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To pinpoint the nature of the sawtooth wave, extensive experiments of methane-air continuous rotating detonation (CRD) are conducted in a hollow chamber around the boundary of the operating range. The time occupancy ratio and the increasing ratio of pressure rise (Δ t i / Δ T i and Δ P i / Δ t i) derived from high-frequency pressure data, and the integral of chemiluminescence intensity (ICI) based on self-luminescence images are proposed as quantitative parameters to distinguish the detonation wave and the sawtooth wave. The results show that the sawtooth wave is a critical unstable mode between conventional isobaric combustion and CRD, propagating with high Δ t i / Δ T i and low Δ P i / Δ t i. Meanwhile, the reaction zone of sawtooth wave is verified to be stabilized in the center of combustor through the self-luminescence observation. Especially, the multiple transition processes from the sawtooth wave to the CRD wave are accurately captured, and simultaneously represented as the change from single-peak waveform to double-peak waveform on ICI, the rise of Δ P i / Δ t i , and the reduction of Δ t i / Δ T i. The significant distinction of the sawtooth wave and the CRD wave in the coupling characteristics is confirmed through Short-Time Fourier Transform processes on ICI and high-frequency pressure. For the CRD wave, the propagation frequency of the reaction zone matches that of the pressure wave, whereas not for the sawtooth wave. It is ascertained that the reaction zone is decoupled with the pressure wave for the sawtooth wave, and the nature of the sawtooth wave is deflagration combined with a weak pressure wave. [ABSTRACT FROM AUTHOR]

Coal is an indispensable fuel and plays an important role in the energy industry. However, coal mining is often accompanied by disasters such as gas explosion, which impose severe restrictions on the efficient exploitation of coal resources. The non-uniformity of combustible gases and obstructions in space are important factors that affect the intensity of explosions. In this study, an experimental investigation of non-uniform and uniform methane–air flame behavior in a duct containing obstacles with different opening scales was investigated. The results showed that for non-uniform methane–air mixtures, the speed of the velocity change under different opening scale obstacle configurations varied owing to the degree of non-uniformity. In addition, the explosion overpressure curves exhibited significant pressure peaks at extended diffusion times, with the largest pressure peak in the configuration with a barrier-opening scale of 50 mm. Furthermore, the backflow phenomenon after the flame passed through the obstacle only appeared in configurations with larger obstacle-opening scales or diffusion times. These findings offer insights into the prevention and management of gas explosions in coal mining and contribute to the efficient and safe development of coal resources. [Display omitted] • The effect of obstacle opening scale on the explosion flame behavior of non-uniform and uniform CH 4 -air mixtures was studied. • Flame velocity was affected by the degree of mixture non-uniformity for different open-scale obstacle configurations. • Backflow flames appear only in configurations with considerable obstacle opening scales or longer diffusion times. [ABSTRACT FROM AUTHOR]

Due to the low chemical activity and large detonation cell size, methane-air continuous rotating detonation (CRD) is hard to be realized in a traditional small annular chamber without extra additives. In order to investigate the characteristics of methane-air CRD wave in a small chamber, three hollow chambers with Laval nozzles are adopted in this research. The methane-air CRD can be realized in a quite small chamber (diameter of 80 mm) under very particular condition. The operating range enlarges with the increase of chamber diameter. The CRD wave propagation frequencies vary apparently with chamber diameter while the propagation velocities stay in a quite narrow range of 1744–1812 m/s, and the relative deviation is within 4%. The CRD wave propagation process is very stable, and the propagation velocity accounts for 99% of the theoretical C-J velocity in the chosen test. Optical observation results illustrate that the reduction in chamber diameter or increase in nozzle contraction ratio will lead to CRD deterioration. By comparing the frequencies of high-frequency pressure and optical observations, the coupling of inducing shock wave and combustion flame is proved quantitatively. • Realizing methane-air continuous rotating detonation (CRD) in a small hollow chamber with diameter of 80 mm. • Analyzing the influences of chamber diameter on CRD realization and propagation characteristics. • Analyzing the flame structure in different chambers though optical observation. • Verifying the couple of inducing shock wave and rotating flame quantitatively. [ABSTRACT FROM AUTHOR]

By installing obstacle plates with different opening shapes in an experimental pipeline, the combined effect of vertical concentration gradients and obstacle shapes on methane-air explosion characteristics was experimentally investigated. The presence of obstacles and concentration gradients induced turbulent flame instabilities, which promoted the positive feedback between the combustible gas flow and combustion process. The combustion process caused the upstream lean methane, lighter than the unburned gas, to propagate through the obstacle and hence bubble-like local disturbances were generated in the central area downstream of the obstacle. Under the action of transverse waves, a "gap" appeared in the lower part of the flame front, and the flame front appeared concave. The effect of the concentration gradient and obstacles on the methane explosion gradually weakened, and hence the wrinkled flame surface gradually stabilized. Results show that the square-shaped opening affected the flame propagation and explosion evolution most significantly. As the concentration gradient increased, the combined effect undermined the positive feedback mechanism, and the maximum explosion overpressure and maximum rate of pressure rise were reached for the square-shaped opening at a low concentration gradient (0.5 %), while for the circular-shaped and quadrant-shaped openings the maximum values were reached at a higher concentration gradient (1.0 %). The effect of obstacles was more significant than the effect of concentration gradients with respect to the flame propagation velocity and rate of pressure rise. [ABSTRACT FROM AUTHOR]

Concentric fuel-rich and fuel-lean mixtures of methane and air were fed in the form of multiple pairs of circumferentially oriented opposing jets that are normal to an air cross flow inside a cylindrical burner to achieve the maximum firing capacity. As two premixed flames developed per each set of concentric jets (while the excess fuel and air diffused into a non-premixed flame wing), combustion via triple flames was thus sustained by a duplicated stagnation impact and a vortical flow field with intense heat recirculation. Setting equivalence ratios of 1.35 and 0.80 for six pairs of opposing jets increased the firing intensity to 100.4 MW/m3 which corresponded to jet flow velocities of 74.5 m/s. While the maximum blow-out velocity corresponded to a combined equivalence ratio of one, the minimum flame length and the peak combustion efficiency were reached at a combined equivalence ratio of 0.6. Switching from normal to inverse triple flames increased the blowout velocity limit from 74.5 to 82.2 m/s but increased the maximum value of the visible flame length/burner diameter ratio from 1.52 to 1.64. [ABSTRACT FROM AUTHOR]

In the present work, LBO limits for methane-air combustion stabilized by a V-Gutter have been predicted by a hybrid method using numerical simulation and empirical relations. The numerical simulations have been carried out to study the stable methane-air combustion and temperature gradients at exit and recirculation region in a model gas turbine afterburner with a planar V-Gutter as a bluff body for four inlet air pressure conditions and three V-Gutter angles. The calculated average exit gas temperature (AEGT) and the average gas temperature in recirculation region have been used for predicting the blowout conditions. An empirical method based on Feature Section Criterion has been used to determine Fuel-Air Ratio (FAR) at blowout conditions very accurately from the numerically calculated average temperature in the central recirculation zone (CRZ).The predicted Fuel-Air Ratio (FAR) at lean blowout conditions has been compared with the experimental results obtained for the same conditions and are found to be in good agreement. [ABSTRACT FROM AUTHOR]

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To evaluate the efficacy of ammonium salt-modified dry water in suppressing the deflagration of methane, in this study, four kinds of ammonium salts were chosen to modify the properties of dry water powders. The physical features of the modified dry water were tested and analyzed. Their suppression characteristics on methane explosion were experimentally explored, and the corresponding deflagration parameters were obtained. Results show that the P max and the (dP/dt) max first diminish and then increase with increasing amounts of modified dry water. The effect of ammonium dihydrogen phosphate-modified dry water (MAP-DW) on the explosion pressure is superior to that of other kinds. The delay effect of MAP-DW and ammonium polyphosphate-modified dry water (APP-DW) on explosion time is better for methane at equivalence ratios of 0.8 and 1.2. Moreover, the explosion suppression mechanism of the modified dry waters was proposed. The findings of this work are helpful for optimizing gaseous deflagration suppressants. [Display omitted] • A new type of explosion suppressant with excellent performance has been prepared. • Ammonium salt-modified dry water has superior inhibitory effect. • The inhibition mechanism is dilution, cooling, and free radical consumption. [ABSTRACT FROM AUTHOR]

Methane-air Continuous Rotating Detonation (CRD) has been firstly achieved in this paper in the hollow chamber with a Laval nozzle, and the diameter of the chamber is just 100 mm. The contraction ratio of the Laval nozzle is a key factor for the CRD realization. CRD can only be obtained when the contraction ratio is no less than 4, but its ER operating range decreases in the increase of contraction ratio from 4 to 10. For all the success cases, the average propagation frequency and velocity are in the ranges of 5.32–5.65 kHz and 1670.48–1774.10 m/s, respectively, and the velocity deficits are less than 10%. Based on the high-speed photography images, the approach of chemiluminescence intensity integral is proposed in this paper, and the propagation characteristics of the flame are analyzed quantitatively. The propagation velocities of the flame and shock wave are agreed well with each other, indicating that the typical feature of detonation wave, i.e., the coupling of the flame and the shock wave, is verified quantitatively. • Methane-air CRD is firstly achieved without additional oxygen and hydrogen. • Realization is in the small hollow chamber with the diameter of 100 mm. • The average velocity of success cases is in the range of 1670.48–1774.10 m/s. • Chemiluminescence intensity integral is proposed as a new image processing method. • The velocity coupling of flame and shock wave is verified quantitatively. [ABSTRACT FROM AUTHOR]

Highlights • Numerical fluxes computed using the Kurganov, Noelle, and Petrova (KNP) scheme. • Proposed criteria for induction times compared with experimental data. • Underprediction of mixture induction time overpredicts detonation parameters. • Evolution of overdriven detonation processes, decoupling into reactive and blast fronts. • Influence of driver conditions on detachment process (failure phenomenon). Abstract This article presents a study concerning the influence of chemical kinetic models on the generation and evolution of methane-air planar detonations. The open source solver rhoCentralRfFoam is applied to perform all numerical simulations. Four chemical kinetic models are considered. Two of them are highly simplified (with, at most, one global chemical reaction and two simple steps), while the other two involve 19 and 53 species, with 57 and 325 chemical reactions, respectively. Insights are provided into methane-air planar detonation waves and the development conditions under which they may or may not become self-sustained. Numerical results showed that regardless of how much time the detonation remains overdriven, without the adequate support it shall always decay to a Chapman-Jouguet (CJ) steady-state value. However, the CJ state may not stand on its own and may continue to decay until the reacting zone decouples from the leading blast. This sort of detonation failure is described and discussed. [ABSTRACT FROM AUTHOR]

Recent explosions with devastating consequences have re-emphasized the relevance of fire safety and explosion research. From earlier works, the severity of the explosion has been said to depend on various factors such as the ignition location, type of a combustible mixture, enclosure configuration, and equivalence ratio. Explosion venting has been proposed as a safety measure in curbing explosion impact, and the design of safety vent requires a deep understanding of the explosion phenomenon. To address this, the Explosion Venting Analyzer (EVA)—a mathematical model predicting the maximum overpressure and characterizing the explosion in an enclosure—has been recently developed and coded (Process Saf. Environ. Prot. 99 (2016) 167). The present work is devoted to methane explosions because the natural gas—a common fossil fuel used for various domestic, commercial, and industrial purposes—has methane as its major constituent. Specifically, the dynamics of methane-air explosion in vented cylindrical enclosures is scrutinized, computationally and experimentally, such that the accuracy of the EVA predictions is validated by the experiments, with the Cantera package integrated into the EVA to identify the flame speeds. The EVA results for the rear-ignited vented methane-air explosion show good agreement with the experimental results.

In this study, the turbulent non-premixed methane-air flame is simulated to determine the effect of air velocity on the length of flame, temperature distribution and mole fraction of species. The computational fluid dynamics (CFD) technique is used to perform this simulation. To solve the turbulence flow, k-ε model is used. In contrast to the previous works, in this study, in each one of simulations the properties of materials are taken variable and then the results are compared. The results show that at a certain flow rate of fuel, by increasing the air velocity, similar to when the properties are constant, the width of the flame becomes thinner and the maximum temperature is higher; the penetration of oxygen into the fuel as well as fuel consumption is also increased. It is noteworthy that most of the pollutants produced are NOx, which are strongly temperature dependent. The amount of these pollutants rises when the temperature is increased. As a solution, decreasing the air velocity can decrease the amount of these pollutants. Finally, comparing the result of this study and the other work, which considers constant properties, shows that the variable properties assumption leads to obtaining more exact solution but the trends of both results are similar.

This paper presents a three dimensional numerical simulation of premixed methane-air low swirl stabilized flames. The computational domain has a simple geometry describing a LBS (low swirl burner) with 50mm of nozzle diameter. RANS Standard κ – ε model to treat turbulence coupled with partially premixed combustion model are used. The purpose is to show the applicability limits and their capacities to predict governing flame parameters by varying swirl intensity and CH4 mass fraction at the inlet, which shows the optimum operating area of the burner in terms of generated energy and flame stability with a particular interest to thermal NOx apparitions. This work is compared and validated with experimental and LES numerical simulation works available in the literature. Results offered good similarity for all flame studied parameters. Swirl number was varied from 0.5 to 1.0 to ensure a wide operating range of the burner. From S=0.6, we observed the onset of recirculation zones, while for the inert flow the appearance of recirculation zones was observed for S=0.9. CH4 equivalence ratio was increased from 0.6 to 1.4. That showed apparition of zones with important NOx mass fraction due to the existence of zones with high temperature. Otherwise, the flow field wasn’t disturbed in terms of recirculation zones apparitions who remained absent for all cases. Actual investigation works to find equilibrium between the maximum of generated temperature and the minimum of NOx emissions for swirled burners. Used models haven’t showed applicability limits, results were clear and precise and offered a significantly gain in computing time and means.

Highlights • An efficient tool to predict the behavior of lean flames near extinction is devised. • Chemistry-turbulence interactions can be fairly captured by using simplified models. • Baseline Reynolds Stress Model (RSM) yields good prediction of mean flow. • Effects of operating parameters on flame structure near lean blowout are reported. Abstract With the forecast of rise in the energy usage, it is imminent that the concentration of toxic emissions such as carbon monoxide (CO) and oxides of nitrogen (NOx) in the ecosystem will increase. One of several industrial techniques to mitigate the emission levels is lean premixed combustion. However, this combustion mode often leads to the problem of Lean Blowout (LBO), which is not well understood. The present study attempts to devise an effective and computation-friendly industrial tool to predict the behavior of lean flames near extinction in a combustion chamber and estimate the lean blowout limits under high temperature and high pressure conditions. Utilizing a tabulated chemistry approach in combination with Reynolds Averaged Numerical Simulation (RANS) turbulence model, extensive validation is performed comparing plain and reacting flow simulation results with the experimental data of laboratory-scale burner at Paul Scherrer Institute (PSI). A modified Flamelet Generated Manifold (FGM) combustion model in conjunction with Reynolds Stress Model (RSM) turbulence model was found to give an accurate prediction of the flow and temperature field inside the combustor. Using this model, the study explores the impact of operational parameters, such as pressure, preheat temperature, turbulence intensity at the inlet and inlet bulk velocity on flame position, temperature, emissions and blowout limits for lean premixed methane-air flames. The combustion model was further applied to the extinguishing flames to study the flame stability limit, which is a very important criterion for an efficient combustor design. The results show that the modified FGM model can reproduce the flame stability curve within 20% of the experimental limit. [ABSTRACT FROM AUTHOR]

Explosion venting offers overpressure protection from potential industrial explosion hazards. This work experimentally studied the non-equilibrium explosion venting of methane–air mixtures in 22L and 110L spherical vessels, under various venting areas and pressures. The results showed that venting area had an obvious influence on explosion venting pressure and the small venting area can hardly protect the vessel from the explosion hazards. When A/V 2/3 is less than 0.00855, pressure will rise after explosion venting and there is linear relationship between the maximum explosion venting pressure and venting pressure. When A/V 2/3 is bigger than 0.025, it is near to the equilibrium venting and independent on the venting pressure. When the A/V 2/3 is up to 0.016, abnormal overpressure occurs under 0.1353 MPa venting pressure. For unconstrained explosion venting, P red in 22L and 110L spherical vessels is equal and (dP/dt) max accords with Cube-Root law for venting in the same A/V 2/3 . The prediction curves of (dP/dt) max vs V 1/3 for unconfined vent different A/V 2/3 and the prediction curves of P red vs venting pressure under different A/V 2/3 are provided. [ABSTRACT FROM AUTHOR]

A numerical study is conducted of methane–air coflow diffusion flames at microgravity (μg) and normal gravity (1g), and comparisons are made with experimental data in the literature. The model employed uses a detailed gas phase chemical kinetic mechanism that includes PAH formation and growth, and is coupled to a sectional soot particle dynamics model. The model is able to accurately predict the trends observed experimentally with reduction of gravity without any tuning of the model for different flames. The microgravity sooting flames were found to have lower temperatures and higher volume fraction than their normal gravity counterparts. In the absence of gravity, the flame radii increase due to elimination of buoyance forces and reduction of flow velocity, which is consistent with experimental observations. Soot formation along the wings is seen to be surface growth dominated, while PAH condensation plays a more major role on centreline soot formation. Surface growth and PAH growth increase in microgravity primarily due to increases in the residence time inside the flame. The rate of increase of surface growth is more significant compared to PAH growth, which causes soot distribution to shift from the centreline of the flame to the wings in microgravity. [ABSTRACT FROM AUTHOR]

This study considers the numerical modeling of sub-breakdown microwave-enhanced combustion in one-dimensional laminar methane-air flames over a range of stoichiometries at standard temperature and pressure. The effect of uniform microwave fields on the laminar flame speed, post-flame temperature and composition is of primary interest here. A model for the non-thermal plasma generated by the microwave field in such flames is constructed based on fitting of electron properties and electron impact reaction rates to pre-computed solutions of the Boltzmann equation, combined with ambipolar diffusion transport for charged species. This model is used with a selection of existing and novel methane-air kinetics schemes to compute laminar flame profiles at a range of microwave field strengths and varying stoichiometry. The trends for laminar flame strength and temperature, peak- and outlet species fractions are compared over this parameter space. While all schemes demonstrate increasing flame speed and temperature with higher field strength and richer flames, significant variations in the form and magnitude of this increase are observed for the various kinetics schemes considered. The laminar flame-speed increase at sub-breakdown electric field strengths is found to be due primarily to electron heating of the gases in both the pre-heat region and across the flame. Analysis of the electron energy transfer shows that the majority of the energy at moderate field strengths is transferred via excitement of vibrational states of nitrogen, followed by water and carbon monoxide. In the pre-heat regime, vibrational excitement of methane is also seen to play an important role for fuel-rich flames. Particular attention is paid to the modification of the flame speed due to the relaxation time of the vibrational N 2 states, which is demonstrated to result in a moderate reduction of the flame speed for lean and stoichiometric flames. A driving concern here is the development of models suitable for use within large-scale three-dimensional numerical simulations and the knowledge gained in comparing the existing schemes is used to derive a range of simplified mechanisms more suited for this purpose. The ability of these reduced models to capture the flame enhancement is demonstrated over a range of flame stoichiometries and sub-breakdown field strengths. [ABSTRACT FROM AUTHOR]

9 batches of unconfined methane-air deflagration tests were conducted to investigate the influential factors on the deflagration load. The methane-air mixture was filled in the cubic tents with different volumes of 1 m 3 , 8 m 3 and 27 m 3 . Methane concentrations varied from 7.5% to 11.5%, covering both lean and rich combustion regimes. The ground overpressure was recorded by using the piezoresistive sensors. An ideal film was validated before the tests. The effects of the methane concentration and gas volume on the deflagration load were analyzed. Based on the results, a new prediction model for the unconfined methane-air deflagration load was proposed. In addition, the model was compared with some existing models. [ABSTRACT FROM AUTHOR]

This paper presents a three dimensional numerical simulation of premixed methane-air low swirl stabilized flames. The computational domain has a simple geometry describing a LBS (low swirl burner) with 50mm of nozzle diameter. RANS Standard k - ε model to treat turbulence coupled with partially premixed combustion model are used. The purpose is to show the applicability limits and their capacities to predict governing flame parameters by varying swirl intensity and CH4 mass fraction at the inlet, which shows the optimum operating area of the burner in terms of generated energy and flame stability with a particular interest to thermal NOx apparitions. This work is compared and validated with experimental and LES numerical simulation works available in the literature. Results offered good similarity for all flame studied parameters. Swirl number was varied from 0.5 to 1.0 to ensure a wide operating range of the burner. From S=0.6, we observed the onset of recirculation zones, while for the inert flow the appearance of recirculation zones was observed for S=0.9. CH4 equivalence ratio was increased from 0.6 to 1.4. That showed apparition of zones with important NOx mass fraction due to the existence of zones with high temperature. Otherwise, the flow field wasn't disturbed in terms of recirculation zones apparitions who remained absent for all cases. Actual investigation works to find equilibrium between the maximum of generated temperature and the minimum of NOx emissions for swirled burners. Used models haven't showed applicability limits, results were clear and precise and offered a significantly gain in computing time and means. [ABSTRACT FROM AUTHOR]

Microcombustion is one of the most essential topics in the development of micropower generators for portable devices, microaerial vehicles and sensors. Although one-dimensional numerical modelling of combustion has been developed, maturing a lot during the last forty years, two-dimensional numerical modelling of combustion, particularly at very small scales has only started in the last decade. In this particular study, two-dimensional numerical simulations of premixed, laminar, lean methane–air flames at atmospheric pressure formed in a 2 mm diameter microcombustor are performed. A skeletal mechanism consisting of 16 species and 41 reactions is employed. This study shows the importance of applying low target residuals for convergence in CFD, which plays a critical role in obtaining an accurate solution in the numerical modelling of microcombustion. The main focus of this work is to investigate the effect of preheating the reactants on the flame structure and stability with adiabatic and non-adiabatic combustor walls. The flame in the 2 mm diameter circular microcombustor under adiabatic wall conditions shows a laminar Bunsen type flame behaviour. On the other hand, under the non-adiabatic wall conditions, a cone shaped high temperature flame zone is observed. Preheating the reactants widens the flame stability at high incoming flow velocities under both conditions, however, under non-adiabatic wall conditions, at low incoming flow velocities and/or low mass flow rates, preheating the reactants contributes neither to flame stability nor combustion characteristics. On the contrary, it results in low combustor exit temperature. Finally, the effects of convective heat transfer coefficients and heat loss, with and without radiation, on the microcombustion are analysed. [ABSTRACT FROM AUTHOR]

Heat transfer from perforated plate burner flame impinging on a flat plate finds importance in industrial (gas fired boiler) and domestic heating (household gas burner and commercial hotel gas burners) applications. A limited study on heat transfer distribution of this kind of burner plate is found in the literature. In the present work, high resolution heat flux is estimated by the inverse heat conduction (IHCP) technique based on use of analytical solution for semi-infinite medium for the impingement plate. Inline, star and staggered holes patterns with three different inter-hole distances (pitch) are considered in the present study. Methane–air premixed flame of Reynolds number varying from 50 to 600 and an equivalence ratio varying from 0.6 to 1.2 is considered. The hole to impingement plate distance is varied from 3 to 7. From the experimental results, it is found that the inline and staggered patterns have the same heat flux averaged over an area of 50 mm × 50 mm for different Reynolds number. The intermediate pitch of 7 mm is the optimal pitch over the entire mixture flow range considered in the present study. The specific fuel consumption for the star pattern is less by 40–60% as compared with the inline pattern for p / d = 1.67, Re = 50–300 and z / d = 3–7. A numerical simulation is carried out using CFD software to explain the shift in the peak heat flux away from the geometric intended location. [ABSTRACT FROM AUTHOR]

To describe the premixed combustion of natural gas in internal combustion engines, the flamelet approach is often assumed to be appropriate. It postulates a thin flame front which is wrinkled by turbulence but locally spreads like a laminar flame. Numerous correlations for the macroscopic turbulent flame speed to the local laminar flame speed exist. In this study seven correlations are validated at engine conditions, i.e., at high pressure (up to 7.0 MPa) and high turbulence (turbulent Reynolds number up to 4.4 · 10 4 ), with the help of measurements from a single cylinder research gas engine. The stoichiometry, the exhaust gas recirculation rate, and the engine speed are varied. Simulations of the detailed reaction kinetics with the diffusive species and energy transport are performed to calculate the laminar flame speed. The flame propagation is described by a zonal cylinder model on the basis of the measured in-cylinder pressure traces, to determine the turbulent flame speed with respect to the thermal expansion of the reacting gas. Additionally, Computational Fluid Dynamics (CFD) simulations of the in-cylinder charge motion are performed to describe the turbulent flow field. It is found that four flame speed correlations including a pressure-dependent term predict flame speed ratios in quantitative accordance to the measurements. This is noteworthy, as these correlations are mostly developed for lower turbulence and at considerable lower pressure. It supports the hypothesis that flamelet-like combustion is still found above the Klimov–Williams criterion. [ABSTRACT FROM AUTHOR]

Abstract: Fundamental flame characteristics derived from counterflow flames are routinely used in chemical kinetic model optimization and validation. This paper reports an experimental and computational investigation aimed at understanding and quantifying the source of uncertainties associated with such characterization of extinction limits of fuel–air mixtures, ranging from low extinction strain rate methane–air flames to high-extinction strain rate ethylene–air flames. In the experiments, two pairs of convergent nozzles with exit diameters of 7.9mm and 14.5mm were used to introduce opposed jets of nonpremixed fuel and air to establish a planar flame in the counterflow mixing region. Velocity profiles and extinction data were measured using both LV and PIV setups. Experiments were conducted at various nozzle separation distances to investigate potential differences in axial velocity profiles along the axial and radial directions and the corresponding local extinction strain rates. The slope of axial velocity in the axial and radial directions at the air outlet boundary was found to increase with decreasing nozzle separation distance. The variation of local extinction strain rate with changes in separation distance was within the uncertainty of experimental data. Using a C1–C4 chemical kinetic model, quasi one-dimensional computations have been performed to quantify the experimentally determined boundary condition effects on the predicted extinction strain rate of counterflow flames. [Copyright &y& Elsevier]

Abstract: Direct initiation experiments were carried out in a 105cm diameter tube to study detonation properties and evaluate the detonability limits for mixtures of natural gas (NG) with air. The natural gas was primarily methane with 1.5–1.7% of ethane. A stoichiometric methane–oxygen mixture contained in a large plastic bag was used as a detonation initiator. Self-supporting detonations with velocities and pressures close to theoretical CJ values were observed in NG–air mixtures containing from 5.3% to 15.6% of NG at atmospheric pressure. These detonability limits are wider than previously measured in smaller channels, and close to the flammability limits. Detonation cell patterns recorded near the limits vary from large cells of the size of the tube to spiral traces of spin detonations. Away from the limits, detonation cell sizes decrease to about 20cm for 10% NG, and are consistent with existing data for methane–air mixtures obtained in smaller channels. Observed cell patterns are very irregular, and contain secondary cell structures inside primary cells and fine structures inside spin traces. [Copyright &y& Elsevier]

Stability of premixed flames is important in applications involving industrial and domestic impingement heat transfer processes, gas turbine combustion chamber and others. Blowoff limits of premixed methane-air flames stabilised on an uncooled Bunsen type burner is considered in the present experimental work. The effect of burner material, wall thickness and burner exit shape on the blowoff limits is presented. Burner materials covered in this study are stainless steel, brass and pyrex. Wall thicknesses considered are 1 mm, 2 mm and 3 mm for pyrex tubes of 10 mm inside diameter. The burner exit shapes covered in this study are circle, triangle, square and hexagon. The operating mixture Reynolds number range is 800 - 4000. It is found that the burners with low thermal conductivity, larger wall thickness and minimal sided polygon shapes provide better lean blowoff stability. Critical velocity gradient parameter defined on the basis of hydraulic diameter collapses the blowoff limits for all shapes covered in the present study. Correlations for Karlowitz number (Ka) are suggested for the blowoff stability for the various cases studied. The Karlowitz number is found to increase steeply with increase in fuel richness when equivalence ratio (φ) is greater than 1. This is attributed to the secondary reaction zones that provide additional heating of the tube at the stabilisation region. For equivalence ratio less than 1, the dependence of Karlowitz number with equivalence ratio is relatively weak. For turbulent flow, the Karlowitz number is found to be independent of the tube material, thickness and shape. [ABSTRACT FROM AUTHOR]

We study the structure of a methane-air edge flame stabilized against an incoming mixing layer. The flame is computed using detailed chemical kinetics, and the analysis is based on computational singular perturbation theory. We focus on examination of the dynamical fast/slow structure of the flame, exploring the distribution of time-scales, the composition of the related specific modes and the effective low-dimensional structure. We also study the importance of chemical/transport processes for both major species and radicals in the flame, analyzing the information available from slow/fast importance indices as compared to reaction flux analysis. Results provide enhanced understanding of the flame, outlining the role of different chemical and transport processes in its observed structure. [ABSTRACT FROM AUTHOR]

Abstract: A numerical study on CH4–air premixed combustion in micro-combustors was undertaken by solving the 2D governing equations. The effects of combustor size and geometry, inlet velocity profile and slip-wall boundary condition on the flame temperature were investigated. The simulation results showed that a larger combustor (d =2mm) gives higher flame temperature only when the flow velocity is below a certain level. With regard to the combustor geometry, a 2D planar channel (H =1mm) represents higher flame temperature than a cylindrical tube with d =2mm (equal hydrodynamic diameter), over the velocity range covered by the present study. In addition, it was noted that the flame temperatures in the cylindrical tube and 2D planar channel are quite close when H =0.65d is satisfied. The fully developed velocity profile applied at the inlet plane was found to have the flames anchored further from the entrance than the uniform profile, but no remarkable difference in terms of flame temperature (≤3K) was observed. A simple analysis of the competing time scales (axial convection and radial diffusion) was presented to address the difference of flame structure between the methane–air and hydrogen–air mixtures. Finally, it was shown that in a combustor with d =1mm the effects of slip-wall boundary are negligible, compared to the bulk velocity and gases temperature. [Copyright &y& Elsevier]

Active material was prepared using industrial and reactive grade magnesium oxide (MgO) with the addition of calcium oxide (CaO). In a fixed packed bed containing particles of active material, the effects of quantity of CaO added, calcination temperature, space velocity, oxygen excess and reactive mixture composition on activity, measured as percentage of methane conversion, were studied. Complete combustion was reached for compositions under the low limit of inflammability which allow avoiding the formation of thermal NOx. Samples prepared with industrial grade MgO showed similar activity to those prepared with reactive grade MgO with the addition of CaO. The materials, of low cost and easy preparation, showed high thermal stability before and after reaction tests. The properties of the prepared solids make them promising materials for natural gas combustion and gas emission treatment systems. [ABSTRACT FROM AUTHOR]

Abstract: X-shaped flames were visually observed when a coaxial burner consisting of two coaxial streams of different premixed streams of methane–air was operated near blowout. High-speed photography showed that a rotating, slanted flamelet was formed; the shape of the flamelet was an asymmetric triangle with a curved surface. The rotational frequency was about 30 Hz and made no sound. Similar X-shaped flames have previously been observed in sudden-expansion tubes. Particle tracks were photographed by introducing seed particles into the flame and using a digital camera synchronized with the rotating flamelet to track the particle motion. The particle tracks show that the streamlines curve on the surface of the passing flamelet and consequently make a spiral flow. Because these flamelets rotate, the burning velocity has a circumferential component. This is different from the behavior reported for rotating polyhedral flames. The rotational frequency is strongly influenced by the mixture composition and gas velocity. The rotational frequency increases with increasing fuel–air ratio and decreases with either increasing inner-gas velocity or decreasing outer-gas velocity. From our tests, as the reactant preheat temperature was increased, the rotational frequency increased from 21–29 Hz at a temperature of 293 K to 27–35 Hz at a temperature of 346 K. The wave speed was higher than the laminar burning velocity. The ratio of the rotational frequencies of 27 Hz (at 346 K) to 21 Hz (at 293 K) is 1.29; this ratio is the same as the ratio of the laminar burning velocity at 346 K to that at 293 K. [Copyright &y& Elsevier]

The growing popularity of natural gas as a eco-friendly fuel, is of paramount motivation of present investigation. In the present paper, the effect of initial temperature on the flame structure have been investigated in which laminar one-dimensional planar propagating flames of CH4/air mixtures is simulated numerically using detailed chemical kinetic scheme and realistic transport models. The burning velocities are fundamentally important in developing models to predict progress of combustion. Hence, the burning velocities as a function of initial temperature of unburnt gas have been computed for stoichiometric mixture. The present predictions of burning velocities are compared with reported experimental data of Stone et al. [Combust. Flame. 114 (1998) 546], Hill and Huang [Combust. Sci. Technol. 60 (1980) 7] and Rallis and Garforth [Combust. Flame 31 (1978) 53]. The present prediction lies within the scatter of experimental data. A correlation in the form of Su/Su,0=(Tu/Tu,0)1.575 has been developed to describe the dependence of initial temperature on the burning velocity for stoichiometric mixture. The structures of flame are investigated in details for initial temperature of 300 and 600 K which clearly indicate that detailed chemical kinetics are essential for prediction of the effects of initial temperature on the burning velocities. The present study will help in designing and developing the regenerative combustion systems. [Copyright &y& Elsevier]

In order to obtain the standing conditions and stationary range of oblique detonation wave in methane-air combustible mixture, we use the Newton-Raphson iteration method to solve the perfect gas flow conservation equations of oblique detonation wave based on detonation theory. We investigate the variation laws of the oblique detonation stationary window and internal mechanism under different methane-air equivalence ratio, incoming flow velocity, initial pressure and initial temperature. The oblique detonation polar curve can be separated into three sections, each of which has different flow characteristics corresponding to different wave morphology. Among them, a stationary oblique detonation wave can be formed when the wedge angle is larger than the wedge angle of Chapman-Jouguet state oblique detonation wave, and smaller than the maximum wedge angle corresponding to the oblique detonation wave without detached. The wedge angle range is called the stationary window of the oblique detonation wave. We found that the oblique detonation is more easily formed when the methane-air mixture is under the oxygen-enriched condition, and the range of oblique detonation wave stationary window becomes larger as the incoming velocity increases. The changes of initial pressure and temperature have little influence on the stationary window. The change law of the stationary window of oblique detonation wave corresponds to the change law of Mach number behind CJ oblique detonation wave. In addition, the standing conditions are mainly related to the coupling of the incoming mixture kinetic energy and heat release. [ABSTRACT FROM AUTHOR]

A series of hydrogen-air, ethylene-air, and methane-air continuous rotating detonation (CRD) tests are performed in the hollow chamber with the variations on nozzle contraction ratio and equivalence ratio. Hydrogen-air, ethylene-air, and methane-air CRDs are achieved with low velocity deficit and high stability in the same laboratory hollow chamber. Larger combustor width, flame stabilization and mixing quality promotion benefited from the hollow chamber, and the prolongation of residence time impacted by the hollow chamber and nozzle are key factors for significant enhancement. Generally, the operating range, propagation frequency, propagation velocity C − J detonation velocity , and stability increase with the fuel detonability increasing. The quantitative analysis approaches, the integral of chemiluminescence intensity and axial distribution of chemiluminescence intensity ratio on high-speed photography images, and flowfield reconstruction based on the time difference calculation method, are proposed and applied. Multiple analysis approaches all show that the length of CRD wave increases with the fuel detonability decreasing, which is mainly attributed to that the hydrocarbon fuels release heat in longer axial distance with relatively lower intensity. In addition, two dominant peak one-wave mode is verified as the single wave coupled with the reflected shock wave, which propagates upstream from nozzle to combustor forepart. • H 2 -air, C 2 H 4 -air and CH 4 -air CRDs are achieved in the laboratory hollow chamber. • Operating range, frequency and stability increases with fuel detonability increasing. • Lengths of CRDs are estimated by axial distribution of chemiluminescence intensity. • Length variation is verified by flowfield reconstruction and pressure distribution. • Two dominant peak one-wave mode has been verified as single wave. [ABSTRACT FROM AUTHOR]

• Simultaneous schlieren and direct photography was used to visualize the ignition of methane-air. • Ignition starts with ignition kernels randomly distributed throughout the mixture. • Above a critical temperature strong ignition occurs randomly away from the reflecting wall. • The strong ignition delay time correlates with an Arrhenius model. • The experimental global activation energy does not match the predicted value for a constant volume. Methane is the primary fuel typically used in dual-fuel compression ignition engines. The ignition of methane-air, equivalence ratios of 0.25, 0.5 and 1.0, was studied using the reflected shock technique. High-speed schlieren and direct photography (taken obliquely to the shock tube axis) in an optically accessible 76 mm square cross-section test section were used to identify ignition sites in three-dimensional space and to obtain the ignition delay time, corroborated with pressure measurements. Tests were carried out at a nominal reflected pressure of 10 bar and temperatures in the range of 880–1500 K. The ignition process started with multiple flame kernels, randomly distributed throughout the mixture, which resulted in a slow rise in pressure. Above a critical reflected shock temperature, this relatively slow deflagration process abruptly terminated with strong ignition, believed to correspond to the onset of a detonation. The location of the detonation initiation was random, and typically occurred away from the end-wall. The minimum temperature for strong ignition was found to be 1015 K, 1054 K, and 1189 K for equivalence ratios of 0.25, 0.5, and 1.0, respectively. The strong-ignition delay time data yielded a global activation energy of 12.9 kcal/mol and 17.2 kcal/mol for equivalence ratios of 0.25 and 0.5, respectively. These experimental values are significantly smaller than constant volume model predictions of 35–36 kcal/mol. [ABSTRACT FROM AUTHOR]