Development of a Self-Atomizing Form of Water 

Next Generation Fire Suppression Technology Program 

Project 4D/4/7 

Final Report to the Strategic Environmental Research and Development Program 

April, 1998 

Richard K. Lyon 

EER, Inc. 
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The goal of this project was to do a laboratory-scale proof of 

concept evaluation of the use of novel type of ice-like materials 

(carbon dioxide hydrate and/or other gas hydrates) as fire 

fighting agents. 

fire fighting agent, it would also have the advantages of being 

clean, environmentally friendly and user safe. Specific 

milestone goals were the demonstration that hydrates are capable 

of self atomization and that this self atomization could be 

delayed briefly. 

milestones were achieved,! carbon dioxide hydrate does not appear 

to be a promising fire klghting agent and no further development 

is recommended. 

If carbon dioxide hydrate were an effective 

While the test results show that these 

15. NUMBER OF PAGES '. susJEcTcf@!"Shydrate, fire, halon, water mist 

16. PRICE CODE 
EXECUTIVE SUMMARY 

A OBJECTIVE 

Broadly the goal of this project was to do a laboratory-scale 

proof of concept evaluation of the use of carbon dioxide hydrate 

and/or other gas hydrates as fire fighting agents. Specific 

milestone goals were the demonstration that hydrates are capable 

of self atomization and that this self atomization could be 

delayed briefly. 

B BACKGROUND 

Halons provide fire protection in many U.S. weapons systems 

currently in service but production of halons has been banned, 

making it necessary to find suitable replacements. Gas hydrates 

such as C02 hydrate are an unusual form of ice, stable only under 

high pressure. Since gas hydrates are unstable at ambient 

pressure, it was speculated that a gas hydrate, when suddenly 

discharged to ambient pressure, might undergo self atomization, 

might disintegrated into fine mist. It was further speculated 

that this fine mist would be an effective fire fighting agent and 

that the self atomization might be delayed, allowing particles of 

hydrate to be thrown substantial distance toward a fire. 

While this research project involved substantial risks that these 

speculations might be incorrect, there was the advantage that C02 

hydrate is obviously user safe and environmentally friendly. 

Thus the risk of discovering an effective fire fighting agent and 

then being unable to use it is avoided. 

C SCOPE 

Section 1 provides background information. A brief theoretical 

examination of the use of fine mists to extinguish fires is given 

in Section 2. The experimental methods used in this research are 

summarized in Section 3. Sections 4 and 5 respectively 

summarizes and discuss the experimental results of this study. 

Section 6 reports the conclusion of this examination and makes 

recommendations. 

D METHODOLOGY 

Theoretical and experimental studies included heat transfer 

analysis video tape analysis of the hydrate discharge process, 

low temperature recovery and examination of hydrate samples at 

ambient pressure and testa with two types of obstructed fires. 

E RESULTS 

The low temperature recovery experiments graphically demonstrated 

that gas hydrates can be metastable, i.e. they can exist briefly 

ii 
at ambient pressure even though they are thermodynamically 

unstable at ambient pressure. The results of the video tape 

experiments indicated that C02 hydrate can undergo delayed self 

atomization, though the efficiency of this process was not 

determined. Tests with two types of obstructed fires did not 

show C02 hydrate to be an effective fire fighting agent. 

F CONCLUSIONS AND RECOMMENDATIONS 

The failure of C02 hydrate to extinguish fires even though it 

does appear to self atomize, may mean that the self atomization 

was poorly efficient. Alternatively it may mean that, for the 

test conditions used, the discharge process allowed the hydrate 

to produce fine mist in the general vicinity of the flame but did 

not provide momentum to carry the mist into the flame. Whatever 

the reason for the failure gas hydrates do not appear to be 

promising as potential replacements for halons. Further research 

based on this approach is not recommended. 

iii 
TABLE OF CONTENTS 

Section 

1. 

1.1 

1.1.1 

1.1.2 

1.1.3 

1.1.4 

1.2 

1.3 

2. 

3. 

3.1 

3.2 

3.3 

3.4 

3.5 

4. 

4.1 

4.2 

4.3 

4.4 

4.5 

5. 

5.1 

5.2 

5.3 

6 

Title 

INTRODUCTION 

BACKGROUND 

DEFINITION OF THE PROBLEM 

CURRENTLY AVAILABLE FINE MIST TECHNOLOGIES 

OTHER APPROACHES TO FINE MIST PRODUCTION 

THE APPROACH TO FINE MIST PRODUCTION 

FOLLOWED IN THIS RESEARCH 

OBJECTIVE 

SCOPE 

THEORETICAL ANALYSIS 

EXPERIMENTAL PROCEDURES 

SAMPLE PREPARATION 

VIDEO CAMERA STUDIES 

CANDLE IN THE WIND EXPERIMENTS 

TESTS WITH AN OBSTRUCTED FIRE 

AMBIENT PRESSURE RECOVERY OF HYDRATES 

RESULTS 

DISCHARGE CHARACTERISTIC OF C02 HYDRATE 

VISUAL OBSERVATIONS 

RESULTS OF THE CANDLE EXPERIMENTS 

RESULTS FOR OBSTRUCTED FLAMES 

OBSERVATIONS ON HYDRATE SAMPLES 

RECOVERED AT AMBIENT PRESSURE 

DISCUSSION 

METASTABILITY OF GAS HYDRATES 

ANALYSIS OF VISUAL OBSERVATIONS 

FIRE EXTINGUISHING TESTS 

CONCLUSIONS AND RECOMMENDATIONS 

Page 

1 

1 

1 

2 

4 

4 

5 

6 

7 

a 

a 

a 

8 

10 

10 

12 

12 

12 

12 

14 

14 

14 

14 

15 

15 

17 

iv 
LIST OF FIGURES 

FIGURE 1 Droplet Evaporation Time in IIOOOC Gas 

FIGURE 2 Droplet Terminal Velocity 

FIGURE 3 Efficiency of Droplets for Removing 

Heat from a Flame's Reaction Zone 

FIGURE 4 Experimental Setup for Testing the Relative 

Effectiveness of Fire Extinguishing Agents 

FIGURE 5 Experimental Setup for Testing Agents 

against and obstructed flame 

FIGURE 6- 

FIGURE 7 Extinction of a Candle Flame by Halon 1211 

Extinction of a Candle Flame by C02 

FIGURE 8 Spray breakup lenghts and regimes for 

nonturblent round liquid jets in still gas. 

FIGURE 9 Spray breakup regime transitions 

Page 

3 

9 

11 

13 

13 

16 

16 

V 
1. INTRODUCTION 

1.1 BACKGROUND 

1.1.1 DEFINITION OF THE PROBLEM 

At one time the fact that halons were highly effective, clean, and 

user safe led to their acceptance as a virtually ideal method of 

extinguishing fires. While cost considerations limited the use of 

halons in some fire protection applications in the civilian sector, 

military acceptance of halons was common. Weapon systems were 

designed in which the use of halons for fire protection was assumed. 

This led to the construction and deployment of weapons systems in 

which retrofitting carbon dioxide or some other fire protection system 

would be difficult and expensive if not impossible. 

The recognition that halons contributed to the destruction of the 

ozone layer led to an international treaty banning their production. 

Since the treaty did allow the use of existing stocks of halons, the 

impact of this ban on the U.S. military was, in the short term, quite 

manageable. All nonessential uses of halons were eliminated and 

halons were recovered and recycled whenever possible. Even with 

strict conservation, however, weapons and other systems presently in 

use will, during their useful lifetimes, need much larger amounts of 

halon than the existing stocks. 

Thus the problem of finding a "suitable replacement" for halons 

involves a number of difficult requirements. The new fire 

extinguishing agent must be: 

basis. 

1) Effective, comparable to halons on both a weight and volume 

2) llCleanlf in that the extinguishing agent must be harmless to 

valuable materials and supplies. Since damage to 

electrical/electronic equipment is a critical issue, the new 

extinguishing agent needs to be nonconducting. Avoiding damage to 

machinery such as jet engines is also important and hence the agent 

also needs to be nonabrasive. 

3) Environmentally friendly, i.e. there must be a substantial 

body of evidence indicating, if not absolutely proving that the new 

fire extinguishing agent causes little or no harm to local ecosystems, 

and contributes virtually nothing to ozone depletion and global warming. I 

4) Nontoxic, i.e. the agent must not cause significant acute 

toxic effects when used properly and there must be a substantial body 

of information indicating little or no reason to be concerned about 

3 
the long term effects of exposure to the agent. 

These are, of course, conflicting requirements. If, for example, a 

proposed new extinguishing agent is not chemically inert, it may be 

very difficult to prove that human exposure to the agent does not 

cause some unfavorable long term health effect. If it is chemically 

inert, then it will accumulate in the atmosphere and contribute to the 

Greenhouse Effect. 

1.1.2 CURRENTLY AVAILABLE FINE MIST TECHNOLOGIES 

The discussion above suggests that a new fire extinguishing agent will 

be unsuitable as a halon replacement if it is not chemically inert and 

if it is. One possible way around this dilemma might be to avoid 

llnew" fire extinguishing agents, i.e. to limit the search to materials 

whose effects on people and the environment are well known. Distilled 

water, for example, is not electrically conductive, is nonabrasive, 

environmentally friendly, and nontoxic. Thus if one could find a way 

to greatly improve the effectiveness of water for extinguishing fires 

this could be a satisfactory solution to the problem. 

There has been a substantial amount of R&D seeking to improve the 

effectiveness of water as a fire fighting agent. At the time that 

halons were developed it was recognized that fine mists of water were 

also a very effective fire fighting agent. As illustrated by the 

calculations shown in Figure 1, the evaporation time of a water 

droplet in a hot environment decreases with decreasing droplet 

diameter. The large droplets generated by the classic fire hose have 

an evaporation time that is much longer than the time they spend 

passing through a flame. Thus they do not remove heat directly from 

the flame but rather do so indirectly by cooling the flame's 

surroundings. This, of course, is inefficient and requires using 

large quantities of water. Conversely a flame is vulnerable to 

cooling by small droplets of water provided one has a means of qetting 

the mist into the flame. 

That, however, has been a problem. Figure 2 shows the calculated 

terminal velocity of droplets settling in air as a function of size. 

Comparing Figures 1 and 2 one sees that droplets small enough to 

effectively remove heat from a flame are also small enough that they 

settle quite slowly. 

can be given is so small and the frictional forces acting on them so 

large that they can not be thrown any great distance. 

As discussed by Tatem et. a1.l 

commercially available,for fire protection. 

Similarly the amount of momentum small droplets 

a number of "fine mist" systems are 

Within these systems the 

'P. A. Tatem, C. Beyler, P. J. DiNenno, E. K. Budnick, G. G. 

Back, S. E. Younis, A Review of Water Mist Technology.for Fire 

Suppression, NRL/MR/6180--94-7624 (AD-A285 738) 

2 
Figure 1 Droplet Evaporation Time 

in 1100 C Gas 

t. milliseconds iooooo 

0.1 ' I t,lIU 

0.1 1 10 100 io00 10000 

Droplet Diameter, microns 

Heat Transfer Coefficient taken 

as 0.0142 watts/cmsqu C 

Figure 2 Droplet Terminal Velocity 

loo> 

1 10 100 

Droplet Diameter, microns 

3 

1000 
solution commonly adapted to the problem of getting the fine mist to 

the fire is to generate a mixture of coarse and fine droplets. 

former can be given enough momentum to carry them to the fire and they 

tend to drag the latter in their wake. 

While this approach allows extinguishing fires with much less water 

than would be required with a conventional fire hose system, the 

amount of water used is still quite large compared with a halon 

system. 

Further improvement seems possible in light of the experiments of B.J. 

McCaffrey’. 

natural gas diffusion flame was examined. 

mist to the flame was avoided by placing the atomizer at the base of 

the flame. Since the natural gas into which the mist was injected all 

went into the flame so did the mist. Mass flow rates of mist 

approximately equal to the flow rate of the fuel were found sufficient 

to extinguish the flame, i.e. in this instance fine mist was fully as 

effective as a halon. 

1.1.3 OTHER APPROACHES TO FINE MIST PRODUCTION 

One possible approach to the problem of getting the fine mist to the 

flame is to generate the mist electrostatically. Since flames are 

electrically conductive, they tend to ground themselves, causing the 

electrically charge mist to be attracted to them. Work along these 

lines is currently being done by Dr. Kelly and coworkers at ZYW Corp, 

Princeton, NJ and by Dr. Berman at AeroChem Princeton, NJ. 

The 

In these experiments the effect of fine water mist on a 

The problem of getting the 

1.1.4 THE APPROACH TO FINE MIST PRODUCTION FOLLOWED IN THIS 

RESEARCH Gas hydrates can form with either of two structures. In 

both of these structures the essential unit is a pentagonal 

dodechedron of water molecules, the gas molecule occupying the space 

in the center of the dodechedron. This structure owes its stability 

to the fact that each of the twenty apices is occupied by an oxygen 

atom bonded to four hydrogen atoms at close to tetrahedral angles, 

i.e. it maximizes hydrogen bonding. 

In the type I structure there are 2 smaller (a, = 3.94A) and 6 larger 

cavities (a2 = 4.30A)for every 46 water molecules. For type I1 there 

are 16 smaller cavities (al = 3.91A)and 8 larger cavities (a2 = 

4.73A)for every 136 water molecules. The type I1 hydrate forms only 

with gas molecules that are too large for the cavities in the type I 

structure. While the number of cavities has a definite stoichiometry, 

the extent to which gas molecules occupy those cavities can vary over 

a significant range. Furthermore mixed hydrates with two or more 

different types of qas klecules fillins the cavities can readily be 

formed . For- the case 

2B. J. McCaf f-rey ,. (19889) 

of a mixed hydrate-of a large molecule such-as 

Combust. Sci. and Tech., 63, 315-335 

4 
CHC13 and a small molecule such as C02, the large molecule occupies 

some of the large cavities and none of the small cavities while the 

small molecule occupies some of both. 

At O°C a pressure of 12.9 atmospheres is necessary to form the C02 

hydrate from pure water. 

needed to form the hydrate increases. 

needed and at greater temperatures no hydrate can be formed by C02 

alone. Mixed hydrates containing substantial amounts of C02 can, 

however, be formed at higher temperatures by using a second gas with a 

hydrate forming ability greater than C02. 

There do not appear to be any reports in the literature as to what 

happens when a gas hydrate is suddenly depressurized. It is, however, 

interesting to note that water under extreme pressure can form 5 solid 

phases in addition to the familiar ice. One of these, Ice 111, is 

blue. While Ice I11 is thermodynamically stable only at pressures in 

excess of 2040 atmospheres, it can be cooled to liquid nitrogen 

temperature, and then be depressurized. At one atmosphere and very 

low temperature, Ice I11 is metastable. It can last for an extended 

period of time if kept cold but it disintegrates into a fine powder of 

ordinary ice on contact with anything warm. 

From a thermodynamic viewpoint the decomposition of a C02 hydrate 

offers two quite different cases. 

from gaseous C02 and liquid water is almost exactly the same as the 

heat for forming ice from the water. 

an initial temperature below O°C is adiabatically depressurized, it 

becomes unstable with respect to decomposition to ordinary ice and 

gaseous C02 with very little temperature change occurring during this 

decomposition. This case would appear to be quite similar to the case 

of Ice I11 in which considerable metastability was possible, i.e. both 

involve the conversion of one solid to another without the formation 

of a liquid phase. 

If, however, the initial temperature is greater than O°C, the hydrate 

becomes unstable with respect to adiabatic decomposition to gaseous 

C02 and a mixture of liquid water and ice, the temperature falling 

during the decomposition to O°C. Since the liquid water can act as a 

catalyst for the conversion of the solid hydrate to ordinary ice the 

hydrate seems less likely to be metastable in this case. 

With increasing temperature the C02 pressure 

At 9.9OC 43.5 atmospheres is 

The heat of forming a C02 hydrate 

Thus, if a pure C02 hydrate at 

1.2 OBJECTIVE 

The goals of this project was to demonstrate that hydrates are capable 

of delayed self atomization and that they are consequently potentially 

useful as replacements for halons in fire fighting. I 

5 
1.3 SCOPE 

Section 1 provides background information. 

examination of the use of fine mists to extinguish fires is given in 

Section 2. The experimental methods used in this research are 

summarized in Section 3. Section 4 summarizes the experimental 

results of this study. Section 5 examines the scientific and 

engineering implications of these results. 

Section 6 reports the conclusion of this examination and makes 

recommendations. While no further work along these lines appears 

justified a possible alternative approach is suggested. 

A brief theoretical 

6 
2. THEORETICAL ANALYSIS 

combustion gases. While the 

evaporation of the droplet removes 

heat from the precombustion gases in 

from postcombustion gases leaving 

the flame, and from the flame 

the temperature gradient region, 

reaction zone itself, in first 

approximation only the latter is 

droplet in extinguishing the flame 

will be proportional to the fraction 

of total heat it removes which comes 

from the reaction zone. 

useful for extinguishing the flame, 

i.e. the effectiveness of the 

The theory of evaporation of small droplets in the vicinity of a flame 

involves a complex set of issues that are beyond the scope of this 

report. There is, however, some useful insight to be gained by going 

beyond the very simplified discussion given above. Let us assume the 

following situation: a droplet of diameter D and initial diameter Do 

travels toward a flame at a velocity of v. Initially the gas through 

which the droplet travels is cool. The flame has a temperature of 

14OOOC. As the droplet approaches the flame it passes through a 

region of thickness a in which the temperature rises from 100°C to 

14OOOC. While the droplet passes though this region, it evaporates, 

its diameter decreasing at a rate of 

1) -dD/dt = 2KATjAH 

where K is the heat transfer coefficient, typically 0.0142 

wattsjcm2°C, 

100°C, and AH is the heat of evaporation of water, 2410 joules/cc. 

AT is the difference between the local temperature and 

0. m 

E o.ms 

.E 0.006 

g 0.004 

0.002 

0 

lqe+-lnN-mN?-rTT 

aaazzzzgggyy a0aLnlN- ULUU 

Droplet Size.cm 

I 

Figure 3 Efficiency of droplets 

for removing heat from a flame,s 

reaction zone, v,a = lo, 1, .l, 

-01 seconds 

Figure 3 shows the results of a set 

of calculations done with this model 

with the assumption that 

ria = 0.01. 

For any given value of via, the time for the droplets to pass through 

the temperature gradient, there is a droplet diameter which maximized 

efficiency of droplets,for removing heat from the flame's reaction 

zone. Droplets much smaller than this optimum value never reach the 

reaction zone and hence are completely ineffective. Those much larger 

have some effect but spend most of their cooling power uselessly in 

the postcombustion gases. 

7 
Thus the problem of making droplets effective for extinguishing fires 

does not appear to be a simple question of getting droplets of some 

"rightll size, but rather a matter of getting the droplet's size and 

the speed with which it enters the flame to match. Large droplets 

fail because they move much more rapidly than their optimum velocity 

while with small droplets it is difficult to get them to move at a 

velocity as high as their optimum. 

3. EXPERIMENTAL PROCEDURES 

3.1 SAMPLE PREPARATION 

500cc stainless steel sample bombs which were equipped with 1/4" ball 

valves (i.e. valves with Cv = 1.4).were used to prepare C02/water 

mixtures which were converted to hydrate. To prepare these mixtures 

the sample bomb was weighed, distilled water was transferred into it 

and it was reweighed. It was then attached to a vacuum line and 

evacuated briefly to remove air. The sample bomb valve was then close 

and the bomb was connected to a C02 cylinder that was equipped with a 

dip leg. After liquid C02 was added to the bomb, it was reweighed and 

placed in a refrigerator to convert the C02/water mixture to hydrate. 

3.2 VIDEO CAMERA STUDIES 

During tests the sample cylinders were held vertically with the valve 

pointed down. A length of 1/4" OD stainless steel tubing on the 

outlet of the ball valve bent to a right angle or to a U shape was 

attached to the outlet end of the ball valve. This allowed the 

discharge to be directed vertically or upward. A video camera was 

used to record what occurred. During subsequent analysis of the video 

tape the discharge time was measured with a stop watch while playing 

the tape at slow speed. Playing speed was measured by comparing stop 

watch readings with time as recorded on the tape by the video camera's 

clock. Distances were measured on the tape by using the PI'S height 

as a reference length. 

3.3 CANDLE IN THE WIND EXPERIMENTS 

Figure 4 shows the experimental setup used in this series of tests. 

In this setup an 0.875" diameter candle is placed inside a 

transparent plastic tube, 24" long by 4" in diameter. The top of this 

plastic tube was connected to a blower while the bottom was filled 

with vertical 4" lengths of pyrex tubing, 6mm OD, 4mm ID. The blower 

was used to pull a flo of air through this tube at a rate of 7.5 cfm, 

as flow straighteners. 

by a sample pump and send first through a rotameter then to a Fuji 

8 

i.e. at a velocity of 7 .43 ftlsec. The 4" lengths pyrex tubing served 

While the bulk of this air flow is vented, some of it is extracted 
.... ~ ..~. -. 

Exti ngui s hge r 

Disc ha rye 

Point 

I 

FIGURE 4 

Experimental Setup for Testing the Relative Effectiveness 

of Fire Extinguishing Agents 

ansparent 

Plastfc 

4cm gap 

I 

9 
NDIR C02 meter whose output was recorded. 

gases exiting the blower could be monitored. 

experiments with Halon 1211, 

an oxidation catalyst at 8OO0C (CuO on high surface area alumina) to 

convert the halon to CO?. 

catalyst is quite efficlent for oxidizing halons. 

Thus the C02 content of the 

the sample was passed through a bed of 

Previous EER studies have shown that this 

For comparison 

3.4 TESTS WITH AN OBSTRUCTED FIRE 

In a literature survey Tatem et. discussed tests of the 

commercially available fine water mist fire extinguishing systems. 

Some test procedures are uninformative, all systems being highly 

effective or highly ineffective. Tests with obstructed flames, 

however, proved quite informative, the percentage effectiveness of 

different systems varying over a substantial range. 

flame test setup a 1' by 1' square obstruction plate was placed 1' 

above a 6 cm diameter pan filled with diesel fuel. The pan was under 

the center of the obstruction plate and the extinguisher discharge 

point was directly over the center of the obstruction plate. 

The experimental setup shown in Figure 5 duplicates this description, 

although, of course, the tests reviewed by Tatem et. al. involve a 

continuous discharge system while the Figure 5 setup was used for 

sample which discharged in a single burst. 

In tests done with the Figure 5 setup the following procedure was 

used: a paper wick and 75 cc of diesel fuel were placed in the 6 cm 

cup. After igniting the wick the fire was allowed to burn for 3 

minutes,the ball valve was opened, and the result observed. 

3.5 AMBIENT PRESSURE RECOVERY OF HYDRATES 

To recover C02 hydrate a 1" OD stainless steel tube with swagelok 

fittings on its ends was used instead of the 500 cc sample bomb. 

Hydrate was prepared as above but then the sample was cooled to -15OC, 

depressurized, the swagelok fittings were undone, and the hydrate 

removed. 

To prepare and recover C3F8 hydrate the same procedure was used except 

that the preparation was done in a Fischer-Porter glass sample bomb. 

In one obstructed 

i 

3P. A. Tatem, C. Beyler, P. J. DiNenno, E. K. Budnick, G. G. 

Back, S. E. Younis, A Review of Water Mist Technology. for Fire 

Suppression, NRL/MR/6180--94-7624 (AD-A285 738) 

10 
kperimental Setup tor- testing Agents against 

an lsbstrcuted flame 

';ample 

Cyli Mer----- 

rlanual 

1 r'4 ball 

Air activated 

1 /4 ball valveUnist r ut 

\\ framewor 

I'bu 1'Aluminum 

\ Plate 

cup 

6cm 

ofdiexl 

diameter 

fuel 1 

--A 

I 

T 

11 

/ 

, 

/ 
4. RESULTS 

4.1 

Samples in which the ratio of C02 to water was low enough to leave a 

significant portion of the water unconverted to hydrate had reasonably 

satisfactory discharge characteristics, i.e. when the ball valve was 

opened the hydratejwater mixture was discharged in a more or less 

continuous flow. 

enough to leave little or no water unconverted did not show 

satisfactory discharge characteristics, i.e. when the ball valve was 

opened the hydrate was discharged in discontinuous spurts. 

DISCHARGE CHARACTERISTIC OF C02 HYDRATE 

Samples in which the ratio of C02 to water was high 

4.2 VISUAL OBSERVATIONS 

A blank experiments was done with 400 cc samples of water in 1000 cc 

sample bombs pressurized with He at 1500psi. This produced a spray 

jet with- a 0.35 second discharge time, 5m tall and roughly 15 cm wide 

at its peak. The droplets formed by this jet showed a readily 

observable settling rate indicating that they were relatively coarse. 

Qualitatively similar behavior was observed at He pressurization of 

1000 and 550 psi. At 228 psi He pressurization, however, the 

discharge resembled that of a common garden hose with little or no 

atomization. 

For a solution of 20 wt% CaC12 in water saturated with C02 at ambient 

temperature and 845 psi, the spray jet pattern was clearly different, 

3m tall and 50 cm wide. The droplets formed by this jet did not show 

an observable settling rate indicating that they were the desired fine 

mist. 

For samples of C02 hydratelwater mixture with an initial 

pressurization of 220 psi the discharge was similar in length and 

width to that of the sample which was pressurized with helium to 228 

but the discharge produced a mist with no observable tendency to 

settle. 

4.3 RESULTS OF THE CANDLE EXPERIMENTS 

Experiments were done with the Figure 4 setup to determine the 

threshold for extinguishing candle flames by C02, Halon 1211, and fine 

mists generated by C02 hydratelwater mixtures. 

Figures 5 and 6 show, these experiments gave a threshold for C02 

between 6.8 and 7.1 vol % and a threshold for Halon 1211 between 1.57 

and 1.74 ~01%. No thresholds were determined for fine mists 

generated by the C02 

flame. 

In another experiment a red laser beam was sent through the 

transparent plastic cylinder. Initially the beam was invisible but 

The results in 

ydratelwater mixtures since under all 

conditions tested thes a mists were not able to extinguish the candle 

12 
Figure 6 Extinction of a Candle Flame 

by C02 

Wt% GO2 in Air 

............................................................... 

.................... 

1 2 3 4 6 6 7 0 

Test Number 

Cylinder cross section = 0.09 squ ft 

Updraft 9 1.43 ft/sec. 

Figure 7 Extinction of Candle Flame 

by Halon 1211 

Vol % Halon 

2, 

.............. 

1 2 3 4 6 6 7 0 

Test Number 

h stays lit ~oes out 

Cylinder cross section 0.09 squ ft 

Updraft 1.43 ftlsec. 

13 
particles of Ca(OH)2 ( 20 to 40 micron size) were scattered in front 

of the plastic cylinder the beam became evident. 

4.4 RESULTS FOR OBSTRUCTED FLAMES 

Three tests were done with C02 

three tests the obstructed flame was not extinguished. 

hydratelwater mixtures and in all 

4.5 OBSERVATIONS ON HYDRATE SAMPLES RECOVERED AT AMBIENT PRESSURE 

Samples of the C02 hydrate looked very much like ice. When they were 

placed on the surface of a hot plate (surface temperature of 26OoC), 

they acted much as did samples ice, i.e. they melted and boiled with 

little or no wetting of the surface. 

A sample of the C02 hydrate placed on an ambient temperature aluminum 

plate ini-tially melted with bubbling but without producing mist, then 

suddenly exploded. After the explosion some fragments of the sample 

remained on the aluminum plate. They melted with bubbling but without 

further explosion. 

A sample of the C3F8 hydrate placed on an ambient temperature aluminum 

plate melted without explosion or mist production but with bubbling 

and a continuous snapping sound. 

5. DISCUSSION 

5.1 METASTABILITY OF GAS HYDRATES 

As discussed above gas hydrates m&&& be an effective fire fighting 

agent if they undergo delayed self atomization, which in turn might 

occur the hydrate were metastable at ambient pressure. One might 

expectthat metastability would or would not lead to self atomization 

depending on the circumstance. If a particle of hydrate is traveling 

through the air at a high velocity, its surface is subject to high 

shear. As that surface begins to convert to liquid water and gaseous 

carbon dioxide, any bubbles forming will be broken into fine droplets. 

Thus for a hydrate particle in flight metastability seems likely to 

lead to atomization. For a particle at rest, however, the situation 

is less clear. If the particle can store sufficient strain energy for 

self atomization, then it might do so. If the stored strain energy is 

not this large, it could still be sufficient to cause the hydrate 

particle to explode into coarse fragments or make snapping sounds as 

it melts. 

While the observations with the recovered hydrate samples are 

consistent with these expectations, the only thing they unambiguously 

show is that the hydrates are capable of metastability. 

I 

14 
5.2 ANALYSIS OF VISUAL OBSERVATIONS 

Figures 8 and 9 are quoted from a review article by Faeth4. Figure 7 

shows the different kinds of spray patterns which occur when a liquid 

is discharge through a hole at various flow rates. Figure 8 shows a 

map of the spray process, i.e. this figure shows ranges of Ohnesorge 

and Weber numbers in which different types of sprays will occur. 

Figure 8 also shows the results of calculations for the experiments 

done with water pressurized with helium. 

observation that samples with helium pressurizations of 1500, 1000 and 

550 psi atomized, the calculations show that these pressurizations 

correspond to the atomization range. The fourth sample, which was 

observed not to atomize, is calculated to be in the wind-induced 

range. 

The theory of spraying presented in the Faeth article does not 

consider the possibility that the liquid being sprayed may contain a 

dissolved.gas. 

water, it is not surprising that the experiments with helium 

pressurized samples were quite consistent with this theory. 

solubility of C02 in water at 2l0C and 845 psi is, however, 34 cc STP 

gas per cc of liquid. The decrease in height, the increase in width 

and the fact that the droplets did not show observable settling are 

all consistent with the assumption that the discharge produced coarse 

droplets which the dissolve C02 subsequently or concurrently atomized 

into fine mist. 

There is, however, the problem that droplets of fine mist are far more 

readily visible that an equal mass of coarse droplets. 

visual observations indicate that spray water saturated with C02 at 

high pressure does indeed produce fine mist, they do not reveal how 

efficiently this happened. 

Since the discharge hydrate/water mixture occurred in the wind induced 

region, had a similar appearance to the helium pressurized discharge 

in this region, but still produced a mist that showed no observable 

settling, the indication of the visual observations is that after a 

delay the hydrate atomized itself into a fine mist. 

however, the data do not tell us how efficient the atomization was. 

Consistent with the 

Given that helium is only very slightly soluble in 

The 

While the 

Here again, 

5.3 FIRE EXTINGUISHING TESTS 

In a previous research project EER examine the ability of water 

saturated with C02 at high pressure to extinguish fires. 

results showed that this was indeed an effective way of extinguishing 

fires, the mechanism for extinguishing the fires was not clear. 

Perhaps the fine mist pFoduced by the C02 saturated water extinguish 

the flame by cooling it or perhaps the jet of C02/mist was simply 

While the 

~ ~~ 

4G. M. Faeth, 23rd Symposium (int.) on Comb., 1345-1352, 

(1990) 

15 
--1 

'' SECOND WIND-INDUCED 

'A ATOYIZATION 

Figure 8 Spray breakup lenghts and regimes for nonturblent round 

liquid jets in still gas. 

. 1 I 

101 102 10.1 10. 10 

*I.# 

Figure 9 Spray breakup regime transitions 

16 
blowing out the flame. 

Both sets of experiments reported above were designed to avoid this 

ambiguity. In both cases the flame was protected from the discharge 

of the hydrate so that mist from the hydrate arrived in the general 

vicinity of the flame, but without momentum from the discharge to 

carry it into the flame itself. Under this condition the mist from 

the hydrate was not effective for extinguishing the flame. 

While the reasons for this failure are not completely clear, two 

possibilities are obvious. The present results suggest that C02 

hydrate does undergo delayed self atomization but they do not show the 

efficiency of this process. If the efficiency of mist production is 

poor, the hydrate will not be an effective extinguishing agent. 

Secondly there is the question of the velocity at which the mist 

enters the flame. Droplets of mist have a diffusion coefficient that 

is very close to zero. For an ideal diffusion flame this would give 

them a near zero velocity of approaching the flame and for a real 

diffusion flame the velocity would be quite low. As the analysis in 

Section 2 shows, fine mist might be ineffective in this situation. 

6 CONCLUSIONS AND RECOMMENDATIONS 

The goal of this research was to demonstrate that gas hydrates, 

especially C02 hydrate, can undergo delayed self atomization and that 

this property makes them potentially useful replacements for the 

halons. While the data indicate the former, the latter now appears to 

be a remote possibility. Accordingly further research along this line 

is not recommended. 

17