Pilot Flame Research Articles

Effects of Annular Combustor Width on the Ethylene-Air Continuous Rotating Detonation

The realization and stable operation of Continuous Rotating Detonation (CRD) in the annular combustor fueled by hydrocarbon-air are still challenging. For further investigation of this issue, a series of ethylene-air CRD tests with the variation of combustor width is conducted, and the effects of combustor width are well analyzed based on high-frequency pressure and high-speed photograph images. The results show that the combustor width plays a significant role in the realization and sustainability of the ethylene-air CRD. In this paper, the critical combustor width for the CRD realization and stable single wave are 20 mm and 25 mm, respectively. In wide combustors, the backward-facing step at the combustor forepart makes the main flow slow down, and thus, the mixing quality is promoted. Besides, the pilot flame at the recirculation zone contributes to sustaining the CRD wave. As the width increases, the propagation mode changes from counter-rotating two-wave mode to single wave mode with higher propagation velocity and stability. The highest propagation velocity reaches 1325.56 m/s in the 40 mm wide combustor, accounting for 71.51% of the corresponding Chapman-Jouguet velocity. Despite large combustor volume, high combustor pressure is obtained in detonation combustion mode indicating that a better propulsive performance could be achieved by CRD.

High-fidelity flame-front wrinkling measurements derived from fractal analysis of turbulent premixed flames with large Reynolds numbers

This work assesses the level of wrinkling that premixed flames exhibit when subjected to turbulence levels in excess of ten times that of previous experimental studies of this nature. This assessment was facilitated by performing fractal analysis of isocontours extracted from simultaneously acquired planar Rayleigh scattering (PRS) and planar laser-induced fluorescence (PLIF) images of OH. Images were collected from 21 separate flames with turbulent Karlovitz (KaT,P) and Reynolds numbers (ReT,0) as large as 210 and 99,000, respectively. Values of the inner-cutoff scale (εi) and fractal dimension (FD) were extracted from two separate isocontours associated with each imaging technique. Results indicate that values of εi normalized by the laminar flame thickness (δL,P) follow the scaling: εi/δL,P∼KaT,P−0.22, which holds for results from prior studies but deviates from those derived from theoretical principles. Cases with main-flame equivalence ratios (ϕm) substantially less than that of their pilot flame (ϕp) did not follow the aforementioned scaling; however, adjusting δL,P and SL values such that they corresponded to the average of ϕm and ϕp was observed to reconcile this discrepancy. Yet, cases with ϕm ∼ ϕp did not deviate much from the scaling above. The influence of isocontour choice and targeted scalar on values of εi and FD was assessed and found to be much more significant for the former than the latter. Additionally, εi and FD exhibit a modest increase with axial distance from the flame base, corroborating prior results.

Capturing multi-regime combustion in turbulent flames with a virtual chemistry approach

Multiple flame regimes are encountered in industrial combustion chambers, where premixed, stratified and non-premixed flame regions may coexist. To obtain a predictive tool for pollutant formation predictions, chemical flame modeling must take into account the influence of such complex flame structure. The objective of this article is to apply and compare two reduced chemistry models on both laminar and turbulent multi-regime flame configurations in order to analyze their capabilities in predicting flame structure and CO formation. The challenged approaches are (i) a premixed flamelet-based tabulated chemistry method, whose thermochemical variables are parameterized by a mixture fraction and a progress variable, and (ii) a virtual chemical scheme which has been optimized to retrieve the properties of canonical premixed and non-premixed 1-D laminar flames. The methods are first applied to compute a series of laminar partially-premixed methane-air counterflow flames. Results are compared to detailed chemistry simulations. Both approaches reproduced the thermal flame structure but only the virtual chemistry captures the CO formation in all ranges of equivalence ratio from stoichiometry premixed flame to pure non-premixed flame. Finally, the two chemical models combined with the Thickened Flame model for LES are challenged on a piloted turbulent jet flame with inhomogeneous inlet, the Sydney inhomogeneous burner. Mean and RMS of temperature and CO mass fraction radial profiles are compared to available experimental data. Scatter data in mixture fraction space and Wasserstein metric of numerical and experimental data are also studied. The analyses confirm again that the virtual chemistry approach is able to account for the impact of multi-regime turbulent combustion on the CO formation.

Characterization of multi-regime reaction zones in a piloted inhomogeneous jet flame with local extinction

Gradient free regime identification (GFRI) is applied to 1D Raman/Rayleigh/LIF measurements of temperature and major species from the intermediate velocity case of the Sydney piloted inhomogeneous jet flame series to better understand the structure of reaction zones and the downstream evolution of multi-regime characteristics. The GFRI approach allows local reaction zones to be detected and characterized as premixed, dominantly premixed, multi-regime, dominantly non-premixed, or non-premixed flame structures, based on flame markers (mixture fraction, chemical mode, and heat release rate) derived from the experimental data. The statistics of chemical mode zero-crossings, which mark premixed reaction zones, and the relative populations of flame structures are shown to be sensitive to the state of mixing in the near field of the flame and to the level of local extinction farther downstream. Multi-regime structures, where premixed and non-premixed reaction zones occur in close proximity and both contribute to overall heat release, account for nearly half the total population at streamwise locations within the first several jet diameters. There is a rapid transition within the near field whereby the relative population of non-premixed and dominantly non-premixed structures grows from 0.05 to nearly 0.5, and the population of premixed and dominantly premixed structures decreases correspondingly as fluid entering the reaction zone becomes progressively fuel-rich. Local extinction and re-ignition bring a resurgence in premixed-type structures, many of which occur at fuel-lean conditions. There are also modest populations of multi-regime structures, having chemical mode zero-crossings at lean conditions, which would not exist in a fully burning jet 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 (NOx) 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.

Soot formation characteristics in a pulverized coal flame formed in a swirling flow

To understand the soot formation characteristics in a pulverized coal flame with a swirling flow, simultaneous imaging of Mie scattering of coal particles and laser induced incandescence (LII) of soot were performed in this study. The pulverized coal flame was stabilized by a hydrogen diffusion pilot flame. The characteristic structures of soot formation in the pulverized coal flame with a swirling flow were analyzed based on a comparison of the experimental results and two-dimensional numerical simulation in the mixing region of coal particles and the oxidizer. The interactions between coal particle clouds and soot formation are discussed in detail. The results clearly show that the averaged radial dispersions of scattering signals from coal particles and of the LII signals from soot are overlapped. The overlapping region appeared nearby the nozzle exit due to the turbulent mixing and the high temperature region formed by swirl-induced recirculation flow. This overlapping region radially expands with increasing the height from the burner. Additionally, the characteristic areas of soot formation were observed in the results of simultaneous imaging of Mie scattering and LII. These areas are 1) streaky soot formation areas around the particle clouds, 2) soot formation areas inside of the particle clouds and 3) soot formation areas around the large coal particles.

Analysis of a Pseudo-active Approach for the Control of Thermoacoustic Instabilities

ABSTRACT This work describes a first characterization study of a novel approach, named ‘pseudo-active’ instability control (PAIC), which might afford the versatility of active control, but with much simpler hardware requirements. The objective is to spontaneously induce oscillations in pilot fuel injection due to the pressure fluctuations in the combustion chamber, in order to produce effects similar to those obtained with active instability control methods. For optimized designs, this interaction could be exploited to damp pressure fluctuations in gas-turbine combustors. The experimental study was performed for different PAIC configurations and under situations of strong, self-sustained pressure fluctuations. The results obtained show good potentialities of the system proposed, and also revealed purely acoustic interactions between the burner and the pilot line, which superimposed on the interaction between the pilot and main flames. Overall, this work provides novel results on the, previously unexplored, influence of the acoustic properties of the pilot fuel injection on the limit cycle as well as first evidence on the rationale and benefits that could be derived from the so-called PAIC strategy.

Fabrication of flame-retardant and smoke-suppressant isocyanate-based polyimide foam modified by silica aerogel thermal insulation and flame protection layers

Because of containing urea groups, flame resistance and smoke releasing behaviors of isocyanate-based polyimide foam (IBPIF) produced using free foaming technology require further improvement. In this work, silica aerogel layers were incorporated into cells of IBPIF through an in situ growth process of silica sol (SS). Compared with silica aerogel particles directly mixed into the foaming slurry, the silica aerogel layers that firmly attached to the pores and surfaces of cells not only provided exceptional thermal insulation and flame protection, but also kept original cellular structure. With increase in ratio of SS mass to IBPIF volume, silica aerogel incorporation dosage was gradually increased. Accompanied by flame resistance was obviously improved and smoke releasing behavior was effectively suppressed. Those were indicated by the improved limiting oxygen index (LOI), decreased heat release rate (HRR), peak of HRR, and specific optical density of smoke (Ds) in trials with pilot flames. Compared with pure IBPIF, when the ratio reached to 5/15 g/cm3, it resulted in LOI increasing from 22.0% to 33.0%, peak of HRR, total smoke production (TSP), and maximum value of Ds decreasing from 174 to 72 kW/m2, 1.11 to 0.37 m2/m2, 45.90 to 17.45, respectively.

Experimental investigation on flame stabilization of a kerosene-fueled scramjet combustor with pilot hydrogen

Flame stabilization in a kerosene-fueled scramjet combustor was investigated experimentally through Schlieren, flame luminosity, and wall pressure measurement, aiming to obtain better insight into combustion characteristics. Experiments were conducted in a direct-connected supersonic combustion facility with inflow conditions of Mach number 2.0, stagnation pressure 0.82 MPa, and temperature 950 K, simulating the flight condition of Mach number 4.0. Results revealed that kerosene was able to be ignited when the equivalence ratio of pilot hydrogen reached 0.080, but was unsuccessful when the equivalence ratio was 0.040. Once ignited, the intense combustion induced high back pressure forcing the flame to spread into the isolator. The pilot flame invariably appeared in the cavity shear layer and attached to the cavity ramp under different equivalence ratios of pilot hydrogen. With the mass flux of pilot hydrogen increased, the kerosene flame located near the cavity ramp was asymmetrical and unstable since it propagated upstream repeatedly. Therefore, the kerosene could be ignited by a suitable equivalence ratio of continuous pilot hydrogen, potentially accompanied with unstable combustion.

Lean Flame Stabilization by Means of a Coaxial Synthetic Jet

In the present paper the effect of a synthetic jet, coaxial with an annular type combustor, has been investigated in order to infer aerodynamic interaction between the two gaseous flows. The experimental investigation has the purpose to find the existence of synthetic jet excitation frequency able to extend the stability limits of a lean air/propane mixture flame. Lean flame is widely adopted in low thermal NOx emission combustors. As it is well known for gaseous premixed flame increasing the air/fuel ratio (lean flame) the flame propagation velocity is decreasing until it is getting comparable to the velocity of supplied fresh gaseous mixture. At this point, the flame is flushed away from the combustor mouth and the combustion process is quenched (blow off). In order to avoid those effects, for very low air/combustible ratio the modern premixed industrial combustors control the flame stability by means of the so called “pilot flame”, a small secondary relatively rich flame. The pilot flame, fed by a few percent of the total amount of supplied combustible, represent a relatively high source of thermal NOx. The experimental investigations have been performed by means of a Particle Image Velocimetry (PIV), in order to obtain instantaneous two dimensional velocity distribution overall the entire gaseous reacting jet, in order to stretch the influences of coherent structures generated in the jet, by means of natural aerodynamic effect and by means of perturbation forced into the flow by means of the synthetic coaxial jet under combustion conditions.

High-Momentum Jet Flames at Elevated Pressure, C: Statistical Distribution of Thermochemical States Obtained From Laser-Raman Measurements

Abstract A detailed investigation on flame structures and stabilization mechanisms of confined high momentum jet flames by one-dimensional (1D)-laser Raman measurements is presented. The flames were operated with natural gas (NG) at gas turbine relevant conditions in an optically accessible high-pressure test rig. The generic burner represents a full scale single nozzle of a high temperature FLOX® gas turbine combustor including a pilot stage. 1D-laser Raman measurements were performed on both an unpiloted and a piloted flame and evaluated on a single shot basis revealing the thermochemical states from unburned inflow conditions to burned hot gas in terms of average and statistical values of the major species concentrations, the mixture fraction and the temperature. The results show a distinct difference in the flame stabilization mechanism between the unpiloted and the piloted case. The former is apparently driven by strong mixing of fresh unburned gas and recirculated hot burned gas that eventually causes autoignition. The piloted flame is stabilized by the pilot stage followed by turbulent flame propagation. The findings help to understand the underlying combustion mechanisms and to further develop gas turbine burners following the FLOX concept.

Investigation of Combustion Characteristics in a Kerosene-Fueled Supersonic Combustor with Air Throttling

The paper presents the results of an experimental and numerical study on the combustion characteristics of a supersonic combustor operation enhanced by pilot hydrogen and air throttling, including ignition and combustion. Wall pressure measurements, schlieren images, and hydroxyl-radical planar-laser-induced-fluorescence (OH-PLIF) are used to obtain information on the flowfield and flame development. With the aid of air throttling, the kerosene is ignited by the pilot flame; and the kerosene flame remains stable even after the pilot hydrogen is switched off. The speed of the shock train generated by the throttling air and combustion is about in the expansion section but only about in the cavity region. The stable flame and flame blowout cases can be separated by a curve in a plot of the coexistence time of pilot hydrogen and throttling air versus the air throttling flux ratio. There is a special case lying on this curve for which the flame is unstable and will be blown out before the throttling air is switched off. It is also found that the position of the thermal throat is the same for all the subsonic combustion cases: about 595 mm downstream of the scramjet entrance.

Numerical Simulation of Laminar and Turbulent Methane/Air Flames Based on a DRG-Derived Skeletal Mechanism

Simulation of turbulent flames using detailed chemical mechanisms is still a challenge in numerical combustion due to the large number of species and the stiffness of the system of governing equations. In this sense, strategies to reduce the size of the detailed model are necessary and one of such models is the well-known directed relation graph (DRG) method. In the present work, a DRG-derived skeletal mechanism developed using only one application for methane/ air simulations is presented and validated for auto-ignition times, laminar flame speed and counterflow flames. The skeletal mechanism is tested for varying the equivalence ratio (ϕ = 0.4, to 3) and pressure (p = 1 to 150 atm). The temperature spans the range from T = 1000 K to T = 2000 K. The relative error, compared with the detailed mechanism, of our proposed model for ignition delay times and flame speed are less than 10% for most of the parameters. The skeletal mechanism is also used to simulate the piloted turbulent jet Sandia Flame D. Results show that this skeletal mechanism can reproduce the main features of laminar and turbulent methane/air flames.

Large eddy simulation of a turbulent non-premixed flame based on the flamelet-generated manifolds approach and a reduced mechanism verification

This work presents the design of a large eddy simulation based on the flamelet-generated manifolds (FGM) approach to evaluate a Sandia Flame D, which is a turbulent non-premixed piloted jet flame. The detailed kinetic mechanism of the Gas Research Institute (GRI3.0, which contains 53 species and 325 elementary reactions), as well as a reduced mechanism, are employed to describe combustion. This reduced mechanism, containing 19 species and 68 elementary reactions, is proposed based on the direct relation graphs with error-propagation (DRGEP) method and a species sensitivity analysis from the GRI3.0. Both the two mechanisms are introduced into Fluent to simulate a Sandia Flame D, and the numerical results are in good agreement with the experimental data, which indicates the accurate predictive capability of the LES/FGM method in combustion modeling and that the proposed reduced mechanism can be applied to the numerical simulation in practical engineering problems.

Comparative study of downwardly propagating CH4/C3H8—air flames along a closed-end tube

Experimental work is reported for premixed flames propagating in tubes. The flames were ignited with a pilot flame and the flame propagation captured with high-speed cameras. Initial measurements were performed characterizing the rig. For downwardly propagating flames to a closed-end, methane and propane were studied. The flames initially propagated steadily, then at approximately a third of the way down the tube, the primary acoustic oscillation sets in, resulting to a change in the flame shape. This was then followed by a plateau of variable length before a more violent secondary acoustic oscillation. In some circumstances, flames were observed to rotate due to the primary acoustic instability. The flame front position growth rate for both methane and propane were similar despite the differences in the fuels. The total acoustic loss time for propane and methane increases from the lean limit with the equivalence ratio, peaks at φ = 1.1 and then decreases as the mixture becomes richer. There was also an increase in the total acoustic loss time as the angular speed of the flame increased. The results showed that the generation of acoustic energy for propane was smaller than that of methane due to the stronger natural damping effect of the former.

Nanoparticle Formation and Behavior in Turbulent Spray Flames Investigated by DNS

This work is a first direct numerical simulation of a configuration closely related to the SpraySyn burner (Schneider et al. in Rev Sci Instrum 90:085108, 2019). This burner has been recently developed at the University of Duisburg-Essen to investigate experimentally nanoparticle synthesis in spray flames for a variety of materials. The present simulations are performed for ethanol and titanium tetraisopropoxide as a solvent and precursor, respectively, in order to produce titanium dioxide nanoparticles. In the direct numerical simulations, the complete scenario leading to the production of well-defined nanoparticles is taken into account, including evaporation of the liquid mixture (solvent and precursor) injected as a spray, multi-step kinetics for gas-phase combustion, and finally nanoparticle synthesis. The employed models are described in this article. Additionally, the impact of the inlet velocity of the pilot flame on the nanoparticle synthesis is investigated. It has been found that increasing this speed delays spray flame ignition, decreases nanoparticle concentration, but leads to a narrower size distribution at early stage.

Analysis of Gas-Assisted Pulverized Coal Combustion in Cambridge Coal Burner CCB1 Using FPV-LES

Gas-assisted pulverized coal combustion in the laboratory-scale Cambridge Coal Burner configuration CCB1 is studied using a large eddy simulation (LES) coupled with a multistream flamelet/progress variable (FPV) model in an Euler–Lagrange framework. The FPV-LES implementation is coupled with an enthalpy correction method for two-phase energy transfer. Both nonreacting and reacting (pure pilot and coal flame) cases are simulated, and the simulation results are compared with the experimental evidence. The agreement of the simulation results with the measurements is found to be reasonable. However, it is necessary to note that the experimental data mainly focus on the far upstream region where coal combustion effects are limited. The flame structures of the pure pilot flame and coal flame are carefully analyzed across the entire simulation domain. Different gas flame modes are identified for the coal flame as a function of the downstream coordinate, namely typical premixed flame (far upstream), double flame (middle), and typical nonpremixed flame (far downstream). The time histories of coal particles are traced to demonstrate the relation between gas flame modes and the devolatilization process. Moreover, the average distance between the particles is used to distinguish between the group combustion mode found near the center of the burner and the individual particle combustion mode located in the outer region of the flame, which is in agreement with experimental observations. This study provides new insights into the flame characteristics of swirling gas-assisted pulverized coal flames and adds to the knowledge database on FPV-LES modeling of pulverized coal combustion.

Cover Feature: Spray‐Flame‐Prepared LaCo1–xFexO3 Perovskite Nanoparticles as Active OER Catalysts: Influence of Fe Content and Low‐Temperature Heating (ChemElectroChem 12/2020)

The Cover Feature shows a spray-flame reactor used to synthesize perovskite catalyst nanoparticles. The blue methane/oxygen pilot flame at the bottom ignites a combustible precursor aerosol injected via a centric nozzle. Nanoparticle formation downstream of the reaction zone leads to metastable, highly active LaCo1–xFexO3 perovskite catalysts with improved oxygen evolution reaction activity. The image was taken by Samer Suleiman. More information can be found in the Aricle by B. Alkan et al.

Analysis of Mass Transport in a Turbulent Flame Using Lagrangian Coherent Structures

The premixed piloted turbulent flame is studied numerically using Lagrangian coherent structures (LCS) to study and analyze the masstransport process from the viewpoint of nonlinear dynamic systems. The mass transport is shown to demonstrate a rich dynamical behavior. Firstly, the numerical simulation results show that the boundaries in the combustion field are delineated by the Lagrangian Coherent Structures. Secondly, the distribution of the attracting LCSs and the OH radical are compared to study the surface of the flame. Finally, the evolution of the LCS was tracked to analyze the vortexes and mass transport near the burner. The results show that the attracting LCS can be considered as a surface of the turbulent flame near the burner. The serial vortexes are regularly generated near the piloted jet, and they are gradually stretched and folded. These vortexes are attracted by the main jet as they move downward in the flow. The main jet goes into the flow field following an attracting LCS and moves forward attracting the piloted jet and the co-flow. However, fluid in the mainstream region has never entered into the piloted jet region, and this region does not exchange substance with the co-flow region. If the flow speed increases, air from the co-flow may pass through the LCS to mix with the material from the piloted jet region. Application of the LCS technique to study mass transport processes provides a new viewpoint for analyzing premixed piloted turbulent flames.

Numerical Study on the Morphology of a Re-Ignited Laminar Partially Premixed Flame with a Co-Axial Pilot Flame

Re-ignited partially premixed flame (PPF) is a quite extensive flame type in real applications, which is directly relevant to the local and global extinction and re-ignition phenomenon. The authors designed a model burner to establish laminar re-ignited PPFs. Numerical simulations were carried out to reveal the morphology of laminar re-ignited PPF. Based on the distributions of temperature, heat release and radicals, the morphologies of re-ignited flames were explored. W-shaped flames were formed under pilot-lean conditions. Line-shaped and y-shaped flames were formed under pilot-rich conditions. Both w-shaped and y-shaped flames had a triple-flame structure. The re-ignited flames can stand beyond the rich flammability limit. Additionally, OH distributions indicated both pilot flame and re-ignited flame well as it rapidly increased near the flame front. OH concentration did not increase visibly while CH2O concentration mildly increased during the mild re-ignition process in the pre-zone of the re-ignited PPF. According to the results of 0-D simulations using closed homogeneous reactor, both OH and CH2O reduced ignition time significantly. The results of this work are helpful for understanding re-ignited PPF more closely.