Flame spread over porous sand beds wetted with propenol

FIRE AND MATERIALSFire Mater. 2011; 35:61–70Published online 13 May 2010 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/fam.1035

Flame spread over porous sand beds wetted with propenol

Jafar Zanganeh and Behdad Moghtaderi∗,†

Priority Research Centre for Energy, Chemical Engineering, School of Engineering, Faculty of Engineering andBuilt Environment, The University of Newcastle, Callaghan, NSW 2308, Australia

SUMMARY

A comprehensive set of experiments was carried out to investigate the spread of flames over porous sandbeds wetted with finite quantities of propenol. The aim was to examine the key characteristics of flamepropagation over porous beds when a limited supply of liquid fuel to the flame zone was available.Experiments were conducted over beds with depths ranging from 13.3 to 39.9 mm and average particlediameters between 0.5 and 5 mm under quiescent, assisted and opposed airflow configurations. It was foundthat the flame spread rate for both assisted and opposed airflow configurations dramatically decreased asthe air speed was increased although the extent of decay in the flame spread rate was more pronounced inthe case of the opposed airflow configuration. The experimental observations were explained by correlatingthe flame spread rate with the Damköhler number and fuel consumption flux. The practical implicationsof the findings are briefly discussed in this paper. Copyright � 2010 John Wiley & Sons, Ltd.

Received 24 August 2009; Revised 4 February 2010; Accepted 7 February 2010

KEY WORDS: flame spread rate; inert porous bed; liquid fuel; fuel consumption flux

INTRODUCTION

Accidental fires in industrial settings, particularly those in petrochemical industries, are responsiblefor considerable loss of life and property in many parts of the world every year. Quite often theaccidental spillage and subsequent ignition of combustible liquids are the cause of accidental firesin large-scale industrial settings (e.g. petrochemical industry). In many such fires, the liquid fuelspills over a porous inert surface (e.g. concrete slab or gravel) penetrating into its open poresand/or cavities. If the fuel is subjected to an ignition source and thereby ignited, a stable flamemay form and subsequently spread over the surface of the porous bed. The characteristics of theflame spread phenomenon over porous beds are significantly different from those over non-poroussolids and liquid pools [1–3]. For this reason, research articles specifically dealing with the subjectof flame spread over inert porous beds have appeared in the open literature [4–19]. Since the early1960s although the number of such publications is quite limited when compared with those ontopics, such as pool fires or combustible porous beds. To name a few, one can list papers by Ishida[4–8], Takeno and Hirano [11, 12], Takeuchi et al. [13], Suzuki [14–16], Chao [1], and Wang [17],who performed a broad range of experimental and theoretical studies on the spread of flames overglass, lead, sands, and steel beads in channels filled up with liquid fuel. They found that the flame

∗Correspondence to: Behdad Moghtaderi, Priority Research Centre for Energy, Chemical Engineering, School ofEngineering, Faculty of Engineering and Built Environment, The University of Newcastle, Callaghan, NSW 2308,Australia.

†E-mail: [email protected]

Contract/grant sponsor: The University of Newcastle, Australia

Copyright � 2010 John Wiley & Sons, Ltd.

62 J. ZANGANEH AND B. MOGHTADERI

spread rate is strongly dependant on the size of the solid particles [6, 12] and that the rate of flamespread is generally slower than over the surface of liquid pools [7, 11].

However, the majority of past studies like those listed above [4–17] has almost exclusivelyfocused on scenarios where an unlimited supply of liquid fuel was available to soak the inertporous bed. In practical situations though there is a limited fuel supply and, as such, the porousbed is not soaked but rather wetted by the liquid fuel which slowly penetrates through the bed.The finite quantity of the liquid fuel in this case has a dramatic impact on the flame altering itspropagation behaviour and/or characteristics. We have shown in our previous publications [4, 5]that in the case of sand beds wetted with finite quantities of propenol, the flame spread decreasesas the bed particle size is increased. Also, deeper sand beds generally lead to slower flame spread.More interestingly, we found that as the air velocity was increased the flame rapidly deceleratedunder both opposed and assisted airflow configurations.

The present paper builds on our previous work and attempts to shed more light on the flamespread phenomenon over porous beds wetted with finite quantities of liquid fuel. The aims wereto (1) obtain a more comprehensive set of flame spread measurements under a wide range ofoptional conditions, (2) quantify the role and relative importance of chemical reaction and transportphenomena, i.e. fluid flow and heat, on the rate of flame spread, and (3) investigate the relationshipamong the physical properties of the bed, the fuel consumption rate, and the flame spread ratewhen finite quantities of fuel are available.

EXPERIMENTAL

Experimental set-up

Experiments were carried out in the set-up schematically shown in Figure 1. The set-up comprisesa rectangular channel mounted on a support frame, a hydraulic jack to raise one end of the channel,a heat exchanger unit fitted to the bottom of the channel, a water bath, a thermocouple grid, aducted fan, a fuel distributor (not shown in Figure 1), a canopy hood connected to an exhaustfan, a high-speed digital camera, and a data acquisition system comprising a data logger and acomputer.

The rectangular channel was 1.5 m long, 0.08 m wide, and 0.04 m deep. It was fixed to thesupport frame at one end through the hinge point A and was mounted on the hydraulic jack at theopposite end. This arrangement allowed us to vary the inclination angle of the channel if needed.A canopy hood was installed above the bed and connected to an exhaust fan to extract the smokeand combustion products from the laboratory. The heat exchanger unit attached to the bottom ofthe channel consisted of a set of copper tubes to maintain the bottom temperature at a constant level(typically the ambient temperature). The temperature of the water was regulated using the waterbath. The external surfaces of the channel were covered with a 5-mm-thick insulating material toprevent heat losses. The thermocouple grid was fitted to the channel to measure the temperature

Figure 1. Schematic representation of the experimental setup.

Copyright � 2010 John Wiley & Sons, Ltd. Fire Mater. 2011; 35:61–70DOI: 10.1002/fam

FLAME SPREAD OVER POROUS SAND BEDS WETTED WITH PROPENOL 63

Figure 2. Schematic representation of the thermocouple assembly.

distribution of the bed. The grid was connected to the data logger and the computer. The embeddedgrid consisted of 24 K-type thermocouples (Ni/Cr–Ni/Al) all with a sheath diameter of 1 mm. Thethermocouples were introduced from one of the side walls of the channel and were arranged ineight columns and three rows with 0.2 m interval between columns and 13.3 mm distance betweenalternative rows (Figure 2). The first row of thermocouples was embedded 1 mm below the surfaceto minimize the re-radiation losses from the thermocouple junction.

The porous bed was constructed by filling the rectangular channel with sand (�=2680kg/m3).Six different particle sizes with median diameters of 0.5, 1, 2, 3, 4, and 5 mm were used. The desiredparticle sizes were obtained through sieving using a sieve shaker. Three different depths (13.3,26.6, and 39.9 mm) were selected to investigate the effect of the bed depth on the flame spreadrate and combustion characteristics. The effect of air flow on the flame spread was simulated usingthe 0.2×0.2m ducted fan connected to a speed controller. Furthermore, a telescopic anemometerwas used to map the air stream over the bed. The airflow velocity could be adjusted over a rangebetween 0 and 2m/s.

Propenol (boiling point 82.4◦C and flash point 12◦C) was used as the liquid fuel in all experi-ments. The fuel was uniformly distributed over the porous bed by the fuel liquid distributor. Theratio of fuel volume to the weight of the sand bed (i.e. fuel volume/sand weight) in all experimentswas kept at about 0.1L/kg. An electronic balance fitted with a timer was used to continuouslymeasure the mass loss of fuel. The balance was tared to zero at the beginning of each experimentand the measured mass loss data were saved into the computer. Ignition was achieved using apilot flame placed near the end of the channel depending upon the desired airflow direction. Theflame speed and hence, the spread rate were digitally determined from high-speed video, imageprocessing as well as visual observation.

Methods and techniques

All experiments were conducted at an ambient temperature of approximately 27◦C. At the beginningof each experiment all thermocouples were calibrated with a standard J -type thermocouple tomake sure they were in correct working order. The channel was then filled with sand particlesto the desired depth consistent with the experimental plan. The surface of the bed was smoothedout using a trowel to make the surface as even as possible without compaction. At this stage theducted fan was turned on in the case of assisted or opposed airflow configurations. The liquidfuel was then distributed uniformly over the bed using the fuel distributor system developed in ourlaboratory as part of this study. After a pause period, the fuel was ignited and flame was let to spreadalong the length of the bed. The progress was captured digitally using a high-speed video camera.

RESULT AND DISCUSSION

Results obtained in this study for the rate of flame spread over porous sand beds in the horizontalorientation under different airflow configurations are shown in Figures 3–9. Figure 3 illustrates



bar charts of the flame spread rate for a 39.9 mm deep bed as a function of particle size underquiescent (i.e. no airflow) conditions. As shown in Figure 3, under quiescent conditions the rate offlame spread decreases as the particle size increases. The highest flame spread rate is about 1.2m/sand corresponds to the bed containing finest particles (0.5 mm). For coarse particles (5 mm) theflame, spread rate is around 0.9m/s, a drop of almost 25% when compared with the flame spreadrate of 0.5 mm particles.

A similar drop in the flame spread rate is observed when the flame is assisted or opposed bythe airflow as illustrated in Figures 4 and 5, respectively. In these figures the rate of flame spreadover a 39.9 mm deep bed has been plotted against the air speed for a range of bed particle sizesbetween 0.5 and 5 mm. As can be seen, regardless of the airflow direction the flame spread ratecorresponding to a given particle size decreases as the airflow velocity increases, although thedecrease is more pronounced under the opposed airflow configuration (Figures 4 and 5). In bothcases a much sharper drop in the flame spread rate is observed when the airflow exceeds 1m/s.At or around an air speed of 1.5m/s, the flame spread approached zero for most cases studied

Figure 3. Bar charts of the flame spread rate versus the particle size of the bed under no airflow configurationfor a 39.9 mm deep bed.

Figure 4. Plots of the flame spread rate against the airflow speed under the assisted airflowconfiguration for a 39.9 mm deep bed.



Figure 5. Plots of the flame spread rate against the airflow speed under the opposed airflowconfiguration for a 39.9 mm deep bed.

in this investigation. Evidently, the drop in the flame spread rate for coarser particles is greater.For example, the flame spread rate with assisted airflow decreases from 0.85 to 0.25m/s as theair speed increases from 0.5 to 1.5m/s for the bed containing 0.5 mm particles, whereas for 5 mmparticles the flame spread rate approaches zero from a rate of 0.6m/s when the air speed increasesfrom 0.5 to 1.5m/s.

Compared with the flame spread under quiescent conditions (Figure 3), the rates of flame spreadcorresponding to assisted and opposed airflow configurations (Figures 4 and 5) are considerablyslower with the opposed airflow having the slowest spread rate under otherwise identical conditions.For any given bed depth, the minimum rate of flame spread observed in our studies corresponded tocoarse particles (i.e. 5 mm) under the opposed airflow configuration. With the exception of 0.5 mmparticles, the spread of flame for other particles sizes under airflow speeds greater than 1.5m/swere impossible in both assisted and opposed airflow configurations.

Figures 3–5 clearly indicate that regardless of the airflow direction and/or speed when finitequantities of liquid fuel are available the rate of flame spread across the surface of an inert porousbed decreases as the size of the bed particles are increased. This is a clear distinction with caseswhere unlimited supply of liquid fuel is available [6, 7, 11, 12]. For example, studies by Hiranoand co-workers [11, 12] have shown that when glass beads are soaked with liquid fuel the flamespread rate may increase or decrease with the bed particle size. For particles larger than a criticaldiameter, the flame spread rate increases with the particle size, whereas for particles smaller thanthe critical diameter, the rate of the flame spread decreases with the particle size [11, 12]. Ourresults do not show the existence of any critical diameter for the porous bed particles when thefuel quantity is limited and, as such, for a given airflow the flame spread rate is always inverselyproportional to the particle size.

The relationships among the flame spread rate, the airflow, and the particle size of the bedobserved in this study can be largely explained if one compares the relative importance of theunderlying chemical reactions and transport phenomena. This can be best achieved if the flamespread results are plotted against the Damköhler number (Da), which is proportional to the ratioof airflow transient time through the flame (tflow which underpins the transport of heat and mass)to the time for completion of chemical reactions, tchem [20]

Da ∝ tflow

tchem∝ 1

V 2a

, (1)

where Va denotes the speed of air.



Figures 6 and 7 illustrate the logarithmic plots of flame spread rate versus Da for a particle sizerange of 0.5 to 5 mm and a bed depth of 39.9 mm. In each case, the flame spread rate (U f ) hasbeen normalized by a theoretical flame spread rate (Utheo) which has been equated to the flamespread rate under quiescent (i.e. no airflow) conditions, U f 0. This is justified since past studies [21]have shown that at airflow speeds close to zero (i.e. Da�O[103]) the U f /Utheo ratio approachesunity, implying that U f 0 =Utheo.

As Figures 6 and 7 show, for both airflow directions the U f /Utheo ratio remains close to unityeven at air speeds as high as 0.1m/s (Da ≈O[102]) indicating that tflow and tchem are approximatelyof the same order of magnitude and hence, under such conditions the effect of airflow on theflame spread is not very dramatic. At higher air speeds (i.e. lower Da) tflow and tchem divergefrom each other, in turn, leading to a decrease in the ratio of U f /Utheo. In the case of assistedairflow for example, (Figure 6) when the air speed is increased to 0.5m/s (Da ≈4) the ratio ofU f /Utheo drops to about 0.7, whereas at the same Damköhler number a bigger drop results underthe opposed airflow conditions, U f /Utheo ≈0.4.

Figure 6. Normalized flame spread rate versus Damköhler number under the assisted airflowconfiguration for a 39.9 mm deep bed.

Figure 7. Normalized flame spread rate versus Damköhler number under the opposed airflowconfiguration for a 39.9 mm deep bed.



The sharpest drops in U f /Utheo for both assisted and opposed airflow configurations are observedfor Da �1 (i.e. Da �O[101]) where tflow and tchem are no longer of the same orders of magnitude.This can happen due to a number of reasons. For example, under assisted airflow conditions, thesupply of the oxidizer (i.e. O2) to the flame front may significantly increase, whereas the supplyof combustible gases from the bed to the flame front may remain unchanged. This imbalance canpotentially slow down or in extreme cases prevent the combustible gas mixture to reach its lowerflammability limit (i.e. fuel lean mixture). The end result can be a much longer chemical reactiontime (i.e. retardation of chemical reaction) and thereby lower Da and flame spread rates. However,under assisted airflow the flame envelop leans towards the bed surface ahead of the flame increasingthe convective and radiative heat transfer between the flame and the bed surface. As a result, theunburned regions of the bed ahead of the flame are preheated faster leading to an increase in therate of fuel evaporation. This phenomenon can partly compensate the retardation of the chemicalreaction due to increased airflow. On the contrary, under opposed airflow conditions the flameleans back towards the regions of the bed that had already experienced the passage of the flameand hence, posses relatively high temperatures. In this case, the increased heat transfer to the beddue to the inclination of the flame cannot influence the preheating process or the rate of fuelevaporation ahead of the flame. Moreover, under opposed airflow conditions the gaseous fuel–airmixture can be completely blown off the surface of the bed if the flow is strong enough, in turn,delaying the gas-phase combustion process. Therefore, on the whole the flame attains a smallerrate of spread under the opposed airflow configuration.

As noted above, the rate of fuel evaporation at the surface of the bed is a key parameter thatinfluences the rate of chemical reaction and, thereby, tchem. The evaporation of the liquid fuel atthe bed surface is underpinned by the rate of fuel supply to the surface, the net heat flux at thebed surface and the thermophysical properties of the fuel (e.g. boiling temperature, specific heat,viscosity, etc.). For a given fuel exposed to a known heat flux, the rate of fuel evaporation is largelydetermined by the supply of the fuel to the surface. For cases, where the bed is soaked in liquidfuel (i.e. unlimited fuel), the supply of the fuel to the surface is largely underscored by the motionof the liquid in the bed [9, 10].

However, when finite quantities of fuel are involved like cases studied in the present investigation,the liquid fuel does exist as a continuous layer throughout the bed and, as such, the rate of fuelsupply to the bed is not necessarily determined by the liquid motion. In these cases, the liquid fueladheres to the surfaces of the beads due to surface tension forming a thin layer. The movement of theliquid fuel through the porous bed towards the bed surface is, therefore, determined by the balancebetween gravitational forces pulling down the liquid and the surface tension and, thereby, capillaryforces. Whether the capillary forces act against the gravity depends on the relative magnitude ofthe cohesive forces between the liquid molecules (i.e. surface tension) and the adhesive forcesbetween the liquid and the surface of the bed particles. If the contact angle is less than 90◦ thenadhesion > cohesion (the liquid wets the bed particles) and, as a result, the liquid rises throughthe bed and travels towards the bed surface (i.e. capillary forces act against gravity). If the contactangle is greater than 90◦ then adhesion < cohesion (the liquid does not wet the particles) and,as a result, the liquid falls in the bed and aids the gravitational pull. Most liquid fuels, however,tend to wet the surface of typical solids and, as such, for these fuels the capillary effect worksagainst the gravity.

For a given fuel and a given heat flux the above force balance is largely a function of the physicalproperties of the porous bed (e.g. depth, particle size, etc.) rather than the liquid motion. Figures 8and 9 illustrate the correlation between the bed particle size and flame spread rate for various beddepths at an air speed of 0.5m/s for assisted and opposed airflow configurations, respectively. Thefamiliar inverse proportionality between the particle size and the flame spread rate for both airflowdirections is observed in Figures 8 and 9, although the variation shown in Figure 8 for the assistedairflow is much more linear than that shown in Figure 9 for the opposed airflow case. Moreover,there is also an inverse relationship between the bed depth and the flame spread rate for bothair flow directions. The results for shallower beds (i.e. 13.3 and 26.6 mm) in particular are muchcloser when compared with those of the 39.9 mm deep bed. On an average the flame spread ratescorresponding to the 39.9 mm bed are 40% slower than those of their 13.3 mm bed counterparts



Figure 8. Plots of the flame spread rate against particle size for a range of bed depth under the assistedairflow configuration for an air speed of 0.5m/s.

Figure 9. Plots of the flame spread rate against particle size for a range of bed depth under the opposedairflow configuration for an air speed of 0.5m/s.

under assisted airflow. Similarly, the difference between 39.9 and 13.3 mm results under opposedairflow is on an average about 59%. The results presented in Figures 8 and 9 indicate that thedeeper the bed and the larger the particle size, the slower the rate of flame spread. This impliesthat the rate of fuel supply to the surface for larger particles and deeper beds should be generallysmaller than those corresponding to shallow beds and/or fine bed particles when finite quantitiesof fuel are present.

To verify the validity of this statement, a series of measurements were carried out to determinethe average fuel consumption rate. For this purpose, the weight of the liquid fuel in the bed wasmeasured before and after the flame spread experiments. The results of the fuel consumptionmeasurements have been summarized in Figure 10 where the fuel consumption rate per unitsurface area (i.e fuel consumption flux expressed in g ·m−2 ·s−1) has been plotted against the beddepth for particle sizes of 0.5, 1, and 5 mm under no airflow conditions. As can be seen, the fuelconsumption flux is indeed inversely proportional to the bed depth and particle size confirmingour earlier statement.



Figure 10. Average fuel consumption flux versus bed depth for particle sizes of 0.5, 1,and 5 mm under no airflow conditions.

The practical implication of the above finding in industrial settings is that the risk of flamespread due to accidental spillage and ignition of a combustible liquid can be greatly minimized ifthe ground is composed of a thick bed of a highly porous non-combustible material (e.g. graveland concrete, etc.) with grain sizes larger than 3 mm and preferably non-wetting properties (i.e. acontact angle of greater than 90◦) in relation to the liquid fuels of interest. Such bed propertieswould assist the gravitational and capillary forces in generating a highly effective downward flowof fuel particles away from the surface in turn starving the bed surface and the reaction zone froma continuous supply of the fuel.

CONCLUSIONS

The flame spread over porous beds of sand wetted with finite quantities of propenol was experimen-tally studied. The aim was to examine the key characteristics of flame propagation over porous bedswhen unlike past studies only finite quantities of fuel were available. The following conclusionscan be drawn based on the experimental results presented in this paper:

• The rate of flame spread under limited fuel supply conditions, unlike its unlimited fuel supplycounterpart, is a monotonic function of parameters such as the air speed, bed particles sizeand bed depth.

• The rate of flame spread under limited fuel supply conditions is inversely proportional withthe air speed, bed particle size, and bed depth.

• Generally, the rates of flame spread for assisted airflow configuration are higher than thosecorresponding to the opposed airflow configuration.

• Much of the observed variation in the flame spread with key controlling parameters (e.g. airspeed, bed particle size, and bed depth) can be explained in terms of the Damköhler numberwhich expresses the relative importance of tflow and tchem.

• The spread of an unwanted industrial fire can be minimized by preparing the ground from athick bed of a highly porous non-combustible material with grain sizes larger than 3 mm andpreferably non-wetting properties.

ACKNOWLEDGEMENTS

The authors acknowledge the financial support provided to them by the University of Newcastle (Australia).



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Flame spread over porous sand beds wetted with propenol

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