1 Introduction
Jet fuel is an important transportation fuel, widely used in aviation industry and sometimes used for special vehicles in ground transportation. The accurate understanding of the jet fuel combustion processes is beneficial for the development of efficient engines. As jet fuel compositions are complex [
1,
2], it is difficult to construct a chemical kinetic model that covers all components. As a result, simple mixtures consisting of a few representative compounds are usually proposed to study the combustion of jet fuels. Fuel auto-ignition is an important property for practical transportation fuels, because fuel ignition propensity determines the subsequent combustion process and the formation of engine-out emissions, especially in those advanced engine combustion concepts. Therefore, ignition delay time is generally selected as a key indicator to evaluate the performance of those proposed surrogate mixtures [
3–
6].
Auto-ignition characteristics of Jet A were extensively studied, and some surrogates, including single-component molecules and multi-component mixtures, were proposed to describe the auto-ignition characteristics of Jet A.
n-decane and
n-dodecane were first proposed as the single-component surrogates due to their similar physical properties to jet fuel, but they could not well reproduce the combustion behaviors of jet fuel [
7,
8]. Subsequently, multi-component surrogates were developed to match the auto-ignition characteristics of jet fuel, including the three-component mixtures, e.g. the mixture of
n-dodecane, methylcyclohexane, and
o-xylene by Mairinger et al. [
9] and the mixture of
n-decane,
iso-octane, and toluene by Dooley et al. [
10], the four-component mixtures, e.g. the mixture of
n-dodecane,
iso-octane,
n-propylbenzene, and 1,3,5-trimethylbenzene by Malewicki et al. [
11], and the five-component mixtures, e.g., the mixture of decalin,
n-dodecane,
iso-cetane,
iso-octane, and toluene by Yu et al. [
12].
Compared with Jet A, RP-3 jet fuel is a different type of jet fuel with changed composition. For example, Mao et al. [
13] compared the compositions of RP-3 jet fuel and Jet A, and found that RP-3 jet fuel had an obviously higher
iso-alkane content (42.9%) than that in Jet A (29%) [
14]. Some existing researches about the auto-ignition behaviors of RP-3 jet fuel and its surrogates are listed as follows. Zhang et al. [
15] proposed a surrogate of 88.7%
n-decane and 11.3% 1, 2, 4-trimethylbenzene on a molar basis, and compared its ignition delays with those of RP-3 jet fuel at temperatures of 1000–1300 K on a shock tube. Yan et al. [
16] proposed a similar two-component surrogate, 92%
n-decane and 8%
n-propylbenzene on a molar basis, and developed a simplified RP-3 jet fuel mechanism based on this surrogate. The ignition delay times of this surrogate and RP-3 jet fuel, measured on a shock tube, were consistent within the temperature range of 1150 K to 1600 K. Liu et al. [
17] studied the ignition delays of a three-component surrogate for RP-3 jet fuel, that is, 66.2%
n-dodecane, 15.8%
n-propylbenzene, and 18% 1, 3, 5-trimethylcyclohexane on a molar basis. Xu et al. [
18] also proposed a surrogate with the same composition but different proportions, i.e., 73%
n-dodecane, 14.7% 1, 3, 5-trimethylcyclohecane, and 12.3%
n-propylbenzene on a mass basis. They also developed a detailed high-temperature chemical mechanism based on this surrogate and the mechanism was validated against the auto-ignition behaviors of RP-3 jet fuel at pressures of 1 bar to 4 bar. Another three-component surrogate was developed by Yu and Gou [
19], including 54.3%
n-dodecane, 32.1% 2, 5-dimethylhexane, and 13.6% toluene on a molar basis. Its ignition delay times well agreed with those of RP-3 jet fuel at the low-temperature region but the comparison became less consistent at the high-temperature region. Recently, Mao et al. [
13] proposed another three-component surrogate, composed of
n-dodecane,
iso-cetane and toluene, and developed a kinetic model based on this surrogate mixture. Some researchers proposed surrogate mixtures of even more components for RP-3 jet fuel. Zheng et al. [
20] presented a four-component surrogate, that is, 40%
n-decane/42%
n-dodecane/13% ethylcyclohexane/5%
p-xylene on a molar basis. Another four-component surrogate was proposed by Yi et al. [
21], which was composed of 15% toluene/18.9% trans-decalin/59.1%
n-decane/7%
iso-cetane on a molar basis. The ignition delay times of this surrogate and the PR-3 jet fuel were compared at atmospheric pressure.
It is noted that the abovementioned studies were mainly focused on the auto-ignition behaviors of RP-3 jet fuel at high temperatures and low pressures, but few researches were designed to characterize its spray auto-ignition behaviors at low-to-intermediate temperatures and high pressures. The fuel combustion chemistry at low-to-intermediate temperatures and high pressures is equally important, as these conditions are typical operation conditions in practical combustion engines. To capture the spray auto-ignition characteristics of RP-3 jet fuel, experiments were conducted on a constant volume combustion chamber (CVCC) in this study, within a temperature range of 808 K to 923 K. Further, the auto-ignition characteristics of two binary-component surrogates and one three-component surrogate were compared with RP-3 jet fuel, as the above literature review indicated that the two-component and three-component surrogates were the simplest and the most widely studied surrogate mixtures. The two-component surrogates are usually the mixtures of n-alkane and aromatic hydrocarbons. Compared with the two-component surrogates, the surrogates with more components also cover iso-alkanes and/or cycloalkanes. The composition of the test RP-3 jet fuel was determined by GC-MS, including 25.1% n-alkanes, 42.7% iso-alkanes, 17.8% cycloalkanes, and 14.4% aromatics by weight. Considering the higher contents of iso-alkanes in RP-3 jet fuel, the three-component surrogate mixture selected in this study uses an iso-alkane molecule as the third component.
2 Methodology
2.1 Experimental apparatus
Fuel ignition tests were conducted on a constant volume combustion chamber (CVCC) facility as shown in Fig. 1 [
4,
22–
24]. The facility was composed of a combustion system, fuel/oxidizer supply systems, and a closed-loop cooling system. The combustion chamber has a volume of 0.473 L and could be heated to a preset initial temperature. The fuel supply system consists of a hydraulic pump, a pressure multiplier, and an electronic diesel fuel injector. The injector consists of six holes, each of which has a diameter of 0.17 mm. The closed-loop cooling system was used to control the temperature of key monitoring elements. Two K-type thermocouples, installed in the stainless steel sheath, were used to measure the temperatures in the chamber and the injector cooling passage. A dynamic pressure transducer located at the bottom of the combustion chamber was used to capture the pressure during the combustion process. A static pressure sensor was used to monitor the chamber pressure prior to fuel injection.
2.2 Test fuels
The physical and chemical properties of RP-3 jet fuel were listed in Table 1, including the molecular weight (MW), H/C ratio, lower heating value (LHV), viscosity, derived cetane number (DCN), density, and flash point. The
n-decane/1,2,4-trimethylbenzene blend proposed by Zhang et al. [
15] was considered as the first model surrogate (Surrogate 1). This surrogate was formulated by capturing the H/C ratio of RP-3 jet fuel, as suggested by Zhang et al. [
15], and has 70% mol
n-decane and 30% mol 1,2,4-trimethylbenzene. The second surrogate (Surrogate 2) is composed of 51% mol
n-decane and 49% mol 1, 2, 4-trimethylbenzene. In spite of its different H/C ratio from RP-3 jet fuel, this surrogate mixture and RP-3 jet fuel have similar DCNs. The surrogate mixture proposed by Mao et al. [
13], which consists of 49.8% mol
n-dodecane, 21.6% mol
iso-cetane, and 28.6% mol toluene, was considered as the third surrogate (Surrogate 3), and it is noted that many physiochemical properties of RP-3 jet fuel and Surrogate 3, such as molecular weight, H/C ratio, viscosity, density, and DCN, are well matched.
2.3 Experimental procedure
Three combustion related time scales, ignition delay (ID), rapid combustion period (RCP), and burn duration (BD) were obtained from the measured pressure traces. The definitions of these combustion related time scales were plotted in Fig. 2. ID is defined as the time from the injector solenoid energizing to the ignition instant, i.e., the time when the pressure increases by 0.2 bar. RCP is defined as the interval between the ignition timing and the moment when the pressure reaches the average of the initial pressure and the maximum pressure. BD is defined as the duration from the ignition to the moment when the combustion pressure rises to the magnitude of 95% of the maximum pressure. The experiments were conducted at the chamber temperatures from 808 K to 923 K, and a chamber pressure of 20 bar. Fuel injection duration was maintained at 2.5 ms and the injection pressure was 1000 bar. Fifteen cycles were conducted for each test condition, and the exhibited pressure traces and combustion related time scales were the average values.
2.4 Heat release rate calculation
Heat release rate was computed from the pressure trace using a zero-dimension model. More details on heat release rate computation can be found in Refs. [
22–
24], and therefore only a brief description is provided here. In this heat release model, the energy conservation equation was applied to obtain a first-law differential equation based on time. With the assumption that the combustible mixture is ideal gas and the state at each constant is in equilibrium state, the total quantity of heat release could be calculated as
where the first term on the right hand side represents the internal energy of the gas mixture in combustion chamber, in which
nk is the mole number of gas mixture,
Cv,k is the averaged specific constant volume heat capacity of gas mixture, and
Tk is the instantaneous temperature of gas mixture in chamber; the second term on the right hand side represents the heat transfer between the gas and the chamber wall described using Newton’s cooling law, in which
A is the internal surface area,
Tw is the temperature of the chamber wall, and
h is the heat transfer coefficient estimated by the Wochini empirical correlation [
28]. Several other assumptions were made when calculating the instant internal energy. First, the fuel and air were well mixed and uniformly distributed. Next, all the fuel components were simultaneously burned. Finally, complete combustion was reached at each constant and only carbon dioxide and water were produced.
3 Results and discussion
Figure 3 demonstrates the combustion pressure traces in the auto-ignition processes of RP-3 jet fuel and the three surrogates at changed chamber temperatures, at an ambient pressure of 20 bar. The uncertainties of combustion pressure traces were also plotted in Fig. 3. All the pressure traces first exhibit a slow rise, followed by a sharp pressure increase. The pressure rise instants of Surrogate 1, either in the slow rise stage or the sharp rise stage, are always the earliest among the test fuels, because its DCN is much higher than those of the other fuels.
On the other hand, the relative phasing of Surrogate 2, Surrogate 3, and RP-3 jet fuel are dependent on ambient temperatures. For the slow rise phasing, a sequence of Surrogate 3<Surrogate 2<RP-3 jet fuel is observed at lower ambient temperatures, but at higher ambient temperatures, the three fuels display negligible differences in their slow pressure rise instants. The slow pressure rise stage is generally considered as a result of fuel low-temperature reactions, which is highly sensitive to the molecular structure and percentage of straight chain alkanes in fuel mixtures [
29,
30]. A comparison of the fuel compositions of RP-3 jet fuel, Surrogate 2, and Surrogate 3, indicates that both two surrogates have higher
n-alkane percentages than RP-3 jet fuel, and as such they exhibit earlier pressure rise instants. Further, in Surrogate 3 formulation, the
n-alkanes are represented by
n-dodecane, which has a longer carbon chain than
n-decane, the molecule selected to represent
n-alkanes in Surrogate 2. As longer carbon-chain hydrocarbons would exhibit a somewhat stronger low-temperature reactivity, it might be reasonable that an earlier pressure rise instant is observed for Surrogate 3 than Surrogate 2. As the ambient temperature increases to above 873 K, the dominant oxidation chemistry is shifted to the high-temperature regime, and as such the slow pressure rise stages, influenced by low-temperature chemistry, are reduced and are almost the same in all the test fuels.
As for the sharp pressure rise stages, the pressure rise instants of Surrogates 2 and 3 are close and slightly earlier than RP-3 jet fuel when the ambient temperature is below 873 K, as depicted in Figs. 3(a)–3(c), because of the earlier first-stage pressure rises of Surrogates 2 and 3 than RP-3 jet fuel. However, as the ambient temperature increases to above 873 K, at which the first-stage pressure rises are almost identical for the three fuels, the pressure rise phasing of Surrogate 3 gradually falls behind that of RP-3 jet fuel, and as such the sharp rise instants of the three test fuels show a sequence of Surrogate 2<RP-3< Surrogate 3. However, the differences in the sharp pressure rises of the three fuels are considered to be trivial, as this sharp rise occurs in such a short period of as less than 1 ms.
Figure 4 illustrates the heat release rate traces of RP-3 jet fuel and the three surrogates at changed chamber temperatures and a constant ambient pressure of 20 bar, in which the heat release phasing of all the test fuels are advanced with increased ambient temperature. Different heat release behaviors are also observed among the test fuels, which are summarized as follows. First, RP-3 jet fuel and Surrogates 2 and 3 exhibit clear two-stage auto-ignition behaviors at low ambient temperatures below 853 K, as in Figs. 4(a)–4(b), but the first-stage heat release gradually diminishes when the ambient temperature increases. On the contrary, Surrogate 1 always exhibits a single-stage auto-ignition behavior at all tested ambient temperatures. Second, Surrogates 1 always shows an earlier heat release than RP-3 jet fuel and Surrogates 2 and 3 at all ambient temperatures. Surrogates 2 and 3 have similar heat release traces at low temperatures, but as the temperature increases, some slight differences appear. That is, Surrogate 2 has a higher peak heat release rate than that of Surrogate 3, with its peak heat release phasing being earlier than Surrogate 3 as well. Third, Surrogates 1 shows far higher peak heat release rates than RP-3 jet fuel and Surrogates 2 and 3 at low ambient temperatures, but the peak heat release rates of RP-3 jet fuel and Surrogates 2 and 3 exceed that of Surrogates 1 when the temperature increases to above 898 K, as in presented in Figs. 4(e)–4(f). Finally, as the ambient temperature increases, the peak heat release rate of Surrogate 1 first increases and then declines, whereas those of RP-3 jet fuel and Surrogates 2 and 3 show a monotonic rising trend.
Figure 5 shows the IDs of RP-3 jet fuel and the three surrogates at an ambient pressure of 20 bar and changed chamber temperatures. According to Refs. [
31,
32], the relations of ID times of fuel spray auto-ignition with temperature could be expressed in an Arrhenius equation as
where
A is the pre-exponential factor,
Ru is the universal gas constant (J/(K∙mol)), and
Ea is the activation energy (J/mol). Therefore, the ID times versus temperature are plotted by fitting the Arrhenius correlation in Fig. 4, in which the ID time versus temperature exhibits a linear relation. The IDs of different fuels are in the order of RP-3 jet fuel>Surrogate 2>Surrogate 3>Surrogate 1. The longest ID time of RP-3 jet fuel is possibly related to its higher density and viscosity, as shown in Table 1, which increases fuel droplet size and extends breakup time, leading to a longer time for combustible mixture formation. This results in a longer physical dominant delay time, an important part of the total ignition delay on a CVCC [
31]. Further, the staged heat release behaviors and later first-stage heat release phasing of RP-3 jet fuel also indicate its weaker low-temperature reactions, which also contributes to its longer ID times. Secondly, Surrogate 1 exhibits the shortest ID times, probably because of its highest straight-chain alkane fraction, as the straight-chain alkanes possess a stronger low-temperature reactivity than branched alkanes and aromatics. The IDs of Surrogates 2 and 3 are between those of RP-3 jet fuel and Surrogate 1, and are quite close at higher temperatures. At lower temperatures, Surrogate 3 exhibits shorter IDs, because of its earlier first-stage pressure rises than those of Surrogate 2, as shown in Fig. 3.
Figure 6 shows the RCPs of RP-3 jet fuel and the three surrogates at a pressure of 20 bar and different chamber temperatures. The trend for the RCPs of RP-3 jet fuel and the surrogates are somewhat different from that of the IDs. The RCPs of different fuels follow an order of Surrogate 3>RP-3>Surrogate 2>Surrogate 1. Despite that the RCPs of the Surrogates 1 are still the lowest, the RCPs of RP-3 jet fuel become shorter than those of Surrogate 3, especially under high temperature conditions. RCP could be considered as an indicator of the combustion rate immediately after ignition occurs. The most intensive heat release processes of Surrogate 1 cause the shortest RCPs. Although Surrogate 3 has an earlier ignition timing than RP-3 jet fuel, yet, its pressure rise rate and heat release intensity after ignition are lower than those of RP-3 jet fuel at higher temperatures, leading to its longer RCPs. Surrogate 2 has closer RCPs to RP-3 than the other surrogates, especially at higher temperatures, as its heat release behaviors are the closest to those of RP-3 jet fuel. The longer RCPs of Surrogate 3 than Surrogate 2 and RP-3 jet fuel might be related to its lower volatility, which can be observed in Table 1. The lower volatility reduces fuel/air mixing rate, and thus decreases the premixed charge formation. As the heat release intensity immediately after ignition is mainly influenced by premixed burn, Surrogate 3 may have a lower premixed burn rate and slightly longer RCPs than Surrogate 2 and RP-3.
Figure 7 shows the BD times of RP-3 jet fuel and the three surrogates at different chamber temperatures. The trend for the BD times for different test fuels is generally consistent with that for the RCPs. However, the BD times of Surrogate 1 do not show a monotonic decreasing trend versus increased temperature, but shows an increasing trend when temperature exceeds 873 K. This non-monotonic trend may have been caused by the decreased ignition delay with increased temperature. Surrogate 1 has the shortest ignition delay, and the further shortened ignition delay leads to less fuel/air mixing time and as such diffusion combustion plays an increasingly important role. As the rate of diffusion combustion is lower than that of premixed combustion, the BD time is therefore extended. The non-monotonic trend in the maximum heat release rates of Surrogate 1 at different ambient temperatures, as shown in Fig. 3, also supports this explanation. In contrast, the maximum heat release rates of RP-3 jet fuel and Surrogates 2 and 3 always increase with increased temperature, coinciding with the monotonic decreasing trend of their BD times. Similar to the trend observed for RCPs, Surrogate 2 has closer BDs to RP-3 than Surrogate 3, especially at higher temperatures.
4 Conclusions
Spray auto-ignition characteristics of RP-3 jet fuel and its three surrogates (Surrogate 1: the 70% mol n-decane/30% mol 1,2,4-trimethylbenzene blend, Surrogate 2: the 51% mol n-decane/49% mol 1, 2, 4-trimethylbenzene blend, and Surrogate 3: the 49.8% mol n-dodecane/21.6% mol iso-cetane/28.6% mol toluene blend) were experimentally investigated on a CVCC. The two-component surrogates (Surrogate 1 and Surrogate 2) possess the same components, but their percentages are different, as the two surrogates were designed to capture the H/C ratio (Surrogate 1) and DCN (Surrogate 2) of RP-3 jet fuel, respectively. The three-component surrogate, Surrogate 3, was able to emulate more physical and chemical properties of RP-3 jet fuel, including molecular weight, H/C ratio, and DCN. The time-resolved pressure traces in the auto-ignition processes of different test fuels were captured at changed ambient temperatures and pressures, and the heat release rates were further derived and compared. Three important time scales describing fuel auto-ignition propensities were also compared and analyzed for RP-3 jet fuel and its three surrogates. Conclusions drawn from the experimental results are as follows:
To capture the spray auto-ignition behaviors of the target fuel, DCN is the most important parameter to match when designing the surrogate formulation. Although Surrogate 1 matches the H/C ratio of the target fuel, its spray auto-ignition behaviors deviate from that of the target fuel, due to their significantly different DCNs.
Surrogates that capture the DCN of the target fuel (Surrogate 2 and Surrogate 3) generally well reproduce the spray auto-ignition behaviors of the target fuel, e.g., the staged pressure rise and heat release at low temperatures. These two surrogates also produce closer ID, RCP, and BD times to RP-3 jet fuel.
However, Surrogate 2 and Surrogate 3 still show some differences in auto-ignition propensities from the target RP-3 jet fuel, at the temperatures deviating from the nominal DCN test condition. For example, at lower ambient temperatures, the two surrogates exhibit earlier and stronger first-stage heat releases than the target fuel. It is also noted that Surrogate 2 has a closer RCP and BD times to the target fuel than Surrogate 3, although the formulation of Surrogate 3 covers more hydrocarbon classes in the target fuel.
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