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In this paper, the effects of the geometric parameters of a rectangular cooling channel on the amount of pyrolysis carbon deposition in the channel unit of real fuel‐cooled plates under the constraints of fixed overall plate shape and total flow rate are studied. A decoupling algorithm for simulating the carbon deposition on surfaces of three‐dimensional (3D) channels is firstly established, and a carbon deposition rate model is extended for 3D channels. Using the developed method, the processes of pyrolysis carbon deposition in five fuel‐cooled plates with various channel parameters are simulated. The effects of structural parameters on heat transfer, pyrolysis reaction, distribution of carbon deposition, and total carbon deposition amount are discussed. The results show that the channel geometric parameters indirectly affect pyrolysis carbon deposition in two ways: by changing the wall temperature where the carbon deposition reaction occurs through affecting the heat transfer and by changing the concentration of precursors through changing the residence time of fuel. A parameter θ related to the rectangular channel structure and the number of channels is defined. Under the geometric constraints of the channel unit, the smaller design parameter θ is beneficial to reduce carbon deposition in the channel. [ABSTRACT FROM AUTHOR]

In this work, three kinds of ANNs are employed to predict the pyrolysis behavior of n-decane. Six-fold cross validation method is adopted to optimize the key parameter of ANNs and evaluate the performance of ANNs. The results indicate that RBFN has best performance in predicting the species distribution and all ANNs have good performance in predicting the bulk temperature. Meanwhile, these results reveal that all ANNs are more suitable for predicting products with high mass fraction than those with less mass fraction, whose errors are also acceptable. [ABSTRACT FROM AUTHOR]

In this research, a global reaction model for the thermal cracking of China No.3 aviation kerosene was modeled based on experimental investigations under supercritical pressure. The result indicates that, when the conversion is lower than 50%, small molecules of products were generated nearly in a constant ratio and the selectivities of them were approximately identical. Therefore, a molecular kinetic model was developed using plug flow reactor model and an isothermal hypothesis. In order to estimate the kinetic parameters, a novel method of online density measuring was used to obtain the precise resident time. The reliability of this molecular kinetic model was validated through the comparison of experimental results with simulated values. [ABSTRACT FROM AUTHOR]

The pyrolysis characteristics of n-decane were investigated in a series of reaction tubes. The results show that at fixed bulk temperature, the wall temperature, and residence time are two crucial contributions to influence the pyrolysis process, and the former has a more important effect on the cracking degree. A two-dimensional numerical model is employed to demonstrate that there exists a thermal boundary layer of the flowing fuel, which enhances the pyrolysis process. Based on the chemical analysis, the wall temperature may accelerate the rate of β-Scission reaction, thus leading to higher alkene content. [ABSTRACT FROM AUTHOR]

• Strained polycyclic hydrocarbons studied in this work have better performances than JP-10 thermodynamically. • The initial pyrolysis mechanism including ring bond breaking and bridge bond breaking was investigated. • The ring opening reactions can be divided into classes depending on the size of the rings and the types of breaking C C bonds. • The rate rules of the initial pyrolysis reactions were obtained. Strained polycyclic hydrocarbons are one of the most important classes of high-energy density fuels. In this work, the thermodynamics and initial pyrolysis kinetics of five strained polycyclic hydrocarbons, which are isomers of JP-10 and are linearly aggregates of two cyclopropyl rings and one cyclobutyl ring, were investigated using quantum chemical computations. The initial pyrolysis reactions include ring bond breaking and bridge bond breaking. The ring bond breaking reactions include a diradical pathway which is the direct C-C bond breaking to produce a diradical and an isomerization pathway involving concert bond breaking and H-atom transfer, and the bridge bond breaking reactions lead to two radicals. Our potential energy calculation shows that the ring bond breaking reactions are much more energetically favorable than the bridge bond breaking reactions, which can be neglected in the initial pyrolysis process of strained polycyclic hydrocarbons. The rate constants of the ring bond breaking reactions were computed using variational transition state theory (VTST) or transition state theory (TST). Our calculated results demonstrated that ring bond breaking reactions of the three-membered ring are competitive with that of the four-membered ring. The studied reactions are divided into classes depending on the size of the rings (three- or four-membered ring) and the types of the carbon atoms of the breaking C-C bond (Cs-Cs, Cs-Ct, Ct-Ct and Cq-Cs). Reaction rate rules are established for each class, which are useful for the mechanism generation for analogous strained polycyclic hydrocarbons. Thermodynamic properties including net calorific values per mole, gravimetric net heat of combustion and impulse time of our studied strained polycyclic hydrocarbons are higher than those of JP-10. A comparison of the initial pyrolysis kinetics of our studied strained polycyclic hydrocarbons with the isomer JP-10 indicates that these compounds may be easier to pyrolyze. [ABSTRACT FROM AUTHOR]

[Display omitted] • The NH 2 radical formed by NH 3 will convert the small hydrocarbons to carbonitrides. • Carbon source for the soot formation is reduced with the NH 3 addition. • NH 3 addition decreases concentrations of key PAH precursors of C 2 H 2 and C 3 H 3. • The coking phenomenon is weakened with the NH 3 addition. Soot formation during the imperfect combustion or pyrolysis of fuels is harmful to engine efficiency·NH 3 has been proved to reduce soot emission effectively in flames. In this work, the effect of NH 3 addition on soot inhibition during n- decane pyrolysis was systematically investigated from a theoretical principle. The MD simulations indicate that the coking phenomenon is weakened with the NH 3 addition. Besides, the NH 3 additives make the concentration peak of the dominant gas products lag. Thus, the concentrations of crucial precursors (acetylene and propargyl) are reduced by NH 3 addition. Moreover, the NH 3 additives can also influence the PAH growth process through the PAHs with N atoms (N-PAHs). Then, the chemical kinetic analysis shows that proper addition of NH 3 additives can meet the heat absorption demand of fuel cracking. Furthermore, the mass fractions of PAHs and soot particles decrease with NH 3 addition, which is consistent with the results from MD simulations. The formation of the main carbonitride species originates from the reaction of CH 3 and C 2 H 5 with NH 2 followed by the dehydrogenation or C-C bond breaking reaction. The carbon source directly involved in the formation of soot particle will be reduced and the growth of the soot particle will be inhibited by preventing the formation of the key reactants, such as the acetylene molecule. [ABSTRACT FROM AUTHOR]

[Display omitted] • Development of coking inhibitors based on blocking coking reaction pathway. • The coking inhibition efficiency of additives exceeds 80% under deeper pyrolysis. • Mechanism of ammonia for inhibiting metal catalytic and bulk phase coking is raised. • Carbides containing N elements on the inner surface affect growth of metal carbides. • Amino groups form p - π conjugates with unsaturated olefins to regulate coking pathway. The coking problem of hydrocarbon fuel pyrolysis is the key factor affecting the application of active cooling technology for high-speed aircraft. The purpose of this work is to investigate the coking inhibition performance and coking mechanism of using ammonia as an additive during the thermal cracking. In this study, the pyrolysis and coking characteristics of supercritical n-decane were experimentally studied using an electrically heated tube. ICP-OES (Inductively Coupled Plasma Optical Emission Spectrometer), GC–MS (Gas Chromatograph Mass Spectrometer), SEM (Scanning Electron Microscope), EDS (Energy Dispersive Spectroscopy), TPO (Temperature Programmed Oxidation), and Raman spectroscopy are applied to characterize the anti-coking performance of the additives. The results show that the additive hardly affects the cracking of the fuel under high-temperature conditions, but the coke mass deposited on the inner surface of the tube is significantly reduced, and the coking inhibition rate can reach 80% at 750 °C. The addition of additives reduced the graphitization degree and oxidative activity of the coke. The maximum oxidation temperature of the coke obtained at 750 °C was reduced by 145 °C, and the I D /I G value of the Raman spectroscopy peak-fitting was 3.29. The new mechanism of the effect of additives on the formation of coking precursors and the deposition of coke on the surface of the tube wall is proposed. Ammonia can react with Ni of the inner metal tube and radicals generated by fuel cracking to form a carbonized film, achieving the effect of inhibiting metal catalytic coking. Meanwhile, the amino radicals generated by ammonia dehydrogenation can regulate the coking reaction path by forming a stable p - π conjugate structure with unsaturated radicals. Therefore, the research will contribute to a deeper comprehension of the coking inhibition mechanism and the development of more effective coking inhibitors, providing new technical support for active cooling technologies. [ABSTRACT FROM AUTHOR]

• Additives show variable effect on fuel pyrolysis in different temperature ranges. • Mechanism for additives inhibiting bulk phase and metal catalytic coking is proposed. • Hydroxyl radicals released by additives can regulate the coking reaction path. • Hydroxyl radicals oxidize the coking precursors to prevent its further growth. • Particulates serving as passivation coating appear on inner surface of the tube. Coking during the endothermic hydrocarbon fuel pyrolysis is a key issue that needs to be solved in the development of regenerative cooling technology for high-speed aircrafts. The compounds containing hydroxyl groups are developed to inhibit coking by regulating coking pathway during the thermal cracking of hydrocarbon fuels. Pyrolytic deposition experiments of n -decane with additives were performed using the method of electrical heating. The anti-coking performance of the additives was characterized by means of gas chromatograph–mass spectrometer (GC–MS), scanning electron microscopy (SEM), temperature programmed oxidation (TPO) and Raman spectroscopy. The results showed that the degree of cracking is reduced at low fuel temperature due to the dilution of the additives, and the additives do not affect the fuel cracking as the fuel temperature increased. With the function of additives, the masses of filamentous and amorphous coke were decreased. A possible mechanism for the additives inhibiting coking has been proposed. The additives containing hydroxyl groups can block the initial coking reaction path to inhibit coking at the source. In general, the hydroxyl radicals released by the additives oxidize the coking precursors to prevent its further growth, and react with the substrate to form particulate matters serving as passivation coating, inhibiting the bulk phase coking and metal catalytic coking simultaneously. This work provides significant ideas for inhibiting coking by regulating coking reaction pathways. [ABSTRACT FROM AUTHOR]

• The formation of the soot nanoparticle undergoes four processes. • Initial pyrolysis provides the reactant source for the formation of soot precursor. • The soot particle undergoes the surface growth and the morphological rearrangement. • Higher temperature can accelerate the formation of soot particle. The chemical-kinetic mechanism of soot formation for the n -decane pyrolysis was comprehensively investigated from theoretical perspective. We found the formation of carbonaceous nanoparticle with a multi-layer closed cylindrical structure needs four processes including initial rings formation, PAH growth, particle nucleation via coalescence of PAH and particle growth by coagulation or surface reactions. The mechanisms of HACA, CAHM and CHRCR play crucial roles in the processes of the formation and growth of PAH molecules. In addition, the temperature effect was discussed and found the higher temperature can accelerate the formation of soot particle, which is in agreement with the experiment findings. [ABSTRACT FROM AUTHOR]

• Pyrolysis and coke of RP-3 in one-side heated rectangular U-bend tube were studied. • Secondary flow in U-bend tube resulted in smaller nonuniformity of temperature field. • Curved structure obviously affected circumferential distribution of pyrolysis products. • High near-wall temperature in straight tube leaded to high yield of coke precursors. • Curved structure was beneficial for restraining coke deposition in the U-bend tube. Pyrolysis and coking of Chinese aviation kerosene RP-3 in a rectangular U-bend tube of nonuniformly heating was numerically simulated. Discussion was held regarding how the bending structure affected temperature distribution, thermal cracking, and pyrolytic deposition. Results indicated that the secondary flow induced by centrifugal forces effectively promoted thermal convection between near-wall and core regions. This effect further decreased the fuel temperature of near-wall region. Furthermore, nonuniform distribution of temperature in the U-bend tube was reduced. More RP-3 reacted along the heated wall in the straight tube, which significantly accelerated the average reaction rate in the cross section and increased the yield of coke precursors formed by secondary reactions. Notably, the curved structure promoted heat transfer and anticoke deposition performance. Under the same inner surface area, the total amount of pyrolytic deposition of U-bend tube was reduced by 17.77 mg during coking duration of 5 min. [ABSTRACT FROM AUTHOR]

[Display omitted] • Effect of secondary flow on fuel pyrolysis and coking in U-bend tube was analyzed. • Wall temperature in the curved section showed an asymmetric V-shaped distribution. • Secondary flow in the curved section affected the uniformity of coking distribution. • Wall coking amount at the inner side was greater than that at the outer side. Curved structures are inevitably involved in the regenerative cooling channel of scramjet engine. In this work, pyrolysis and coking characteristics of supercritical RP-3 aviation kerosene fuel were experimentally studied using an electrically heated U-bend tube with a curved structure and a straight tube. Furthermore, numerical simulations were conducted to obtain detailed information of local flow, heat transfer, pyrolysis, and coking deposition. The results showed that increasing system pressure and outlet temperature can improve the gas yield and conversion of the fuel and then improve the chemical heat sink and total heat sink. The wall temperature in the curved section of the U-bend tube showed an asymmetric V-shaped distribution due to the secondary flow caused by centrifugal force. The secondary flow effectively strengthened the heat transfer near the outer side of the curved section. As a result, the wall temperature at the outer side was about 17–56 K lower than that at the inner side. Meanwhile, the secondary flow in the curved section affected the uniformity of coke deposition distribution. It was found by scanning electron microscope images and surface energy spectrum analysis that the amount of wall coke deposition at the inner side was much greater than that at the outer side. This was because compared with the outer side, the inner side of the curved section had lower flow rate, higher wall temperature and greater concentration of coking precursor. [ABSTRACT FROM AUTHOR]

• ·A characteristic pressure-drop model of reacting flow was established and validated. • ·Two types of negative slopes of pressure-drop were measured and simulated. • ·The effects of pyrolysis on negative slope of pressure-drop were analyzed. • ·The effects of pyrolysis on flow excursion in parallel channels were discussed. The non-monotonic relation between pressure-drop and flow-rate leading to the flow instability of fuel with pyrolysis was investigated. A characteristic pressure-drop model of n -decane coupled pyrolysis reactions under supercritical pressure was established. The model includes one-dimensional flow control equation, species equation, RK-PR equation of state, high-pressure viscosity model and a reaction mechanism consisting of 10 species and 15 reactions. The non-isothermal friction factor formula with wall correction coefficient fitted by experimental data was used in the model. To verify the model, the pressure-drops at different flow-rates were measured in a horizontal stainless steel tube with the inner diameter of 1.93 mm under different back pressures (3, 4 and 5 MPa). The results show that the model can accurately capture the non-monotonicity of characteristic pressure-drop. Under 3 MPa condition, the characteristic pressure-drop curve of n -decane has a pseudo-critical negative slope (PCNS) region and a reaction negative slope (RNS) region caused by the pyrolysis. With the increase of pressure, the PCNS region disappears but the RNS region still exists. The high pressure makes the onset of RNS region appear earlier due to the accelerated initial reaction rate. The total pressure-drop is separated and analyzed based on the model mathematically. In the PCNS region, the derivative of acceleration pressure-drop to mass flux is negative, whereas the derivative of friction pressure-drop to mass flux is positive. In the RNS region, the two derivatives are both negative because of the sharp decrease in density and the increase in friction factor caused by the pyrolysis reaction. The two parallel channels' instability with constant total mass flow rate is analyzed based on the characteristic pressure-drop curves for reacting and non-reactive flow. The results indicate that the fuel pyrolysis reaction not only expands the range where the flow excursion may occur but also makes it difficult for the flow-rate to reach stable state again after the flow excursion caused by pseudo-critical instability. [ABSTRACT FROM AUTHOR]

Methylcyclohexane is usually adopted as a surrogate component of aviation fuels to study its properties and reactions. Under the condition of 3.5/4.0 MPa and the highest temperature of 1023 K, the pyrolysis reaction of methylcyclohexane was investigated in a horizontal tube with the inner diameter of 0.2 cm and the length of 90 cm. An electric heating method and orifice online density measuring device were introduced to obtain the product distribution and density along the tube. The pyrolysis products were analyzed by gas chromatography. The main cracked products were C 7 alkenes, alkylcyclopentanes, alkylcyclohexenes, aromatic compounds and small molecular hydrocarbons. Different from the n-alkanes pyrolysis, methylcyclohexane pyrolysis do not obey proportional product distribution model in the whole conversion range. As some pyrolysis products can be produced and consumed simultaneously, the cracked products were classified into primary and secondary products, and therefore primary and secondary reactions were proposed to describe the methylcyclohexane pyrolysis. For methylcyclohexane, the primary reaction occurs accompanying the secondary reaction from the initial stage. In the hypothesis of plug flow reactor and first order reaction, the activation energy for primary and secondary reactions can be determined as 208.8 kJ/mol and 174.3 kJ/mol respectively. • A two-step reaction mechanism of methylcyclohexane pyrolysis was proposed. • A novel density measuring method was used to accurately obtain density data. • The selectivity of products was varied in the initial stage. • Primary and secondary reactions occurred simultaneously even at low conversion. [ABSTRACT FROM AUTHOR]

In order to inhibit the surface coking, TiO 2 coatings were prepared on the 310S stainless steel surface by the metal-organic chemical vapor deposition method in a horizontal hot-wall tubular reactor under atmospheric pressure condition. According to the coating deposition kinetics, the apparent activation energy of 29.30 kJ/mol was obtained in the low temperature range from 250 to 400 °C due to the domination of the surface reaction. However, the coating deposition rate declined at temperatures above 400 °C because of the simultaneous generation of TiO 2 powder in the gas phase. The morphology, elemental and phase composition of TiO 2 coatings were characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction, respectively. The results show that the surface TiO 2 particle size increased with the rising temperature, and anatase phase was achieved above 350 °C with the preferential growth of crystal structure from (101) face at 350 °C to (220) face at 450 °C. Correspondingly, the coating thickness increased with the deposition time, and the coating with the thickness of about 2 μm covered the substrate surface completely, with the elemental composition in atomic percentage of Cr, Fe and Ni being close to 0, respectively. Based on the parallel experiments of cyclohexane pyrolysis for each TiO 2 coating specimen, it is indicated that the best performance was achieved on TiO 2 coating deposited at 250 °C for 80 min with an anti-coking ratio of 64.0%. The ratio was 48.0% for that deposited at 450 °C for 30 min. And the coating deposited at 450 °C for 30 min has better effect on inhibiting the formation of filamentous carbon. [ABSTRACT FROM AUTHOR]

• The different coking mechanism leads to the change of coke characteristics. • Relationship between coke characteristics and formation mechanism is established. • Low reactive filamentous carbon caused by metal catalytic coking is graphitic. • Granular carbon has higher reactivity, which is conducive to remove. • Simplified kinetics models of coke production and removal are constructed. Coking during the hydrocarbon fuel pyrolysis is a key problem that needs to be considered. To get more insights into coke, this work explored deposit formation of hydrocarbon fuels from the coke characteristics. Pyrolytic deposition experiments of n -decane and HF-01 were experimentally performed by using electric heating stainless steel and TiN-coated tubes to investigate the different growth pathway of coke. The morphology, graphitization degree and mass of coke produced under different experimental conditions were analyzed by means of scanning electron microscopy, Raman spectroscopy and carbon burn-off method, respectively. Results reveal that the differences in physical properties and reactivity of coke between different tubes are due to the different growth mechanism. The graphitization degree of coke can distinguish the main growth mechanism. It is also found that the low reactive filamentous carbon with I D /I G value below 2.7 produced by metal catalytic coking is the primary cause of pipeline blockage. TiN coating effectively inhibits the production of filamentous carbon, meanwhile, the coking inhibition efficiency can reach above 70%. The simplified coking kinetic models and semi-empirical coke combustion models are established to explore the coke formation and oxidation process. The models are performed to investigate the coke characteristics, for further exploring and understanding intrinsic hydrocarbon fuel pyrolysis coking mechanism. [ABSTRACT FROM AUTHOR]

In this study, titanium nitride (TiN) coatings at different deposition temperatures have been designed as anti-coking coatings during cyclohexane steam pyrolysis. The TiN coatings were deposited on 310S foils (10 mm × 10 mm × 0.9 mm) between 750 and 950 °C by a chemical vapor deposition (CVD) method. To evaluate the anti-coking property of TiN coatings, cyclohexane was employed for cracking in a tubular quartz reactor. The as-deposited and coked TiN coatings were characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, Raman spectroscopy and X-ray diffraction. The results show that the deposition temperature has a great influence on the morphology and microstructure of CVD TiN coatings. The structures of TiN coatings with apparent differences tend to lie on (220) preferred orientation as the deposition temperature increases. When the deposition temperature is below 850 °C, the TiN coatings cannot cover the substrate surface completely and some filamentous coke was observed. However, the anti-coking ratios of TiN coatings at different deposition temperatures are all above 90% during cyclohexane pyrolysis at 770 °C for 1.5 h. In summary, the TiN coatings at high deposition temperatures can completely cover the cracks and gaps on the rough surface of blank substrate, and thus provide a very good protective layer to prevent the substrate from metal dusting and severe coking. Moreover, the Raman spectra show that the dust-like coke on the surface of TiN coatings has a higher degree of graphitization than the filamentous coke on 310S substrate, which also proves the good anti-coking performance of TiN coatings. [ABSTRACT FROM AUTHOR]

Highlights: [•] The chemical vapor deposition TiN coating shows a good ant-coking performance. [•] The products distribution of the pyrolysis of fuels is influenced by TiN coating. [•] The influence of TiN on cracking behavior is not effect of promotion or inhibition. [•] The solid, liquid and gaseous products which formed extreme condition were studied. [ABSTRACT FROM AUTHOR]

The fuel structure is an important controlling factor during the process of soot formation. Alkylated cyclohexane is a typical surrogate fuel with more complex structure than alkanes or aromatic fuels. Five alkylated cyclohexanes (i.e., cyclohexane, methylcyclohexane, ethylcyclohexane, 1,3-dimethylcyclohexane, 1,3,5-trimethylcyclohexane) and the n- alkane were selected to investigate the effect of the alkyl substituent on sooting tendency during the pyrolysis process from the theoretical principle. The reaction energy barriers profiles of alkylated cyclohexanes obtained by the density functional theory (DFT) illustrate the length and number of the alkyl substituent will affect the production of alkenes fragments which finally decompose to C 2 H 2 and C 3 H 3 species. The molecular dynamics simulations show the abundance of C 2 H 2 and C 3 H 3 fragments from the initial pyrolysis of fuels will accelerate the sooting tendency, which follow the trend of cyclohexane < methylcyclohexane < ethylcyclohexane ≈ 1,3-dimethylcyclohexane < 1,3,5-trimethylcyclohexane, in agreement with experimental findings. We hope this study can provide a more comprehensive understanding of the effect of fuel structure on sooting tendency and ideas for soot inhibition. [Display omitted] • Main decomposition pathways are directly influenced by the structure of fuels. • The alkyl substituents affect the production of alkene fragments. • The number of C 2 H 2 and C 3 H 3 fragments accelerate the sooting tendency. • The length and number of the alkyl substituents increase the sooting tendency. [ABSTRACT FROM AUTHOR]