Combustion and Flame最新文献

Non-vertical turbulent non-premixed jet flame occur in combustion systems and by accidental release of fuels caused fires. Understanding the combustion dynamics of these asymmetric diffusion jet flames is needed to characterize the flame morphology and entrainment, which is a foundation for further detailed analysis of the physics and implications. This paper investigates experimentally and theoretically the flame morphological characteristics of non-vertical turbulent jets at both positive- and negative inclined angles in a systematic way. A total of 168 experimental cases were considered involving various initial fuel flow rates, inclined angles and nozzle diameters. Dimensional analysis was performed taking into account the controlling parameters of the physical mechanisms, namely the momentum flux, the fuel mass flow rate, the flame buoyancy, the stoichiometric ratio and the jet initial inclined angle, which together determine a characteristic length scale and a characteristic volumetric flow rate. These two characteristic parameters provide successful non-dimensional correlations for the location of the flame tip. A new integral model was also developed physically considering the momentum and continuity equations along the trajectory to predict the flame tip, as well as the lowest point for negative inclined jets. By comparing the experimental correlations with numerical prediction, the effective air entrainment coefficients were derived for different jet initial inclined angles, which showed little change with inclined angle from positive to negative, but the constant relating the average values to the integrals of the (Gaussian) radial profiles decreased as the initial angle decreased from positive to negative. This change is related to the variation of the profile properties normal to the trajectory as well as to mass detrainment laterally escaping from the jet flame especially for negative inclined angles. Finally, the local Richardson number and the flame lowest point for negative inclined angles was numerically predicted and compared well with the experimental results.

Knowledge of the underlying physical mechanisms responsible for the triggering of high-frequency transverse combustion dynamics is of fundamental importance in the development of heavy-duty gas turbine combustors, aircraft engine afterburners, and bipropellant liquid rocket engines. Detailed information about three-dimensional thermoacoustic interactions and local flame dynamics, however, remains largely unknown and unanticipated, mainly because high-amplitude transverse mode instabilities are challenging to excite and detect in well-controlled sub-scale laboratory environments. To overcome this impasse, here we exploit a spatially tailored rectangular injector assembly consisting of ten equidistant horizontal slit nozzles to eliminate the complications of out-of-plane flame dynamics characterization. A total of 56 datasets of self-induced instabilities were acquired over a wide range of operating conditions to understand spatiotemporal phase dynamics and important mode shapes, in conjunction with 2D Rayleigh angle reconstruction and phase-resolved OH PLIF-based local flame front identification. Experimentally, we show that high-frequency transverse instabilities are excited only under high temperature and high thermal power conditions, manifested as non-evanescent pressure fluctuations at 6.50 kHz strongly coupled to the second-order tangential mode of the rectangular combustion chamber. Two vertically-oriented pressure nodal planes and the characteristic phase transition perpendicular to the horizontal slit injector direction are accurately measured and reconfirmed by Helmholtz simulations in terms of their interpositions and spatial orientation. Remarkably, the periodic formation of co-propagating coherent structures and concomitant local flame displacement/pinch-off are revealed to play an important role in driving the high-frequency hydrogen combustion dynamics.

The thermal behaviors of high-temperature particle (HTP) and low-temperature particle (LTP) are investigated based on the bench-scale impinging entrained-flow coal-water slurry (CWS) gasification experimental platform with a modified visualization system. The size and velocity distribution of both particles, and the evolution of HTP are analyzed through the algorithmic and precise processing of the image sequences. In addition, in-situ temperature diagnosis of the particles during the reaction process were realized. The typical evolution process and the temperature of HTP in char oxidation stage are obtained. The results show that the concentration of HTP in the gasifier is greater than LTP, but the particle size is relatively small. Particles moving at the low speed (0–2 m/s) account for the largest proportion of both HTP and LTP. The char oxidation process lasts over 300 ms and can be divided into three reaction stages. During the reaction, the peak temperature at the center of the particle can reach more than 2000 K. The average temperature of the particles gradually increased, reaching a peak in reaction stage II (1500 K) followed by a gradual decrease. The particle temperature is affected by O/C and is prone to experience swelling and bubbling phenomena during char oxidation.

In this work, the phenomenon of cycle-to-cycle variability (CCV) of combustion in a spark ignition engine is investigated to give a deeper understanding of CCV generation. The main objective is to localize within the cylinder and all along the engine cycle the flow variabilites and identify some driving mechanisms originating in the flow structures and leading to combustion variabilites. In Part I , the application of empirical mode decomposition methods combined with topology-based techniques to the LES flow results allowed the extraction of the large-scale flow motion from the small-scale turbulence and the follow-up of their evolution during compression stroke [1]. A link was then established between the combustion process and the tumble formation and destabilization near BDC. In the present paper, the overall tumble motion development during compression and intake strokes is quantitatively analyzed, and links are built between different engine phases to establish the cause-and-effect chain. Other CCV factors, such as spray injection and exhaust gas recirculation, were not included in the current study. However, the developed methodology for in-cylinder flow analysis could be used in studies on other engine configurations to improve the development of engine designs.

Novelty and significance statement

In this work, the cycle-to-cycle variability (CCV) of combustion in a spark ignition engine is investigated to give a deeper understanding of CCV generation. The present study focuses on CCV caused by the stochastic nature of internal turbulent flow structures. LES approach is chosen due to its ability to capture CCV, and advanced flow analysis tools are developed and applied to LES results to characterize instantaneous flow structures of different scales in the three-dimensional domain and separately quantify their impacts on combustion.

A first important finding is that flow-wall interactions near BDC determine the tumble evolution.

A second novelty is the characterization of several 3D dominant flow interactions during intake yielding large-scale flow variability.

A third novelty and important finding is that links are found between the flow organization during intake, the tumble development, and destabilization during early compression and the combustion. Throughout our analyses starting from the spark timing and going back to the early intake phase, a cause-and-effect chain is finally established between the development of in-cylinder flow and the combustion variability.

In this paper we consider a classical model of gasless combustion in a one dimensional formulation under the assumption of ignition temperature kinetics. We study the propagation of flame fronts in this model when the initial distribution of the solid fuel is a spatially periodic function that varies on a large scale. It is shown that in certain parametric regimes the model supports periodic traveling fronts. An accurate asymptotic formula for the velocity of the flame front is derived and studied. The stability of periodic fronts is also explored, and a critical condition in terms of parameters of the problem is derived. It is also shown that the instability of periodic fronts, in certain parametric regimes, results in a propagation-extinction-conduction-reignition pattern which is studied numerically.

Novelty and significance statement: This work provides a closed form asymptotic description of periodic traveling fronts in a gasless combustion model with step-wise ignition temperature kinetics with a slowly varying concentration field. The stability analysis is performed, and the range of applicability of asymptotic formulas is given. A new propagation-extinction-conduction-reignition regime is identified. This regime emerges exclusively due to periodicity of the concentration field.

Due to the characteristics of heat absorption and decomposition, endothermic hydrocarbon fuels (EHFs) have been widely used in scramjets for thermal protection and heat recirculation. The understanding of ignition characteristics of EHFs is of great importance for their safe and efficient utilization. In this paper, the ignition processes of EHFs were numerically simulated at atmospheric pressure and with an initial temperature of 500 K. Three different ignition stages were identified based on the chemical heat release and flame kernel propagation. A 3-component kerosene surrogate model composed of n-dodecane, methyl cyclohexane and m-xylene was adopted, as well as the corresponding chemical kinetic model with 369 species and 2691 reactions. Results showed that the discrepant decomposition characteristics of n-alkanes and cycloalkanes affected the chemical heat release and propagation during the ignition process. Two-stage exothermic characteristic was observed in the time evolutions of chemical heat release rate and fuel decomposition. The mass production of molecules and accumulation of radicals dominated the first and second exothermic peaks, respectively. Furthermore, the minimum ignition energies (MIEs) of EHFs with various methyl cyclohexane were determined to quantify the effect of fuel composition on ignition performance. Characteristically, the MIE dramatically decreased from 10.2 to 2.15 mJ when 20% n-dodecane was replaced by methyl cyclohexane. However, it was slightly increased as methyl cyclohexane continued to increase. Analyses from both physical and chemical aspects were conducted to elaborate the dependence of MIE on fuel composition. The dominant effects of flame-dynamic and chemical effects on different ignition stages were analysed. The faster propagation speed and stronger endothermic ability of methyl cyclohexane led to the nonlinear variation of MIEs. The results in this study provide useful guidance for composition optimization and safety evaluation of EHFs.

An improved consistent soot model is proposed and applied to evaluate the effect of 2-butanone addition (10 % to 50 % on a volume basis represented as case 1 to case 5) to ethylene fuel on soot formation using a counterflow burner configuration. The predictive capability of the suggested soot model is verified by assessing its performance against existing experimental data on soot formation (SF) configuration-type ethylene counterflow flames at diverse strain rates and various fuel additives. The proposed soot model comprises 55 inception reactions with temperature-dependent collision efficiency and 10 condensation reactions from 10 PAH species (from naphthalene to larger PAHs up to coronene), including modified HACA surface growth and oxidation reactions. 2-butanone is produced as a byproduct during the pyrolysis of biomass and the microbiological fermentation of agricultural waste. It holds various benefits as a prospective biofuel for spark ignition (SI) engines. Limited information exists regarding its sooting characteristics due to a lack of available soot measurements. The simulations are conducted for the soot formation (SF) type counterflow flames with a fixed fuel and oxidizer jet velocity. The proposed soot model can effectively replicate both the qualitative and quantitative aspects of the experimental trends and shows a better agreement than the existing models available in the literature. The soot volume fraction (SVF) and the particle number density (PND) decrease with increasing the 2-butanone concentration in the binary fuel mixture. The PAH concentration decreases with increasing 2-butanone addition in the fuel mixture. The peak SVF and the maximum temperature are reduced by ∼22.7 % and ∼3.6 %, with a 40 % increase in the 2-butanone portion in the fuel mixture from case 1 to case 5. Increasing the 2-butanone content in the fuel mixture decreases the inception rate, HACA rate, and condensation rate while it increases the oxidation rate.

Understanding, modeling, and reducing the cycle-to-cycle variability (CCV) of combustion in internal combustion engines (ICE) is a critical challenge to design engines of high efficiency and low emissions. A high level of CCV may contribute to partial burn, misfire, and knock in extreme engine cycles, which affects engine performance and eventually damages the engine. The origins of CCV have been studied both experimentally and numerically, and the variability of in-cylinder aerodynamics is recognized as one of the most important sources of CCV. However, a detailed and quantitative explanation of how in-cylinder flow CCV is generated is not yet clear. The objective of the present study is to develop a methodology to localize inside the chamber of a spark-ignition engine (SIE) the origins of flow variabilities and to identify some driving mechanisms leading to combustion variabilities. Multi-cycle wall-modeled large-eddy simulations (LES) for the TU Darmstadt optical engine under fired conditions are performed using the CFD solver Converge 3.0. The evolution of organized large-scale structures and the small-scale turbulence of the in-cylinder flow are analyzed using a developed methodology that includes the empirical mode decomposition (EMD) method adapted for 2D and 3D flow fields, and a vortex identification tool ${\Gamma }_{3p}$. The contributions of different parts of the flow to CCV are quantified. In Part I of this work, the LES framework is validated against experimental data, and CCV of large-scale structures is characterized at spark timing. In Part II, the overall flow development during compression and intake strokes are quantitatively analyzed, and links are built between different engine phases to establish the cause-and-effect chain. Other CCV factors, such as spray injection and exhaust gas recirculation, are not included in the current study. However, the developed methodology for in-cylinder flow analysis could be used in studies on other engine configurations to improve the development of engine designs.

Novelty and significance statement

In this work, the cycle-to-cycle variability (CCV) of combustion in a spark ignition engine is investigated to give a deeper understanding of CCV generation. The present study focuses on CCV caused by the stochastic nature of internal turbulent flow structures. LES approach is chosen due to its ability to capture CCV. The LES methodology was validated in a motored case in Ding et al. (2023). In the present study, it is validated in a reactive case against experimental in-cylinder pressures and velocity fields.

A first novelty is the application of EMD methods combined with topology-based techniques to reactive LES results to characterize flow structures of different scales in the three-dimensional domain and to quantify separately their impacts on combustion.

A second n