2783

Twenty-Seventh Symposium (International) on Combustion/The Combustion Institute, 1998/pp. 2783–2790

PILOTED IGNITION OF A SLICK OF OIL ON A WATER SUBLAYER:THE EFFECT OF WEATHERING

NEIL WU, GILLES KOLB and JOSE L. TORERODepartment of Fire Protection Engineering

University of Maryland

College Park, MD 20742-3031, USA

Piloted ignition of a slick of oil on a water sublayer has been experimentally studied. The objective ofthis work is to provide a tool that will serve to assess a fuel’s ease to ignite under conditions that arerepresentative of oil spills. The fuel is exposed suddenly to external radiation to increase its temperatureuntil ignition occurs. The strength and geometrical placement of the pilot were chosen to minimize gas-phase induction time and heat feedback from the pilot to the fuel surface. Temperature measurements,flow visualization, and ignition delay time are used to characterize piloted ignition, and an existing one-dimensional heat transfer model is used to correlate the experimental results. Two different crude oils andSAE 30W oil were used for these experiments. Crude oils were tested in their natural state and at differentlevels of weathering. It was observed that the ignition delay time is a strong function of the flow structuresformed both in the liquid and gas above the pool. Piloted ignition is inhibited by premature boiling of thewater sublayer, and weathering significantly increases the ignition delay time. It was determined that acritical heat flux for ignition could be obtained and better serve as a parameter to characterize the fuelpropensity to ignite in the presence of a strong pilot. The minimum heat flux that will permit ignitionbefore boiling of the water sublayer occurs also needs to be considered.

Introduction

Burning of an oil spill is of interest because ofoffshore exploration, production, and transportationof petroleum [1]. In the case of an accidental spill atsea, in situ burning can provide an effective meansfor the removal of an oil slick, reducing negative en-vironmental impact. The efficiency of the ignitionand burning process is crucial for the successfulelimination of the crude oil.

Information available on burning of a thin fuellayer on a water sublayer is quite limited. Thin-layerboilover [1–3] has been found generally to enhanceburning rate, although Koseki et al. [1] noted thatboiling at the fuel–water interface can limit flamespread. The effect of the minimum fuel layer thick-ness necessary for sustained combustion has beenstudied extensively [2,3]. Several models have beendeveloped to describe the heat losses from a poolfire to the supporting water layer [4] and to attemptdescription of in-depth absorption of radiation bythe fuel layer [5]. Flame spread across the liquid fuelsurface has also been emphasized, and excellent re-view papers have been published by Glassman [6]and Ross [7]. Glassman summarizes the extensiveliterature on ignition; however, it is clear that littleattention has been drawn to characterizing the ig-nition process of liquid fuels on a water sublayer.

Ignition behavior of petroleum fractions has notbeen studied beyond flash and fire points under qui-escent conditions [6,8,9]. Weathering and oil–water

emulsions on the flash and fire points have yet to bestudied. Flash or fire point tests do not incorporatethe effects that high heat insult has on the nature ofthe fuel—that is, emulsions break down when sub-ject to a high heat flux—thus, they are of reducedapplication for an oil spill scenario. Furthermore,heat transfer toward the water sublayer is entirelydependent on the fuel properties and can precludeignition. It therefore needs to be incorporated whencharacterizing the ignition process. To our knowl-edge, the only study that addresses the effect ofweathering and formation of emulsions on ignitionunder conditions pertinent to the oil-spill scenario isdue to Putorti et al. [10]. This study quantified thenecessary heat flux for ignition of various liquid fuels,and emphasis was placed on the ignition delay timeof weathered and emulsified samples.

Most accidental and deliberate burns of spilled oilat sea suffer from the effects of wind and waves.Volatiles tend to evaporate rapidly with time (weath-ering), and mixing tends to form oil–water emulsionsmaking the oil difficult to ignite. Consequently, al-teration of the physical or chemical properties of theoil can require additional energy for ignition. Severalstudies reported have attempted to characterizeweathering and emulsions typical of oil-spill scenar-ios [11].

In situ burning of an oil spill requires the fuel toignite and that ignition to be followed by spread andeventually lead to mass burning. Many studies have

2784 FIRE SAFETY RESEARCH

shown that ignition is not always followed by spread[12] and therefore, is not sufficient to guarantee ef-ficient removal of the oil slick. The need to under-stand the three stages necessary for the efficient re-moval of crude oil has resulted in the choice to usea modified version of the LIFT (ASTM-E-1321) [13]apparatus to characterize the burning process.

The overall objective of this study is to character-ize the entire burning process, but in the presentwork emphasis will be given to piloted ignition. Thischoice does not provide optimal conditions for thestudy of each individual element, but it is justifiedin the general context of this problem. This studywill use two different crude oils, as representative ofthose commonly transported by oil tankers, and SAE30W oil as a reference of a better characterized fuel.Crude oils will be studied in their natural state andsubject to different levels of weathering. The for-mation of emulsions and their effect on ignition willbe a subject of future study, but it goes beyond theobjectives of the present work.

Background

Fuel properties vary when subject to a strong heatinsult and therefore need to be evaluated under “fireconditions” [14]. The concept of minimum heat fluxfor ignition has been commonly applied to solid fu-els, and ignition behavior can be predicted or mea-sured using small, bench-scale experiments. In asimilar fashion, ignition behavior of liquid fuels canbe studied, and evaluation of the “fire properties”allows ranking of fuels in various states: natural,weathered, and emulsified. The scale dependencywill always be a matter of controversy. Thus, large-scale tests remain a necessity for validation.

The mechanisms leading to gas-phase ignition canbe described as follows. The liquid bed is consideredinitially at ambient temperature, Ti. After suddenlyimposing an external heat flux ( ), the temperatureq̇9eof the bed rises until the surface reaches the pyrol-ysis or evaporation temperature (Tp). The time re-quired for the fuel surface to attain Tp will be re-ferred as the pyrolysis time, tp. After attaining Tp,the vapor (pyrolysate) leaves the surface, is diffusedand convected outward, mixes with the ambient ox-idizer, and creates a flammable mixture near thesolid surface. This period will be referred here asthe mixing time, tm. If the mixture temperature isincreased, the combustion reaction between the fuelvapor and the oxidizer gas may become strongenough to overcome the heat losses to the solid andambient, thus becoming self-sustained, at whichpoint flaming ignition will occur. This period corre-sponds to the induction time, ti.

Extending the analysis proposed by Fernandez-Pello [15], the ignition time (tig) will be given thenby

t 4 t ` t ` t (1)ig p m i

Introducing a pilot reduces the induction timemaking it negligible when compared with tp and tm.Furthermore, mixing has been commonly consid-ered as a fast process compared with heating of thefuel. Therefore, the fuel and oxidizer mixture be-comes flammable almost immediately after pyrolysisstarts. Thus, pyrolysis temperatures and times arecommonly referred to as ignition temperature andignition time [14], and equation 1 simplifies to

t 4 t (2)ig p

and Tig can be defined as Tp. Although such a defi-nition is not physically correct [16], it can be veryuseful in some practical applications, because it pro-vides a reference parameter that could serve to char-acterize ignition.

The flow over the fuel surface will control mixingof fuel and oxidizer as well as the transport of thismixture toward the pilot (tm), and therefore, it canhave a significant effect on tig and on the validity ofequation 2. Equation 2 could be extrapolated only ifthe experimental conditions at which the ignition de-lay time is obtained satisfy the assumption that tm 'ti K tp. Slight changes in the flow structure, espe-cially for a horizontal configuration, can strongly af-fect tm without changing tp significantly. This effectis least significant as the external heat flux ap-proaches the critical heat flux for pyrolysis (q̇9 'e

) and tp → `. This is important because it impliesq̇90,pthat the error incurred in the experimental deter-mination of tig (due to the unknown nature of theflow) will decrease as approaches .q̇9 q̇9e 0,p

To obtain tp the fuel and water bed are assumedto be one thermally thick material with propertiescorresponding to an unknown combination of bothliquids. The bed is assumed a semiinfinite slab, andthus all convective and thermocapillary motion inthe bed is neglected. This assumption is not neces-sarily correct [7], and will be addressed later, but willbe accepted as a possible source of error. Through-out the heating process, the fuel layer is assumedinert, with negligible pyrolysis before ignition. Thesolution to the one-dimensional transient heating ofa semiinfinite slab is given by Carslaw and Jaeger[17], and an elaboration of all additional assumptionsand the derivation pertaining to this study are givenby Quintiere [14].

The boundary condition for this solution is im-posed by heat balance at the surface, which needsto incorporate convective heat losses, re-radiation,in-depth absorption, and the fraction of the externalheat flux not absorbed [18]. Losses result in a min-imum external heat flux necessary, , to attain Tp.q̇90,pFor , the surface will attain thermal equilib-q̇9 q̇9e 0,prium at TEQ , Tp. A linearized heat transfer coeffi-cient, h, is commonly used to describe heat transferat the surface and all heat loss terms can be reduced

PILOTED IGNITION OF OIL: WEATHERING EFFECT 2785

Fig. 1. Schematic of the experimental apparatus.

to h(Tp 1 Ti). Values for h have been shown to varywith orientation and environmental effects. Exam-ples of typical values found in the literature are: 8.0W/m2K for natural turbulent convection and a ver-tical sample [14], 13.5 W/m2K for a horizontal ori-entation [18], and up to 15.0 W/m2K obtained byMikkola and Wichman [19] while conducting exper-iments on a vertical orientation with wood.

These assumptions lead to the following solutionfor the attainment of the pyrolysis temperature as afunction of time

h(T 1 T ) 4 q̇9[1 1 exp(at )erfc(at )] (3)p i e p p

where tp is the time necessary to attain Tp at thesurface, a 4 a(h/k)2, “a” is the thermal diffusivityand k is the thermal conductivity. It needs to benoted that k and a are not the fuel or water prop-erties but an equivalent set of properties that in-cludes the contribution of both liquids. If 4q̇9e

, Tp is expected to be reached when t → `, andq̇90,pthe critical heat flux that would lead to pyrolysis canbe derived from equation 3 and is given by

q̇9 4 h(T 1 T ) (4)0,p p i

For k it can be assumed thatq̇9 q̇9e 0,p

2 1/2[1 1 exp(at )erfc( at )] ' (at )!p p pp!

which leads to the approximate expression valid forshort times (tp)

2p q̇90,pt 4 (5)p 1 24a q̇9e

Equation 5 is of great practical importance becauseit shows that a plot of as a function of the cor-11/2tpresponding will be linear for . , and fromq̇9 q̇9 q̇9e e 0,pthe slope of this line, the value of a can be deter-mined. The fire literature generally refers to a as aglobal thermal property of the material.

In the present study, the magnitudes of tm and tiwill be addressed, but the geometry will be chosento make them minimal. Thus ' and, fromq̇9 q̇90,p 0,igequation 2, tp ' tig.

Methodology

The LIFT requires the sample to be positionedvertically, and a uniform radiant flux is suddenly ap-plied. For this study, the LIFT had to be rotated 908to a horizontal position (Fig. 1). A fan, capable ofinducing an airflow velocity of 0.1 m/s, was placedat the trailing edge of the sample. A homogeneousflow was guaranteed by means of a duct placed infront of the fan. The duct was filled with steel woolsandwiched between two honeycomb plates.

A radiation shield (marinite board) was placed infront of the panel before the sample was introducedto its test position (Fig. 1). Once the sample hadbeen placed, the radiation shield was removed andtime recording started. It was observed that increas-ing the pilot size reduced the ignition delay time,and increasing the distance from the fuel surface hadan opposite result. Thus, the choice of size and lo-cation of the pilot resulted from a systematic studythat minimized the effect of heat feedback from the

2786 FIRE SAFETY RESEARCH

Fig. 2. (a) Incident radiant flux distribution for the LIFT apparatus, (b) Ignition sample thermocouple temperaturesfor SAE 30W oil. Upper temperatures curves correspond to surface temperatures in the corner, side, and center of thetray. Lower temperature curves correspond to temperature 5 mm below fuel surface in the corner, side, and center ofthe tray.

pilot to the fuel surface and guaranteed best repeat-ability of the results. Premature ignition by a pilotflame has been addressed by several authors whoshowed similar observations [6]. This systematicstudy led to a small propane diffusion flame (20 mmin height) established on a 4-mm stainless-steel noz-zle being used as an ignition pilot. The nozzle wasplaced 10 mm above the fuel surface plane in thecenterline and 10 mm downstream of the trailingedge of the ignition tray (Fig. 1).

The fuel tray was placed under a 200-mm squareplate with a 100-mm square hole in the middlewhere the fuel was located. Details on this plate andits use will be provided in later sections.

The fuel was placed in 100-mm cubical stainlesssteel trays with one open side. Two different ignitiontrays were used for the tests. The first tray containedan interior lip measuring 5 mm surrounding theedge of the open end. The second tray, of similarconstruction, had no interior lip. Previous studies onignition using the cone-calorimeter [10] are usedthroughout this work as reference data. In these ex-periments Putorti et al. used a tray of square sectionof similar dimensions to the ones used in the presentstudy and with a 5-mm interior lip.

The radiant panel forms an angle of 158 with thesample, with the objective of producing a heat fluxdistribution like the one shown in Fig. 2. Despitethe inclination, the incident heat flux for the regionup to 150 mm is relatively uniform (Fig. 2a), provid-ing a constant heat flux boundary condition for the

fuel–air interface. Thermocouple measurementshave been used to characterize the temperature evo-lution of the fuel and have shown negligible differ-ences at several locations along the fuel surface (Fig.2b). Detailed hardware characteristics, typical heatflux distributions, and experimental procedures in-volving the LIFT have been well documented [14]and will not be repeated here.

Experimental Results and Discussion

The Effect of Temperature Gradients Between theFuel and the Container

The container has significant effects on the for-mation of recirculation currents inside the liquid.Temperature gradients in the fuel surface inducethermocapillary motion combined with natural con-vection [7], enhancing heat transfer inside the liquidand resulting in a more homogeneous temperaturedistribution and, thus, in longer ignition delay times.Eight chromel-alumel thermocouples (0.5 mm in di-ameter) were placed in the liquid bed with the tipat the axis of symmetry. The thermocouples werespaced to provide a finer grid close to the surfaceand to cover the entire depth of the tray (distancefrom the thermocouple to the surface: 0 mm, 3 mm,6 mm, 10 mm, 18 mm, 43 mm, 68 mm, and 93 mm).

Heat from the radiant panel increases the tem-perature of the fuel but also of the container. The

PILOTED IGNITION OF OIL: WEATHERING EFFECT 2787

Fig. 3. Smoke visualization of sample tray (a) in the ab-sence of flush floor, (b) surrounded by a flush floor, and (c)surrounded by a flush floor with a weak draft.

inclusion of a 5-mm lip increased the solid surfacereceiving radiation from the panel. Temperatures ofthe upper part of the tray were observed to be sig-nificantly higher than those observed for the no-liptray. In contrast, the fuel surface temperature wasfound to be consistently higher for the no-lip tray,whereas the temperatures recorded by the thermo-couples deeper in the fluid were higher for the liptray. Recirculation inside the liquid bed resulted ina 30% increase in the ignition delay time when thetray with a lip was used. For identical fuel layerthickness and external heat flux, boiling occurred fas-ter in the tray with an interior lip.

A fine metallic powder was used to coat the sur-face of the fuel with both trays. Observations of theflow in the lip tray indicated increasing eddy activityof the fuel layer as the temperature difference be-tween the container and the fuel increased. Theseflow patterns were found to be restricted to approx-imately 10 mm from the tray in the absence of aninterior lip. Motion of the powder was observed tobe almost negligible in a circle approximately 80 mmin diameter. This issue is worth an independent

study but escapes the objectives of the present work.By selecting the no-lip configuration, effect of heattransfer from the tray was considered minimized.

Flow Structures Above the Fuel Surface

To study the flow characteristics over the fuel sam-ple, a 0.5 W red diode laser (SDL-820) was used tocreate a light sheet for visualization of the smokeemerging from the fuel surface. The images havebeen processed using an EPIX video card. To obtaina clear image of the flow structure, as evidenced bythe smoke, a threshold was established below whichall pixels were assigned a 0 value. Fig. 3 shows a setof three typical images.

In the absence of a flush floor surrounding the fueltray, clear eddies could be observed at the edges ofthe tray (Fig. 3a). It was observed that these eddiescould grow and cover the entire surface of the fueltray. When a floor surrounded the fuel tray (as shownin Fig. 1), the eddies disappeared and a random up-ward flow of gases was observed (Fig. 3b). As a con-sequence of the decreased mixing of fuel and air, theignition delay time increased by approximately 20%over the no-floor case. By introducing a flush floorand 0.1 m/s flow parallel to the surface, a boundarylayer was formed and all the eddies were eliminated(Fig. 3c). Although the ignition delay time remaineddependent on the magnitude of the flow, this con-figuration allowed for the greatest repeatability.

Ignition Delay Time

To calibrate the apparatus, SAE 30W oil was firstused for the ignition tests. This fuel was used tomake possible comparisons with previously reportedresults on ignition delay time [10] and also becauseof the high flash point (approximately 250 8C). Thehigher flash point results in a longer ignition delaytime, providing a longer period to observe the dif-ferent processes affecting ignition. Radiation absorp-tion is also lower with SAE 30 oil favoring boiling ofthe water bed. Although the viscosity of SAE 30 oilis generally higher than that of crude oils, it is verysensitive to temperature (approximately 1 2 1012

at 0 8C and 1 2 1015 at 100 8C), reaching compa-rable values after only a small temperature increase.

The results from these experiments are presentedin Fig. 4 together with data obtained for the samefuel by Putorti et al. [10]. Following equation 5, theignition delay time is presented as t11/2. It can beobserved that, although the ignition delay time sig-nificantly differs from the values found by Putorti etal., the data converge to a unique critical heat fluxfor ignition. Because these experiments were con-ducted using different ignition procedures and un-der different environmental conditions, tm is ex-pected to be different, thus affecting the ignitiondelay time. On the contrary, tp should not be affected

2788 FIRE SAFETY RESEARCH

Fig. 4. Ignition delay times ofSAE 30W oil for various externalfluxes.

Fig. 5. Critical heat flux and timefor boiling using SAE 30W oil.

Fig. 6. Ignition delay times forvarious external heat fluxes for crudeoils.

PILOTED IGNITION OF OIL: WEATHERING EFFECT 2789

Fig. 7. Critical heat flux for igni-tion ( ) for ANS and Cook Inletq̇90,ig

crude oils weathered at various dif-ferent levels (% mass loss).

if convective losses are similar in magnitude. As theexternal heat flux approaches , tm becomes neg-q̇90,igligible compared with tp, and all data converge to aunique point ( ' 5 kW/m2), as observed in Fig.q̇90,ig4. This linear dependency corresponds well withdata reported in the literature for solid fuels [14,19]and serves to validate the previously mentioned as-sumptions.

For the particular case of an oil slick on a waterbed, the water underneath the fuel might attain boil-ing before ignition occurs. It was observed that onceboiling started, ignition of the fuel was precluded.Heating of the bed can be treated as heating of asemiinfinite solid, and temperature distributions canbe predicted quite accurately [4]. The analytical pre-diction of a characteristic time to boiling goes be-yond the scope of this work. But the determinationof a minimum heat flux that will lead to boiling( ) before ignition can occur is of great practicalq̇90,Bimportance. Therefore it needs to be included as acomplement to the critical heat flux for ignition.

As the external heat flux decreases, the tempera-ture gradient at the surface decreases, and thermalpenetration increases before the surface attains Tp.If the thermal wave can increase the water tempera-ture at the fuel/water interface to the boiling pointbefore the surface reaches Tp, boiling will preventignition from occurring. The minimum heat flux thatwill allow the surface temperature to reach Tp beforeboiling is given by and presented in Fig. 5. Asq̇90,Bthe fuel layer decreases in thickness, the heat wavewill reach the water faster, allowing a shorter avail-able time for the surface to reach Tp and conse-quently requiring a higher temperature gradient atthe surface (higher ).q̇90,B

Crude Oils and the Effect of Weathering

A series of tests were conducted with two crudeoils. Fig. 6 shows ignition delay times for different

external heat fluxes obtained for ANS crude oil anddata reported by Putorti et al. [10]. The data pre-sented are an average of at least five experimentsconducted under identical conditions. It was ob-served that ANS crude oil in its natural state ignitedat ambient temperature; therefore, no external heatflux was necessary. Flash points for this type of fuelhave been reported as low as 19 8C [8], showingagreement with the preceding observation. Whenweathered, the ignition delay time decreases as theheat flux increases, and a linear dependency be-tween the external heat flux and is obtained.11/2tigThe intercept with the horizontal axis will providethe critical heat flux for ignition. Fig. 6 shows thatthe critical heat flux for ignition will increase withweathering. The experimental data from Putorti [10]fit well with the data collected in the present work.

The critical heat flux for ignition ( ) as obtainedq̇90,igfrom figures such as Fig. 6 is presented in Fig. 7.Results are presented for Cook Inlet and ANS crudeoils for different fuel layer thicknesses. Figure 7shows a discrepancy between the critical heat fluxfor ignition for 3 mm as opposed to almost identicalvalues obtained for 8- and 15-mm layers. For layersthicker than 8 mm, the effect of fuel layer thicknesswas mostly manifested in the curves being truncatedby boiling before attaining . The increasing valueq̇90,igof with mass loss shows that weathering makesq̇90,igignition more difficult. The increasing slope of thecurve points toward the possibility of an asymptoticvalue at which the crude oil will not ignite. Based onthe values for , ANS crude oil was observed toq̇90,igbe more prompt to ignite than Cook Inlet crude oil.Cook Inlet ignited without an external heat flux fora mass loss rate smaller than 10%, and ANS crudeoil for a mass loss smaller than 7%. The resultspresented are representative of all other casesstudied.

2790 FIRE SAFETY RESEARCH

Conclusion

To study piloted ignition of a slick of oil on a watersublayer, a modified LIFT apparatus is used. Igni-tion delay times and a critical heat flux for ignitioncan be extracted using this testing methodology. Theignition delay time is a strong function of the flowstructures formed both in the liquid and gas abovethe pool. Piloted ignition is inhibited by prematureboiling of the water sublayer. The minimum heat fluxthat will permit ignition before boiling of the watersublayer needs to be considered. The critical heatflux for ignition will serve as an appropriate param-eter to characterize the fuel propensity to ignite.Cook Inlet, ANS crude oils, and SAE 30W oils wereused for these experiments, and crude oils weretested in their natural state and at different levels ofweathering. Weathering of the crude oils signifi-cantly increases the ignition delay time. The dataextracted can be used as a tool to rank fuels in vari-ous states: natural and weathered.

Acknowledgments

Funding for this work was provided by NIST. The tech-nical comments, encouragement and support of Dr. Ma-rino di Marzo are greatly acknowledged.

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