NATL INST. OF STAND & TECH

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PUBLICATIOI^S

NBSIR 83-2689

Thermal Radiative Ignition of

Liquid Fuels By A CO 2 Laser

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Fire Research

Washington, DC 20234

May 1983

Sponsored by:

Air Force Office of Scientific Research

Building 410, Bolling Air Force Base

Washington, DC 20332

NBSIR 83-2689

THERMAL RADIATIVE IGNITION OF

LIQUID FUELS BY A CO 2 LASER

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Takashi Kashiwagi

Thomas J. Ohiemiller

Takao Kashiwagi

Walter W, Jones

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Fire Research

Washington, DC 20234

May 1983

Sponsored by:

Air Force Office of Scientific Research

Building 410, Bolling Air Force Base

Washington, DC 20332

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldriga, Socrotsry

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
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CONTENTS

Page

LIST OF FIGURES iv

Abstract 1

1. INTRODUCTION 3

2. OBSERVATIONS OF LIQUID FUEL BEHAVIOR AND VAPOR GENERATION DURING

CO

2 LASER IRRADIATION 3

2.1 Background 3

2 .2 Experimental Apparatus 4

2.3 Results and Discussion 7 2.3.1 Range of Laser Flux 7 2.3.2 Parametric Study of Phenomena 7 2.3.3 Laser Flux 11

2.3.4 Laser Incident Angle 11

2.3.5 Absorption Coefficient of Liquid 12

2.4 Sequence of Phenomena and Underlying Processes 14

2.4.1 Idealized Sequence 14

2.4.2 Key Processes in Vapor Generation 20

3. RADIATIVE IGNITION MECHANISM OF A LIQUID FUEL USING HIGH

SPEED HOLOGRAPHIC INTERFEROMETRY 27

3.1 Background 27

3.2 Experimental Apparatus 30

3.3 Results and Discussion 36

3.3.1 Observation from High Speed Motion Pictures 36

3.3.2 The Growth of the Vapor Plume and the

Location of Ignition 38

3.3.3 Distributions of Temperature and Fuel

Vapor Concentration 41

4. DEVELOPMENT OF ABSORPTION COEFFICIENT MEASUREMENT

TECHNIQUE AT ELEVATED TEMPERATURES 43

4.1 Background 43

4.2 Experimental Apparatus 44

5. SUMMARY 45

6. ACKNOWLEDGMENT 47

7. REFERENCES 48

iii
1 1ST OF FIGURES

Page

Figure 1. Schematic illustration of experimental apparatus for

observation of liquid behavior and vapor generation 53

Figure 2. Top view direct picture and side view schlieren picture;

left columns, top view; right columns, side view. Decane,

laser flux of 740 W/cm^, incident laser angle of 30 degrees.

A, appearance of hole due to first bubble in top view;

B, corresponding sudden release of liquid vapor. Time gap

about 30 msec 54

Figure 3a. Top view picture, decane, laser flux of 410 W/cm^ and

incident laser angle of 30 degrees; B, appearance of hole

due to first bubble. Time gap, about 350 msec 55

Figure 3b. Side view schlieren picture, same test as Figure 3a.

A, appearance of laminar vapor plume; B, sudden release of

liquid vapor by bubble burst 56

Figure 4. Side view closeup direct picture near the decane surface,

laser flux of about 2500 W/cm^ with incident laser angle of

30 degrees from right to left 57

Figure 5. Side view direct picture to show the location of the first

appearance of flame. Decane at a laser flux of about

2500 W/cm^ with incident laser angle of 30 degrees from

left to right 58

Figure 6. Top view picture of decene with laser flux of 910 W/cm^

and incident laser angle of 90 degrees. Time gap is about

18 msec 59

Figure 7. Idealized sequence of bubble development; the sequence

repeats with continuing irradiation but the starting

condition is no longer as simple as that in the first

sketch 60

Figure 8. Schematic illustration of experimental set-up of high speed

two-wavelength holographic interferometer 61

Figure 9. Data analysis scheme from 16 mm movie frames 62

Figure 10. Interf erogram reconstructed by computer for blue light with

1-decene 63

Figure 11. Computer calculated contour plot of peaks of fringes,

1-decene and blue light 64
Page

Figure 12. Typical results of the interferogram motion pictures made

at 500 frames per second at wavelength 632.8 nm, peak laser

flux at 260 W/cm^ with various environmental gases: (a) air

(b) 40% 02/60% N2 (c) N2 65

Figure 13. Interf erogram motion pictures made at 500 frames per second

at wavelength 632.8 nm, peak laser flux at 520 W/cm^ with air

environment 66

Figure 14. Comparison in the growth of the fuel vapor plume height with

three different environmental gases at three different peak

laser fluxes 67

Figure 15. Comparison of temperature and fuel vapor concentration

distributions between air and nitrogen environments at

(a) 120 msec, (b) 140 msec, (c) 158 msec from the start of

the CO 2 laser irradiation, ignition delay time 160 msec,

peak laser flux at 260 W/cm^ 68

Figure 16. Schematic illustration of infrared absorption spectra

measurement apparatus 69

Figure 17. Picture of infrared absorption spectra measurement

apparatus 70

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Abstract

This report summarizes progress in the study of the ignition mechanism of

a liquid fuel by a CW CO

2 laser; the period covered is from October 1, 1979 to

September 30, 1982. It describes (1) new observations of liquid fuel behavior

near and at the liquld/air interface during the laser Irradiation with incident fluxes from 260 to 2500 W/cm , (2) new time-resolved measurements of

temperature and vapor concentration distributions in the gas phase using a

newly-developed, high speed, two-wavelength holographic interferometer, and

(3) the development of a technique to measure infrared absorption spectra of

fuel vapors at elevated temperatures.

High speed photographs reveal the complex behavior of n-decane and

1-decene surfaces immediately after the onset of incident CO

2 laser

irradiation. In time sequence one sees the formation of a radial wave, a

central surface depression, bubble nucleation/growth/bursting, followed by

complex surface motion and further bubbling; typically several (or many)

bubble cycles precede ignition. An increase in either the flux or the fuel

absorptivity accelerates the whole sequence of events leading to a dominance

of bubbling phenomena and decreasing the ignition delay. An increase in

oxygen concentration in the surrounding gas phase decreases the distance

between the liquid surface and the location of the first appearance of visible

emission. However, the distance is independent of peak laser flux within the

range studied. Also, oxygen in the gas phase significantly Increases the

growth rate of the plume shortly after the appearance of fuel vapor at the

same peak laser fluxes. Prior to ignition, there are two global chemical

reaction stages in the gas phase. The first is a relatively slow process

1
occurring shortly after the appearance of the plume, at temperatures up to

about A00-450^C* The other Is a significantly faster process occurring just

before Ignition at temperatures above 450-500^C.

2
1 . INTRODUCTION

High power lasers are now being developed as tactical weapons. Such

lasers pose a threat to aircraft integrity since they can ignite aircraft fuel

after fuel tank penetration causing a fire or explosion. However, the

ignition of flammable liquids by laser radiation has been studied little and

is not well understood. The objective of this study is to obtain a fundamental understanding of the mechanism of the radiative ignition process to aid

in the development of future design guidelines for the Improvement of aircraft

survivability.

This paper is the final progress report on this study for the period from

October 1979 to September 1982. The paper consists of three parts; (1) new

observation of liquid fuel behavior near and at the liquid/alr interface

during the laser irradiation, (2) new time-resolved measurement of temperature

and vapor concentration distributions in the gas phase during the laser

irradiation, and (3) the development of a technique to measure infrared

absorption spectra of fuel vapors at elevated temperatures.

2. OBSERVATIONS OF LIQUID FUEL BEHAVIOR AND VAPOR GENERATION

DURING CO

2

LASER IRRADIATION

2.1 Background

In previous studies (1,2), the change in ignition delay time for n-decane

fuel was measured for various CO2 laser fluxes, laser incident angles and

sizes of the liquid container. It was evident from those studies that several

3
key processes In the Ignition Interval required further clarification. One of

them, the behavior of the vaporizing liquid fuel and its coupling to the

build-up of vapor in the gas phase during the pre-ignition period, is reported

in this section.

This section reports various observations from high-speed photographs of

n-decane and 1-decene during laser irradiation. The formation, growth and

collapse of bubbles in the liquid and the growth of a vapor plume in the gas

are described. The primary physical processes in the overall behavior are

deduced from these observations by simple qualitative and order of magnitude

analyses.

2.2 Experimental Apparatus

A schematic illustration of the experimental apparatus is provided in

Figure 1. The Coherent Radiation Model 41 CO2 laser* emits an approximately

6-7 mm diameter beam (at 1/e points). Beam power was varied from 195 to 340

watts in the fundamental mode which has a nearly Gaussian power distribution

across the beam. The Incident flux distribution at the sample surface

position was measured prior to experiment by traversing a water-cooled,

horizontally-mounted calorimeter with 0.25 mm diameter sensing element. The

maximum value of the Incident flux measured in this manner is reported as the

incident radiant flux in this study.

In order to adequately describe materials and experimental procedures it is

occasionally necessary to identify commercial products by manufacturer's name

or label. In no instance does such identification imply endorsement by the

National Bureau of Standards nor does it imply that the particular product or equipment is necessarily the best available for that purpose.

4
A rotating mirror was used as a shutter to provide a step-function onset

of irradiance; it then remained open through the ignition event. The onset of

laser irradiation, i.e. , t = 0, was determined by the signal from a sensitive

pyroelectric detector collecting part of the laser beam scattered from a

mirror. The signal change turned off a neon lamp inside the high speed camera

as an event mark on the film through electronic switching circuitry. However,

comparison of this event mark with the onset of events on corresponding

picture frames revealed some ambiguity in quantifying each sequence precisely

in terms of time from the onset of the laser irradiation. Consequently,

timing was frequently referenced to the onset of a surface wave believed to

result both from photon pressure and the expansion of the liquid, as described

below. Further improvements in procedures to determine the start of the

irradiation will be necessary in future studies.

Two high speed cameras (HYCAM) were used simultaneously in each

experiment. One of them photographed the liquid surface behavior from the

top. This camera was focused on the surface and mounted as shown in Figure 1

so that the illumination seen by this camera was specularly reflected light

from an approximately 10 cm high diffusing screen illuminated in turn by a

tungsten-halogen lamp. This illumination technique provides graded but high

contrast for detecting changes in surface inclination. The second camera

photographed either the behavior of fuel vapors in the gas phase made visible

by a schlieren system as shown in Figure 1 or the behavior of the liquid

beneath the surface by direct photography. In the Schlieren photography, the

He-Ne laser beam was cut horizontally from the bottom by a knife edge. Kodak

4X negative film for high speed photography was used for all pictures in this

study; most prints were made at enhanced contrast.

5
The dimensions of the liquid container were 5.5 cm x 7,8 cm by 5.5 cm

deep. These dimensions are large enough so as not to affect the Ignition

delay time (1). Two optical grade quartz flats were used as observation

windows In two sides of the container. The container was filled to the top

with the liquid fuel. Two liquids, n-decane of 99,6% purity supplied by

Fisher Scientific Co. and 1-decene of approximately 99% purity supplied by

Sigma Chemical Co. , were used as fuel samples. The two fuels were chosen to

be as similar as possible in all properties except absorptivity with respect

to the CO

2 laser radiation; 1-decene has a substantially higher effective

absorptivity (approximately 50 cm”^ vs. 16 cm~^) (2) in the liquid and

probably also in the vapor. No special precautions such as degassing and

further filtering of particulates were taken to assure the absence of

potential nuclei in the fuels. In nearly all tests the bulk of the fuels was

clean enough to exhibit practically no scattering of the He/Ne laser light

used for the Schlieren photography. On the other hand, no precautions were

taken to prevent room air dust particles, that could serve as bubble nuclei,

from falling on the top surface of the liquids during a test.

Ignition was defined by the first light emission detected by a

photomultiplier (maximum sensitivity at wavelength 340 nm) , the output of

which was recorded by an oscillographic recorder with maximum resolution

of ± 1 msec. At the onset of flaming, a step-function-like output of the

photomultiplier was obtained. This provided a well defined measurement of the

time of ignition.

6
2,3 Results and Discussion

2.3.1 Range of Laser Flux

In the authors' previous study using decane (1,2), the range of laser

fluxes was from 900 to 4000 W/cm and Ignition delay time was in the range of

0,06 ~ 1,0 sec. Since the objective of the present study is detailed observation of behavior of the liquid and of vapor build-up in the gas phase, low

laser fluxes in the range of 350 - 950 W/cm were used to make the behavior

slow enough to be more easily observed. Also, decene was added as a second

liquid fuel to examine the effect of absorption coefficient on the overall

behavior. With the present conditions, ignition delay times of decane are in

the range of 0.35 ~ 1.0 sec and those of decene, 0,040 0.085 sec,

2.3.2 Parametric Study of Phenomena

Three parameters, laser flux. Incident laser angle with respect to the

liquid surface and absorption coefficient of the liquid, were selected in this

study. Top view and side view pictures were taken with various combinations

of these parameters. Typical examples are shown in this paper. Pictures of

decane are shown in Figure 2 for an incident laser flux of 740 W/cm and in

Figure 3a and 3b for 410 W/cm . The dark band in the side view pictures is

due to the liquid meniscus at the window of the container. Surface behavior

as seen from above and formation of the vapor cloud in the gas phase as seen

from the side are shown for decane with an incident laser angle of 30°, In

Figure 2, the top view pictures and side view Schlleren pictures are shown

side by side in correlated time sequence. The left columns of pictures are of

7
the top view and the right columns are of the side view. For the low laser

flux case, top view pictures are shown in Figure 3a and side view Schlieren

pictures in Figure 3b. In Figures 3a and 3b, the correlation between the top

view pictures and the side view Schlieren pictures is not well established due

to the lack of a precise determination of the time when the laser starts to

irradiate the decane surface, as described in the previous section (and

differences In camera framing rates, as well). The top view pictures in both

figures indicate the formation of a faint ring-shape wave motion moving slowly

outward radially. As time Increases after the laser Irradiation begins, the

shape of the ring becomes clearer; the center part of the ring Is concave.

With further Increase in time, the top view pictures show a further surface

depression at the center of the ring. This further depression Is circular and

its diameter is about the same as that of the incident laser beam. At the

same time, the pictures at low flux Indicate the development of small scale

complex wave motions between the area of the further depression and the

ring. In Figure 3b, side view Schlieren pictures show the formation and

growth of a plume-like cloud of decane vapor; the plume appears stable and

shows no turbulent eddies. Decane, with its high molecular weight (142), is

heavier than the surrounding air despite its elevated temperature (boiling

point 174°C). The formation and the Initial growth of the vapor plume is thus

due to the momentum implicit In the jet-like, rapid vaporization of decane

Instead of to buoyancy. Therefore, It is probable that the further central

surface depression and the subsequent small wavy motions are caused by this

momentiffli (see Section 2.4). In Figure 2, side view Schlieren pictures

corresponding to the appearance of the further central depression do not show

any plume-like cloud. (Note that it has much less time to move up from the

surface.) This Is probably due to a low sensitivity setting of the Schlieren

8
system for Figure 2. Further discussion of this early sequence of events is

given in Section 2,4,

In Figure 2, shortly after the appearance of the further central surface

depression, a black circle appears in the center of the ring, A similar black

circle also appears much later in Figure 3a, The corresponding side view

Schlieren picture in Figure 2 Indicates a sudden release of decane vapor into

the gas phase. The black circle becomes larger with time and the vapor cloud

of decane becomes more extensive and turbulent. Side view direct close-up

pictures of decane near the surface, shown in Figure 4, indicate extremely

rapid growth of a bubble (its diameter is approximately 3 mm) beneath the

surface. These pictures were taken with different conditions from those of

Figures 2 and 3; they are for decane, a laser incident angle of 30° from right

to left and a laser flux of about 2600 W/cm , Some properly lighted sequences

of top photos clearly show that even at much lower fluxes than this the bubble

nucleates, grows and bursts between movie frames (l.e, , in ^ 2/3 ms); therefore, the observed black circle in Figures 2 and 3 is an open hole left by the

bubble. Shortly after the open hole reaches its maximum size it starts to

shrink; this is made complicated by the appearance of a second smaller bubble

in the side wall of the first as shown in Figure 4 (a high flux phenomenon;

see Section 4), Pictures in the middle of the fourth column of this figure

show many small bubbles a few mm beneath the surface. This phenomenon may be

due to entrained bubbles caused by the bursting of the first bubble (3,4),

The decane surface contour becomes very complex and, except at low fluxes, a

second bubble forms before these complex motions damp out. Thus the cycle of

formation and collapse of a bubble sandwiched between periods of no bubble is

repeated; however, the frequency of bubbling is not exactly regular. The top

9
view pictures in Figure 2 and Figure 3a indicate the formation and radial

outward movement of waves initially generated by the surface depression (see

Section 2,4) and later further enhanced by rapid bubble format ton/collapse and

the associated violent vaporization process. These waves interact so that the

surface structure becomes extremely complex as shown in the last columns of

Figure 2. At the same time, the side view Schlieren pictures indicate violent

vaporization, with decane vapor and liquid decane droplets being thrown in all

directions.

Gas phase ignition generally occurs during this complex behavior in the

liquid and gas phase. The onset of ignition is controlled by the amount of

vapor, the degree of mixing of vapor with air, and the amount of absorption of

the laser energy by the vapor. This is illustrated in Figure 5 which shows

the location of the first appearance of flame with respect to the irradiated

area. In this figure, a side view of the decane surface is seen through the

window; the surface is the broad horizontal line across each frame (broadened

by the meniscus at the window). The upper part of each frame shows the gas

phase; the lower part is liquid decane. The incident laser beam comes from

left to right at an angle of 30 degrees with respect to the surface. The

small white area of flame that first appears in the bottom of the second

column of frames is clearly seen to be in the gas phase about 1 cm above the

surface and about 2 cm to the left of the irradiated area (indicated by the

bubble on the right side of the pictures). The flame spreads upward from the

point of its first appearance, presumably assisted by buoyancy, and then

spreads toward the irradiated area of the decane surface along the laser beam.

10
The observed effects of three parameters, laser flux, Incident laser

angle and the value of absorption coefficient of the liquid, on the behavior

of the liquid surface, bubble formation and vaporization are summarized in

Table 1. All information except ignition delay times was obtained from a

total of twenty eight separate movies. Since the onset of the laser irradiation is not precisely defined in these pictures and also since some of the

phenomena are not sharply visible in some movies, values listed in this table

may vary ± 2 ~ 3 msec.

2.3.3 Laser Flux

Higher laser flux shortens the time to the first appearance of the

central surface depression associated with the momentum of the stable, nonturbulent vaporization plume and to the appearance of the first bubble. This

is clearly demonstrated by comparison of pictures shown in Figures 2 and 3.

The frequency of the bubble formation/collapse cycle tends to increase with

Increases in laser flux. It also appears qualitatively that the laser flux

does not have a significant effect on the size of the bubbles. The pictures

show that higher laser flux causes the behavior of the liquid surface to

become complex earlier and vaporization tends to become more vigorous.

2.3.4 Laser Incident Angle

Top and side view pictures show that the qualitative behavior of the

liquid phase at incident laser angles of 30 and 90 degrees does not differ

significantly regardless of total laser power level. Any differences are

caused mainly by the change in the laser flux due to the change in the

11
Irradiated area with the Incident angle. One noticeable difference between

the two angles occurs at high fluxes after the first bubble bursts but before

the hole It leaves has collapsed. As was noted previously, in such high flux

cases a second bubble can start to form in the wall of the hole left by the

first bubble and this complicates the collapse of the first. As shown in

Figure 4, the left side of the original bubble hole develops a bulge with the

Incident laser angle of 30 degrees from right to left. At 90 degrees, the

bottom part of the original bubble hole shows this bulge.

Since ignition occurs at the location where the incident laser beam

Interacts with the appropriate mixture of fuel vapor and air, the location of

the first appearance of the flame is expected to change with the incident

laser angle. However, this idea is not supported by the fact that, once

bubbling starts, fuel vapor is thrown into the gas phase over almost the

entire hemisphere by the violent vaporization process and not into a specific

direction dependent on the incident laser angle.

2.3.5 Absorption Coefficient of Liquid

The effect of the value of the laser absorption coefficient of the liquid

on liquid behavior during the ignition period and also on the overall ignition

delay was studied by a comparison of high speed pictures and the measured

Ignition delay time with decane and decene. The value of the absorption coefficient of decane with respect to the CO2 laser is 16 cm~^ (2) and that of

decene is about 50 cm~^ (5). Since the only difference between the two molecules is one carbon-carbon double bond at the end of the carbon backbone for

decene Instead of all carbon-carbon single bonds for decane, the physical and

12
chemical characteristics of the two liquids are close to each other; the

normal boiling temperature of decane is 174, 1°C versus 170,5^C for decene (6),

the difference in molecular weight is two; thermal properties are close to

each other. Oxidation characteristics are expected to be quite similar,

judging from the analogous pair, pentane and pentene. Flammability limits for

pentane are 1,4 - 7,8% fuel in air; for pentene they are 1,65 - 7,7% fuel in

air (7), Therefore, the difference between the two liquids for this study is

mainly the difference in the value of absorption coefficient. Also, it is

expected that a similar difference in absorption coefficient applies to vapors

from the two liquids.

Top view pictures of decene during the ignition period are shown in

Figure 6, The sequence of phenomena, l,e,, circular wave motion, the central

depression, bubble formation and complicated surface motion, is virtually the

same as that of decane as seen in Figures 2 and 3, The distinctive difference

between pictures with decene and decane is the generally shorter time scale of

events with decene compared to that of decane. This is also indicated in

Table 1 in a comparison of various times for the same experimental conditions, It is interesting to note that the times to the appearance of the

central depression and to the appearance of the first bubble for decene can be

a factor of two to four shorter than those for decane. There are exceptions;

The heat capacities of n-decane are 314,8 and 234,8 J/mole °C for the liquid

and gas, respectively. Comparable values for 1-decene are 300,6 and

224,0 J/mole °C (6), The heat of vaporization of n-decane is 45,6 kJ/mole,

whereas the value for 1-decene is 42,7 kJ/mole, The combined "endothermicity" of the heat of vaporization and sensible heat to reach the boiling

point for decene is less than 10% below that for decane. However, the

difference in absorption coefficients at 10,6 /zm, which indicates the

difference in the depth of heating by the laser, is over 300%, Therefore,

the difference between decane and decene with regard to the vaporization

process is mainly a difference in the value of radiation absorption

coefficient.

13
for example, the times to appearance of the first bubble at a laser peak, flux

of 730 W/cm are comparable for both liquids. The reasons for the few discrepancies of this nature are unclear. Ignition delay times for decene are at

least one order of magnitude shorter than those for decane. These trends

Indicate that the differences in absorption coefficient between the liquids

and the vapors can both have a major effect on ignition delay time.

Other differences between the two liquids are the size of the bubbles and

the bubble frequency. Bubbles for decene tend to be smaller than those for

decane as shown in Table 1, although the quantitative difference needs further

definition. Also, bubble frequency for decene tends to be higher than that

for decane under the same experimental conditions.

2,4 Sequence of Phenomena and Underlying Processes

2,4,1 Idealized Sequence

Figure 7 shows a series of sketches, in cross-section, of what are

believed to be the major steps in the radiation-induced vaporization

process. The starting condition, for the first bubble only, is a quiescent

liquid at uniform temperature (room temperature). Recall that the incident

beam profile is roughly Gaussian and therefore is strongly peaked in the

center.

In the second sketch of Figure 7, two changes are apparent. A wave is

spreading outward radially from the locus of the beam center and the

temperature for some depth below the surface has increased. The expected

14
depth of the thermal wave will be examined more closely below. It is probable

that the initial radial surface wave is due at least in r rn tv . . ;ton pressure that suddenly exerts a sustained force on the surface. The static depth

of a depression in the liquid surface induced by photon pressure is calculated

by equating the static liquid head to the rate of momentum loss of the

photons; the result is

where is the photon energy flux, c is the speed of light, g is the

acceleration due to gravity and p is the liquid density. For a peak radiant

i_i 2 flux of the order of 1000 W/cm (high end of current tests), the value of hp

is about 5 micrometers. As a static depression extending smoothly over the

beam diameter, this would probably not be visible, even with the rather high

contrast specular surface lighting used here. However, we have a dynamic

situation here in which we view the surface response to the sudden onset of a

force; this produces a wave whose steep sides contribute to its visibility.

An additional contribution to this early wave may come from the transient

volumetric expansion of the liquid as it heats. Decane expands about 10% in

heating up 100®C; this would cause a static surface height increase of about

10% of the radiation absorption depth over the heating time and a radial

outward displacement over the same Interval of about 10% of the beam diameter. This may be more visible than the direct photon pressure effect; at

least the static height would be greater but the dynamic situation is

unclear. The identification of the wave with these early effects is important

because, as was noted, the wave onset has been used as a zero time reference

in several tests.

15
In the third sketch of Figure 7, the surface has reached a sufficient

temperature for vaporization to become appreciable. This vapor flux is

visible as the stable, non-turbulent plume in some of the Schlieren photographs (e.g., Fig, 3b). This vapor absorbs some of the incoming radiation as

will be discussed below. If the speed of this plume is sufficient, the

reaction force against the surface can cause a further depression. Balancing

the static and dynamic heads gives, for this surface depression h^,

h

R

2

2PlE

( 2 )

For an upward vapor plume speed, Vp, of 75 cm/sec (roughly that obtained from

Fig. 3b) and plume density, p, taken as that of decane vapor at its boiling

point, one finds \ is about 150 microns. A central depression is frequently

visible in the top surface photographs for varying periods before the first

bubble appears.

In cases where this depression (plus that due to photon pressure)

persists for an extended period (tens of milliseconds) prior to bubble formation, it appears that some instability exists that causes small wavelength

surface waves to grow in amplitude especially around the periphery of the

depression. This may be a result of some coupling between wave-induced fluid

motion, the vapor flux (which causes the waves) and the highly non-uniform

radiant flux profile on the liquid surface.

In the fourth sketch of Figure 7, the near-surface layer of the liquid

has achieved appreciable superheating because the preceding vaporization

process from the top surface has not been able to remove a sufficient fraction

16
of the Incoming radiant energy. In the next sketch, this superheating results

in bubble nucleation. The amount of superheating that occurs prior to nucleation will depend on the radiant flux, absorption coefficient of liquid, and on

the purity of the liquid. There are two types of nucleation, homogeneous and

heterogeneous, the latter caused by Impurities or solid particulates in the

liquid. In the absence of heterogeneous nucleation, the liquid can be heated

to near the superheat limit which is typically constrained by the kinetics of

homogeneous bubble nucleation and growth (8-10). The value is typically

around 0.89 of the critical temperature; for decane this translates to 285°C

(11). Since the normal (1 atm) boiling point of decane is 174®C, this means

that one can exceed the boiling point by 111°C before achieving homogeneous

nucleation. While we have no definitive measure of the actual amount of

superheating achieved here before the first bubble nucleates, it is probable

that the superheat limit was not approached. Recent optical measurements in

the same experimental set-up using a somewhat lower flux (260 W/cm ) gave a

decane vapor temperature of 200*^C just above the irradiated surface (with no

visible bubble nucleation); this implies a superheat of 25°C or less for this

flux (the vapors absorb the laser radiation after they emerge from the surface

and therefore continue to heat) although the sub-surface liquid that did not

vaporize in this case could have gotten hotter. The higher fluxes of this

work present the potential for greater superheating but it is doubtful that we

had the necessary absence of heterogeneous nuclei to permit this. For

example, one can estimate that particles slightly too small to make their

presence known by scattering of the He/Ne laser light can still nucleate

bubbles in decane with about 25®C of superheating. Furthermore, we may have

had larger particles settled into the top surface of the liquid; the surface

was not protected from room dust which was visibly present in the air by

17
scattering of the He/Ne light. Finally we note that dissolved gases, which we

took no precaution to remove, could also promote nucleation (homogeneous)

before the superheat limit is approached (12,13). Various heterogeneous

nuclei do tend to initiate bubbles at temperatures well below those needed for

homogeneous nucleation (8). It is interesting to note that the first bubble

can produce new nuclei (entrained air bubbles) and lessen the amount of

superheating necessary to generate subsequent bubbles (3,4).

The radiant flux range used in this study is at least three orders of

magnitude lower than that used to cause optical cavitation (14).

Although the mechanism of nucleation per se has been studied extensively,

most studies Involved nucleation near the heated solid wall or in a homogeneously heated liquid without a wall (14). There is little information

available about nucleation at and near the free liquid surface as encountered

in the present study.

The last sketch in Figure 7 indicates the consequence of the

superheating: when nucleation finally occurs below the surface, bubble growth

occurs with great speed. Most (and perhaps all) of the top surface photographs are consistent with bubble growth and breakage in less than one millisecond. The dynamics of bubble growth have been analyzed in detail for the

case where the bubble is immersed in an infinite liquid (15). The growth

process here is considerably altered by the proximity to the free surface but

one conclusion from that work appears quite pertinent here: the first few

milliseconds of bubble growth are heavily influenced by Inertial forces. We

estimate below that the thickness of the superheated layer is of the order of

18
100 ym for decane (less for decene). The bubble must originate Ln this thin

layer and, as it expands, it stretches the liquid above it, thinning it

considerably. Since this top layer is both thinning rapidly and vaporizing

rapidly from both sides while continuing to absorb some of the incoming

radiation, it is plausible that it breaks in less than a millisecond. Despite

the fact that the bubble breaks quickly, the hole it produces does not disappear with equal speed because of the momentum Imparted to the liquid (and

the weaker forces now countering the presence of the hole). The material that

was above the region of nucleation is tossed up and out, frequently forming

droplets that may be tossed several bubble diameters away (and out of the

laser beam path). Since much of this material was superheated and its surface

area per unit volume is sharply Increased, there is an "explosive" release of

fuel vapor that is turbulent in nature, spreading more randomly than the

preceding stable, non-turbulent vapor plume. The impulsive downward force due

to bubble expansion and subsequent vapor release causes a persisting (up to

tens of milliseconds) hole or depression in the liquid. This movement

stretches, mixes and largely erases the thermal layer previously established

in the liquid over a diameter about equal to the maximum size of the bubbleinduced hole. This in turn largely terminates the vaporization process

because the upper liquid layer is cooled. Collapse of the hole is induced by

gravity and surface tension once the outward/downward movement of the liquid

is brought to a halt. Since the laser irradiation continues (somewhat

diminished by vapor absorption), the stage is set for repetition of the heatup and bubble initiation/growth/collapse sequence abetted by any bubbles left

behind; unfortunately, the Initial condition is now considerably more complex.

19
Repetition of this cycle feeds the gas phase with slow vaporization

punctuated by rapid multi-directional vapor emissions. As this vapor builds

up in the gas phase it absorbs the laser radiation and heats up. Ultimately

it will ignite at some point where the proper combination of temperature plus

fuel and oxygen concentration persists for a time sufficient to permit thermal

runaway. Achievement of this condition is clearly dependent on the whole

history of complex cycles like that just described.

2.4.2 Key Processes In Vapor Generation

In the above section, the complex behavior during the vapor generation

process was described. To fully rationalize these observations, it is necessary ultimately to develop models describing vapor generation as an Integral

part of a predictive model of radiative ignition. However, the observed

phenomena are too complex to be modeled in detail. In this section, we

attempt to determine the dominant processes describing vapor generation by

order of magnitude analysis; this can serve as a guide for future modeling

efforts. It is of interest to attempt to assess what thermal processes

dominate the heat-up time between bubble bursts. Consider first the period

before the first bubble. The relative importance of heat conduction and indepth radiation absorption is indicated by the ratio of their characteristic

lengths.

Here t is time, and is the absorptivity of the CO

2 laser radiation in the

(3)

liquid; recall that it is about 16 cm~^ for decane and roughly three times

20
higher for decene. The thermal dif fuslvitles, Kj^, of decane and decene are

both about 8*10 cm /sec (7). Then the above ratio becomes 0.45 t ' for

decane and 1.4 t^^^ for decene. In all but one case (Table I), the time to

the first bubble was 60 msec or less for decane so that the above ratio is

typically less than about 0.1; for decene, first bubble times were 10 to

50 msec depending on radiant flux so the above ratio ranges from 0.14 to

0.3. For the high flux cases which are of primary Interest here heat conduction is a minor process compared to in-depth absorption. Since the average

time between successive bubbles is generally less than the time to the first

bubble, this statement is generally valid at later times as well (each bubble

tends to reset the clock to zero). Conduction may be important in dissipating

some of the thermal energy that is mixed randomly by bubble growth/collapse.

One can show also that radial conduction due to the non-uniform laser flux Is

unimportant in the high flux cases since the thermal layer is thin compared to

the laser beam diameter.

There are two principal sources of convective motion prior to the first

bubble buoyant flow and surface tension gradient flow (16). The buoyant flow

is a result of the non-one-dimens ional heating of the top of the liquid. Here

we follow the work of Reference 16 but note that in the present case inertial

forces are more significant than viscous forces in balancing the buoyancy

force. Consider a radial flow proceeding upward and then outward from the

center of the laser beam impact point. From a steady state balance of

buoyancy and fluid inertia one gets the following (the steady state balance is

an upper limit on the present dynamic case).

dv.

L'B dx

~

Pt V. (4)

21
This Is approximated as follows.

2

( 5 )

so

1/2

( 6 )

Here Vg is the buoyantly Induced upward flow velocity, is the liquid

density, is the change of liquid density with temperature, and Tg is the

liquid boiling point. The use of Rj^, the laser beam radius, as the characteristic length in the upward flow is rough so we can only estimate Vg crudely.

Vg = 8 cm/sec. Recall that this is a steady state upper limit so that,

roughly the average transient value acting on events over a time scale of

(Rb/vg) = 40 msec is about half this or 4 cm/sec. Then only for bubble delay

times of the order of 10 msec is this motion small; for delays of the order of

100 msec it is substantial, causing a full replacement of the liquid beneath

the beam profile.

Consider next the flow driven by the surface tension gradient that is a

consequence of the non-uniform laser beam profile. The steady state balance

is now between the surface tension gradient force and the inertia of the

radial outward flow. Roughly, for unit volume of liquid undergoing this

radial motion, we have

Inserting typical numbers for decane (about the same as decene) one finds

22
2

( 7 )

or

V

s [ ) 1/2

(8 )

Here (3a/3T) is the change in surface tension with temperature;, the other

symbols are the same as before except that v_ is the radial outward flow

velocity. Inserting typical values for decane (decene is again similar), one

finds the surface tension Induced velocity, v. = 3 cm/sec. Again this is the

steady state upper limit so, roughly, half this value is that pertinent to our

transient heat-up process. Since this flow is in the same direction as the

buoyant flow, this result provides a small reinforcement to the previous

conclusion that these fluid motions become appreciable on a time scale of

about 100 msec and are small perturbations on a 10 msec time scale.

This conclusion is reinforced by the experimental behavior of a piece of

foam plastic placed on the liquid surface about 1 cm away from the irradiated

area in several early high flux tests. This object was always pushed to the

side of the container during a 2-3 seconds laser irradiation but its rate of

movement was negligible during the time sequence of Interest here.

Again, since the main concern here is high flux, short delay behavior, we

can assume for now that the above fluid motions are not significant. Then in

the period before the first bubble the dominant energy transport mode is indepth radiation absorption which follows Beer's law. In this case the

governing equation is

(9)

with

23
and

T = T at 00 X =

00

T = T at t = 0

There is the effective absorptivity (corrected for angle of radiation

impingement; note that Qj^ and have equal and opposite cosine corrections

for Incident angle which therefore cancel out), r^^ is surface reflectivity

(generally about 5%) and is the liquid heat capacity. Equation (9) can be

solved using Laplace transformation and the temperature distribution is simply

(1 - r ) -ax

T=T + QTCiTte ( 10)

and the surface temperature is

(1 - r ) T„ = T + Q. t S ~ pc L

Li Li

This temperature distribution and the surface temperature variation hold well

only until the surface reaches the boiling point. Beyond this, the surface

will tend to be held at the boiling point because nucleation is unnecessary

there. For deeper molecules (presumably several mean free paths and beyond)

nucleation is necessary to escape from the liquid phase and the continued net

energy influx causes superheating. The result is a temperature profile

perturbed Increasingly from that in Eq, (10) by a downward dip near the

surface (Fig. 7). The dip is the net result of in-depth radiation absorption,

surface evaporation and conduction of heat from the interior to

endothermically vaporizing surface. The depth of the dip grows with

increasing heat flux but the thickness of the layer affected by conduction

decreases so Eq, (10) is less perturbed for higher fluxes. Using Eq. (10) as

a first approximation and assuming a superheat prior to nucleation of 25°C

(consistent with the preceding discussion), we can roughly estimate the depth

of the superheated layer prior to the first bubble. The result, which will be

24
more nearly correct for the highest fluxes, is approximately 100 microns for

decane; it is about 1/3 of this for decene. This implies that the bubble size

will be less for decene by roughly the same factor which is consistent with

Table I.

Once the first bubble grows and bursts, we immediately have a new form of

convective motion in the liquid. As was noted previously, the initial motion

induced by the bubble is largely that of radial expansion but it quickly

becomes more complex because of the surface proximity. Some of the Schlieren

pictures appear to Indicate that the net short term effect of the bubble is to

push a segment of the upper thermal layer (about equal in diameter to the

bubble in width and one to two diameters deep) down into the cold bulk of the

fluid and sometimes to entrain clusters of small bubbles; this is qualitatively consistent with other observations (3,4,15). The speed of the

generation of subsequent bubbles depends on the product of flux level and

liquid absorptivity (see equation (10)). If the product is high (as in the

high flux decene case of Table I), the result, already mentioned, is a second

bubble in the bottom (or side) of the hole left by the first, despite any

convective/conductive cooling this fluid may have experienced during the

growth of the original bubble. The further result is quite complex mixing

motion in the liquid that should lead to erratic high frequency bubbles.

If the product (Q *a ) is low (as in the low flux decane case of

Li Li Table I), the bubble-induced hole will collapse and at least partially mix a

layer of the order one to two times its original diameter. The mixing, if

total, would fully reset the heating process to zero and another bubble woyld

not follow until a time roughly estimable from Eq. (10) (solved with

25
^S— ^BOIL^» residual sub-surface convection due to bubble collapse would

lengthen the Interval. This behavior for the small (Q »a ) case would imply a

Lj Lj bubble interval comparable to the delay time to the first bubble. This

behavior is only roughly what one sees for decane at low fluxes, as indicated

by Table I.

These bubble-induced liquid motions complicate the determination of

bubble frequency considerably. This frequency is of Interest because it is

probable that it is half the key to the rate of fuel vaporization; the other

half of the key is the amount of fuel vaporized in the bubble growth/burst/

collapse sequence (plus any vaporization between bubbles).

For now, we note only that there are three idealized limiting cases of

vaporization behavior that facilitate analysis of the gas phase vapor buildup. The first is the purely stable, non-turbulent vaporization case in which

a bubble never occurs (or, if it does, the bubble size is small compared to

all other characteristic lengths in the problem). The ignition processes

observed here begin this way but quickly get more complex. Such relatively

simple behavior has been observed with solid fuel ignition by radiation

(17,18). Since the plume here is fairly mono-directional its interaction with

the laser beam depends strongly on the incident angle of the beam; such

behavior has been seen experimentally (1).

The second limiting case is that of very low frequency bubbling as

described above for the liquid. To the gas this presents a sequence of

widely-spaced vapor pulses that might be approximated by delta functions.

Transport in the gas is convective and increasingly multi-directional; the

26
bursting bubbles generate a non-dlrectlonal vapor plume and between bursts,

the negative buoyancy of heavy fuel vapors forces them down, spreading them

out away from the incident beam.

The third limiting case corresponds to high frequency, large bubble

bursts as described above for the liquid. To the gas this presents a nearly

constant, non-directlonal vapor source. One would expect a more or less

hemispherical envelope of vapor to accumulate over the region where the beam

is incident. Since the gradients would be primarily radial, the interaction

with the laser beam becomes nearly the same regardless of its Incident angle

although the location of the first appearance of the flame would change with

the incident angle; this trend at high fluxes is also seen experimentally (1).

These limiting cases are more amenable to at least approximate

analysis. The general behavior seen in the present experiments tends to fall

between the second and third limits described above.

3. RADIATIVE IGNITION MECHANISM OF A LIQUID FUEL

USING HIGH SPEED HOLOGRAPHIC INTERFEROMETRY

3.1 Background

In most of the previous experimental studies, ignition delay time was

selected and measured as the characteristic feature of radiative ignition.

Since the Ignition delay time represents an integration of the complex interactions of heat and mass transfer and chemical reactions, this information is

not sufficient to clarify the fundamental mechanisms of radiative ignition.

27
Furtheraore, in the case of high intensity radiation (e.g., from lasers) the

complicated behavior, such as bubble nucleation and thermal expansion in the

superheated layer near the liquid surface, renders the systematic analysis of

Lgnition extremely difficult. In the previous section, a qualitative model of

bubble formation, growth and collapse in the liquid was presented together

with the coupling of these processes to the build-up of vapor in the gas phase

during the ignition period. The onset of runaway gas phase chemical reactions

leading to ignition was found to be determined by the local temperature, fuel

vapor and oxygen concentration.

Therefore, a holographic two -wave length Interferometer was used for the

measurement of distribution of temperature and fuel species concentration.

The advantages of this optical method are its non-lntruslve nature, the

capability of high time resolution and, instead of point by point measurements, Information about the whole field can be obtained by the evaluation of

Interferograms. Since the test chamber is Illuminated twice within a short

space of time, only the temporal changes caused by the phenomena are

measured. This means that high quality windows, lenses and mirrors are not

necessary in contrast to conventional interferometric methods, such as MachZehnder and Michelson interferometry. Therefore, the necessary optical

arrangements are much cheaper; the experimental setup becomes much more

flexible; the time needed for adjustment of the equipment and taking

measurements is also greatly reduced.

All interferometric methods measure variations in the refractive index.

When the refractive index is changed simultaneously by temperature, species

concentration or pressure, a conventional single wavelength interferometer

28
cannot directly evaluate the obtained interference patterns. In this study,

temperature and species concentration change simultaneously. Since the

refractive index varies with wavelength, temperature and concentration distribution can be evaluated from two interference patterns at two different

wavelengths. This application to the mass and heat transfer processes was

first attempted by Ross and El-Wakll (19) using a modified Mach-Zehnder interferometer for the study of the steady state vaporization and combustion of

liquid fuels. More recently, Mayinger and Panknln (20,21) made further

improvement in this technique using holographic interferometry with two lasers

for the study of temperature and species concentration of vaporizing naphthalene in the boundary layer. Although their experiments were steady state,

they proved clearly that the technique is feasible and has great potential to

study heat and mass transfer processes. Mutoh et al . (18) attempted to

measure temperature and fuel vapor concentration distributions in the gas

phase during the ignition period of polymethylmethacrylate (PMMA) by CO

2 laser

beam irradiation using two-wavelength Mach-Zehnder interferometry. However,

published results appear not to have sufficient time resolved sequential and

spatially accurate information. To understand the mechanism of radiative

ignition, the work by Mutoh et al, (18) is also limited to only air as an

environmental gas.

In this study, holographic two-wavelength interferometry is developed to

attempt measuring time-dependent sequential distributions of temperature and

fuel vapor concentration from the start of the laser irradiation to the onset

of Ignition, The effect of Initial gas phase oxygen on the ignition is also

studied by varying the concentration of oxygen. Preliminary results of the

measurement and of the effect of oxygen are reported in this paper.

29
3,2 Experimental Apparatus

The optical set-up of the two-wavelength holographic Interf eroraeter is

schematically described in Figure 8, The entire set-up except a movie camera

Is mounted on an airsuspended optical bench to Isolate the system from any

external vibration. In this study, beams from a He-Ne laser (632.8 nm) and an

Ar-ion laser (488 nm) are used as light sources. A temperature controlled

etalon was installed in the Ar-ion laser to avoid mode hopping. The two beams

are superposed at a cube prism beam-splitter, then this combined beam is split

into two. One of them is called the reference beam and the other is called

the object beam. Both beams are then expanded to parallel waves by telescopes

which consist of a microscope objective and a collimating lens. Pinholes with

10 ym diameter are used for both beams at the focal point of the microscope

objective lens to remove any fringes on incoming laser beams due to dust

scattering. The object beam passes through the test section, in which will be

the temperature and vapor concentration field to be examined, whereas the

reference beam passes directly onto the photographic plate and interferes with

the object beam to make a hologram. Kodak 649 F glass plate was used for the

photographic plates. It may be mentioned that not only the object beam is

reconstructed by the reference beam of the same wavelength, but also that a

false object wave is obtained. This unwanted wave, however, emerges at a

different angle from the hologram and can therefore easily be separated from

the original wave.

The unique feature of this experiment is its ability to temporarily

record interference patterns generated by two wavelengths of light caused by

changes in temperature and fuel vapor concentration resulting from the start

30
of the CO

2 laser irradiation onto the 1-decene surface up to the event of

Ignition, The basic procedure for recording these patterns is described as

follows. Installing a camera shutter at the exit of the laser, a hologram is

taken at the glass plate for each laser beam separately without the CO2 laser

irradiation, A hologram for each wavelength is superposed on the glass plate.

The plate is developed in a liquid gate without moving it or developed in a

darkroom after removing it. In the latter case, the developed plate must be

replaced at the same position precisely as it was during the first exposure.

After the plate is developed, both lasers are turned on and the reference beam

and object beam again interfere at the hologram. The diffracted wave front

from the hologram due to the reference beam can interfere with the object wave

front passing through the hologram. When there is no change in the test

section, if the hologram plate is slightly tilted, the infinite fringe pattern

occurs (22), By controlling the amount of the tilt angle and direction of the

tilt, number of fringes and direction of fringes can be adjusted. In this

study, typically about 70 to 80 fringes nearly parallel to the sample surface

are created in the observation field. With changes in temperature and fuel

vapor concentration during the ignition period, interference fringe shifts

occur in the field. Since changes in temperature and fuel vapor concentration

occur rapidly, changes in the Interference fringe pattern for both wavelengths

must be recorded simultaneously. As shown in Figure 8, the time-dependent

fringe shift of both wavelengths is recorded on 16 mm color movie film

(Eastman Ektachrome Video News Film 7239) using a Locam Model 50 high speed

movie camera which is a pin-registered. Intermittent transport camera. The

advantage of using this camera is that the film is stationary as each frame is

exposed rather than in motion as in other cameras. Therefore, the fringe

pattern taken by this camera is sharper. Although its disadvantage is a

31
relatively low maximum framing rate of 500 frames/sec, this framing rate is

high enough for this study. From this color film, the two interference fringe

patterns for wavelengths of 488 nm and 632.8 nm are obtained from each frame

using two color filters or interference filters.

A test chamber was constructed to avoid the effect of any air movement in

the room on the experiment and also to permit study of the effect of initial

oxygen concentration on the distributions of temperature and fuel vapor

concentration. Since the holographic technique does not require high optical

quality windows commercial PMMA sheet was used to construct the chamber.

A NaCil window was used on the top of the chamber for the transmission of the

CO

2 laser beam into the chamber.

The conversion from the measurement of fringe shift to values of

temperature and fuel vapor concentration is well documented in previous

publications (19,21,22) and only a brief discussion is given here. If the

temperature and fuel concentration distributions can be assumed to be axisymmetrical around the axis passing through the center of the vertical CO2

laser beam, the index of refraction n(z, r) for the wavelength of X can be

expressed as follows ( 22 ):

ti^(z, r)

X rR (dN/dx)dx

’^Xo TT ^r ,2 2,1/2

(x - r )

( 11 )

where n^^ is the refractive index for the surrounding environmental gas such

as the nitrogen, air (21% O

2

) or 40% 02 / 60% N

2 used in this study. In

cylindrical coordinates (r,z) in the hot fuel vapor plume, z is the height

from the liquid surface, r the radial distance from the axis, and x the dummy

32
path variable of integration, respectively. R is the radius of the hot fuel

plume and N. Is the number of fringe shifts seen in an Interf erogram. In

addition, the following relationship between the refractive index and molar

refractivity of the mixed gas (the environmental gas and the fuel vapor) can

be obtained from the following expression assuming perfect gas behavior ( 21 )

2 ^ 3

(r,z) = RT(r,z)

(M

fX “aX>

C^(r,z) + M

aX ( 12 )

Here, 6 , = n, - n, , and M , are the molar ref ractlvlties of the fuel

* X X Xo fX aX

vapor and environmental gas at the wavelength X, and is the molar concentration of the fuel vapor. T, P, and R denote thermodynamic temperature,

pressure and the universal gas constant, respectively. The values of Mf

»

and ^

3 ^

2

* calculated using data from reference 23, are listed in

Table 2 for the three environmental gases used in this study.

Table 2.

O Molar Refractivity (cm /mole)

Wavelength (nm) 488 632.8

1 -decene vapor 49.603 48.885

N

2 4.500 4.459

Air 4.413 4.370

40% 02 / 60% N

2 4.334 4.289

Equation (11) can be written for two different wavelengths, X^^ and X

2

. Ihen

the temperature and 1 -decene vapor mole fractions can be expressed as

33
T

T

o (13)

1 - I- T RF

3 o

C

f (14)

where

and F

The scheme used for data analysis from a 16 mm movie frame to deduce

distributions of temperature and fuel vapor mole fraction is summarized in

Figure 9. After the blue and red interf erograms are separated by filters, a

black and white negative film of 10 x 12.5 cm is made for each wavelength

interferogram. The light transmission through the film is measured over the

entire frame by a computerized automatic scanning densitometer. The range of

the spatial resolution of the densitometer is 25 200 ym, and the number of

digitized transmittance values for each frame is typically 1.5 million points;

these are stored on magnetic tape. A typical interf erogram reconstructed by

the computer is shown in Figure 10, which is exactly the same as the original

Interf erogram except at the top of the plume where a fringe line becomes wider

and less clear. In defining the locations of the peaks of the fringes, peaks

within one scan are determined by taking the derivative of quadratic profiles

fitted to a sequence of data points in an Interval which is 40% of the peak

transmittance (above the background transmittance value). The search for the

34
next peak In the same scan Is initiated beyond the point at which the last

sequence terminated. Once we obtain all possible peaks within a scan, their

orders are matched with those from the previous scan. Unmatched peaks are

eliminated. A typical result for a computer-generated contour of fringe peaks

with 1-decene and blue light is shown in Figure 11. Further improvement in

the determination of fringe contour is required to obtain more accurate

measurement of fringe shift. This is currently in progress. For this report,

the amount of each fringe shift was measured manually from movie frame

enlargements projected on a large screen. The accuracy of this fringe

measurement technique is at best ±0.1 fringe because of the difficulty in

locating the peak of each fringe. Another difficulty is correcting for the

difference between the two wavelengths in the location of the base fringe

lines. These factors result in estimated uncertainties in the temperature and

fuel vapor concentration measurements of about 5 ~ 10% near the liquid

surface, increasing to about 10 ~ 15% at the top of the vapor plume as long as

the plume is axlsymmetric. Our preliminary work using the automatic scanning

densitometer indicates that the order of the accuracy of the fringe shift

measurement can be improved. Much more accurate results for temperature and

fuel vapor concentration are therefore expected in the near future. The

reproducibility of the detailed shape of distributions of temperature and

vapor concentration measurements is roughly 10%. The major cause of the

scatter is due to two effects; variation of the CO2 laser power flux distribution and somewhat irregular plume growth shortly after the appearance of

vapor. However, this scatter does not change the qualitative conclusions

reached here.

35
3,3 Results and Discussion

3,3,1 Observation from High Speed Motion Pictures

The experiments were carried out at peak CO

2 laser fluxes of 260, 520 and

780 W/cm , Three different gas environments, nitrogen, air, and a 40% 02 / 60%

N

2 mixture, were used to study the effects of oxygen concentration. The

effects of incident laser radiant flux and oxygen concentration in the gas

phase on the distribution of temperature and vapor concentration and the

location of the first appearance of the flame were studied. Typical results

are shown in Figure 12, These high speed pictures are the negative prints

based on the red laser beam. The liquid surface can be seen at the bottom of

each frame. The pictures shown in Figure 12(a), (b) and (c) were taken at the

same radiant heat flux of 260 W/cm , which is the lowest heat flux used in

this experiment. With this flux, the formation and growth of a steadily rising

plume-like cloud of decene vapor can be observed in each environment. It is

found that the time from the start of the CO

2 laser irradiation to the

appearance of vapor is approximately the same («40 msec) for the three

environmental gases. Subsequently, the plume height grows more rapidly and

also becomes wider. With the 40% ©

2

/ 60% N

2 environment, visible light

emission is observed from the upper part of the plume nearly 120 msec after

the start of the laser irradiation. One frame prior to this, rapid expansion

of the plume is observed. This causes the upper part of the plume to become

fuzzy as shown in Figure 12(b), If ignition is defined as the first

appearance of the visible emission, this time period of 120 msec corresponds

to the Ignition delay time.

36
In air, the height of the steadily rising vapor plume (about 26 mm) at

ignition is taller than that in 40% 62/60% N2 (about 18 mm). It is clear that

Ignition is Initiated near the top surface of the plume in an umbrella-shaped

zone, although the amount of light emission there is much less than that in

40% 02/60% N2» Then the flame propagates downward rapidly through the flammable gas mixture. Note the steep gradient in the Interference fringe shifts

which can be seen near the top surface of the hot vapor plume for several

frames before ignition; this indicates very high temperatures in this

region. More quantitative discussion of this will be given later. With

nitrogen as the environmental gas, nearly the same behavior as with air can be

seen. Of course, there is no ignition because of the absence of oxygen.

With an Increase in peak laser flux, the vaporization process becomes

more complex, with the formation of bubbles followed by violent throw-off of

liquid droplets and vapor from bursting bubbles. This behavior of the liquid

surface directly affects the growth of the vapor plume. This complexity is

unique to liquid fuels; the plume growth pattern is Independent of laser peak

flux for a solid fuel (24), At peak laser fluxes of 520 W/cm^ and 780 W/cm^,

the shape of the vapor plume becomes extremely complicated and nonaxlsymmetric. Such behavior is shown in Figure 13 for 520 W/cm , The initial

stage of growth of the vapor plume is random in direction and is controlled by

the bubbling of the liquid, A discussion of the formation of bubbles and

their characteristics is given in a previous section of this report. After

about 60 msec of laser irradiation, the upper part of the vapor cloud forms a

p reasonably stable plume similar to those in Figure 12 at 260 W/cm , Although

the lower part of the plume retains an asymmetric and complicated vapor

distribution, it appears that the upper part of the plume proceeds to Ignition

37
and the subsequent umbrella-shaped ignition patterns are similar to those in

Figure 12. The present approach (which assumes symmetry around the center

axis of the plume and the CO2 laser beam) might allow analysis of the upper

part of the plume where the actual Lgnitlon event occurs. However, if the

entire plume needs to be analyzed, some other approach such as tomography

would be required.

The vapor plume shown in Figure 12 is narrower than that from the solid

fuel (PMMA) studied previously (24). The molecular weight of MMA vapor is 100

which is less than that of 1-decene vapor (MW 142). The average density of

the generated gas mixtures for either 1-decene or MMA at the central, nearsurface part of the plume is higher than that of the surrounding air even when

the temperature effect is included. This is confirmed by the change in the

direction of the fringe shift from the lower part of the plume (upward) to the

upper part (downward) as shown in Figure 12. Therefore, in the process of

vapor plume growth, the initial plume is formed by the momentum of the vaporization from the surface against its negative buoyancy. It acts like a jet of

weak momentum. In jets, the radial spreading rate increases with lower

molecular weight of the Injected gas and this is why the vapor plume of 1-

decene is narrower than that of PMMA.

3.3.2 The Growth of the Vapor Plume and the Location of Ignition

The effects of oxygen concentration in the environmental gas and peak

laser flux on the growth of the vapor plume and on the location of ignition

are described in this section. The growth of the fuel vapor plume is shown in

Figure 14 as a function of oxygen concentration and peak laser flux. The peak

38
height of the plume was selected as the representative value for examining the

2 growth history. As seen from these figures, the plume growth for 260 W/cm

and 520 W/cm is the same, but at 780 W/cm , a reverse trend between air and

40% ©2 occurs in the early stage of irradiation. It is not clear why this

reverse trend for the oxygen effect occurs, but it may be that the violent

bubbling behavior disturbs the consistent growth of the plume at this peak

laser flux.

There are three regimes in the growth of the plume as shown in

Figure 14. They are: a short early buildup stage (most obvious at the lowest

flux), followed by a constant growth regime with nearly constant slope and,

finally, a sudden rapid growth regime with increasing slope. The effect of

the ambient oxygen concentration on the growth rate of the plume appears

clearly in the last two regimes and becomes stronger with Increase in oxygen

concentration at the same peak laser fluxes. Since the difference in the size

of the vapor plume between the nitrogen environment and an oxygen-containing

one must be based on gas phase oxidative chemical reactions, the contribution

from such gas phase chemical reactions appears much earlier than was

previously thought (25). Further discussion about the gas phase chemical

reactions will be given in the next section together with the results on

temperature and fuel vapor concentration.

The rate of growth of the vapor plume increases with peak laser flux; the

average speed of plume growth in the constant growth regime is about 19 cm/sec

for 260 W/cm^, 34 cm/sec for 520 W/cm^ and 48 cm/sec for 780 W/cm^ with air.

This is expected, because the rate of vaporization of the fuel increases with

an Increase in peak laser flux. Also the rate of expansion of the gases in

39
the plume tends to become larger because there is a larger amount of

absorption of the CO

2 laser beam by the gases in the plume with higher peak

laser flux. However, the above growth rates are roughly 40% smaller than

those with PMMA under the same experimental conditions (24).

Other Important results shown in Figure 14 are the effects of oxygen

concentration and of peak laser flux on the location of ignition. In this

work, we have defined ignition as the first measurable emission in the motion

picture frames. This location is generally near the top of the vapor plume,

and it is reasonable to assume that the height of the vapor plume at ignition

Is proportional to the distance from the location of ignition to the liquid

surface. The difference in the plume height at Ignition shown in Figure 14

for various conditions can thus represent the change in the location of

Ignition. The figure shows that an increase in oxygen concentration lowers

the location of ignition toward the surface. Surprisingly, however, an

Increase in peak laser flux does not change the location of ignition; it

remains at about 26 ~ 27 mm above the fuel surface. The first effect is also

observed with PMMA, but the distance of ignition from the surface tends to

increase with peak laser flux for PMMA (24). We believe that differences in

the vaporization process between decene and PMMA and also convective motions

in the liquid are responsible for this different trend for the effect of peak

laser flux on the location of ignition. Further discussion of the location of

ignition will be given in the next section together with the results of

temperature and fuel vapor concentration.

40
3.3.3 Distributions of Temperature and Fuel Vapor Concentration

The temperature and fuel vapor concentrations were calculated using the

procedure described in Section 3.2. Values of the refractive Indices were

evaluated by the onion skin technique (22) for solving Eq. (11). Three

O typical frames were selected from the movie picture for a flux of 260 W/cm as

an example: 120, 140 and 158 msec after the start of the CO2 laser irradiation. As discussed in the previous section, these values belong to the

constant plume growth stage, the middle of the rapid growth stage and just

before the ignition, respectively.

The comparison of temperature and fuel vapor concentration distributions

between air and nitrogen environments at the three different times after the

start of the CO2 laser irradiation are shown in Figure 15(a), (b) and (c).

Figures 15 clearly Indicate that the temperature in air is higher than that in

nitrogen in each stage and especially in the upper part of the plume. Also,

the fuel vapor concentration in air is slightly lower than that In nitrogen.

The differences in temperature and fuel vapor concentration between the two

environmental gases are the result of chemical oxidation reactions. In the

case of 120 msec, the difference in maximum temperature between the two gases

reaches nearly 100°C near the center of the plume. With the approach to

ignition, the temperature of the upper part in the plume increases rapidly in

the case of air. These results are consistent with the characteristics of the

growth rate of the vapor plume shown in Figure 14. From an examination of

Figure 14 and Figure 15(a), (b) and (c), it is reasonable to consider that

there are two global regimes of gas phase chemical reactions with air: one is

that of slow, rather steady reactions starting at an early stage with a

41
temperature up to about AOO ~ 450®C corresponding to constant growth of the

plume; the other is that of much faster chemical reactions occurring near

ignition with the temperature above A50 ~ 500°C, coinciding with rapid growth

of the plume. In the same manner as shown in the authors' previous paper for

solid fuels (2A) , this classification also confirms the important role of

chemical reactions and their complexity, similar to the multi-stage chemical

reaction regimes prior to ignition reported by Gray, et al . (26). It should

be noted, especially, that at 158 msec (just before ignition), the highest

temperature region (near 800°C) exists near the top center part of the fuel

vapor plume, where the fuel concentration is between 1 and 5%. Since this

range of concentration is within the flammability limits of decene (0.8% and

5% at room temperature, as predicted from (27), and wider at elevated temperatures), it is reasonable to expect that runaway chemical reactions will occur

in this region. If the visible emission follows the runaway chemical reactions, the first appearance of visible emission (ignition) is expected to be

in the shape of an umbrella. Subsequently, the flame propagates rapidly

through the part of the gas mixture that is within the flammability limit in

an outer envelope and resembles a flame envelope as shown In Figure 12.

Another Interesting aspect is the absorption of the incident CO

2 laser

beam by the fuel vapor in nitrogen atmosphere. As shown by the dashed lines

This raises the question of the accuracy of the measurement technique upon

the addition of oxidation products such as CO, CO2 and H

2O to the assumed two

molecule system. In Eq. (12), additional terms appear consisting of the mole

fraction of these molecules and their ref ractivitles. Molar ref ractivlties

for CO, CO2 and H2 O are 5.062, 6.720 and 3.80A at A88 nm and A. 991, 6.6A5 and

3.7A8 at 633 nm, respectively (23). Wavelength dependency of these values is

much closer to that of the surrounding gases used in this study than that for

1-decene. Also, concentration of these products would be small during the

Ignition period. Therefore, the effect of additional oxidative products on the fringe shift is expected to be small and the accuracy of the measurement

is the same as discussed in Section 3.2.

A2
in Figure 15, the temperature along the CO

2 laser beam reaches over 300°C in

the center of the plume. This greatly exceeds the fuel vapor temperature near

the liquid surface which is 10-30®C over the boiling temperature of 1-decene

(170.5°C). Since gas phase oxidation cannot occur, the temperature rise

observed in the center of the plume is due to absorption of the CO

2 laser

beam, a conclusion consistent with the previous studies (28). This implies

that the absorption of the CO

2 laser radiation by the fuel vapor cannot be

neglected, and it appears that this heating is the key process for raising the

gas phase temperature high enough to initiate the first slow global chemical

reactions in the fuel vapor.

4. DEVELOPMENT OF ABSORPTION COEFFICIENT MEASUREMENT

TECHNIQUE AT ELEVATED TEMPERATURES

4.1 Background

As described in the previous sections and our previous studies (28,29),

it was found that one of the key mechanisms for Ignition of liquid fuels by a

CO

2 laser is absorption of the incident CO2 laser beam by evolved fuel vapor

in the gas phase above the irradiated surface. Since the boiling temperatures

(150 200°C) of liquid fuels are much lower than the decomposition temperatures of solid fuels (300 ~ 600®C), the supplemental gas phase temperature

increase caused by direct absorption of the incident laser energy for ignition

of liquid fuels is critical in raising the gas phase temperature enough to

Initiate gas phase oxidation reactions and to reach a runaway condition. The

amount of the absorption is determined by the absorption coefficient of the

fuel vapor at the laser wavelength. There is extensive published absorption

43
data for CO, CO

2 and H

2 O (30,31), but those for hydrocarbon molecules are

limited to methane (31,32), Data on elevated temperature absorption spectra

for large hydrocarbon molecules, which are liquids at room temperature, are

extremely limited. Therefore, the measurement of the Infrared absorption

spectra of liquid fuel vapors at elevated temperatures is essential in order

to estimate the amounts of absorbed energy and temperature increase that

Influence the Ignitability of liquid fuel vapors,

4,2 Experimental Apparatus

A single beam infrared absorption measurement system has been

constructed; a schematic illustration is given in Figure 16 and a photograph

Is shown in Figure 17, The system consists of a well regulated black body

(~1070°C) as a radiant source, a heated cell with a water jacket (cell length

is 15 cm, and a monochromator with a pyroelectric detector. The system

provides low resolution spectra which might not be as accurate as using a CO2

laser as the light source due to the narrowness of the laser line. However,

the system can measure absorption spectra from 2 to 11 pm using three different snap-in type gratings. This can cover various different kinds of laser

lines such as deuterium flouride, carbon dioxide, hydrogen fluoride, using the

same light source. The cell can be heated to 600°C, Since ambient water

vapor and carbon dioxide are good absorbers in the wavelength range of

Interest, the system is enclosed in an air-tight, water-cooled box, and waterpumped, dry nitrogen is purged through the box and the monochromator during

the measurement. At first, the reference signal is measured by scanning the

monochromator with high purity nitrogen in the heated cell at the desired

elevated temperature. The signal is stored in a laboratory computer. Then,

44
the sample signal is measured by repeating the same scan with an appropriate

concentration of a test gas brought to one atmosphere pressure by nitrogen in

the cell at the same temperature as the first scan. The absorption spectrum

can be calculated from the difference in the signal between the two scans.

Preliminary experiments with CO2 gas at elevated temperatures are in progress

as a basis for comparison with previous studies. At present, a vapor generator for liquid fuels is under construction. Absorption spectra of decane and

1 -decene at various temperatures and concentrations will be measured in the

future.

5 . SUMMARY

The radiative ignition mechanism of a liquid fuel by a CO2 laser has been

investigated by observing liquid behavior near the surface and measuring the

distributions of temperature and fuel vapor concentration in the gas phase

during the ignition period. The conclusions obtained from these experiments

are as follows:

High speed photographs reveal complex behavior of the liquid surface

immediately after the incident CO2 laser irradiation. In time sequence, the

formation of a radial wave, a central surface depression, bubble nucleation/

growth/bursting, followed by complex surface motion and further bubbling, were

observed; typically several (or many) bubble cycles precede ignition. It

appears that volumetric liquid expansion and photon pressure cause the initial

radial wave movement, and the subsequent surface depression is caused by the

reaction force of plume-like vaporization. Soon the sub-surface layer of the

liquid has achieved appreciable superheating which results in bubble

nucleation probably on heterogeneous nuclei.

45
There are clear trends of behavior with increasing incident laser flux or

fuel absorptivity; increases in either accelerate the whole sequence of

events, lead to a dominance of bubbling phenomena and decrease the ignition

delay. An order of magnitude analysis indicates that the motion in the liquid

caused by temperature gradients does not have significant affects on the

vaporization process at least in the high end of the flux range In this study.

There are three idealized limiting cases of vaporization behavior that

facilitate analysis of the gas phase vapor build-up and subsequent ignition.

The first Is the purely stable, non-turbulent vaporization case in which a

bubble never occurs. The second limiting case is that of very low frequency

bubbling superposed on the stable plume vaporization, which appears at medium

laser fluxes. The third limiting case corresponds to high frequency, large

bubble bursts. To the gas phase this presents a roughly constant, nondirectional vapor source. These limiting cases are more amenable to at least

approximate theoretical analysis.

An increase in oxygen concentration in the environmental gas phase

decreases the distance between the liquid surface and the location of the

first appearance of visible emission which is defined as ignition in this

study. However, the distance is independent of an increase in peak laser flux

within the range used in this study. Oxygen in the gas phase significantly

increases the growth rate of the plume shortly after the appearance of fuel

vapor at the same peak laser fluxes.

Prior to ignition, there are two global chemical reaction stages in the

gas phase, similar to those for a solid fuel: one is a relatively slow

46
process starting at an early stage after the appearance of the plume with

temperatures up to about 400-450°C; the other is a significantly faster

process occurring just before ignition at temperatures above 450-500°C.

A significant temperature increase (more than 100°C) was found in the

center of the plume in a nitrogen atmosphere, and is attributed to the

absorption of the incident CO

2 laser beam by fuel vapor.

6 . ACKNOWLEDGMENT

The authors would like to thank Mr, William H, Wooden for his assistance

in conducting experiments.

47
7. REFERENCES

1. Kashlwagl, T, "Ignition of a Liquid Fuel Under High Intensity

Radiation," Comb. Sci. Tech. 21, p. 131 (1980).

2. Kashiwagi, T. , Baum, H, R. , and Rockett, J. A. "Ignition of a Liquid

Fuel Under High Intensity Radiation," AFOSR-TR-80-0476 (1980).

3. Bergman, T. and Mesler, R. "Bubble Nucleation Studies, Part 1:

Formation of Bubble Nuclei in Superheated Water by Bursting Bubbles,"

AIChE J., 27, p. 851 (1981).

4. Carroll, K, and Mesler, R. "Bubble Nucleation Studies, Part 2: Bubble

Entrainment by Drop-Formed Vortex Ring," 27, p. 853 (1981).

5. Texas A and M University, Selected Infrared Spectral Data , American

Petroleum Institute Research Project 44, Vol. 3 (1977),

6. Handbook of Chemistry and Physics , Edited by R, C, Weast, The Chemical

Rubber Co. Press (1969).

7 . Handbook of Tables for Applied Engineering Science , The Chemical Rubber

Co. Press, p. 388 (1973).

8. Cole, R. "Boiling Nucleation" in Advances in Heat Transfer

(J. P. Hartnett and T. F. Irvine, Eds.), Academic Press, Vol. 10 (1974).

48
9.

Blander, M. and Katz, J, L, "Bubble Nucleation in Liquids," AIChE J.

,

21, p. 833 (1975).

10, Reid, R. C, "Superheated Liquids," American Scientist, Vol. 64, p. 146

(1976).

11, Porteous, W, and Blander, M. "Limits of Superheat and Explosive Boiling

of Light Hydrocarbons, Halocarbons, and Hydrocarbon Mixtures," AIChE J,

,

Vol. 21, p. 560 (1975).

12, Ward, C, A., Balakrishnan, A., and Hooper, F. C. "On the Thermodynamics

of Nucleation in Weak Gas-Liquid Solutions," J. Basic Eng,, D92, p. 695

(1970).

13, Mori, Y, , Hljikata, K, , and Nagatanl, T, "Effects of Dissolved Gas on

Bubble Nucleation," Int. J, Heat Mass Transfer, 19, p. 1153 (1976).

14, Lauterborn, W, and Bolle, H, "Experimental Investigations of CavitationBubble Collapse in the Neighborhood of a Solid Boundary," J. Fluid Ifech.

,

72, p. 391 (1975).

15, Hsu, Y. Y. and Graham, R. W, Transport Processes in Boiling and TwoPhase Systems , McGraw-Hill Book Co,, (1976),

16, Murad, R. J, , Larendola, J, , Isoda, H. , and Summerfield, H. "A Study of

Some Factors Influencing the Ignition of a Liquid Fuel Pool," Comb,

Flame, 15, p. 289 (1970).

49
17. Ohlemiller, T. J. and Summerf ield, M. "Radiative Ignition of Polymeric

Materials in Oxygen/Nitrogen Mixture," Thirteenth Symposium

(International) on Combustion, The Combustion Institute, p. 1087 (1971),

18. Mutoh, N. , Hirano, T, , and Akita, K. "Experimental Study of Radiative

Ignition of Polymethylmethacrylate," Seventeenth Symposium

(International) on Combustion, The Combustion Institute, p, 1183 (1978).

19. El-Wakil, M, M, and Ross, P. A, "A Two-Wave length Interferometric

Technique for the Study of Vaporization and Combustion on Fuels," Liquid

Rockets and Propellants , Progress in Astronautics and Rocketry, Vol. 2,

Academic Press, New York (1960).

20. Maylnger, F, and Panknin, W, "Holography in Heat and Mass Transfer,"

Proceeding of Fifth International Heat Transfer Conference, Vol, 6, p, 28

(1974).

21. Mayinger, F. and Panknin, W, "Holographic Two-Wave lengths Interferometry

for Measurement of Combined Heat and Mass Transfer," Combustion

Measurements (R. Goulard, Ed.), Academic Press, New York, p. 270 (1976).

22. Vest, C, M, Holographic Interferometry , John Wiley and Sons, New York,

Chapter 6 (1979).

23. Gardiner, W, C., Hidaka, Y, , and Tanzawa, T. "Ref ractivity of Combustion

Gases," Combust, Flame, 40, p. 213 (1981),

50
24, Kashiwagl, T. and Kashiwagi, T. "A Study of the Radiative Ignition

Mechanism of a Solid Fuel Using Holographic Interferometry."

AIAA-82-0238, AIAA 20th Aerospace Sciences Meeting, Orlando, Florida,

January 1982,

25, Ohlemiller, T, J, and Summerfield, M, "Radiative Ignition of Polymeric

Materials in Oxygen/Nitrogen Mixtures," Thirteenth Symposium

(International) on Combustion, The Combustion Institute, p, 1087 (1971),

26, Gray, P. , Griffiths, J. F., Hasko, S. M,, and Llgnola, P, G, Proc. Roy.

Soc. London A374, p. 313 (1981),

27, Zabetakis, M, G, "Flammability Characteristics of Combustible Gases and

Vapors," Bulletin 627, Bureau of Mines, 1965,

28, Kashiwagi, T, Combust. Flame 34, p. 231 (1979).

29, Kashiwagi, T. "Radiative Ignition Mechanism of Solid Fuels," Fire

Safety J. 3, p. 185 (1981).

30, Hottel, H. C, and Sarofim, A, F, Radiative Transfer , McGraw-Hill Book

Co., Chapter 6, 1967.

31, Edwards, D, K. and Balakrishnan, A. "Thermal Radiation by Combustion

Gases," Int, J, Heat Mass Transfer, 16, p. 25 (1973).

51
32, Lee, R. H, C, and Happel, J, "Thermal Radiation of Methane Gas," I/EC

Fundamentals 3, p. 167 (196A).

52
o

ca Co

•H

4-> CO>M

(U

CO -QO

53

Fig.

1.

Schematic

illustration

of

experimental

apparatus

for

liquid

behavior

and

vapor

generation
15mm

aiuji o03 CN (/) E

54

Fig.

2.

Top

view

direct

picture

and

side

view

schlieren

picture;

lef

columns,

top

view;

right

columns,

side

view.

Decane,

laser

flux

of

740

W/cm^,

incident

laser

angle

of

30

degrees.

A,

appearance

of

hole

first

bubble

in

top

view;

B,

corresponding

sudden

release

of

liquid v

Time

gap

about

30

msec
15mm

Fig. 3a. Top view picture, decane, laser flux of 410 W/cnt^ and incident laser

angle of 30 degrees; B, appearance of hole due to first bubble. Time gap,

about 350 msec 55
— —

j 17mm U —

IwHHMMi IIHHHHHf

mHh flBHI

MM MBM

- f

«• »

Fig. 3b. Side view schlieren picture, same test as Figure 3a. A, appearance

of laminar vapor plume; E, sudden release of liquid vapor by bubble burst

56
Fig. 4. Side view closeup direct picture near the decane surface, laser flux

of about 2500 W/cm^ with incident laser angle of 30 degrees from right to left

57
Time

Fig. 5. Side view direct picture to show the location of

of flame. Decane at a laser flux of about 2500 W/cm^ with

angle of 30 degrees from left to right

L

the first appearance

incident laser

58
15mm

Spreading

wave

Central

depression

Bubble

Time

gap

Fig. 6. Top view picture of decene with laser flux of 910 W/cm^ and incident

laser angle of 90 degrees. Time gap is about 18 msec

59
Non-uniform flux

t = 0

Gas

y

7’7 / / / / /

-y

Liquid

Wave induced by

photon pressure

and liquid

expansion

Laminar vapor

plume formation as boiling point

is approached

\

/

Superheating in cross-hatched

region

Bubble nucleation

in superheated

layer

Bubble grows and \

bursts quickly, I destroying most

| of thermal layer '

60

1 *-T

Temp, distribution

, , ^oc below beam center

X

X

TTb

T

Fig.

7.

Idealized

sequence

of

bubble

development;

the

sequence

repeats

with

continuing

irradiation

but

the

starting

condition

is

no

longer

as

simple

as

that

in

the

first

sketch
AIRSUSPENDED

OPTICAL

BENCH

61

Fig.

8.

Schematic

illustration

of

experimental

set-up

of

high

speed

two-wavelength

holographic

interferometer
DATA ANALYSIS SCHEME

Fig. 9. Data analysis scheme from 16 mm movie frames

62
63

reconstructed

by

computer

for

blue

light

with
64

Fig.

11.

Computer

calculated

contour

plot

of

peaks

of

fringes,

1-decene

and

blue

light
start

of

CO2

laser

irradiation 0 CO IS 3B <«Ou0 3p

5. HH0 E£ 0iTl

ijb,.

p P V-l

*> : .•4?< 3 3 3 ^ 3 P 3 'H

(3S oEo

CO

CM

COCM 0) 6 M o 3

•u 12 o

•H

O

O.VO

CM co

•H

4-) OBB 4-1 cdJ-4 }-< bO 01 O CO M CO 0) m

<U cO 4J (U

Cl cu 2

Q.

7-* CD

B ^ ^ B

Q£ 4J W 00 5 o c^CO

CO vO

•p iH r3 3

CO bO 0) 3 3 3

r—I

tH 0) CO > O CO •H ;? D- p

Eh cO '3 • 3 3 O

CMO

iH <U

CO

bi) 3

•H 3 t

65
E

E

Fig. 12b. Typical results of the interferogram motion pictures made at 500 frames

per second at wavelength 632.8 nm, peak laser flux at 260 W/cm^ with 40% ©2

/ 60% N2

.

66
46

msec

after

CO2

laser

irradiation

67

PEAK

LASER

FLUX

260

W/cm2,

1-DECENE, N2

Fig.

12c.

Typical

results

of

the

interf

erogram

motion

pictures

made

at

500

frames

per

second

at

wavelength

632.8

nm,

peak

laser

flux

at

260

W/cm^

with N2.
o>

CDo%

raS.

Q. <cLoro

O) oCJ

•c

ra

*-> c/3

A'

<c

IS

OJ-l CO

4-1 T3 c c 03 o B o c

03 o

03 )-l •1^

>-> >

03 c

CX 03

03

03 •H B CO CO

Ui -C CM 4J O 3 Ou~i CMe

4-J

U

CO 3

03 •o

O

CO CM B

03 4-1 03 CO

)-i 3 X

4-1

D

u r-4 •w U-l ex

Xi e 03 o 03 •H CO

4-1 OE

CO

G 03 CO ex

)-l bO

—

OB

l-i

C

03

<4-1 C30

l-i • 03 CM

4-1m

C vO M

4-1 • bOc

03 • 03 bC >

•H CO

(Xi 3
520

W/cm

69

TIME

AFTER

LASER

IRRADIATION

(msec)

Fig.

14.

Comparison

in

the

growth

of

the

fuel

vapor

plume

height

with

three

different

environmental

gases

at

three

different

peak

laser

fluxes
1-DECENE

PEAK

LASER

FLUX

AT

260

W/cm

70

Fig.

15.

Comparison

of

temperature

and

fuel

vapor

concentration

distributions

between

air

and

nitrogen

environments

at

(a)

120

msec,

(b)

140

msec,

(c)

158

msec

from

the

start

of

the

CO?

laser

irradiation,

ignition

delay

time

160

msec,

peak

laser

flux

at

260

W/cm^.
7

1. BLACK BODY

2. CHOPPER

3. CONCAVE MIRRORS

4. HEATED CELL WITH

WATER COOLED JACKET

5. WATER COOLED SHIELDS

6. FLAT MIRROR

7. MONOCHROMATOR

8. PYROELECTRIC DETECTOR

9. WATER COOLED SLIT

Fig. 16. Schematic illustration of infrared absorption spectra measurement

apparatus

71
Picture

of

infrared

absorption

spectra

measurement

apparatus
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BIBLIOGRAPHIC DATA

SHEET fSee /nstructions)

1. PUBLICATION OR

REPORT NO. NBSIR 83-2689

2. Performinj Or(zui. Report No. 3. Publication Date May 1983

4. TITLE AND SUBTITLE

Thermal Radiative Ignition of Liquid Fuels by a CO

2

Laser

5. AUTHOR(S)

Takashi Kashiwagi, Thomas J. Ohlemiller, Takao Kashiwagi and Walter W. Jones

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

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No

AFOSR-ISSA-82-00014

S. Type of Report & Period Covered

Final progress report

Oct. 1, 1979-Sept. 30, 1 >82

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

Air Force Office of Scientific Research

Building 410, Bolling Air Force Base

Washington, DC 20332

10.

SUPPLEMENTARY NOTES

I I 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

bibliography or literature survey, mention it here)

This report summarizes p^rogress in the study of the ignition mechanism

of a liquid fuel by a CW CO 2 laser; the period covered is from October

1, 1979 to September 30, 1982. It describes (1) new observations of

liquid fuel behavior near and at the liquid/air interface during the

laser irradiation with incident fluxes from 260 to 2500 W/cm^, (2) new

time-resolved measurements of distributions of temperature and vapor

concentration in the gas phase using a newly-developed, high speed,

two-wavelength holographic interferometer and (3) the development of a technique to measure infrared absorption spectra of fuel vapors at

elevated temperatures.

12.

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

absorption; decanes; decenes; electromagnetic absorption; holographic interferometry;

ignition; infrared radiation; vaporization

13.

AVAILABILITY

Jj^J Unlimited

I I For Official Distribution. Do Not Release to NTIS

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

20402.

Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

84

15. Price

$11.50

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