Flame Angle Research Articles

Optimum Multinozzle Configuration for Minimizing the Rayleigh Integral During High-Frequency Transverse Instabilities

Abstract This paper develops a formalism for optimizing nozzle location/configuration with respect to combustion stability of high-frequency transverse modes in a can combustor. The stability of these acoustically noncompact flames was assessed using the Rayleigh integral (RI). Several key control parameters influence RI—flame angle, swirling strength, nozzle location, as well as nozzle location with respect to the acoustic mode shape. In this study, we consider a N-around-1 configuration such as typically used in a multi-nozzle can system and study the overall stability of this system for different natural transverse modes. Typically, such nozzles are distributed in a uniformly circular manner for which we study the overall RI, and for cases where RI > 0, we optimize the nozzle distribution that can reduce and minimize RI. For a fixed geometry such as a circular configuration, the analysis shows how the flame's parameters must vary across the different nozzles, to result in a relatively stable system. Additionally, for a fixed set of flame parameters, the analysis also indicates the noncircular distribution of the N nozzles that minimizes RI. Overall, the analysis aims to provide insights on designing nozzle locations around the center nozzle for minimal amplification of a given transverse mode.

The Effect of Biogas Purification Process Using Calcium Oxide-Based Sorbents on the Diffusion Flame Combustion Characteristics

Biogas is an alternative energy source that could solve two problems at once, the problem of environmentally friendly energy needs and the problem of waste treatment. One of the sources of biogas is obtained from anaerobic bacterial fermentation of cow dung waste. The biogas fermentation process produces impurities such as carbon dioxide (CO2) as a combustion inhibitor. Carbon dioxide will inhibit the combustion reaction, resulting in incomplete combustion. The biogas purification process is needed to reduce the amount of carbon dioxide in the biogas. The purification process is carried out using an absorbent compound of calcium oxide (CaO) to bind carbon dioxide contained in the biogas. Thus, the purpose of this study was to analyze the effect of the biogas purification process using calcium oxide on the characteristics of the diffusion flame produced by combustion. The research was conducted experimentally using the physicochemical-absorption method of purification by flowing biogas through a purificator device that contained a purification solution. After passing through the purification solution, the biogas was regulated at a fuel flow rate of 3 liters/min and then proceed to the bunsen burner. The results showed that purification affected increasing the characteristics of the diffusion flame combustion due to the reduced amount of carbon dioxide in the biogas. This is indicated by increasing the purification molarity, it also increases the flame speed of combustion and the flame angle, as well as a decrease in the flame height.

A combined oscillation cycle involving self-excited thermo-acoustic and hydrodynamic instability mechanisms

The paper examines the combined effects of several interacting thermo-acoustic and hydrodynamic instability mechanisms that are known to influence self-excited combustion instabilities often encountered in the late design stages of modern low-emission gas turbine combustors. A compressible large eddy simulation approach is presented, comprising the flame burning regime independent, modeled probability density function evolution equation/stochastic fields solution method. The approach is subsequently applied to the PRECCINSTA (PREDiction and Control of Combustion INSTAbilities) model combustor and successfully captures a fully self-excited limit-cycle oscillation without external forcing. The predicted frequency and amplitude of the dominant thermo-acoustic mode and its first harmonic are shown to be in excellent agreement with available experimental data. Analysis of the phase-resolved and phase-averaged fields leads to a detailed description of the superimposed mass flow rate and equivalence ratio fluctuations underlying the governing feedback loop. The prevailing thermo-acoustic cycle features regular flame liftoff and flashback events in combination with a flame angle oscillation, as well as multiple hydrodynamic phenomena, i.e., toroidal vortex shedding and a precessing vortex core. The periodic excitation and suppression of these hydrodynamic phenomena is confirmed via spectral proper orthogonal decomposition and found to be controlled by an oscillation of the instantaneous swirl number. Their local impact on the heat release rate, which is predominantly modulated by flame-vortex roll-up and enhanced mixing of fuel and oxidizer, is further described and investigated. Finally, the temporal relationship between the flame “surface area,” flame-averaged mixture fraction, and global heat release rate is shown to be directly correlated.

Flame trajectory of a non-vertical turbulent buoyant jet flame

This paper presents a theoretical and experimental study on the flame trajectory of a non-vertical turbulent diffusion flame in still air. A series of propane turbulent flame tests (inner diameter of the burner: d=13 mm) with different initial Froude numbers Fr0 (31.4, 125.5, 282.3, 501.8, and 784.1) and initial flame angles θ0 (60∘, 45∘, 30∘, 15∘, 0∘, −30∘, and −45∘) are performed. The flame motion is physically described based on the Favre-averaged governing equations. The effects of buoyancy and streamline curvature on turbulent mixing are characterized by the mixing length theory. By theoretical derivations, a Richardson number (Ria) is proposed to characterize the flame asymmetry, and a flame trajectory predictive model coupled with the effect of flame asymmetry is established. Experimental results indicate that under the same conditions, the flame curvature first increases and then decreases (eventually tends to be zero) with the continuous increase of Fr0, and increases gradually with the decrease of θ0. A flame intermittency fitting method and a temperature fitting method based on the normal bi-Gaussian distribution are proposed to determine the flame trajectory under different flame asymmetry. These methods are validated by experimental data. It is found that in the near field where the flame is curved, the peak trajectory deviates from the flame midline. In the far field, as the flame recovers to be symmetric, the peak trajectory tends to coincide with the flame midline. Furthermore, experimental data verify that the proposed model can accurately predict the flame trajectory under different flame asymmetry. Finally, results indicate that, under the same conditions, the effect of flame asymmetry on the flame trajectory first increases and then decreases (eventually disappears) with the continuous increase of Fr0, and increases continuously with the decrease of θ0. Comparison between model prediction and experimental data verifies that Ria is the key parameter for the flame asymmetry.

Experimental study of the downward flame spreading characteristics of building facade flexible polyurethane with a lateral environment wind effect

This work looks into the relationship between the downward burning behavior of flexible polyurethane (FPU) and horizontal wind speed. The flame geometry, burning rate, flame radiation, and temperature field under different wind speeds were studied experimentally. The results showed that the flame front on the windward side inclined obviously, and the flame front on the leeward side shrank gradually with the increase of wind speed and disappeared when the wind speed increased to a critical value. The burning rate and fuel melting/dripping increased first and then decreased because of the reduced flame radiation angle coefficient and the cooling and flame stripping effects induced by increased wind speed. Froude number and flame angle are positively correlated with the wind speed. The relationship between the flame angle and dimensionless velocity index is significantly lower than that of pool fire due to the inference of backplane on airflow field. The temperature at leeward side is higher than that on windward side. The peak of radiant heat flux decreases with the increase of wind speed because the inclination of the flame surface reduces the radiation area. The results of this study will permit assessments of high-rise building fire safety issues in actual windy environments.

Hybrid Large Eddy Simulation/Reynolds-Averaged Navier–Stokes Analysis of a Premixed Ethylene-Fueled Dual-Mode Scramjet Combustor

Hydrocarbon fuels offer optimal high energy per volume for scramjet applications for sustained hypersonic flight but require additional residence time due to slower ignition delays (compared with hydrogen fuel). The injection of ethylene at the start of the isolator of a dual-mode scramjet combustor, operating in ramjet mode, allows sufficient mixing to achieve efficient premixed turbulent combustion. A cavity flameholder anchors the flame, supplying sufficient radicals to sustain a stable flame in the high-speed environment. This work investigates flame structure and stabilization limits of a new configuration with a scaled-down cavity embedded in the flow path of the combustor with a strut and insert. The cavity is reduced in size by one-third to enable eventual direct numerical simulations of the flame stabilization process. This work, however, focuses on modeling the full isolator/combustor geometry using a hybrid large eddy simulation/Reynolds-averaged Navier–Stokes simulation strategy. Particle image velocimetry, planar laser-induced fluorescence, coherent anti-Stokes Raman spectroscopy, and pressure measurements are compared with numerical predictions to analyze and characterize the conditions within the combustor, including flame structure, flow velocities, species composition, and wall pressure. An adjustable air throttle in the extender is able to control the placement of the shock train in the isolator and maintain a stable flame at various equivalence ratios. The simulations show reasonably good agreement with the experimental scalar and velocity data and predict a flame angle consistent with premixed turbulent flame-speed correlations.

Effect of wall boundary conditions on the nonlinear response of turbulent premixed flames

Fully compressible Large Eddy Simulations (LES) are performed to investigate the effect of wall boundary conditions on the nonlinear response of turbulent premixed flames. For the unforced flames, the flame length and flame angle from the present LES results in the isothermal case are in better agreement with experimental results than those in the adiabatic case. For the forced flames, both amplitudes and phases of nonlinear flame response at moderate frequency are well reproduced in the isothermal case. While in the adiabatic case, the amplitudes are well reproduced, the prediction of phases is slightly underestimated. At high frequency, LES results from the isothermal case provide reasonable agreement with the experimental results in general, while the adiabatic case gives unreasonable prediction of nonlinear flame response. Results show that wall boundary conditions affect the spatial distributions of heat release fluctuations by changing the temperature field, which is affected by the evolutions of the flame structure and flow field in the nonlinear oscillation cycle. At moderate frequency, heat release fluctuations in different parts of the combustor are in phase, and wall boundary conditions have limited influence on the global flame response, while at high frequency, wall boundary conditions have significant influence on the global flame response since the heat release fluctuations in different parts of combustor are out of phase. Accurate prediction of flame response at high frequency needs accurate calculation of the wall temperature.

On the non-monotonic behaviour of fire spread

A conceptual model based on the dynamic interaction between fire, the fuel bed and the surrounding flow to explain the non-monotonic or intermittent behaviour of fires is proposed. According to the model, even in nominally permanent and uniform boundary conditions, the fire-induced flow modifies the geometry of the flame and its rate of spread. After an initial acceleration, there is a reduction in the rate of spread followed by one or more cycles of growth. Carefully controlled experiments of fires in slopes and canyons show that the evolution of fire properties, namely flame angle and rate of spread, have high-frequency oscillations superimposed on the low-frequency fire growth cycle described above.

Significance of the Direct Excitation Mechanism for High-Frequency Response of Premixed Flames to Flow Oscillations

Abstract Premixed flames are sensitive to flow disturbances, which can arise from acoustic or vortical fluctuations. For transverse instabilities, it is known that a dominant mechanism for flame response is “injector coupling,” whereby pressure oscillations associated with transverse waves excite axial flow disturbances. These axial flow disturbances then excite heat release oscillations. The objective of this paper is to consider another mechanism—the direct sensitivity of the unsteady heat release to transverse acoustic waves—and to compare its significance relative to the induced axial disturbances, in a linear framework. The rate at which the flame adds energy to the disturbance field is quantified using the Rayleigh criterion and evaluated over a range of control parameters, such as flame length and swirl number. The results show that radial modes induce heat release fluctuations that always add energy to the acoustic field, whereas heat release fluctuations induced by mixed radial-azimuthal modes can add or remove energy. These amplification rates are then compared to the flame response from induced axial fluctuations. For combustor-centered flames, these results show that the direct excitation mechanism has negligible amplification rates relative to the induced axial mechanism for radial modes. For transverse modes, the fact that the nozzle is located at a pressure node indicates that negligible induced axial velocity disturbances are excited; as such, the direct mechanism dominates. For flames that are not centered on pressure nodes, the direct mechanism for mixed modes dominates for certain nozzle locations and flame angles.

Stabilization mechanisms of CH4 premixed swirled flame enriched with a non-premixed hydrogen injection

High-fidelity Large Eddy Simulations (LES) are performed to study the effect of hydrogen injection on a lean turbulent CH4/Air premixed flame. An Analytically Reduced Chemistry (ARC) mechanism is used to achieve a detailed description of CH4/Air-H2 chemistry. First, a validation of this kinetic scheme against the detailed GRI-Mech 3.0 mechanism is presented considering both simplified and complex transport properties. When hydrogen is added to the mixture, large variations of the mixture Prandtl and of the N2 Schmidt numbers are observed depending on the local species concentrations, features that are missed by simplified models. LES is then applied to study the structure and stabilization mechanisms of a lean (ϕ = 0.8) premixed CH4/Air swirled flame enriched with hydrogen by using different transport modeling strategies. First, the fully premixed CH4/Air case is considered and results are found to validate the LES approach. In agreement with experiments, a classical V-shape flame is stabilized in the low-velocity region near the flame holder created by a central recirculation zone (CRZ). Then, hydrogen enrichment is achieved injecting 2% of the CH4 thermal power with a central fuel injection lance. Both premixed and diffusion flame branches are present in this case, impacting flame stabilization and flame angle. The flame root of the main premixed flame is stabilized by a diffusion flame kernel created by the injected hydrogen reacting with the oxygen in excess of the premixed stream. Moreover, the H2 consumed with the remaining oxygen in burnt gases leads to the formation of a second flame branch inside the CRZ which is responsible of an increase of the flame angle. Given the high concentration of hydrogen, an impact of the molecular transport models is observed on the flame lift-off height highlighting the importance of using complex transport properties in any LES involving hydrogen combustion.

Characteristics of anchoring locations and angles for GOX/GCH4 flames of an annular pintle injector

Several studies regarding the combustion characteristics of a pintle injector, which is one of techniques for throttling liquid rocket engines, have been conducted; however, the primary focus was on the performance instead of on the flame. To study the fundamental flame characteristics and provide a useful design guide for a pintle injector, gaseous methane and oxygen flames under the laminar condition were visualized by CH* chemiluminescence and Schlieren imaging techniques. The pintle injector used in this study had a fully opened annular injection area, and the combustion chamber was designed as a square cylinder with two windows to visualize the flames. The mass flow rates of methane and oxygen were controlled to change the injection condition, and the pintle opening distance was changed as a geometrical variation. The flame shapes could be classified globally into two types: a shear layer flame, located between two injected propellants and a tip-attached flame located near the pintle tip. The flame angles were measured from deconvoluted flame images, and measured angles were compared with spray angles from previous studies. In conclusion, the flame-anchoring locations and flame shapes were found to be critical for the injection momentum flow rate. Additionally, a new non-dimensional number, the pintle number, Kp, for the normalized angle is proposed, and the pintle number has a strong relation with the flame and spray angles. It is expected that the pintle number can be used to design a pintle injector.

Combining analytical models and LES data to determine the transfer function from swirled premixed flames

A methodology is developed where the acoustic response of a swirl stabilized flame is obtained from a reduced set of simulations. Building upon previous analytical flame transfer functions, a parametrization of the flame response is first proposed, based on six independent physical parameters: a Strouhal number, the mean flame angle with respect to the main flow direction, the vortical structures convection speed, a swirl intensity parameter, a time delay between acoustic and vortical perturbations, as well as a phase shift between bulk and local velocity signals. It is then shown how these parameters can be deduced from steady and unsteady simulations. The methodology is applied to a laboratory scale premixed swirl stabilized flame exhibiting features representative of real aero-engines. In this matter, cold and reactive flow Large Eddy Simulations are first validated by comparing results with reference data from experiments. The high fidelity simulations are seen to be able to capture the flame structure and velocity profiles at different locations while forced flame dynamics for the frequency range of interest also match the experimental data. From the same analytical transfer function model, three methodologies of increasing complexity are presented for the determination of the model parameters, depending on the available data or computational resources. A first estimation of the flame acoustic response is obtained by evaluating parameters from a single stationary flame simulation in conjunction with analytical estimations for the acoustic-convective time delay. Flame dynamics and swirl related parameters can then be determined from a series of robust treatments on pulsed simulations data to improve the model accuracy. It is shown that good qualitative agreement for the flame transfer function can be obtained from a single non-forced simulation while quantitative agreement over the frequency range of interest can be obtained using additional reactive or non-reactive pulsed simulations at one single forcing frequency corresponding to a local gain minimum. The method also naturally handles different perturbation levels.

Investigation on the Flame Front and Flow Field in Acoustically Excited Swirling Flames with and without Confinement

ABSTRACT Open and confined dimethyl ether/air swirling flames were investigated with an acoustically excited swirling burner. Simultaneous particle-image velocimetry and formaldehyde planar laser-induced fluorescence were measured using a high repetition rate burst-mode laser at 20 kHz. Time-averaged velocity field, vorticity and flame brush cloud were compared under different confinements and acoustic excitation conditions. The integrated fluorescence intensities are similar while the open flame has dramatically large fluorescence areas. Time-resolved velocity field and flame brush were measured to investigate the continuous evolution of swirling flame front and acoustically induced vortex. The flame brush was found to be much dispersed in the open flame due to the mixing effect from the surrounding air. Flame root and flame angle were extracted and analyzed statistically using the fluorescence images. Different stages of acoustically induced vortex were observed in both confined and open cases. The vortex trajectory along the outer shear layer determines that the flame tip can perceive the vortex instead of the flame base. The flame regions affected and unaffected by acoustically induced vortex indicate different perturbation mechanisms for the heat release.

Experimental investigation on varying flame characteristics of benzoic resin solid fuel pellets

This paper discusses about the varying flame characteristics of a solid fuel known as benzoic resin as a function of surface temperature and time. Benzoic resin is a naturally occurring biomass and the solid fuel is synthesized using different species of tree bark from the genus Styrax. Hence, an induction heating steel plate fitted with liquefied petroleum gas (LPG) flame ignitor was employed to evaluate these characteristics. Furthermore, a mathematical model is developed to estimate and predict the changing flame behavior of benzoic resin solid fuel. Experiments were executed for different heating surface temperatures such as 70 °C, 110 °C, 130 °C, 160 °C, 180 °C, 210 °C, 240 °C, and 270 °C. The experimental results revealed that there was a change in the flame behavior at 70 °C and 110 °C surface temperatures. The nature of the flames were transition and turbulent, but the laminar flame was detected only at the induction heating surface temperatures of 160 °C and 270 °C. Also, the laminar flame angle is 90° under uniform surface heat flux. Moreover, increase in heat release rate was observed for increasing combustion time.

Design and development of polybenzoxazine-POSS hybrid materials from renewable starting materials for low k and low surface free energy applications

The introduction of nano-sized polyhedral oligomeric silsesquioxane (POSS) derivatives into polymer matrices have received significant interest in recent years mainly because of the inherent properties of POSS which include low coefficient thermal expansion, low density, low thermal conductivity, low dielectric constant, high oxidation resistance, and low surface free energy. In view of this, an attempt has been made to develop thermally stable, flame retardant, low dielectric, and lower surface free energy polybenzoxazine-POSS (PBz-POSS) hybrid prepared using renewable agro-waste (cardanol, furfurylamine and stearylamine) as starting materials. A new approach involving thiol-ene click reaction has been adopted to introduce mercapto functional POSS (SH-POSS) into cardanol benzoxazines (Bz-C-F and Bz-C-S). The thermal, dielectric, flame-retardant, and contact angle studies of the prepared PBz-POSS hybrids were investigated using modern characterization techniques. The results from thermal, flame, dielectric and water contact angle studies clearly indicate that the developed renewable, cost-effective and eco-friendly PBz-POSS hybrid materials can be exploited as coatings and sealants as well as for low dielectric microelectronic applications to substitute the currently used petroleum-based raw materials.

Development and participatory evaluation of fireline intensity and flame property models for managed burns on Calluna-dominated heathlands

BackgroundPrescribed burning plays an important role in the management of many ecosystems and can also be used to mitigate landscape-scale fire risk. Safe and effective application of prescribed fire requires that managers have a robust understanding of potential fire behavior in order to decide on the appropriate tools and tactics for any burning operation. Shrubland ecosystems, including heaths and moors, are known to exhibit intense fire behavior under marginal burning conditions under which fire would not be expected to spread in other vegetation types. This makes developing fire behavior predictions for such systems important. Traditional managed burning is widely used as a tool in Calluna vulgaris (L.) Hull-dominated heath and moorland landscapes in northwest Europe, but in some regions, especially the United Kingdom, there is significant debate over fire use. Despite the controversy, there is general agreement on the need to (1) understand relationships between fuel structure and potential fire behavior, and (2) improve burning practice to optimize potential trade-offs between different ecosystem services. Our aim was to provide knowledge to improve management practice by developing models of potential fireline intensity and flame length. We conducted 27 burns in three developmental stages of Calluna with different stand structures and estimated fireline intensity, flame length, flame height, and flame angle. Flame properties were assessed using photographs and visual observation. We evaluated our models using a participatory research approach for which conservation and land managers submitted basic observations on fire behavior and fire weather for their burns.ResultsFireline intensity and flame height increased significantly across age-related Calluna phases. Regression modeling revealed that fireline intensity could be adequately estimated by a combination of fuel height and wind speed, with taller fuels and higher wind speeds related to more intense fires. Predictions were, however, improved by accounting for live fuel moisture content. Flame length and height were modeled as a function of fireline intensity using standard approaches, but adequately performing models for flame angle could not be established. Evaluation data provided by land managers was noisy, but their qualitative assessments of fire behavior and estimates of flame length were significantly correlated with predictions from our models.ConclusionsFire intensities and flame properties seen in northern Calluna heathlands are similar to those encountered in shrublands associated with climates and fuels more commonly perceived as representing high fire danger. The results demonstrated that our models perform tolerably well although there is substantial uncertainty in their predictions. The models were used to develop a fire behavior nomogram that can provide an indication of potential fireline intensity and flame length prior to commencing a burn.

Numerical Study on Combustion Characteristics of Wall-type Tangential Boiler

The research object of this paper is a 600MW wall-tangentially fired boiler of a power plant. In order to understand the combustion characteristics of the boiler, this paper first analyzes the flow characteristics in the furnace at 600MW to make familiar with the flow in the furnace. Combined with the flow characteristics in the furnace, the combustion conditions of the boiler under three different loads of 600MW, 480MW and 360MW are analyzed. It can be seen from the simulation results that as the boiler load increases, the temperature level in the furnace increases significantly, the flame length increases and the ignition distance of the pulverized coal jet decreases slightly. The burnout distance of the pulverized coal from the burnout zone to the flame angle is increased. The O2 concentration gradually decreases, CO2 concentration gradually increases and unburned carbon loss increases. Combined with the boiler thermal design parameters, it can be seen that the simulation results are basically the same as the design combustion parameters, which indicates that the simulation results can more accurately depict the combustion characteristics of the boiler.

Experimental Study on Downslope Fire Spread over a Pine Needle Fuel Bed

The downslope fire represents a percentage of wildland fireline while the heat transfer mechanism of this process is poorly understood. In this study, the experiments were carried out in a fuel bed of dead pine needles with the slopes of − 30°, − 20°, − 10° and 0° for 0.4 and 0.8 kg/m2 fuel loads. Flame length, flame angle, temperatures over the fuel bed, flow speed at the fuel bed surface, radiation heat flux near the end of the fuel bed were measured. The rate of spread shows a parabolic shape which decreases firstly and then increases from 0° to − 30°. The combustion interface, reconstructed from the temperature histories of two vertical thermocouples, was perpendicular to the fuel bed under all slope conditions for two fuel loads. The measured radiation heat flux is higher at − 30° slope than level ground, which is attributed to higher flame emissivity. A quasi-physical model was developed to describe the heat transfer mechanism of downslope fire spread. The calculation results show that the flame radiation dominated the downslope fire spread process and the combustion zone radiation should not be neglected in the near flame region.

An improved test method for refrigerant flammability limits in a 12 L vessel

The current study presents an improved test method for refrigerant flammability limit measurements. Such measurements are essential for determining refrigerant safety classifications. Predicated on expert interviews and experiments, several changes to ASTM E681 and related standards are recommended, as follows. The 12 L glass vessel should be replaced with transparent polycarbonate. The orientation of the electrode supports and the temperature probe should be changed from vertical to horizontal. All penetrations should be removed from the rubber stopper, it should be weighted for a total mass of 2.5 kg, and the initial pressure should be 90 kPa absolute. The flame angle should be plotted versus refrigerant concentration to obtain flammability limits. Finally, the vessel pressure should be measured during each test. These changes are relatively easy to implement and they will improve the test precision and reproducibility without significantly changing previously established flammability limits.

Flame Spread Characteristics of Inclined Extruded Polystyrene Thermal Insulation Material

Abstract A set of small-scale flame spread experiments was conducted to explore the effects of sample orientation on the flame spread characteristics of extruded polystyrene (XPS) insulation material. The obtained data were employed to show the variation law of flame angle, pool fire zone length and flame spread rate when the tilt angle of XPS material changes from -90° to 90°. The flame angle decreased when the inclined angle increased from -90° to 90° and there are two different variation trends of flame angle in different inclined angle ranges. Meanwhile, both pool fire length and flame spread rate of the inclined XPS material depend on gravity controlled flow of flammable and molten drops produced by heating XPS material. Furthermore, the fitting relationship between flame spread rate and tilt angle was put forward when the tilt angle was less than or equal to 0°. This work is a supplement to the fire risk study on the XPS insulation materials and system.