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Comparative assessment of the explosion characteristics of alcohol–air mixtures
Journal of Loss Prevention in the Process Industries 37 (2015) 91e100
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Comparative assessment of the explosion characteristics of alcoholeair mixtures Qianqian Li, Yu Cheng, Zuohua Huang* State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an, 710049, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 April 2015 Received in revised form 13 June 2015 Accepted 8 July 2015 Available online 15 July 2015
Explosion characteristics of five alcoholeair (ethanol, 1-butanol, 1-pentanol, 2-pentanol and 3-pentanol) mixtures were experimentally conducted in an isochoric chamber over wide ranges of initial temperature and pressure. The effect of temperature and pressure on the different explosion behaviors among these alcohols with various structures were investigated. Results show that the peak explosion pressure is increased with the decrease of temperature and increase of pressure. Maximum rate of pressure rise is insensitive to the temperature variation while it significantly increases with the increase of initial pressure. Among the 1-, 2-, and 3-pentanoleair mixtures, 1-pentanol has the highest values in peak explosion pressure and maximum rate of pressure rise and 2-pentanol gives the lowest values at the initial pressure of 0.1 MPa. These differences tend to be decreased with the increase of initial pressure. Among the three primary alcoholeair (ethanol, 1-butanol and 1-pentanol) mixtures, a similar explosion behavior is presented at the lean mixture side because of the combined effect of adiabtic temperature and flame propagation speed. At the rich mixture side, 1-pentanol gives the highest values in peak explosion pressure and maximum rate of pressure rise and ethanol gives the lowest values. This phenomenon can be interpretated from the combining influence of heat release and heat loss, since the flame speeds of ethanol-, 1-butanol-, 1-pentanoleair mixtures are close at rich mixture side. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Pentanol isomers Primary alcohols Explosion characteristics Combustion phase
1. Introduction The interest in bio-alcoholic renewable fuels is increasing because of energy shortages and serious environmental pollution. In the past decades, extensive studies on bio-alcohols suggested that alcohols addition potentially favors to the reduction of emissions, especially the particulate matter (Gautam and Martin, 2000; Gautam et al., 2000; Yucesu et al., 2006; Koc et al., 2009; Ozsezen and Canakci, 2011; Surisetty et al., 2011). Low alcohols like ethanol with high octane number and relatively low cost have been successful in the practical use as the gasoline additives. However, low alcohols still have their disadvantages such as low energy content and high hygroscopicity. This leads to an inconvenient storage and transportation and restricts their wide applications in engines. Meanwhile, low alcohols favor to the knock resistance in the spark ignition (SI) engines but they are bad for compression ignition (CI) engines. Recent study on the higher alcohols indicated that 1-pentanol presents a negative temperature coefficient (NTC)
* Corresponding author. E-mail address: [email protected] (Z. Huang). http://dx.doi.org/10.1016/j.jlp.2015.07.003 0950-4230/© 2015 Elsevier Ltd. All rights reserved.
behavior in the intermediate temperature regime, which indicates their potential of fuels for the CI engines (Heufer et al., 2013). An engine study on the 1-pentanol/diesel blend reported that, compared with pure diesel, the blend exhibited the comparative engine performance, lower particulate emissions, and even better combustion characteristics for the blending ratio of pentanol up to 25% (Campos-Fernandez et al., 2013; Wei et al., 2014). Li et al. (2015) tested pure pentanol in a conventional diesel engine and achieved the ultralow NOX and smoke emissions without exhaust gas recirculation (EGR), demonstrating its good potential applicability. Fundamental combustion research can help the clean and efficient utilization of alcohols. Up to now, extensive studies concentrated on the low alcohols. Konnov et al. (2011) and Bradley et al. (2009) respectively studied the laminar combustion characteristics of ethanol with heat flux method and spherically propagating flame. Gu et al. (2010), Veloo and Egolfopoulos (2011) and Wu and Law (2013) subsequently measured the laminar flame speeds of butanol isomers using different apparatuses at the initial pressures of 0.1e0.75 MPa. Cancino et al. (2011) measured the ignition delay times of ethanol-containing gasoline surrogates in a shock tube at
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the temperatures from 720 to 1220 K. Moss et al. (2008) and Stranic et al. (2012) studied the auto-ignition characteristics of butanol isomers at the elevated temperatures and the pressures of 1.5e43 atm. Fundamental researches on high alcohol of pentanol fuels started in the latest years. Li et al. (2013a,b) comparatively studied the laminar combustion and chemical kinetic characteristics of 1-, 2-, and 3-pentanol, and reported that 1-pentanol gives the highest flame speed, followed by 3-pentanol and 2-pentanol. Tang et al. (2013) studied the ignition delay characteristics of 1pentanol, iso-pentanol and 2-methyl-1-butanol using a shock tube, indicating 1-pentanol has the shortest ignition delay. These studies provided valuable insight into the difference among the isomers. Alcohols are flammable fuels and the alcohol vapors might burn or explode, leading to disasters. Such accidents could happen in the case of fuel evaporation or leakage especially under a high temperature, resulting in properties losses and even human casualties (Dorofeev et al., 1995; Wang et al., 2014a,b). Therefore, the safety issue claims high concern over the fuel utilization, storage and transportation, calling for the demand of investigation on the explosion characteristics to assess the potential explosion hazard. In the previous studies, the main attention was focused on the gaseous fuels like methane, natural gas and ethylene, etc. (Dahoe, 2005; Razus et al., 2007, 2011; Tang et al., 2009, 2014; Zhu et al., 2012; Movileanu et al., 2013; Wang et al., 2014a,b). Their studies demonstrated that the explosion parameters are strongly dependent on initial pressure, initial temperature and fuel/air ratios. Until now, limited researches have been performed on the explosion properties of alcohols. Chang et al. (2009) determined the critical explosion properties of the tolueneemethanol mixtures at varied blending ratios and initial oxygen concentrations using a closed spherical vessel, and provided a triangular flammability diagram for the explosive hazard region. Cammarota et al. (2012) studied the explosion characteristics of pure ethanol and ethanolehydrogeneair mixtures at different equivalence ratios and temperatures using a cylindrical chamber. Zhang et al. (2009) studied the explosion characteristics of methanoleairediluent mixtures in a constant chamber, from evaluating the peak combustion pressure, flame development duration and combustion duration. Shimy (1970)
proposed the formulas for the prediction of the ignition temperature and flammability of alcohols and hydrocarbons. To our knowledge, no report was published for the influence of structures and carbon chain length on the explosion characteristics of alcohols. In this study, the explosion characteristics of 1-, 2-, and 3pentanoleair mixtures were studied at initial temperatures from 393 to 433 K and initial pressures from 0.1 to 0.75 MPa using a constant combustion vessel. Explosion hazard was assessed by the explosion parameters such as explosion pressure, deflagration index, maximum rate of pressure rise, combustion duration. The discrepancies among the isomers were analyzed and the effects of the initial conditions on the explosion characteristics were discussed. Meanwhile, a comparative assessment on the explosion characteristics of ethanol-, 1-butanol- and 1-pentanoleair mixtures at different equivalence ratios was also made to evaluate the influence of carbon chain length on the potential risk and hazard of these primary alcohols. 2. Experimental setup and data acquisition Fig. 1 gives the sketch of the experimental setup. It is composed of ignition system, heating system, constant volume vessel, data acquisition system and inlet/exhaust system. The constant volume vessel is a stainless cylinder with a diameter of 180 mm and length of 210 mm. The mixture in the vessel is ignited by the centrally located electrodes with the spark produced by a standard capacitive discharge ignition system. The spark energy is 45 mJ which is higher than the minimum ignition energy of all the mixtures involved in present study. Heating tapes are wrapped around the chamber to heat the chamber. The temperature in the chamber is monitored by a thermocouple with an accuracy of ±1 K. The combustion pressure is acquired with a pressure transducer (Kistler 7001) at a sample-rate of 100 kHz connected with a Charge Amplifier (Kistler 5011), and the pressure data was recorded with an oscilloscope. Other control components such as the intake and outlet valves are all mounted on the chamber. When the chamber was heated to the target temperature, the chamber was vacuumed and repeatedly flushed with dry air to remove the residual gases from the previous experiment. The
Fig. 1. Experimental setup.
Q. Li et al. / Journal of Loss Prevention in the Process Industries 37 (2015) 91e100 Table 1 Properties of fuels involved in the study. Chemical name
Purity
Molecular formula
Ethanol
99.8%
C2H6O
1-Butanol
99.7%
C4H10O
1-Pentanol
>99%
C5H12O
2-Pentanol
>99%
C5H12O
3-Pentanol
>99%
C5H12O
Structure
Table 2 Deflagration index classes (NFPA 68, 2002). Level
Deflagration index (KG)/MPa m s1
St St St St
0 1e20 20e30 >30
0 1 2 3
mixtures at specified equivalence ratios were prepared based on the partial pressure of each component. Liquid fuel was injected into the chamber with a micro-syringe and five minutes were awaited to ensure its full evaporation. Dry air of 21% O2 and 79% N2 was filled into the chamber. Time intervals of both 5 min and 30 min were awaited to make the uniform mixing, and no significant difference could be observed. Thus, 5 min were adopted in
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present study. The oscilloscope was initiated before the mixture was ignited so that the whole combustion process can be recorded. In this experiment, the initial temperatures were chosen as 393 K, 433 K, 473 K. The initial pressures were 0.1 MPa, 0.25 MPa, 0.5 MPa, 0.75 MPa. The equivalence ratio is ranging from 0.6 to 1.8. For each condition, at least three tests were made to examine the repeatability of the experiment. Purity of O2 and N2 are over 99.95% and fuel properties are listed in Table 1. Explosion parameters such as peak explosion pressure (Pmax/P0), maximum rate of pressure rise (dP/dt)max, and deflagration index (KG), etc., are used to assess the explosion characteristics. These parameters provide key information of an explosion process, assessing safety of combustion chamber and designing the relief device against the explosion damage. The explosion pressure and rate of pressure rise can be directly obtained from the pressure history. Since pressure variation depends on experimental conditions, especially the vessel volume, thus, maximum rate of pressure rise is always normalized with vessel volume on the basis of cubicroot law through KG ¼ ðdP=dtÞmax V1=3 , where KG is deflagration index and indicates the explosion intensity (De Smedt et al., 1999; Tang et al., 2009; Cammarota et al., 2012). The law is generally used in spherical vessels. However, as described in European Standard guideline and NFPA 68 (2002), it can also be used in cylinder vessel with the length to diameter ratio (L/D) lower than 2. For present cylinder vessel, L/D is 1.17, complying with the standard. It is assumed the deflagration index depends only on the mixture properties regardless of chamber volume, and could scale the explosion hazard (NFPA 68, 2002). The explosion index is scaled as shown in Table 2, and higher values indicate more robust explosion. Combustion duration and flame development duration, reflecting the combined effect of heat release and flame propagation speed. Combustion duration is the time interval between ignition and
Fig. 2. Histories of P/P0 and dP/dt at elevated temperatures and pressures for 1-pentanoleair mixtures.
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Fig. 3. Pmax/P0 and Pe/P0 versus f at f ¼ 1.0 and three initial temperatures for 1pentanoleair mixtures with error bars presented. Line: Pe/P0; Symbol: Pmax/P0.
peak explosion pressure (Zhang et al., 2014) and flame development duration is the time from ignition to 7% pressure rise (De Smedt et al., 1999). 3. Results and discussion 3.1. Pressure evolution and deflagration 3.1.1. 1-Pentanol-air mixture Fig. 2 plots the histories of explosion pressure (P/P0) and rate of pressure rise (dP/dt) of 1-pentanoleair mixtures at elevated pressures and temperatures. The explosion pressure increases progressively until the rate of pressure rise obtain the maximum value. Then the explosion pressure keeps increasing but the rate of pressure rise gradually decreases, due to the heat loss of the flame front to the vessel wall and the reduced fuel consumption near the end of combustion (Dahoe, 2005; Saeed and Stone, 2004). This behavior was also reported for methane and natural gas combustion (Tang et al., 2014; Zhang et al., 2014). Fig. 2a and b give the explosion pressure and rate of pressure rise versus time at three temperatures, P0 ¼ 0.1 MPa and f ¼ 1.0. The peak explosion pressure (Pmax/P0) is the maximum value of explosion pressure, as indicated in Fig. 2a. With the increase of temperature, the peak explosion pressure is decreased due to the decrease of the total fuel mass. However, the maximum rate of pressure rise varies little. Besides, the time intervals between ignition and peak explosion pressure are decreased. This is because the species in flames at
elevated temperatures are more reactive, resulting in the promotion of the whole reactions and hence the faster flame speed. Specifically, previous study (Li et al., 2013a,b) reported the laminar flame speed of 1-pentanol-air mixture increases from 58.8 cm s1 to 79.0 cm s1 with the initial temperature increasing from 393 to 473 K at the equivalence ratio of 1.0. Fig. 2c and d shows the explosion pressure and rate of pressure rise versus time at four initial pressures for the 1-pentanoleair mixtures. Peak explosion pressure increases with the increase of initial pressure because total amount of heat release is increased as the increased fuel mass. Peak explosion pressure and peak value of the rate of pressure rise reach early with the decrease of pressure because of the increased flame speed at decreased pressure. The maximum combustion pressure in a constant volume chamber can be simulated based on the adiabatic assumption through thermal equilibrium. The adiabatic equilibrium pressure, Pe, is calculated from the equilibrium module in CHEMKIN. The mechanism used for the simulation was developed by Togbe et al. (2011). Fig. 3 illustrates the peak explosion pressure (Pmax/P0) and normalized maximum equilibrium pressure (Pe/P0) versus equivalence ratio at atmospheric pressure and three initial temperatures. Error bars are presented for Pmax/P0 to show the experimental uncertainties. It is seen the experimental uncertainties are small at most conditions. Big uncertainties are just presented for some fuel rich mixtures. Pe/P0 gives a similar behavior to that of the adiabatic temperature, and it decreases with the increase of initial temperature. The adiabatic temperature and flame propagation speed all give their peak values around f ¼ 1.1, resulting in the peak values of Pmax/P0 and Pe/P0 fall between f ¼ 1.0 and f ¼ 1.2. Leaner or richer mixtures make the drop of pressure. Real combustion is not under the absolute adiabatic condition. It accompanies both radiant and convective heat losses to the wall, leading to the lower Pmax/P0 than Pe/P0. It is noted that the difference between Pe/P0 and Pmax/P0 is remarkably increased at highly rich mixtures. This is because soot formed at rich mixtures due to the absence of oxygen, highly promoting the heat loss through the continuum radiation to the vessel wall. With the mixture becoming richer, more soot formed in the flame, causing larger difference between Pe/P0 and Pmax/P0. Fig. 4 shows the maximum rate of pressure rise, (dP/dt)max, and deflagration index, KG, versus equivalence ratio at elevated temperatures and pressures for the 1-pentanoleair mixtures. Maximum rate of pressure rise and deflagration index reach their peaks at f around 1.1, and they decrease at both lean and rich mixtures. As shown in Fig. 4a, (dP/dt)max and KG give an approximate value at varied initial temperatures, indicating that (dP/dt)max and KG are insensitive to the variation of temperature. Rate of pressure rise is affected by both flame speed and heat release. Flame speed increases monotonically with the increase of
Fig. 4. (dP/dt)max and KG versus f at elevated temperatures and pressures for 1-pentanoleair mixtures.
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Fig. 5. P/P0 and dP/dt at f ¼ 1.0, P0 ¼ 0.1 MPa and T0 ¼ 433 K for 1-, 2-, and 3-pentanoleair mixtures.
Fig. 6. Pmax/P0 versus f at T0 ¼ 433 K and two pressures for 1-, 2-, and 3-pentanoleair mixtures.
temperature (Li et al., 2013a,b), but the decrease of total fuel mass results in the decreased heat release and offsets the effect of flame speed on the explosion pressure. With the increase of initial pressure as shown in Fig. 4b, (dP/dt)max and KG increase dramatically, especially around equivalence ratio of 1.1. This differs from the flame speed, but agrees with the mass burning flux when taking into account of the effect of density. It is noted that deflagration index is less than 20 MPa m s1 at f ¼ 1.0 and the initial pressure of 0.1 MPa, belonging to the first class of deflagration index (NFPA 68,
2002), and low potential of explosion hazard. However, KG exceeds 30 MPa m s1 around the stoichiometric ratio at the pressures higher than 0.5 MPa and enters the highest class of deflagration index. Therefore, high attention needs to be taken at elevated pressures. 3.1.2. Pentanol isomereair mixtures Fig. 5 shows the evolution of the explosion pressure (P/P0) and rate of pressure rise (dP/dt) at f ¼ 1.0, T0 ¼ 433 K and P0 ¼ 0.1 MPa
Fig. 7. (dP/dt)max versus f at different temperatures and pressures for 1-, 2-, and 3-pentanoleair mixtures.
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Fig. 8. P/P0 and dP/dt at T0 ¼ 393 K and P0 ¼ 0.1 MPa for ethanol, 1-butanol and 1-pentanoleair mixtures.
for the 1-, 2-, and 3-pentanoleair mixtures. Among three pentanoleair mixtures, the time to reach peak values of Pmax/P0 and (dP/ dt)max are advanced in the order of 1-pentanol, 3-pentanol and 2pentanol. The difference between Pmax/P0 of 2-pentanol and 3pentanoleair mixtures is small. As reported in the previous study (Li et al., 2013a,b), 1-pentanol-air mixture gives the largest flame speed and the highest adiabatic temperature among 1-, 2-, and 3pentanoleair mixtures. Thus, 1-pentanol-air mixture yields the highest explosion pressure. For 2-, and 3-pentanoleair mixtures, 3pentanol has close adiabatic temperature with 2-pentanol, but always exhibits 2 cm s1 higher flame speed than 2-pentanol does at
normal pressure, Li et al., 2013a,b), leading to the slightly higher Pmax/P0 and (dP/dt)max of 3-pentanol. Fig. 6 gives the peak explosion pressure of 1-pentanol and its two isomereair mixtures versus equivalence ratio at initial temperature of 443 K and two initial pressures. At initial pressure of 0.1 MPa, Pmax/P0 of 1-pentanol-air mixture gives the highest value, followed by 3-pentanol and 2-pentanol. Pmax/P0 gives its maximum values between 1.1 and 1.2. At a high pressure of 0.5 MPa, Pmax/P0 of three isomereair mixtures present close values at all equivalence ratios. For the mixtures with equivalence ratio larger than 1.2 and pressure higher than 0.5 MPa, the flame front is unstable and is
Fig. 9. Comparison among three alcohol flames with different parameters including Pe/P0, Pmax/P0, Tad, laminar flame speed, and mole fractions of H2O and CO2 at P0 ¼ 0.1 MPa and T0 ¼ 393 K.
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values at around f ¼ 1.1, in which 1-pentanol gives the highest value and 2-pentanol gives the lowest value. As shown in Fig. 7b, when initial pressure is elevated to 0.5 MPa, (dP/dt)max and KG increase dramatically, but the difference among these isomers tends to be decreased, which is similar to the variation of peak explosion pressure. This pressure dependent behavior is resulted from the decreased difference among the flame speeds of the isomereair mixtures at elevated pressure. As indicated in previous study (Li et al., 2013a,b), 1-, 2-, 3-pentanoleair mixtures respectively gives the maximum laminar flame speed of 67.2, 61.5, 63.8 cm s1 at 0.1 MPa, and 47.2, 43.9, 46.4 cm s1 at 0.5 MPa and 433 K. The difference is decreased from 5.7 to 3.3 cm s1 between 1-pentanol and 2-pentanol, and from 2.4 to 0.8 cm s1 between 1-pentanol and 3-pentanol, with the initial pressure elevated from 0.1 to 0.5 MPa.
Fig. 10. (dP/dt)max and KG versus f at different temperatures and pressures for ethanol, 1-butanol, and 1-pentanoleair mixtures.
easily developed into a cellular structure, resulting in a remarkable increase in flame propagation speed and pressure. Experiments conducted under these conditions needs to be careful and vessel safety design is required. Fig. 7 shows the comparison among 1-, 2-, and 3-pentanoleair mixtures on the maximum rate of pressure rise and the deflagration index versus equivalence ratio at T0 ¼ 433 K and two initial pressures. As shown in Fig. 7a, (dP/dt)max and KG reach their peak
3.1.3. Ethanol, 1-butanol and 1-pentanoleair mixtures Fig. 8 gives the explosion pressure and rate of pressure rise versus time at f ¼ 1.0, P0 ¼ 0.1 MPa, T0 ¼ 393 K for the ethanol, 1butanol and 1-pentanoleair mixtures. The phase of Pmax/P0 and (dP/dt)max are all slightly postponed with the increase of carbon chain length, reflecting the gradually decreased flame speed. Pmax/ P0 of ethanol- and 1-butanoleair mixtures shows an approximate values and that of 1-pentanol-air mixture is slightly higher than them. Ethanol gives the highest (dP/dt)max and 1-pentanol gives the lowest value. In general, the difference among the three mixtures is small. Fig. 9a shows the peak explosion pressures (Pmax/P0) of ethanol, 1-butanol and 1-pentanoleair mixtures versus equivalence ratio at T0 ¼ 393 K and P0 ¼ 0.1 MPa. Behavior at f ¼ 1.0 in Fig. 8 is identical to all fuel lean mixtures. For fuel rich mixtures, Pmax/P0 decreases
Fig. 11. Combustion duration for 1-, 2-, and 3-pentanoleair mixtures.
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monotonically in the order of 1-pentanol, 1-butanol and ethanol. To interpret the reason, the calculated adiabatic temperatures and laminar flame speeds are provided in Fig. 9b and c, respectively (Li et al., 2013a,b). For fuel lean mixtures, the ethanol-air mixture gives the lowest adiabatic temperature and the largest flame speed, resulting in a slightly lower Pmax/P0 compared to 1-pentanol-air mixture. For fuel rich mixtures, the ethanol-air mixture has an approximate laminar flame speed to the 1-butanol and 1-pentanol mixtures but lower adiabatic temperature, resulting in the lower explosion pressure. To further interpret the difference between 1butanol and 1-pentanol, the equilibrium explosion pressure, Pe/ P0, is calculated and plotted in Fig. 9a. The mechanisms used here for ethanol, 1-butanol and 1-pentanol were proposed by Marinov (1999), Sarathy et al. (2012) and Togbe et al. (2011), respectively. Pe/P0 is much higher than Pmax/P0, however, it can accurately capture the trend of Pmax/P0 among the three mixtures. Similar to the adiabatic temperature, Pe/P0 of 1-butanol and 1-pentanol-air mixtures are approximate in the absence of heat loss. However, heat could be lost in combustion vessel through conduction, convection and radiation. The flame temperature is much higher than the air mixture outside the vessel. Heat could be lost through conduction and convection after the flame front touches the vessel wall. Besides, the main products in alcohol flames, are all triatomic gases with high radiation intensity will enhance the heat loss by radiation. Due to the heat loss, Pmax/P0 gives lower value than Pe/P0 over all equivalence ratios. It is noted that three alcohol flames present approximate values of Pmax/P0 and Pe/P0 at lean mixtures. At the rich mixtures, Pmax/P0 decreases in the order of 1-pentanol, 1-
butanol and ethanol, similar with Pe/P0. However, the difference among the three flames are much significant for Pmax/P0 than for Pe/ P0 at the rich mixtures, revealing the most heat is lost in ethanol flame. To illustrate this phenomenon, the mole fractions of the main products, CO2 and H2O, in the rich alcohol flames are given in Fig. 9d. Though CO is also important product in the rich fuel flames, it couldn't radiate heat and is not discussed here. As seen in Fig. 9d, the mole fractions of CO2 and H2O all decrease in the order of ethanol, 1-butanol and 1-pentanol, indicating the heat loss through radiation decreases in the same order. Fig. 10 gives the maximum rate of pressure rise and deflagration index versus equivalence ratio at P0 ¼ 0.1 MPa and T0 ¼ 393 K for the ethanol, 1-butanol and 1-pentanoleair mixtures. Similar to Pmax/P0, the three alcoholeair mixtures give the approximate values in (dP/dt)max and KG at fuel lean side. At fuel rich side, 1pentanol gives the highest and ethanol gives the lowest value. For all mixtures at f ¼ 1.0, the deflagration index is less than 10 MPa m s1, indicating low potential explosion hazard. 3.2. Combustion phase Combustion phase is an index reflecting the heat release and is also an important parameter to characterize the combustion process. Variation in combustion phase can directly influence fuel/air mixing, energy conversion efficiency, combustion and emissions. In a practical combustion device, combustion phase is adjusted to the variation of initial conditions to achieve high energy efficiency. Thus, combustion phase is a parameter for combustion control. In
Fig. 12. td versus f at different temperatures and pressures for 1-, 2-, and 3-pentanoleair mixtures.
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Fig. 13. tc and td versus f at T0 ¼ 393 K and P0 ¼ 0.1 MPa for ethanol, 1-butanol, and 1-pentanoleair mixtures.
this study, the combustion duration (tc) and flame development period (td), are determined. Their definitions can be referenced to previous publications (De Smedt et al., 1999; Zhang et al., 2014). Fig. 11 gives the combustion duration of 1-, 2-, and 3pentanoleair mixtures versus f at different temperatures and pressures. Combustion duration increases with the decrease of temperature and the increase of pressure. Flame propagation speed is decreased with the decrease of initial temperature and the increase of initial pressure, resulting in the increase of time to reach the peak explosion pressure. At f ¼ 1.0 as shown in Fig. 11c, combustion duration increases in the order of 1-pentanol, 3-pentanol and 2-pentanol. At elevated pressure (0.5 MPa) as shown in Fig. 11d, the three mixtures gives the approximate values. Combustion duration gives its minimum value around f ¼ 1.1, corresponding to the position of the largest flame speed. Fig. 12 shows the flame development period (td) versus f for 1-, 2-, and 3-pentanoleair mixtures at elevated temperatures and pressures. Similar to the combustion duration, flame development period gives its minimum value around f ¼ 1.1. It decreases with the increase of temperature and the decrease of pressure. Change of td with the increase of initial pressure is more significant at highly lean mixtures compared to that at stoichiometric equivalence ratio. The mixtures with low flame speed gives long td. At highly lean mixtures, flame speed is lower at atmospheric pressure and even much lower at elevated pressures, resulting in the increased difference of td to the variation of pressures. Small differences are presented for the three pentanol isomereair mixtures. Fig. 13 plots tc and td versus f for the ethanol, 1-butanol and 1pentanoleair mixtures at P0 ¼ 0.1 MPa and T0 ¼ 393 K tc and td show the approximate values at lean and stoichiometric mixtures side, and large difference at rich mixture side. Ethanol gives the longest tc and td, while 1-pentanol gives the shortest tc and td. 4. Conclusions A comparative study on the explosion characteristics of five alcohol (ethanol, 1-butanol, 1-pentanol, 2-pentanol, and 3pentanol)eair mixtures at elevated temperatures and pressures was conducted using a constant volume chamber. The main conclusions are as follows: 1. Peak explosion pressure of 1-pentanol-air mixture increases with the decrease of initial temperature and the increase of initial pressure. Maximum rate of pressure rise and deflagration index are insensitive to the variation of initial temperature. They
remarkably increase with the increase of pressure, demonstrating a high potential explosion hazard at elevated pressures. Combustion duration and flame development period of 1pentanoleair mixtures give their minimum values at f ¼ 1.1. 2. 1-Pentanoleair mixtures give the highest peak explosion pressure and maximum rate of pressure rise among 1-, 2-, and 3pentanoleair mixtures at atmospheric pressure. At elevated pressure of 0.5 MPa, the difference among the three isomereair mixtures is decreased. 3. For ethanol, 1-butanol and 1-pentanoleair mixtures, because of the combined effect of adiabatic temperature and flame speed, peak explosion pressure, maximum rate of pressure rise, combustion duration and flame development period give the approximate values at fuel lean mixtures. At fuel rich mixtures, 1-pentanol gives the highest peak explosion pressure and maximum rate of pressure and the shortest combustion duration and flame development period. Their difference in heat loss is responsible for this. Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant No. 51406159, 91441203 and 50876085), the National Basic Research Program (2013CB228406), and the China Postdoctoral Science Foundation (2014M560774). English editing by Jason Cai is greatly acknowledged. References Bradley, D., Lawes, M., Mansour, M.S., 2009. Explosion bomb measurements of ethanoleair laminar gaseous flame characteristics at pressures up to 1.4 MPa. Combust. Flame 156, 1462e1470. Cammarota, F., Benedetto, A.D., Sarli, V.D., Salzano, E., 2012. The effect of hydrogen addition on the explosion of ethanol/air mixtures. Chem. Eng. Trans. 26. Campos-Fernandez, J., Arnal, J.M., Gomez, J., Lacalle, N., Dorado, M.P., 2013. Performance tests of a diesel engine fueled with pentanol/diesel fuel blends. Fuel 107, 866e872. Cancino, L.R., Fikri, M., Oliveira, A.A.M., Schulz, C., 2011. Ignition delay times of ethanol-containing multi-component gasoline surrogates: shock-tube experiments and detailed modeling. Fuel 90, 1238e1244. Chang, Y.M., Lee, J.C., Chan, C.C., Shu, C.M., 2009. Fire and explosion properties examinations of toluene-methanol mixtures approached to the minimum oxygen concentration. J. Therm. Anal. Calorim. 96, 741e749. Dahoe, A.E., 2005. Laminar burning velocities of hydrogeneair mixtures from closed vessel gas explosions. J. Loss Prev. Process Ind. 18, 152e166. , R., Berghmans, J., 1999. Comparison of two De Smedt, G., De Corte, F., Notele standard test methods for determining explosion limits of gases at atmospheric conditions. J. Hazard. Mater. 70, 105e113. Dorofeev, S.B., Sidorov, V.P., Efimenko, A.A., Kochurko, A.S., Kuznetsov, M.S., Chaivanov, B.B., Matsukov, D.I., Pereverzev, A.K., Avenyan, V.A., 1995. Fireballs
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from deflagration and detonation of heterogeneous fuel-rich clouds. Fire Saf. J. 25, 323e336. Gautam, M., Martin, D.W., 2000. Combustion characteristics of higher-alcohol/ gasoline blends. Proc. Inst. Mech. Eng. Part A J. Power Energy 214, 497e511. Gautam, M., Martin, D.W., Carder, D., 2000. Emissions characteristics of higher alcohol/gasoline blends. Proc. Inst. Mech. Eng. Part A J. Power Energy 214, 165e182. Gu, X.L., Huang, Z.H., Wu, S., Li, Q.Q., 2010. Laminar burning velocities and flame instabilities of butanol isomers-air mixtures. Combust. Flame 157, 2318e2325. Heufer, K.A., Bugler, J., Curran, H.J., 2013. A comparison of longer alkane and alcohol ignition including new experimental results for n-pentanol and n-hexanol. Proc. Combust. Inst. 34, 511e518. Koc, M., Sekmen, Y., Topgul, T., Yucesu, H.S., 2009. The effects of ethanol-unleaded gasoline blends on engine performance and exhaust emissions in a sparkignition engine. Renew. Energy 34, 2101e2106. Konnov, A.A., Meuwissen, R.J., de Goey, L.P.H., 2011. The temperature dependence of the laminar burning velocity of ethanol flames. Proc. Combust. Inst. 33, 1011e1019. Li, Q., Hu, E., Zhang, X., Cheng, Y., Huang, Z., 2013a. Measurements of laminar flame speeds and flame instability analysis of 2-methyl-1-butanoleair mixtures. Fuel 112, 263e271. Li, Q., Hu, E., Cheng, Y., Huang, Z., 2013b. Laminar flame speeds and flame instabilities of pentanol isomereair mixtures at elevated temperatures and pressures. Energy Fuels 27, 1141e1150. Li, L., Wang, J., Liu, H., 2015. Combustion and emissions of compression ignition in a direct injection diesel engine fueled with pentanol. Energy 80, 575e581. Marinov, N.M., 1999. A detailed chemical kinetic model for high temperature ethanol oxidation. Int. J. Chem. Kinet. 31, 183e220. Moss, J.T., Berkowitz, A.M., Oehlschlaeger, M.A., Biet, J., Warth, V., Glaude, P.A., Battin-Leclerc, F., 2008. An experimental and kinetic modeling study of the oxidation of the four isomers of butanol. J. Phys. Chem. A 112, 10843e10855. Movileanu, C., Razus, D., Oancea, D., 2013. Additive effects on the rate of pressure rise for ethyleneeair deflagrations in closed vessels. Fuel 111, 194e200. NFPA 68, 2002. Guide for Venting Deflagrations. National Fire Protection Agency. Ozsezen, A.N., Canakci, M., 2011. Performance and combustion characteristics of alcohol-gasoline blends at wide-open throttle. Energy 36, 2747e2752. Razus, D., Movileanu, C., Brinzea, V., Oancea, D., 2007. Closed vessel combustion of propyleneeair mixtures in the presence of exhaust gas. Fuel 86, 1865e1872. Razus, D., Brinzea, V., Mitu, M., Movileanu, C., Oancea, D., 2011. Temperature and pressure influence on maximum rates of pressure rise during explosions of propaneeair mixtures in a spherical vessel. J. Hazard. Mater. 190, 891e896. Saeed, K., Stone, C.R., 2004. Measurement of the laminar burning velocity for mixtures of methanol and air from a constant-volume vessel using a multzone model. Combust. Flame 139, 152e166.
Sarathy, S.M., Vranckx, S., Yasunaga, K., Mehl, M., Osswald, P., Metcalfe, W.K., Westbrook, C.K., Pitz, W.J., Kohse-Hoinghaus, K., Fernandes, R.X., Curran, H.J., 2012. A comprehensive chemical kinetic combustion model for the four butanol isomers. Combust. Flame 159, 2028e2055. Shimy, A.A., 1970. Calculating flammability characteristics of hydrocarbons and alcohols. Fire Technol. 6, 135e139. Stranic, I., Chase, D.P., Harmon, J.T., Yang, S., Davidson, D.F., Hanson, R.K., 2012. Shock tube measurements of ignition delay times for the butanol isomers. Combust. Flame 159, 516e527. Surisetty, V.R., Dalai, A.K., Kozinski, J., 2011. Alcohols as alternative fuels: an overview. Appl. Catal. A Gen. 404, 1e11. Tang, C., Huang, Z., Jin, C., He, J., Wang, J., Wang, X., Miao, H., 2009. Explosion characteristics of hydrogenenitrogeneair mixtures at elevated pressures and temperatures. Int. J. Hydrogen Energy 34, 554e561. Tang, C., Wei, L., Man, X., Zhang, J., Huang, Z., Law, C.K., 2013. High temperature ignition delay times of C5 primary alcohols. Combust. Flame 160, 520e529. Tang, C., Zhang, S., Si, Z., Huang, Z., Zhang, K., Jin, Z., 2014. High methane natural gas/ air explosion characteristics in confined vessel. J. Hazard. Mater. 278, 520e528. Togbe, C., Halter, F., Foucher, F., Mounaim-Rousselle, C., Dagaut, P., 2011. Experimental and detailed kinetic modeling study of 1-pentanol oxidation in a JSR and combustion in a bomb. Proc. Combust. Inst. 33, 367e374. Veloo, P.S., Egolfopoulos, F.N., 2011. Flame propagation of butanol isomers/air mixtures. Proc. Combust. Inst. 33, 987e993. Wang, H., Lu, C., An, G., Xiong, C., Zhou, Y., Ren, L., Wang, X., Xie, L., Wang, G., Wang, F., Sun, L., 2014a. Assessment method of liquid fuel field explosion hazardousness. Proced. Eng. 84, 394e401. Wang, Z.R., Ni, L., Liu, X., Jiang, J.C., Wang, R., 2014b. Effects of N2/CO2 on explosion characteristics of methane and air mixture. J. Loss Prev. Process Ind. 31, 10e15. Wei, L., Cheung, C.S., Huang, Z., 2014. Effect of n-pentanol addition on the combustion, performance and emission characteristics of a direct-injection diesel engine. Energy 70, 172e180. Wu, F., Law, C.K., 2013. An experimental and mechanistic study on the laminar flame speed, markstein length and flame chemistry of the butanol isomers. Combust. Flame 160, 2744e2756. Yucesu, H.S., Topgul, T., Cinar, C., Okur, M., 2006. Effect of ethanol-gasoline blends on engine performance and exhaust emissions in different compression ratios. Appl. Therm. Eng. 26, 2272e2278. Zhang, Z., Huang, Z., Wang, X., Zheng, J., Miao, H., Wang, X., 2009. Combustion characteristics of methanoleair and methanoleairediluent premixed mixtures at elevated temperatures and pressures. Appl. Therm. Eng. 29, 2680e2688. Zhang, B., Xiu, G., Bai, C., 2014. Explosion characteristics of argon/nitrogen diluted natural gaseair mixtures. Fuel 124, 125e132. Zhu, C., Lin, B., Jiang, B., 2012. Flame acceleration of premixed methane/air explosion in parallel pipes. J. Loss Prev. Process Ind. 25, 383e390.
Contents lists available at ScienceDirect
Journal of Loss Prevention in the Process Industries journal homepage: www.elsevier.com/locate/jlp
Comparative assessment of the explosion characteristics of alcoholeair mixtures Qianqian Li, Yu Cheng, Zuohua Huang* State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an, 710049, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 April 2015 Received in revised form 13 June 2015 Accepted 8 July 2015 Available online 15 July 2015
Explosion characteristics of five alcoholeair (ethanol, 1-butanol, 1-pentanol, 2-pentanol and 3-pentanol) mixtures were experimentally conducted in an isochoric chamber over wide ranges of initial temperature and pressure. The effect of temperature and pressure on the different explosion behaviors among these alcohols with various structures were investigated. Results show that the peak explosion pressure is increased with the decrease of temperature and increase of pressure. Maximum rate of pressure rise is insensitive to the temperature variation while it significantly increases with the increase of initial pressure. Among the 1-, 2-, and 3-pentanoleair mixtures, 1-pentanol has the highest values in peak explosion pressure and maximum rate of pressure rise and 2-pentanol gives the lowest values at the initial pressure of 0.1 MPa. These differences tend to be decreased with the increase of initial pressure. Among the three primary alcoholeair (ethanol, 1-butanol and 1-pentanol) mixtures, a similar explosion behavior is presented at the lean mixture side because of the combined effect of adiabtic temperature and flame propagation speed. At the rich mixture side, 1-pentanol gives the highest values in peak explosion pressure and maximum rate of pressure rise and ethanol gives the lowest values. This phenomenon can be interpretated from the combining influence of heat release and heat loss, since the flame speeds of ethanol-, 1-butanol-, 1-pentanoleair mixtures are close at rich mixture side. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Pentanol isomers Primary alcohols Explosion characteristics Combustion phase
1. Introduction The interest in bio-alcoholic renewable fuels is increasing because of energy shortages and serious environmental pollution. In the past decades, extensive studies on bio-alcohols suggested that alcohols addition potentially favors to the reduction of emissions, especially the particulate matter (Gautam and Martin, 2000; Gautam et al., 2000; Yucesu et al., 2006; Koc et al., 2009; Ozsezen and Canakci, 2011; Surisetty et al., 2011). Low alcohols like ethanol with high octane number and relatively low cost have been successful in the practical use as the gasoline additives. However, low alcohols still have their disadvantages such as low energy content and high hygroscopicity. This leads to an inconvenient storage and transportation and restricts their wide applications in engines. Meanwhile, low alcohols favor to the knock resistance in the spark ignition (SI) engines but they are bad for compression ignition (CI) engines. Recent study on the higher alcohols indicated that 1-pentanol presents a negative temperature coefficient (NTC)
* Corresponding author. E-mail address: [email protected] (Z. Huang). http://dx.doi.org/10.1016/j.jlp.2015.07.003 0950-4230/© 2015 Elsevier Ltd. All rights reserved.
behavior in the intermediate temperature regime, which indicates their potential of fuels for the CI engines (Heufer et al., 2013). An engine study on the 1-pentanol/diesel blend reported that, compared with pure diesel, the blend exhibited the comparative engine performance, lower particulate emissions, and even better combustion characteristics for the blending ratio of pentanol up to 25% (Campos-Fernandez et al., 2013; Wei et al., 2014). Li et al. (2015) tested pure pentanol in a conventional diesel engine and achieved the ultralow NOX and smoke emissions without exhaust gas recirculation (EGR), demonstrating its good potential applicability. Fundamental combustion research can help the clean and efficient utilization of alcohols. Up to now, extensive studies concentrated on the low alcohols. Konnov et al. (2011) and Bradley et al. (2009) respectively studied the laminar combustion characteristics of ethanol with heat flux method and spherically propagating flame. Gu et al. (2010), Veloo and Egolfopoulos (2011) and Wu and Law (2013) subsequently measured the laminar flame speeds of butanol isomers using different apparatuses at the initial pressures of 0.1e0.75 MPa. Cancino et al. (2011) measured the ignition delay times of ethanol-containing gasoline surrogates in a shock tube at
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the temperatures from 720 to 1220 K. Moss et al. (2008) and Stranic et al. (2012) studied the auto-ignition characteristics of butanol isomers at the elevated temperatures and the pressures of 1.5e43 atm. Fundamental researches on high alcohol of pentanol fuels started in the latest years. Li et al. (2013a,b) comparatively studied the laminar combustion and chemical kinetic characteristics of 1-, 2-, and 3-pentanol, and reported that 1-pentanol gives the highest flame speed, followed by 3-pentanol and 2-pentanol. Tang et al. (2013) studied the ignition delay characteristics of 1pentanol, iso-pentanol and 2-methyl-1-butanol using a shock tube, indicating 1-pentanol has the shortest ignition delay. These studies provided valuable insight into the difference among the isomers. Alcohols are flammable fuels and the alcohol vapors might burn or explode, leading to disasters. Such accidents could happen in the case of fuel evaporation or leakage especially under a high temperature, resulting in properties losses and even human casualties (Dorofeev et al., 1995; Wang et al., 2014a,b). Therefore, the safety issue claims high concern over the fuel utilization, storage and transportation, calling for the demand of investigation on the explosion characteristics to assess the potential explosion hazard. In the previous studies, the main attention was focused on the gaseous fuels like methane, natural gas and ethylene, etc. (Dahoe, 2005; Razus et al., 2007, 2011; Tang et al., 2009, 2014; Zhu et al., 2012; Movileanu et al., 2013; Wang et al., 2014a,b). Their studies demonstrated that the explosion parameters are strongly dependent on initial pressure, initial temperature and fuel/air ratios. Until now, limited researches have been performed on the explosion properties of alcohols. Chang et al. (2009) determined the critical explosion properties of the tolueneemethanol mixtures at varied blending ratios and initial oxygen concentrations using a closed spherical vessel, and provided a triangular flammability diagram for the explosive hazard region. Cammarota et al. (2012) studied the explosion characteristics of pure ethanol and ethanolehydrogeneair mixtures at different equivalence ratios and temperatures using a cylindrical chamber. Zhang et al. (2009) studied the explosion characteristics of methanoleairediluent mixtures in a constant chamber, from evaluating the peak combustion pressure, flame development duration and combustion duration. Shimy (1970)
proposed the formulas for the prediction of the ignition temperature and flammability of alcohols and hydrocarbons. To our knowledge, no report was published for the influence of structures and carbon chain length on the explosion characteristics of alcohols. In this study, the explosion characteristics of 1-, 2-, and 3pentanoleair mixtures were studied at initial temperatures from 393 to 433 K and initial pressures from 0.1 to 0.75 MPa using a constant combustion vessel. Explosion hazard was assessed by the explosion parameters such as explosion pressure, deflagration index, maximum rate of pressure rise, combustion duration. The discrepancies among the isomers were analyzed and the effects of the initial conditions on the explosion characteristics were discussed. Meanwhile, a comparative assessment on the explosion characteristics of ethanol-, 1-butanol- and 1-pentanoleair mixtures at different equivalence ratios was also made to evaluate the influence of carbon chain length on the potential risk and hazard of these primary alcohols. 2. Experimental setup and data acquisition Fig. 1 gives the sketch of the experimental setup. It is composed of ignition system, heating system, constant volume vessel, data acquisition system and inlet/exhaust system. The constant volume vessel is a stainless cylinder with a diameter of 180 mm and length of 210 mm. The mixture in the vessel is ignited by the centrally located electrodes with the spark produced by a standard capacitive discharge ignition system. The spark energy is 45 mJ which is higher than the minimum ignition energy of all the mixtures involved in present study. Heating tapes are wrapped around the chamber to heat the chamber. The temperature in the chamber is monitored by a thermocouple with an accuracy of ±1 K. The combustion pressure is acquired with a pressure transducer (Kistler 7001) at a sample-rate of 100 kHz connected with a Charge Amplifier (Kistler 5011), and the pressure data was recorded with an oscilloscope. Other control components such as the intake and outlet valves are all mounted on the chamber. When the chamber was heated to the target temperature, the chamber was vacuumed and repeatedly flushed with dry air to remove the residual gases from the previous experiment. The
Fig. 1. Experimental setup.
Q. Li et al. / Journal of Loss Prevention in the Process Industries 37 (2015) 91e100 Table 1 Properties of fuels involved in the study. Chemical name
Purity
Molecular formula
Ethanol
99.8%
C2H6O
1-Butanol
99.7%
C4H10O
1-Pentanol
>99%
C5H12O
2-Pentanol
>99%
C5H12O
3-Pentanol
>99%
C5H12O
Structure
Table 2 Deflagration index classes (NFPA 68, 2002). Level
Deflagration index (KG)/MPa m s1
St St St St
0 1e20 20e30 >30
0 1 2 3
mixtures at specified equivalence ratios were prepared based on the partial pressure of each component. Liquid fuel was injected into the chamber with a micro-syringe and five minutes were awaited to ensure its full evaporation. Dry air of 21% O2 and 79% N2 was filled into the chamber. Time intervals of both 5 min and 30 min were awaited to make the uniform mixing, and no significant difference could be observed. Thus, 5 min were adopted in
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present study. The oscilloscope was initiated before the mixture was ignited so that the whole combustion process can be recorded. In this experiment, the initial temperatures were chosen as 393 K, 433 K, 473 K. The initial pressures were 0.1 MPa, 0.25 MPa, 0.5 MPa, 0.75 MPa. The equivalence ratio is ranging from 0.6 to 1.8. For each condition, at least three tests were made to examine the repeatability of the experiment. Purity of O2 and N2 are over 99.95% and fuel properties are listed in Table 1. Explosion parameters such as peak explosion pressure (Pmax/P0), maximum rate of pressure rise (dP/dt)max, and deflagration index (KG), etc., are used to assess the explosion characteristics. These parameters provide key information of an explosion process, assessing safety of combustion chamber and designing the relief device against the explosion damage. The explosion pressure and rate of pressure rise can be directly obtained from the pressure history. Since pressure variation depends on experimental conditions, especially the vessel volume, thus, maximum rate of pressure rise is always normalized with vessel volume on the basis of cubicroot law through KG ¼ ðdP=dtÞmax V1=3 , where KG is deflagration index and indicates the explosion intensity (De Smedt et al., 1999; Tang et al., 2009; Cammarota et al., 2012). The law is generally used in spherical vessels. However, as described in European Standard guideline and NFPA 68 (2002), it can also be used in cylinder vessel with the length to diameter ratio (L/D) lower than 2. For present cylinder vessel, L/D is 1.17, complying with the standard. It is assumed the deflagration index depends only on the mixture properties regardless of chamber volume, and could scale the explosion hazard (NFPA 68, 2002). The explosion index is scaled as shown in Table 2, and higher values indicate more robust explosion. Combustion duration and flame development duration, reflecting the combined effect of heat release and flame propagation speed. Combustion duration is the time interval between ignition and
Fig. 2. Histories of P/P0 and dP/dt at elevated temperatures and pressures for 1-pentanoleair mixtures.
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Fig. 3. Pmax/P0 and Pe/P0 versus f at f ¼ 1.0 and three initial temperatures for 1pentanoleair mixtures with error bars presented. Line: Pe/P0; Symbol: Pmax/P0.
peak explosion pressure (Zhang et al., 2014) and flame development duration is the time from ignition to 7% pressure rise (De Smedt et al., 1999). 3. Results and discussion 3.1. Pressure evolution and deflagration 3.1.1. 1-Pentanol-air mixture Fig. 2 plots the histories of explosion pressure (P/P0) and rate of pressure rise (dP/dt) of 1-pentanoleair mixtures at elevated pressures and temperatures. The explosion pressure increases progressively until the rate of pressure rise obtain the maximum value. Then the explosion pressure keeps increasing but the rate of pressure rise gradually decreases, due to the heat loss of the flame front to the vessel wall and the reduced fuel consumption near the end of combustion (Dahoe, 2005; Saeed and Stone, 2004). This behavior was also reported for methane and natural gas combustion (Tang et al., 2014; Zhang et al., 2014). Fig. 2a and b give the explosion pressure and rate of pressure rise versus time at three temperatures, P0 ¼ 0.1 MPa and f ¼ 1.0. The peak explosion pressure (Pmax/P0) is the maximum value of explosion pressure, as indicated in Fig. 2a. With the increase of temperature, the peak explosion pressure is decreased due to the decrease of the total fuel mass. However, the maximum rate of pressure rise varies little. Besides, the time intervals between ignition and peak explosion pressure are decreased. This is because the species in flames at
elevated temperatures are more reactive, resulting in the promotion of the whole reactions and hence the faster flame speed. Specifically, previous study (Li et al., 2013a,b) reported the laminar flame speed of 1-pentanol-air mixture increases from 58.8 cm s1 to 79.0 cm s1 with the initial temperature increasing from 393 to 473 K at the equivalence ratio of 1.0. Fig. 2c and d shows the explosion pressure and rate of pressure rise versus time at four initial pressures for the 1-pentanoleair mixtures. Peak explosion pressure increases with the increase of initial pressure because total amount of heat release is increased as the increased fuel mass. Peak explosion pressure and peak value of the rate of pressure rise reach early with the decrease of pressure because of the increased flame speed at decreased pressure. The maximum combustion pressure in a constant volume chamber can be simulated based on the adiabatic assumption through thermal equilibrium. The adiabatic equilibrium pressure, Pe, is calculated from the equilibrium module in CHEMKIN. The mechanism used for the simulation was developed by Togbe et al. (2011). Fig. 3 illustrates the peak explosion pressure (Pmax/P0) and normalized maximum equilibrium pressure (Pe/P0) versus equivalence ratio at atmospheric pressure and three initial temperatures. Error bars are presented for Pmax/P0 to show the experimental uncertainties. It is seen the experimental uncertainties are small at most conditions. Big uncertainties are just presented for some fuel rich mixtures. Pe/P0 gives a similar behavior to that of the adiabatic temperature, and it decreases with the increase of initial temperature. The adiabatic temperature and flame propagation speed all give their peak values around f ¼ 1.1, resulting in the peak values of Pmax/P0 and Pe/P0 fall between f ¼ 1.0 and f ¼ 1.2. Leaner or richer mixtures make the drop of pressure. Real combustion is not under the absolute adiabatic condition. It accompanies both radiant and convective heat losses to the wall, leading to the lower Pmax/P0 than Pe/P0. It is noted that the difference between Pe/P0 and Pmax/P0 is remarkably increased at highly rich mixtures. This is because soot formed at rich mixtures due to the absence of oxygen, highly promoting the heat loss through the continuum radiation to the vessel wall. With the mixture becoming richer, more soot formed in the flame, causing larger difference between Pe/P0 and Pmax/P0. Fig. 4 shows the maximum rate of pressure rise, (dP/dt)max, and deflagration index, KG, versus equivalence ratio at elevated temperatures and pressures for the 1-pentanoleair mixtures. Maximum rate of pressure rise and deflagration index reach their peaks at f around 1.1, and they decrease at both lean and rich mixtures. As shown in Fig. 4a, (dP/dt)max and KG give an approximate value at varied initial temperatures, indicating that (dP/dt)max and KG are insensitive to the variation of temperature. Rate of pressure rise is affected by both flame speed and heat release. Flame speed increases monotonically with the increase of
Fig. 4. (dP/dt)max and KG versus f at elevated temperatures and pressures for 1-pentanoleair mixtures.
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Fig. 5. P/P0 and dP/dt at f ¼ 1.0, P0 ¼ 0.1 MPa and T0 ¼ 433 K for 1-, 2-, and 3-pentanoleair mixtures.
Fig. 6. Pmax/P0 versus f at T0 ¼ 433 K and two pressures for 1-, 2-, and 3-pentanoleair mixtures.
temperature (Li et al., 2013a,b), but the decrease of total fuel mass results in the decreased heat release and offsets the effect of flame speed on the explosion pressure. With the increase of initial pressure as shown in Fig. 4b, (dP/dt)max and KG increase dramatically, especially around equivalence ratio of 1.1. This differs from the flame speed, but agrees with the mass burning flux when taking into account of the effect of density. It is noted that deflagration index is less than 20 MPa m s1 at f ¼ 1.0 and the initial pressure of 0.1 MPa, belonging to the first class of deflagration index (NFPA 68,
2002), and low potential of explosion hazard. However, KG exceeds 30 MPa m s1 around the stoichiometric ratio at the pressures higher than 0.5 MPa and enters the highest class of deflagration index. Therefore, high attention needs to be taken at elevated pressures. 3.1.2. Pentanol isomereair mixtures Fig. 5 shows the evolution of the explosion pressure (P/P0) and rate of pressure rise (dP/dt) at f ¼ 1.0, T0 ¼ 433 K and P0 ¼ 0.1 MPa
Fig. 7. (dP/dt)max versus f at different temperatures and pressures for 1-, 2-, and 3-pentanoleair mixtures.
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Fig. 8. P/P0 and dP/dt at T0 ¼ 393 K and P0 ¼ 0.1 MPa for ethanol, 1-butanol and 1-pentanoleair mixtures.
for the 1-, 2-, and 3-pentanoleair mixtures. Among three pentanoleair mixtures, the time to reach peak values of Pmax/P0 and (dP/ dt)max are advanced in the order of 1-pentanol, 3-pentanol and 2pentanol. The difference between Pmax/P0 of 2-pentanol and 3pentanoleair mixtures is small. As reported in the previous study (Li et al., 2013a,b), 1-pentanol-air mixture gives the largest flame speed and the highest adiabatic temperature among 1-, 2-, and 3pentanoleair mixtures. Thus, 1-pentanol-air mixture yields the highest explosion pressure. For 2-, and 3-pentanoleair mixtures, 3pentanol has close adiabatic temperature with 2-pentanol, but always exhibits 2 cm s1 higher flame speed than 2-pentanol does at
normal pressure, Li et al., 2013a,b), leading to the slightly higher Pmax/P0 and (dP/dt)max of 3-pentanol. Fig. 6 gives the peak explosion pressure of 1-pentanol and its two isomereair mixtures versus equivalence ratio at initial temperature of 443 K and two initial pressures. At initial pressure of 0.1 MPa, Pmax/P0 of 1-pentanol-air mixture gives the highest value, followed by 3-pentanol and 2-pentanol. Pmax/P0 gives its maximum values between 1.1 and 1.2. At a high pressure of 0.5 MPa, Pmax/P0 of three isomereair mixtures present close values at all equivalence ratios. For the mixtures with equivalence ratio larger than 1.2 and pressure higher than 0.5 MPa, the flame front is unstable and is
Fig. 9. Comparison among three alcohol flames with different parameters including Pe/P0, Pmax/P0, Tad, laminar flame speed, and mole fractions of H2O and CO2 at P0 ¼ 0.1 MPa and T0 ¼ 393 K.
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values at around f ¼ 1.1, in which 1-pentanol gives the highest value and 2-pentanol gives the lowest value. As shown in Fig. 7b, when initial pressure is elevated to 0.5 MPa, (dP/dt)max and KG increase dramatically, but the difference among these isomers tends to be decreased, which is similar to the variation of peak explosion pressure. This pressure dependent behavior is resulted from the decreased difference among the flame speeds of the isomereair mixtures at elevated pressure. As indicated in previous study (Li et al., 2013a,b), 1-, 2-, 3-pentanoleair mixtures respectively gives the maximum laminar flame speed of 67.2, 61.5, 63.8 cm s1 at 0.1 MPa, and 47.2, 43.9, 46.4 cm s1 at 0.5 MPa and 433 K. The difference is decreased from 5.7 to 3.3 cm s1 between 1-pentanol and 2-pentanol, and from 2.4 to 0.8 cm s1 between 1-pentanol and 3-pentanol, with the initial pressure elevated from 0.1 to 0.5 MPa.
Fig. 10. (dP/dt)max and KG versus f at different temperatures and pressures for ethanol, 1-butanol, and 1-pentanoleair mixtures.
easily developed into a cellular structure, resulting in a remarkable increase in flame propagation speed and pressure. Experiments conducted under these conditions needs to be careful and vessel safety design is required. Fig. 7 shows the comparison among 1-, 2-, and 3-pentanoleair mixtures on the maximum rate of pressure rise and the deflagration index versus equivalence ratio at T0 ¼ 433 K and two initial pressures. As shown in Fig. 7a, (dP/dt)max and KG reach their peak
3.1.3. Ethanol, 1-butanol and 1-pentanoleair mixtures Fig. 8 gives the explosion pressure and rate of pressure rise versus time at f ¼ 1.0, P0 ¼ 0.1 MPa, T0 ¼ 393 K for the ethanol, 1butanol and 1-pentanoleair mixtures. The phase of Pmax/P0 and (dP/dt)max are all slightly postponed with the increase of carbon chain length, reflecting the gradually decreased flame speed. Pmax/ P0 of ethanol- and 1-butanoleair mixtures shows an approximate values and that of 1-pentanol-air mixture is slightly higher than them. Ethanol gives the highest (dP/dt)max and 1-pentanol gives the lowest value. In general, the difference among the three mixtures is small. Fig. 9a shows the peak explosion pressures (Pmax/P0) of ethanol, 1-butanol and 1-pentanoleair mixtures versus equivalence ratio at T0 ¼ 393 K and P0 ¼ 0.1 MPa. Behavior at f ¼ 1.0 in Fig. 8 is identical to all fuel lean mixtures. For fuel rich mixtures, Pmax/P0 decreases
Fig. 11. Combustion duration for 1-, 2-, and 3-pentanoleair mixtures.
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monotonically in the order of 1-pentanol, 1-butanol and ethanol. To interpret the reason, the calculated adiabatic temperatures and laminar flame speeds are provided in Fig. 9b and c, respectively (Li et al., 2013a,b). For fuel lean mixtures, the ethanol-air mixture gives the lowest adiabatic temperature and the largest flame speed, resulting in a slightly lower Pmax/P0 compared to 1-pentanol-air mixture. For fuel rich mixtures, the ethanol-air mixture has an approximate laminar flame speed to the 1-butanol and 1-pentanol mixtures but lower adiabatic temperature, resulting in the lower explosion pressure. To further interpret the difference between 1butanol and 1-pentanol, the equilibrium explosion pressure, Pe/ P0, is calculated and plotted in Fig. 9a. The mechanisms used here for ethanol, 1-butanol and 1-pentanol were proposed by Marinov (1999), Sarathy et al. (2012) and Togbe et al. (2011), respectively. Pe/P0 is much higher than Pmax/P0, however, it can accurately capture the trend of Pmax/P0 among the three mixtures. Similar to the adiabatic temperature, Pe/P0 of 1-butanol and 1-pentanol-air mixtures are approximate in the absence of heat loss. However, heat could be lost in combustion vessel through conduction, convection and radiation. The flame temperature is much higher than the air mixture outside the vessel. Heat could be lost through conduction and convection after the flame front touches the vessel wall. Besides, the main products in alcohol flames, are all triatomic gases with high radiation intensity will enhance the heat loss by radiation. Due to the heat loss, Pmax/P0 gives lower value than Pe/P0 over all equivalence ratios. It is noted that three alcohol flames present approximate values of Pmax/P0 and Pe/P0 at lean mixtures. At the rich mixtures, Pmax/P0 decreases in the order of 1-pentanol, 1-
butanol and ethanol, similar with Pe/P0. However, the difference among the three flames are much significant for Pmax/P0 than for Pe/ P0 at the rich mixtures, revealing the most heat is lost in ethanol flame. To illustrate this phenomenon, the mole fractions of the main products, CO2 and H2O, in the rich alcohol flames are given in Fig. 9d. Though CO is also important product in the rich fuel flames, it couldn't radiate heat and is not discussed here. As seen in Fig. 9d, the mole fractions of CO2 and H2O all decrease in the order of ethanol, 1-butanol and 1-pentanol, indicating the heat loss through radiation decreases in the same order. Fig. 10 gives the maximum rate of pressure rise and deflagration index versus equivalence ratio at P0 ¼ 0.1 MPa and T0 ¼ 393 K for the ethanol, 1-butanol and 1-pentanoleair mixtures. Similar to Pmax/P0, the three alcoholeair mixtures give the approximate values in (dP/dt)max and KG at fuel lean side. At fuel rich side, 1pentanol gives the highest and ethanol gives the lowest value. For all mixtures at f ¼ 1.0, the deflagration index is less than 10 MPa m s1, indicating low potential explosion hazard. 3.2. Combustion phase Combustion phase is an index reflecting the heat release and is also an important parameter to characterize the combustion process. Variation in combustion phase can directly influence fuel/air mixing, energy conversion efficiency, combustion and emissions. In a practical combustion device, combustion phase is adjusted to the variation of initial conditions to achieve high energy efficiency. Thus, combustion phase is a parameter for combustion control. In
Fig. 12. td versus f at different temperatures and pressures for 1-, 2-, and 3-pentanoleair mixtures.
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Fig. 13. tc and td versus f at T0 ¼ 393 K and P0 ¼ 0.1 MPa for ethanol, 1-butanol, and 1-pentanoleair mixtures.
this study, the combustion duration (tc) and flame development period (td), are determined. Their definitions can be referenced to previous publications (De Smedt et al., 1999; Zhang et al., 2014). Fig. 11 gives the combustion duration of 1-, 2-, and 3pentanoleair mixtures versus f at different temperatures and pressures. Combustion duration increases with the decrease of temperature and the increase of pressure. Flame propagation speed is decreased with the decrease of initial temperature and the increase of initial pressure, resulting in the increase of time to reach the peak explosion pressure. At f ¼ 1.0 as shown in Fig. 11c, combustion duration increases in the order of 1-pentanol, 3-pentanol and 2-pentanol. At elevated pressure (0.5 MPa) as shown in Fig. 11d, the three mixtures gives the approximate values. Combustion duration gives its minimum value around f ¼ 1.1, corresponding to the position of the largest flame speed. Fig. 12 shows the flame development period (td) versus f for 1-, 2-, and 3-pentanoleair mixtures at elevated temperatures and pressures. Similar to the combustion duration, flame development period gives its minimum value around f ¼ 1.1. It decreases with the increase of temperature and the decrease of pressure. Change of td with the increase of initial pressure is more significant at highly lean mixtures compared to that at stoichiometric equivalence ratio. The mixtures with low flame speed gives long td. At highly lean mixtures, flame speed is lower at atmospheric pressure and even much lower at elevated pressures, resulting in the increased difference of td to the variation of pressures. Small differences are presented for the three pentanol isomereair mixtures. Fig. 13 plots tc and td versus f for the ethanol, 1-butanol and 1pentanoleair mixtures at P0 ¼ 0.1 MPa and T0 ¼ 393 K tc and td show the approximate values at lean and stoichiometric mixtures side, and large difference at rich mixture side. Ethanol gives the longest tc and td, while 1-pentanol gives the shortest tc and td. 4. Conclusions A comparative study on the explosion characteristics of five alcohol (ethanol, 1-butanol, 1-pentanol, 2-pentanol, and 3pentanol)eair mixtures at elevated temperatures and pressures was conducted using a constant volume chamber. The main conclusions are as follows: 1. Peak explosion pressure of 1-pentanol-air mixture increases with the decrease of initial temperature and the increase of initial pressure. Maximum rate of pressure rise and deflagration index are insensitive to the variation of initial temperature. They
remarkably increase with the increase of pressure, demonstrating a high potential explosion hazard at elevated pressures. Combustion duration and flame development period of 1pentanoleair mixtures give their minimum values at f ¼ 1.1. 2. 1-Pentanoleair mixtures give the highest peak explosion pressure and maximum rate of pressure rise among 1-, 2-, and 3pentanoleair mixtures at atmospheric pressure. At elevated pressure of 0.5 MPa, the difference among the three isomereair mixtures is decreased. 3. For ethanol, 1-butanol and 1-pentanoleair mixtures, because of the combined effect of adiabatic temperature and flame speed, peak explosion pressure, maximum rate of pressure rise, combustion duration and flame development period give the approximate values at fuel lean mixtures. At fuel rich mixtures, 1-pentanol gives the highest peak explosion pressure and maximum rate of pressure and the shortest combustion duration and flame development period. Their difference in heat loss is responsible for this. Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant No. 51406159, 91441203 and 50876085), the National Basic Research Program (2013CB228406), and the China Postdoctoral Science Foundation (2014M560774). English editing by Jason Cai is greatly acknowledged. References Bradley, D., Lawes, M., Mansour, M.S., 2009. Explosion bomb measurements of ethanoleair laminar gaseous flame characteristics at pressures up to 1.4 MPa. Combust. Flame 156, 1462e1470. Cammarota, F., Benedetto, A.D., Sarli, V.D., Salzano, E., 2012. The effect of hydrogen addition on the explosion of ethanol/air mixtures. Chem. Eng. Trans. 26. Campos-Fernandez, J., Arnal, J.M., Gomez, J., Lacalle, N., Dorado, M.P., 2013. Performance tests of a diesel engine fueled with pentanol/diesel fuel blends. Fuel 107, 866e872. Cancino, L.R., Fikri, M., Oliveira, A.A.M., Schulz, C., 2011. Ignition delay times of ethanol-containing multi-component gasoline surrogates: shock-tube experiments and detailed modeling. Fuel 90, 1238e1244. Chang, Y.M., Lee, J.C., Chan, C.C., Shu, C.M., 2009. Fire and explosion properties examinations of toluene-methanol mixtures approached to the minimum oxygen concentration. J. Therm. Anal. Calorim. 96, 741e749. Dahoe, A.E., 2005. Laminar burning velocities of hydrogeneair mixtures from closed vessel gas explosions. J. Loss Prev. Process Ind. 18, 152e166. , R., Berghmans, J., 1999. Comparison of two De Smedt, G., De Corte, F., Notele standard test methods for determining explosion limits of gases at atmospheric conditions. J. Hazard. Mater. 70, 105e113. Dorofeev, S.B., Sidorov, V.P., Efimenko, A.A., Kochurko, A.S., Kuznetsov, M.S., Chaivanov, B.B., Matsukov, D.I., Pereverzev, A.K., Avenyan, V.A., 1995. Fireballs
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