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A 1 1 1 b I) 3 1 b b ^ - r

NBSIR 85-3266

A Preliminary Analysis of Oil-Slick

Combustion

Indrek S. Wichman

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Fire Research

Gaithersburg, MD 20899

November 1985

Final Report

Prepared for: Minerals Management Service

Department of the Interior

Reston, VA 22091

-QC

100

U56

85-3266

1985

NBSIR 85-3266

A PRELIMINARY ANALYSIS OF OIL-SLICK

COMBUSTION

Indrek S. Wichman

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Fire Research

Gaithersburg, MD 20899

November 1985

Final Report

Prepared for: Minerals Management Service

Department of the Interior

Reston, VA 22091

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
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TABLE OF CONTENTS

Page

LIST OF FIGURES iv

Abstract . 1

1. INTRODUCTION 1

2. LITERATURE REVIEW 2

3. THE PHYSICAL MODEL 5

4. SUGGESTED EXPERIMENTS 8

5. ACKNOWLEDGMENTS 9

6. REFERENCES 9

-ill-
LIST OF FIGURES

Page

Figure 1. The probable distribution of oil in a typical

arrangement of slicks 11

Figure 2. The initial (acceleratory) burning regime.

The oil thermal thickness is effectively infinite 12

Figure 3. The transition regime between acceleratory growth

and final slowdown. Thickness effects are important

in this stage 13

Figure 4. The formation of a ring fire. Extinction occurs when

the radiation and gas-phase heat release can no longer

sustain the required volatile mass efflux 14

Figure 5. A qualitative illustration of the burning process

described by Figs. 2-4. The time (t) is the process

variable 15

-iv-
A PRELIMINARY ANALYSIS OF OIL-SLICK COMBUSTION

Indrek S. Wichman

Abstract

This preliminary study of oil-slick combustion contains a literature

review, a formulation of a physical model of oil-slick burning, and some

suggested experiments. The theoretical model is divided into three stages:

(1) an ignition and acceleratory-growth cycle, (2) a slowdown regime, in which

finite slick thickness effects become important, and (3) an extinction cycle.

The proposed experiments emphasize the use of the Fire Research Laboratory

located on the NBS grounds.

Keywords: buoyant flow, burning rate, combustion efficiency, combustion

theory, experiments, ignition, liquid spills, oils.

1 . INTRODUCTION

This report presents results of three tasks in this preliminary study of

oil-slick combustion. Section 2 contains a review of literature that is

relevant, directly or indirectly, to the combustion of oil slicks. Section 3

contains the formulation of a physical model of oil-slick burning, and Section

4 contains a discussion of some (potential) supporting experiments. The

research literature relating directly to the combustion of oil slicks is

scant. However, the literature dealing with various specialized aspects of

oil-slick combustion is extensive (e.g., studies of pool-fire radiation,

buoyant plumes, etc.). Since most of these subproblems are fairly well understood the literature review conducted in Section 2 is not exhaustive, just

- 1 -
sufficient to motivate the development of the physical model of Section 3.

The mathematical model(s) to be solved during the course of this study will be

derived from the physical model. The experiments discussed in Section 4

support the development of the physical model discussed in Section 3. Small

to medium scale experiments are given special emphasis herein.

2. LITERATURE REVIEW

A review of the literature on oil-slick combustion shows that relatively

little work has been done to advance the fundamental understanding of the

ignition and spread of flames over oil slicks. However, a large body of

literature exists for the numerous (component) subproblems of oil-slick

combustion, such as oil-slick spreading over a calm sea, flame spreading over

a thin layer of combustible liquid, the radiant characteristics of a buoyant

plume over a burning liquid pool, etc. It is believed here that these various

component models can be combined, in some way, to produce a rational means for

understanding this complex problem.

Some examples of reports dealing directly with oil-slick combustion are

those of Brzustowski and Twardus [1], Buist, Ross and Twardus [2], Ross, et al

[3] and the DOE review [4], The simple model of Brzustowski and Twardus [1]

describes the one-dimensional quasi-steady burning of a combustible fuel layer

over water. The water underneath the slick is assumed to be a constanttemperature heat sink that eventually extinguishes the fire. The effects of

wind deflection of the flame are included in an ad-hoc manner. There are no

comparisons between theory and experiment. The studies of the Energetex group

(Buist, Ross and Twardus [2], Ross, et al [3]) are an attempt to describe oil

- 2-
slick combustion over the entire range of slick sizes. Their theoretical

analyses borrow heavily from previous studies using the same methodology (see,

e.g.. Fay [5], in which the spreading of oil slicks is described through

simple dimensional arguments). Such an approach is useful for preliminary

order-of-magnitude estimates, but is unlikely to produce highly-accurate

predictions of burn efficiencies and other physical quantities. Thus, the

theoretical analyses of Ross, et al [3] lead only to parametric correlations,

of the type found by using dimensional analysis. In addition to their

modeling efforts, they have conducted small, medium and large-scale experiments. The data for their small and medium-scale tests appear to be much more

reliable than their large-scale test data. The effects of wind-herding are

discussed in connection with the large-scale tests.

The DOE review [4] is a very useful compilation of relevant work on oilslick removal by combustion, and contains an extensive bibliography. Many

chapters are devoted to practical engineering and economic discussions; one

chapter discusses previous (up to 1979) modeling efforts and experiments, and

some of the fundamental physical principles of oil-spill burning.

The research literature of combustion contains numerous studies of

idealized problems, various aspects of which are relevant to the study of oilslick burning. Research (up to the late 1970's) dealing with the structure of

buoyancy-induced turbulent flow fields over large (greater than ~ 0.2 m

diameter) pool fires has been reviewed by Hertzberg, et al [6]. More recent

studies have been conducted by McCaffrey [7]. The applicability and usefulness of integral methods is discussed at length, and comparisons with experiment are favorable. Estimates of the horizontal inflow velocites are made,

- 3-
and compare reasonably well with previous experiments. Although pool-fire

analyses apply only to stationary (non-growing) fires over inf initely-thick

liquid fuels, it is expected that these calculations, suitably modified, will

apply in the initial growth stages of the fire over the slick. Such modifications will have to be made during the course of this study.

Another fundamental problem that has received considerable attention is

the spread of flames over thin layers of combustible liquids (in pans). The

group headed by I. Glassman at Princeton University has produced a series of

papers discussing the controlling mechanisms of spread, specifically,

processes controlling the advancement of the flame over the unburnt fuel.

Their work is summarized in the paper of Glassman and Dryer [8], where it is

shown experimentally and theoretically that the physical mechanism governing

flame advance is the gradient (in the direction of spread) of the surface

tension. The flame spread rate is given (approximately) by u ~ Oj^h/p,

where a, is the strearawise gradient of the surface tension, h is the fuel

layer depth and p is the fuel viscosity. In addition, convincing arguments

are made for the importance (in the flame-spreading process) of convective

flows in both the gas and the liquid fuel. Since the experiments of the

Princeton group were small-scale, the effects of radiation were not

emphasized.

However, for flame spreading under any conditions other than small (or

laboratory) scale, the influences of radiation will be important. The radiant

preheating of the unburnt fuel ahead of the flame is expected to play a major

role in determining the spread rate and hence the fuel consumption rate. The

subject of radiation in fires has been reviewed by deRis [9]. The effects of

-4-
radiation on pool fires has been discussed by Burgess and Hertzberg [10]; a

simplified model of pool-fire radiation has been constructed by Orloff [11].

A very useful concept, that of a cylindrical flame, was used to model the

effects of flame radiation on the surroundings (Dayan and Tien [12]). The

effects of radiation will be included in the model of flame spread developed

here.

3. TEE PHYSICAL MODEL

The modeling efforts of this investigation are based on the physical

assumption that an actual oil slick is a nonhomogeneous , variable-thickness

layer, whose removal by combustion is most effectively performed by igniting

its thicker areas. Such an assumption is consistent with the transient nature

of oil spillage, with field observation, and with the detailed mathematical

analyses of Cox, et al (see, e.g., Foaa and Cox [13], DiPietro and Cox [14],

DiPietro, Huh and Cox [15]), who show that even under highly idealized conditions oil slick spreading is accompanied by the growth of an exceedingly thin

layer (monolayer) along the outer edge. Thus, an actual oil slick most likely

resembles the physical configuration illustrated in Fig. 1, where regions of

thick (say, of the order of 1 cm) oil splotches (less than ~ 10 meters in

diameter) are surrounded by large regions of very thin monolayer. The combustion of such a slick obviously cannot be accomplished by generating a single

large-scale fire over it (since the monolayer cannot support combustion) but

must instead be removed by lighting smaller (local) fires in regions of the

slick that are thick enough to burn. Thus, the theoretical and experimental

emphasis of this investigation is on the small and medium-scale processes

(small-scale means ~ 10 ^ meter in diameter, medium-scale means ~ 1 meter in

-5 -
diameter; large-scale fires are assumed to be ~ 10 meters or more in

diameter), since the large-scale processes are believed to be unimportant.

This simplifies the problem considerably, since (as discussed in Section 2)

large-scale experiments are costly and difficult to control, even under ideal

conditions .

The theoretical model describing the aforementioned physical processes is

anticipated to consist primarily of three distinct burning regimes. The first

is an ignition and acceleratory spread (or initial growth) regime, in which

the fire spreads radially from the ignition source until a medium scale (say ~ 1 m diameter) fire is established over the slick. The second regime

exhibits a slowdown in the rate of fire growth, because of fuel depletion,

heat losses, etc.; the first signs of flame quenching are evident in this

burning stage. The third regime has quenching in the center (ring fire) and a

decreasing mass burning rate.

Figure 2 illustrates the physical configuration for the ignition/

acceleratory spread (initial growth) regime. The gas-phase flame is initially

laminar, becoming increasingly turbulent as it grows. The mass efflux of

combustible gases from the gasifying fuel continually increases; in this

regime the slick thickness is effectively infinite (i.e., the oil slick

appears to be an infinitely deep pool). After ignition, while the fire is

still quite small, the initial radiant preheating of the unbumt liquid (ahead

of the fire) is assumed negligible, so that initial spread is controlled by

conduction processes near the flame foot. The combined effects of air

entrainment, oil circulation and the oil surface tension gradient (Glassman

and Dryer [11]) produce the observed spread rate. As the fire grows, radiant

-6 -
preheating becomes more important. The oil slick, is now heated to a wider

radius, and flame spreading is accelerated.

Such growth is expected to continue until the fuel supply in the

vigorously-gasifying region (in the center of the fire) begins to dwindle, see

Fig. 3. Thus, the second burning stage (in which the fire plume is assumed to

be fully turbulent) contains the influences of finite slick thickness.

Combustion is assumed to proceed until the slick vanishes. This assumption is

necessary for two reasons; (1) the oil is assumed homogeneous, so that

volatile burn-off is not the combustion-limiting factor and (2) experimental

studies have shown (e.g., Hall [16]) that the water beneath the slick does not

behave as a pure heat sink, since the heat flux from the burning slick establishes a nearly continuous temperature gradient at the slick-water interface.

Thus, the effects of thermal quenching are not considered in this study. The

decline in the rate of increase of the volatile mass flux from the burning

surface is accompanied by a decline in the rate of increase of the radiant

preheating of the unburnt fuel (there is a delay between these two processes,

since the fuel leaving the surface at time t is combusted at a later time,

t + At). The fire extinguishes first in the middle, forming a ring fire (see

Fig. 4).

In this final stage the radiant flux and the volatile mass flux both

decrease. The rate at which the mass burning rate decreases depends upon how

quickly the quenching diameter approaches the flame-foot diameter (see Fig.

4). The diameter at which extinction occurs allows the calculation of the

total oil-slick volume that can be combusted (under ideal conditions) by one

igniter.

-7 -
The combustion regimes discussed here can be represented diagramatically

,

as shown in Fig. 5. In Fig. 5, t (time) is the process variable. Ignition

occurs at t=0, and at tj , the first loop is entered. This loop represents the

initial growth regime, in which slick thickness effects are negligible, and

the mass efflux, radiant heating and the spread rate all increase. At t

2 thickness effects become important, and the process leaves the first cycle,

enters the second (the slowed-growth regime) and then (at t 3

) the final

quenching cycle. Quenching occurs at t^, when insufficient heat is available

for vaporization of the unburnt fuel.

4. SUGGESTED EXPERIMENTS

All supporting experiments can be performed at the research facility

located in Building 205 on the NBS grounds. The size of the experimental

facility is suitable for conducting small and medium-scale tests. Preliminary

testing will emphasize the fundamental understanding of slick spreading and

burning, to facilitate the associated modeling effort.

The combustible liquid will be poured over a layer of water contained in

a transparent tank (roughly 2 m x 2 m in area), so that visual recording of

subsequent events can be made. Particle tracking of the water motion is

performed by seeding the water with visible particulates; of special interest

should be the view from directly below the slick. The depth of the tank

should be no more than approximately 20 to 30 cm, for reasons of safety,

because temperature gradients become vanishingly small with increasing depth,

and because the fluid-dynamical effects of circulation (if any) are expected

to be confined to the near-surface region.

-8-
Initial experiments will be conducted with fuels whose behavior is well

understood (such as heptane, for example); such fuels can be modified to

simulate oil burning by adding suitable quantities of thickening agents.

Effects such as air entrainment into the fire and flame-induced motion of the

oil and water can easily be assessed through visualization techniques; quantitative measurements will be more difficult to make. The fire growth rate and

spread rate are readily evaluated, while measurements of the radiant heat flux

to the surface would be more complex; such measurements may require the positioning of numerous heat flux gauges in various locations around the fire.

Experimental burn efficiencies are evaluated by dividing the weight of the

residue by the weight of the original (unburned) fuel.

5 . ACKNOWLEDGMENTS

The author wishes to thank Mr. Ed Tennyson and Mr. John Gregory of the

Minerals Management Service, DOI, for their interest in and support of this

study. Discussions of this problem with J. Quintiere (NBS) and T. Terai

(Kyoto University) have proved very useful.

6. REFERENCES

[1] Brzustowski, T.A. and Twardus, E.M., A study of the burning of a slick

of crude oil on water, 19th Symposium (International) on Combustion, The

Combustion Institute, pp. 847-854 (1982).

[ 2 J Buist, I. A., Ross, S.L. and Twardus, E.M., Burning uncontained oil

slicks; large scale tests and modeling, in Proceedings of the 8th Annual

Arctic Marine Oilspill Program, June 18-20, 1985.

[3] Ross, S.L. and Energetex Engineering, In-situ burning of uncontained oil

slicks, prepared for the Environmental Emergencies Technology Division,

Ottawa, Ontario (1985).

-9 -
[4] U.S. Department of Energy, Combustion: an oil spill mitigation tool,

August 1979.

[5] Fay, J.A., The spread of oil slicks on a calm sea, in Oil on the Sea

(D.P. Hoult, Ed.), Plenum, pp. 53-63 (1969).

[6] Hertzberg, M. , Cashdollar, K., Litton, C. and Burgess, D., U.S. Bureau

of Mines Report 8263 (1978).

[7] McCaffrey, B.J., Momentum implications for buoyant diffusion flames,

Comb, and Flame 52_, 149 (1983).

[8] Glassman, I. and Dryer, F., Flame spreading across liquid fuels, Fire

Safety J. _3> 123 (1981).

[9] deRis, J., Fire radiation - a review, 17th Symposium (International) on Combustion, The Combustion Institute, pp. 1003-1016 (1979).

[10] Burgess, D. and Hertzberg, M. , Radiation from pool flames, in Heat

Transfer in Flames (Afgan and Beer, Eds.), Chapter 27 (1974).

[11] Orloff, L., Simplified radiation modeling of pool fires, 18th Symposium

(International) on Combustion, The Combustion Institute, pp. 549-561

(1981).

[12] Dayan, A. and Tien, C.L., Radiant heating from a cylindrical fire

column, Comb. Sci. Tech. 9_, 41 (1974).

[13] Foda, M. and Cox, R.G., The spreading of thin liquid films on a waterair interface, J. Fluid Mech. 101 , 33 (1980).

[14] DiPietro, N.D. and Cox, R.G., The containment of an oil slick by a boom

placed across a uniform stream, J. Fluid Mech. 96 , 613 (1980).

[15] DiPietro, N.D., Huh, C. and Cox, R.G., The hydrodynamics of the

spreading of one liquid on the surface of another, J. Fluid Mech. 84 , 529 (1978).

[16] Hall, A.R., Pool burning; a review. Rocket Propulsion Establishment

Technical Report 72/11 (1972).

- 10-
'.•.It - 11 -

Figure

1.

The

probably

distribution

of

oil

ina

typical

arrangement

of

slicks
- 12-

Figure

2.

The

initial

(acceleratory

)

burning

regime.

The

oil

thermal

thickness

is

effectively

infinite
Fire

plume

- 13-

Figure

3.

The

transition

regime

between

acceleratory

growth

and

final

slowdown.

Thickness

effects

are

important

in

this

stage
Ring

- 14 -

Figure

4.

The

formation

of

a

ring

fire.

Extinction

occurs

when

the

radiation

and

gas-phase

heat

release

can

no

longer

sustain

the

required

volatile

mass

efflux
Flame

spread

rate

Finite-thickness

Flame

spread

rate

increases

effects

decreases

- 15-

Figure

5.

A

qualitative

illustration

of

the

burning

process

described

by

Figs.

2-4.

The

time

(t)

is

the

process

variable
N BS-H4A 'rev. 2 -tc) U.S, DEPT. OF COMM. 1. PUBLICATION OR l2i Performing Organ. Report No. BIBLIOGRAPHIC DATA

REPORT NO. SHEET (See Instructions) NBSIR-85/3266

3. Publication Date November 1985

4. TITLE AND SUBTITLE

A Preliminary Analysis of Oil-Slick Combustion

5. AUTHOR(S)

Indrek S. Wichman

6. PERFORMING ORGANIZATION (If joint or other than NB5, see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

7. Contract/Grant No.

Gaithersburg, MD 20899

9.

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

U.S. Department of the Interior

Minerals Management Service

647 National Center

Reston, VA 22091

I. Type of Report & Period Covered

Final

10.

SUPPLEMENTARY NOTES

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

11.

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

bi bliography or literature survey, mention it here)

This preliminary study of oil,-slick combustion contains a literature review, a formulation of a physical model of oil-slick burning, and some suggested experiments

.

The theoretical model is divided into three stages; (1) an ignition and acceleratorygrowth stage, (2) a slowdown regime, in which finite slick thickness effects become

important, and (3) an extinction cycle. The proposed experiments emphasize the use

of the Fire Research Laboratory, located on the NBS grounds.

12.

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

Buoyant Flow; Burning Rate; Combustion Efficiency; Combustion Theory; Experiments;

Ignition; Liquid Spills; Oils

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