Characterization of the effects of pressure and hydrogen concentration on laminar burning velocities of methane–hydrogen–air mixtures

The aim of the present work was to characterize both the effects of pressure and of hydrogen addition on methane/air premixed laminar flames. The experimental setup consists of a spherical combustion chamber coupled to a classical shadowgraphy system. Flame pictures are recorded by a high speed camera. Global equivalence ratios were varied from 0.7 to 1.2 for the initial pressure range from 0.1 to 0.5 MPa. The mole fraction of hydrogen in the methane + hydrogen mixture was varied from 0 to 0.2. Experimental results were compared to calculations using a detailed chemical kinetic scheme (GRIMECH 3.0). First, the results for atmospheric laminar CH₄/air flames were compared to the literature. Very good agreements were obtained both for laminar burning velocities and for burned gas Markstein length. Then, increasing the hydrogen content in the mixture was found to be responsible for an increase in the laminar burning velocity and for a reduction of the flame dependence on stretch. Transport effects, through the reduction of the fuel Lewis number, play a role in reducing the sensitivity of the fundamental flame velocity to the stretch. Finally, when the pressure was increased, the laminar burning velocity decreased for all mixtures. The pressure domain was limited to 0.5 MPa due to the onset of instabilities at pressures above this value.     

On the determination of laminar flame speed from low-pressure and super-adiabatic propagating spherical flames

The outwardly propagating spherical flame (OPF) method is popularly used to measure the laminar flame speed (LFS). Recently, great efforts have been devoted to improving the accuracy of the LFS measurement from OPF. In the OPF method, several assumptions are made. For examples, the burned gas is assumed to be static and in chemical equilibrium. However, these assumptions may not be satisfied under certain conditions. Here we consider low-pressure and super-adiabatic propagating spherical flames, for which chemical non-equilibrium exists and the burned gas may not be static. The objective is to assess the chemical non-equilibrium effects on the accuracy of LFS measurement from the OPF method. Numerical simulations considering detailed chemistry and transport are conducted. Stoichiometric methane/air flames at sub-atmospheric pressures and methane/oxygen flames at different equivalence ratios are considered. At low pressures, broad heat release zone is observed and the burned gas cannot quickly reach the adiabatic flame temperature, indicating the existence of chemical non-equilibrium of burned gas. Positive flow in the burned gas is identified and it is shown to become stronger at lower initial pressure. Consequently, the LFS measurement from OPF at low pressures is not accurate if the burned gas is assumed to be static and at chemical equilibrium. For super-adiabatic spherical flames, the burned gas speed is found to be negative due to the local temperature overshoot at the flame front. Such negative speed of burned gas can also reduce the accuracy of LFS measurement. It is recommended that the direct method measuring both flame propagation speed and flow speed of unburned gas should be used to determine the LFS at low pressures or for mixtures with super-adiabatic flame temperature.     

Effects of radiation heat loss on laminar premixed ammonia/air flames

Ammonia (NH₃) direct combustion is attracting attention for energy utilization without CO₂ emissions, but fundamental knowledge related to ammonia combustion is still insufficient. This study was designed to examine effects of radiation heat loss on laminar ammonia/air premixed flames because of their very low flame speeds. After numerical simulations for 1-D planar flames with and without radiation heat loss modeled by the optically thin model were conducted, effects of radiation heat loss on flame speeds, flame structure and emissions were investigated. Simulations were also conducted for methane/air mixtures as a reference. Effects of radiation heat loss on flame speeds were strong only near the flammability limits for methane, but were strong over widely diverse equivalence ratios for ammonia. The lower radiative flame temperature suppressed the thermal decomposition of unburned ammonia to hydrogen (H₂) at rich conditions. The equivalence ratio for a low emission window of ammonia and nitric oxide (NO) in the radiative condition shifted to a lower value than that in the adiabatic condition.     

Effects of cryogenic temperature on premixed hydrogen/air flame propagation in a closed channel

As a carbon-free fuel, hydrogen has received significant attention recently since it can help enable low-carbon-economy. Hydrogen has very broad flammability range and very low minimum ignition energy, and thereby there are severe safety concerns for hydrogen transportation and utilization. Cryo-compressed hydrogen is popularly used in practice. Therefore, it is necessary to investigate the combustion properties of hydrogen at extremely low or cryogenic temperatures. This study aims to assess and interpret the effects of cryogenic temperature on premixed hydrogen/air flame propagation and acceleration in a thin closed channel. Different initial temperatures ranging from normal temperature (T₀ = 300 K) to cryogenic temperature (T₀ = 100 K) are considered. Both one- and two-dimensional hydrogen/air flames are investigated through transient simulations considering detailed chemistry and transport. It is found that when the initial temperature decreases from T₀ = 300 K to T₀ = 100 K, the expansion ratio and equilibrium pressure both increase substantially while the laminar flame speeds relative to unburned and burned gasses decrease moderately. The one-dimensional flame propagation is determined by laminar flame speed and thereby the combustion duration increases as the initial temperature decreases. However, the opposite trend is found to happen to two-dimensional flame propagation, which is mainly controlled by the flame surface area increase due to the no-slip side wall constraint and flame instability. Based on the change in flame surface area, three stages including the initial acceleration, steady burning and rapid acceleration are identified and investigated. It is demonstrated that the large expansion ratio and high pressure rise at cryogenic temperatures can significantly increase the flame surface area in early stage and promote both Darrieus-Landau instability (hydrodynamic instability) and Rayleigh-Taylor instability in later stage. These two instabilities can substantially increase the flame surface area and thereby accelerate flame propagation in hydrogen/air mixtures at cryogenic temperatures. The present study provides useful insights into the fundamental physics of hydrogen flames at extremely low temperatures, and is closely related to hydrogen safety.     

Experimental and kinetic investigations of the effect of H2/CH4/C2H4 addition on the burning properties of practical jet fuel

Laminar flame speeds of premixed jet fuel/air with the addition of hydrogen, methane and ethylene are measured in a constant-volume bomb at an initial temperature of 420 K, initial pressure of 3 atm, equivalence ratios of 0.6–1.5 and gas mass fractions of 0–50%. The experimental results show that the addition of hydrogen and ethylene can significantly improve the laminar flame speed of the liquid jet fuel, while the addition of methane shows a weak inhibitory effect, and these effects are relatively remarkable on the fuel-rich conditions. The laminar flame speed of the dual fuels/air is linearly dependent on the additional gas mass fraction. A kinetic analysis indicates that the gas addition causes both thermodynamic and chemical kinetic effects on the laminar flame speed of the dual fuels/air. The adiabatic temperature increases and decreases with the addition of hydrogen/ethylene and methane, respectively. A sensitivity analysis shows that the reactions concerning to the H, CH₃ and C₂H₃ radicals become significant with the addition of hydrogen, methane and ethylene, respectively, and that the different values of the rate of product (ROP) of these species via the critical reactions lead to a different promotional or inhibitory effect on the fuel-rich and fuel-lean conditions.     

Effect of Soret diffusion on lean hydrogen/air flames at normal and elevated pressure and temperature

The influence of Soret diffusion on lean premixed flames propagating in hydrogen/air mixtures is numerically investigated with a detailed chemical and transport models at normal and elevated pressure and temperature. The Soret diffusion influence on the one-dimensional (1D) flame mass burning rate and two-dimensional (2D) flame propagating characteristics is analysed, revealing a strong dependency on flame stretch rate, pressure and temperature. For 1D flames, at normal pressure and temperature, with an increase of Karlovitz number from 0 to 0.4, the mass burning rate is first reduced and then enhanced by Soret diffusion of H₂ while it is reduced by Soret diffusion of H. The influence of Soret diffusion of H₂ is enhanced by pressure and reduced by temperature. On the contrary, the influence of Soret diffusion of H is reduced by pressure and enhanced by temperature. For 2D flames, at normal pressure and temperature, during the early phase of flame evolution, flames with Soret diffusion display more curved flame cells. Pressure enhances this effect, while temperature reduces it. The influence of Soret diffusion of H₂ on the global consumption speed is enhanced at elevated pressure. The influence of Soret diffusion of H on the global consumption speed is enhanced at elevated temperature. The flame evolution is more affected by Soret diffusion in the early phase of propagation than in the long run due to the local enrichment of H₂ caused by flame curvature effects. The present study provides new insights into the Soret diffusion effect on the characteristics of lean hydrogen/air flames at conditions that are relevant to practical applications, e.g. gas engines and turbines.     

Laminar flame speed of methane/air stratified flames under elevated temperature and pressure

Flame propagation under mixture stratification is relevant to a wide range of applications including gas turbine combustors and internal combustion engines. One of the local stratification effects is known as the back-support effect, where the laminar flame speed is modified when a premixed flame propagates into gradually richer or leaner mixtures. A majority of previous studies have focused on the propagation of methane/air stratified flames under standard temperature and pressure. However, stratified combustion often occurs under elevated temperature and pressure in practical applications, which may influence the characteristics of the back support effect through modified reaction pathways. This study performs numerical simulations of stratified laminar counterflow flames under an Atmospheric Temperature and Pressure (ATP) condition and an Elevated Temperature and Pressure (ETP) condition and examines the influence of elevated temperature and pressure on the back-support effect. Reaction flow analyses were extensively conducted to elucidate the difference in the primary reaction pathway between the two conditions. When scaled by the stratification Damköhler number, the back-support effect on the rich-to-lean stratified flame is weaker under the ETP condition than the ATP condition in the stoichiometric to lean region. This is due to increased contribution from reactions involved with OH radicals under the ETP condition, which leads to lower H₂ reproduction in the reaction zone than under the ATP condition. The contribution from OH radicals is increased under the ETP condition because the conversion of H into OH is enhanced. These results suggest that the back-support effect may become negligibly small in practical combustors operating under elevated temperature and pressure due to (1) the flame being less sensitive to stratification because of the thinner flame, and (2) the lower H₂ reproduction that deteriorates the radical production that drives the back-support effect.     

Effects of Soret diffusion on the laminar flame speed and Markstein length of syngas/air mixtures

The effects of Soret diffusion on premixed syngas/air flames at normal and elevated temperatures and pressures are investigated numerically including detailed chemistry and transport. The emphasis is placed on assessing and interpreting the influence of Soret diffusion on the unstretched and stretched laminar flame speed and Markstein length of syngas/air mixtures. The laminar flame speed and Markstein length are obtained by simulating the unstretched planar flame and positively-stretched spherical flame, respectively. The results indicate that at atmospheric pressure the laminar flame speed of syngas/air is mainly reduced by Soret diffusion of H radical while the influence of H₂ Soret diffusion is negligible. This is due to the facts that the main reaction zone and the Soret diffusion for H radical (H₂) are strongly (weakly) coupled, and that Soret diffusion reduces the H concentration in the reaction zone. Because of the enhancement in the Soret diffusion flux of H radical, the influence of Soret diffusion on the laminar burning flux increases with the initial temperature and pressure. Unlike the results at atmospheric pressure, at elevated pressures the laminar flame speed is shown to be affected by the Soret diffusion of H₂ as well as H radical. For stretched spherical flame, it is shown that the Soret diffusion of both H and H₂ should be included so that the stretched flame speed can be accurately predicted. Similar to the laminar flame speed, the Markstein length is also reduced by Soret diffusion. However, the reduction is found to be mainly caused by Soret diffusion of H₂ rather than that of H radical. Moreover, the influence of Soret diffusion on the Markstein length is demonstrated to decrease with the initial temperature and pressure.     

Combustion of bimodal nano/micron-sized aluminum particle dust in air

The combustion of bimodal nano/micron-sized aluminum particles with air is studied both analytically and experimentally in a well-characterized laminar particle-laden flow. Experimentally, an apparatus capable of producing Bunsen-type premixed flames was constructed to investigate the flame characteristics of bimodal-particle/air mixtures. The flame speed is positively affected by increasing the mass fraction of nano particles in the fuel formulation despite the lower flame luminosity and thicker flame zone. Theoretically, the flames are assumed to consist of several different regimes for fuel-lean mixture, including the preheat, flame, and post flame zones. The flame speed and temperature distribution are derived by solving the energy equation in each regime and matching the temperature and heat flux at the interfacial boundaries. The analysis allows for the investigation of the effects of particle composition and equivalence ratio on the burning characteristics of aluminum-particle/air mixtures. Reasonable agreement between theoretical results and experimental data was obtained in terms of flame speed. The flame structure of a bimodal particle dust cloud may display either an overlapping or a separated configuration, depending on the combustion properties of aluminum particles at different scales. At low percentages of nano particles in the fuel formulation, the flame exhibits a separated spatial structure with a wider flame regime. At higher nano-particle loadings, overlapping flame configurations are observed.     

A numerical study on extinction behaviour of laminar micro-diffusion flames

We conducted a numerical study on the fluid dynamic, thermal and chemical structures of laminar methane–air micro flames established under quiescent atmospheric conditions. The micro flame is defined as a flame on the order of one millimetre or less established at the exit of a vertically-aligned straight tube. The numerical model consists of convective–diffusive heat and mass transport with a one-step, irreversible, exothermic reaction with selected kinetics constants validated for near-extinction analyses. Calculations conducted under the burner rim temperature 300 K and the adiabatic burner wall showed that there is the minimum burner diameter for the micro flame to exist. The Damköhler number (the ratio of the diffusive transport time to the chemical time) was used to explain why a flame with a height of less than a few hundred microns is not able to exist under the adiabatic burner wall condition. We also conducted scaling analysis to explain the difference in extinction characteristics caused by different burner wall conditions. This study also discussed the difference in governing mechanisms between micro flames and microgravity flames, both of which exhibit similar spherical flame shape.     

Numerical simulation of premixed H2–air cellular tubular flames

The detailed flame structure of laminar premixed cellular flames in the tubular domain is simulated in 2D using a fully-implicit primitive variable finite difference formulation that includes multicomponent transport and detailed chemical kinetics. Numerical results for H₂/air flames are presented and compared against spatially resolved experimental measurements of temperature and chemical species including atomic H and OH. The experimental results compare well for flame structure and cell number, despite the numerical model under-predicting the peak temperature by 200 K. Numerical experiments were performed to assess the ability for cellular tubular flames to impact experimental and numerical investigations of practical flames. The cellular flame structure is found to provide a highly sensitive geometry that is useful for validating diffusive transport modelling approximations. This capability is exemplified through the development of a simple and accurate approximation for thermal diffusion (i.e. the Soret effect) that is suitable for practical combustion codes.     

Role of hydroxyl production and heat release in the two-zone fuel-rich adiabatic dimethyl ether/air flames at atmospheric pressure

Fuel-rich laminar adiabatic flames of premixed dimethyl ether/air mixtures at a high initial temperature and atmospheric pressure have been studied by numerical simulation and sensitivity analysis. These flames, having two heat release zones, are of great interest as an unusual and little-studied subject. We have investigated the chemical processes occurring in the two zones and analysed the mechanism of heat release in the flame. It has been found that the key reactions that have a significant influence on the flame speed are those involving dimethyl ether and the products of its incomplete oxidation. Calculation of the heat release rate confirms the presence of two heat release zones in the flame. A comparison of the reactions making a major contribution to the heat release with those significantly affecting the flame speed indicates that the main factor determining the flame speed is the formation of hydroxyls, rather than heat release. Analysis of the flame speed sensitivity shows that in the case of a two-zone structure of the flame, its speed is mainly determined by the reactions taking place in the low-temperature zone. That is, the cool zone with a higher temperature gradient is the leading one.     

Effects of compression and stretch on the determination of laminar flame speeds using propagating spherical flames

The effects of flow compression and flame stretch on the accurate determination of laminar flame speeds at normal and elevated pressures using propagating spherical flames at constant pressure or constant volume are studied theoretically and numerically. The results show that both the compression-induced flow motion and flame stretch have significant impacts on the accuracy of flame speed determination. For the constant pressure method, a new method to obtain a compression-corrected flame speed (CCFS) for nearly constant pressure spherical bomb experiments is presented. Likewise, for the constant volume method, a technique to obtain a stretch-corrected flame speed (SCFS) at elevated pressures and temperatures is developed. The validity of theoretical results for both constant pressure and constant volume methods is demonstrated by numerical simulations using detailed chemistry for hydrogen/air, methane/air, and propane/air mixtures. It is shown that the present CCFS and SCFS methods not only improve the accuracy of the flame speed measurements significantly but also extend the parameter range of experimental conditions. The results can be used directly in experimental measurements of laminar flame speeds.     

Measurements of self-excited instabilities and nitrogen oxides emissions in a multi-element lean-premixed hydrogen/methane/air flame ensemble

Understanding the distinguishing physical properties of multi-element lean-premixed high hydrogen content flames is expected to be integral to the development of carbon-neutral, and ultimately carbon-free, gas turbine combustion systems. Despite their fundamental importance, the thermoacoustic and emission-related characteristics of such small-scale flame ensembles are not thoroughly understood, particularly for the full range of 0 to 100% hydrogen content blended with methane fuel. Here we investigate the structure and collective behavior of a multi-element lean-premixed hydrogen/methane/air flame ensemble using measurements of nitrogen oxides emissions and self-excited instability, combined with OH* and OH PLIF flame visualizations. Our results indicate that the system's responses can be classified into several distinctive stages according to their static and dynamic stability, including flame blowoff and thermoacoustically stable regions under relatively low hydrogen concentration conditions, low-frequency self-excited instabilities in intermediate hydrogen concentration, and triggering of intense pressure perturbations at about 1.7 kHz under high- or pure hydrogen combustion conditions. While the low-frequency combustion dynamics are dominated by axisymmetric translational movements of parallel flame fronts, the higher frequency response originates from significant lateral modulations accompanied by small-scale vortical rollup and flame surface annihilation due to front merging and pinch-off. Longitudinal-to-transverse dynamic transition is observed to play a mechanistic role in kinematically accommodating higher-frequency heat release rate fluctuations, and this newly identified mechanism suggests the possibility of high-frequency transverse modes, if such lateral motions are strong enough to induce inter-element flame interactions. In contrast to the substantial differences in thermoacoustic properties for different fuel compositions, the total nitrogen oxides emissions are found to depend primarily on adiabatic flame temperature; the influence of fuel composition is limited to approximately 20% under the inlet conditions considered.     

Numerical study of the effects of gravity on soot formation in laminar coflow methane/air diffusion flames under different air stream velocities

Numerical simulations of laminar coflow methane/air diffusion flames at atmospheric pressure and different gravity levels were conducted to gain a better understanding of the effects of gravity on soot formation by using relatively detailed gas-phase chemistry and complex thermal and transport properties coupled with a semi-empirical two-equation soot model. Thermal radiation was calculated using the discrete-ordinates method coupled with a non-grey model for the radiative properties of CO, CO₂, H₂O, and soot. Calculations were conducted for three coflow air velocities of 77.6, 30, and 5 cm/s to investigate how the coflowing air velocity affects the flame structure and soot formation at different levels of gravity. The coflow air velocity has a rather significant effect on the streamwise velocity and the fluid parcel residence time, especially at reduced gravity levels. The flame height and the visible flame height in general increase with decreasing the gravity level. The peak flame temperature decreases with decreasing either the coflow air stream velocity or the gravity level. The peak soot volume fraction of the flame at microgravity can either be greater or less than that of its normal gravity counterpart, depending on the coflow air velocity. At sufficiently high coflow air velocity, the peak soot volume fraction increases with decreasing the gravity level. When the coflow air velocity is low enough, soot formation is greatly suppressed at microgravity and extinguishment occurs in the upper portion of the flame with soot emission from the tip of the flame owing to incomplete oxidation. The numerical results provide further insights into the intimate coupling between flame size, residence time, thermal radiation, and soot formation at reduced gravity level. The importance of thermal radiation heat transfer and coflow air velocity to the flame structure and soot formation at microgravity is demonstrated for the first time.     

Quantitative temperature imaging at elevated pressures and in a confined space with CH_4/air laminar flames by filtered Rayleigh scattering

Laminar methane/air premixed flames at different pressures in a newly developed high-pressure laminar burner are studied through Cantera simulation and filtered Rayleigh scattering(FRS).Different gas component fractions are obtained through the detailed numerical simulations.And this approach can be used to correct the FRS images of large variations in a Rayleigh cross section in different flame regimes.The temperature distribution above the flat burner is then presented without stray light interference from soot and wall reflection.Results also show that the extent of agreement with the single point measurement by the thermocouple is 6%.Finally,this study concludes that the relative uncertainty of the presented filtered Rayleigh scattering diagnostics is estimated to be below 10% in single-shot imaging.     

Computational and experimental study of oxygen-enhanced axisymmetric laminar methane flames

Three axisymmetric laminar coflow diffusion flames, one of which is a nitrogen-diluted methane/air flame (the ‘base case’) and the other two of which consist of nitrogen-diluted methane vs. pure oxygen, are examined both computationally and experimentally. Computationally, the local rectangular refinement method is used to solve the fully coupled nonlinear conservation equations on solution-adaptive grids. The model includes C2 chemistry (GRI 2.11 and GRI 3.0 chemical mechanisms), detailed transport, and optically thin radiation. Because two of the flames are attached to the burner, thermal boundary conditions at the burner surface are constructed from smoothed functional fits to temperature measurements. Experimentally, Raman scattering is used to measure temperature and major species concentrations as functions of the radial coordinate at various axial positions. As compared to the base case flame, which is lifted, the two oxygen-enhanced flames are shorter, hotter, and attached to the burner. Computational and experimental flame lengths show excellent agreement, as do the maximum centreline temperatures. For each flame, radial profiles of temperature and major species also show excellent agreement between computations and experiments, when plotted at fixed values of a dimensionless axial coordinate. Computational results indicate peak NO levels in the oxygen-enhanced flames to be very high. The majority of the NO in these flames is shown to be produced via the thermal route, whereas prompt NO dominates for the base case flame.     

Experimental and numerical study of product gas characteristics of ammonia/air premixed laminar flames stabilized in a stagnation flow

Ammonia has widely attracted interest as a potential candidate not only as a hydrogen energy carrier but also as a carbon free fuel for internal combustion engines, such as gas turbines. Because ammonia contains a nitrogen atom in its molecule, nitrogen oxides (NO_x) and other pollutants may be formed when it burns. Therefore, understanding the fundamental product gas characteristics of ammonia/air laminar flames is important for the design of ammonia-fueled combustors to meet stringent emission regulations. In this study, the product gas characteristics of ammonia/air premixed laminar flames for various equivalence ratios were experimentally and numerically investigated up to elevated pressure conditions. In the experiments, a stagnation flame configuration was employed because an ammonia flame can be stabilized by using such a configuration without a pilot flame. The experimental results showed that the maximum NO mole fraction was about 3,500 ppmv, at an equivalence ratio of 0.9 at 0.1 MPa. The NO mole fraction decreased as the equivalence ratio increased. In addition, the maximum value of the NO mole fraction decreased with an increase in mixture pressure. Furthermore, it was experimentally clarified that the simultaneous reduction of NO and unburnt ammonia can be achieved at an equivalence ratio of about 1.06, which is the target equivalence ratio for emission control in rich-lean two-stage ammonia combustors. Comparison of experimental and numerical results showed that even though the reaction mechanisms employed have been optimized for predicting the laminar burning velocity of ammonia/air flames, they failed to satisfactorily predict the measured species in this study. Sensitivity analysis was used to identify elementary reactions that control the species profiles but have negligible effects on the burning velocity. It is considered that these reaction models need to be updated for accurate prediction of product gas characteristics of ammonia/air flames.     

Tabulated chemistry approaches for laminar flames: Evaluation of flame-prolongation of ILDM and flamelet methods

The present study considers the performance of tabulation methods for numerical simulation of complex chemical kinetics in laminar combusting flows and compares their predictions to results obtained by direct calculation. Two tabulation methods are considered: the Flame Prolongation of Intrinsic low-dimensional manifold (FPI) method and Steady Laminar Flamelet Model (SLFM). The FPI method is of current interest as it is a potentially unifying approach capable of dealing with both premixed and non-premixed flames for gaseous fuels. SLFM tabulation methods are popular for non-premixed flames and form a good basis for comparing the performance of the FPI approach. The performance of each method is also evaluated by comparing the results to the direct simulation of the laminar flames using two chemical kinetic schemes: simplified chemistry involving five species and one reaction and detailed chemistry involving 53 species and 325 reaction steps. As part of the evaluation process, the computational cost of each method is also assessed. The laminar flames considered in this study include: freely propagating laminar premixed flames, a two-dimensional axisymmetric methane–air opposed-jet diffusion flame, and a two-dimensional axisymmetric methane–air co-flow diffusion flame. Both tabulation methods are implemented in a parallel adaptive mesh refinement (AMR) framework for solving the complete set of governing partial differential equations. These equations are solved using a fully-coupled finite-volume formulation on body-fitted multi-block quadrilateral mesh. Significant improvements in terms of reduced computational requirements, as measured by both storage and processing time, are demonstrated for the tabulated methods.     

Numerical investigation of flame–vortex interactions in laminar cross-flow non-premixed flames in the presence of bluff bodies

Flame stabilisation in a combustor having vortices generated by flame holding devices constitutes an interesting fundamental problem. The presence of vortices in many practical combustors ranging from industrial burners to high speed propulsion systems induces vortex–flame interactions and complex stabilisation conditions. The scenario becomes more complex if the flame sustains after separating itself from the flame holder. In a recent study [P.K. Shijin, S.S. Sundaram, V. Raghavan, and V. Babu, Numerical investigation of laminar cross-flow non-premixed flames in the presence of a bluff-body, Combust. Theory Model. 18, 2014, pp. 692–710], the authors reported details of the regimes of flame stabilisation of non-premixed laminar flames established in a cross-flow combustor in the presence of a square cylinder. In that, the separated flame has been shown to be three dimensional and highly unsteady. Such separated flames are investigated further in the present study. Flame–vortex interactions in separated methane–air cross flow flames established behind three bluff bodies, namely a square cylinder, an isosceles triangular cylinder and a half V-gutter, have been analysed in detail. The mixing process in the reactive flow has been explained using streamlines of species velocities of CH₄ and O₂. The time histories of z-vorticity, net heat release rate and temperature are analysed to reveal the close relationship between z-vorticity and net heat release rate spectra. Two distinct fluctuating layers are visible in the proper orthogonal decomposition and discrete Fourier transform of OH mass fraction data. The upper fluctuating layer observed in the OH field correlates well with that of temperature. A detailed investigation of the characteristics of OH transport has also been carried out to show the interactions between factors affecting fluid dynamics and chemical kinetics that cause multiple fluctuating layers in the OH.