation of emissivity at different
temperatures was measured. The influence of wavelength
and source temperature on absorptivity and reflectivity was
also studied.
The measured emissivity of silica fabric varies from
0.76 at 150°C to 0.79 at 630°C. The emissivity of glass
fabric and silica fabric combination reduces from 0.89 at
95°C to 0.56 at 270°C. The effective absorptivity was
estimated for different source temperatures. The silica cloth
exhibits higher absorptivity at larger wavelengths. Its
absorptivity is pretty low at high temperatures because
energy level associated with lower wavelengths is low. The
spectral absorptivity of stellite is higher at lower
wavelengths resulting in higher effective absorptivity at
high temperatures. These measured parameters were used
in detailed thermal design of base region in liquid engines.
ACKNOWLEDGMENTS
Authors wish to acknowledge the spectral
measurements of reflectivity and absorptivity by
ASD/VSSC. Authors wish to acknowledge the support
provided by Mr. Sreekumar. R, Mr. Rajiv. R, Mr. Harilal
and Mr. Nagaraj of AHTD to carry out experiments.
REFERENCES [1] L. del Campo, R. B. Perez-Saez, L. Gonzalez-
Fernandez, X. Esquisabel, I. Fernandez, P. Gonzalez-
Martin, M. J. Tello, “Emissivity Measurements on
Aeronautical Alloys”, Journal of Alloys and
Compounds, 489, 2010, pp. 482–487.
[2] John H. Leinhard IV and John H. Leinhard V, 2004, A
Heat Transfer Book, 3rd Edition, Phlogiston Press.
[3] Xiaodong He, Yibin Li, Lidong Wang, Yue Sun and
Sam Zhang, “High Emissivity Coatings for High
Temperature Application: Progress and Prospect”,
Thin Solid Films, 517, 2009, pp. 5120–5129.
[4] Saeed Moghaddam, John Lawler and Joseph Currano,
“Novel Method of Total Hemispherical Emissivity”,
Journal of Thermophysics and Heat Transfer, 21 (1),
2007.
[5] Samuel Boyden and Yuwen Zhang, “Prediction of
Temperature Dependent Absorptivities of Metallic
Materials at 1.06 µm and 10.6 µm”, AIAA 2005-5211.
[6] Brandon W. Olson and Harikishin P. Bakhtiani,
“Thermal Characterization of Emisshield”,
AIAA 2007-417.
[7] Robert Siegel and John R. Howell, Thermal Radiation
Heat Transfer, Hemisphere Publishing Corporation.