Combustion and Flame最新文献

Starting with a detailed-chemistry description involving 20 elementary steps for hydrogen oxidation and 40 elementary steps for ammonia oxidation, it is shown that systematic application of sensitivity analyses of premixed flames under typical gas-turbine combustion conditions reduces the description to 12 elementary steps for hydrogen oxidation, 4 of them being reversible, and an additional 19 steps for ammonia oxidation, 6 of them being reversible, yielding reasonable predictions for auto-ignition and deflagration processes. Subsequent introduction of steady-state approximations for chemical intermediates, afforded by the high-pressure conditions existing in gas-turbine combustion chambers, effectively reduces the fuel-oxidation description in systems utilizing H${}_2$-NH${}_3$ fuel mixtures to two global steps for deflagrations, namely, 2H${}_2+O_2\rightleftharpoons$ 2H${}_2$O and 4NH${}_3$ ＋ 3O${}_2$ $\rightleftharpoons$ 2N${}_2+6H_2$O. Analytical expressions for the associated overall rates, involving the local temperature and the O${}_2$, H${}_2$, NH${}_3$, N${}_2$, and H${}_2$O concentrations, are derived through selective truncation of the steady-state expressions, resulting in a simplified chemistry description that can facilitate future numerical analyses based on direct-numerical and large-eddy simulations.

Novelty and significance statement

A new short mechanism involving only 31 elementary reactions between 16 reactive species has been derived for hydrogen-ammonia oxidation under conditions of pressure, temperature and dilution typically found in gas-turbine burners. Introduction of steady-state assumptions for all intermediate species leads to a two-step mechanism that is shown to predict burning rates with sufficient accuracy. The proposed mechanism can significantly reduce computational times in future direct-numerical and large-eddy simulations.

This study presents the first kinetic mechanism for methyl tert-butyl ether (MTBE) containing low-temperature chemistry, as well as the first mechanism for 2,2-dimethoxypropane (DMP). The mechanisms have been validated against ignition delay time experiments conducted in a high-pressure shock tube and rapid compression machine. The rapid compression machine conditions were set to 20 and 40 bar for MTBE, and 10 and 20 bar for DMP in a temperature range of 584 to 946 K. Shock tube experiments have been performed for DMP at 20 and 40 bar for stoichiometric and fuel-lean conditions in air at temperatures ranging from 913 to 1173 K. A pronounced negative temperature coefficient regime with two-stage ignition has been observed for both fuels. The developed mechanism consists of reaction rate constants that were primarily modeled in analogy to iso-octane and dimethoxymethane, and calculated thermodata on the G4//B3LYP-D3BJ/def2-TZVP level of theory. Simulations have been performed to analyze the fuel oxidation at different temperatures. Over the full temperature range, H-atom abstraction occurs mainly on the $\alpha$-side for DMP and on the $\beta$-side for MTBE. At low temperatures, both fuels isomerize to the peroxy radical. The dominant MTBE radicals then tend to produce cyclic ether, while the DMP radicals react with $O_2$, enabling significant chain branching and explaining the higher reactivity of DMP. With rising temperature, $\beta$-scission of the fuel radicals and unimolecular elimination reactions start to dominate the oxidation process.

Novelty and Significance Statement

The novelty of this research is the first observation of a two-stage ignition of methyl-tert butyl ether (MTBE) in an RCM and a discussion of its fundamental ignition chemistry, which is based on a developed detailed kinetic mechanism. In addition, 2,2-dimethoxypropane (DMP) has been investigated experimentally and theoretically to explain the difference in reactivity to MTBE despite their strong molecular similarity. The experiments include RCM and shock tube experiments. The kinetic model is based on rate constant analogies and newly calculated thermo data on the G4//B3LYP-D3BJ/def2-TZVP level of theory. This work is significant, as MTBE is still a widely used octane booster and the developed model could help to improve engine simulations. Furthermore, the findings of DMP provide insights into future fuel design.

Hydrothermal carbonization (HTC) can transform a wide range of biomass into renewable biofuels. Hydrochar, the solid fuel resulting from HTC, has a similar energy density to low-rank coals. Despite an extensive body of literature detailing the thermogravimetric analysis (TGA) of hydrochar under slow pyrolysis and oxidation, there remains a limited understanding of hydrochar's behavior in realistic combustion settings. In this work, we integrate combustion experiments and TGA to understand the combustion characteristics of a model hydrochar fuel. Cellulose-based hydrochars produced at different temperatures along with solvent-extracted chars are tested in a Hencken burner across varying temperatures and oxygen concentrations in the oxidizer gas. Simultaneous CH* chemiluminescence, particle image velocimetry, and two-color pyrometry are used to measure ignition delay time and identify homogeneous/heterogeneous ignition mechanisms and combustion phases. Incorporating TGA with combustion results shows that the ignition mode and combustion processes are strong functions of surrounding gas temperature and oxygen mole fraction. The level of carbonization of the hydrochar dictates the ignition delay time and combustion modes. Further, the presence of a tar-like secondary char on the as-carbonized hydrochars leads to more rapid ignition due to high volatility.

Scramjet combustors feature ultra-high-speed turbulent reacting flows with high temperatures and intense luminescence. Measurement of velocity fields under such extreme conditions presents great challenges. The present work demonstrated a seedless velocimetry approach using focusing schlieren images (FSIs) of high spatiotemporal resolution in a scramjet engine. Two fuel mass flow rates (Case1 and Case2) were investigated with corresponding global equivalence ratios of 0.27 and 0.13, respectively. The FSIs enabled by the employment of a high-speed pulsed LED light source are characterized by an effective exposure of 100 ns, and a 500-ns frame-straddling time interval with full resolution of 1280 × 800 pixels recorded at 76 kHz. The 100-ns exposure allows for capturing of transient high-speed flow motion without blurring, and the 500-ns time interval ensures an appropriate spatiotemporal correlation between subsequent schlieren images for high-speed reacting flows. A wavelet-based optical flow velocimetry (wOFV) algorithm was developed and applied to the FSIs. In contrast to the correlation-based algorithms widely employed in PIV for distinct particles, the wOFV algorithm suits better FSIs with continuous variation in brightness. The maximal velocities in the main duct of the scramjet combustor were measured to be approximately 550 m s^-1 and 1100 m s^-1 for Case 1 and Case2, suggestively corresponding to subsonic and supersonic combustion modes, respectively. The measured velocity inside the cavity is generally below 200 m s^-1 for both cases. Recirculation regions and their dynamic motions inside the cavity were well resolved. In summary, the development of the novel velocimetry approach holds great potential for applications in extreme flow conditions.

Novelty and Significance Statement

: Present work demonstrates a seedless velocimetry approach based on high-speed frame-straddling focusing schlieren imaging coupled with the novel wavelet optical flow velocimetry (wOFV) algorithm. Velocity field measurement realized in a scramjet combustor with high spatiotemporal resolution (1280 × 800 pixels at 38 kHz) for the first time, showing great abilities to accommodate wide velocity range and to resolve dynamic flow characteristics with potentials for broader future applications.

Ammonium perchlorate (AP) is a commonly used ingredient in solid propellants and explosives. As a result, many experimental and theoretical studies have investigated AP to elucidate its condensed phase and gas phase decomposition behavior. In various experimental efforts, including both fast and slow thermolysis, the results indicate that major species such as H₂O, O₂, NO₂, N₂O, Cl₂, HCl, ClO₂, ClO, HOCl, and HClO₄, etc., are the products of decomposition of AP. The objective of this work is to analyze the decomposition of AP in the condensed phase using density functional theory (DFT) at the B3LYP/6-311++G(d,p) level and CBS-QB3 methods. The polarizable continuum model, using the integral equation formalism variant (IEFPCM), was used to account for the condensed phase. Three pathways – 1) NH₃ oxidation, 2) NH₂ consumption, and 3) HNO₃ and NOx formation - are identified, which explain the experimentally observed major gas phase spices. The study focuses on the oxidation reactions involving NH₃, unveiling complex sequences of reactions leading to the formation of various nitrogen and chlorine-containing species. The oxidation pathways include reactions with HClO₂ and O₂, resulting in the production of radicals such as NH₂, Cl, and OH. The subsequent reactions explore the interactions of these radicals, leading to the formation of HNO₃ and other nitrogen oxide (NOx) compounds. These reactions can assist in the development of a detailed liquid-phase chemical kinetics mechanism of AP-containing propellants.

Chemiluminescent emissions from NH₂*, NH*, NO*, and OH* during the pyrolysis and oxidation of ammonia (NH₃) are quantitatively characterized to gain insight into their reaction mechanisms. Time profiles of light emitted from high-temperature reactions of NH₃/Ar and NH₃/O₂/Ar mixtures have been measured behind reflected shock waves at temperatures of 2300–2600 K and pressures of 1.6–1.9 bar in a high-repetition-rate shock tube. The emission intensities have been calibrated based on the well-characterized OH* chemiluminescence in a hydrogen-oxygen system and converted to photon emission rates for quantitative comparison with kinetic simulations. A kinetic model describing the pyrolysis and oxidation of ammonia and reactions of excited species has been constructed by combining the reaction mechanisms proposed in recent modeling studies. With only modest updates of the thermodynamic functions and the rate constants for the formation and quenching of excited species, the observed chemiluminescence profiles could be reasonably reproduced, with a few exceptions. The rate of production analysis indicates that NH₂* is produced by the reaction of NH₃ with H as well as thermal excitation of NH₂, that the energy transfer reactions from ³N₂ to NH and NO are responsible for the formation of NH* and NO*, respectively, and that the formation of OH* is competitively contributed by the reactions of H with N₂O, ³N₂ with OH, and NH with NO. Remaining discrepancies between the experiment and modeling are noted, and potential directions for further model improvement are discussed.

Nitrocyclohexane (NCH) is a model nitroalkane compound with naphthenic structural groups for studying the combustion properties of nitro-based energetic fuels, and also demonstrates significant potential applications for advanced engines. However, accurate description of the detailed kinetic mechanisms for pyrolysis and combustion of NCH receives little attentions. Herein, the detailed pyrolysis and oxidation mechanism of NCH is studied by using quantum chemistry calculations and chemical kinetic theory. The potential energy surfaces of the CN bond dissociation reaction, HONO elimination reaction, abstraction reactions of NCH as well as the subsequent β-scission reactions are calculated using G4//M06–2X/6–311++G(d, p) method, which are subsequently employed for reaction rate constant computations via transition state theory (TST) and RRKM/master-equation (RRKM/ME) method. The ab initio chemical kinetic study results are used to develop a detailed kinetic mechanism to predict the pyrolysis experimental results of NCH, which are performed using a single-pulse shock tube (SPST). The SPST experiment is performed at pressure of 10 bar, temperature ranging from 1070 to 1560 K, reaction time of around 1.77 microsecond with fuel concentrations of 0.5 % diluted by Argon gas. The developed detailed kinetic mechanism exhibits reasonable prediction results of the pyrolysis product distributions together with literature ignition delay times across a wide range of temperature, pressure, and equivalence ratio conditions. Rate-of-production analysis is conducted to gain insight into the pyrolysis chemical kinetics of NCH. The present work should be valuable for the study of the combustion chemistry of nitro-alkane energetic fuels.

This study presents experimental and modeling results of the 1,2,4-trimethylbenzene (T124MBZ) oxidation in a jet-stirred reactor at equivalence ratios of 0.4 and 2.0, temperature range of 450–1000 K, and pressure of 12.0 atm. Compared with the oxidation at atmospheric pressure, a pronounced negative temperature coefficient (NTC) behavior of T124MBZ was observed between 622 K and 773 K under fuel-lean condition. A detailed chemical kinetic model involving 865 species and 5092 reactions was developed. In addition, the model was validated against experimental results of laminar burning velocities and ignition delay times. The present model reasonably reproduces these experimental data. The dominant consumption channels for T124MBZ are H-abstraction reactions on methyl groups by OH radicals. The H-abstraction reactions on the 1- and 2- methyl sites by OH radicals are the most promoting in pre- and mid- NTC phases, and H₂O₂(+M) = 2OH(+M) is the most promoting in the post-NTC phase, while the reaction on the 4-methyl site is the most inhibiting across pre-, mid-, and post-NTC phases. This difference arises from the fact that the dominant consumption pathway for 2,4-dimethylbenzyl and 2,5-dimethylbenzyl involves H-transfer, while the 4-methyl site cannot undergo an H-transfer reaction due to the absence of adjacent methyl groups. The competition between the two consumption channels of ·QOOH radicals (decomposition and second O₂ addition), resulting in the reduction of OH radicals as temperature increases, leads to diminished OH production. This causes the emergence of pronounced NTC behavior at high pressure.