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Temperature characteristics of a filtration combustion wave propagating upstream against a fresh mixture in a one-layer porous medium is studied experimentally using thermal imaging. The design of a model porous burner allows one to obtain data on the time evolution of the temperature distribution of the solid phase at the pore scale. It is shown that the macroscale propagation of a flame is accompanied by local distortions of its front and the emergence and movement of hot spots and low-temperature regions at the scale of several pores. The most common pattern of the local dynamic behavior of the filtration combustion wave is identified and described. [ABSTRACT FROM AUTHOR]

This paper presents experimental investigations of the polyurethane foam influence on the combustion dynamics of hydrogen-air flames propagating in a channel with a sudden change in cross-section (i.e. expansion). The channel is open at both ends. Porous media of various lengths and pore size are considered. The porous inserts are placed downstream of the sudden expansion, inside the diagnostic section of dimensions 20 × 40 mm. A Schlieren visualization technique is used to monitor flame shape and propagation dynamics. Various equivalence ratios ranging from 0.3 to 1.0 are tested. The results show that depending on the equivalence ratio, porous length and pore size, the mixture can either propagate throughout the foam or be quenched. In propagating regime, it is found that the output velocity just behind the foam increases linearly with porous matrix length, indicating that the tortuous flow within the foam plays a significant role in the propagation of the flame. These results could be used both to increase the efficiency of gaseous combustion and to ensure the explosion safety of the gas equipment. [ABSTRACT FROM AUTHOR]

Thickened flame models are prolific in the literature and offer an effective method of resolving flame dynamics on coarse LES meshes. The current state of the art relies heavily on the use of efficiency functions to compensate for impaired wrinkling of the thickened flame. However in practice these functions can involve parameters that are difficult to determine, perform poorly outside of certain ranges or require a posteriori analysis to evaluate performance. An alternative based on a generalised thickening is evaluated across a range of canonical configurations. The approach is demonstrated to perform well across a large range of thickening factors in capturing phenomena such as localised quenching and pinch off as well as generation of flame surface. Including good performance even in the case of large flame dynamics under acoustic forcing where the model has a clear advantage over DNS in achieving grid independence. Finally the approach is unified into an Large Eddy Simulation/Adaptive Mesh Refinement framework and applied to a turbulent Bunsen flame. The results show that even if the internal flame structure is poorly resolved on the original mesh, the global system behaviour is well predicted and compares favourably with other approaches. [ABSTRACT FROM AUTHOR]

A nonlinear phenomenological model of two coupled oscillators is proposed, which is able to describe the rich nonlinear behaviour stemming from an unstable pure intrinsic thermo-acoustic (ITA) mode of a simple combustion system. In an experimental bifurcation analysis of a pure ITA mode, it was observed that for increasing mean upstream velocity the flames loose stability through a supercritical Hopf bifurcation and subsequently exhibit limit cycle, quasi-periodic and period-2 limit cycle oscillations. The quasi-periodic oscillations were characterised by low frequent amplitude and frequency modulation. It is shown that a phenomenological model consisting of two coupled oscillators is able to reproduce qualitatively all the different experimentally observed regimes. This model consists of a nonlinear Van der Pol oscillator and a linear damped oscillator, which are nonlinearly coupled to each other. Furthermore, a parameter estimation of the model parameters is conducted, which reveals a good quantitative match between the model response and the experimental data. Hence, the presented phenomenological dynamical model accurately describes the nonlinear self-excited acoustic behaviour of premixed flames and provides a promising model structure for nonlinear time-domain flame models. [ABSTRACT FROM AUTHOR]

The interaction mechanism of internally-staged-swirling stratified flame is complex, and the pilot flame has a manifest influence on flame stability. To study this, a series of experimental investigations for the pilot flame has been carried out in a model swirl combustor by only supplying the pilot fuel. The CH* chemiluminescence images of the pilot flame are acquired by a high-speed camera with a CH* bandpass filter, whose dynamic characteristics are identified by image statistical analysis and proper orthogonal decomposition (POD) analysis. And the fast algorithm based on matrix theory proposed in this paper increases the operation efficiency and operability of POD. With the pilot equivalence ratio Φ increase, the pilot flame gradually shows an unstable state, whose POD energy distribution is significantly different. In the unstable state, the flame dynamics include three modes—spiral motion mode, flame shedding mode, and axial oscillation mode, whose formation reasons have also been further analyzed in combination with the experimental characteristics. And the fast Fourier transform (FFT) analysis of the time coefficients for the first four POD modes indicates all the dominant frequency is 280 Hz, which means the model combustor is in resonance. In addition, a sensitivity analysis based on the different image resolutions further reveals the robustness of the POD method and its optimization direction. These results emphasize the important influence of the pilot fuel flow rate on the stability of the pilot flame. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study on dynamic response of a forced low-swirl methane/air premixed flame with external acoustic excitation over a wide range of driving frequency. Global flame response in terms of gain and phase delay between flame intensity and incoming velocity perturbation is determined. Local flame response is investigated in detail at three typical frequencies: 55 Hz, 105 Hz and 155 Hz. The effect of swirl number on the flame response is also discussed. Proper orthogonal decomposition is applied to identify the large coherent structures in the forced flame. Experimental results show flame response gain exhibits a successive of valleys and peaks which is dependent on swirl number. Time delay decreases as swirl number is increased. The low-swirl flame oscillates back and forth mainly in the axial direction at low excitation frequency and it turns into radially dominated direction at high frequency. Flame intensity fluctuation is mainly dominated by the tail of the flame at 55 Hz and 155 Hz while the flame response is controlled by a combined effect of the base and tail region at 105 Hz. Further POD analysis shows symmetric, anti-symmetric and helical modes in the flame. The most energetic modes (mode 1 and mode 2) feature a symmetric wave-like structure at low excitation frequency while it tends to be in antisymmetric modes at high frequency. [ABSTRACT FROM AUTHOR]

Turbulent combustion models for Large Eddy Simulation (LES) aims at predicting the flame dynamics. So far, they have been proven to predict correctly the mean flow and flame properties in a wide range of configurations. A way to challenge these models in unsteady situations is to test their ability to recover turbulent flames submitted to harmonic flow modulations. In this study, the Flame Transfer Function (FTF) of a CH4/H2/air premixed swirled-stabilized flame submitted to harmonic flowrate modulations in a non-adiabatic combustor is compared to the response computed using the Filtered TAbulated Chemistry for LES (F-TACLES) formalism. Phase averaged analysis of the perturbed flow field and flame response reveal that the velocity field determined with Particle Image Velocimetry measurements, the heat release distribution inferred from OH* images and the probability of presence of burnt gases deduced from OH-Planar Laser Induced Fluorescence measurements are qualitatively well reproduced by the simulations. However, noticeable differences between experiments and simulations are also observed in a narrow frequency range. A detailed close-up view of the flow field highlight differences in experimental OH* and numerical volumetric heat release rate distributions which are at the origin of the differences observed between the numerical and experimental FTF. These differences mainly originate from the outer shear layer of the swirling jet where a residual reaction layer takes place in the simulations which is absent in the experiments. Consequences for turbulent combustion modeling are suggested by examining the evolution of the perturbed flame brush envelope along the downstream distance of the perturbed flames. It is shown that changing the grid resolution and the flame subgrid scale wrinkling factor in these regions does not alter the numerical results. It is finally concluded that the combined effects of strain rate and enthalpy defect due to heat losses are the main factors leading to small but sizable differences of the flame response to coherent structures synchronized by the acoustic forcing in the outer shear layer of the swirling flow. These small differences in flame response lead in turn to a misprediction of the FTF at specific forcing frequencies. [ABSTRACT FROM AUTHOR]

The effects of flow structure and flame dynamics on combustion instabilities in a lean premixed swirl combustor were numerically investigated using Large eddy simulation (LES) by varying the inlet geometry of combustor. The dynamic k -equation and G-equation flamelet models were respectively employed as the LES subgrid models of turbulence and combustion. The divergent half angle (α) in the combustor inlet was varied systematically from 30° to 90° to quantify the effect of inlet geometry on the combustion instabilities. This variation caused considerable deformation in recirculation zones in terms of their size and location, leading to significant changes in flame dynamics. Analysis of unsteady pressure distributions in the combustor showed that the largest damping caused by combustion instabilities takes place at α = 45°, and the amplitude of acoustic pressure oscillation is largest at α = 30°. Examination of local Rayleigh parameters indicated that controlling flame-vortex interactions by modifying inlet geometry can change the local characteristics of combustion instabilities in terms of their amplification and suppression, and thus serve as a useful approach to reduce the instabilities in a lean premixed swirl combustor. These phenomena were studied in detail through unsteady analysis associated with flow and flame dynamics. [ABSTRACT FROM AUTHOR]

The results of modeling of the combustion of a gas-air mixture in a closed volume filled with high-porosity medium have been given. A comparison of the calculation results to experimental data has shown their qualitative agreement. It has been established that a quasistationary regime of propagation of the flame is possible in the case of fairly large relative length of the system and specific surface of the porous medium. Rapid deceleration of the flame is attributed to the decrease in the area of its surface because of the quenching on the reactor walls and the intense heat exchange between the hot combustion products and the porous medium. Stabilization of the flame propagation occurs when the heat release in gaseous combustion and the heat loss during the heat exchange between the combustion products and the porous medium are approximately equal. [ABSTRACT FROM AUTHOR]