High-pressure Combustion Research Articles

Data-Driven Model Learning and Control of RCCI Engines based on Heat Release Rate

Reactivity controlled compression ignition (RCCI) technology not only offers high thermal efficiency but also produces low nitrogen oxides (NOx) and soot emissions. However, it is imperative to control the combustion in RCCI engines to prevent high pressure rise rates and combustion instability. In this study, a model-based control framework is developed to optimize the RCCI operating mode. To this end, the effects of variations in the premixed ratio, start of injection timing and fuel equivalence ratio on the combustion dynamics are analyzed by examining the heat release rates. Three distinct heat release rate patterns are identified together with two transition zones. Heat release rate traces are grouped together as a function of fractions of early and late heat release rates. Based on a classification algorithm, the fractions of early and late heat release rate are identified as scheduling variables for the data-driven modeling of an RCCI engine. Linear regression is used to model the fractions of early and late heat release. These models are then used to train linear parameter varying (LPV) models using least-squares support vector machine (LS-SVM). Using the learned LPV model, a model predictive controller (MPC) scheme is then developed for a 2-liter 4-cylinder RCCI engine to control combustion phasing (CA50) and indicated mean effective pressure (IMEP) while limiting the maximum pressure rise rate (MPRR) to avoid engine knocking. The simulation results show that the designed controller is capable of limiting MPRR below 6 bar/CAD while tracking CA50 and IMEP with average errors of 1.2 CAD and 6.2 kPa, respectively.

High-pressure Spray and Combustion Characteristics of Ammonia Jets under Diesel-Like Conditions

High-pressure Spray and Combustion Characteristics of Ammonia Jets under Diesel-Like Conditions

Development and study of the oxygen-fuel high pressure combustion chamber

Development and study of the oxygen-fuel high pressure combustion chamber

Effect of equivalence ratio on spark ignition combustion of an air-assisted direct injection heavy-fuel two-stroke engine

In the small aviation field, the research of direct injection (DI) heavy-fuel two-stroke engines has attracted much attention. The safety and unity of aviation heavy-fuel (i.e., aviation kerosene and diesel) is better than those of gasoline, which is more suitable for small aviation engines. In this paper, the influence of air-assisted injection on the formation and combustion of aviation kerosene direct injection mixture is studied on an aviation heavy-fuel direct injection spark ignition (DISI) two-stroke engine. Firstly, the spray simulation model of three-dimensional air-assisted injector was established by Computational Fluid Dynamics (CFD) software. And the effectiveness of the spray model was verified by comparing the simulation results with the experimental data of constant volume bomb spray. The combustion system CFD model of the two-stroke engine was further established to study the heavy-fuel spark ignition combustion at different ignition timings and equivalence ratios. The results show that when the ignition advance angle is 30°CA BTDC, higher combustion pressure and faster flame propagation speed can be achieved without knock. Aviation kerosene is less prone to knock than diesel and can achieve better combustion performance under the condition of rich mixture.

Numerical investigation of the effect of injection strategy on a high-pressure isobaric combustion engine

High-pressure isobaric combustion adopted in the double compression expansion engine (DCEE) has the prospect to achieve higher thermal efficiency compared to conventional diesel combustion. This work numerically explored the effects of various injection strategies on the combustion and emission characteristics of isobaric combustion. The study developed a mathematical model to predict the injection rate profile. After validations, extensive simulations were conducted with a peak pressure of up to 300 bar – mimicking the high-pressure unit of DCEE. Several major engine design parameters such as the exhaust recirculation gas (EGR) rate, engine speed, injection strategy, and intake pressure were varied and evaluated. The results demonstrated that a higher EGR rate resulted in a higher exhaust loss but a lower heat transfer loss owing to the lower combustion temperature, so the thermal efficiency exhibited a firstly growing and then declining trend. Besides, a higher engine speed generated a higher thermal efficiency due to the shorter combustion duration and thus lower heat transfer loss. Consequently, a peak thermal efficiency of 47.5% was achieved at EGR = 50% and 1800 rpm. The high-pressure cylinder performance can also be improved with an appropriate introduction of the isochoric combustion, but its impact on the whole DCEE setup needs further investigation.

The effect of preheating temperature on PAH/soot formation in methane/air co-flow flames at elevated pressure

Preheating technology is widely used to improve the emission or efficiency of combustors, such as diesel engines and gasifiers operating at high pressures. Soot is an unwanted by-product, but its formation is unavoidable in high-pressure diffusion combustion. In this study, the effect of preheating temperature on polycyclic aromatic hydrocarbon (PAH) and soot formation in methane/air co-flow flames was comprehensively investigated in the pressure range of 1–5 bar. The temperature of inlet gas ranges from 295 K to 573 K. The soot, PAH, and OH* concentrations were obtained using planar laser-induced incandescence, planar laser induced fluorescence, and chemiluminescence techniques, respectively. The experimental results reveal that soot and PAH formation is greatly enhanced at higher pressure or with a higher preheating temperature of inlet gas. At a fixed preheating temperature, the peak/integrated soot volume fraction follows a power law with pressure. As pressure increases, the enhancement of soot formation by preheating temperature is suppressed. As the preheating temperature is raised from 295 K to 573 K, the integrated soot volume fraction is increased by 33.7 times at 1.5 bar, but the difference narrows to 2.3 times at 5 bar. OH* signal increases with preheating temperature at 1 bar, but the difference becomes indistinguishable at higher pressure. Further, the experimental results were utilized to evaluate the soot modeling, PAH and soot formation under experimental conditions are examined using four different kinetic mechanisms. While the soot trend along different pressure and preheating temperature is qualitatively captured, the quantitative predictions vary depending on the mechanisms. Specifically, KAUST and Narayanaswamy-Blanquart-Pitsch (NBP) mechanisms overpredict the soot volume fraction while DRL and Appel-Bockhorn-Frenklach (ABF) mechanisms underpredicts the soot volume fraction. In terms of the PAH spatial distribution, only DRL and ABF mechanisms show the ability to capture the experimental observations, that is the peak PAH appears in the flame centerline. The reaction pathway analysis indicates both fuel-pyrolysis chemistry and PAH growth chemistry should be accounted for the discrepancy.

Effects of radiation reabsorption on the laminar burning velocity of methane/air and methane/hydrogen/air flames at elevated pressures

Due to the development of high-load and high-pressure combustion equipment, it is crucial to increase the accuracy of laminar burning velocity prediction under elevated pressures. However, the radiation ability of methane under high pressures is ignored, which may cause large discrepancies and fluctuations in the simulation of burning velocity. In this study, the HITRAN 2016 database was employed to build the Statistical Narrow-Band (SNB) model of methane at elevated pressures. The effects of radiation reabsorption on the laminar burning velocity were investigated in methane/air and methane/hydrogen/air flames using different radiation models. The simulation was conducted within a wide equivalence ratio (ϕ = 0.6–1.4) and the calculated pressures were up to 20 atm. Comparing the results of different radiation models revealed that the laminar burning velocity was promoted by radiation reabsorption. For varying equivalence ratios at P = 5 atm, in the case of ϕ = 0.6–1.2, the preheat-induced chemical effect by radiation reabsorption was the major factor affecting the burning velocity, controlled by R290 CH2 + O2<=greater than2H + CO2 and R86 2OH<=>O + H2O. And the direct radiation effect played the key role with the equivalence ratio between 1.2 and 1.4, enhanced by increasing concentration of methane. For methane/air flames under stoichiometric conditions, the effects of radiation reabsorption increased with pressure. The preheat-induced chemical effect, related to the consumption of H radical through R35 H + O2 + H2O<=>HO2 + H2O and R43 H + OH + M<=>H2O + M, was significant at P = 1–10 atm. And the direct radiation effect was the dominant factor above 10 atm by increasing optical thickness. Besides, the change of preheat-induced chemical effect by radiation reabsorption caused by H2 addition was examined. The results clarified that the dominant reactions remained the same, while the range of pressures where each reaction dominated changed as 20% H2 was added, owing to the higher sensitivity to H2 addition for R43 comparing to R35.

Experimental and kinetic modeling study of di-n-propyl ether and diisopropyl ether combustion: Pyrolysis and laminar flame propagation velocity

To explore the fuel isomeric effect on ether combustion characteristics, pyrolysis, and laminar flame propagation of di-n-propyl ether (DPE) and diisopropyl ether (DIPE) were investigated. A new kinetic model of DIPE was developed based on our recent DPE model [Fuel 298 (2021) 120797]. The pyrolysis experiments were carried out in two jet-stirred reactors at near-atmospheric pressure with good agreement on the measurements using synchrotron vacuum ultraviolet photoionization mass spectrometry and gas chromatography/mass spectrometry. The decomposition profile of DIPE showed a faster trend than that of DPE, which can be attributed to the faster alcohol elimination reaction of DIPE. The dominant roles of alcohol elimination reaction and H-abstraction reactions in fuel consumption explain the production of fuel-specific oxygenated species, i.e. n-propanol and propanal in DPE and i-propanol, acetaldehyde and acetone in DIPE. The laminar burning velocities of DPE and DIPE were also measured in a high-pressure constant-volume cylindrical combustion vessel at the initial temperature of 373 K and pressures of 1–10 atm. It was found that the linear DPE propagated faster than DIPE with the branched structure under all investigated conditions. Rate of production analysis and sensitivity analysis were also performed to elucidate the key radicals and reactions responsible for the remarkable reactivity of isomeric fuels. Fuel structures have a great influence on the distribution of radical pools, resulting in the easy formation of active radicals in DPE flames such as vinyl and ethyl and stable radicals in DIPE flames like methyl and allyl. The successive decomposition reactions of these dominant radicals promote the DPE flame and inhibit the DIPE flame propagation respectively, which explains the higher laminar burning velocities and reactivity of DPE than that of DIPE. Furthermore, the present model was also examined against the literature data, including pyrolysis and oxidation in the jet-stirred reactor and flow reactor.

Morphological and nanostructure characteristics of soot particles emitted from a jet-stirred reactor burning aviation fuel

Aircraft engine soot emissions have been a growing concern because of their adverse impacts on global climate and human health. In this study, soot emissions from a Jet-Stirred Reactor (JSR) which could be considered as a basic aero-engine combustor were investigated under different combustion conditions including varying combustion pressures, fuel flow rates, and inlet air temperatures. The emitted soot particles were collected onto quartz fiber filters and then examined using high-resolution transmission electron microscopic (HRTEM) and Raman techniques. The HRTEM images and Raman spectra of the filter-laden soot samples were analyzed to determine the morphology and the nanostructure characteristics of the soot particles. It turned out that compared to the reference point, both high combustion pressure and high flow rate led to an increase in primary particle size. The HRTEM and Raman studies showed that there was no amorphous carbon present in the soot particles generated under high combustion pressure. Inlet air temperature exhibited a significant influence on the size and the morphology of soot particles; a large quantity of amorphous carbon with a lower degree of graphitization was generated under the low inlet temperature condition presumably due to poor fuel spray breakup and atomization in the JSR. In addition, we found that nanostructure parameters, such as soot fringe length and fringe tortuosity could be strongly dependent upon the residence time and the inlet air temperature.

Study on the ignitability of a high-pressure direct-injected methane jet using a diesel pilot, a glow-plug, and a pre-chamber

Natural gas is a promising alternative fuel for internal combustion engines, it allows for a reduction of engine-out emissions without impairing high engine efficiencies. Although this approach is already utilized from small to large engine classes, it is almost exclusively based on the combustion of a premixed, homogeneous charge. For ignition, small engines use standard spark-plugs or pre-chambers, while large and lean-operated engines use pre-chambers and pilot injections. Direct high-pressure gas injection is a more recent, alternative way to operate gas engines which offers benefits compared to premixed operation such as high compression ratio, high combustion pressures, lean operation, quantity regulation, among others. However, in contrast to diesel direct injection, the compression temperatures are too low for the auto-ignition of the gas jets. Therefore, an additional ignition system is required, usually a pilot injection system is used. In this study, the usability and performance of three ignition strategies for direct injected high-pressure gas jets have been investigated in an optically accessible test-rig that is able to operate at engine-like conditions. The first type of ignition system is a pilot injection with a liquid fuel, the second is a glow-plug located near the main gas jet, and the third system is a pre-chamber with a nozzle hole aimed at the main gas jet. Results show that all three strategies are feasible options under the studied conditions. Ignition by a pilot fuel injection is a safe and reliable way to ignite the main fuel. The glow-plug is less reliable and leads to high cycle-to-cycle variation. The best option in the present study is the pre-chamber, it is very reliable, delivers the highest peak cylinder pressure and exhibits the lowest cyclic variability. The good performance is attributed to the intense mixing of the main gas jet with the hot jet exiting the pre-chamber.

Theoretical analysis of power generation applying supercritical steam and high-pressure combustion chamber consuming biomass slurry

Theoretical analysis of power generation applying supercritical steam and high-pressure combustion chamber consuming biomass slurry

Simulation of high-pressure methane-oxygen combustion with a new reduced chemical mechanism

The modeling and simulation of methane-oxygen combustion at high pressure requires a dedicated kinetic mechanism to obtain a precise description of the fuel decomposition. However, using the detailed mechanisms literature in high-fidelity simulations, like direct numerical simulations, is not possible due to the high numerical cost. Accordingly, a reduced chemical mechanism is required, and the one proposed in this study contains 17 species and 44 reactions to encompass a very large range of pressure (P∈[1,100] bar) and equivalence ratio (ϕ∈[0.2,14]). The Optimized and Reduced Chemistry method, that is based on directed relation graph with error propagation, is applied to the RAMEC kinetic scheme, and validations are performed for a set of canonical test-cases: auto-ignition delay simulation, one-dimensional laminar premixed flame freely propagating and one-dimensional counterflow diffusion flame. A very good agreement is obtained by comparison with the RAMEC detailed mechanism. To further evaluate the ability of the reduced mechanism, a premixed flame expanding in a homogeneous isotropic turbulence at high pressure (P=56 bar) is performed. The temporal evolution of the flame front is detailed from its ignition, revealing the flame behavior and its interaction with turbulence. The new reduced chemical mechanism performs well with a relative error of 4% for the maximum of temperature at each time step and even less if the difference observed in the auto-ignition times between the two mechanisms is considered. Finally, a substantial gain is found when using the reduced scheme on three-dimensional cases, the simulations being 8 times faster than those using the detailed scheme. A dedicated post-processing tool based on the contour tracing labeling method is developed to measure the flame length.

Ultrafast-laser-absorption spectroscopy in the mid-infrared for single-shot, calibration-free temperature and species measurements in low- and high-pressure combustion gases.

This manuscript presents an ultrafast-laser-absorption-spectroscopy (ULAS) diagnostic capable of providing calibration-free, single-shot measurements of temperature and CO at 5 kHz in combustion gases at low and high pressures. Additionally, this diagnostic was extended to provide 1D, single-shot measurements of temperature and CO in a propellant flame. A detailed description of the spectral-fitting routine, data-processing procedures, and determination of the instrument response function are also presented. The accuracy of the diagnostic was validated at 1000 K and pressures up to 40 bar in a heated-gas cell before being applied to characterize the spatiotemporal evolution of temperature and CO in AP-HTPB and AP-HTPB-aluminum propellant flames at pressures between 1 and 40 bar. The results presented here demonstrate that ULAS in the mid-IR can provide high-fidelity, calibration-free measurements of gas properties with sub-nanosecond time resolution in harsh, high-pressure combustion environments representative of rocket motors.

Methane oxidation to methanol over copper-containing zeolite

Methane oxidation to methanol over copper-containing zeolite

Obtaining pseudo-OH* radiation images from CFD solutions of transcritical flames

The quantitative comparison of experimental data and results from CFD simulations is still an ongoing challenge in the investigation of high pressure combustion in rocket combustion chambers. This is due to the extreme environment which develops in liquid propellant rocket engines, which represent a challenge for experimental data collection. OH* radiation emitted from the flame has often been designated as an indicator of the combustion zone, because of its relative ease of detection with appropriate cameras. A method was developed to compare OH* radiation originating from cryogenic oxygen-hydrogen flames in an experimental combustor with the CFD simulation results. Pseudo-OH* images were obtained from CFD results of two combustors. The method consists in obtaining the path of a ray of light by a reverse ray tracing algorithm and sampling the thermodynamic properties along the path of the ray, simulating the emission and absorption spectra in the wavelength range of interest, in this case of OH* emission during combustion. The spectral radiance is then determined by solving the differential radiative transfer equation. Finally, the total radiance is calculated integrating the spectral radiance. The results obtained applying this method are then compared with former results of two test cases, a laminar and a turbulent flame, and with the related experimental data. An improvement of the comparison with the experimental data was achieved in terms of the prediction of self-absorption, which was underestimated in previous works by a factor of 15, and in terms of radiance near the injection plane, where difference is estimated to be about 40% when including refraction. The method allows for more direct comparison between 3D CFD results and 2D experimental images collected by the optical setup and probes.

Experimental and kinetic modeling study on NH3/syngas/air and NH3/bio-syngas/air premixed laminar flames at elevated temperature

Ammonia (NH3) can help achieve large-scale storage and long-distance transportation of renewable energy. The co-firing of NH3 with syngas and bio-syngas and increasing the initial temperature of reactants are economical and effective methods to enhance the reactivity of NH3 flames in boilers and gas turbines. An accurate kinetic model is essential for developing corresponding burners. The current kinetic models are not accurate enough in temperature dependence. Laminar burning velocities (SL) of NH3/syngas/air and NH3/bio-syngas/air mixtures were measured at various H2 contents in syngas and NH3 contents in the mixtures, equivalence ratios of 0.7~1.4, atmospheric pressure, and elevated temperature up to 423 K in a high-pressure constant-volume combustion vessel. A detailed kinetic model was developed for NH3/syngas combustion and validated based on the literature data. The experimental results show that slightly rich flame, high mole fractions of syngas/bio-syngas, and elevated initial temperature contribute to increasing the SL. Kinetic modeling analysis was performed for interpreting the effect of fuel composition, equivalence ratio and initial temperature on the laminar flame propagation characteristics. The elevated H2 content in syngas provides a stronger chemical effect and the enhanced laminar flame propagation of NH3/syngas/air and NH3/bio-syngas/air mixtures is dominated by chemical effect. Due to the presence of diluent (CO2 and N2), thermal effect has a negative influence on the enhancement of NH3/bio-syngas/air flame propagation. Although diluent hardly changes the NH3 consumption pathways, it will significantly reduce the NH3 consumption. Different NH3 consumption pathways are preferred by lean and rich flame. Sensitivity analysis shows that NH2 chemistry is favored by rich flames. Since NH2 + NH = N2H2 + H promotes the production of H radical in rich flames, it has the greatest influence on the SL of rich flames. H2 is more capable of improving the temperature dependence of NH3/syngas/air flame than CO. The increased importance of NH3 chemistry accounts for the strong temperature dependence of NH3/bio-syngas/air flame.

Numerical and experimental study on high-speed hydrogen–oxygen combustion gas flow and aerodynamic heating characteristics

The need to increase the payload capacity of the rockets motivates the development of high-power rocket engines. For a chemical propulsion system, this results in an increasing thermal load on the structure, especially the combustion chamber and nozzle must be able to withstand the extreme thermal load caused by high-temperature and high-pressure combustion gas. In order to protect the structure from the effect of increasing heat flux, it is necessary to counteract such effect with more advanced thermal management technology. This requires us to accurately predict the aerodynamic heating of the structure by high-temperature and high-speed combustion gas. In this study, a high-temperature combustion gas tunnel developed in the laboratory is used to produce high-speed combustion gas. Combined with the results of numerical calculation, the flow and aerodynamic heating characteristics of air and hydrogen–oxygen combustion gas under the same total temperature and pressure are analyzed and compared. The comparison revealed that the combustion gas flow in the nozzle has higher static temperature, velocity, and smaller Mach number. When the combustion gas flows around the sphere, the shock standoff distance and stagnation pressure are smaller than those of air, and the wall heat flux is much larger than that of air. The active chemical reaction in the combustion gas makes the aerodynamic heating of the structure more severe. Finally, through the analysis of a large amount of data, a semi-empirical formula for the heat flux of the stagnation point heated by a high-speed hydrogen and oxygen equivalent ratio combustion gas is obtained.

Enhancement of ammonia combustion with partial fuel cracking strategy: Laminar flame propagation and kinetic modeling investigation of NH3/H2/N2/air mixtures up to 10 atm

The low combustion intensity of ammonia (NH3) raises great research needs of combustion enhancement strategies for its practical applications. Considering the high proportion of hydrogen in cracked gas of NH3, partial fuel cracking is a feasible strategy to enhance NH3 combustion. This work reports an experimental and kinetic modeling study on the laminar flame propagation of partially cracked NH3/air mixtures (NH3/H2/N2/air mixtures) up to 10 atm. Laminar burning velocities (LBVs) of partially cracked NH3/air mixtures are measured at various cracking ratios and equivalence ratios using a high-pressure constant-volume cylindrical combustion vessel. Our recently reported NH3/syngas model is updated to simulate the experimental results, which shows satisfactory performance on predicting the partially cracked NH3/air LBVs in this work, as well as the LBV and speciation data of NH3 and NH3/H2 combustion in literature. modeling analysis is performed to provide insight into effects of equivalence ratio, cracking ratio and pressure on laminar flame propagation of partially cracked NH3/air mixtures. The modified fictitious diluent gas method reported in our recent work is adopted to separate thermal and other effects in enhancement of NH3 combustion. The analysis results indicate that thermal effect only plays a minor role in the enhancement of laminar flame propagation of NH3 in partial fuel cracking strategy, while chemical effect should be significant for the enhanced laminar flame propagation. Due to the presence of H2 in cracked gas, reaction H + O2 (+M) = HO2 (+M) shows enhanced importance and makes the laminar flame propagation of partially cracked NH3/air mixtures more pressure-dependent. Furthermore, NO formation characteristics with increasing cracking ratio is also numerically investigated, which shows that fuel NO is the major NOx formation source. The results reveal a dramatic non-monotonic behavior of NO formation as the cracking ratio increases, which originates from the transition of NH3 chemistry to cracked gas chemistry.

Auto-Ignition and Numerical Analysis on High-Pressure Combustion of Premixed Methane-Air mixtures in Highly Preheated and Diluted Environment

ABSTRACT This work investigates both autoignition and combustion characteristics in highly preheated and diluted combustion of a laminar premixed stoichiometric CH4/O2/N2 mixture in a cylindrical combustor operating at elevated pressures. The analysis was carried out for a range of operating parameters, including reactant preheat temperatures of 1100–1500 K, combustor pressures of 1-10 atm, and in a highly diluted mixture, achieved by decreasing the oxygen content in the oxidizer from 21% to 3% on volume basis. Simulations were conducted using the laminar premixed adiabatic PFR (plug flow reactor) model of Ansys Chemkin Pro. Two-dimensional pictorial representation was performed using the finite volume-based CFD code Ansys Fluent 19.2. Finite-rate chemistry with the detailed chemical mechanism GRI Mech 3.0 was used for combustion analysis. Results showed that OH and HCO mole fractions decreased with increasing combustor pressure and N2 dilution (or decreased O2 content), while the mole fractions increased with reactant temperature. It was also found that, by reducing the oxygen content in the mixture, the flame stabilized far away from the combustor inlet. In contrast, an increase in combustor pressure and reactant temperature stabilized the flame toward the combustor inlet. These flame stabilization characteristics at different locations of the combustor are explained in terms of ignition delay time, which were calculated using the closed homogenous reactor (CHR) model available in the Ansys Chemkin Pro package. The flame peak temperature decreased with increased N2 dilution and increased by increasing the reactant temperature. Moreover, the peak temperature varied marginally when the combustor pressure was increased. Finally, a regime diagram was prepared to show the various combustion modes, such as HiTAC, MILD combustion, and the no ignition region as a function of O2 content and reactant temperature for different operating pressures. The CO and NO emission were reduced with an increase in pressure in the MILD combustion region.

Study of soot formations in co-flow laminar diffusion flames of n-heptane and oxygenated aromatic biofuels from atmospheric condition to 2.3 bar

Anisole and benzyl alcohol, both are lignin based oxygenated aromatic biofuels as a pair of isomers with benzene rings possessing different oxygen functional groups. This work investigated the characteristics of soot formation in laminar co-flow diffusion flames with n-heptane and its blends with anisole and benzyl alcohol from atmospheric to elevated pressures. With the high-pressure burner and pressurized Twin-Lines liquid fuel evaporization (TLLFE) system, a series of experiments were carried out to measure the sooting tendency by utilizing the Laser induced incandescence (LII) combining with Line-of-sight attenuation (LOSA) technique. The experimental results show that with the addition of anisole and benzyl alcohol, the soot formation in flames is enhanced, especially for the addition of benzyl alcohol. Moreover, the elevated pressure can enhance the soot formation in n-heptane and n-heptane/anisole flames, and the integrated soot volume fractions (SVFs) follow power law relationships with pressure. The power dependence on pressure for n-heptane/anisole flame is larger in comparison with that of n-heptane flame. These findings provide basic understanding of the soot formation in co-flow laminar diffusion flames of n-heptane and oxygenated aromatic biofuels.