In spark ignition engines, local or global flame extinction may occur after the ignition spark deactivates, posing challenges to engine stability. Identifying and predicting this phenomenon is crucial for addressing combustion instability. This study investigates the acoustic extinction of a partially premixed methane-air counterflow flame. The focus is on the impact of fuel-air mixing on flame behavior under acoustic waves, using non-intrusive CH* chemiluminescence. One-dimensional steady flame simulations, alongside flame chemiluminescence and spectrometry, are employed to discern differences in flame structure among various mixing regimes. The stagnation point of the reactant jet varies with the momentum flux ratio of upper and lower nozzle flows while mixing effects alter temperature and CH* radical distributions. Experimental results reveal that at low frequencies (~20 Hz), the non-premixed flame, due to its mixing time scale, exhibits lower stability and extinguishes at lower acoustic pressure levels compared to the other flames. Mixing effects on extinction are notable up to 55 Hz, beyond which they become negligible. CH* chemiluminescence analysis of the partially premixed flame shows reduced thickness, area, and average heat release rate at low frequencies compared to the steady-state flame. With increasing frequency, these parameters increase compared to the steady-state flame; however, higher-frequency acoustic waves have minimal impact on flame structure.

In this study a 2D flamelet method is presented based on premixed regime. This method can be considered as a combination of two existing reduced chemistry approaches; the flamelet and manifold approach. This method shares the idea with the flamelet approach that a multi-dimensional flame may be considered as a set of one-dimensional flames (flamelet method) and the flame structure is considered by some control variables. By this method, the data base of chemical variables is constructed in terms of progress and controlling variables. During flame simulation, conservation equations have to be solved for the controlling variables only and the properties are interpolated from the data base. In this research, 2D flamelet generated manifold (FGM) is applied for laminar counterflow flames with two parameters (progress variable and mixture fraction). Test results of premixed counterflow flame shows that detailed chemistry computations are reproduced very well by using 1DFGM with only progress variable. Predictions of temperature and major species mass fractions using 2DFGM have shown very good agreement with detailed chemistry computations especially at high strain rates. Using the FGM method, the computation time has been reduced several times in simulating flames, demonstrating the enormous potential of the method.

Hydrogen plays a key role in reducing reliance on fossil fuels in various energy production systems. While its use offers a promising alternative to carbon-based fuels, it presents significant challenges. The transition from carbon-rich hydrocarbons to hydrogen-based energy systems requires a thorough understanding of its dynamic effects. This study investigates the impact of hydrogen addition on the dynamic behavior of a partially premixed methane-air counterflow flame through experimental analysis and one-dimensional numerical simulations. The study examines the steady-state flame structure and response without acoustic excitation, assessing the influence of added hydrogen on the flame's heat release rate and thermal region thickness. Employing an acoustically excitable counterflow flame burner setup, the effects of increasing hydrogen content on methane-air flames are explored, and their dynamic response is analyzed using the CH* radical chemiluminescence method. The findings reveal that while hydrogen addition reduces the intensity of CH* radical emission in the absence of acoustic excitation, exposure to acoustic waves amplifies oscillations in the heat release rate and CH* radical emission intensity. This underscores the enhanced flame response function with hydrogen addition. 

This study investigates the stability of a surface flame burner using a photodiode and data acquisition system. The light intensity fluctuations were measured by the photodiode and, using fast Fourier transform, they were transferred from the temporal to the frequency space. To illustrate the dynamic behavior of premixed flames, flames are divided into two regions of cellular flames and surface flames. This classification is dependent on the flow rate and the equivalence ratio. In surface flames, as the flow rate increases, the oscillation frequency also increases because the hot burned gas velocity increases. In cellular flames, as the flow rate increases, oscillation frequency decreases. At identical flow rates, the sharp decrease in the oscillation frequency indicates the appearance of cellular flames so we can find the transition from the surface flame to the cellular flame. At a constant flow rate, with an increase in the equivalence ratio, there is no increase in the oscillation frequency, the transition from the cellular flame to the surface flame occurs. The initiation of the transition from the cellular flame to the surface flame occurs at flow rates of 1. 1, 1. 2, 1. 3, 1. 4, 1. 5, 1. 6 m3/h and at equivalence ratios of 0. 6, 0. 62, 0. 62, 0. 64, 0. 66, and 0. 67, respectively. The location of the transition corresponds to the start of the liftoff zone based on the image processing. This research is innovative because it is possible to evaluate flame stability using a non-intrusive method without disturbing the flame shape and damaging the flame regime.

Global warming and high fossil fuel consumption increase the application of the low-Nox burner with lean-premixed combustion. However, this type of combustion is more susceptible to thermoacoustic instabilities. Then, a fundamental understanding of the lean-premixed combustion is essential. The aim of this work is the numerical study of a premixed propane-air V-flame stabilized on the designed flame holder with focus on the stability and blowout analysis using ANSYS Fluent. Different turbulent models in cold flow simulation are investigated and for steady flame with lower computational cost and for transient flame dynamics are selected. The combustion models including FR/ED, EDC with 2-step mechanism and EDC with CHEMKIN reduced kinetics with 28 species and 114 reactions are used for simulation. The lean and rich limits for V-flame are predicted and the combustion stability range is determined as using SAS turbulent model and EDC reduced mechanism. The numerical stability limit covers the experimental range. The experimental tests have higher turbulent intensity than numerical model, leading to the difference in the blowout threshold. The flame dynamics on the blowout limit is investigated by instantaneous temperature field and radicals. The NOx emission varies with the mean flame temperature and is higher in the lean combustion than the rich cases. The reason of this phenomenon is incomplete mixing of the fuel and air, leading to the anchoring of the flame on the one side of the flame holder more than the other side and the hot regions formation which results in higher amount of NOx. As the equivalence ratio decreases, the flame fragments are separated locally due to the high strain rate formed with turbulence-flame interaction and transferred downstream with flow velocity. The V-flame surface was enhanced due to the vortex interaction with flame front. With a further reduction in the fuel amount, the heat release by the V-shaped flame area is not sufficient to sustain the burning of the rest of the flame anchoring on the bluff body. Then, the mean temperature immediately reduces lower than ignition temperature, while the radicals are less than the stable combustion, leading to the global flame extinction.

An experimental study has been carried out to investigate the effects of inlet velocity, equivalence ratio, and acoustic forcing on flame lengths and flame center lengths in a dump combustor. A premixed gas of ethylene and air was supplied to a combustor through an inlet section and an acoustic driver was used to generate acoustic forcing to simulate unstable combustion. By changing these parameters, combustion tests were performed and flame images were taken using an ICCD camera with a bandpass filter corresponding to a CH* chemiluminescence band. Flame lengths/flame center lengths were obtained from the flame images and were analyzed with respect to dimensional parameters. For a more general finding, the flame length and flame center length were normalized by the inlet width. The dimensional parameters were also replaced with nondimensional parameters such as the Reynolds number, Strouhal number, Damkö hler number, and normalized inlet velocity fluctuation, since dimensional parameters have a complex influence on these non-dimensional parameters. The normalized flame lengths and flame center lengths could be expressed well as a function of the non-dimensional parameters. It was found that an increase in the Reynolds number and a decrease in the Strouhal number, Damkö hler number and normalized inlet velocity fluctuation caused the flame length/flame center length to become greater.

This paper presents a computational fluid dynamics (CFD) analysis of nitrogen monoxide (NO) and nitrogen dioxide (NO2) formation in a biodiesel turbulent non-premixed flame via Eulerian-Lagrangian concept. The model includes governing conservation equation of mass, momentum and energy, and equations representing the tabulation and transport of NOX (NO+NO2). Discrete Ordinates (DO) was exploited for modeling of heat radiation that increases the accuracy of the computational simulation in prediction of NO. The temperatures, and concentration of NOX are presented and compared with experimentation. The proposed model is able to accurately predict the formation of NOX under many circumstances. However, some discrepancies between the model and experimental data exist because of the surrogate combustion mechanism employed for biodiesel fuel and absence of prompt NO in the combustion mechanism employed in CFD codes. 

An experimental study was conducted on the response of V-shaped and M-shaped premixed flames to external acoustic excitation. Low emission premixed combustors with anchored V-flames are susceptible to combustion instabilities. V-flames attach to an anchoring rod at the centre of the burner, whereas M-flames attach to the anchoring rod as well as the burner rim. This is in contrast to conical flames which are anchored only on the burner rim. As a consequence, V-and M-shaped flames are more sensitive to external flow perturbations. In the present study, a mixture of propane-air was used. The mixture equivalence ratio was changed to determine the stability range of V-and M-shaped premixed flames. The results indicated that M-shaped flames have a narrower stability range compared to that of V-flames. This limited the equivalence ratios range for acoustic excitation experiments. In order to study the flame response to external acoustic excitations, a loud speaker was placed upstream of the flame to generate acoustic waves in a certain range of frequencies. Pressure fluctuations caused by the acoustically excited flame were measured by a microphone placed downstream of the burner. Flame imaging was done with a CCD high speed camera during acoustic excitations. These images were used for flame response determination. Image processing was accomplished using the MATLAB software to obtain the amplitude and phase of the flame response. Variations of the flame shape and the intensity of light emitted by the flame were examined during a period of excitation for various values of equivalence ratios and flow rates. The results indicated that the vortex shedding and roll-up due to the velocity perturbations play an important role on the flame response. The phase response of flames evolves quasi-linearly with excitation frequency, indicating a certain time delay for the fluctuations to reach the flame surface. Also, in a certain range of excitation frequency, the flame gain response exhibits more sensitivity to variation of the equivalence ratio. Observations of the V-flame behavior during a period of excitation have led to determination of the effects of nonlinear parameters on the flame response.

Natural gas combustion is one of the primary sources of harvesting energy for various processes and has gained a wide attention during the past decade. One of the most recent applications of natural gas combustion can be found in non-premixed combustion of methane in a coflow burner system. One of the main environmental concerns that arises from the natural gas combustion is the formation of NO produced by thermal NO and prompt NO mechanisms. Current paper is devoted on an examination of a 2D numerical simulation of turbulent non-premixed coaxial methane combustion in air enclosed by an axisymmetric cylindrical chamber to study the effects of species concentrations of reactants on NO formation, their individual contributions, and the chamber outlet temperature. A finite-volume staggered grid method is utilized to solve conservation equations of mass, energy, momentum, and species concentrations. In order to handle radiation heat transfer, discrete transfer method is used to solve radiation equation. Utilizing weighted-sum-of-gray-gases model, based on the newly obtained high-temperature molecular spectroscopic data, local variations of species absorption coefficients are taken into account. To calculate NO concentration, a single- or joint-variable probability density function in terms of a normalized temperature, mass fractions of species, or a combination of both is employed. Plus, published relevant experimental data are used to validate temperature and species concentration fields. It is shown that a decrease in N2concentration contributes to reducing NO. More importantly for higher O2 mass fraction, thermal NO formation becomes the dominant mechanism responsible for NO emission.

A new mathematical model is introduced to predict structure of premixed flame propagating in combustible systems, with uniformly distributed volatile fuel particles and air. In the present paper the flame structure is divided into four zones that consists of a preheat zone, an extensive particles vaporization zone, an asymptotically thin reaction zone, and finally a post flame zone. It is presumed that the fuel particles vaporize first to yield a gaseous fuel, which is subsequently oxidized. The study involves the Damkohler number (Da<1.0), the ratio of chemical reaction rate to vaporization rate, and the Zeldovich number (1.0<<Ze), the nondimensional form of activation energy, as essential parameters. Finally, with considering unity Lewis number and neglecting the latent heat of vaporization, for several equivalence ratios and several diameters of particles the analysis yields results for the mass fraction of the fine-solid particles, mass fraction of the fuel in the gaseous phase and nondimensional temperature. This prediction is in agreement with experimental observations of fine particles combustion.