Effect of Pressure on Pyrolytic and Oxidative Coking of JP-10 in Near-Isothermal Flowing Reactor

Abstract

JP-10 (exo-tetrahydrodicyclopentadiene) is a high-energy-density hydrocarbon broadly used in advanced aerospace propulsion as a regenerative cooling fluid; in this study, we aimed to clarify how fuel pressure affects its thermal degradation (oxidative and pyrolytic) in near-isothermal flowing reactor. Experiments were performed under oxidative conditions (wall temperature 623.15 K, p = 0.708–6.816 MPa) and pyrolytic conditions (wall temperature 793.15 K, p = 2.706–7.165 MPa); carbon deposits were quantified by LECO analysis, oxidation activity was assessed by temperature-programmed oxidation (TPO), and morphology was performed by FESEM and EDS. Results show that oxidative coking is minimal (5.37–14.95 μg·cm²) and largely insensitive to pressure in the liquid phase (1.882–6.816 MPa), whereas at 0.708 MPa (gas/phase-change conditions), deposition increases, implicating phase and local heat-transfer effects. Under oxidative conditions, deposits are predominantly amorphous carbon with a disordered structure, formed at relatively low temperatures, with only a few fiber-like metal sulfides identified by EDS. In contrast, under pyrolysis conditions, the deposits are predominantly carbon nanotubes, exhibiting well-defined tubular morphology formed at elevated temperatures via metal-catalyzed growth. The pyrolysis coking yield is substantially higher (66.88–221.89 μg·cm⁻²) and increases with pressure. The findings imply that the pressure influences the coking of JP-10 via phase state under oxidative conditions and residence time under pyrolytic conditions, while basic morphologies of coke deposits remain similar; operationally, maintaining the working pressure higher than the saturated vapor pressure can mitigate oxidation coking associated with phase transitions, and minimizing residence time can mitigate pyrolytic coking.

1. Introduction

When aircraft is cruising at speeds above Mach number 5, the aerodynamic heating of the nose cone, leading edges, and internal engine surfaces can raise wall temperatures to 2000–3000 °C [1]. Existing materials such as high-temperature alloys are far from adequate for such extreme environments. Consequently, active or passive thermal protection systems are indispensable. Among these, the regenerative cooling schemes using fuel as a coolant have become a highly promising thermal protection strategy for the thermal management of hypersonic aircraft [2,3]. Endothermic hydrocarbon fuel can absorb the heat from high-temperature walls through both physical sensible heat and chemical heat sink and then burn and release the absorbed heat in the combustion chamber, which improves the overall efficiency of the engine.

JP-10 (exo-Tetrahydrodicyclopentadiene) is a highly promising high-density hydrocarbon fuel. With its high-volume calorific value and low freezing point, it has become the preferred coolant for regenerative cooling systems in new generation aircraft [4]. However, the thermal degradation of the fuel produces a large amount of coke deposits at high temperature, which not only drastically reduces the heat transfer efficiency, but also may block the micro-cooling channels, posing a critical threat to flight safety.

The thermal degradation of fuel is a complex process [5], which can be divided into three stages: autoxidation, intermediate stage, and pyrolysis [6]. It is influenced by factors such as fuel properties [7], temperature [8,9], dissolved oxygen [10], metal wall [11,12], residence time [13], and fuel pressure. However, the impact of pressure on the coking of fuels has yielded inconsistent results in previous studies. Key findings are summarized below:

①. Vranos and Marteney [14]: Jet A and No. 2 heating oil, pressures 0.68~2.04 MPa, outlet temperature 533 K, a preheat temperature 422 K; the coking rate increased with pressure.

②. Chin and Lefebvre [15]: Jet A and No. 2 heating oil, pressures > 1.5 MPa, no effect of pressure on deposition rates; JP-5 fuel, pressures 1.72–5.5 MPa, no effect on deposit formation rates; JP-4, under boiling conditions, higher deposits; limited influence of pressure on deposition rates once it exceeds a minimum threshold.

③. Ervin et al. [16]: Neat Jet-A fuel F3219, temperature 650 °C, pressure 3.89 MPa, 5.27 MPa or 6.31 MPa; no effect on pyrolytic deposition.

Taylor [17] indicated that the coking of Fuel A was unaffected by pressure, but the coking rate decreased with increasing pressure within the range of 1.82 to 6.97 MPa and temperatures between 371 and 538 °C for Fuel B. Jones et al. [18,19,20,21] also reported that pressure had no effect on the coking rate. Conversely, Shikhman et al. [22] observed that the coking of n-octane and RT jet fuel decreased with increasing pressure. Watt et al. [23] studied the local and total coke deposits of oxygenated and deoxygenated fuels and revealed that the coking behavior of LeRC67-1, LeRC67-2, and RAF178-64 varied with pressure. Zhao et al. [24] investigated RP-3, finding that the cracking conversion of China RP-3 fuel is proportional to pressure when pyrolysis occurs sufficiently. Ju Y. et al. [25] studied that RP-3 was heated from 127 °C to 427 °C in a stainless-steel pipe and demonstrated that increased pressure had a certain negative effect on the coking of RP-3. Therefore, the effect of pressure on hydrocarbon fuel coking is complex, and findings related to one type of fuel may not necessarily apply to another.

The effect of pressure on the thermal cracking of JP-10 has been investigated, mainly focused on the cracking product distribution, conversion rate, and mechanism. Xing et al. [26] reported that the relative content of ethane or propene decreased, but the content of methane, ethane, or propane increased with increasing pressure, and the major gaseous and liquid components in the products. Wang Y [27] further revealed that the rate of the cracking reaction decreased due to diffusion limitation when JP-10 entered the near-critical or supercritical state from the gas phase, leading to a decrease in the overall conversion rate. Research by Vandewiele et al. [28] confirmed that JP-10 could generate more than 70 intermediate species in the 930–1080 K interval, providing a rich source for subsequent coke precursors. Other studies [29,30] showed that increasing pressure led to an increase in fluid density and a decrease in flow rate, thereby extending the residence time of fuel and reactive fragments in the high-temperature zone and promoting polymerization, condensation, and dehydrogenation reactions. From the point of view of chemical equilibrium, high pressure promoted the generation of large molecular aromatics and coking [31].

Recent studies also highlight the relationship between fuel deposition behaviors in thermal environments and operating conditions. Guo et al. [32] demonstrated that RP-3 flames exhibit oxygen concentration affecting soot morphology and nanostructure evolution. Hu et al. [33] further showed that varying oxygen concentration significantly alters soot formation pathways in ethylene and ethane flames. The study of RP-3 by Zhang et al. [34] showed that the increase in pressure would promote the formation of pyrolytic deposition. The phase transition from liquid to gas state of fuel greatly accelerated the precipitation and deposition of insoluble substances under subcritical pressure. These results emphasize that carbon deposition is highly sensitive to operating conditions.

The deposits not only significantly reduce the wall heat transfer coefficient but also may block the micro-cooling channels, posing a direct threat to the safe operation of the regenerative cooling system [35,36]. In summary, the pressure effect on the coking of JP-10 remains poorly resolved, especially the lack of research on the variation in coke amount with pressure. Further research is required to carry out on the oxidative and cracking coking of JP-10, especially the effect of pressure on coking of JP-10.

Therefore, the aim of this study was to investigate the effect of pressure on coke deposition during oxidative and cracking degradation of JP-10, and to quantitatively characterize the amount of coking and microscopic morphology during the process. The experiments were conducted in a near isothermal flow device and the effect of elemental sulfur on the coking morphology was considered. This study provides fundamental data for the reliable application of JP-10 in future regenerative cooling systems for high-speed vehicles.

2. Materials and Methods

2.1. Test Apparatus

Pyrolytic and oxidative coking was performed in a near-isothermal flowing reactor, as shown in Figure 1, described by He et al. [37]. The flow reactor was primarily composed of a copper heat exchanger and a stainless-steel test section. This cylindrical exchanger, with a length of 500 mm and a diameter of 600 mm, enclosed a 500 mm-long stainless steel (SS316L) test tube, purchased from Beijing Xiongchuan Valve Manufacturing Co., Ltd. (Beijing, China), with an outer diameter of 3.175 mm and an inner diameter of 2.175 mm. Heating was provided by eight identical electrical cartridge heaters embedded within the copper block. For uniform thermal distribution, four heaters were arranged circumferentially at one end of the exchanger, and others were distributed in the same way at the other end. The wall temperature of the test section was measured using 9 type-K thermocouples, purchased from OMEGA Engineering (Shanghai) Co., Ltd. (Shanghai, China), at about 5 cm intervals along the tube. The uniformity of wall temperature can be controlled within 2 °C and the uncertainty of pressure is 2.5 kPa. A programmable preheater (red dotted box), Zhenjiang Chuxin Electric Heating Equipment Co., Ltd. (Zhenjiang, China), was installed in the front section of the reaction tube. When the preheating temperature reached the target temperature and stabilized for 10 min, the reaction section would be turned on to ensure the inlet temperature of the reaction section. Once the wall temperature of the reactor tube stabilized at the target temperature for 2 h, the saturated air fuel was introduced. After the liquid was discharged, the back-pressure valve reached the target pressure, and the test time began after stabilization.

Experiments were carried out for 6 h at a volumetric flow rate of 1 mL/min, measured under ambient conditions. To investigate the effect of fuel pressure on oxidative coking, experiments were conducted at a constant preheat temperature of 533.15 K and a wall temperature of 623.15 K, with fuel pressure varied from 0.708 to 6.816 MPa. Analogous tests for pyrolytic coking were performed at the same preheat temperature, but with the wall temperature elevated to 793.15 K, fuel pressure varied between 2.706 MPa and 7.165 MPa.

2.2. Materials

JP-10 was purchased from Liming Chemical Research Institute (Luoyang, Henan Province, China) with a mass content of 99%. The sulfur content of the JP-10 fuel, as measured by micro-coulometer, was 1.7 μg/mL, which is typical for commercially available material. No further purification was performed prior to use. The main components (wt%) of 316L stainless steel pipe were as follows: 70.10% Fe, 0.01% C, 0.67% Mn, 0.032% P, 0.002% S, 0.31% Si, 16.73% Cr, 10.10% Ni, 2.03% Mo, 0.065% N.

2.3. Analysis of Coke Deposit

The reactor tube was sectioned into ten 50 mm-long segments. Each segment was rinsed with hexane followed by alcohol to remove residual fuel and soluble precursors and subsequently dried in a vacuum oven at 353 K for 2 h to ensure complete solvent evaporation. The remaining solid deposits were quantified as coke using a LECO RC612 analyzer, LECO Corporation (St. Joseph, MI, USA) through temperature-programmed oxidation (TPO). To obtain the amount of oxidative coke deposit, the samples were heated from 100 °C to 900 °C at a rate of 30 °C/min and held at 900 °C for 5 min. The rate of high-purity oxygen was 750 mL/min. For pyrolytic coke deposit, the samples were heated from 100 °C to 750 °C at a rate of 30 °C/min and held at 750 °C for 3 min. All experiments were repeated under identical conditions to assess reproducibility. Statistical analysis by Student’s t-test yielded a mean reproducibility of 3.5% and a relative uncertainty of 15% at a 95% confidence level.

Surface morphologies of carbon deposits were characterized by a field emission scanning electron microscope (FESEM, Hitachi High-Tech Corporation (Tokyo, Japan)) equipped with a high-performance X-ray energy spectrometer (EDS). The location where the samples are characterized by FESEM is at the 45–50 cm section.

3. Results

3.1. Effect of Fuel Pressure on Oxidative Degradation

3.1.1. Amount of Oxidative Coke Deposits of JP-10

Figure 2 shows the oxidative coke amount of JP-10 at wall temperature of 623.15 K and pressures between 0.708 MPa and 6.816 MPa. Under these conditions, oxidative degradation of JP-10 yields relatively low coke deposits (5.37–14.95 μg/cm²). In the internal pressure from 2.831 MPa to 6.816 MPa, increases in pressure exhibit a negligible effect on the coke amount of JP-10 in the intermediate stage. This behavior is primarily attributed to the fact that JP-10 is an incompressible liquid over this pressure range. Accordingly, increases in pressure produce only minor changes in the Reynolds number, mass transfer characteristics, and residence time [18].

The amount of oxidative coking of JP-10 was 14.95 μg/cm² at 0.708 MPa, more than twice the amount of coking at 2.831 MPa. At an inlet temperature of 533.15 K and 0.708 MPa, the fuel was in the liquid phase because the fuel pressure exceeded the saturated vapor pressure of JP-10. However, as the fuel flowed along the tube and was heated to the wall temperature of 623.15 K, the rising saturation pressure of JP-10 eventually exceeded the fuel pressure in the downstream section, leading to vaporization of the fuel at the outlet. The amount of oxidative coking of JP-10 in the gas phase was significantly higher than that in the liquid phase at the same volumetric flow rate and temperature.

3.1.2. TPO and FESEM Analysis of Oxidative Deposits

Figure 3, Figure 4, Figure 5, Figure 6, Figure 7, Figure 8 and Figure 9 show the TPO profiles and FESEM images of deposits on SS316L in the intermediate stage at different fuel pressures.

As can be seen from Figure 3a, the TPO profile only gives one CO₂ emission peak near 395 °C, while the TPO profile in Figure 4a, Figure 5a, Figure 6a, and Figure 9a gives a wide CO₂ emission peak in the temperature range of 295~353 °C, 320~385 °C, 320~395 °C, and 278~349 °C, respectively. This shows that the oxidative coke deposits of JP-10 have similar activity at different fuel pressures. Figure 7a and Figure 8a both show two CO₂ emission peaks in different temperature ranges, indicating the coke deposits have different oxidation reactivity or structure. The low-temperature CO₂ emission peaks are in the temperature range of 274~351 °C and 295~360 °C, respectively, while the high-temperature emission peaks are 550 °C and 537 °C, respectively. The low-temperature CO₂ emission peak is attributed to oxidation of deposits with high reactivity, whereas the high-temperature peak corresponds to the oxidation of less reactive or more structurally ordered carbonaceous deposits.

Figure 3b, Figure 4b, Figure 5b, Figure 6b, Figure 7b, Figure 8b and Figure 9b show that the morphologies of the coke deposits vary slightly with fuel pressure. In Figure 3b, Figure 4b, Figure 5b, Figure 6b and Figure 9b, the deposits have a similar morphology, which are spherical carbon particles with diameters of about 10–60 nm. The low oxidation temperature of these carbon spheres is responsible for the low-temperature CO₂ emission peak. Especially in Figure 3b and Figure 4b, a large number of carbon particles wrap around the metal tube wall surface, and the particle size of the coke is relatively small. In Figure 7b and Figure 8b, carbon particles were also found, which explains the existence of the low-temperature CO₂ emission peaks in Figure 7a and Figure 8a. Besides carbon particles, other morphologies of coke were also discovered. The coke deposits are characterized by a fibrous morphology with dimensions of 30–50 nm in diameter and 400 nm⁻¹ μm in length. This structure differs significantly from pyrolytic coke, indicating a different formation mechanism. As the fuel temperature was below the pyrolysis temperature of JP-10 [38], these ordered structures are not products of pyrolysis. The leading hypothesis is that they are metal sulfides, formed via reactions between sulfur compounds in the fuel and the metal surface. Elemental analysis via EDS (Figure 10), which confirmed the presence of sulfur, provides supporting evidence that these fiber-like cokes are metal sulfides. These metal sulfides are embedded in the carbon layer, as described by Venkataraman et al. [39] and Xu et al. [40], and their formation process is a chemical vapor deposition process. These results indicate that the oxidative coking products of JP-10 under different fuel pressures are mainly amorphous carbon, and sulfide plays an important role in the formation of fiber-like cokes in this process.

3.2. Effect of Fuel Pressure on Pyrolysis Degradation

3.2.1. Amount of the Pyrolytic Coking of JP-10

Figure 11 shows the pyrolytic coke yield of JP-10 fuel on SS316L surface when the wall temperature is 793.15 K and the fuel pressure is between 2.706 Mpa and 7.165 Mpa. It can be seen from Figure 11 that the pyrolytic coke yield of JP-10 ranges from 66.88 μg∙cm⁻² to 221.89 μg∙cm⁻², which is several times or even dozens of times larger than the oxidation coke amount. It is noteworthy that the pyrolytic coke yield exhibits a first plateau, then a rapid increase beyond 4.716 Mpa. The coke amount of JP-10 changes very gently in the range of 2.706 Mpa–4.716 Mpa. When the fuel pressure increases by 2.01 Mpa, the pyrolytic coke yields of JP-10 increase by 7%. However, the pyrolytic coke yield increases rapidly from 4.716 Mpa to 7.165 Mpa, displaying that the pyrolytic coke yields at 7.165 Mpa are more than twofold higher compared to that at 4.716 Mpa. At the same temperature, the formation of this phenomenon is closely related to the flow velocity, flow pattern, and residence time of fuel.

Table 1. The value of V, Re and t of JP-10 at wall temperature of 793.15 K and different pressures.

p/Mpa	V/m∙s−1	Re	t/s
2.706	0.058	449.2	8.85
4.023	0.033	403.8	15.53
4.716	0.025	381.0	20.11
5.913	0.017	339.2	29.88
7.165	0.013	290.5	40.02

shows the flow rate, Reynolds number, and residence time of JP-10 in the reaction tube under different fuel pressures. The thermophysical parameters of JP-10, which are required to calculate the Reynolds number, are from Supertrapp 3.2. The residence time is calculated according to Equation (1):

$$t=15{\displaystyle \frac{\pi d^{2}L}{V}}$$

(1)

where t is the residence time of the fluid in the reaction tube, s; d is inner diameter of the reaction tube, mm; L is length of the reaction tube, m; V is volumetric flow rate, with a value of 1 mL/min.

Table 1 shows that as the fuel pressure increases, the flow rate of JP-10 fuel decreases, the Reynolds number decreases, the residence time increases, and the fuel is in the laminar phase throughout the experimental pressure range. At 2.706 Mpa, JP-10 is in the initial stage of decomposition and pyrolysis conversion rate is low, therefore the yields of high-molecular-weight compounds constitute a low proportion of the total products. As the fuel pressure further increases, especially when the pressure is greater than the critical pressure, JP-10 fuel becomes a supercritical fluid. Residence time is an important factor, affecting the cracking of the supercritical fluid. The coke precursors exhibit sustained accumulation in inner surface of reaction tube. This promotes progressive deposition of high-molecular-weight compounds on inner surfaces, resulting in a sharp increase in the amount of coking.

3.2.2. TPO and FESEM Analysis of Pyrolytic Deposits

Figure 12, Figure 13, Figure 14, Figure 15 and Figure 16 show the TPO profiles and FESEM of the pyrolytic coke of JP-10 when the fuel pressure is 2.706 Mpa, 4.023 Mpa, 4.716 Mpa, 5.913 Mpa and 7.165 Mpa, respectively. Two CO₂ emission peaks are given from Figure 12a, Figure 13a, Figure 14a, Figure 15a and Figure 16a: a low-temperature peak around 300 °C, attributed to the oxidation of amorphous carbon with high reactivity, and a high-temperature peak around 500 °C, associated with the oxidation of more ordered carbon structures. No additional CO₂ emission peaks were found above 500 °C. It can be seen from Figure 12b, Figure 13b, Figure 14b, Figure 15b and Figure 16b that the pyrolytic coke of JP-10 is mainly carbon nanotubes, with a small amount of granular carbon particulates. Similar tubular morphologies have been widely reported in previous studies of JP-10 [41], supporting the identification as carbon nanotubes. The carbon nanotubes are caused by pyrolysis of JP-10 because the experimental temperature is above the pyrolysis temperature of JP-10. No sulfur signal is detected in the EDS spectrum (Figure 17), which also proves that the coke deposits are carbon nanotubes rather than metal sulfide.

By comparing Figure 12b, Figure 13b, Figure 14b, Figure 15b and Figure 16b, it can be known that the fuel pressure has little effect on the morphologies of the pyrolytic coke of JP-10 in the temperature range of 2.706–7.165 Mpa. The pyrolytic cokes are carbon nanotubes across the investigated pressures, with one end of the carbon fiber connected to metal particles. They oxidized in the temperature around 500 °C, correlating with the high-temperature peak on the TPO profiles (Figure 12a, Figure 13a, Figure 14a, Figure 15a and Figure 16a). A small amount of amorphous carbon is also detected. The amorphous carbon oxidized at the temperature around 300 °C, corresponding to the low-temperature peak on the TPO profiles (Figure 12a, Figure 13a, Figure 14a, Figure 15a and Figure 16a).

In Figure 12b, the carbon nanotubes are between 20 nm and 100 nm in diameter and between 1 μm and 3 μm in length. In Figure 13b, the carbon nanotubes are between 40 nm and 80 nm in diameter and between 1 μm and 3 μm in length. Below them, particles with irregular shapes are packed close to the tube surface. In Figure 14b, a large number of loose honeycomb-shaped cokes are attached to the wall of the tube, which are essentially amorphous carbon, and carbon fibers are embedded in these amorphous carbons. The diameter of the carbon fibers is 100 nm~170 nm and the length is 1 μm~3 μm. In Figure 15b, the carbon nanotubes are between 40 nm and 100 nm in diameter and between 2 μm and 10 μm in length, and the carbon particles are between 80 nm and 250 nm in diameter. In Figure 16b, a large number of carbon nanotubes are shown. They are not evenly distributed but aggregated in a certain area. The carbon nanotubes exhibit diameters of approximately 180 nm and lengths ranging from 1 μm to 5 μm. Irregular carbon particles with different sizes are also found closed to the surface of tube. In summary, the pyrolytic coke of in morphology is similar at 793.15 K and various fuel pressure, and the main differences are only reflected in the slight changes in the diameter and length of carbon fibers/particles.

4. Discussion

In this study, the sudden increase in the coke amount at 0.708 Mpa and at 623.15 K may be attributed to two factors. First, when the saturated vapor pressure of JP-10 (≈1.6033 MPa, see Ref. [42]) exceeds the operating fuel pressure, phase transition occurs, and a portion of the fuel enters the vapor phase. Under such conditions, coke precursors in the gas phase can condense or deposit more readily onto the reactor wall, while entraining oxidative products with relatively high boiling points. This promotes heterogeneous nucleation and accelerates deposition. Second, gas bubbles adhering to the tube wall reduce local heat transfer efficiency, resulting in a local temperature rise that accelerates the oxidative reactions and enhances fouling. The differences compared with Marteney’s study [21] highlight the role of fuel-specific thermophysical properties. From a practical perspective, maintaining the operating pressure of JP-10 above its saturated vapor pressure is critical to suppress vapor–liquid transition and mitigate oxidative coking.

Under pyrolysis conditions, pyrolytic cokes are mainly carbon nanotubes and a small amount of amorphous carbon, which is different from that discovered in Xie’s work [41], as well as that in Pan’s work [36]. Xie’s work shows that coke deposits of JP-10 fuel on the SS316L surface at 600 °Cand 4 MPa are mainly irregular carbon and few carbon nanofibers. The experimental temperature in Xie’s work is higher than that in this paper. At higher temperatures, JP-10 is more likely to produce hydrogen-depleted compounds, which undergo polymerization and cyclization reactions, resulting in the formation of larger particles of carbon and preventing the growth of filamentous carbon. Pan’s work demonstrates that the pyrolytic cokes of JP-10 on nickel-based superalloy are primarily amorphous carbon along with few coke filaments. Whereas in the present study using stainless steel (SS316L) tubes, the deposits exhibit more ordered structures. This contrast highlights the significant influence of wall material composition on coke morphology and formation mechanisms. The characteristics of coke are attributed to the rapid formation of polycyclic aromatic hydrocarbons rather than the wall catalysis. The main morphology of pyrolytic coke in this work is consistent with metal-catalyzed carbon growth. Coke precursors first adsorb on the metal surface, then undergo dehydrogenation/polymerization, and finally carbon precipitates onto the metal particle to form elongated fibers [43,44,45].

5. Conclusions

In this paper, oxidative and pyrolytic coking of JP-10 was systematically investigated, and the influence of pressure on both was also evaluated.

This result demonstrates that the oxidative coking of JP-10 is relatively low, ranging from 5.37 to 14.95 μg·cm⁻² at a temperature of 623.15 K and pressures between 0.708 and 6.816 MPa. In contrast, pyrolytic coking exhibits a significantly higher range, measuring between 66.88 and 221.89 μg·cm⁻² at a temperature of 793.15 K and pressures varying from 2.706 to 7.165 MPa.

Oxidative coking yield is not effective in fuel pressure in the liquid phase, whereas it increases significantly in the gas phase. In contrast, pyrolytic coking is positively correlated with pressure and experiences a marked acceleration under supercritical conditions due to prolonged residence time.

From a morphological perspective, oxidative deposits primarily consist of amorphous carbon, along with filamentous structures and metal sulfides. Conversely, pyrolytic deposits are predominantly characterized by carbon nanotubes that form through a metal catalytic mechanism, but non-metallic sulfides, with a small amount of amorphous carbon particles, exist.

These findings indicate that operating pressure, wall temperature, phase state, and supercritical conditions are key factors influencing both the severity of coking and the morphology of deposits. In practical applications, such as regenerative cooling systems in hypersonic aircraft, these results suggest that maintaining JP-10 above its saturated vapor pressure prior to pyrolysis can suppress phase change induced oxidative coking, while minimizing residence time in high-temperature regions can reduce pyrolytic coking.

The limitations of this study are as follows: it conducted in a laboratory-scale near-isothermal flow reactor. TEM and Raman characterization were not available, and the reactor walls may differ from full-scale engine materials. Therefore, extrapolation to operational systems should be made with caution, and further studies are recommended under representative flight conditions.