Stability Of Diffusion Flames Research Articles

Influence of Heat Loss on the Stability of Diffusion Flames in a Narrow-Channel Shear Flow

ABSTRACTUnder shear-enhanced diffusion (Taylor dispersion), a planar diffusion flame has been found recently to undergo a new cellular instability in mixtures with Lewis number Le>1 under adiabatic conditions, which may explain the formation of so-called flame streets in microcombustion experiments. Since heat losses are expectedly significant in such experiments, it is important to examine how they influence this Taylor dispersion-induced cellular instability. The paper is mainly dedicated to clarifying this issue, using a linear stability analysis and time-dependent numerical simulations, based on a two-dimensional depth-averaged model accounting for heat losses and Taylor dispersion. The investigation is focused on near-extinction conditions where the planar diffusion flame is known to become unstable, that is near the two extinction points a non-adiabatic diffusion flame possesses, namely the so-called diffusive extinction point occurring at a relatively small Damköhler number and the heat-loss extinction point, occurring at a larger Damköhler number. The results confirm that the cellular instability in Le>1 mixtures survive in the presence of heat losses, provided the Péclet number Pe exceeds a critical value. Stability regime diagrams in the Le-Pe plane are presented, identifying the three types of instabilities encountered, namely, the cellular instability aforementioned, an oscillatory planar instability and a non-oscillatory planar instability. The time-dependent numerical simulations demonstrate the development of the instabilities predicted by the linear stability analysis. In particular, flames undergoing the cellular instability are found to evolve ultimately into one of the three states: an extinguished state, an apparently stable steady cellular structure, and an unsteady state with complex dynamics involving triple-flame propagation, splitting and merging of cells. In general, for the cellular structures generated by the instability to be stabilized, larger values of Pe and Le are found to be required, in order to counteract quenching by heat losses.

Stability of diffusion flames under shear flow: Taylor dispersion and the formation of flame streets

Diffusion flame streets, observed in non-premixed micro-combustion devices, align parallel to a shear flow. They are observed to occur in mixtures with high Lewis number (Le) fuels, provided that the flow Reynolds number, or the Peclet number Pe, exceeds a critical value. The underlying mechanisms behind these observations have not yet been fully understood. In the present paper, we identify the coupling between diffusive-thermal instabilities and Taylor dispersion as a mechanism which is able to explain the experimental observations above. The explanation is largely based on the fact that Taylor dispersion enhances all diffusion processes in the flow direction, leading effectively to anisotropic diffusion with an effective (flow-dependent) Lewis number in the flow direction which is proportional to 1/Le for Pe≫1. Validation of the identified mechanism is demonstrated within a simple model by investigating the stability of a planar diffusion flame established parallel to a plane Poiseuille flow in a narrow channel. A linear stability analysis, leading to an eigenvalue problem solved numerically, shows that cellular (or finite wavelength) instabilities emerge for high Lewis number fuels when the Peclet number exceeds a critical value. Furthermore, for Peclet numbers below this critical value, longwave instabilities with or without time oscillations are obtained. Stability regime diagrams are presented for illustrative cases in a Le−Pe plane where various instability domains are identified. Finally, the linear analysis is supported and complemented by time dependent numerical simulations, describing the evolution of unstable diffusion flames. The simulations demonstrate the existence of stable cellular structures and show that the longwave instabilities are conducive to flame extinction.

Anchored and Lifted Diffusion Flames Supported by Symmetric and Asymmetric Edge Flames

Numerous combustion applications are concerned with the stabilization of diffusion flames formed by injecting gaseous fuels into a co-flowing stream containing an oxidizer. The smooth operation of these devices depends on the attachment and lift-off characteristics of the edge flame at the base of the diffusion flame. In this paper, we address fundamental issues pertinent to the structure and dynamics of edge flames, which have attributes of both premixed and diffusion flames. The adopted configuration is the mixing layer established in the wake of a splitter plate where two streams, one containing fuel and the other oxidizer, merge. The analysis employs a diffusive-thermal model which, although it excludes effects of gas expansion, systematically includes the influences of the overall flow rate, unequal strain rates in the incoming streams, stoichiometry, differential and preferential diffusion, heat loss and gas–solid thermal interaction, and their effect on the edge structure, speed, and temperature. Conditions when the edge flame is anchored to the plate, lifted-off and stabilized in the flow, or blown-off, are identified. Two stable modes of stabilization are observed for lifted flames; the edge flame either remains stationary at a specified location or undergoes spontaneous oscillations along a direction that coincides with the trailing diffusion flame.

The Speed and Temperature of an Edge Flame Stabilized in a Mixing Layer: Dependence on Fuel Properties and Local Mixture Fraction Gradient

ABSTRACT Edge flames are of fundamental importance to the attachment and stabilization of diffusion flames in many practical devices, and serve as transient structures for establishing combustion in locally-quenched regions of turbulent diffusion flames. In this study, we consider an edge flame sustained in the mixing layer of two streams, one containing fuel and the other oxidizer, and examine its structure and its thermal and dynamic properties over a wide range of flow conditions, using a diffusive-thermal (constant-density) model. In addition, we allow for the fuel Lewis number to vary from sub- to super-unity values, corresponding to light and moderately heavy fuels, or fuels suitably diluted by inert species, with unity oxidizer Lewis number as appropriate for combustion in air. Two regimes have been identified and examined; a thermal-interaction regime where there is appreciable heat loss from the edge flame to the splitter plate, and a freely-standing regime where the edge flame lifts off considerably and the plate plays no role on its stabilization. The objective is to correlate important edge-flame properties, including standoff distance and edge speed and temperature, with global parameters such as overall flow rate and total heat conducted from the flame to the plate, and local parameters such as local mixture fraction gradient at the edge of the flame.

A numerical investigation of CO2 dilution on the thermochemical characteristics of a swirl stabilized diffusion flame

The turbulent combustion flow modeling is performed to study the effects of CO2 addition to the fuel and oxidizer streams on the thermochemical characteristics of a swirl stabilized diffusion flame. A flamelet approach along with three well-known turbulence models is utilized to model the turbulent combustion flow field. The k-ω shear stress transport (SST) model shows the best agreement with the experimental measurements compared with other models. Therefore, the k-ω SST model is used to study the effects of CO2 dilution on the flame structure and strength, temperature distribution, and CO concentration. To determine the chemical effects of CO2 dilution, a fictitious species is replaced with the regular CO2 in both the fuel stream and the oxidizer stream. The results indicate that the flame temperature decreases when CO2 is added to either the fuel or the oxidizer stream. The flame length reduction is observed at all levels of CO2 dilution. The H radical concentration indicating the flame strength decreases, following by the thermochemical effects of CO2 dilution processes. In comparison with the fictitious species dilution, the chemical effects of CO2 addition enhance the CO mass fraction. The numerical simulations show that when the dilution level is higher, the rate of the flame length reduction is more significant at low swirl numbers.

Laser-induced incandescence versus photo-acoustics: implications for qualitative soot size diagnostics

Laser-induced incandescence at high-repetition rates can in principle be used to resolve the temporal evolution of soot processes. The intrusive character of this technique, however, requires due care of historical effects associated with multiple exposures of individual soot particles to laser light. On the other hand, repetitive heating and cooling opens up an independent, acoustic detection channel. We illustrate a photo-acoustic soot volume fraction measurement, and show that the comparison to simultaneously recorded laser-induced incandescence provides qualitative information on soot growth. Experiments are performed on a propane-fueled, co-flow stabilized diffusion flame, and signals are collected at varying heights above the burner deck. Results show a clear correlation between the laser-induced incandescence and photo-acoustic signals; small deviations are interpreted as a qualitative indicator for the particle size.

A conceptual model of the flame stabilization mechanisms for a lifted Diesel-type flame based on direct numerical simulation and experiments

This work presents an analysis of the stabilization of diffusion flames created by the injection of fuel into hot air, as found in Diesel engines. It is based on experimental observations and uses a dedicated Direct Numerical Simulation (DNS) approach to construct a numerical setup, which reproduces the ignition features obtained experimentally. The resulting DNS data are then used to classify and analyze the events that allow the flame to stabilize at a certain Lift-Off Length (LOL) from the fuel injector. Both DNS and experiments reveal that this stabilization is intermittent: flame elements first auto-ignite before being convected downstream until another sudden auto-ignition event occurs closer to the fuel injector. The flame topologies associated to such events are discussed in detail using the DNS results, and a conceptual model summarizing the observation made is proposed. Results show that the main flame stabilization mechanism is auto-ignition. However, multiple reaction zone topologies, such as triple flames, are also observed at the periphery of the fuel jet helping the flame to stabilize by filling high-temperature burnt gases reservoirs localized at the periphery, which trigger auto-ignitions.

Experimental and numerical study on bluff-body and swirl stabilized diffusion flames

Bluff-body and swirl flow are commonly utilized to stabilize diffusion flames in industrial applications, such as gas turbines, ramjets and furnaces. Flame stabilization mechanisms of these two kinds of burners are similar with each other: the interaction between the recirculation zone and the fuel jet. In the present paper, flow fields within flames stabilized by combinations of swirl flow and bluff-body were captured using high-speed PIV; while the flame structures were visualized by high-speed CH2O PLIF, CH∗ chemiluminescence and broadband chemiluminescence. The global CO emissions from the flames were captured as well. In addition, based on the CFD software OpenFOAM, simulations were adopted to better understand the interactions between flames and flow structures. Flames stabilized by bluff-bodies with different diameters (db = 14 mm and 20 mm), or only by swirl flow without a bluff-body, were studied. All reacting experiments were carried out with a constant mass flow rate of the central fuel jet (with thermal power 2.01 kW) and a constant mass flow rate of the total air flow (m = mt + ma = 200 ln/min). The swirl strength was controlled by the mass flow rate ratio of the tangential to the axial air flow. The geometrical swirl number was altered between Sg = 0 and Sg = 4.08. Simulation results matched well with experimental data, especially in predicting the spatial distribution of CH2O. The position of the outer recirculation zone would be affected by the size of the bluff-body and the swirl strength. In addition, the recirculation zone determined the flame structures and the global CO emission levels. With a larger bluff-body, the air driven recirculation zone located more upstream near the burner exit. Flame prone to be more stable with a larger bluff-body and/or a stronger swirl flow. Flame was observed propagating into the upstream region in cases without a bluff-body or in cases with the small bluff-body (db = 14 mm), when the swirl strength was sufficiently strong. The mechanism for the diffusion flame ‘flashback’ was proposed. Flames in cases with a larger swirl number were shorter while its CO emission levels were higher.

Advances in understanding combustion phenomena using non-premixed and partially premixed counterflow flames: A review

Counterflow flames provide an ideal platform for understanding the flame structure and as a model to study the effect of physical and chemical perturbations on the flame structure. This article reviews the advances made in the understanding of combustion dynamics and chemistry through experimental and numerical studies in counterflow non-premixed and partially premixed flames. Key contributions on fundamental aspects such as extinction, ignition and effect of perturbations on the stability of diffusion flames are first summarized and analysed. The review then focuses on the progress made in the understanding of the effect of inert particles and flame suppressants on the flame characteristics. A review of detailed studies on edge flames facilitates further understanding of local quenching and re-ignition phenomena in highly turbulent flames. The influence of radiation model and unsteady flow-conditions on the flame kinetics and dynamics along with work on NOx kinetics has been discussed. The review also outlines that specific experiments need to be carried out over a wide range of conditions for further understanding and validation of numerical models.

Global stability analysis of lifted diffusion flames

This work describes the development of a method for the global hydrodynamic stability analysis of diffusion flames. The low-Machnumber (LMN) Navier–Stokes (NS) equations for reacting flows are solved together with a transport equation for the mixture fraction describing the local composition of the fluid. The equations are solved by the spectral-element code NEK5000 with Legendre polynomial reconstruction of degree twelve and second-order accurate Runge-Kutta time integration scheme. In order to compute the base flow for the stability analysis, a selective frequency damping approach has been employed. The global stability analysis has been performed by a matrix-free time-stepper algorithm applied to the LMN-NS equations, using an Arnoldi method to compute the most unstable modes. Moreover, a numerical linearization of the governing equation is employed, which allows one to study the stability of diffusion flames without the direct evaluation and storage of the linearized operator. Therefore, a remarkable reduction of the storage capacity is achieved and a more flexible numerical approach is obtained. The numerical model has been validated by comparison with the results for the axisymmetric diffusion flame available in the literature.

Stabilization of diffusion flames of impacting and opposing fuel jets

Stability of diffusion combustion of fuel jets impacting on targets and interaction of two jets (pure propane–butane and the same fuel with addition of carbon dioxide) are experimentally studied. Data on combustion failure are obtained for various variants of jet interaction, as well as information about the composition of combustion products, air-to-fuel ratio, and completeness of combustion before combustion failure. It is shown that the domain of stable combustion of impacting and opposing jets is expanded by more than an order of magnitude as compared to combustion of individual free jets.

Stabilization of Diffusion Flames of Impacting and Opposing Fuel Jets

Stabilization of Diffusion Flames of Impacting and Opposing Fuel Jets

The impact of hydrogen enrichment and bluff-body lip thickness on characteristics of blended propane/hydrogen bluff-body stabilized turbulent diffusion flames

At the beginning of this study, the well-known turbulent bluff-body stabilized diffusion flame of HM1 is simulated by a coupled flamelet/radiation approach. The HM1 flame comprises a CH4:H2 [50:50 Vol.] jet flame at a Reynolds number of 15,800. The results showed reasonable agreement for the flow field and species. Afterwards, the abovementioned approach is employed to investigate the effects of hydrogen addition on bluff-body stabilized flames of propane–hydrogen. Adding hydrogen to the blended fuel of propane/hydrogen shifts the recirculation zone outwards the bluff-body and thus culminates in increased flame length. Besides this, the flame length is predicted to be enhanced with decreasing the lip thickness of the bluff-body configuration. The CO emission level is found to be decreased with hydrogen addition in near-burner and far field regions which might be attributed to the decrease of inflow carbon atoms. The local radiative heat power reveals higher values for fuel blends with decreased contents of hydrogen at the recirculation and jet-like zones. This might be attributed to the increased local heat release rate due to breaking further carbon bonds.

Diffusion Flame Stability of Low Calorific Fuels

Mechanisms related to diffusion flame stabilization have been the subject of several studies within the last decades due the industrial and scientific interests. Information on flame stability is of fundamental importance in energy efficiency and safety regarding industrial applications. Thus, an experimental study was performed in order to examine the flame characteristics and regions of stability limits. In this study, a representative burner of industrial applications was employed, which allows the stabilization of several combustion regimes. The lift-off and blow-out flame regimes were investigated for different proportions of carbon dioxide in natural gas. In this way, an analysis of the calorific fuel influence on the flame stability was performed based on the measurements and a comparison with classical literature models was done. The fuel dilution by adding carbon dioxide was found to decrease the soot production, leading to lower flame heights and also, lower lift-off and blow-out limits. Results obtained from this study encourage future works which consider flames in large scale, in order to equate to industrial applications.

The Effect of Percentage of CO<sub>2</sub> on the Biogas Counterflow Diffusion Flame Stability

This study aims to determine the effect of the percentage of CO2 in the biogas to the characteristics of biogas diffusion flame. Counterflow configuration was used in this study to investigate detail structure of diffusion flame and the flame stability of biogas. The concentration of CO2 in the biogas was varied from 0% to 50% while the mass flow rate of the reactants was varied from 4 to 14 L / min. The results showed that the CO2 in the biogas fuel affect the diffusion flame characteristics, especially the area of luminous yellow flame formed in the fuel flow. In the low oxygen concentration, percentage of CO2 did not affect the diffusion flame stability. However, the flame stability was more influenced by the rate of diffusion between fuel and oxygen. Therefore, the combustion of biogas is more recommended to be done in the low oxygen concentration.

Investigation of noise radiation from a swirl stabilized diffusion flame with an array of microphones

Next generation of combustors are expected to provide significant improvement on efficiency and reduced pollutants emission. In such combustors, the challenges of local flow, pressure, chemical composition and thermal signatures as well as their interactions will require detailed investigation for seeking optimum performance. Sensor networks with a large number of sensors will be employed in future smart combustors, which will allow one to obtain fast and comprehensive information on the various ongoing processes within the system. In this paper sensor networks with specific focus on an array of homogeneous microphones are used examine the spectral characteristics of combustion noise from a non-premixed combustor. A non-premixed double concentric swirl-flame burner was used. Noise spectra were determined experimentally for the non-premixed swirl flame at various fuel–air ratios using an array of homogeneous condenser microphones. Multiple microphones positioned at discrete locations around the turbulent diffusion flame, provided an understanding of the total sound power and their spectral characteristics. The growth and decay coefficients of total sound power were investigated at different test conditions. The signal coherence between different microphone pairs was also carried out to determine the acoustic behavior of a swirl stabilized turbulent diffusion flame. The localization of acoustic sources from the multiple microphones was examined using the noise spectra. The results revealed that integration of multiple sensors in combustors and reactors does allow one to obtain additional information which can be further tailored for the quest to develop advanced combustors.

Thermo-acoustic behavior of a swirl stabilized diffusion flame with heterogeneous sensors

Next generation combustors are expected to be significantly more efficient while reducing pollutants and eliminating carbon emissions. In such combustors, the challenges of local flow, pressure, chemical composition and thermal signatures as well as their interactions require understanding to seek for optimum performance of the system. The current practice of using a single sensor to measure certain parameters at a single location cannot provide sufficient information to achieve desirable and optimum overall performance of the combustor. A high density sensor network with a large number of sensors will be required in future smart combustors to obtain detailed information on the various ongoing processes within the system. As an initial step towards the development of such sensor networks, the effect of mean and fluctuating temperature distribution on the distribution of acoustic sources within the flame has been examined by using a thermocouple and condenser microphone using swirl stabilized diffusion flames. The measurement of high frequency temperature signal allowed observation of characteristic mean and fluctuating temperatures, and thermal stratification characteristics from within the flame. Specifically mean and fluctuating temperatures, integral and micro-thermal time scales have been determined at various spatial locations in the flame. Investigation of the thermal field and their effect on the localization of acoustic sources in the two flames formed at different equivalence ratios has been examined. The thermal characteristics data obtained provided a better insight on the thermal behavior of co-swirl diffusion flames. Noise spectra for varying air–fuel ratios were determined. Results of time average and fluctuating temperature and sound pressure level spectra showed noise emission in flames to lie near to the regions of high temperature which result in pressure fluctuations within the flame. The results are complemented with 3D CFD simulations that supported the localization of the acoustic sources within the turbulent diffusion flames.

Experimental realization and characterization of unstretched planar one-dimensional diffusion flames

A burner configuration that allows the creation of nearly unstretched one-dimensional diffusion flames has recently been introduced. This paper presents the first detailed characterization of flames produced in such a burner to assess how well they approach the idealized one-dimensional configuration, which is used extensively for the development of theoretical models for diffusion flame stability. Particular emphasis is directed at quantifying the remaining inhomogeneities in the burner, such as residual flame stretch or variations in the boundary conditions, and identifying possible means to minimize them. Measurements made include the velocity field, stable species concentration profiles and temperature distribution throughout the burning chamber. Additionally, the flame position in the burner is determined as a function of mixture strength, reactant transport properties and bulk flow velocity. All the measurements are found to be in good agreement with a simplified one-dimensional flame model. However, the boundary conditions and their physical location have to be determined experimentally on both sides of the flame, which complicates comparison with results from theoretical and numerical models. Despite these limitations, we conclude that this new burner type provides a good experimental approximation of the one-dimensional idealized construct.

Effect of Low Frequency Burner Vibrations on the Characteristics of Jet Diffusion Flames

Mechanical vibrations introduced in diffusion flame burners significantly affect the flame characteristics. In this experimental study, the effects of axial vibrations on the characteristics of laminar diffusion flames are investigated systematically. The effect of the frequency and amplitude of the vibrations on the flame height oscillations and flame stability is brought out. The amplitude of flame height oscillations is found to increase with increase in both frequency and amplitude of burner vibrations. Vibrations are shown to enhance stability of diffusion flames. Although flame lifts-off sooner with vibrations, stability of the flame increases.

Simulation of spray development and turbulent combustion processes in low and high speed diesel engines by the CMC-ISR model

Simulation is performed to analyse the characteristics of turbulent spray combustion in conventional low and high speed diesel engine conditions. Turbulence–chemistry interaction is resolved by the Conditional Moment Closure (CMC) model in the spatially integrated form of an Incompletely Stirred Reactor (ISR). After validation against measured pressure traces, characteristic length and time scales and dimensionless numbers are estimated at the locations of sequentially injected fuel groups. Conditional flame structures are calculated for sequentially evaporated fuel groups to consider different available periods for ignition chemistry. Injection overlaps the combustion period in the high rpm engine, while most combustion occurs after injection and evaporation are complete in the low rpm engine. Ignition occurs in rich premixture with the initial peak temperature at the equivalence ratio around 2–4 as observed in Dec [2]. It corresponds to the most reactive mixture fraction of the minimum ignition delay for the given mixture states. Combustion proceeds to lean and rich sides in the mixture fraction space as a diffusion process by turbulence. The mean scalar dissipation rates (SDRs) are lower than the extinction limit to show stability of diffusion flames throughout the combustion period.