NAT'L INST. OF STAND & TECH

AlllDb S2T1Q7

NBS NBSIR 81-1647

PUBLICATIONS

LASER-INITIATED COMBUSTION STUDIES OF

SELECTED ALUMINUM, COPPER, IRON, AND NICKEL ALLOYS

James W. Bransford

Alan F. Clark

Thermophysical Properties Division

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

April 1981

100

,1156

81-1647

1981

c ,2

HATIOJIAJ- nine.*'

Of 9TAHDABDS

y t lBflART NBSIR 81-1647 JUL z 0 1981

LASER-INITIATED COMBUSTION STUDIES OF

SELECTED ALUMINUM, COPPER, IRON, AND NICKEL ALLOYS

James W. Bransford

Alan F. Clark

Thermophysical Properties Division

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

April 1981

O*

**

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
\
CONTENTS

Page

LIST OF TABLES v

LIST OF FIGURES vi

ABSTRACT viii

I. INTRODUCTION ..... ... 1

II. EXPERIMENTAL 6

III. RESULTS 11

A. Aluminum Alloys 1100, 2219, 6061, and Tensile-50 11

B. Stainless Ferrous Alloys 304, 347, and 21-6-9 ... 31

C. Nickel Alloy Inconel 600 42

D. Copper Alloys 46

1. Beryllium-Copper 46

2. Bronze 47

IV. CONCLUSIONS AND RECOMMENDATIONS 55

A. Conclusions 55

1. Aluminum Alloys 55

a. Ignition 55

b. Combustion 56

2. Stainless Ferrous Alloys and Nickel Alloys 56

a. Ignition 56

b. Combustion 57

3. Copper Alloys 57

a. Ignition 57

b. Combustion 58

4. Areas of Uncertainty 58

iii
B . Recommendations 59

V. ACKNOWLEDGMENTS , , 60

Appendix A. Temperature Measurement Using the Thermal Radiation

Spectrum 61

Appendix B. Two-Color Pyrometer Theory 64

by Key Nguyen and M. C. Branch

I. INTRODUCTION 64

II. THEORY OF THE MEASUREMENT METHOD 65

III. DETERMINATION OF THE SPECTRAL EMISSIVITY RATIO 68

IV. DESCRIPTION OF THE APPARATUS 71

A. Two-Color Pyrometer 71

B. Spectral Emissivity Ratio Measuring Device 73

V. THE VALIDITY OF THE THERMAL EQUILIBRIUM ASSUMPTION 78
LIST OF TABLES

PAGE

Table 1. FACTORS AFFECTING METAL IGNITION AND COMBUSTION 4

Table 2. MAJOR COMPONENTS OF ALLOYS STUDIED 5

Table 3. COMBUSTION TEMPERATURES 16

Table B-l. SPECIFICATIONS OF ARGON ION LASER 76

Table B-2. POWER OUTPUT OF THE ARGON ION LASER 77

v
LIST OF FIGURES

PAGE

Figure 1. Experimental setup. ..... 7

Figure 2. General instrument hook-up 8

Figure 3a. Continuum spectra for Aluminum Alloy 1100. 400 to 800 nm wavelength range. . . ....... 13

Figure 3b. Bandhead spectra for Aluminum Alloy 1100 13

Figure 4. Graph of thermal emission spectrum for temperature

determination 14

Figure 5. Graph of thermal emission spectrum for temperature

determination 15

Figure 6 . Vapor pressure of Al and AI 2 O 3 17

Figure 7. 1100 Aluminum Alloy combustion rate and radiant intensity . . 19

Figure 8 . Fraction of 1100 Aluminum Alloy remaining during combustion . 20

Figure 9. Log fraction remaining for 1100 Aluminum Alloy 21

Figure 10. 2219 Aluminum Alloy combustion rate and radiant intensity . . 22

Figure 11. Fraction of 2219 Aluminum Alloy remaining during combustion . 23

Figure 12. Log fraction remaining for 2219 Aluminum Alloy 24

Figure 13. 6061 Aluminum Alloy combustion rate and radiant intensity . . 25

Figure 14. Fraction of 6061 Aluminum Alloy remaining during combustion . 26

Figure 15. Log fraction remaining for 6061 Aluminum Alloy. ...... .27

Figure 16. Tensile-50 Aluminum Alloy combustion rate and radiant

intensity 28

Figure 17. Fraction of Tensile-50 Aluminum Alloy remaining during

combustion. 29

Figure 18. Log fraction remaining for Tensile-50 Aluminum Alloy 30

Figure 19. 304 Stainless Steel combustion rate and radiant intensity . . 33

Figure 20. Fraction of 304 Stainless Steel remaining during combustion . 34

Figure 21. Log fraction remaining for 304 Stainless Steel 35

Figure 22. 347 Stainless Steel combustion rate and radiant intensity . . 36

vi
Figure 23. Fraction of 347 Stainless Steel remaining during combustion . . 37

Figure 24. Log fraction remaining for 347 Stainless Steel 38

Figure 25. 21-6-9 Stainless Steel combustion rate and radiant intensity. . 39

Figure 26. Fraction of 21-6-9 Stainless Steel remaining during

combustion. . 40

Figure 27. Log fraction remaining for 21-6-9 Stainless Steel 41

Figure 28. Inconel 600 combustion rate and radiant intensity 43

Figure 29. Fraction of Inconel 600 remaining during combustion 44

Figure 30. Log fraction remaining for Inconel 600. 45

Figure 31. Beryllium-Copper combustion rate and radiant intensity 48

Figure 32. Fraction of Beryllium-Copper alloy remaining during

combustion 49

Figure 33. Log fraction remaining for Beryllium-Copper alloy 50

Figure 34. Bronze combustion rate and radiant intensity 52

Figure 35. Fraction of Bronze alloy remaining during combustion 53

Figure 36. Log fraction remaining for Bronze alloy 54

Figure Bl. Schematic of temperature measurement system 72

Figure B2. Chopper disk 74

Figure B3. Alternator disk 74

Figure B4. Characteristic conduction times of Al-1100, Ni and

Inconel 600 80

vii
LASER-INITIATED COMBUSTION STUDIES OF SELECTED

ALUMINUM, COPPER, IRON, AND NICKEL ALLOYS*

James W. Brans ford and Alan F. Clark

National Bureau of Standards

Boulder, Colorado 80303

The results of combustion studies at atmospheric pressure on ten metal

alloys are presented. The alloys studied were aluminum alloys 1100, 2219,

6061, and tensile-50; 304, 347 and 21-6-9 stainless steel; inconel 600;

beryllium-copper and a bronze. It was found that once ignition was achieved

all alloys would generally burn to completion. The overall combustion process

appears to obey a first-order rate process. Preliminary conclusions are

presented along with recommendations for future work.

Key words: alloys; combustion; combustion rate; ignition; metals; oxygen

atmosphere; temperature measurement.

*Certain commercial equipment, instruments, and materials are identified in

this paper to adequately specify the experimental procedure and materials

studied. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the material

or equipment identified is necessarily the best available for the purpose.

viii
I. INTRODUCTION

The advent of large scale use of liquid and gaseous oxygen by the defense

establishments in the late nineteen forties and early nineteen fifties resulted

in the clear recognition that serious materials compatibility problems existed.

These problems were centered around organic compounds used for seals, lubricants and other components, and around the metallic alloys of aluminum, magnesium and titanium. In order to select the most oxygen-compatible materials, an

impact test method and apparatus was developed at the Army Ballistic Missile

Agency during the mid nineteen fifties. This method was, in general, adequate

for selecting materials used in rocket and ground support systems at that time

and later for the early manned space program. However, as higher stress levels

were reached in various rocket and ground support system components - from

increased pressure, flow velocities, component operating speeds, etc. - it was

recognized that the original test apparatus and methods were probably inadequate. Thus, several new test methods and apparatus were developed to extend

the pressure and temperature range and to utilize other ignition techniques.

However, the impact tester, modified to operate at high pressure, is still the

primary device used to evaluate materials for oxygen service. Several versions

of the high-pressure oxygen impact tester have been used to evaluate space

shuttle engine materials.

As the testing of the space shuttle main engine materials progressed it

was found that all metallic materials passed the applicable impact test except

the aluminum alloys. These aluminum alloys fracture readily upon impact

whereas the other high strength engine metallic materials do not. As

1
individual component and engine testing commenced, facility and components

began to fail through metal ignition and combustion. Several of the failures

were very extensive and costly. With these failures it became readily apparent

that detailed knowledge of the metal ignition and combustion processes was

seriously lacking. Consequently this study of the metal ignition and combustion processes was begun by the National Bureau of Standards sponsored by

NASA's George C. Marshall Space Flight Center.

In initiating a study on metal ignition and combustion, several major

aspects to the problem should be understood at the beginning. These are:

1. At room temperature, the reaction

Metal + Oxygen = Metal oxide + Energy

is spontaneous, to the right, for every metal element except gold.

For the metal elements that are used to form high strength alloys

this spontaneity to the right continues for at least one oxide, and

usually more, to very high temperatures. Thus the tendency to

oxidize at high rates (burn) is strong.

2. The metal-oxygen reaction is heterogeneous involving gas, liquid, and

solid phases. The reaction products are usually refractory and thus

complicate the description and study of the chemical process due to

latent heats, and melting and boiling points.

3. High pressure complicates the experimental procedure used to study

metal combustion. Equipment that can ordinarily be used in the open

2
must be adapted for use in pressure chambers and shielded from heat

and splatter, or alternative means of study must be found. Examples

of this are the need to adapt an electronic balance for high pressure

use or the potential involvement of the containment vessel itself.

4. A large number of parameters are involved in the ignition and combustion process. Many of these parameters are temperature dependent, and

this dependency is only poorly known, if known at all. Table 1

contains an extensive list of such parameters.

The initial effort has been to apply methods previously utilized in the

study of titanium combustion to metals and alloys of interest to space applications. A listing of the materials studied is shown in Table 2. The methods

employed to date are high speed cinematography, emission spectra, combustion

intensity, combustion temperature, and mass uptake. Schlieren and Hilbert

transform methods will be employed later to determine flame distribution.

In general, it was found that all alloys, except bronze, were somewhat

difficult to ignite, especially inconel and beryllium-copper. This difficulty

was caused by the reflectivity of the specimen and by the oxide layer formed

during the heat-up period. However, once ignited, the alloys generally burned

with vigor and essentially to completion. The manner of combustion varied from

the "skating sun" of the aluminum alloys to a very active type with ejection of

material by the steel alloys and inconel to the quiet combustion of the bronze.

The following sections will discuss in turn the experimental setup,

results, conclusions and recommendations resulting from the work to date.

3
TABLE

1:

FACTORS

AFFECTING

METAL

IGNITION

AND

COMBUSTION

H il e* oB 4

metal metal

oxide
TABLE 2: MAJOR COMPONENTS OF ALLOYS STUDIED

% Composition

Aluminum Alloys Ferrous Alloys

Nickel

Alloy Copper Alloys

1100 2219 6061

Tens . -50 304 347 21-6-9

Inconel

600 2% Be Bronze

A1 98.7 92.8 96.2 91.3

Be 1.9

Cr 19 18 20 16

Cu 0.2 6.3 0.28 0.1 98 89.9

Fe 1.0 0.3 0.7 68 70 63 7

Mg f—

5

« o

t 0.5

Mn 0.3 0.2 2 9.5 0.2

Nb 1

Ni 10 11 7 76 0.4

Si CN o • 0.6 8.0 1 0.15 0.2

Sn 8

Zn 0.1 0.1 0.3 1.4

5
II. EXPERIMENTAL

The initial experiments have been caried out in the open using the

arrangement of Figure 1. Ten alloys were studied. These were the aluminum

alloys 1100, 2219, 6061, and Tensile-50; 304, 347 and 21-6-9 stainless steels;

inconel 600; bronze and beryllium-copper. The 40 to 300 mg specimens were

blanketed with flowing oxygen and irradiated with a focused 100 W CW CO2

laser beam. Since the alloys studied are generally difficult to ignite at

ambient pressure conditions, the specimens were insulated from the high thermal

conductivity graphite support block with a thin granular layer of aluminum

oxide or asbestos material. These two materials have high melting points and

do not oxidize; therefore, it is believed they will not change the ignition or

combustion characteristics of the alloys. Even with this insulating layer most

materials, notably inconel and beryllium-copper, were still difficult to

ignite. A large part of this difficulty can be explained by the high

reflectivity and emissivity of the alloys and alloy oxides and the high

temperature that must be achieved for ignition. These temperatures are

generally the melting point of aluminum oxide for the aluminum alloys and the

alloy melting point for the other materials. Typical heating times before

ignitions varied from about 15 seconds to more than a minute.

The general experimental arrangement is shown schematically in Figure 2.

The photodiode used in this study is a silicon PIN photovoltaic device with a

spectral response that peaks at approximately 850 nm. This type of response

curve makes the diode a good device for detecting the beginning of the thermal

runaway oxidation of the alloys studied. Thus, the amplified photodiode signal

6
Carbon

Dioxide

Laser

Figure

1.

Experimental

setup.

can be used as a trigger source for other instrumentation as well as a moniter

of the relative radiant intensity of the burning sample.

Spectra were obtained using a Tektronix J20/7J20 rapid scanning spectrometer (RSS). This unit, which has two diffraction gratings, has a first order

spectral response range from 300 nm to 1100 nm. The spectrum can be scanned in

intervals as large as 400 nm or as small as 4 nm. The region to be studied is

focused, by an external lens, onto the entrance slit of the J20 unit. The

light passing through the slit is diffracted by a grating onto the surface of a

vidicon tube constructed of charge-storage diodes. An electron beam scans the

vidicon and ultimately an analog signal representing time-resolved spectral

power (incident optical power versus wavelength) is produced. This signal can

then be recorded photographically from an oscilloscope or digitally by a digital oscilloscope. Spectra thus obtained were used to identify reacting vapor

species or to determine combustion temperatures.

Combustion rates can be estimated from the mass increase of the burning

specimen. To obtain this mass increase an electronic balance, modified to

increase its response time, was used. Since the combustion process usually had

a very violent period with the ejection of particles, a thin glass hood with

openings for the laser beam and oxygen, was placed over the specimen. Even

with this hood a small amount of material was lost because of vapor and particles ejected through the holes by the specimen during its most active combustion period. The particle ejection was also detected by the balance and the

resulting spikes in the mass signal partially obscures some portions of the

mass-gain curve. The balance was also slightly sensitive to heat transmitted

9
through the pan assembly to the suspension. This heat sensitivity was generally noticeable only for the aluminum alloys which required longer heating times

before ignition and also have a higher heat of combustion than the other

alloys. The maximum mass loss was approximately 20 percent for one aluminum

alloy sample to only a few percent for a bronze. The deviation from stoichiometric weight for each alloy is noted on each mass increase graph as the ratio

W^/Wg. Where W^ is the initial weight of the alloy sample and Wg is

the total weight of the oxidation products.

The behavior of the alloys before and after ignition was determined by

high-speed cinematography. Framing rates were approximately 300 fps. A highspeed tungsten balanced (3200 K) color film was used.

Combustion temperatures were measured by using the Rapid Scanning Spectrometer. The development of the procedure by which the combustion temperature is

determined from a radiant intensity vs. wavelength scan is given in Appendix A.

Since the electronic recording equipment used was only capable of recording a

total of four scans, multiple scans at fixed time intervals were recorded on a

single film. From these scans one was selected that represented the highest

combustion radiant intensity. The radiant intensity was then determined at

selected wavelengths. Those data was then used with a linear least squares

procedure to determine the combustion temperature at the time of highest radiant intensity.

10
III. RESULTS

As stated previously five types of measurements were made on each alloy

studied. The results of these measurements will be discussed below for each

alloy or alloy system.

A. Aluminum Alloys 1100, 2219, 6061 and Tensile-50

The four aluminum alloys had essentially the same ignition and combustion

characteristics. This similarity can be seen in the combustion mass increase

and radiant intensity traces in Figures 7, 10, 13, and 16 and in the high speed

movies. The combustion sequence, for these alloys, can be broken into two

separate stages; the induction or heat-up stage and the combustion stage.

During the induction stage, the sample develops a tight thick oxide layer.

This layer has sufficient strength to maintain the original sample shape after

the sample temperature has increased beyond the alloy melting point. Ignition

appears to occur when the oxide melts or fractures near the oxide melting point

from internal pressure and/or thermal stresses and exposes the molten alloy to

the oxygen atmosphere. The ignition spot is usually small but spreads rapidly

over the sample melting the oxide layer and converting the sample into a molten

drop. The beginning of the combustion stage is marked by a vigorous combustion

usually accompanied by the ejection of small molten drops of material. Since

the molten oxides produced by the combustion, predominately AI 2 O

3

, are not

miscible with the molten alloys, two phases soon separate with the lower density alloy floating on the oxides. This phase separation continues with the

molten

11
alloy "skating" on the oxide surface until combustion is complete. A great

deal of smoke is generated during the combustion stage along with a fine spray

and some larger droplets of the molten alloy. The spray droplets generally are

ejected with a momentum too low to be picked up distinctly by the electronic

balance; however, the larger droplets can be detected and are seen as sharp

spikes in the mass electrical signal.

Spectra of the combustion region show strong vibrational band heads in the

emission spectra from 440 run to 560 nm wavelength as shown in Figure 3a; an

expanded bandhead is shown in Figure 3b. These band heads have been attributed

to the gaseous A10 molecule [Zhdanova and Sokolov]. The A10 molecule is primarily produced in the A1 vapor - O2 gaseous reaction zone above the surface

of the liquid alloy sample and is subsequently oxidized to AI 2 O3

.

In spite of the band heads, sufficient continuum exists so that a combustion temperature can be determined. Figure 3a shows a typical oscilloscope

photograph that was used for this purpose and Figures 4 and 5 show typical

graphs of data obtained from several photographs. Table 3 summarizes the

results for a number of these measurements. The temperature values given are

for the maximum stable combustion temperature. This value is usually reached

within several tenth's of a second after ignition and then slowly decreases

until combustion is complete. Occasionally flare temperatures, generally at

the start of combustion, appear to have higher temperatures. However, these

last for too short a time to be measured by the present instrumentation. The

temperature of the molten sample should be approximately 2300 K (see Figure 6 ),

the approximate boiling point at our altitude of the aluminum alloys

12
Figure 3a. Continuum spectra for Aluminum Alloy 1100. 400 to 800 nm wavelength range.

Figure 3b. Bandhead spectra for Aluminum Alloy 1100.

13
INC I

)+5#LN(N

)

1.0 IS 2.0 2.5

l/WRVELENBTH

Figure 4. Graph of thermal emission spectrum for temperature

determination

14
LNC i

H£*LNCW>

Tensile -50 Aluminum Alloy

Wavelength Range 600 nm to 800 nm Temperature = 3348 K

- 1 3 T

i

i

-IB 1

j - 1 9 i*

i

-21 1

1/HHVELfNETH

Figure 5. Graph of thermal emission spectrum for temperature

determination.

15
TABLE 3: COMBUSTION TEMPERATURES

Aluminum Alloy 2219

Temp . , K ± Temp

.

S'? +1 3327 102 3.06

3515 62 1.77

3469 65 1.86

3452 42 1.20

3275 71 2.17

3430 40 1.17

304 Stainless Steel

Temp., K ± Temp. ± %

2558 19 0.72

2497 45 1.82

2690 14 0.52

2628 25 0.96

2645 10 0.39

2689 35 1.28

Aluminum Alloy 6061

Temp., K ± Temp

.

± %

3400 78 2.30

3271 80 2.46

3534 40 1.13

3303 87 2.63

3282 51 1.56

3041 59 1.94

347 Stainless Steel

Temp., K ± Temp

.

+1 2545 27 1.07

2699 28 1.02

2487 18 0.74

2520 20 0.78

2690 17 0.62

2732 39 1.42

Aluminum Alloy Tensile-50

Temp . , K ± Temp

.

± %

3348 77 2.29

3456 49 1.41

3408 79 2.32

3318 60 1.81

3451 81 2.34

3462 48 1.38

21-6-9 Stainless Steel

Temp., K ± Temp

.

± %

2520 12 0.47

2470 32 1.31

2630 21 0.80

2553 27 1.04

2629 16 0.61

2678 28 1.05

Bronze

Temp . , K ± Temp

.

1+ S'? 2072 15 0.72

2105 17 0.80

2202 15 0.70

2192 13 0.61

2244 14 0.64

2173 16 0.75

Inconel

Temp., K ± Temp

.

± %

2513 16 0.65

2491 23 0.91

2527 30 1.17

2631 33 CsJ • 2579 13 0.49

2642 30 1.14

BeCu

Temp . , K ± Temp. ± %

2441 23 0.96

2497 11 0.45

2839 17 0.59

2533 22 0.86

2720 14 0.53

2479 19 0.76

Aluminum Alloy 1100

Temp., K ± Temp. ± %

3607 71 1.96

3731 59 1.57

3513 84 2.40

3499 37 1.05

3331 63 1.90

3392 43 1.28

16
Log

P,Pa

Figure 6. Vapor pressure of A1 and Al^O^.

17
studied. This temperature could not be confirmed since the RSS presently cannot differentiate between the combustion zone and the surface of the molten

sample. The combustion zone temperatures, as determined by the procedure of

Appendix A, ranged between 3000 K to 3700 K.

Figures 7a, 10a, 13a and 16a show examples of the weight gain curves

obtained for each aluminum alloy. These curves show a nonstoichiometric weight

gain. There are three reasons for this. First, the combustion was generally

slightly incomplete, less than 1 percent; alloy particles were present on or

near the bottom of the combustion residue. Second, microscopic oxide particles

and some ejected particles escaped from the covered combustion region. Third,

the weight sensor is temperature sensitive and this shows up as a weight loss.

Overall the final indicated weights were approximately 80 percent to 90 percent

of stoichiometric. If the assumption is made that the material lost is proportional to the weight gained - only oxide is lost - then the curves can be used

to construct plots of weight of alloy burned or remaining versus time. Figures

8, 11, 14 and 17 show fraction of alloy remaining versus time curves constructed from Figures 7a, 10a, 13a and 16a.

The combustion kinetics for our experimental arrangement were thought to

be first-order. To show this, logarithmic plots of fraction of alloy remaining

versus time were made, Figures 9, 12, 15, and 18. As can be seen there are one

or more linear regions indicating first-order or nearly first-order combustion

kinetics for the experimental setup.

18
Relative

Radiant

Intensity

I

Weight,

mg

7a. W./W = 0.903

i s

Time, s

Figure 7. 1100 Aluminum Alloy combustion rate and radiant intensity

19
FRRCTION

REMRININS

I J *

0.9..

9

• •• 0.0 ..

0.7..

0

0.S..

0.S..

0.H..

0.3..

0 . 2 ..

•• 0. I ..

• e

• • * •• e

1

.

B 1 1 H i —

H

-l 0 ! 2 3 H 5

TIHE/5

Figure 8. Fraction of 1100 Aluminum Alloy remaining during combustion.

20
LOE

FRRCTIQN

REMRININE

0.0 -

-0.H ..

-0.E ..

-0.0 ..

-I.0..

- 1.2 ..

-I .H ..

-! .B ..

- 1.0 ..

-2.0 .. -I

4 1 1 -4— 1

l 2 3 H 5

TIME/

5

Figure 9. Log fraction remaining for 1100 Aluminum Alloy.

21
Relative

Radiant

Intensity

j

Weight

,

mg

Figure 10. 2219 Aluminum Alloy combustion rate and radiant intensity.

22
FRHCT

i

DN

REMAINING

l.0 T

0.9..

a. a..

0.7 ..

0.6 ..

0.S..

0.H..

0.3..

0 . 2 ..

0.1 ..

0.0 i- —f— 1 j

-10 12

T I ME/ 5

+

3

+- 1 H 5

Figure 11. Fraction of 2219 Aluminum Alloy remaining during combustion.

23
LOG

FRACTION

REMAINING

i.i

*“ 0.2 ..

-0.H ..

”B.E „

”0.0 ..

“•I .0 ..

-*! .2 ..

”1 . H ..

-I .0 ..

+— 1 B I 2 1 H E

1 1 ME/ 5

Figure 12. Log fraction remaining for 2219 Aluminum Alloy.

24
Relative

Radiant

Intensity

I

Weight

13a. W./W = 0.773

1 s

13b.

Time, s

Figure 13. 6061 Aluminum Alloy combustion rate and radiant intensity.

25
FRfKT

SON

FOR I

NS

NS

« o *

0.9

0.6

7 X

0.G ..

0.5 ..

0.H

3

'.2 X

0. I

+

2

TS0E/5

Figure 14. Fraction of 6061 Aluminum Alloy remaining during combustion.

26
LOG

FRRCTIQN

REMAINING

-0.2 ..

~0. H ..

~0.E ..

-0.B ..

-I.0..

“i .2 ..

“I .H ..

-I.E ..

+— 1

> —f— i 0 1 2 3 H £

T l ME/ 5

Figure 15. Log fraction remaining for 6061 Aluminum Alloy.

27
Relative

Radiant

Intensity

Weight

16a. W./W = 0.878

1 s

16b.

Time, s

Figure 16. Tensile-50 Aluminum Alloy combustion rate and radiant

intensity.

28
FRRCT

I

ON

REMRiMNS

1 .0 T

0.9 ..

0.0 ..

0.7 ..

0.B ..

0.5 ..

0.H ..

0.3 ..

0 . 2 ..

0 . 1 ..

0.0. —* | —

I

— 4—~**~-**~*

I -1 0 1 2 3 H 5

T 1 HE/ 5

Figure 17. Fraction of Tensile-50 Aluminum Alloy remaining during

combustion.

29
LOS

FRACTION

FORI

NINE

I

0.2

“0. H ..

-0.E 1

-0.8 ..

-I .0

-I .2 i

-I .H ..

-I .5 1

T

~ S .0 j.

I

1— > - - t

* - —

h

i 1 2 3 H £

T5HE/5

Figure 18. Log fraction remaining for Tensile-50 Aluminum Alloy.

30
B. Stainless Ferrous Alloys 304, 347 and 21-6-9

The three ferrous alloys had essentially the same ignition and combustion

characteristics and, like the aluminum alloys, the combustion sequence can be

divided into the same two stages. The alloys were usually difficult to ignite

and had somewhat irregular combustion characteristics as evidenced by the radiant intensity curves of Figures 19b, 22b and 25b. Regardless of the variation

in combustion intensity over the combustion period the alloys generally had two

distinct combustion phases during the combustion stage. During the induction

stage the samples would form a gray to black oxide layer. Ignition appears to

begin when a combustion front, weak orange in color, advances, top to bottom,

over the solid sample. However, this front appears to weaken momentarily; then

a bright incandescence front forms at the top of the sample. Sample melting

follows the advance of this front. The initial (orange) combustion zone

appears as a shoulder on the radiant intensity curves, e.g. see Figure 19b. It

has not been determined at this time whether some surface melting accompanies

this initial combustion zone or not. Thus, it is not known at this time if the

transition from oxidation to combustion takes place just before or at the alloy

melting point. The second stage of the combustion sequence can be divided into

two phases; the active combustion phase which is characterized by violent drop

movement with the ejection of large droplets of material and a moderately

active phase which is characterized by minimal drop activity and the ejection

of small spray particles. The two phases are separated by a short period of

greatly reduced activity. During the initial part of the combustion phase the

surface tension of the drop is sufficient to form a well-defined sessile drop;

however, in the latter part of the combustion phase the drop wets the graphite

31
surface and spreads out. There appears to be no liquid phase separation thus

we conclude that the molten alloy and oxides are miscible or otherwise

thoroughly mixed.

The combustion region was scanned with the RSS to determine if there were

any emission lines which would indicate vapor reactions occuring. No emission

lines in the 400 nm to 1100 nm spectral region were observed and no vapor reaction regions were observed on the movie film indicating no vapor-phase combustion was present. Temperatures calculated from spectral scans from 400 nm to

800 nm and 500 nm to 900 nm indicated maximum combustion temperatures between

2500 K and 2700 K, Table 3. Since no reacting vapor is considered to be

present, the temperatures measured are those of the molten reacting material.

Weight-gain measurements were made in order to determine the combustion

rate. Examples of typical results are shown in Figures 19a, 22a and 25a. As

can be seen, the ejection of drops and spray by the burning sample is a complicating factor in obtaining an interpretable curve. However, these curves were

used to generate fraction remaining and log fraction remaining curves, Figures

20, 21, 23, 24, 26, and 27. The best curves were obtained from 304 stainless

steel. The overall combustion kinetics for the experimental setup, as determined from log plots, appear to be first-order with two distinct slopes. This

change in slope may indicate a change in the oxidation mechanism and occurs

when approximately one half of the sample is consumed.

32
Relative

Radiant

Intensity

Weight

1 9a

19b,

141 .5 mg

5.50 s

W./W = 0.949

i s

1 07 mg

4 6

Time, s

10

Figure 19. 304 Stainless Steel combustion rate and radiant intensity

33
FRRCT

ION

RtMR

ININS

0.9

0.0

0.7 ..

0.E ..

0.S ..

B. 1

! ..

0.3 ..

0.2 ..

0. i ..

0.0

-I

+—— - I 1 H

I 2 3 H S'

T S HE/

5

Figure 20. Fraction of 304 Stainless Steel remaining during combustion.

34
LOB

FRRCT I

ON

REMRININB

0.0 -

“0.2 ..

~0.M ..

“0.E ..

~0.0 ..

“ 1.0 ..

- 1.2 ..

-i.H ..

-I.E ..

- 1.

0

..

-2.0 +

0

*—— 1 1 1 H

l 2 3 H 5

T l HE/ 5

Figure 21. Log fraction remaining for 304 Stainless Steel.

35
Relative

Radiant

Intensity

I

Weight

22a. W./W = 0.971

1 s

7 9 11

Time,

13

s

15 17

Figure 22. 347 Stainless Steel combustion rate and radiant intensity

36
FRACTION

FOR

INI

NS

i .0

B.S ..

0.0 ..

0.7 1

0.5 ..

S

0. H ..

0.3 ..

0.2 ..

0 . 1 ..

0.0

-I 0

+ —I 4 +

i 2 3 H

T i ME/ 5

Figure 23. Fraction of 347 Stainless Steel remaining during combustion.

37
LOS

FRACTION

REMRININS

•8.H ..

8.9 ..

-I .0

.2 ..

.9

* 2.1 I—

2

TIME/ S

H

Figure 24. Log fraction remaining for 347 Stainless Steel

38
Relative

Radiant

Intensity

Weight

25a. W./W = 0.942

i s

10.10 s

25b.

Time, s

Figure 25. 21-6-9 Stainless Steel combustion rate and radiant intensity.

39
FRBCTIDN

REHR1NINS

Ie « i Ofp • a 9 © «

6

0.9..

0.8 ..

0.7 ..

0.6 ..

0.E „.

0.H ..

0.3 ..

0.2 ..

0 . I ..

0.8 f

-I i

*——H§——H*-

I 2 3

TiHE/S

H——i—

H

H E

Figure 26. Fraction of 21-6-9 Stainless Steel remaining during

combustion.

40
LOB

FRACTION

REMRINING

-0.2 ..

-0.H ..

-0.G ..

-0.B ..

-I .0 ..

- 1.2 ..

-I .H ..

-2.0 +

0

^ 1 —I 1 —

I

l 2 3 H 5

T l ME/ 5

Figure 27. Log fraction remaining for 21-6-9 Stainless Steel.

41
C. Nickel Alloy Inconel 600

This material proved to be quite difficult to ignite due to reflectivity

of the alloy and the build up of a heavy oxide layer. However, once the sample

ignited the combustion would usually go to completion. The material's combustion characteristics were very close to the ferrous alloys in that the combustion had irregular radiant intensity traces and ejected significant amounts of

material. Figure 28b shows such a trace. The molten drop of material was

usually very active and ejected considerable amounts of material during the

combustion period. Usually there would be a quiescent period between the

initial active phase and a final spray phase. It appears that the material

must melt before ignition and sustained combustion can take place. The materials that comprise the molten drop appear to be completely mixed; no phase

separation is evident.

The RSS was used to scan the combustion zone from 400 nm to 1100 nm. No

emission lines were seen in this region and no vapor reaction regions were seen

on film indicating no vapor-phase combustion. Combustion temperatures determined by using the RSS are in the same range as that for the ferrous alloys,

2500 to 2700 K (Table 3).

Weight-gain measurements were made in an attempt to determine the combustion rate. A typical result is shown in Figure 28a. As with the ferrous

alloys the ejection of material presents some problems in obtaining an interpretable curve. However, since most of the material ejected was caught, it is

believed that the curve in Figure 28a is a reasonably accurate representation

42
Relative

Radiant

Intensity

|

Weight

28a. W./W = 0.956

1 s

1 9 11 13 15 17

Time, s

Figure 28. Inconel 600 combustion rate and radiant intensity.

43
FRRCTiDN

REMB

ININS

i.b T

0.B ..

0.0 ..

0

.

7 ..

1.

G ..

BS „

0.H ..

0.3 ..

0.2 ..

0.0 f -t--- H f

i 2 3 H

HMDS

H

S

Figure 29. Fraction of Inconel 600 remaining during combustion.

44
LOS

FRACTION

REMAINING

T

-0.2 ..

-0.H ..

-0.E ..

-0.0 ..

-I .0 ..

-1.2 ..

-I .H ..

-i .5 ..

-i .a ..

-2.0 4——i ~H— h— fr- -10 12 3

TIME/S

-j ( H 5

Figure 30. Log fraction remaining for Inconel 600.

45
of the combustion event. From the weight gain, curve fraction remaining, and

log fraction remaining curves were made, Figures 29 and 30. If the initial

irregularity of the log fraction remaining curve is disregarded two approximately linear regions can be observed. These regions indicate an overall

first-order reaction for the experimental arrangement. Like the stainless

steels, a change in slope occurs at about the 50 percent consumed point, indicating a possible oxidation mechanism change.

D. Copper Alloys

1. Beryllium-Copper

The beryllium-copper alloy was one of the two most difficult alloys to

ignite. This alloy also had the most unusual radiant intensity trace. Figure

31b shows a typical example. The radiant intensity is approximately one eighth

that of the ferrous alloys.

The induction phase of the combustion sequence is much like the other

alloys studied. At some point after the sample has reached an incandescence

temperature, a surface combustion front will advance from top to bottom and

melt the sample. Combustion, with minimum drop activity, will proceed from

this point with a small amount of spray ejected from the molten sample.

Combustion is fast and of medium luminosity except for the initial flash. The

oxides and unreacted alloy appear to mix completely; no phase separation is

evident

.

46
The combustion region was scanned from 400 nm to 1100 nm, using the RSS,

for emission lines or bands from reacting vapor species. None were found nor

was any vapor-combustion region evident on movie film indicating no vapor-phase

combustion. Temperature measurements using the RSS gave maximum combustion

temperatures between 2440 K and 2840 K (Table 3) for the molten sample.

Weight-gain measurements were made in order to determine the combustion

rate. An example of a typical result is shown in Figure 31. Spikes, in the

electrical signal, caused by the ejection of spray and droplets is a minor

problem; however, the low stoichiometric factor (ratio of molecular weight of

the oxides to the molecular weight of the alloy) does present a minor resolution problem. From the weight-gain curve, fraction remaining and log fraction

remaining curves were made, Figures 32 and 33. It can be seen from the log

fraction remaining curve that several approximately linear regions exist. This

would indicate overall first-order reactions rates for the experimental setup.

2. Bronze

The bronze used in this study was easy to ignite. The combustion had a

low luminous intensity with little to moderate amounts of spray and droplets

produced. Drop activity during combustion was low. The oxides and unreacted

alloy appear to be well mixed; no phase separation is evident.

The combustion zone was scanned from 400 nm to 1100 nm, using the RSS, for

emission lines or bands from reacting vapor species. None was found nor was

47
Relative

Radiant

Intensity

1

Weight

31a. W./W = 0.979

1 s

10.16 s

31b.

Time, s

Figure 31. Beryllium-Copper combustion rate and radiant intensity.

48
FRACTION

REMAIN

I

N5

0.9 ..

e.e ..

0.7 ..

|

0.0 ..

0.5 ..

0.H ..

|

s

i

I 0.3 i

f

l

0.3 ..

0. S ..

0.0 4—

-i 0

i

— ——+— —- -i

l 2 3 H 5

TiMF/5

Figure 32. Fraction of Beryllium-Copper alloy remaining during

combustion

.

49
LOG

FACTION

FOR

I

!N

NS

0.Z

~0.2 ..

“0.B ..

“0.0 ..

“ 1.0 ..

j “i.ni

~ 1 . B

“i .a ..

“2.0 .. “I

4-

0

1 1 HE/ 5

4-

H

4

5

Figure 33. Log fraction remaining for Beryllium-Copper alloy.

SO
any vapor-combustion region evident on movie film indicating no vapor combustion was present. Maximum combustion temperature measurements were also made

using the RSS. The values ranged from 2075 K to 2250 K (Table 3).

Oxygen-uptake measurements were made in order to determine the combustion

rate. An example of a typical results is shown in Figure 34. The material

appears to have a high surface oxidation rate before the onset of combustion as

determined from preignition observations. Essentially all of the material can

be accounted for from the electronic balance data. Fraction remaining and log

fraction remaining plots were made from the curve of Figure 34a (Figures 35 and

36). The log plot has predominately linear regions indicating first order

combustion kinetics for the experimental arrangement.

51
Relative

Radiant

Intensity

Weight

34a

.

W./W = 0.998

l s

0 2 4 6 8 10

Time, s

Figure 34. Bronze combustion rate and radiant intensity.

52
rRRCTiON

RCMRINING

| a.s |

0 .a 1

0.7 il

0.6 i

i

I

B.si

<

B.2 ..

j

13. I ..

i

0.0 .. -I

4— ~H +*—Hf— 4 1 0 l 2

' 3 H £

T l ME/ 5

Figure 35. Fraction of Bronze alloy remaining during combustion.

53
j -•0.2 i

i

1

~0.H i?

I

[

*

| ~0.E 1

i

i4

_I

i

}

> ~i .H i

ij

ij -i .5 i

i

j

!

-2 . IS i

H S

T i ME/ H

Figure 36. Log fraction remaining for Bronze alloy.

54
IV. CONCLUSIONS AND RECOMMENDATIONS

A. Conclusions

For this preliminary report, only tentative conclusions can be made on the

ignition and combustion characteristics of metals in high-pressure oxygen.

However, trends may be anticipated from chemical knowledge, past experiments,

and the experiments reported herein. These anticipated trends are discussed

below for each alloy group.

1. Aluminum Alloys

a. Ignition

Ignition of aluminum and aluminum alloys normally does not occur at

one atmosphere until the melting point of aluminum oxide is reached. This

is due to the dense uniform nature of the oxide produced during heat-up,

even on the molten metal surface, and to the fact that the vapor pressure

of the molten alloy does not exceed one atmosphere until about the oxide

melting point; thus a sufficient amount of metal vapor is not available

for ignition until this point is reached. If other means are provided for

removing the protective oxide layer, e.g. mechanical scraping, then

ignition will occur at temperatures below the oxide melting point. For an

undisturbed surface, the change in ignition point temperature may not vary

55
significantly with pressure; however, for a disturbed surface the ignition

point may decrease significantly.

b. Combustion

For the experimental setup used in the experiments reported herein,

the measured combustion temperatures are about 1000 K greater than the

boiling point of the alloys, generally between 3300 to 3500 K (Table 3).

This would indicate that the alloy is vaporizing then reacting with the

oxygen. As system pressure is increased the boiling point of the alloy

will increase, thus the molten alloy temperature, and the reaction rate

will increase. Whether the reaction zone will collapse to the molten surface cannot be predicted with accuracy but may take place. In any event

the reaction rates should increase considerably with the increase of

oxygen pressure as will the combustion temperature.

2. Stainless Ferrous Alloys and Nickel Alloys

a. Ignition

The ignition temperature of these materials appear to be at or just

below the materials' melting point. If the ignition takes place on a

solid surface then we could expect a decreasing ignition temperature with

pressure. This may also apply if ignition takes place at the alloy

melting point. Whether mechanically disturbing the surface will effect

the ignition point significantly cannot be predicted at this time.

56
b Combustion

.

The combustion temperatures of these alloys ranged from approximately

2400 K to 2700 K. If the combustion rate-determining process is oxygen

diffusion controlled then an increase in pressure will increase the reaction rate and also the combustion temperature. Observing the intensity of

the combustion process reported herein it can be anticipated that the combustion will be considerably more intense and active as the oxygen is

increased, possibly to the point where the intensity is almost explosive

in nature.

3. Copper Alloys

a. Ignition

The ignition characteristics of the two copper alloys studied, a

bronze and a beryllium-copper, were quite different. The bronze appeared

to ignite at or just before melting, whereas the beryllium-copper appeared

to ignite, after the alloy melted, at a fracture point on the oxide surface or at the oxide melting point. The high melting point of BeO is

considered to be the reason for the difference in ignition characteristics

between the two alloys.

For the bronze, if the ignition starts in the solid phase it can be

expected that the ignition temperature would decrease for increasing

57
oxygen pressure. This may also be the case if the ignition takes place at

a liquid phase.

For the beryllium-copper alloy no prediction on the pressure dependence of the ignition point can be made at this time.

b. Combustion

The combustion temperatures for the two alloys were not, on the average, vastly different - the bronze being 2165 K and the beryllium-copper

being 2585 K - as might be expected from the completely different compositions. Since no vapor combustion was evident, it may be assumed that the

combustion is diffusion controlled and that increased pressures would

increase the combustion temperature and reaction rate. However, the

predominant oxides, CuO and CU2 O, are not refractory oxides and will

decompose, thus limiting the combustion temperatures; beryllium-copper,

due to the refractory nature of BeO, may not be limited in its combustion

temperature until considerably higher temperatures are reached.

4. Areas of Uncertainty

There are several areas which require a more detailed explanation than we

can make at this time. These areas are:

1. The absence of emission spectra during the combustion of the ferrous,

nickel and copper alloys.

58
2. The existance of several combustion rate constants (see figure 21 for

example) each of which is apparently first-order or very nearly so.

3. The mechanism for the ejection of drops of material from the combusting parent mass.

Each of these areas probably requires a detailed knowledge of the chemical

reaction mechanisms and the effect of the different chemical species upon one

another in both a chemical and physical sense.

B. Recommendations

At present, a high pressure chamber has been designed in order to extend

the studies into the high pressure region. Also the development of a highspeed two-color optical pyrometer has been undertaken in order to obtain

accurate surface temperatures. With these two devices work can then begin on

the determination of the high pressure ignition and combustion characteristics

of the metals and alloys of interest. The present plans, and our

recommendations, for the continuation of the present study call for the

following action:

1. Procurement of the high pressure chamber and support equipment.

2. Procurement of optical equipment for the pyrometer development.

59
3. Concurrent with the above procurements, study, at one atmosphere, the

combustion temperature pattern by optical Hilbert transform

photography

.

4. Upon receipt of high pressure chamber begin elevated pressure

experiments. Obtain spectra, combustion rate, combustion temperature,

and combustion behavior data.

5. Concurrent with the activity of items 4 and receipt of optical

components, begin development of the two color pyrometer.

With successful implementation of these projects the program experimental

objectives can be met.

In addition to the above recommendations we also suggest that effort be

expended to design experiments and theoretical models that will explain the

material ejection mechanisms, existance of several rate constants and the

absence of spectra during the combustion of several of the materials.

V. ACKNOWLEDGMENTS

The authors would like to express their gratitude to Mr. John C. Moulder

for his helpful discussion with and training of personnel performing this

study; to Mr. James D. Breuel for his assistance in performing the experiments;

to the George C. Marshall Space Flight Center, Mr. John G. Austin, Jr., technical representative, for funding this program.

60
Appendix A: Temperature Measurements Using the Thermal Radiation Spectrum

Starting with the Planck radiation equation [1]

8rrhc

A5

he

. AkT ..-l

(e - 1) ( 1 )

where A = wavelength, m

T = absolute temperature, Kelvin

h = Planck’s constant, 6.626196 x 10“34 j. s k = Boltzman's constant, 1.380622 x 10“^3 J/K

c = velocity of light in vacuum, 2.997925 x 10$ m/

s

a wavelength range may be chosen such that the Planck equation may be

approximated by Wien’s approximation, that is

he

XkT

e

he

IkT

1 for A <<

he

kT

* ( 2 )

Our wavelength range of 0.4 to 1.1 cm meets this requirement. The Planck

equation thus reduces to

W

c

1 A,T IF

C

2

/ At

e where (3)

c. = S'n'hc and c_ = — . 1 2 k

[1] Light, Vol. 2; R. W. Ditchburn; Interscience Publishers.

61
Since the above equation applies only to a black body it must be modified to

apply to a gray body. This is easily accomplished by multiplying by the

spectral emissivity, £,. The Wien approximation then becomes

W(A,T) = e(A) c

x A

C

2

/ At

e (4)

This equation expressed in a logarithmic form yields

In W(X,T) = In £( A) + In c^ - 5 In A - c^/AT. (5a)

which can also be written as

In W( A,T) + 5 In

-

c

2 A

= -f+ In e(A) + In c . X

^ (5b)

If the assumption is made that the spectral emissivity is constant or reasonably so then the above equation has a linear dependence of (In W + 5 In A ) on

1/A- The temperature can then be determined from the slope of a plot of In W +

5 In A vs . 1 / A.

There are two major sources of nonlinearity. The first is the change in

the value of emissivity with wavelength. This change is usually small over the

visible and near infrared regions. Thus the change in the value of In e(A)

will be small and not contribute significantly to a nonlinearity in the curve.

The second source of nonlinearity is in the determination of the values of

radiant energy, W(A,T). If the value of W(A,T) is in error by a constant multiplication factor (for example a calibration error) then this introduces a

62
constant error and shifts the curve along the vertical axis. This shift does

not affect the value of the slope, thus an accurate temperature can be determined. If the value of W(A,T) is in error by an additive error (for example,

an error introduced in reading a photograph) then a small nonlinear perturbation to the curve can result. This type of error can usually be minimized by

using least squares techniques in the data reduction.

63
Appendix B: Two-Color Pyrometer Theory

by Key Nguyen and M. C. Branch

I. INTRODUCTION

Due to the large exothermic heat released and the transient nature of chemical reactions in metal combustion, any proposed method for measuring ignition

and combustion temperatures faces two basic problems: high and rapidly changing

temperatures and an oxidative environment. Such problems confine the measurement methods to radiation pyrometry. There are several advantages in employing

radiation methods to temperature measurement. These are: (1) radiation methods

confine disturbances of the system to the radiant energy leak emitted toward the

detector; (2) theoretically there is no upper limit for measurable target temperature; (3) temperature measurements can be made on inaccessible targets; and

(4) short response-time temperature measurements can be made. The last advantage is satisfied because the signal acquisition time is restricted only to the

response time of the radiation detector, sampling electronics and recording

methods. For the case of a photomultiplier detector, the response time is in

the order of 10 ns.

One of the most commonly used methods of radiation temperature measurement

is two-color pyrometry. In this method, the radiation emitted from the target

is detected at two different wave lengths, ^ and ^ • From the ratio of

radiation fluxes, the target temperature is then calculated (see eq . (B4)).

Even though the method is quite versatile, there are still many disadvantages inherent to the method. For example, it must always be assured that the target has

64
to approach gray-body condition for the two wavelengths at which the measurements are made and that due to these restrictions, the measured surface temperature from a non-gray target obtained from two-color pyrometry tends to be higher

than the true temperature. This is a characteristic feature of a two-color

pyrometer. The reason is attributed to the fact that for most metals, the eraissivity at shorter wavelengths has a larger numerical value than that at longer

wavelengths (see eq . (B4)). Furthermore, for most real surfaces the emissivity

is generally not only a function of wavelength but also of target temperature,

direction and surface condition. This largely accounts for the fact that emissivity values listed in various reference books sometimes differ greatly. With

this in mind, it is obvious that to reduce errors in temperature measurements

resulting from the gray-body assumption, a method has to be devised to determine

the spectral emissivity ratio e( ,T)/£( ^ >T)

.

The temperature measurement system described herein is a modification of

the method suggested by DeWitt and Kunz [1]. It is modified in order to have a

short response time, in the order of 1 millisecond.

II. THEORY OF THE MEASUREMENT METHOD

For a black body, the spectral radiance is related to the temperature and

wavelength by the Planck radiation equation,

N(X,b) = (c^ A tt) / [exp (c

2 /At)- 1] (Bl)

65
where

N(A) _ Radiation emitted at wavelength A, per unit surface area, per

unit wavelength interval, per unit solid angle.

C]_ = 3.74 x 10”12 w*m^

C 2 = 1.43 m*k

T _ Absolute temperature of radiating body, K

A _ Wavelength of emitted radiation, m

For the case of real surface or non-black body, the spectral radiance is

proportional to that of black body at the same temperature by

the Planck radiation equation can be approximated by the mathematically simpler

form known as Wien's law

N( A) = e(A,T) N( A, b) (B2)

where e(A,T) is the spectral monochromatic emissivity.

For small values of AT such that

e

C 2 /AT » 1

-c 2 /AT

(B3)

66
For a body radiating at , and X 2 with corresponding spectral monochromatic emissivities e(X^,T) and e(X2 >T), it can be shown that the body

temperature T is given by

lT

ln[e(X

1 ,T)/e(X2 ,T)]

CA

-

where T r , the ratio temperature is given as

1 1 1 "l 1

’

T

r X,T

L

1

S

1

A

0 T

2 S 2j \

(BA)

(B5 )

in which Ts i and T S 2 are the "apparent monochromatic temperatures" at wavelength X^ and X

2

. In other words, Ts x and Ts2 are the temperatures of a blackbody which at defined wavelengths X^ and X

2 has the same radiation intensity

as the body under consideration.

The above relations form the basis of "two-color" pyrometry. It is evident

from eq . (B4) that for a graybody (where by definition emissivity is independent

of wavelength), the spectral emissivity ratio e(Xx ,T)/e(X2 »T) is unity and

consequently, the second term of eq . (B4) is zero. In general, not all radiating bodies behave like graybodies, and hence the contribution of the spectral

emissivity ratio to the measured temperatures has to be determined.

67
III. DETERMINATION OF THE SPECTRAL EMISSIVITY RATIO

For a system in thermal equilibrium, Kirchhoff's law is applicable; that

is,

e(X,T) = a(X,T) (B6)

where a(X,T) is the absorptivity. Hence, it is possible to determine the spectral emissivity ratio e( , T)/ e(

A

2

, T) by direct measurement of the spectral

absorptivity ratio, a( X]_ ,T)/ a( X2 , T) . Of course the determination of the spectral emissivity ratio must be carried out under the same geometric conditions as

that of the ratio temperature Tr as described by eq . (B4).

If the target is irradiated by a spectral line source, the energy E(X)

incident upon the target will cause a temperature rise At, proportional to the

absorbed energy.

AT = k

l a(X,T) E( X

)

(B7)

or, by Kirchhoff's law.

AT = k e(X,T) E(X) (B8)

68
If this process is performed at two different wavelengths, A]. and A

2 ,

then the spectral monochromatic emissivity ratio given in terms of the spectral

irradiances and their corresponding temperature rises is

e(X 1} T) E(A

2

) At

l e(A

2

> T)

=

’ AtJ

(B9)

It should be emphasized here that to formulate the above relation, it is

assumed that the proportionality constant is the same for both conditions.

This assumption is valid when the temperature rises, AT]_ and At£, have the

same order of magnitude and are much smaller than the target temperature.

Evidently to minimize the disturbances on the target, any temperature rise

caused by laser irradiation has to be made as small as possible, and the lower

limit of the temperature rise is restricted by the detectability of the radiation detector. With the extremely sensitive low-noise photomultipliers available it is expected that a temperature rise of approximately 0.05 K can be

detected

.

The temperature rises, AT]_ and AT2 , can be observed at a different

wavelength A0 to minimize the effects of reflection from the target. The

signal output S from the radiation detector will be proportional to the spectral

radiance from the target

S = k

2 R(A

q

) t(A

o

) e(A

q

,T) N(A

q

,T) (BIO)

69
where R(A Q

) is the absolute responsivity of the radiation detector, t(X

q

) the

overall transmittance of the optical system, and k/? a proportionality constant

which includes the effect of geometrical conditions.

By taking the differentials on both sides of the above relation, the change

in signal output for a change in temperature of the target can be expressed as

AS

S

AT (Bll )

Solving for AT and then substituting into eq . (B9), this equation becomes

E(A

2

) AS

1 /S]_ T

2

2

c(A

2 ,T) = E(x

t

)

* as

2 /s2

* ^2

(B12)

At a high frequency of temperature measurement of the order of 1000 Hz, it

is assumed that the temperature of the target essentially remains the same

during the irradiation at two different wavelengths. Thus, T^ = T2 and then

it follows that S^ = S

2

, reducing the above relation to

e(X

1 ,T) E(a2

) e(\ v V

=

E( Xl )

* AS^

(B 13 )

70
The determination of the spectral emissivity ratio then essentially becomes

the measurement of the incident laser energy upon the target at two different

wavelengths, and the measurement of the corresponding change in photomultiplier

signal output at a third wavelength.

IV. DESCRIPTION OF THE APPARATUS

A schematic diagram of the apparatus is shown in Fig. Bl. The components

of the apparatus can be separated basically into two different groups according

to their functions: two-color temperature and spectral emissivity ratio

measurements

.

A. Two-Color Pyrometer

The heart of the system is the two-color pyrometer, which measures the

"apparent monochromatic temperatures” Tsi and Ts2 at two different wavelengths and A. 2

*

The radiation flux from the target is focussed by an achromatic lens ,

(300 mm focal length, 82 mm diameter) on a circular field stop inclined 6° to

the optical axis with unity magnification to minimize spherical aberration. The

emerging beam is then collimated by the lens L 2 (100 mm focal length, 26.5 mm

diameter), and the aperture is limited by the circular stop. The collimated

beam with a beam diameter of 7 mm, then passes through a system of cube beamsplitters, rotating chopper disk and mirror.

71
interference

Filter
The chopper disk as shown in Fig. B2 has six large apertures each subtending 30° at the center of the disk. The chopper disk is rotated at 200 rps by a

synchronous motor. With this arrangement, it is possible to chop the radiation

flux from the target at a frequency of 1200 Hz. At this frequency, the chopping

disk opens the radiation flux from the target for 417 ys, then closes for 417 ys

.

The radiation fluxes reflected from the cube beamsplitter B4 are then

focused by lenses L4 and L5 , which have the same characteristics and dimension as lens L 2 , on the radiation detectors. These consist of two photomultipliers viewing the radiation fluxes through interference filters with wavelengths centered on 488 and 514.5 nm. The interference filters have a half

height band width and peak transmittance of 3 nm and 50 percent, respectively.

These two wavelengths are chosen such that to avoid any emission line or band

spectra coming from the target. A preliminary study of spectra of burning

Aluminum 1100, 2219, 6061 and Tensile-50, Inconel 600, Stainless Steel 304, 347,

and 21-6-9, Beryllium-Copper and Bronze indicates that no emission line or band

spectra exist at these two wavelengths. In addition, these two wavelengths are

selected due to the inherent characteristics of argon-ion laser, which is to be

discussed in the next section. The photomultipliers used to provide signaldetection are head-on types, with a peak response at 400 nm and a typical rise

time of 15 ns.

B. Spectral Emissivity Ratio Measuring Device

For irradiating the target, an argon-ion laser is used as a spectral line

source. The laser is operated at all line operation, that is, the output beam

73
74

Figure

B2

.

Chopper

disk.
oscillates at eight different frequencies simultaneously. Furthermore, the

laser is restricted to operation in TEMOQ Mode. This mode is selected to

minimize diffraction loss, divergence, and, more importantly, to focus the beam

to the smallest possible spot. The specifications and the spectral lines and

their corresponding power outputs of the argon-ion laser are shown in Table B1

and B2, respectively.

As seen in Table B2, the power outputs in the various spectral lines of the

argon-ion laser are quite different. At the spectral lines of 488.0 and 514.5

nm, the power outputs are largest and almost equal. It is always preferable to

operate the laser at its maximum power output in order to achieve maximum output

stability. The direct operating power outputs in the range of 25 mW to 145 mW

can then be easily achieved by using a variable neutral density filter.

Due to the above considerations and the absence of emission line and band

spectra coming from the target, the spectral lines of 488.0 nm and 514.5 nm are

selected to irradiate the target in order to determine the spectral emissivity

ratio

.

To fill the aperture of the pyrometer, the output beam of the argon-ion

laser is expanded by a beam expander by a factor of 5. The beam expander also

provided spatial filtering for the laser beam. The expanded beam is then passed

through a variable neutral density filter whose function is to control the beam

power output.

To select 488.0 nm and 514.5 nm spectral lines, a system of cube beamsplitters and interference filters which have the same dimensions and characteristics

75
TABLE Bl. SPECIFICATIONS OF ARGON ION LASER

Beam Diameter: 1.4 mm at l/e^ points

Beam Divergence: 0.8 mrad

Bore Configuration: Segmented BeO with Integral Symmetrical Gas Return Channels

Resonator Construction: Composition^ (Composite Rod Ion

Resonator Construction)

Excitation: Current Regulated DC

Cavity Length: 1.15 m

Cavity Configuration: Flat/Long Radius Spherical

Output Polarization: 1:100, Electric Vector Vertical

Noise

Less than 0.3% rras - 10 Hz to 2 MHz

(Measurement is made with a wide

band photodiode driving a resistive

load. The AC noise voltage is

measured on an rms voltmeter with 10

Hz to 2 MHz bandwidth.)

Light Regulation: (With optional Model 236.) Long

term amplitude stability (1 second

to 10 hours), better than 0.1% rms.

Power Measurement: Integral, thermally stabilized

(+ 0.1°C), and wavelength corrected

over spectral range from 450.0 to 700.0 nm. Accuracy is + 5%, full

scale. 1-, 3-, 10-Watt Scales.

Input jack for external power

measurement

.

Frequency Stability: (With optional 423.) + 75 MHz.

76
TABLE B2 . POWER OUTPUT OF THE ARGON ION LASER

WAVELENGTH POWER, W

676.4 (Red)

647.1 (Red)

568.2 (Yellow)

530.9 (Green)

520.8 (Green)

514.5 (Green) 1.400

501.7 (Green) .200

496.5 (Green) .300

488.0 (Blue) 1.300

482.5 (Blue)

476.5 (Blue) .500

476.2 (Blue)

472.7 (Blue) .100

468.0 (Blue)

465.8 (Indigo) .050

461.9 (Indigo)

457.9 (Indigo) .150

351.1/363.8 UV

350.7/356.4 UV

Total Power 4.000

77
as the ones used in the two-color pyrometer is employed. The expanded laser

beam is directed onto a cube beamsplitter which passes 50 percent and reflects

50 percent of the beam. The split beams are then directed to interference

filters, one to a filter centered on 488.0 nm and the other to a filter centered

at 514.5 nm. Before the target is irradiated with these spectral lines, one

condition has to be fulfilled. For example, the beam from the laser is usually

plane polarized and it is necessary to depolarize the beam to correspond to the

unpolarized conditions of the real radiating target. To achieve this a quarter

wave retardation plate is inserted in the individual beam path with the retarder

principal axis making an angle of + 45° with the electric field vector of the

laser beam. The emergent beams are then circularly polarized. These two beams

with wavelengths centered on 488.0 nm and 514.5 nm then travel parallel to each

other, pass through the rotating alternator disk, and enter the two-color

pyrometer optical path and are focussed in the same manner that the target is

viewed

.

The alternator disk, shown in Figure B3, is used to alternate the irradiation of two spectral lines upon the target. The increase in spectral irradiance

as a result of laser flux incident upon the target is measured by a head-on

photomultiplier at a wavelength centered on 650.0 nm.

V. THE VALIDITY OF THE THERMAL EQUILIBRIUM ASSUMPTION

As mentioned earlier, the determination of the spectral emissivity ratio

e( Xi ,T)/e(

A

2 ,T) by direct measurement of the spectral absorptivity ratio is

valid only if the target achieves a state of thermal equilibrium. Only in this

78
case is Kirchhoff's law applicable. Although the heating effects due to laser

irradiation of the target are, in general, not in thermal equilibrium with each

other, no appreciable heating of the target occurs at least within the investigated power limits of 25 mW to 145 mW during the absorption of the incident

laser irradiation, and Kirchoff’s law is valid.

A state of local thermal equilibrium can be assumed if at some distance

from the surface of the target the characteristic conduction time is less than

the duration of the laser irradiation. The characteristic conduction time of

the target r, with a characteristic dimension d = 100 m is calculated from the

following relation

T — —— (B14) a

where a is the thermal diffusivity of the target. Typical values of characteristic conduction time of Al-1100, Ni and Inconel-600 are presented in Figure 4.

To satisfy the local thermal equilibrium assumption, the duration time of

laser irradiation has to be adjusted according to the specimen. For example, in

the case of Inconel-600, the duration time of laser irradiation is approximately

2.5 msec, whereas it is 0.2 msec for Al-1100.

[1] D. P. DeWitt and H. Kunz, "Theory and Technique for Surface Temperature

Determinations by Measuring the Radiance Temperatures and the Absorptance Ratio

for Two Wavelengths," Temperature - Its Measurement and Control In Science and

Industry, Vol. 4, Part I, pp. 599-610 (1972).

79
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80

TEMPERATURE
NBS-114A (rev. 2 -ac) U.s. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Date BIBLIOGRAPHIC DATA

SHEET (See instructions)

REPORT NO.

NBSIR 81-1647 April 1981

4. TITLE AND SUBTITLE

Laser-Initiated Combustion Studies of Selected Aluminum, Copper, Iron, and Nickel

Alloys

5. AUTHOR(S)

J. W. Bransford and A. F. Clark

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

NATIONAL bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Con tract/ Grara No. Government Order

H-43201B

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City. State, ZIP)

George C. Marshall Space Flight Center

National Aeronautics and Space Administration

Marshall Space Flight Center, AL 35812

10.

SUPPLEMENTARY NOTES

Q21 Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bliography or literature survey, mention it here)

The results of combustion studies at atmospheric pressure on ten metal

alloys are presented. The alloys studied were aluminum alloys 1100, 2219,

6061, and tensile-50; 304, 347 and 21-6-9 stainless steel; inconel 600;

beryllium-copper and a bronze. It was found that once ignition was achieved

all alloys would generally burn to completion. The overall combustion process

appears to obey a first-order rate process. Preliminary conclusions are presented along with recommendations for future work.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

alloys; combustion; combustion rate; ignition; metals; oxygen atmosphere;

temperature measurement. —

13. AVAILABILITY 14. NO. OF

PRINTED PAGES

[X5| Uni imited

| | For Official Distribution. Do Not Release to NTIS

1 Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

89

15. Price

[X^COrder From National Technical Information Service (NTIS), Springfield, VA. 22161 $9.50

USCOMM-DC 6043-P80