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This work investigates the ignition and flame development processes of low reactivity fuel combustion under compression ignition conditions based on the large eddy simulation approach. The chemical explosive mode analysis (CEMA) is employed to characterize the local combustion features, including gas-liquid fuel zone, auto-ignition, diffusion-assisted, extinction, cool flame and post-ignition zone, among which auto-ignition and post-ignition are found to play a key role in the overall heat release process. The local flame propagation modes in gasoline compression ignition (GCI) are determined by quantifying the relative magnitude of diffusion/chemistry at a representative progress variable in the pre-ignition zone. The results show that autoignition fronts and deflagration waves exist simultaneously in the ignition and intense high temperature heat release (HTHR) stages, but autoignition fronts dominate. In addition, the chemical kinetic processes of four heat release periods are analyzed. The heat release during the ignition period is found to be dominated by the reactions CH3+ H (+M) <=> CH4 (+M) and CH3CHO + H <=> CH2CHO + H2. The reaction CH2OH + OH <=> CH2O + H2O always plays an important role in the heat releases during the other three combustion stages including intense HTHR, moderate HTHR and post-combustion. [ABSTRACT FROM AUTHOR]

Large-eddy simulation with the thickened flame model (LES/TFM) is conducted to simulate a three-dimensional dual swirl partially premixed methane/air gas turbine model combustor. Finite-rate chemistry is described by a skeletal chemical mechanism consisting of 16 species and 41 reactions. Flame sensors based on formyl radical (HCO) and chemical explosive mode analysis (CEMA) are proposed and implemented within the TFM framework. The two sensors are designed for multi-step chemical kinetic models to avoid thickening the low-intensity heat release rate (HRR) region. One-dimensional freely-propagating laminar premixed flames are first employed to assess the two new sensors. The HCO-based sensor can successfully avoid the low-intensity HRR region for lean flames, but can fail under fuel-rich conditions. The CEMA-based sensor can robustly avoid low-intensity HRR regions under both fuel-lean and fuel-rich conditions. A second test case using a hydrogen/air reheat burner further demonstrates the robustness of the CEMA-based sensor. The two new sensors are subsequently applied to the gas turbine model combustor, and effects of different flame sensors are studied. Baseline results from the HCO-based sensor are first compared with experimental measurements to validate the LES/TFM solver. The mean and r.m.s. velocity, temperature, and mass fractions of O 2 and CO agree reasonably well with the experiment, although the mixture fraction within the inner recirculation zone (IRZ) is under-predicted. The predicted mean and r.m.s. temperature and species profiles are comparable at most locations, except near the IRZ where the CEMA-based sensor predicts the largest fluctuations by only thickening the chemically explosive regions. After optimisation, only 15% of overhead in computational cost is imposed when the CEMA sensor is employed on-the-fly. Future work includes further reduction of the computational cost of the CEMA-based sensor. [ABSTRACT FROM AUTHOR]

Large eddy simulation is applied for the combustion process of n-heptane spray flame in methane environment. The influence of methane (CH4) concentration on the auto-ignition process and flame structures are discussed in detail. By increasing the CH4 concentration in the ambient gas, auto-ignition is delayed obviously at a low initial temperature because of the delayed first-stage ignition. While at a high temperature, the auto-ignition time is less influenced by the addition of CH4. However, after the addition of CH4, the concentration of the mixture where the high-temperature ignition is initialed becomes much richer and it is independent of the initial temperatures. In addition, the chemical explosive mode analysis (CEMA) method shows that the mixture in high-temperature regions is non-explosive without CH4 addition, and the abrupt transition between the explosive and non-explosive mixture at the tip of the reacting spray is observed by adding CH4 in the ambient gas, indicating that a flame front is developing downstream. This flame front exists for a long time at low CH4 concentration. The eigenvalue of the Jacobian matrix for the CH4/air mixture in the ambient gas is increased significantly with the addition of CH4, indicating that the combustion process is dominated by auto-ignition. Finally, results show that the development of the flame is accelerated by the addition of CH4 in the ambient gas. [ABSTRACT FROM AUTHOR]

The paper presents computational results of a turbulent non-premixed flame with large extinction, using the large eddy simulation probability density function methodology. The simulation captures local flame extinction at different flame locations, and major species predictions show good agreement with experimental data. A chemical explosive mode analysis is used to determine the flame structure including the explosive modes and the Damköhler number distribution. Due to the nature of the turbulence combustion model, statistical information on the sub-grid flame structure is available: such as sub-grid explosive modes. The analysis suggests that, in the present flame, sub-grid structures are only relevant close to the inlet nozzle, and that downstream extinction is governed by large-scale interactions. [ABSTRACT FROM PUBLISHER]

In this work, the dynamic combustion characteristics in a scramjet engine were investigated using three diagnostic data analysis methods: DMD (Dynamic Mode Decomposition), STFT (Short-Time Fourier Transform), and CEMA (Chemical Explosive Mode Analysis). The data for the analyses were obtained through a 2D numerical experiment using a DDES (Delayed Detached Eddy Simulation) turbulence model, the UCSD (University of California at San Diego) hydrogen/oxygen chemical reaction mechanism, and high-resolution schemes. The STFT was able to detect that oscillations above 50 kHz identified as dominant in FFT results were not the dominant frequencies in a channel-type combustor. In the analysis using DMD, it was confirmed that the critical point that induced a complete change of mixing characteristics existed between an injection pressure of 0.75 MPa and 1.0 MPa. A combined diagnostic analysis that included a CEMA was performed to investigate the dynamic combustion characteristics. The differences in the reaction steps forming the flame structure under each combustor condition were identified, and, through this, it was confirmed that the pressure distribution upstream of the combustor dominated the dynamic combustion characteristics of this scramjet engine. From these processes, it was confirmed that the combined analysis method used in this paper is an effective approach to diagnose the combustion characteristics of a supersonic combustor. [ABSTRACT FROM AUTHOR]