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Biodiesel oil pool fire under air crossflow conditions: Burning rate, flame geometric parameters and temperatures
International Journal of Heat and Mass Transfer 149 (2020) 119164
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International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/hmt
Biodiesel oil pool fire under air crossflow conditions: Burning rate, flame geometric parameters and temperatures Rafael Gialdi Salvagni a, Maria Luiza Sperb Indrusiak b, Felipe Roman Centeno b,∗ a b
Graduate Program in Mechanical Engineering, Universidade do Vale do Rio dos Sinos, Av. Unisinos, n. 950, São Leopoldo, RS, Brazil Department of Mechanical Engineering, Federal University of Rio Grande do Sul, Rua Sarmento Leite, n. 425, Porto Alegre, RS, Brazil
a r t i c l e
i n f o
Article history: Received 4 August 2019 Revised 11 November 2019 Accepted 2 December 2019
Keywords: Biodiesel Pool fire Burning rate Flame geometry Air crossflow
a b s t r a c t The replacement of fossil fuels with renewable ones is an irreversible worldwide trend. More specifically, biodiesel made either from animal fat residue or vegetable oil is being increasingly used. Biodiesel storage tanks are as prone to hazardous fires as other fuel tanks. In this article we present the results of an experimental investigation of biodiesel B100 pool fires in reduced scale submitted to an air crossflow. Regular and infrared images were used to obtain flame geometric parameters and temperature fields for conditions ranging from quiescent air to crossflow velocities of up to 4.0 m/s. Mass burning rates were also obtained for the same air flow conditions. Results were compared with a previous study of regular diesel pool fires conducted with the same experimental apparatus and correlations from literature. It was shown that, compared to diesel fires, biodiesel fires presented longer flames and higher temperatures both in the flame and plume regions as well as downstream of the pool. This indicates that biodiesel storage devices are likely more prone to dangerous fire events than diesel ones. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction Pool fires are a proper way to study the behavior of hazardous fires in fuel storage tanks as well as fuel leakages either onshore or offshore. Mass burning rates obtained from pool fire experiments are related to heat release rates and, together with flame geometry, are key parameters to ascertain the potential impact of these fires in multiple situations such as storage tank parks and fuel plants. Literature has been continuously presenting new approaches to the theme as in the work of Kuang et al. [1] which studied the influence of lip height in the burning rates of n-heptane square pool fires under crosswind. They observed that, for pools of 15 and 20 cm in width and small lip heights, the burning rate first increased with wind velocity, then decreased and finally increased again. Flame sag of square n-heptane pool fires in crosswind was studied by Zhang et al. [2] with a methodology similar to the present article to determine the geometry of the flame. For pools with the rim at ground level, one of the main parameters is the flame base drag length. For example, Tang et al. [3] studied rectangular hydrocarbon pools of various aspect ratios and crosswind velocities up to 4.5 m/s. They found that the drag length increased with wind speed until a critical point and then
∗
Corresponding author. E-mail address: [email protected] (F.R. Centeno).
https://doi.org/10.1016/j.ijheatmasstransfer.2019.119164 0017-9310/© 2019 Elsevier Ltd. All rights reserved.
decreased, with some unburnt fuel being blown away. The critical velocity depended on the burner aspect ratio and heat release rate. Another condition that can enhance the hazardous potential of storage tank fires is the presence of a water layer at the bottom of the tank. When this water exceeds its boiling temperature an eruptive vaporization occurs, dragging fuel droplets and enhancing flame size and radiative transfer rate. Ping et al. [4] obtained flame geometric characteristics of this phenomenon under crosswind conditions. According to Moreno et al. [5], biodiesel production processes may be mistakenly considered safer than conventional diesel production just by being a green fuel and this misconception leads to concerns about accident hazards in the biodiesel production chain. They found that most frequent accidents are related to fire in production and storage areas. Indeed, they analyzed world data from 2003 to 2017 and concluded that 66% of the accidents are related to storage tanks and process buffers. Calvo Olivares et al. [6] conducted a similar study addressing the period from 2003 to 2013, with similar outcomes. They found that during that period, 73% of the accidents occurred in the storage and processing areas and most of them during normal operation of the plant. After analyzing data from essays with three biodiesel samples, Shibata et al. [7] concluded that the spontaneous ignition risk of biodiesel is higher than that of vegetable oils. According to Jones [8], increased fire hazard in biodiesel plants in comparison to un-
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Nomenclature D Fr g Lf m˙ Uc U∞ Uc, mod T T∗ Tmeas Tamb Tmax
pool pan diameter (m) Froude number gravitational acceleration (m/s²) flame length (m) mass burning rate (kg/(m²•s)) characteristic air velocity (m/s) air crossflow velocity (m/s) modified characteristic air velocity (m/s) Temperature (K) dimensionless temperature measured temperature with the thermographic camera (K) room temperature (K) higher measured temperature for each position analyzed (flame or plume) (K)
Greek Symbols θ flame tilt angle (°) ρa ambient air density (kg/m³) ρg fuel vapor density (kg/m³)
Lf = 70 D
ρa gD
cos θ =
U −0.49 U ∞ ∞ Uc
;
Uc
≥1
(1)
;
Uc, mod
Lf = D
−0.5
U∞
(2)
U∞ >1 Uc, mod
−0.19
m˙
ρa gD
(3)
0.06
U∞
(4)
Uc, mod
Moorhouse [22] experimentally studied LNG pool flames and proposed correlations for flame tilt angle and length considering pools of 6.1 m × 6.1 m to 15.2 m × 12.2 m and U∞ from 1.8 m/s to 14.4 m/s:
U −0.272 U ∞ ∞
Lf = 4.7 D
;
Uc
cos θ = 0.7
F r −0.11
Tilt angle and flame length empirical correlations in LNG pool fires of 1.8 m to 24.4 m in diameter and air crossflow velocity (U∞ ) of 1.3 m/s to 7.9 m/s were obtained by Atallah and Raj [19] apud [18]:
cos θ = 0.87
processed plant oil was due to the esterification with methanol, which enhanced the reactivity of the product. Diesel burning behavior investigations are commonly found in literature [e.g. 9–13] but the corollary cannot be said of biodiesel. One such study by Rahman et al. [14] obtained flame height, mass burning rate and flame temperature profile in quiescent conditions for biodiesel produced from palm oil. They observed that all three characteristics are directly proportional to the pool diameter. Oyelaran et al. [15] obtained physicochemical properties of mango seed biodiesel and its blends with diesel using ASTM standards. They concluded that the property values obtained were within those accepted for use as renewable fuel. Chaudhary et al. [16] studied pool fires of jatropha biodiesel and blends with diesel in a ventilated compartment using a 0.6 m pan. The main results were that the burning rate of biodiesel was higher than that of diesel, plume centerline temperature increased and combustion efficiency decreased with the increase of biodiesel fraction in the blend. They also observed that the combustion characteristics of blended fuels cannot be predicted accurately based on blending ratio. Rantuch et al. [17] produced biodiesel from cooking sunflower oil waste using methanol for transesterification. The resulting product was properly filtered and characterized. Determination of combustion characteristics was performed using a cone calorimeter with external heat flux of 20 kW/m2 and 25 kW/m2 . Results indicated that a higher heat flux increased fire hazards and smoke production. Two geometric parameters are used for flame geometry characterization: flame length (Lf ) and tilt angle (θ ), according to Fig. 1. Lam and Weckman [18] made a comparison of various correlations for flame geometrical parameters from literature with their experimental results. Some of these correlations are employed in the present article. The correlations for flame length and tilt angle are functions of the air crossflow velocity (U∞ ) and either the characteristic wind velocity (Uc = gm˙ D/ρa )1/3 ) or the modified characteristic wind velocity (Uc, mod = (gm˙ D/ρa )1/3 ) [19]. Thomas et al. [20], and Thomas [21] studied fires in forestry materials. They used wooden cribs and incident velocities of 1.5 m/s to 5.6 m/s. Their tilt angle and length correlations proposed were, respectively:
0.86
m˙
m˙
0.121
≥1
Uc
U −0.114 ∞
ρa gD
Uc
(5)
(6)
Ferrero et al. [23] studied the effect of boilover on flame parameters in fires of thin layers of gasoline and diesel over water in large pool fires of 1.6 m to 6.0 m diameter for U∞ = 0 m/s to 2.3 m/s. They observed that flame length increased during water ebullition but the tilt angle was not affected. The correlations obtained from their experimental data were:
cos θ = 0.92
U −0.26 U ∞ ∞ Uc
Lf = 4.201 D
m˙
;
Uc
0.181
≥1
U −0.082 ∞
ρa gD
Uc
(7)
(8)
Tang et al. [24] studied the effects of reduced pressure on combustion parameters of acetone pool fires. They used rectangular pool fires of aspect ratios ranging from 1 to 8, with sides of 0.07 m to 0.58 m. The cross-wind velocities varied from 0.0 m/s to 3.0 m/s. For normal pressure (100 kPa), the experimental results correlated well with Eq. (9):
cos θ = 1.06
U −0.73 U ∞ ∞ Uc
;
Uc
≥1
(9)
The correlation for flame length obtained by [25] from experiments using 0.1 m to 0.3 m square pools of acetone with U∞ = 0.0 m/s to 2.5 m/s as a function of the Froude number 2 /gD) is given by: (F r = U∞
Lf = 0.8F r 0.5 + 2.8 D
(10)
Results from diesel pool fires under crosswind conditions have been presented by the authors in a previous article [13]. Considerations about the increase in the production and use of biodiesel, due to its environmental advantages over fossil fuels, naturally extended the research to biodiesel behavior under the same conditions. Based on that, the current article aims to present geometric characteristics, mass burning rates and temperatures (flame and plume gaseous regions, and adjacent to the pool) of a biodiesel pool fire with a crossflow in an aerodynamic channel. The pool was 11 cm in diameter with its burning classified as convection-driven
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Fig. 1. Experimental setup: test section, pool, thermocouples and cameras (IR and DSLR).
(referring to the heat feedback to the pool surface, which drives the fuel evaporation and then the burning process) and the wind velocity ranged from 0.0 m/s (quiescent air) to 4.0 m/s. From the author’s best knowledge, there is no previous research reported in the literature concerning the fire behavior of a convection-driven biodiesel pool fire subjected to air crossflow. 2. Methodology Biodiesel is produced through chemical reactions from oil plants like soybean, cooking oil waste or animal fatty residue. Biodiesel B100 was used in this research, which means a 100% biodiesel without fuel blend. Biodiesel B100 is produced from soybean oil (59.5%), animal fat (35%), canola oil (5%) and recovered oil (0.5%), and presents an HHV of 39,934 kJ kg−1 . Biodiesel is considered safer than diesel because its flash point (446 K) is higher than that of diesel (347 K according to [26]). For this reason, biodiesel samples had to be pre-heated to 393 K (value obtained experimentally) before the experiments. When the biodiesel was pre-heated to temperatures lower than 393 K, it would not ignite or, when ignited, combustion was not sustained. This article used the same aerodynamic channel of [13]. Fig. 1 presents the general disposition of the experimental setup, with location of the main instruments. Previous measurements using a pitot tube coupled to a digital manometer assured that the flow profile at the channel exit and consequently incident on the flame was quite uniform with a boundary layer much thinner than the pool height. Instruments used during the experiments were the same as in [13]: Infrared (IR) and DSLR cameras, digital thermometer with Ktype thermocouples, digital manometer, Pitot tube, digital scale, thermo-hygrometer and a data acquisition system. Mass burning rates were measured by the digital scale positioned below the pool, with the interposition of an insulation layer over the scale (not shown in Fig. 1 for clarity). The DSLR camera video recording was used to obtain flame geometric parameters. When needed, fuel volume inside the pool was kept constant by a feeding syringe with the fuel level maintained at 10 mm below the upper pool rim. Flame fluctuations, which occur irrespective the constant incident velocity, determined the use of average values for each experiment. Post-processing of the video recording frames was performed following the same procedure described in [13]. A sensitivity test of geometric parameters results (flame length and tilt angle) relatively to the number of frames extracted from the video recordings indicated that 300 frames are enough for stabilized results. Geometry measurement errors were determined with a confidence interval of 95%. Flame detachment occurred for the highest crossflow velocities and the detached part, not only the continuous flame body, was also considered to compute geometric parameters.
Flame geometry results obtained by applying the empirical correlations described by Eqs. (1)–(10) for flame tilt angle and length were used for comparison with the present experimental data. Flame and plume IR-measured temperatures were taken from images provided by the thermographic camera software, which considered the maximum transient temperature data at each acquisition window. Two acquisition windows were placed respectively in the hottest region of the flame and of the plume (30 mm downstream the flame tip in the centerline trend direction). Results for each value of U∞ considered the average over more than 300 samples for each window. The temperature evaluation resulting from the thermographic method described should be considered only qualitatively. This was a result of the fixed emissivity value set in the thermographic camera – in contrast to the real emissivity variation over the flame and the plume. However, it was adequate to observe the influence of crossflow on pool fire thermal behavior. Since the temperatures measured by infrared techniques were not supposed to be the actual flame and plume temperatures, dimensionless temperatures were defined. The dimensionless temperature, T∗ , for both flame and plume regions (T f∗ and Tp∗ , respectively) were obtained in the usual form as in, e.g., [27] and [28]:
T∗ =
Tmeas − Tamb Tmax − Tamb
(11)
Temperatures for diesel from [13] were also rendered dimensionless for comparison purposes with present results. Lastly, temperatures downstream to the pool fire were measured with four K-type thermocouples, positioned as indicated in Fig. 1. 3. Results and discussions The experiments for mass burning rate, flame geometry, flame and plume temperatures and downstream temperature were conducted with ambient temperature ranging between 298 K and 302 K. With the exception of the mass burning rate experiment, all other experiments were conducted with fuel replacement to maintain a steady fuel level in the pool. The results for biodiesel were compared with those for diesel, which utilized the same experimental setup as presented in [13]. 3.1. Mass burning rate Fig. 2(a) shows the mass burning time history in the pool for three incident air velocities (U∞ = 0 m/s, 2 m/s and 4 m/s) for biodiesel and diesel while Fig. 2(b) depicts the evolution of mass burning rate with respect to air crossflow velocity for both fuels. As seen in Fig. 2(b), biodiesel mass burning rate presents a smooth behavior. In a quiescent environment, the biofuel burning rate is higher than diesel, decreasing as the air velocity increases until
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Fig. 2. (a) Fuel mass loss history and (b) mass burning rate as a function of air crossflow velocity (U∞ ). Results for diesel [13] and biodiesel (present study).
2 m/s. Then, for higher velocities the biofuel burning rate increases again. As described in detail in [13], the burning process of a pool fire is strongly influenced by the heat feedback of the flame to the pool surface. According to the pool diameter, the heat feedback can be classified among four categories of heat transfer mechanisms. The pool fire investigated in this study was classified as turbulent and convective. The variations in mass burning rate according to the air velocity shown in Fig. 2 is a combination of these heat transfer mechanisms. Air crossflow introduced additional air entrainment into the flame and changed its geometry, thus modifying the flame-pool surface heat feedback. Thermal buoyancy, fluid dynamics and shear stresses were the main phenomena conducting pool fires in open air with air crossflow. The increase of air crossflow velocity increased the turbulence at the reactive region, thus enhancing the intensity of combustion and the flame temperature. The flame tilt enhanced the feedback heat transfer and, due to these two combined heat transfer effects, the mass burning rate tended to increase at sufficiently high enough velocities. On the other hand, incoming air crossflow was usually much cooler than the burning region and chilled the liquid fuel surface and the flame. Thus, at low velocities, there was a trend to reduce the mass burning rate. The results for biodiesel presented good qualitative agreement with those obtained by Hu et al. [29] for methanol, Hu et al. [30] for heptane, and Jiang and Lu [31] for aviation fuel. Fig. 2(a) and (b) show that the mass burning rate of biodiesel is less influenced by air velocity than that of diesel. A possible reason for that would be the different composition of those fuels. While diesel is basically a chain of Carbon and Hydrogen, the biodiesel chain also includes Oxygen, meaning that the additional air entrained by the crossflow air affects less its burning rate. Additional considerations could be extracted from the respective physical properties and flame-pool heat exchange behavior of biodiesel and diesel. Biodiesel presents flash point and vaporization temperatures higher than diesel ones, thus the cooling effect of the air cross flow is more effective for biodiesel burning rates at the higher air velocities. Preheating of the biodiesel samples before experiments to temperatures higher than that of diesel could be responsible for the smoother variation of the experimental values at the lowest air velocities. As a result of these considerations, the maximum biodiesel burning rate measured in this experiment was 4.1% higher than its minimum, while the corresponding value for diesel was of 142.3%.
3.2. Flame geometric parameters The behavior of the geometric parameters of the biodiesel flame submitted to air crossflow is presented in Figs. 3 (flame tilt angle, θ ) and 4 (flame length, Lf ). Figs. 3(a) and 4(a) show comparisons between experimental data for diesel [13] and for biodiesel (present study), while Figs. 3(b) and 4(b) depict comparisons between the present experimental data and results calculated with correlations from literature. The biodiesel flame, when submitted to air crossflow, tended to tilt and shorten. Fig. 3(a) shows that the tilt becomes very pronounced once the air velocity departs from zero, grows slightly until U∞ = 2.0 m/s and almost stabilizes at θ = 70° for U∞ > 2.0 m/s. The flame length, shown in Fig. 4(a) is 340 mm in quiescent air and decreases asymptotically to about 175 mm as air velocity increases. The length of the biodiesel flame is larger than that of the diesel, indicating that the oxidation process takes more time to occur as there is more fuel vapor to burn. Effectively, as can be seen in Fig. 2(b) for low air crossflow velocities, the mass burning rates for biodiesel are generally higher indicating that the vaporization process releases more fuel vapor for the biodiesel flame than in the case of diesel. As the air velocity increases and the mass burning rate of biodiesel becomes lower than that of diesel, the flame length approaches that of diesel, but it is still larger. The comparison of experimental data for flame tilt angle with results from correlations presented in Eqs. (1), (3), (5), (7) and (9) resulted in good agreement for Eqs. (5) and (7), as shown in Fig. 3(b). The mean and maximum deviations (calculated between results from correlations and experimental data) considering all but zero air velocities were 4% and 15% for Eq. (5) (fitted for LNG [22]) and 5% and 8 % for Eq. (7) (fitted for diesel and gasoline [23]). Fig. 4(b) presents the comparison between the experimental data for biodiesel flame length and correlations presented in Eqs. (2), (4), (6), (8) and (10). The best agreement is obtained with Eq. (6), originally fitted for large LNG pools and U∞ = 1.8 m/s to 14.4 m/s [22]. The mean deviation between experimental data and results obtained from Eq. (6) were 11% (for air velocities of 0.5 m/s to 2.5 m/s) and 26% (air velocities of 3.0 m/s to 4.0 m/s). The largest deviation, 31%, was found at an air velocity of 3.0 m/s and the lowest one, 6%, at 2.0 m/s, with a mean deviation of 17% over all crossflow velocities. The second best agreement is found with Eq. (8) (fitted for large pools of diesel and gasoline, for U∞ = 0 m/s to 2.3 m/s [23]), providing mean and maximum deviations of 20% and 39%, respectively. These results demonstrated the major impor-
R.G. Salvagni, M.L.S. Indrusiak and F.R. Centeno / International Journal of Heat and Mass Transfer 149 (2020) 119164
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Fig. 3. Flame tilt angle (θ ) × Air crossflow velocity (U∞ ): (a) experimental data for diesel [13] and for biodiesel (present study), (b) present experimental data (biodiesel) and results calculated with correlations from literature.
Fig. 4. Flame length (Lf ) × Air crossflow velocity (U∞ ): (a) experimental data for diesel [13] and for biodiesel (present study), (b) present experimental data (biodiesel) and results calculated with correlations from literature.
tance of the crossflow velocity as compared to fuel and pool size for prediction of both geometric parameters (flame length and tilt angle) since the results obtained using correlations from [22] were as good as, or even better than, those using correlations from [23]. 3.3. Flame and plume temperatures The flame and plume dimensionless IR temperatures (T f∗ and Tp∗ , respectively) obtained from thermographic images are presented on Fig. 5(a) and (b). Those figures also show the results for diesel [13], rendered dimensionless for comparison purposes. The ambient temperature (Tamb ) in Eq. (11) was considered as 300 K, while Tmax was 912.4 K for computing T f∗ (flame region) and 477.1 K for Tp∗ (plume region) for both diesel and biodiesel. The maximum temperatures measured with the IR camera during the experiments with biodiesel were higher than the maximum temperatures obtained when burning diesel [13], so the values of Tmax used in Eq. (11) were that of biodiesel flame and plume. Due to the characteristics of the thermographic equipment, temperatures obtained do not represent the actual temperatures of flame and plume and should be taken only for comparing their behavior when influenced by the air crossflow and by different burning fuels. Fig. 5 presents IR-measured dimensionless temperatures for diesel [13] and biodiesel (present study) as functions of the air
crossflow velocity (U∞ ) at the flame region (T f∗ ), Fig. 5(a), and at the plume region (Tp∗ ), Fig. 5(b). As observed in Fig. 5(a), for U∞ ≥ 1.0 m/s, the growing trend of the dimensionless temperature at the flame region, T f∗ , with air crossflow velocity is similar for diesel and biodiesel. Additionally, T f∗ values are also higher for biodiesel, which can be attributed to the O2 in the biodiesel molecules contributing to the oxidation process and leading to higher temperatures. Another factor that may contribute to the higher temperatures when burning biodiesel is the higher flash point temperature of biodiesel compared to diesel. Both fuels needed preheating to enhance the evaporation of fuel to initiate the combustion process: in the current experiments biodiesel was preheated to 393 K, while diesel in [13] was preheated to 323 K (i.e., a preheating temperature difference of 70 K, or 22%). On the other hand, at the lowest crossflow velocity range (U∞ = 0 m/s and 0.5 m/s), Fig. 5(a) shows different T f∗ behavior for biodiesel and diesel. For biodiesel, T f∗ first decreases from U∞ = 0 m/s to U∞ = 0.5 m/s, while for diesel it presents an ascending behavior in this U∞ range. Such difference on the T f∗ behavior can be attributed to the preheating temperature difference between diesel and biodiesel. As U∞ increased to 1.0 m/s and beyond, the convective heat exchanges from/to the flame, fuel surface and pool pan tended to cool down the liquid fuel, con-
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Fig. 5. IR-measured dimensionless temperature (T∗ ) × Air crossflow velocity (U∞ ): (a) Flame region (T f∗ ), (b) Plume region (Tp∗ ). Results for diesel [13] and biodiesel (present study).
Fig. 6. Temperature in the region adjacent to the pool at several thermocouple positions (TC_) × Air crossflow velocity (U∞ ): results for (a) biodiesel (present study) and (b) diesel [13].
sequently leading to a similar behavior of T f∗ for both diesel and biodiesel. Finally, it is interesting to note that T f∗ for biodiesel was 52% higher than for diesel for quiescent air, while for U∞ = 0.5 m/s to 4.0 m/s such difference was around 28%. These results indicated that both biodiesel composition and preheating contributed to the T f∗ rise for quiescent air, while the convective heat exchanges reduced the preheating importance on T f∗ for higher crossflow velocities. The IR-measured dimensionless temperatures at the plume region (Tp∗ ) shown in Fig. 5(b) are also higher for biodiesel (20% higher than diesel on average, for the whole U∞ range). Unlike T f∗ , the plume dimensionless temperature, Tp∗ , presents the same descending trend for both fuels within the whole U∞ range of this experiment. Such behavior was a consequence of the mixture of fresh air with the hot plume gases (the higher U∞ , the fresher air entrainment into the plume).
(present study) and Fig. 6(b) for diesel (data from [13]), reproduced here for comparison purposes. Fig. 6 shows that biodiesel pool fire presents a behavior like diesel ones, with higher temperatures closer to the pool for all crosswind velocities. Temperatures for biodiesel are higher than for diesel pool fires, which agrees with results for flame and plume temperatures of Fig. 5 (T f∗ and Tp∗ ). Both fuels present temperature oscillations from U∞ = =0 m/s to U∞ = =2.0 m/s. This was a result of flame tilt angle growing with air velocity and promoting radiative heat transfer and temperature enhancement to the surroundings of the pool trailing edge. At the same time, the additional air amount entrained in the plume reduced its temperature. The dynamic balance between these two phenomena was responsible for the oscillatory behavior of the adjacent temperature. On Fig. 6 for U∞ > 2.0 m/s the balance between these two phenomena is stable and the temperature profile becomes nearly independent of the air crossflow velocity.
3.4. Temperature downstream the pool
4. Conclusions
Fig. 6 presents temperature distributions in the region adjacent to the pool for several thermocouple locations (TC_50 to TC_200, as shown in Fig. 1, positioned 50 mm, 100 mm, 150 mm and 200 mm downstream from the pool rim) as a function of the air crossflow velocity, U∞ . Fig. 6(a) depicts results for biodiesel
This article presented an experimental analysis of biodiesel pool fires under air crossflow in controlled laboratory conditions. The experiments intended to simulate actual fire conditions in fuel storage tanks in open air. The experiments were conducted in the outlet section of an aerodynamic channel with a reduced-scale
R.G. Salvagni, M.L.S. Indrusiak and F.R. Centeno / International Journal of Heat and Mass Transfer 149 (2020) 119164
pool. Results for mass burning rate, flame geometry and temperatures of flame, plume and region adjacent to the pool were obtained for quiescent air and air crossflow velocities up to 4 m/s. The outcomes were compared with previous results for diesel under similar experimental conditions and correlations from literature. The main results observed were that the biodiesel mass burning rate decreased with the increase of air crossflow velocity up to U∞ = 2 m/s and increased for U∞ > 2 m/s and the flame tilt angle rose up to 70° at U∞ = 2 m/s, becoming constant for U∞ > 2 m/s. The flame length decreased proportionally to the air velocity increase, but the values were larger than those of diesel. The flame geometric values (length and tilt angle) obtained experimentally were compared with results obtained from correlations available in the literature. Despite being formulated from experiments with different conditions and fuels, some of them presented good agreement. For the flame tilt angle, the least mean deviation was of 4%, and for the flame length it was 17%. The IR-measured dimensionless temperature at the biodiesel flame region increased with U∞ , while it was reduced at the plume region. The results were qualitatively equivalent to the results obtained for diesel pool fires [13], nevertheless the temperatures for biodiesel were higher than that obtained for diesel. These results, as well as the results for mass burning rate, were in agreement with the conclusions of Chaudhary et al. [16]. As a concluding remark, it is worth to comment the importance of research addressing biofuel fires, like the present one, from the safety and fire hazard point of views. Although this study was carried out in small scale and under laboratory conditions, it was possible to observe the importance of the analysis with this type of fuel. Considering the data obtained, biodiesel storage devices are likely more prone to more dangerous fire events than diesel ones, due to the higher temperatures and longer flames. These findings agree with the considerations presented by Moreno et al. [5] and Calvo Olivares et al. [6]. A better knowledge of the behavior of biodiesel under fire conditions is of utmost importance for strategic fire-fighting planning and as an auxiliary tool for decisionmaking in projects of fuel storage tank parks to prevent the fire from spreading to neighboring tanks or other structures. Declaration of Competing Interest None. Acknowledgment Author FRC thanks CNPq for research grant 400472/2016-3. References [1] C. Kuang, L. Hu, X. Zhang, Y. Lin, L.W. Kostiuk, An experimental study on the burning rates of n-heptane pool fires with various lip heights in cross flow, Combust. Flame 201 (2019) 93–103 https://doi.org/10.1016/j.combustflame. 2018.12.011. [2] X. Zhang, X. Zhang, L. Hu, R. Tu, M.A. Delichatsios, An experimental investigation and scaling analysis on flame sag of pool fire in cross flow, Fuel 241 (2019) 845–850 https://doi.org/10.1016/j.fuel.2018.12.020. [3] F. Tang, Q. He, J. Wen, Effects of crosswind and burner aspect ratio on flame characteristics and flame base drag length of diffusion flames, Combust. Flame 200 (2019) 265–275 https://doi.org/10.1016/j.combustflame.2018.11.011. [4] P. Ping, J. Zhang, D. Kong, Z. Xu, H. Yang, Experimental study of the flame geometrical characteristics of the crude oil boilover fire under cross air flow, J. Loss Prev. Process Ind. 55 (2018) 500–511 https://doi.org/10.1016/j.jlp.2017.12. 005. [5] V.C. Moreno, E. Danzi, L. Marmo, E. Salzano, V. Cozzani, Major accident hazard in biodiesel production processes, Saf. Sci. 113 (2019) 490–503 http://dx.doi. org/10.1016/j.ssci.2018.12.014.
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[6] R. Calvo Olivares, S.S. Rivera, J.E. Núñez Mc Leod, Database for accidents and incidents in the biodiesel industry, J Loss Prev. Process Ind 29 (2014) 245–261 https://doi.org/10.1016/j.jlp.2014.03.010. [7] Y. Shibata, H. Koseki, O. Shimizu, Spontaneous ignition of biodiesel: a potential fire risk, Therm. Sci. 12 (2008) 149–158 https://doi.org/10.2298/TSCI0802149S. [8] J.C. Jones, Esterification: a possible exacerbation of the fire hazards of biodiesel, J. Fire Sci. 28 (2010) 304–305 https://doi.org/10.1177/ 0734904110362345. [9] R.M. Leite, F.R. Centeno, Effect of tank diameter on thermal behavior of gasoline and diesel storage tanks fires, J. Hazard. Mater. 342 (2018) 544–552 https: //doi.org/10.1016/j.jhazmat.2017.08.052. [10] F. Ferrero, M. Muñoz, J. Arnaldos, Effects of thin-layer boilover on flame geometry and dynamics in large hydrocarbon pool fires, Fuel Process. Technol. 88 (2007) 227–235 https://doi.org/10.1016/j.fuproc.2006.09.005. [11] S. Sudheer, S.V. Prabhu, Measurement of flame emissivity of hydrocarbon pool fires, Fire Technol. 48 (2012) 183–217 http://dx.doi.org/10.1007/ s10694-010-0206-5. [12] H.C. Hottel, Review of certain laws governing the diffusive burning of liquids, Fire Res. Abstracts Rev. 1 (1959) 41–43. [13] R.G. Salvagni, F.R. Centeno, M.L.S. Indrusiak, Burning rate, flame geometry and temperature of convection-controlled circular diesel oil pool fire under air crossflow conditions, J. Hazard. Mater. 368 (2019) 560–568 https://doi.org/10. 1016/j.jhazmat.2019.01.087. [14] M.R.A. Rahman, R. Mohan, K.Y. Leong, S.N.A. Baharom, Characteristics of the biodiesel palm oil methyl ester pool fire, in: MATEC Web of Conferences, 74, 2016, p. 15, doi:10.1051/matecconf/2016740 0 015. [15] O.A. Oyelaran, O.E. Omofumni, A.O. Fagbemigun, O. Balogun, Fuel and physicochemical properties of mango (Mangifera indica) seed biodiesel and its blends with diesel, Agric. Eng. Int. 20 (2018) 108–115. [16] A. Chaudhary, A. Gupta, S. Kumar, R. Kumar, Pool fires of jatropha biodiesel and their blends with petroleum diesel, Exp. Therm. Fluid Sci. 101 (2019) 175–185 https://doi.org/10.1016/j.expthermflusci.2018.10.021. [17] P. Rantuch, L. Blinová, A. Bartošová, J. Martinka, I. Wachter, Burning process characterization of biodiesel pool fires, J. Fire Sci. 37 (2019) 3–17 https://doi. org/10.1177/0734904118807627. [18] C.S. Lam, E.J. Weckman, Wind-blown pool fire, part II: comparison of measured flame geometry with semi-empirical correlations, Fire Saf. J. 78 (2015) 130–141 https://doi.org/10.1016/j.firesaf.2015.08.004. [19] S. Atallah, P. Raj, Radiation from LNG fires, LNG Safety Program, Interim Report on Phase II Work, Project IS-3-1, American Gas Association, Batelle Columbus Laboratories, 1974. [20] P. Thomas, R. Pickard, H. Wraight, On the Size and Orientation of Buoyant Diffusion Flames and the Effect of Wind, Fire Research Notes 516, Fire Research Station, Joint Fire Research Organization, Boreham Wood, UK, 1963 https://www.iafss.org/publications/frn/516/-1. [21] P. Thomas, Fire Spread in Wooden Cribs: Part III, the Effect of Wind, Fire Research Notes 600, Fire Research Station, Joint Fire Research Organization, Boreham Wood, UK, 1965 https://www.iafss.org/publications/frn/600/-1. [22] J. Moorhouse, Scaling criteria for pool fires derived from large scale experiments, in: The Assessment of Major Hazards, 1982, pp. 165–179. [23] F. Ferrero, M. Muñoz, J. Arnaldos, Effects of thin-layer boilover on flame geometry and dynamics in large hydrocarbon pool fires, Fuel Process. Technol. 88 (2007) 227–235 https://doi.org/10.1016/j.fuproc.2006.09.005. [24] F. Tang, L. Hu, X. Zhang, X. Zhang, M. Dong, Burning rate and flame tilt characteristics of radiation-controlled rectangular hydrocarbon pool fires with cross air flows in a reduced pressure, Fuel 139 (2015) 18–25 https://doi.org/10.1016/ j.fuel.2014.07.093. [25] F. Tang, L. Li, K. Zhu, Z. Qiu, C. Tao, Experimental study and global correlation on burning rates and flame tilt characteristics of acetone pool fires under cross air flow, Int. J. Heat Mass Transf. 87 (2015) 369–375 https://doi.org/10.1016/j. ijheatmasstransfer.2015.04.019. [26] American Petroleum Institute (API), API Publication 4261: Alcohols and Ethers: A Technical Assessment of Their Application as Fuel and Fuel Components, second ed., API, Washington, 1988. [27] R.L. Thompson, E.J. Soares, Viscoplastic dimensionless numbers, J. NonNewtonian Fluid Mech. 238 (2016) 57–64 https://doi.org/10.1016/j.jnnfm.2016. 05.001. [28] X. Shen, J. Lu, J. Ding, W. Wang, Transient heat transfer performance of molten salt tubular receiver, Energy Procedia 158 (2019) 541–546, doi:10.1016/ j.egypro.2019.01.149. [29] L. Hu, S. Liu, W. Peng, R. Huo, Experimental study on burning rates of square and rectangular gasoline and methanol pool fires under longitudinal air flow in a wind tunnel, J. Hazard. Mater. 169 (2009) 972–979, doi:10.1016/j.jhazmat. 2009.04.050. [30] L. Hu, S. Liu, Y. Xu, D. Li, A wind tunnel experimental study on burning rate enhancement behavior of gasoline pool fires by cross air flow, Combust. Flame 158 (2011) 586–591 https://doi.org/10.1016/j.combustflame.2010.10.013. [31] P. Jiang, S. Lu, Effects of cross air flow on mass loss rates of circular aviation fuel pool fires in large open space, Procedia Eng. 62 (2013) 309–316 https: //doi.org/10.1016/j.proeng.2013.08.069.
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International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/hmt
Biodiesel oil pool fire under air crossflow conditions: Burning rate, flame geometric parameters and temperatures Rafael Gialdi Salvagni a, Maria Luiza Sperb Indrusiak b, Felipe Roman Centeno b,∗ a b
Graduate Program in Mechanical Engineering, Universidade do Vale do Rio dos Sinos, Av. Unisinos, n. 950, São Leopoldo, RS, Brazil Department of Mechanical Engineering, Federal University of Rio Grande do Sul, Rua Sarmento Leite, n. 425, Porto Alegre, RS, Brazil
a r t i c l e
i n f o
Article history: Received 4 August 2019 Revised 11 November 2019 Accepted 2 December 2019
Keywords: Biodiesel Pool fire Burning rate Flame geometry Air crossflow
a b s t r a c t The replacement of fossil fuels with renewable ones is an irreversible worldwide trend. More specifically, biodiesel made either from animal fat residue or vegetable oil is being increasingly used. Biodiesel storage tanks are as prone to hazardous fires as other fuel tanks. In this article we present the results of an experimental investigation of biodiesel B100 pool fires in reduced scale submitted to an air crossflow. Regular and infrared images were used to obtain flame geometric parameters and temperature fields for conditions ranging from quiescent air to crossflow velocities of up to 4.0 m/s. Mass burning rates were also obtained for the same air flow conditions. Results were compared with a previous study of regular diesel pool fires conducted with the same experimental apparatus and correlations from literature. It was shown that, compared to diesel fires, biodiesel fires presented longer flames and higher temperatures both in the flame and plume regions as well as downstream of the pool. This indicates that biodiesel storage devices are likely more prone to dangerous fire events than diesel ones. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction Pool fires are a proper way to study the behavior of hazardous fires in fuel storage tanks as well as fuel leakages either onshore or offshore. Mass burning rates obtained from pool fire experiments are related to heat release rates and, together with flame geometry, are key parameters to ascertain the potential impact of these fires in multiple situations such as storage tank parks and fuel plants. Literature has been continuously presenting new approaches to the theme as in the work of Kuang et al. [1] which studied the influence of lip height in the burning rates of n-heptane square pool fires under crosswind. They observed that, for pools of 15 and 20 cm in width and small lip heights, the burning rate first increased with wind velocity, then decreased and finally increased again. Flame sag of square n-heptane pool fires in crosswind was studied by Zhang et al. [2] with a methodology similar to the present article to determine the geometry of the flame. For pools with the rim at ground level, one of the main parameters is the flame base drag length. For example, Tang et al. [3] studied rectangular hydrocarbon pools of various aspect ratios and crosswind velocities up to 4.5 m/s. They found that the drag length increased with wind speed until a critical point and then
∗
Corresponding author. E-mail address: [email protected] (F.R. Centeno).
https://doi.org/10.1016/j.ijheatmasstransfer.2019.119164 0017-9310/© 2019 Elsevier Ltd. All rights reserved.
decreased, with some unburnt fuel being blown away. The critical velocity depended on the burner aspect ratio and heat release rate. Another condition that can enhance the hazardous potential of storage tank fires is the presence of a water layer at the bottom of the tank. When this water exceeds its boiling temperature an eruptive vaporization occurs, dragging fuel droplets and enhancing flame size and radiative transfer rate. Ping et al. [4] obtained flame geometric characteristics of this phenomenon under crosswind conditions. According to Moreno et al. [5], biodiesel production processes may be mistakenly considered safer than conventional diesel production just by being a green fuel and this misconception leads to concerns about accident hazards in the biodiesel production chain. They found that most frequent accidents are related to fire in production and storage areas. Indeed, they analyzed world data from 2003 to 2017 and concluded that 66% of the accidents are related to storage tanks and process buffers. Calvo Olivares et al. [6] conducted a similar study addressing the period from 2003 to 2013, with similar outcomes. They found that during that period, 73% of the accidents occurred in the storage and processing areas and most of them during normal operation of the plant. After analyzing data from essays with three biodiesel samples, Shibata et al. [7] concluded that the spontaneous ignition risk of biodiesel is higher than that of vegetable oils. According to Jones [8], increased fire hazard in biodiesel plants in comparison to un-
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Nomenclature D Fr g Lf m˙ Uc U∞ Uc, mod T T∗ Tmeas Tamb Tmax
pool pan diameter (m) Froude number gravitational acceleration (m/s²) flame length (m) mass burning rate (kg/(m²•s)) characteristic air velocity (m/s) air crossflow velocity (m/s) modified characteristic air velocity (m/s) Temperature (K) dimensionless temperature measured temperature with the thermographic camera (K) room temperature (K) higher measured temperature for each position analyzed (flame or plume) (K)
Greek Symbols θ flame tilt angle (°) ρa ambient air density (kg/m³) ρg fuel vapor density (kg/m³)
Lf = 70 D
ρa gD
cos θ =
U −0.49 U ∞ ∞ Uc
;
Uc
≥1
(1)
;
Uc, mod
Lf = D
−0.5
U∞
(2)
U∞ >1 Uc, mod
−0.19
m˙
ρa gD
(3)
0.06
U∞
(4)
Uc, mod
Moorhouse [22] experimentally studied LNG pool flames and proposed correlations for flame tilt angle and length considering pools of 6.1 m × 6.1 m to 15.2 m × 12.2 m and U∞ from 1.8 m/s to 14.4 m/s:
U −0.272 U ∞ ∞
Lf = 4.7 D
;
Uc
cos θ = 0.7
F r −0.11
Tilt angle and flame length empirical correlations in LNG pool fires of 1.8 m to 24.4 m in diameter and air crossflow velocity (U∞ ) of 1.3 m/s to 7.9 m/s were obtained by Atallah and Raj [19] apud [18]:
cos θ = 0.87
processed plant oil was due to the esterification with methanol, which enhanced the reactivity of the product. Diesel burning behavior investigations are commonly found in literature [e.g. 9–13] but the corollary cannot be said of biodiesel. One such study by Rahman et al. [14] obtained flame height, mass burning rate and flame temperature profile in quiescent conditions for biodiesel produced from palm oil. They observed that all three characteristics are directly proportional to the pool diameter. Oyelaran et al. [15] obtained physicochemical properties of mango seed biodiesel and its blends with diesel using ASTM standards. They concluded that the property values obtained were within those accepted for use as renewable fuel. Chaudhary et al. [16] studied pool fires of jatropha biodiesel and blends with diesel in a ventilated compartment using a 0.6 m pan. The main results were that the burning rate of biodiesel was higher than that of diesel, plume centerline temperature increased and combustion efficiency decreased with the increase of biodiesel fraction in the blend. They also observed that the combustion characteristics of blended fuels cannot be predicted accurately based on blending ratio. Rantuch et al. [17] produced biodiesel from cooking sunflower oil waste using methanol for transesterification. The resulting product was properly filtered and characterized. Determination of combustion characteristics was performed using a cone calorimeter with external heat flux of 20 kW/m2 and 25 kW/m2 . Results indicated that a higher heat flux increased fire hazards and smoke production. Two geometric parameters are used for flame geometry characterization: flame length (Lf ) and tilt angle (θ ), according to Fig. 1. Lam and Weckman [18] made a comparison of various correlations for flame geometrical parameters from literature with their experimental results. Some of these correlations are employed in the present article. The correlations for flame length and tilt angle are functions of the air crossflow velocity (U∞ ) and either the characteristic wind velocity (Uc = gm˙ D/ρa )1/3 ) or the modified characteristic wind velocity (Uc, mod = (gm˙ D/ρa )1/3 ) [19]. Thomas et al. [20], and Thomas [21] studied fires in forestry materials. They used wooden cribs and incident velocities of 1.5 m/s to 5.6 m/s. Their tilt angle and length correlations proposed were, respectively:
0.86
m˙
m˙
0.121
≥1
Uc
U −0.114 ∞
ρa gD
Uc
(5)
(6)
Ferrero et al. [23] studied the effect of boilover on flame parameters in fires of thin layers of gasoline and diesel over water in large pool fires of 1.6 m to 6.0 m diameter for U∞ = 0 m/s to 2.3 m/s. They observed that flame length increased during water ebullition but the tilt angle was not affected. The correlations obtained from their experimental data were:
cos θ = 0.92
U −0.26 U ∞ ∞ Uc
Lf = 4.201 D
m˙
;
Uc
0.181
≥1
U −0.082 ∞
ρa gD
Uc
(7)
(8)
Tang et al. [24] studied the effects of reduced pressure on combustion parameters of acetone pool fires. They used rectangular pool fires of aspect ratios ranging from 1 to 8, with sides of 0.07 m to 0.58 m. The cross-wind velocities varied from 0.0 m/s to 3.0 m/s. For normal pressure (100 kPa), the experimental results correlated well with Eq. (9):
cos θ = 1.06
U −0.73 U ∞ ∞ Uc
;
Uc
≥1
(9)
The correlation for flame length obtained by [25] from experiments using 0.1 m to 0.3 m square pools of acetone with U∞ = 0.0 m/s to 2.5 m/s as a function of the Froude number 2 /gD) is given by: (F r = U∞
Lf = 0.8F r 0.5 + 2.8 D
(10)
Results from diesel pool fires under crosswind conditions have been presented by the authors in a previous article [13]. Considerations about the increase in the production and use of biodiesel, due to its environmental advantages over fossil fuels, naturally extended the research to biodiesel behavior under the same conditions. Based on that, the current article aims to present geometric characteristics, mass burning rates and temperatures (flame and plume gaseous regions, and adjacent to the pool) of a biodiesel pool fire with a crossflow in an aerodynamic channel. The pool was 11 cm in diameter with its burning classified as convection-driven
R.G. Salvagni, M.L.S. Indrusiak and F.R. Centeno / International Journal of Heat and Mass Transfer 149 (2020) 119164
3
Fig. 1. Experimental setup: test section, pool, thermocouples and cameras (IR and DSLR).
(referring to the heat feedback to the pool surface, which drives the fuel evaporation and then the burning process) and the wind velocity ranged from 0.0 m/s (quiescent air) to 4.0 m/s. From the author’s best knowledge, there is no previous research reported in the literature concerning the fire behavior of a convection-driven biodiesel pool fire subjected to air crossflow. 2. Methodology Biodiesel is produced through chemical reactions from oil plants like soybean, cooking oil waste or animal fatty residue. Biodiesel B100 was used in this research, which means a 100% biodiesel without fuel blend. Biodiesel B100 is produced from soybean oil (59.5%), animal fat (35%), canola oil (5%) and recovered oil (0.5%), and presents an HHV of 39,934 kJ kg−1 . Biodiesel is considered safer than diesel because its flash point (446 K) is higher than that of diesel (347 K according to [26]). For this reason, biodiesel samples had to be pre-heated to 393 K (value obtained experimentally) before the experiments. When the biodiesel was pre-heated to temperatures lower than 393 K, it would not ignite or, when ignited, combustion was not sustained. This article used the same aerodynamic channel of [13]. Fig. 1 presents the general disposition of the experimental setup, with location of the main instruments. Previous measurements using a pitot tube coupled to a digital manometer assured that the flow profile at the channel exit and consequently incident on the flame was quite uniform with a boundary layer much thinner than the pool height. Instruments used during the experiments were the same as in [13]: Infrared (IR) and DSLR cameras, digital thermometer with Ktype thermocouples, digital manometer, Pitot tube, digital scale, thermo-hygrometer and a data acquisition system. Mass burning rates were measured by the digital scale positioned below the pool, with the interposition of an insulation layer over the scale (not shown in Fig. 1 for clarity). The DSLR camera video recording was used to obtain flame geometric parameters. When needed, fuel volume inside the pool was kept constant by a feeding syringe with the fuel level maintained at 10 mm below the upper pool rim. Flame fluctuations, which occur irrespective the constant incident velocity, determined the use of average values for each experiment. Post-processing of the video recording frames was performed following the same procedure described in [13]. A sensitivity test of geometric parameters results (flame length and tilt angle) relatively to the number of frames extracted from the video recordings indicated that 300 frames are enough for stabilized results. Geometry measurement errors were determined with a confidence interval of 95%. Flame detachment occurred for the highest crossflow velocities and the detached part, not only the continuous flame body, was also considered to compute geometric parameters.
Flame geometry results obtained by applying the empirical correlations described by Eqs. (1)–(10) for flame tilt angle and length were used for comparison with the present experimental data. Flame and plume IR-measured temperatures were taken from images provided by the thermographic camera software, which considered the maximum transient temperature data at each acquisition window. Two acquisition windows were placed respectively in the hottest region of the flame and of the plume (30 mm downstream the flame tip in the centerline trend direction). Results for each value of U∞ considered the average over more than 300 samples for each window. The temperature evaluation resulting from the thermographic method described should be considered only qualitatively. This was a result of the fixed emissivity value set in the thermographic camera – in contrast to the real emissivity variation over the flame and the plume. However, it was adequate to observe the influence of crossflow on pool fire thermal behavior. Since the temperatures measured by infrared techniques were not supposed to be the actual flame and plume temperatures, dimensionless temperatures were defined. The dimensionless temperature, T∗ , for both flame and plume regions (T f∗ and Tp∗ , respectively) were obtained in the usual form as in, e.g., [27] and [28]:
T∗ =
Tmeas − Tamb Tmax − Tamb
(11)
Temperatures for diesel from [13] were also rendered dimensionless for comparison purposes with present results. Lastly, temperatures downstream to the pool fire were measured with four K-type thermocouples, positioned as indicated in Fig. 1. 3. Results and discussions The experiments for mass burning rate, flame geometry, flame and plume temperatures and downstream temperature were conducted with ambient temperature ranging between 298 K and 302 K. With the exception of the mass burning rate experiment, all other experiments were conducted with fuel replacement to maintain a steady fuel level in the pool. The results for biodiesel were compared with those for diesel, which utilized the same experimental setup as presented in [13]. 3.1. Mass burning rate Fig. 2(a) shows the mass burning time history in the pool for three incident air velocities (U∞ = 0 m/s, 2 m/s and 4 m/s) for biodiesel and diesel while Fig. 2(b) depicts the evolution of mass burning rate with respect to air crossflow velocity for both fuels. As seen in Fig. 2(b), biodiesel mass burning rate presents a smooth behavior. In a quiescent environment, the biofuel burning rate is higher than diesel, decreasing as the air velocity increases until
4
R.G. Salvagni, M.L.S. Indrusiak and F.R. Centeno / International Journal of Heat and Mass Transfer 149 (2020) 119164
Fig. 2. (a) Fuel mass loss history and (b) mass burning rate as a function of air crossflow velocity (U∞ ). Results for diesel [13] and biodiesel (present study).
2 m/s. Then, for higher velocities the biofuel burning rate increases again. As described in detail in [13], the burning process of a pool fire is strongly influenced by the heat feedback of the flame to the pool surface. According to the pool diameter, the heat feedback can be classified among four categories of heat transfer mechanisms. The pool fire investigated in this study was classified as turbulent and convective. The variations in mass burning rate according to the air velocity shown in Fig. 2 is a combination of these heat transfer mechanisms. Air crossflow introduced additional air entrainment into the flame and changed its geometry, thus modifying the flame-pool surface heat feedback. Thermal buoyancy, fluid dynamics and shear stresses were the main phenomena conducting pool fires in open air with air crossflow. The increase of air crossflow velocity increased the turbulence at the reactive region, thus enhancing the intensity of combustion and the flame temperature. The flame tilt enhanced the feedback heat transfer and, due to these two combined heat transfer effects, the mass burning rate tended to increase at sufficiently high enough velocities. On the other hand, incoming air crossflow was usually much cooler than the burning region and chilled the liquid fuel surface and the flame. Thus, at low velocities, there was a trend to reduce the mass burning rate. The results for biodiesel presented good qualitative agreement with those obtained by Hu et al. [29] for methanol, Hu et al. [30] for heptane, and Jiang and Lu [31] for aviation fuel. Fig. 2(a) and (b) show that the mass burning rate of biodiesel is less influenced by air velocity than that of diesel. A possible reason for that would be the different composition of those fuels. While diesel is basically a chain of Carbon and Hydrogen, the biodiesel chain also includes Oxygen, meaning that the additional air entrained by the crossflow air affects less its burning rate. Additional considerations could be extracted from the respective physical properties and flame-pool heat exchange behavior of biodiesel and diesel. Biodiesel presents flash point and vaporization temperatures higher than diesel ones, thus the cooling effect of the air cross flow is more effective for biodiesel burning rates at the higher air velocities. Preheating of the biodiesel samples before experiments to temperatures higher than that of diesel could be responsible for the smoother variation of the experimental values at the lowest air velocities. As a result of these considerations, the maximum biodiesel burning rate measured in this experiment was 4.1% higher than its minimum, while the corresponding value for diesel was of 142.3%.
3.2. Flame geometric parameters The behavior of the geometric parameters of the biodiesel flame submitted to air crossflow is presented in Figs. 3 (flame tilt angle, θ ) and 4 (flame length, Lf ). Figs. 3(a) and 4(a) show comparisons between experimental data for diesel [13] and for biodiesel (present study), while Figs. 3(b) and 4(b) depict comparisons between the present experimental data and results calculated with correlations from literature. The biodiesel flame, when submitted to air crossflow, tended to tilt and shorten. Fig. 3(a) shows that the tilt becomes very pronounced once the air velocity departs from zero, grows slightly until U∞ = 2.0 m/s and almost stabilizes at θ = 70° for U∞ > 2.0 m/s. The flame length, shown in Fig. 4(a) is 340 mm in quiescent air and decreases asymptotically to about 175 mm as air velocity increases. The length of the biodiesel flame is larger than that of the diesel, indicating that the oxidation process takes more time to occur as there is more fuel vapor to burn. Effectively, as can be seen in Fig. 2(b) for low air crossflow velocities, the mass burning rates for biodiesel are generally higher indicating that the vaporization process releases more fuel vapor for the biodiesel flame than in the case of diesel. As the air velocity increases and the mass burning rate of biodiesel becomes lower than that of diesel, the flame length approaches that of diesel, but it is still larger. The comparison of experimental data for flame tilt angle with results from correlations presented in Eqs. (1), (3), (5), (7) and (9) resulted in good agreement for Eqs. (5) and (7), as shown in Fig. 3(b). The mean and maximum deviations (calculated between results from correlations and experimental data) considering all but zero air velocities were 4% and 15% for Eq. (5) (fitted for LNG [22]) and 5% and 8 % for Eq. (7) (fitted for diesel and gasoline [23]). Fig. 4(b) presents the comparison between the experimental data for biodiesel flame length and correlations presented in Eqs. (2), (4), (6), (8) and (10). The best agreement is obtained with Eq. (6), originally fitted for large LNG pools and U∞ = 1.8 m/s to 14.4 m/s [22]. The mean deviation between experimental data and results obtained from Eq. (6) were 11% (for air velocities of 0.5 m/s to 2.5 m/s) and 26% (air velocities of 3.0 m/s to 4.0 m/s). The largest deviation, 31%, was found at an air velocity of 3.0 m/s and the lowest one, 6%, at 2.0 m/s, with a mean deviation of 17% over all crossflow velocities. The second best agreement is found with Eq. (8) (fitted for large pools of diesel and gasoline, for U∞ = 0 m/s to 2.3 m/s [23]), providing mean and maximum deviations of 20% and 39%, respectively. These results demonstrated the major impor-
R.G. Salvagni, M.L.S. Indrusiak and F.R. Centeno / International Journal of Heat and Mass Transfer 149 (2020) 119164
5
Fig. 3. Flame tilt angle (θ ) × Air crossflow velocity (U∞ ): (a) experimental data for diesel [13] and for biodiesel (present study), (b) present experimental data (biodiesel) and results calculated with correlations from literature.
Fig. 4. Flame length (Lf ) × Air crossflow velocity (U∞ ): (a) experimental data for diesel [13] and for biodiesel (present study), (b) present experimental data (biodiesel) and results calculated with correlations from literature.
tance of the crossflow velocity as compared to fuel and pool size for prediction of both geometric parameters (flame length and tilt angle) since the results obtained using correlations from [22] were as good as, or even better than, those using correlations from [23]. 3.3. Flame and plume temperatures The flame and plume dimensionless IR temperatures (T f∗ and Tp∗ , respectively) obtained from thermographic images are presented on Fig. 5(a) and (b). Those figures also show the results for diesel [13], rendered dimensionless for comparison purposes. The ambient temperature (Tamb ) in Eq. (11) was considered as 300 K, while Tmax was 912.4 K for computing T f∗ (flame region) and 477.1 K for Tp∗ (plume region) for both diesel and biodiesel. The maximum temperatures measured with the IR camera during the experiments with biodiesel were higher than the maximum temperatures obtained when burning diesel [13], so the values of Tmax used in Eq. (11) were that of biodiesel flame and plume. Due to the characteristics of the thermographic equipment, temperatures obtained do not represent the actual temperatures of flame and plume and should be taken only for comparing their behavior when influenced by the air crossflow and by different burning fuels. Fig. 5 presents IR-measured dimensionless temperatures for diesel [13] and biodiesel (present study) as functions of the air
crossflow velocity (U∞ ) at the flame region (T f∗ ), Fig. 5(a), and at the plume region (Tp∗ ), Fig. 5(b). As observed in Fig. 5(a), for U∞ ≥ 1.0 m/s, the growing trend of the dimensionless temperature at the flame region, T f∗ , with air crossflow velocity is similar for diesel and biodiesel. Additionally, T f∗ values are also higher for biodiesel, which can be attributed to the O2 in the biodiesel molecules contributing to the oxidation process and leading to higher temperatures. Another factor that may contribute to the higher temperatures when burning biodiesel is the higher flash point temperature of biodiesel compared to diesel. Both fuels needed preheating to enhance the evaporation of fuel to initiate the combustion process: in the current experiments biodiesel was preheated to 393 K, while diesel in [13] was preheated to 323 K (i.e., a preheating temperature difference of 70 K, or 22%). On the other hand, at the lowest crossflow velocity range (U∞ = 0 m/s and 0.5 m/s), Fig. 5(a) shows different T f∗ behavior for biodiesel and diesel. For biodiesel, T f∗ first decreases from U∞ = 0 m/s to U∞ = 0.5 m/s, while for diesel it presents an ascending behavior in this U∞ range. Such difference on the T f∗ behavior can be attributed to the preheating temperature difference between diesel and biodiesel. As U∞ increased to 1.0 m/s and beyond, the convective heat exchanges from/to the flame, fuel surface and pool pan tended to cool down the liquid fuel, con-
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Fig. 5. IR-measured dimensionless temperature (T∗ ) × Air crossflow velocity (U∞ ): (a) Flame region (T f∗ ), (b) Plume region (Tp∗ ). Results for diesel [13] and biodiesel (present study).
Fig. 6. Temperature in the region adjacent to the pool at several thermocouple positions (TC_) × Air crossflow velocity (U∞ ): results for (a) biodiesel (present study) and (b) diesel [13].
sequently leading to a similar behavior of T f∗ for both diesel and biodiesel. Finally, it is interesting to note that T f∗ for biodiesel was 52% higher than for diesel for quiescent air, while for U∞ = 0.5 m/s to 4.0 m/s such difference was around 28%. These results indicated that both biodiesel composition and preheating contributed to the T f∗ rise for quiescent air, while the convective heat exchanges reduced the preheating importance on T f∗ for higher crossflow velocities. The IR-measured dimensionless temperatures at the plume region (Tp∗ ) shown in Fig. 5(b) are also higher for biodiesel (20% higher than diesel on average, for the whole U∞ range). Unlike T f∗ , the plume dimensionless temperature, Tp∗ , presents the same descending trend for both fuels within the whole U∞ range of this experiment. Such behavior was a consequence of the mixture of fresh air with the hot plume gases (the higher U∞ , the fresher air entrainment into the plume).
(present study) and Fig. 6(b) for diesel (data from [13]), reproduced here for comparison purposes. Fig. 6 shows that biodiesel pool fire presents a behavior like diesel ones, with higher temperatures closer to the pool for all crosswind velocities. Temperatures for biodiesel are higher than for diesel pool fires, which agrees with results for flame and plume temperatures of Fig. 5 (T f∗ and Tp∗ ). Both fuels present temperature oscillations from U∞ = =0 m/s to U∞ = =2.0 m/s. This was a result of flame tilt angle growing with air velocity and promoting radiative heat transfer and temperature enhancement to the surroundings of the pool trailing edge. At the same time, the additional air amount entrained in the plume reduced its temperature. The dynamic balance between these two phenomena was responsible for the oscillatory behavior of the adjacent temperature. On Fig. 6 for U∞ > 2.0 m/s the balance between these two phenomena is stable and the temperature profile becomes nearly independent of the air crossflow velocity.
3.4. Temperature downstream the pool
4. Conclusions
Fig. 6 presents temperature distributions in the region adjacent to the pool for several thermocouple locations (TC_50 to TC_200, as shown in Fig. 1, positioned 50 mm, 100 mm, 150 mm and 200 mm downstream from the pool rim) as a function of the air crossflow velocity, U∞ . Fig. 6(a) depicts results for biodiesel
This article presented an experimental analysis of biodiesel pool fires under air crossflow in controlled laboratory conditions. The experiments intended to simulate actual fire conditions in fuel storage tanks in open air. The experiments were conducted in the outlet section of an aerodynamic channel with a reduced-scale
R.G. Salvagni, M.L.S. Indrusiak and F.R. Centeno / International Journal of Heat and Mass Transfer 149 (2020) 119164
pool. Results for mass burning rate, flame geometry and temperatures of flame, plume and region adjacent to the pool were obtained for quiescent air and air crossflow velocities up to 4 m/s. The outcomes were compared with previous results for diesel under similar experimental conditions and correlations from literature. The main results observed were that the biodiesel mass burning rate decreased with the increase of air crossflow velocity up to U∞ = 2 m/s and increased for U∞ > 2 m/s and the flame tilt angle rose up to 70° at U∞ = 2 m/s, becoming constant for U∞ > 2 m/s. The flame length decreased proportionally to the air velocity increase, but the values were larger than those of diesel. The flame geometric values (length and tilt angle) obtained experimentally were compared with results obtained from correlations available in the literature. Despite being formulated from experiments with different conditions and fuels, some of them presented good agreement. For the flame tilt angle, the least mean deviation was of 4%, and for the flame length it was 17%. The IR-measured dimensionless temperature at the biodiesel flame region increased with U∞ , while it was reduced at the plume region. The results were qualitatively equivalent to the results obtained for diesel pool fires [13], nevertheless the temperatures for biodiesel were higher than that obtained for diesel. These results, as well as the results for mass burning rate, were in agreement with the conclusions of Chaudhary et al. [16]. As a concluding remark, it is worth to comment the importance of research addressing biofuel fires, like the present one, from the safety and fire hazard point of views. Although this study was carried out in small scale and under laboratory conditions, it was possible to observe the importance of the analysis with this type of fuel. Considering the data obtained, biodiesel storage devices are likely more prone to more dangerous fire events than diesel ones, due to the higher temperatures and longer flames. These findings agree with the considerations presented by Moreno et al. [5] and Calvo Olivares et al. [6]. A better knowledge of the behavior of biodiesel under fire conditions is of utmost importance for strategic fire-fighting planning and as an auxiliary tool for decisionmaking in projects of fuel storage tank parks to prevent the fire from spreading to neighboring tanks or other structures. Declaration of Competing Interest None. Acknowledgment Author FRC thanks CNPq for research grant 400472/2016-3. References [1] C. Kuang, L. Hu, X. Zhang, Y. Lin, L.W. Kostiuk, An experimental study on the burning rates of n-heptane pool fires with various lip heights in cross flow, Combust. Flame 201 (2019) 93–103 https://doi.org/10.1016/j.combustflame. 2018.12.011. [2] X. Zhang, X. Zhang, L. Hu, R. Tu, M.A. Delichatsios, An experimental investigation and scaling analysis on flame sag of pool fire in cross flow, Fuel 241 (2019) 845–850 https://doi.org/10.1016/j.fuel.2018.12.020. [3] F. Tang, Q. He, J. Wen, Effects of crosswind and burner aspect ratio on flame characteristics and flame base drag length of diffusion flames, Combust. Flame 200 (2019) 265–275 https://doi.org/10.1016/j.combustflame.2018.11.011. [4] P. Ping, J. Zhang, D. Kong, Z. Xu, H. Yang, Experimental study of the flame geometrical characteristics of the crude oil boilover fire under cross air flow, J. Loss Prev. Process Ind. 55 (2018) 500–511 https://doi.org/10.1016/j.jlp.2017.12. 005. [5] V.C. Moreno, E. Danzi, L. Marmo, E. Salzano, V. Cozzani, Major accident hazard in biodiesel production processes, Saf. Sci. 113 (2019) 490–503 http://dx.doi. org/10.1016/j.ssci.2018.12.014.
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