Experimental study on ignition and combustion ... - Wiley Online Library

FIRE AND MATERIALS Fire Mater. 2014; 38:409–417 Published online 28 June 2013 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/fam.2191

Experimental study on ignition and combustion characteristics of typical oils Xiao Chen1,2, Shouxiang Lu1,*,†, Changhai Li1, Jiaqing Zhang1,2 and Kim Meow Liew2 1

State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei 230026, China Department of Civil and Architectural Engineering, City University of Hong Kong, Kowloon 999077, Hong Kong

2

ABSTRACT The impact of radiant heat flux on ignition and combustion behavior of typical oils (diesel, lubricating oil, and aviation kerosene) was conducted in a cone calorimeter. A circular steel pan with a diameter of 10 cm was used to contain diesel, lubricating oil, and aviation kerosene without water sublayer. Using the standard oxygen consumption method, we obtained ignition time, heat release rate, mass loss rate, extinction coefficient, CO, and CO2 yield, and average specific extinction area was calculated from the extinction coefficient. Janssens’ method was adopted in this study to deal with ignition time and radiant heat flux under a 0.55 power rule. Results show that the fitting through Janssens’ method is good for ignition time of diesel, lubricating oil, and aviation kerosene and radiant heat flux. Moreover, heat release rate, mass loss rate, and CO/CO2 ratio appear to positively correlate with radiant heat flux, whereas average specific extinction area varies in a certain range. Copyright © 2013 John Wiley & Sons, Ltd. Received 14 February 2012; Revised 21 February 2013; Accepted 29 May 2013 KEY WORDS:

typical oils; radiant heat flux; ignition time; combustion characteristic; cone calorimeter

1. INTRODUCTION Since the cone calorimeter was invented in the 1980s, oxygen consumption principle used by benchscale apparatus has been developed for use in fire testing and research [1]. As a standardized test instrument, the cone calorimeter has been used to measure ignition and combustion characteristics of various materials, for example, wood [2, 3], PMMA [4, 5], and polyurethanes [6]. The ignition and combustion characteristics of liquid fuels have been experimentally studied by using cone calorimeter in the past few decades. Apte [7] and Iwata [8] studied the effect of fuel type on combustion characteristics of pool fires. Apte [7] observed that specific smoke extinction area (SEA) varied with CO and CO2 generation, which vary with fuel type, whereas Iwata [8] discovered by testing 14 different crude oils that heat release rate (HRR), mass loss rate (MLR), flame radiation, and smoke yield seem to have a linear relationship with crude oil density. Several researchers [9–13] have also reported that different radiant heat flux may be a factor in ignition time. Elam [9] studied flammability of spilled crude oils and discovered that ignition time and external radiant heat flux were in an inverse power exponent relationship. Grand [10] focused on industrial fluids and employed the empirical equation of ignition time to process the relationship between ignition time and heat flux. Hshieh [11, 12] focused on ignition time of silicone fluids under different radiant heat fluxes. Suzuki [13] presented an equation for estimating ignition time of insulation fluids, especially the less-flammable transformer fluids. Many procedures have been *Corresponding to: Shouxiang Lu, State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230027, China. † E-mail: [email protected] Copyright © 2013 John Wiley & Sons, Ltd.

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developed to describe the relationship between ignition time and radiant heat flux [14] for thermally thick solid. Furthermore, some other factors will affect the ignition time, such as sample size [15] and sample thickness [16–18]. Due to their practical applications in vehicles, vessels, and aircraft, typical oils, such as diesel, lubricating oil, and aviation kerosene have attracted substantial attention. However, little research has focused on testing of ignition time and combustion characteristic of these oils under different radiant heat fluxes using cone calorimeter. This paper reports results of an experimental study in accordance with ISO5660 [19], using a cone calorimeter and an improved pan device, undertaken to test combustion characteristics of typical oils. The major goal was to explore ignition and combustion characteristics of diesel, lubricating oil, and aviation kerosene under different radiant heat fluxes. Next, Janssens’ procedure was used to discuss ignition time of these oils, and some ignition parameters were considered. Moreover, the effects of radiant heat flux on HRR, MLR, average specific extinction area, and CO/CO2 ratio of the three typical oils were also studied.

2. EXPERIMENT 2.1. Materials Typical oils, that is, diesel, lubricating oil, and aviation kerosene, widely employed in transportation, were used in bench-scale fire tests. Thermal properties of these oils are listed in Table I. (1) Diesel: Generally used in transport vehicles. Its composition is a mixture of different hydrocarbons, containing C9 ~ C18 alkanes, naphthenes, and aromatic hydrocarbons. It is commonly divided into two types: light diesel and heavy diesel. 0# diesel (one of the light diesels) was tested in this study. (2) Lubricating oil: Used as a liquid lubricant to protect machines and components. Its composition is usually base oil and additives, and the main chemical components include high boiling point, high molecular weight hydrocarbons, and non-hydrocarbon mixtures. (3) Aviation kerosene: This is used as the main fuel for aviation turbine engines. It is a complex mixture of hydrocarbons, containing alkanes, naphthenes, and aromatic hydrocarbons. It weighs 0.81 kg/L and has a high flash point.

2.2. Cone calorimeter Experiments were conducted with the cone calorimeter in the State Key Laboratory of Fire Science, China. The cone calorimeter [1] principally consists of a combustion chamber, load cell, oxygen sampling system, gases measure system, exhaust device, and data acquisition system. The tests were carried out by using two steel pans. The inner circle pan was 100 mm in diameter, whereas the outer square pan was 140 mm in length. A piece of gypsum board (135 135 10 mm) was placed between the inner pan and the outer pan to prevent heat transfer. The main function of the outer pan was to avoid spattering of liquid onto the load cell. Additionally, this allowed the inner pan to be fixed in the test center, whereas the control device was mounted on the outer pan. Table I. Thermal properties of the oils tested.

Oils Diesel Lubricating oil Aviation kerosene

Ignition temperature Tig (K)

Ambient temperature Tamb (K)

Density r (kg.m3)

Thermal conductivity l (W.m1.K1)

Specific heat C (kJ.kg1.K1)

530 638 483

298 298 298

850 870 810

0.17 0.11 0.15

2.1 1.88 2.01

Copyright © 2013 John Wiley & Sons, Ltd.

Fire Mater. 2014; 38:409–417 DOI: 10.1002/fam

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IGNITION AND COMBUSTION CHARACTERISTICS OF TYPICAL OILS

Schematic diagram and view of the pan is shown in Figure 1(a) and (b). Volume of oil tested was 150 ml, namely, the liquid depth was 19 mm, and no further oil was added during tests. Tests of 12.7 and 25.4 mm liquid depth were added to explore the relationship between liquid depth and ignition time. To prevent the influence of thin layer boilover [20] on the test results, the inner pan did not contain a water sublayer. The pan filled with oil was placed onto the load cell to wait for the liquid level to be stable before conducting experiments. With the pan apparatus reconfigured, the data acquisition system was used to obtain a series of data, such as HRR, MLR, ignition time, extinction coefficient, concentrations of CO, and CO2, at a sampling rate of 1 s. All tests were conducted at an ambient temperature of 25 C and relative humidity of 70%. The air flow rate in the exhaust pipe was set to 24 l/s, whereas the distance between the liquid surface of the inner pan and the cone heater was fixed at 5.5 cm [21]. Depending upon various properties of oils tested, radiant heat flux varied from 5 to 40 kW/m2 in steps of 5 kW/m2. Before each test, a radiant heat flow meter was used to calibrate radiant heat flux. Whereas aviation kerosene was tested under a low radiant heat flux (5–25 kW/m2) for safety reasons, range of radiant heat flux for diesel and lubricating oil was 5–40 kW/m2. After the basic data were collected for 60 s, all samples were ignited by using an electronic spark combined with the cone heater. The electronic spark was removed when samples were ignited, and the time of ignition was then recorded. The samples were considered not ignited after a duration of 600 s. At the end of the experiments, no oil residue was found in the inner pan. Every test was conducted twice, and the experimental data used in this paper were the average value of the two tests administered.

3. RESULTS AND DISCUSSION 3.1. Ignition time Many researchers [22] have made some approximations about ignition times based on a series 00 expansion before and obtained power rules between ignition time tig and radiant heat flux q_ e , for instance, a second approximation in a power of 0.5 equation [22, 23], at the same time, Janssens [24, 25] developed the procedures by a statistical approach and found a good approximation in a power of 0.55 equation, as shown in Equation (1): 2

q_

00

00

e

!0:55 3 lrC 00 5 ¼ q_ cr 41 þ 0:73 2 hig t ig

(1)

00

where q_ e is radiant heat flux (kW m 2), q_ cr is the critical heat flux for ignition (kW m 2), hig is the heat transfer coefficient from the surface at ignition (kW m 2 K 1), tig is the ignition time (s), and lrC is material thermal inertia (kW2 m- 4 K 2 s 1). Babrauskas [3,22] highlighted that the procedures Janssens developed are the best way to analyze ignition data for thermally thick solids. He also provides the general guidance in considering radiant ignition of liquids: ‘for the thick layer liquid, both the theory and the engineering treatment are nearly identical to the case of solid combustibles’ [22]. Therefore, typical oils in the present study are considered as thermally thick materials.

Figure 1. Pan used in the tests: (a) schematic diagram of the pan and (b) view of the pan. Copyright © 2013 John Wiley & Sons, Ltd.


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Figure 2 shows the relationship between ignition time and radiant heat flux according to Janssens’ procedure. A linear relationship is evident in the transformed form of ignition time and radiant heat 00 flux. X-axis intercept is determined as the critical radiant heat flux q_ cr , and the inverse of slope is 00 defined as a constant, Big. The results can be represented for aviation kerosene as q_ cr ¼ 0:45kWm2 , 00 Big = 126.3 and for lubricating oil as q_ cr ¼ 5:94 kWm2 , Big = 432.9. However, diesel shows a different behavior: its critical radiant heat flux is less than zero, the result would suggest that diesel do not require any radiant heat flux and an electronic spark would ignite it without limitation of time. We can also obtain from the tests that diesel and aviation kerosene cannot be ignited below a radiant heat flux of 10 kW/m2 within the limited time. However, lubricating oil can be ignited under a radiant 00 heat flux of 20 kW/m2 until a long time (about 505 s). The minimum flux q_ min is defined as the half between lowest ignited heat flux and highest heat flux at which no ignition happens for a long time [26]. In view of this, we can infer that the minimum radiant heat flux for ignition of lubricating oil is larger than diesel and aviation kerosene. Delichatsios [27] also developed a simpler calculation 00 00 formula to obtain the minimum flux, and he suggested the ratio q_ cr =q_ min is 0.7. The depth of liquid also has been considered as a factor to ignition time in this paper, as illustrated in Figure 3. Depths on the ignition time of typical oils under radiant heat flux of 25 kW/m2 are 12.7, 19, and 25.4 mm, and depths of SAE 30 and SAE 50 under radiant heat flux of 30 kW/m2 in Putorti’s [18]

Figure 2. The radiant ignition of typical oils.

Figure 3. The effect of depth on the ignition time of typical oils, SAE 30, and SAE 50 [18]. Copyright © 2013 John Wiley & Sons, Ltd.



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study is 10,15 and 42 mm. These data indicate that the ignition time becomes smaller with the depth of fuel increasing, and a maximum difference value of these oils in Figure 3 is approximate 25%. 3.2. Heat release rate Heat release rate is the main parameter obtained from bench-scale tests, which are regarded as approximating full-scale results [1,28]. Measurement apparatus based on the oxygen consumption principle (cone calorimeter) is widely employed to measure the HRR of different materials [12,29, 30]. Thus, the cone calorimeter is an effective tool for study of HRRs of typical oils. Figure 4 illustrates HRR of diesel (a), lubricating oil (b), and aviation kerosene (c) tested by the cone calorimeter under different radiant heat fluxes. A significant trend apparent from Figure 4 is that the peak HRR becomes larger with increase in radiant heat flux, but the time spent on the entire combustion process is inversely proportional. From Figure 4(a), we can see that the trend of variation of HRR of diesel is symmetrical, and the time it takes for HRR to rise is close to the fall time required. For lubricating oil and aviation kerosene, it is evident from Figure 4(b) and (c) that the time taken in the rising stage is longer than the time taken in the declining stage, and the HRR in the rising stage moves more smoothly and steadily than in the declining stage. Under the same radiant heat flux, HRR curves of these typical oils evolve differently because of their intrinsic attributes. We can see that the maximum and average values of HRR are linearly correlated with radiant heat flux (Figure 5). The maximum HRR values were the peak value during the whole burning period, and the average HRR values were calculated over the whole burning period of after ignition. The maximum HRR value of aviation kerosene can be seen to be larger than that of diesel and lubricating oil under the same radiant heat flux. Under a radiant heat flux of 20 kW/m2, the maximum HRR value of aviation kerosene was equal to 1463 kW/m2, whereas the maximum HRR values of diesel and lubricating oil

Figure 4. Heat release rate of diesel (a), lubricating oil (b), and aviation kerosene (c) tested in the cone calorimeter under different radiant heat fluxes. Copyright © 2013 John Wiley & Sons, Ltd.


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were 693 and 393 kW/m2, respectively. The average HRR values show the same trend as the maximum HRR values. The linear curves fit the maximum and average values (Figure 5). The slope of the calculated line in aviation kerosene is slightly steeper than others. The results indicate that aviation kerosene burns more intensely than diesel and lubricating oil as radiant heat flux increases. Generally, lighter components of fuel evaporate first, when the fuel is ignited, and then burn in the flame region, whereas heavier components constantly sedimentate and enrich [31]. In the present work, we also test the MLR. Figure 6 demonstrates that average values of MLR of the three oils vary with radiant heat flux. The average MLR values were calculated over the whole burning period after ignition. With the increase in radiant heat flux, average MLR values of diesel, lubricating oil, and aviation kerosene increased, and there is good linear correlation in case of all the three. When under the same radiant heat flux (e.g., 20 kW/m2), the average MLR value of lubricating oil is the lowest, and the average MLR value of aviation kerosene is higher than that of diesel. After comparing properties of these three typical oils (Table I), we can infer that higher density corresponds to lower MLR, owing to sedimentation and enrichment of heavier components, which has been discussed by Iwata [8]. In addition to density being a factor on MLR, there are many other factors, such as lip height [32, 33] and pool size [15]. Future studies could seek to examine these factors.

Figure 5. Variation in maximum and average heat release values with radiant heat flux.

Figure 6. Variation in average mass loss rate values with radiant heat flux. Copyright © 2013 John Wiley & Sons, Ltd.



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3.3. Smoke obscuration Extinction coefficient K (in 1/m) is calculated as follows [34]: 1 I0 K ¼ ln I l

(2)

where l is the diameter of the exhaust pipe (m), I0 is the initial light intensity launched by the laser light source, and I is the light intensity transmitted through the exhaust pipe. According to ASTM E 1354 standard [35], the average specific extinction area is considered as significant variable when talk about smoke obscuration, and it can be expressed in the following equation, X sf ðAvgÞ ¼

i

V_ i k i Δt i

(3)

mi mf

where V_ i is the air flow rate in the exhaust pipe (L/s), ki is the extinction coefficient (m1), Δti is the sample rate (s), mi is the initial specimen mass (kg), and mf is the final specimen mass (kg). Figure 7 demonstrates variations of the average specific extinction area (SEA) with radiant heat flux. As we can see from bars extending above the horizontal line, the impacts of radiant heat flux on average SEA for these typical oils are quite uncertain. With the radiant heat flux increasing, the average SEA of diesel varies in the range of 872–1028 m2/kg, lubricating oil varies from 813 to 930 m2/kg, and the variation range of aviation kerosene is 604–743 m2/kg. In an earlier work [7], the SEA was a function of the fuel type and independent on the pool size. We can infer that average SEA values for diesel are larger than that of lubricating oil and aviation kerosene under radiant heat flux of 20 and 25 kW/m2 due to its lower density. 3.4. The CO/CO2 ratio The CO/CO2 ratio is the ratio of CO steady value and CO2 steady value after ignition. Variations in ratios of CO to CO2 for diesel, lubricating oil, and aviation kerosene with radiant heat flux are illustrated in Figure 8. It is evident from Figure 8 that with increase in radiant heat flux, CO/CO2 ratios of the three typical oils have an increasing trend, and there are good linear relationships between radiant heat flux and CO/CO2 ratios. The slope of the straight line that fits CO/CO2 ratio of diesel under different radiant heat fluxes is shallower than the others; CO/CO2 ratio of diesel varied within the range of 0.041–0.043. We can

Figure 7. Smoke production-average specific extinction area. Copyright © 2013 John Wiley & Sons, Ltd.


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Figure 8. Variation in ratio (CO/CO2 yield) with radiant heat flux.

infer that radiant heat flux has little influence on CO/CO2 ratio of diesel within the range considered. Under different radiant heat flux, CO/CO2 ratio of lubricating oil varied from 0.03 to 0.035, whereas the variation in CO/CO2 ratio of aviation kerosene was 0.028–0.036. McCaffrey et al. [36] presented that the variation in molar CO/CO2 ratio of crude oils was from 0.02 to 0.07, whereas Iwata et al. [8] studied CO/CO2 ratios of several types of crude oils and provided the smaller range (0.022–0.027). Apte [7] discovered that CO/CO2 ratio of aviation turbine fuel was 0.0105. The CO/ CO2 ratios in our study are within the range reported by McCaffrey et al. [36], but much higher than the results of Iwata et al. [8] and Apte [7].

4. CONCLUSION An experimental study on ignition and combustion characteristics of three typical oils (diesel, lubricating oil, and aviation kerosene) was conducted considering radiant heat flux. Ignition time and several combustion parameters were measured in a cone calorimeter. Ignition time changed during the tests with different radiant heat flux. Radiant heat flux and the ignition times of diesel, lubricating oil, and aviation kerosene were fitted well in a 0.55 power rule through Janssens’ method. Effect of liquid depth on ignition has also been considered, and a maximum 25% difference value of ignition time is found in this paper. Moreover, several combustion parameters were observed. The maximum and average value of HRR, average MLR, and CO/CO2 ratio appear to be linear with radiant heat flux. However, average specific extinction area changes in some range and varies with oil density. ACKNOWLEDGEMENT

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