Single-Pulse Shock Tube Pyrolysis Study of RP-3 Jet Fuel and Kinetic Modeling.

A single-pulse shock tube study of the pyrolysis of two different concentrations of Chinese RP-3 jet fuel at 5 bar in the temperature range of 900-1800 K has been performed in this work. Major intermediates are obtained and quantified using gas chromatography analysis. A flame-ionization detector and a thermal conductivity detector are used for species identification and quantification. Ethylene is the most abundant product in the pyrolysis process. Other important intermediates such as methane, ethane, propyne, acetylene, butene, and benzene are also identified and quantified. Kinetic modeling is performed using several detailed, semidetailed, and lumped mechanisms. It is found that the predictions for the major species such as ethylene, propene, and methane are acceptable. However, current kinetic mechanisms still need refinement for some important species. Different kinetic mechanisms exhibit very different performance in the prediction of certain species during the pyrolysis process. The rate of production (ROP) is carried out to compare the differences among these mechanisms and to identify major reaction pathways to the formation and consumption of the important species, and the results indicate that further studies on the thermal decomposition of 1,3-butadiene are needed to optimize kinetic models. The experimental data are expected to contribute to a database for the validation of mechanisms under pyrolytic conditions for RP-3 jet fuel and should also be valuable to a better understanding of the combustion behavior of RP-3 jet fuel.

Introduction

Jet fuel is widely used in current aeroengines for propulsion. To achieve high combustion efficiency and low pollution emission, a better understanding of the combustion chemistry of jet fuels is of significant importance toward the development of advanced combustors in aeroengines.[1] Thus, many researchers focused on studying the combustion properties of jet fuels and developing combustion chemical kinetic mechanisms. In particular, the development of a high fidelity chemical kinetic mechanism requires a wide range of kinetic targets from experiments for mechanism validation, such as ignition delay time, laminar flame speed, and species profiles.[2] Hence, many relevant studies have been performed by researchers to measure the basic combustion properties of jet fuels during the past decades.[3−9] Although jet fuels mainly consist of hydrocarbon compounds, the real components of jet fuels used in different countries are still different depending upon their origin oil resource, exploitation, and refining processes. The different compositions in jet fuels certainty affect the combustion properties. During the past decades, the combustion properties for a wide range of jet fuels, including widely used Jet A, JP-10, JP-8, Jet-A POSF 4658, and alternative kerosene, have been experimentally studied.[3−15] The ignition, extinction, flame speed, and species profiles were measured, covering a wide range of combustion conditions. Detailed chemical kinetic mechanisms for these fuels were also developed and validated by the traditional fuel surrogate approach, which employs several representative hydrocarbon compounds and the corresponding detailed kinetic mechanisms to mimic the physical and combustion properties of real fuels.[16,17] Another different approach entitled as the “HyChem” concept has been proposed recently to model the combustion of practical fuels.[18−22] The essential idea is to decouple the overall oxidation processes into a lumped fuel pyrolysis scheme and a detailed kinetic mechanism describing the oxidation of small pyrolysis fragments. Following this concept, a better understanding of the fuel pyrolysis process is a prerequisite step to develop kinetic mechanisms.

Besides the need to develop combustion kinetic mechanisms, the pyrolysis of jet fuels has also been the subject of interest because it now plays an important role in the development of regenerative cooling systems in advanced hypersonic aircrafts, which employs the endothermic pyrolysis process of jet fuels before combustion to relieve the great heat load.[23−27] Hence, the pyrolysis process of typical jet fuels was studied using a wide range of facilities.[28−31] Traditional experimental approaches to study fuel pyrolysis include the jet-stirred reactor and the flow reactor.[32] However, such reactors were mainly operated under low-temperature conditions. The shock tube facility can reach high-temperature conditions to study fuel pyrolysis, and the pyrolysis process of several jet fuels was mainly studied using the Stanford shock tube apparatuses.[18,20,21,29] However, it requires expensive laser to measure the species profiles. In contrast, the single-pulse shock tube (SPST) is a well-established reactor for post-shock sampling and analysis.[33] The designs of the SPST have been implemented by several groups and recently adopted to study the fuel pyrolysis processes.[30,34] An SPST usually has a quenching rate of ∼6 × 106 K/s after the residence time; thus, it can be assumed that the reaction is frozen because of rapid quenching.[33,35] Hence, it becomes a useful approach to study the high-temperature fuel pyrolysis process.

Compared with Jet-A, JP-10, JP-8, etc., the RP-3 kerosene as one of the most important civil aviation fuels in China is a different type of jet fuel with a different composition and thus different combustion characteristics.[11,14,15,36] Although RP-3 jet fuel is mainly produced and used in China, the research on its combustion characteristics is still important since it is also widely used for international airlines and aeroengines around the world. However, it has received little attention. Up to now, although a series of detailed and reduced kinetic mechanisms of RP-3 were proposed,[11,15,36−42] the validation kinetic targets have been mainly limited to several global combustion properties such ignition and laminar flame speed. Table summarizes representative experimental studies on RP-3 jet fuel, and it can be found that the pyrolysis of RP-3 has received little attention. Thus, it is of great importance to understand the fuel pyrolysis of RP-3, especially under high-temperature conditions.

Table 1

Previous Experimental Studies of Pyrolysis and Combustion of RP-3 Jet Fuel

year	type of experiment	conditions	type of data	reference
2012	shock tube	T = 820–1500 K; p = 5.5, 11, 22 atm; φ = 0.5, 1.0, 1.5	ignition	Liang et al.[43]
2015	shock tube	T = 650–1500 K; p = 1–20 atm; φ = 0.2, 1.0, 2.0	ignition	Zhang et al.[11]
2015	shock tube	T = 1100–1600 K; p = 1, 2, 3 atm; φ = 0.5, 1.0, 1.5	ignition	Zeng et al.[38]
2015	counterflow flame	T0 = 403 K; p = 1 atm; φ = 0.7–1.4	laminar flame speed	Zheng et al.[39]
2016	constant volume combustion chamber	T0 = 390, 420, 450 K; p = 1, 3, 5, 7 atm; φ = 0.6–1.6	laminar flame speed	Ma et al.[40]
2017	shock tube	T = 1670–2200 K; p = 2, 5 atm; φ = 5, 10, 20	soot	He et al.[44]
2019	shock tube	T = 1113–1600 K; p = 0.1–1 atm; φ = 0.5–1.5	ignition	Chen et al.[14]
2019	shock tube rapid compression machine	T = 624–1437 K; p = 10, 15, 20 atm; φ = 0.5–1.5	ignition	Mao et al.[15]
2021	shock tube	T = 920–1590 K; p = 2, 10 atm; φ = 0.5, 1.0, 2.0	ignition	Yang et al.[45]

Based on the above considerations, this work intends to provide complementary data of RP-3 jet fuel and to aid in the development and validation of chemical mechanisms describing the pyrolysis and combustion of RP-3 at high temperatures using SPST experiments and kinetic modeling.

Experimental Methods

In this work, the pyrolysis experiment of RP-3 jet fuel is performed in a SPST at North University of China. Mixtures of fuel/bath gas (argon, Ar)/internal standard (krypton, Kr) are prepared in stainless steel mixture tanks according to Dalton’s law of partial pressure, and the prepared mixture is maintained for at least 12 h before experiments to ensure complete vaporization and homogeneity. The mixing tank, shock tube, and sampling tube are heated and maintained at a temperature of 398 K throughout the experiments to avoid the adsorption of RP-3. The heating system is controlled by seven thermocouples placed along the mixing tank, shock tube, and sampling tube. Each thermocouple has an independent electrical circuit to provide a uniform temperature of the setup. The purities of Ar and Kr used in the experiment are 99.99%. Table lists the detailed experimental conditions, and we have studied the pyrolysis of 0.02% and 0.05% fuel at 5 bar in an SPST. The jet fuel is provided by the Aviation Fuel and Chemical Airworthiness Certification Centre of CAAC in China, and the purity is larger than 99.9%. Helium is used as the driver gas in the SPST, and the purity is also 99.99%. Since the pyrolysis process of jet fuel is endothermic, all experiments are performed under high-diluted conditions to make the temperature stable during the experimental process.

Table 2

Pyrolysis Experimental Conditions in This Work

fuel	xFuel (mol %)	XAr (mol %)	Xkr (mol %)	Avg. P5 (bar)	T5 range (K)
RP-3	0.02	99.48	0.5	5.02	1025–1677
	0.05	99.45	0.5	5.01	981–1719

The employed SPST is composed of a 1.5 m driver section and a 3.05 m driven section with an inner diameter of 44 mm. The driver and driven sections of the tube are separated using a polycarbonate diaphragm. A very important component of the SPST named dump tank is connected near to the diaphragm section of the driven tube by a manual ball valve with a diameter of 44 mm. It is a pressure vessel that can consume the reflected shock waves and ensure that the reaction mixture is only under a single heated condition. This vessel is connected to an Inficon PSG500 Pirani gauge and an Inficon CDG100 (1000 Torr) capacitance gauge and has connections for evacuation and argon. The experimental procedure and product analysis method are almost the same as those described by Nagaraja et al.[33,46] and Panigrahy et al.[47] and will only be briefly described here. Before each experiment, the ball valve of the dump tank is closed. The driven section and dump tank is evacuated to 5 × 10–2 Pa using a rotary pump and a turbo molecular pump, and the pressure is monitored using an MKS 901P Pirani gauge. Once the ultimate vacuum is achieved, the driven section is charged with the fuel mixture to the desired pressure, which is monitored using the Inficon CDG100 (10 and 1000 Torr) capacitance manometer. This pressure is obtained from the Gaseq program[48] based on the shock jump equations. The dump tank is filled with Ar to the same pressure as the driven section. Thereafter, the evacuated driver section is filled with He to the required pressure, using a digital pressure gauge. Before the polycarbonate diaphragm ruptured, the dump tank valve is opened manually and then the diaphragm is burst with a manual acting four-pronged blade. Consequently, there is a lag between the opening of the ball valve of the dump tank and bursting of the diaphragm. To evaluate the effect of this on the test gas compositions, a comparison between the present facility and the shock tube at the National University of Ireland, Galway (NUIG), that used a solenoid valve connecting the dump tank and the driven section is performed. The time between the opening of the solenoid valve and the bursting of the diaphragm is 800 ms for NUIG facility. However, it is found that the two facilities provide the same results with the same experimental conditions for propene and propane pyrolysis.[33,47] Thus, the effect of the lag between the opening of the ball valve of the dump tank and the bursting of the diaphragm can be neglected. The incident shock velocity is measured using four PCB 113B21 (pressure rise time less than 1.0 μs) piezoelectric pressure transducers mounted on the sidewall of the driven section at known locations and one Kistler 603CBA piezoelectric pressure transducer mounted on the endwall of the driven section. The Kistler is also used to record the pressure–time profiles (P5). All pressures are recorded using two digital TiePie Handyscope HS4 oscilloscopes. The shock velocity is calculated by linearly extrapolating the four known velocities to the endwall. The reflected shock wave pressure and temperature are determined using the one-dimensional normal shock relations employed by the program Gaseq[48] from the initial temperature, initial pressure, measured incident shock velocity, and thermodynamic properties of the fuel/Ar/Kr mixtures. Pyrolysis time is defined as the time interval between the arrival of the reflected shock wave and the 80% of the pressure signal recorded by the Kistler pressure sensor. The calculated pressures are verified using the endwall Kistler pressure sensor.

The shock-heated products are sampled from the endwall using a solenoid valve through a 3 mm inner diameter tube that protrudes 10 mm into the ST and are analyzed using an Agilent 7820A GC. The dead volume is minimal (71 mm3) because of the small diameter of the sampling tube; therefore, it is assumed that the effect of the unreacted mixture is negligible. An FID and a thermal conductivity detector are used for reaction products. Kr is used as an internal standard gas, and the system was calibrated using a 16 gas GC standard obtained from Beijing Haipubeifen Gas Industry, Ltd., China. The calibrated standard provides the sensitivity of the detector for each species and is used to calculate the concentration of the pyrolytic products. For species with no calibration standard, the effective carbon number method is used to estimate the concentrations.

The uncertainty of the experimental temperature is estimated using a standard error analysis procedure based on the uncertainty in the shock attenuation and nonideal shock reflection from the interactions between the shock wave and boundary layer. The uncertainties in reflected temperatures are calculated based on the uncertainties in shock velocities of the shock tube and are approximately ±2% based on calculations by Petersen et al.[49] The uncertainty in calibrated species concentrations calculated using the repetitive sampling of the standard gas is approximately ±10%, and the estimated species concentration, calculated using the effective carbon number method, is approximately ±20%. The uncertainty in reactant mole fractions is ±0.02%. For the uncertainty in the residence time, we performed five experiments with different temperatures during the SPST debugging period and repeated three times for each experiment to measure the average residence time. The results show that the uncertainty is less than 5% for all the five experiments. Thus, the uncertainty in the residence time is ±5%. In addition, because the actual composition of RP-3 jet fuel is unknown, it is hard to calculate the carbon balance. Furthermore, the formation of soot and large polycyclic aromatic compounds under high-temperature conditions, which are beyond the detectability in this work, also significantly affects the carbon balance. However, based on our previous work on the pyrolysis studies of fuel molecules with accurate chemical compositions,[33,47] the uncertainty can be controlled within the range of 85–115% for all experimental conditions. Generally, the carbon balance mainly relies on the absorption in the mixture tank, shock tube, and sampling tube and the GC analysis method. The previous analysis on other fuels indicates that the experimental system is accurate to determine the carbon balance. Thus, the uncertainty of the carbon balance can be controlled within 15%. It is worth noting that although some of the large hydrocarbon compounds and soot-relevant compounds are not detected and measured in this work, the measured product distributions of small C1–C4 molecules are critical in the development of detailed mechanisms for jet fuels, since they are the foundations affecting the global high-temperature combustion properties of jet fuels.[19,21] The uncertainty of the properties from experimental procedures is generally consistent with that of the other related facilities.[33,46,47]

Kinetic Modeling

During the past decades, several detailed chemical kinetic mechanisms have been developed for various jet fuels. Almost all of them are based on the surrogate fuel approach. Another different approach, as demonstrated previously, was the HyChem method. The two kinds of methods were employed to develop the detailed combustion mechanisms for traditional jet fuels, including Jet A, JP-8, JP-10, and also alternative fuels. Detailed kinetic mechanisms were also developed for RP-3 fuels using the surrogate fuel approach. Based on the compositions of RP-3 fuel, the three-component surrogate model composed of n-dodecane, 1,3,5-trimethylcyclohexane, and n-propylbenzene is now widely used.[9,32,37,45] Detailed kinetic mechanisms for the surrogate model were developed and validated against experimental results and are employed in this work for kinetic modeling studies. In addition, the CRECK mechanism developed by Pelucchi et al.[50] was previously modified to model the ignition of RP-3[15] and is also employed to check its performance for SPST pyrolysis results. Because of the similar physical properties between RP-3 and Jet A, the HyChem model for Jet A is also employed for kinetic modeling. Table lists the adopted combustion kinetic mechanisms for kinetic modeling.

Table 3

RP-3 Surrogate Models and Their Associated Mechanisms Used in This Work

model	surrogate fuel (by mol)	kinetic mechanism
Tian’s mechanism[9,32]	n-dodecane 66%, 1,3,5-trimethylcyclohexane 18%, n-propylbenzene 16%	462 species, 3170 reactions
Xu’s mechanism[37]		2237 species, 7959 reactions
Yang’s mechanism[45]		793 species, 4358 reactions
modified CRECK mechanism[15]	n-dodecane 49.76%, iso-cetane 21.63%, toluene 28.61%	223 species, 5689 reactions
HyChem (A2 Jet A) model	C11H22	119 species, 841 reactions

Kinetic modeling is performed using the Cantera software,[51] assuming a closed homogeneous batch reactor at constant volume. The residence/reaction time approach is adopted to simulate the SPST results since this approach is simple and the predicted results show no significant differences compared with the method based on the actual recorded pressure profiles, as discussed by Han et al.[35]

Results and Discussion

SPST Experimental and Kinetic Modeling Results

Figures and 2 show the major species profiles as a function of temperature for 0.02% and 0.05% fuel pyrolysis experiment at 5 bar and kinetic modeling results using different combustion reaction mechanisms. From the formation of products, it can be seen that the pyrolysis of RP-3 under the studied conditions begins above 1100 K. For both the pyrolysis conditions, ethylene (C2H4) is the most abundant product. The mole fraction of C2H4 tends to be stable as a function of temperature around the temperature range of 1200–1500 K. The concentration of C2H4 decreases when the temperature is higher than 1500 K, while the concentration of acetylene (C2H2) increases significantly as the temperature rises, probably indicating that a large number of C2H4 may convert to C2H2. Methane (CH4) and propene (C3H6) are other two major pyrolysis products. However, the yield tendencies of CH4 and C3H6 as a function of temperature are different. The concentration of CH4 gradually increases as a function of temperature and reaches the highest values around 1500 K. Then, the concentration of CH4 slightly decreases as the temperature increases. The concentration of C3H6 increases sharply and reaches the highest value around 1300 K and then it also decreases sharply as the temperature increases. From Figures and 2, it can be seen that the species profiles along the temperature for the mole fractions of C2H4, CH4, C3H6, and C2H2 can be reasonably captured by the selected five combustion reaction mechanisms, except that Xu’s mechanism exhibit large deviation in predicting the concentration of CH4. Qualitatively, Tian’s mechanism exhibits an overall better performance compared with the other four mechanisms. The modified CRECK mechanism based on the lumped approach and Yang’s mechanism also well predict the species profiles. It is interesting to find that the HyChem mode derived from experiments for Jet A also shows reasonable prediction results for major species, including C2H4 and CH4, and only slightly overestimates the concentration of C3H6.

Figure 1

Species profiles as a function of temperature for 0.02% fuel pyrolysis experiment at 5 bar and kinetic modeling results.

Figure 2

Species profiles as a function of temperature for 0.05% fuel pyrolysis experiment at 5 bar and kinetic modeling results.

Besides C2H4, CH4, C3H6, and C2H2, ethane (C2H6) is also detected in large quantities. It is shown that the mole fraction profile of C2H6 along the temperature is very similar to that of C2H4. However, it can be seen that only the two detailed mechanisms, namely, Tian’s and Yang’s mechanisms, can well predict the measured species profiles of C2H6. The lumped high-temperature submechanism for Jet A in the HyChem model did not consider the formation of C2H6, and the formation of C2H6 was mainly controlled by the transformation from the other products, which significantly overestimates the formation of C2H6.

The other C3 species including propane, allene (aC3H4), and propyne (pC3H4) are also detected. However, the quantities of propane are small. All the five mechanisms predict very little propane. The species profiles of aC3H4 and pC3H4 are similar. It is shown that Yang’s mechanism and the HyChem model exhibit good performance in predicting the concentrations of the two species. For C4 species, 1-butene, 2-butene, 1,3-butadiene, and vinyl acetylene are detected. However, the quantities of 1-butene and 1-butene are very small and quickly consumed under the studied conditions. Hence, only under specific conditions, 1-butene and 1-butene are detected with small quantities. It is worth noting that the modified CRECK mechanism exhibits the best performance in predicting the concentrations of 1,3-butadiene. As the precursor toward the formation of soot, benzene is detected in this work; however, the quantities are very small.

ROP Analysis

Based on Figures and 2, although the selected different mechanisms show reasonable prediction results for major pyrolysis products, none of the mechanisms can give satisfactory results against all the SPST experimental measurements. Further optimization of the detailed mechanisms or the development of new surrogate model is still needed for RP-3 jet fuel.[52,53] To facilitate the optimization of detailed combustion mechanisms and explain the reaction paths for the formation of major products, we use ROP analysis via the selected combustion mechanisms to compare their differences and gain useful information for future research. Table lists the ROP analysis results for the dominant reactions corresponding to the formation and consumption of the major species using selected combustion mechanisms at 1400 K and 5 bar with a reaction time at 1.68 ms.

Table 4

Dominant Reactions Related to the Consumption and Production of Major Species from ROP Analysis

species	HyChem model		CRECK mechanism		Tian’s mechanism		Yang’s mechanism
C2H4	C2H4 + H(+M) = C2H5(+M)	72%	C2H2 + C2H4 = C4H6	20%	C2H4 + H(+M) = C2H5(+M)	83%	C2H4 + H(+M) = C2H5(+M)	56%
			C2H4 + H(+M) = C2H5(+M)	24%
	C3H6 + H = C2H4 + CH3	20%	nC3H7 = C2H4 + CH3	18%	C3H6 + H = C2H4 + CH3	12%	C3H6 + H = C2H4 + CH3	33%
			C2H6 = H2 + C2H4	24%
	C2H4 + H = C2H3 + H2	43%	C2H4 + H ⇒ H2 + C2H3	62%	C2H4 + H = C2H3 + H2	67%	C2H4 + M = H2 + H2CC + M	77%
	C2H4 + CH3 = C2H3 + CH4	50%	C2H4 + CH3 ⇒ CH4 + C2H3	22%	C2H4 + CH3 = C2H3 + CH4	28%	C2H4 + H = C2H3 + H2	11%
C3H6	C3H6 + H = aC3H5 + H2	27%	H + CH2CHCH2(+M) = C3H6(+M)	52%	C3H6 + H = CH3CCH2 + H2	64%	C3H6 + H = aC3H5 + H2	32%
	aC3H5 + H(+M) = C3H6(+M)	19%	C3H6 + H = H2 + CH2CHCH2	13%	C3H6 + H = aC3H5 + H2	13%	C3H6 + H = C2H4 + CH3	25%
	C3H6 + H = C2H4 + CH3	16%	CH3 + C2H3(+M) = C3H6(+M)	11%
	aC3H5 + C3H6 ⇒ C5H6 + H2 + CH3	12%	nC3H7 = C3H6 + H	12%	C3H6 + H = C2H4 + CH3	9%	aC3H5 + H(+M) = C3H6(+M)	16%
CH4	CH4 + H = CH3 + H2	21%	C7H8 + CH3 = CH4 + C7H7	46%	CH4 + H = CH3 + H2	31%	CH4 + H = CH3 + H2	37%
	C2H4 + CH3 = C2H3 + CH4	17%	CH4 + H = H2 + CH3	17%	C2H6 + CH3 = C2H5 + CH4	31%	C2H4 + CH3 = C2H3 + CH4	18%
	C5H6 + CH3 = C5H5 + CH4	16%	aC3H4 + CH3 = CH4 + C3H3	11%	C2H4 + CH3 = C2H3 + CH4	10%	pC3H4 + CH3 = C3H3 + CH4	12%
	C3H6 + CH3 = aC3H5 + CH4	11%
C2H2	C2H2 + CH3 = pC3H4 + H	36%	C2H2 + H(+M) = C2H3(+M)	33%	C2H2 + CH3 = pC3H4 + H	63%	C2H2(+M) = H2CC(+M)	52%
	C2H3(+M) = C2H2 + H(+M)	35%	C2H2 + C2H4 = C4H6	20%			C2H2 + CH3 = pC3H4 + H	25%
	C3H3 + C2H2 = C5H5	28%	C3H3 + CH2CHCH2 ⇒ C2H2 + C4H6	14%	C2H2 + H(+M) = C2H3(+M)	31%	C2H3(+M) = C2H2 + H(+M)	19%
C2H6	C2H6 + H = C2H5 + H2	78%	C2H6 = H2 + C2H4	76%	C2H6 + H = C2H5 + H2	25%	C2H6 + H = C2H5 + H2	88%
	C2H6 + CH3 = C2H5 + CH4	15%	C2H6 + H = H2 + C2H5	15%	C2H6 + CH3 = C2H5 + CH4	67%	C2H6 + CH3 = C2H5 + CH4	12%
aC3H4	aC3H4 + H = aC3H5	84%	aC3H4 + H(+M) = CH2CHCH2(+M)	39%	aC3H4 + H = aC3H5	55%	aC3H4 + H = aC3H5	89%
	aC3H4 + CH3 = iC4H7	11%	iC4H7 = aC3H4 + CH3	35%	aC3H4 + H = CH3CCH2	33%
			H + CH2CHCH2 = H2 + aC3H4	18%
	pC3H4 = aC3H4	46%	aC3H4 + H = H2 + C3H3	38%	pC3H4 = aC3H4	35%	pC3H4 = aC3H4	38%
			H + C3H3(+M) = aC3H4(+M)	10%	aC3H4 + H = C3H3 + H2	16%
	pC3H4 + H = aC3H4 + H	43%	aC3H4 = pC3H4	30%	aC3H4 + CH3 = C3H3 + CH4	19%	pC3H4 + H = aC3H4 + H	47%
					pC3H4 + H = aC3H4 + H	13%
pC3H4	pC3H4 = aC3H4	44%	aC3H4 = pC3H4	85%	pC3H4 + H = CH3CCH2	61%	pC3H4 = aC3H4	34%
	pC3H4 + H = aC3H4 + H	41%			pC3H4 = aC3H4	25%	pC3H4 + H = aC3H4 + H	43%
							pC3H4 + H = CH3CCH2	15%
	C2H2 + CH3 = pC3H4 + H	80%	pC3H4 + H = C2H2 + CH3	57%	pC3H4 + H = C3H3 + H2	13%	C2H2 + CH3 = pC3H4 + H	83%
	pC3H4 + CH3 = C3H3 + CH4	17%	pC3H4 + H = H2 + C3H3	21%	pC3H4 + H = aC3H4 + H	10%	pC3H4 + CH3 = C3H3 + CH4	14%
					C2H2 + CH3 = pC3H4 + H	68%
C4H6	C4H612 = C4H6	55%	C3H3 + CH2CHCH2 ⇒ C2H2 + C4H6	67%	sC4H7(+M) = C4H6 + H(+M)	34%	sC4H7(+M) = C4H6 + H(+M)	58%
	C4H6-2 = C4H6	24%	sC4H7 = C4H6 + H	12%	CH3 + C5H5 ⇒ C4H6 + C2H2	18%
	C4H7 = C4H6 + H	13%	C4H8-1 = H2 + C4H6	11%	sC5H9 = CH3 + C4H6	17%	C4H612 = C4H6	19%
					sC4H7 = C4H6 + H	14%
	H2CC + C2H4 = C4H6	22%	C2H2 + C2H4 = C4H6	76%	C4H6 + H = nC4H5 + H2	13%	H2CC + C2H4 = C4H6	15%
					C2H4 + C2H3 = C4H6 + H	16%
	C4H6 + H = C2H4 + C2H3	44%	C4H6 + H ⇒ H2 + C4H5	10%	C4H6 + CH3 = nC4H5 + CH4	18%	C4H6 + H = iC4H5 + H2	10%
	C4H6 + CH3 = iC4H5 + CH4	10%			C4H6 + CH3 = iC4H5 + CH4	27%	C4H6 + H = C2H4 + C2H3	53%
C4H4	C4H4 + H = nC4H5	14%	C4H5 = C4H4 + H	31%	iC4H5 = C4H4 + H	50%	C4H4 + H = nC4H5	30%
	C4H4 + H = iC4H5	74%	C3H3 + iC4H7 ⇒ C3H6 + C4H4	55%	nC4H5 = C4H4 + H	34%	C4H4 + H = iC4H5	62%
	H2CC + C2H2(+M) = C4H4(+M)	33%	2C2H2 = C4H4	32%	H2CC + C2H2(+M) = C4H4 (+M)	35%	C2H3 + C2H2 = C4H4 + H	38%
			C4H4 = H2 + C4H2	13%
	C2H3 +C2H2 = C4H4 + H	34%	C4H4 + C3H3 ⇒ C7H7	17%	C4H4 + H = nC4H3 + H2	19%
	C4H4 + H = nC4H3 + H2	13%	C4H4 + H ⇒ H2 + C4H3	19%	C4H4 + H = iC4H3 + H2	28%	C4H4 + H = nC4H3 + H2	21%
	C4H4 + H = iC4H3 + H2	19%	C2H2 + C2H3 = C4H4 + H	11%	2C2H2 = C4H4	13%	C4H4 + H = iC4H3 + H2	31%

For the formation and consumption of C2H4, it can be seen that dominant reactions in the HyChem model and Tian’s and Yang’s mechanisms are very similar, while the controlling reactions in the modified CRECK mechanism are different, resulting in the different prediction results for C2H4, as shown in Figures and 2. The reaction C2H2 + C2H4 = C4H6 contributing 20% to the formation of C2H4 in the modified CRECK mechanism also shows a significant effect in the consumption of C4H6, as shown in Table . However, this decomposition reaction for C4H6 only exists in the CRECK and Tian’s mechanisms with different rate constants. For the prediction results and ROP analysis of C4H6, it can also be concluded that the reaction C2H2 + C2H4 = C4H6 should be the chief reason for the different modeling results of C2H4 and C4H6 from the CRECK mechanism compared with the other mechanisms. The different reaction channels for the thermal decompositions of C4H6 existing in different mechanisms require further experimental or high-level theoretical calculations.

ROP analysis under the studied conditions indicates that propene (C3H6) is mainly consumed through decomposition and abstraction reactions. The CRECK mechanism reveals that the decomposition reaction of C3H6 to the formation of H and CH2CHCH2 is dominant, while the other three mechanisms indicate that the reactions with H are important besides the decomposition to aC3H5 + H. A similar reactivity trend is also found for C2H6, as shown in Table . The decomposition reaction of C2H6 to C2H4 + H2 is dominant in the CRECK mechanism, while the abstraction reactions by the H/CH3 radical is dominant in the other three mechanisms.

The dominant reactions to the formation of C2H2 from different mechanisms also exhibit large differences. Besides the decomposition reaction C2H3(+M) = C2H2 + H(+M) existing in all mechanisms, the formation of C2H2 from pC3H4 via C2H2 + CH3 = pC3H4 + H contributes only 6% in the CRECK mechanism, which is much lower than that from the other three mechanisms, especially Tian’s mechanism. The reaction C2H2(+M) = H2CC(+M) contributes about half percent to the formation of C2H2 in Yang’s mechanism probably because of the large amount formation of H2CC from the consumption reaction of C2H4 + M = H2 + H2CC + M, as shown in Table . The dominant consumption reaction of C4H6 decomposition also contributes a lot to the formation of C2H2 besides C2H4 in the CRECK mechanism.

The formation of CH4 is mainly through the abstraction reactions by the CH3 radical on H2, C2H4, C2H6, C3H6, aC3H4, pC3H4, and the other intermediates, which are different by using different mechanisms. A major difference in the CRECK mechanism is that the abstraction reaction from toluene (C7H8) by the CH3 radical contributes greatly to the formation of CH4, probably because of the large amount of toluene, which is difficult for pyrolysis in the surrogate model, indicating that the development of the surrogate model also significantly affects the prediction results in modeling the real jet fuel pyrolysis process.

For the two isomers of C3H4, namely, allene (aC3H4) and propyne (pC3H4), it is shown that the aC3H4 is formed mainly through the aC3H5, CH3CCH2, and CH2CHCH2 radicals, resulting from the consumption of C3H6. Thus, the dominant reactions to the formation of aC3H4 are also directly related to the dominant reactions controlling the consumptions of C3H6, for example, the large amount formation of CH2CHCH2 from H + CH2CHCH2(+M) = C3H6(+M) in the consumption of C3H6, resulting in the large contribution of reaction aC3H4 + H(+M) = CH2CHCH2(+M) in the formation of aC3H4 from the CRECK mechanism. The formation of pC3H4 is mainly through the isomerization reaction of aC3H4 together with its reaction with H except for Tian’s mechanism because of the large formation of CH3CCH2 from C3H6, which can be easily converted to pC3H4.

The consumption rate of C4H6 is slightly larger than the formation rate. All the mechanisms indicate that the consumption reactions are mainly decomposition and abstraction reactions but with different products and contributions. As previously demonstrated, the major difference is the different reaction channels and related products in the direct decomposition of C4H6, which affects the prediction results, especially for C2H4 and C2H2. The dominant reactions to the formation of C4H6 from the four mechanisms are also different. However, most of them are related to the transformation of C3 and C4 species. The formation of C4H4 shows a close relation with the consumption reaction products from C4H6. The decomposition reactions of nC4H5 and iC4H5 radicals control the formation of C4H6 in the HyChem model, Tian’s, and Yang’s mechanisms, which is different from that in the CRECK mechanism. In addition, the major products from the decomposition of C4H4 are also different from these mechanisms. Overall, although the previously developed detailed/lumped mechanisms employed in this work to model the pyrolysis of RP-3 jet fuels exhibit good performance for the major detected species, ROP analysis indicates that further improvements in the developed mechanisms and the surrogate model are still needed. The current experimental data provide additional targets to further develop a comprehensive detailed mechanism for RP-3 jet fuel.

Conclusions

The pyrolysis of jet fuel is not only crucial in the development of combustion kinetic models but also plays an important role in the development of regenerative cooling systems in advanced hypersonic aircrafts. For this purpose, this work reports the first SPST experimental study on the pyrolysis of RP-3 jet fuel at 5 bar in highly diluted fuel/Ar mixtures. The experimental results are compared with the predictions of a series of kinetic models developed recently. SPST experimental results indicate that ethylene is the most abundant product. Methane, ethane, and propene also largely exist. Acetylene increases significantly as the temperature rises. Other C3/C4 species including allene, propyne, and 1,3-butadiene are also detected as important intermediates. Propane and benzene are found to be in very small quantities. Several kinetic models are employed to predict the experimental results, and it is shown that none of these models can well capture all the measured species profiles as a function of temperature. Detailed ROP analysis indicates that the dominant reactions controlling the formation of the measured species are related to C0–C4 species. The reaction channels of thermal decompositions of 1,3-butadiene are different among the selected kinetic models, and experimental or high-level theoretical studied on the decomposition of 1,3-butadiene requires urgent treatment. ROP analysis also reveals that the selection of components in the development of the surrogate model for RP-3 jet fuel is also important in the prediction of some species. The present work not only contributes a database for the validation of mechanisms for RP-3 jet fuels but also provides important suggestions in the development of detailed kinetic models for jet fuels.