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Detailed chemical kinetic models for the low-temperature combustion of hydrocarbons with application to gasoline and diesel fuel surrogates

Modeling of turbulent dilute spray combustion

The main goal of this paper is the extension of a general semidetailed kinetic scheme of hydrocarbon oxidation to heavy fuels. Taking previous n-pentane and n-heptane kinetic modeling experience a step further, a lumped approach was systematically applied to n-decane, n-dodecane, and n-hexadecane. These semidetailed submodels for higher n-alkanes are directly derived from the complete set of primary propagation reactions, automatically generated by the MAMOX++ program on the basis of a small set of reference kinetic parameters. This proposed approach and lumped model are supported by a wide range of comparisons with a large variety of experimental measurements relating to stirred and flow reactors, premixed and diffusion flames, and fuel droplet combustion under microgravity conditions. The general agreement obtained in the overall range of conditions further confirms and supports the soundness of the small set of kinetic parameters used as reference values for the different classes of the primary propaga...

Wide-Range Kinetic Modeling Study of the Pyrolysis, Partial Oxidation, and Combustion of Heavy n-Alkanes

Autoignition of Jet-A and mixtures of benzene, hexane, and decane in air has been studied using a heated shock tube at mean post-shock pressures of 8.5 ± 1 atm within the temperature range of 1000–1700 K with the objective of identifying surrogate fuels for aviation kerosene. The influence of each component on ignition delay time and on critical conditions required for strong ignition of the mixture has been deduced from experimental observations. Correlation equation for Jet-A ignition times has been derived from the measurements. It is found that within the scatter of experimental data dilution of n -decane with benzene and n -hexane leads to slight increase in ignition times at low temperatures and does not change critical temperatures required for direct initiation of detonations in comparison with pure n -decane/air mixtures. Ignition times in 20% hexane/80% decane (HD), 20% benzene/80% decane (BD) and 18.2% benzene/9.1% hexane/72.7% decane (BHD) mixtures at temperature range of T  ≅ 1450–1750 K correlate well with induction time of Jet-A fuel suggesting that these mixtures could serve as surrogates for aviation kerosene. At the same time, HD, BD and BHD surrogate fuels demonstrate a stronger autoignition and peak velocities of reflected shock front in comparison with Jet-A and n -decane/air mixtures.

Autoignition of surrogate fuels at elevated temperatures and pressures

Soot formation is compared in turbulent diffusion flames burning a commercial Diesel and two Diesel surrogates containing n-decane and α-methylnaphthalene. A burner equipped with a high-efficiency atomisation system has been specially designed and allows the stabilisation of liquid fuels flames with similar hydrodynamics conditions. The initial surrogate composition (70% n-decane, 30% α-methylnaphthalene) was previously used in the literature to simulate combustion in Diesel engines. In this work, a direct comparison of Diesel and surrogates soot tendencies is undertaken and relies on soot and fluorescent species mappings obtained respectively by Laser-Induced Incandescence (LII) at 1064 nm and Laser-Induced Fluorescence at 532 nm. LIF was assigned to soot precursors and mainly to high-number ring Polycyclic Aromatic Hydrocarbons (PAH). The initial surrogate was found to form 40% more soot than the tested Diesel. Consequently, a second surrogate containing a lower α-methylnaphthalene concentration (20%) has been formulated. That composition which presents a Threshold Soot Index (TSI) very close to Diesel one is also consistent with our Diesel composition that indicates a relatively low PAH content. The spatially resolved measurements of soot and fluorescent soot precursors are quite identical (in shape and intensity) in the Diesel and in the second surrogate flames. Furthermore the concordance of the LII temporal decays suggests that a similar growth of the primary soot particles has occurred for Diesel and surrogates. In addition, the comparison of the LII fluence curves indicates that physical/optical properties of soot contained in the different flames might be similar. The chemical composition present at the surface of soot particles collected in Diesel and surrogate flames has been obtained by laser-desorption ionisation time-of-flight mass spectrometry. An important difference is found between Diesel and surrogate samples indicating the influence of the fuel composition on soot content.

Experimental comparison of soot formation in turbulent flames of Diesel and surrogate diesel fuels

To provide an aircraft propulsion system which can secure the optimum thrust and thrust vector for flight conditions, as well as the optimum sectional area for the engine, and which is highly compatible with the environment. An electrical generator is coupled to a turbofan engine, the electrical generator is driven by output power of the turbofan engine to output electric power, and an electromagnetic driving fan is driven by the electric power. On the other hand, after bringing each of coils in the electromagnetic driving fan to a superconductive state, liquid hydrogen is introduced to a heat exchanger, collects the energy of exhaust as heat, is then vaporized, and thereafter supplied to a combustor and to a fuel cell. Further, the electromagnetic driving fan is changed in its rotational phase by a rotating mechanism portion, is made movable in a width direction of a wing and a wing chord direction by a slide mechanism portion, and can be stored inside or outside the wing by a storage mechanism portion.

Aircraft propulsion system

Pressure effects on combustion of methanol and methanol/dodecanol single droplets and droplet pairs in microgravity

Spontaneous ignition of isolated two-component fuel droplets has been experimentally studied. The components were n-decane (ND)/1-methylnaphthalene (MN), or ND/1,2,4-trimethylbenzene (TMB). Both are n-alkane/aromatic mixtures, and therefore are candidates for model fuels of multicomponent commercial fuels refined from crude petroleum. ND showed two-stage ignition behavior, while the other two fuels (aromatic hydrocarbons) did not and were less reactive. A suspended droplet was suddenly brought to a high temperature for ignition. Observation by a Michelson interferometer detected cool-flame appearance as well as hot-flame appearance. The ambient gas was air, and the droplet diameter was 0.7 mm. The experimental conditions that were varied were ambient temperature, Ta, (500–1000K), ambient pressure, Pa, (0.1–2.0 MPa), and the initial mole fraction of ND in the liquid phase, Z (0–1). In the case Z≠1, Pa was fixed to 0.3 MPa. Ignition delays for cool-flame and hot-flame appearance (t1 and ttot) and the difference between them (t2) were measured, and ignition types (no ignition, cool-flame only, singlestage ignition, and two-stage ignition) were classified on a “Z-Ta” map. It is common to ND/MN and ND/TMB that two-stage ignition region is narrowed and finally vanishes as Z decreases, and that t1, t2, and ttot all increase monotonically as Z decreases. However, TMB was affected more than MN because of its higher volatility. (Normal boiling point: ND 447.3 K, MN 517.9 K, TMB 442.5 K) Numerical simulation with a fully transient one-dimensional model was employed to help the interpretation.

Effects of dilution by aromatic hydrocarbons on staged ignition behavior of n-decane droplets

This paper presents results of an experimental investigation on acoustic effects on combustion of single droplets and droplet pairs in microgravity. The ambient gas was air at atmospheric temperature and pressure, with octane as the fuel. A loudspeaker at the bottom of the chamber produced the acoustic field. Experimental results of single droplets showed that at low frequency and small to moderate acoustic intensities the evaporation rate increases, and the burning rate constant is nearly proportional to the product of frequency, f, and square of displacement, Xa2, fXa2. At higher acoustic intensities, the burning rate constant either remains constant or decreases, and in some cases, flame extinction occurs at a finite droplet diameter. The burning rate constant for a droplet pair is consistently lower than that for a single droplet. At lower frequencies, the burning rate constant reaches a maximum at an intermediate acoustic intensity. At higher frequencies, the burning rate constant increases monotonically with increasing acoustic intensity. The flame size decreases as a result of interactions, as does the critical spacing that indicates a merged flame around the droplet pair versus individual flames surrounding the droplets. The results also show that interactions stabilize the flame, in that droplet pairs burn to completion under conditions in which the flame surrounding a single droplet extinguishes at a finite droplet diameter.

Keiichi Okai

Papers

Aircraft propulsion system

Pressure effects on combustion of methanol and methanol/dodecanol single droplets and droplet pairs in microgravity

Effects of dilution by aromatic hydrocarbons on staged ignition behavior of n-decane droplets

Combustion of single droplets and droplet pairs in a vibrating field under microgravity

Design Study on Hypersonic Engine Components for TBCC Space Planes