Combustion and Flame最新文献

Due to the current interest in biomass derived carbon neutral fuels and fuel additives for gasoline engines and to the growing need of understanding the fundamentals of gas phase combustion in the context of wildfires, this study investigates the combustion behavior of key components of biomass pyrolysis oils, namely the three methylanisole isomers (ortho-, meta- and para-methylanisole). Specifically, this study presents the first experimental ignition delay time measurements for such fuels using a shock tube and a rapid compression machine. Ignition experiments were carried out for stoichiometric fuel/air mixtures (φ = 1) at compressed pressure p_c = 10 and 20 bar, covering a temperature range T_c = 880–1220 K. A kinetic model based on previous efforts in the area of oxygenated aromatic hydrocarbon fuels is proposed to reproduce and interpret the experimental findings, specifically focusing on capturing the observed different reactivity of the three isomers. To this aim, thermodynamic properties of primary intermediates and bond dissociation energies were calculated highlighting relevant differences originated from the relative position of the O–CH₃ and –CH₃ substituents. In addition, an isomerization pathway specific to the ortho isomer was theoretically investigated and found to motivate the observed higher reactivity with respect to the meta and para isomers

Ammonia-coal co-combustion can significantly reduce CO₂ emissions from pulverized coal boiler. However, since ammonia is a blended high-nitrogen fuel, an inevitably increases the risk of high NOx emissions. Therefore, in-depth research on the transformation path of fuel-N during ammonia-coal co-combustion is the key to achieving low-nitrogen combustion. Since naturally occurring minerals in coal impact the migration and transformation of fuel-N, we report an experimental study coupled with quantum chemistry calculations to study the generation of nitrogen oxides during ammonia-coal co-combustion, in the presence of the inherent mineral Fe. The experimental results showed that under all the temperatures and ammonia co-firing ratios studied in this work, ammonia-coupled Fe-impregnated pulverized coal inhibited NO generation compared to coal without Fe impregnation. Theoretical calculations provided the possible existing forms of Fe in this system, and revealed the molecular pathways for the oxidation of ammonia-N to the nitrogen-containing intermediates of HNO and NCO, as influenced by the presence of mineral Fe. It was found that with impregnated Fe, the activation energy of the rate-determining step for the oxidation of fuel-N was about 30–40 kJ/mol higher than that without Fe impregnation; thus, Fe reduced the oxidation rate of fuel-N. The theoretical calculations elaborated the mechanism of the inhibited generation of nitrogen oxides with mineral Fe. The results indicated the enhancement of binding energy between nitrogen products and the surface of coal char.

Laser ignition (LI) is promising for green combustion of lean-fuel mixtures with controllable ignition timing and location. It was recently discovered that despite the inferior energy deposition and low thermal temperature in femtosecond (fs) laser-induced plasma, fs laser pulses can achieve a robust ignition of lean-fuel mixture through forming a “line” kernel by filamentation. Here, to clarify fs-LI mechanism, we investigated a dual-color (DC: 800 nm at 1.5 mJ and 400 nm at 0.43 mJ, ∼50 fs) fs-LI of a lean methane/air mixture with an equivalence ratio of φ = 0.87. An optical emission spectroscopy study was conducted to probe the N₂⁺ and OH emissions and characterize the ignition success rate. It was demonstrated that fs-LI can be achieved at a lower minimum ignition energy (MIE) (<0.46 mJ) by the DC scheme than that (>0.7 mJ) by a single-color (SC: 800 nm at 2.0 and 2.4 mJ, ∼50 fs) scheme, indicating a strong wavelength effect on the successful ignition. A pump-probe measurement was carried out to reveal the effect of the ionization enhancement on the successful ignition. It was found that only when the two-color fs pulses are temporally overlapped, the OH yield is strongly enhanced and the MIE is decreased. By comparing the variation trend of the fluorescence intensity of OH with that of the direct ionization product N₂⁺, we ascribed fs-LI to a non-resonant photochemical ignition mechanism, in which the enhancement in the multiphoton/tunnel ionization of the lean-fuel mixture by the high-energy 400-nm photon can increase the yields of the reactive radicals through various dissociation and chain reaction pathways, and thus result in the successful ignition at the micro-joule level. This work unravels the essential role of the non-resonant photochemical ignition mechanism in fs-LI, and provides a promising route for the ignition of lean-fuel engines by compact ultrashort-pulsed lasers in the filamentation regime.

With good mechanical properties and high specific impulse, nitrate ester plasticized polyether (NEPE) is potentially well suited for future high-energy propulsion systems. In this study, ignition and combustion behaviors of NEPE propellants with different graphene oxide (GO) addition contents were investigated under rapid thermal stimulus using an optical rapid compression machine (RCM). Ignition sensitivity was studied by examining the pressure evolutions and high-speed images at different temperatures and pressures. Results show that the lowest ignition temperature for all NEPE propellants decreases with the increase of pressure. At fixed pressure NEPE propellant without GO (GO0) has the lowest ignition temperature among all propellants, indicating that addition of GO decreases the ignition sensitivity of NEPE propellant. Interestingly, with the increase of GO ratio (GO/(GO + CL-20)) from 0.5 % to 2.0 %, the lowest ignition temperature presents a nonmonotonic increase. The addition of 0.5 % and 1.5 % have excellent desensitized effects, which have the potential to be applied in future propulsion systems. Moreover, the hot spots for all samples locate left or right surface of propellant, strong convective heat transfer by the rapid movement of pistons in RCM occurs. Secondly, the burning rate of GO0 is around 11 mm/s at 40 bar and 940 K, as observed by high-speed images. Note that burning rates are increased by 30–45 % with the addition of GO. In addition, simultaneous thermal analysis (STA) experiments were used to reveal the overall reactivity of the samples and NEPE propellant combustion kinetic process under rapid thermal stimulus was illustrated.

This study is the first to present the emission characteristics of the fuel staging method for a pure ammonia-air flame using a coaxial tangential injection combustor. The primary combustor used in this study had the same swirl number of 3.3 as in a previous study; however, the dimension of the combustor was increased to examine at a higher thermal load. The emissions of NO, NO₂, N₂O, and NH₃ were measured simultaneously under an elevated pressure. Consequently, secondary ammonia injection (fuel staging) was confirmed to be effective for NO_x abatement even under pressurized conditions. In addition, excessive secondary ammonia injection exhibited an additional NO_x reduction effect coupled with the generation of unburned N₂O and NH₃. The optimized amounts of NO_x, N₂O, and NH₃ were 83, 23, and 11 ppm (@15 % O₂, dry), respectively. The results of this study are expected to be useful as significant design and operating data for pure ammonia gas turbines.

This study presents a numerical simulation of detonation waves in insensitive high explosives (IHE). A direct numerical simulation (DNS) method of detonation waves propagation was developed. It solves two-dimensional reactive Euler equations by using a semi-discrete node-centered finite-volume (NCFV) scheme on triangle mesh. Employing ZND model analytical solution as the initial condition, the upper and lower boundary conditions were designed as local sonic equilibrium conditions. The DNS method was validated using steady detonation wave propagation experimental results for tri-amino-tri-nitro-benzene (TATB) based explosives. The two-dimensional steady detonation propagation of the circular arc experiments was completed using a non-embedded technique (electric and optical fibre probe velocimetry). The comparison results demonstrate that the numerical method can provide a good prediction of the pseudo-steady-state detonation wave front propagation and the angular speed.

A possible method for a flammability reduction of the polymeric materials is the introduction of flame retardants into their composition. An effective search for optimal flame retardants and an understanding of their inhibition mechanisms requires the extensive information on the chemistry of their transformation in flames. A new data on chemical flame structure of a premixed H₂/O₂/Ar flame doped with 1000 ppm of triphenyl phosphate (TPP) at a pressure of 1 atm was obtained with a molecular-beam mass-spectrometry technique. Among intermediate products of the TPP decomposition were identified: several large phosphorus containing compounds, small phosphorus containing species (PO, PO₂, HOPO, HOPO₂), cyclic hydrocarbons (benzene, toluene, phenol), phenyl and phenoxy radicals. The detailed kinetic mechanism proposed earlier for the thermal degradation of TPP was updated with several new reactions including reactions with common flame radicals H/OH/CH₃. Reaction rate constants were calculated using the Rice-Ramsberger-Kassel-Marcus theory and potential energy surfaces obtained by the DLPNO-CCSD(T)/cc-pVQZ//ωb97xd/6-31G(d) method. A comparison between experimental data and simulation results with the new model has shown a satisfactory qualitative and quantitative agreement, which confirmed that, the TPP decomposition occurs in flame according to the proposed scheme.

Methane has become increasingly popular in rocket propulsion, but low stability and limited flammability range have always been a concern about methane-powered systems. Many stabilization methods have been developed to change the geometrical or flow characteristics of the burner. However, most of these efforts have yet to be practically successful due to cost and compatibility issues. Alternatively, other methods such as microwave, dielectric barrier, and nanosecond repetitive pulse (NRP) discharges have been proven to be efficient by modifying the kinetic and transport pathways. NRP discharges have shown promising results as one of the most effective low-temperature plasma (LTP) methods. In this paper, chemiluminescence imaging is used to study the effect of NRP discharge on liftoff and blowout, as the important stabilization parameters, by recording the liftoff height and liftoff/blowout velocities under a wide range of discharge ($f$=0–10 kHz and $V$=11–19 kV) and jet velocity ($\nu$=2–60 m/s). Depending on these parameters, four different discharge regimes of corona, diffuse, filamentary, and arc were observed. The results have shown that high-intensity plasma in a filamentary discharge regime can provide a significant advantage in delaying the liftoff conditions, but no improvements in the blowout were observed. It was also found that NRP discharge can reduce the liftoff height. To explore the cause of the increased stability, a parametric numerical study is conducted using detailed plasma-assisted methane kinetic modeling coupled to a 1D opposed diffusion flame simulation. Results show that the extinction limits of diffusion flames can be dramatically enhanced by LTP due to the local formation of high radical and excited species concentrations, with subsequent recombination leading to increased temperature and higher reactivity in the flame zone. In addition, a 1D laminar flame speed evaluation shows that the plasma-generated active species can dramatically increase the flame speed, which, in turn, reduces the lifted flame height above the burner surface.

Variable-thrust cryogenic propellant rocket engines are gaining significant research interest in space exploration. However, low-frequency unstable combustion is still a challenging scenario, especially in fuel-rich preburner and low-thrust operating conditions of deep variable thrust. To deeply understand the mechanism of low-frequency unstable combustion, chemiluminescence images of CH* and background light images of the spray were obtained synchronously by chemiluminescence imaging and laser background light imaging, respectively. Both the spray and flame stabilization of liquid oxygen/methane swirl coaxial injector were studied through the continuous regulation of liquid oxygen mass flow rate. The results showed that low-frequency unstable combustion occur in both the start-up stage and the throttled stage under the fuel-rich condition at a frequency of 39.1∼48.1 Hz and an amplitude of 30% of the average combustor pressure. It is found that the two-phase flow instability of liquid oxygen is likely to induce spray and flame instability, resulting in low-frequency unstable combustion. The dimension subcooling degree of liquid oxygen is an important factor affecting unstable combustion. When the dimensionless subcooling degree is larger than 0.7, the low-frequency unstable combustion is suppressed. On the other hand, as the mixing ratio decreases, the flame oscillation mode gradually transforms from the longitudinal oscillation mode to contraction/expansion mode. Flame filling up and flashback processes can be observed in both flame oscillation modes, and the entropy coupling mechanism of the oscillation mode is explained in detail. Furthermore, an oscillation period is determined to include four processes: propellants filling up and flame liftoff; heat release of the combustion products and entropy disturbance; acceleration of the entropy wave through the nozzle creating an acoustic disturbance; and flame flashback, in which the heat release time of combustion products and entropy disturbance is the longest.

Normal shock wave reflection driven detonation initiation was investigated in a 7.63-cm square cross-section shock tube using novel stereo visualization. High-speed schlieren video captured shock reflection and detonation onset through the sidewall of the shock tube, and simultaneously, self-luminous imaging through the end wall enabled the determination of the location of detonation onset in three-dimensional space. Tests were carried out with nitrogen- diluted stoichiometric ethylene-oxygen. Weak and strong detonation initiation modes were observed at the low and high end of the reflected shock temperature range tested, respectively. A novel reflected shock bifurcation driven weak detonation initiation mode was observed at intermediate temperatures where a flame, ignited at one, or multiple, reflecting wall corners, accelerated along the channel wall corner reaching the bifurcated reflected shock wave. The flame rapidly spread through the bifurcation stagnation bubble transitioning to detonation. The reflected shock temperature range that this new weak initiation mode was observed depends on the reflected shock pressure. For low reflected shock pressures, typical of historic detonation initiation shock reflection studies, this weak bifurcation mode was not observed. Nonreactive three-dimensional Navier-Stokes simulations showed that the reflected shock bifurcation generates flow conditions along the channel wall corners that can explain the observed flame propagation from the end wall to the stagnation bubble, and the subsequent rapid spreading in the stagnation bubble.