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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: ramprabhu_m@vssc.gov.in

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-

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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-

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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

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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

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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,

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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.

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