Flame Mode Research Articles

Experimental investigation on ignition and cross-flame performance of dual-annular combustor

The ignition and co-flame process of the dual annular combustor(DAC) was studied using high speed camera in order to investigate the mechanism of flame stabilization and propagation in the combustor. Particle Laser Velocimetry (PIV) combined with high speed camera were used to measure the key properties of ignition kernels, including orientation, growth direction, movement velocity, flame propagation speed. Results showed the flow field structure obtained by PIV includes the primary air jet, the dilution air jet, the inner/outer recirculation zones, the central body jet, and the middle area of the central body. The ignition process could be categorized into the four stages: kernel generation at the end of the igniter, kernel movement at the recirculation zone, kernel growth at the head, and burner-scale flame establishment at the combustor, the flame had significantly different performance in each stage. The time of each stage decreases with the increase in the inlet Reynolds number, which significantly reduced the ignition delay time. Additionally, as the excess air coefficient increased, the ignition of pilot stage become more complex and the ignition delay and the cross-flame time increased. The relative location of the flame center and the centroid confidence interval of the flame path was used to judge ignition success. The flame path analysis showed that the transverse propagation mode of flame in the head is the determining stage of ignition. The splitting direction and the flame statement were related to the flame's position and shape in the flow field, the transverse flame splitting played a vital role in the ignition process. Two typical co-flame mechanisms existed in the cross-flame process of the DAC as follows: the flame tongue co-flame mechanism under small working conditions and the heat reflux combined small flame co-flame mechanism under large working conditions.

Rotating gliding arc discharge induced flame oscillation near the lean blowout limit

This work investigates the rotating gliding arc (RGA) assisted swirling methane combustion characteristics near the lean blowout limit by utilizing the high-speed videography. As the flow rate and the global equivalence ratio were varied, four flame modes, namely self-sustained flame, RGA-sustained stable flame, RGA-induced oscillating flame, and RGA anchored flame modes were detected. In particular, the oscillating flame mode with an oscillating frequency of 2∼4 Hz occurs with the air flow rates above 13 L/min and the equivalence ratio between 0.52 and 0.56. It is an RGA-induced low-frequency flame oscillation phenomenon. The RGA discharge characteristics can impact the oscillating behaviors that with the increase of the transformer input voltage, the flame oscillation frequency is enhanced and the averaged flame area becomes larger to result in a higher combustion efficiency. Besides, increasing the wall temperature of the combustion chamber promotes the combustion during the oscillating stage. Based upon the experimental and numerical results this RGA-induced low-frequency oscillating flame are further interpreted by a three-staged mechanism. It implies that the first stage, during which the fuel/oxidant mixture is refreshed in the chamber and the temperature increases through recirculating flow gradually, determines the oscillating frequency. The fast accumulation of released energy from plasma discharge and partial fuel burning by recirculation can accelerate the temperature growth in the recirculation zone to increase the oscillation frequency. In all, this unique oscillating phenomenon and related explanations indeed broaden our knowledge of oscillating combustion behaviors and GA assisted combustion.

Experimental Study of Thermal and Fire Reaction Properties of Glass Fiber/Bismaleimide Composites for Aeronautic Application.

Thermal behavior and fire reaction properties of aerial glass fiber (GF)/bismaleimide (BMI) composites were tested using thermogravimetric analysis (TGA), thermogravimetric coupled with Fourier transform infrared spectroscopy (TG-FTIR), cone calorimeter, limiting oxygen index, and smoke density chamber. The results showed that the pyrolysis process was one stage in a nitrogen atmosphere with the prominent volatile components of CO2, H2O, CH4, NOx, and SO2. The release of heat and smoke increased with the increase in heat flux, while the time required to reach hazardous conditions decreased. The limiting oxygen index decreased monotonically from 47.8% to 39.0% with increasing experimental temperature. The maximum specific optical density within 20 min in the non-flaming mode was greater than that in the flaming mode. According to the four kinds of fire hazard assessment indicators, the greater the heat flux, the higher the fire hazard, for the contribution of more decomposed components. The calculations of two indices confirmed that the smoke release in the early stage of fire was more negative under flaming mode. This work can provide a comprehensive understanding of the thermal and fire characteristics of GF/BMI composites used for aircraft.

Quenching modes of local flame–wall interaction for turbulent premixed methane combustion in a constant volume vessel

The quenching mode of local flame–wall interaction (FWI) is investigated for its response to different levels of turbulence intensity as well as its effect on quenching distance, wall heat flux, and near-wall reaction. For that, direct numerical simulations of turbulent premixed methane combustion in a constant volume vessel are carried with initial Karlovitz numbers (Ka) of 1.0, 10.0, and 30.0. Local flame–wall quenching positions are identified based on the local fuel consumption speed during the turbulent combustion process, and the local FWI events have been classified into four quenching modes according to the flame–wall geometric relationships of quenching positions, namely head-on quenching (HOQ), oblique-wall quenching, side-wall quenching (SWQ), and back-on quenching (BOQ). The results show that in the case with higher initial Ka, the flame surface shows a more complicated wrinkled structure due to the flame–turbulence interaction. Meanwhile, the local quenching distance defined based on the identified quenching position is strongly influenced by the near-wall flow, and the range of the local quenching mode extends further to BOQ. However, for all three cases, HOQ and near-HOQ modes account for the majority of local FWI. Wall heat flux and heat release rate (HRR) of near-wall reaction yield high values for the FWI under HOQ or BOQ and are low for SWQ. In addition, there is a discrepancy in the near-wall transportation of some species under different quenching modes, which further leads to the difference in FWI-induced near-wall reaction regarding its total and elementary HRR.

IDDES simulation of hydrogen-fueled supersonic combustion based on dynamic zone flamelet model

Based on the dynamic zone flamelet model (DZFM), the turbulent combustion of the DLR supersonic combustor with a wedge-shaped flame holder is numerically studied by IDDES. The mixing-relevant flow structures, combustion characteristics, and reaction paths were analyzed. The significant variation of flame modes, turbulence-chemistry interaction modes, and correlations between species (YOH) and mixture fraction suggests that applying a single flamelet for the whole physical space may introduce remarkable errors in describing the reacting states. Dynamically partitioning the flow field based on multiple zone division indices can effectively reduce conditional fluctuations and make the first-order closure assumption more valid. The results also demonstrate the necessity of a low-Re modification to DZFM, even for supersonic combustion.

Effect of H2 addition on the local extinction, flame structure, and flow field hydrodynamics in non-premixed bluff body stabilized flames

We examined the effect of hydrogen (H2) enrichment on the primary fuel methane (CH4) in a canonical non-premixed bluff-body stabilized burner operating under typical central jet-dominated flame mode. In the chosen mode of operation, globally, the flow field and flame feature three important successive spatial zones: the recirculation zone, the neck zone, and the jet-like flame zone. The flame is exposed to a higher stretch rate in the neck zone in such a configuration and eventually undergoes local extinction. Such local extinction and subsequent re-ignition/reconnection of broken flame branches have substantial implications for the hydrodynamic instability of the coaxial annular air shear layer. It is well known that H2 addition increases the flame extinction strain rate (κext) and thus alters the local extinction phenomenon. To understand this, we performed experiments at 0%, 10%, 20%, 30%, 50%, 80%, and 100% hydrogen proportion in the H2-CH4 blend. High repetition rate (5 kHz) Particle Image Velocimetry and OH Planar Laser Induced Fluorescence (PLIF) measurements are simultaneously implemented to gain quantitative insight into the flow field and flame structure. A detailed analysis performed over the instantaneous OH–PLIF datasets reveals the absence of local extinctions in flames with H2 enrichment >30% due to an increased extinction strain rate (κext). Furthermore, it is found that H2 enrichment plays a significant role in the reconnection/re-ignition of the broken flame branches formed during the local extinction. For instance, a high reconnection probability is observed in flames with an H2 addition of ≥20%. Consequently, variations in the mean reaction zone height are witnessed for different H2 enrichment levels. Further analysis of the influence of variation in reaction zone height on flow field hydrodynamics is explored using Proper Orthogonal Decomposition (POD) and Continuous Wavelet Transform (CWT). The results obtained from POD and CWT indicated the suppression of vortex shedding at the annular air shear layer for H2 addition greater than 20% and irregular wrinkling of flame fronts. Thus, they quantified the beneficial effect of H2 addition in turbulent flame stabilization.

Pressure gradient effect on flame–vortex interaction in lean premixed bluff body stabilized flames

This investigation considers the effect of axial pressure gradient on the dynamics of flame–vortex interaction for a lean premixed bluff body stabilized flame. Large eddy simulations (LESs) of four different combustor geometries generated through combustor wall adjustments that resulted in mild to strong pressure gradients are studied. A bluff body stabilized combustor for a propane/air flame is analyzed first. The results are compared with all available experimental data with the purpose of validating the LES methodology used in OpenFOAM and obtaining a base solution for the study of the pressure gradient effect on flame–vortex interaction. The role of the pressure gradient on flame structure, emission characteristics, vortex dynamics, and flame stability is presented. The mild favorable pressure gradient due to the decelerated flow in diffuser configurations influences flame–vortex dynamics by suppressing flame-induced vorticity sources, baroclinic torque and dilatation, and hence resulting in augmented hydrodynamic instabilities. The sustained hydrodynamic instabilities maintain the large flame wrinkles and sinusoidal flame mode in the wake region. The nourished near-lean blowoff dynamics also affect the emission characteristics, and the emission of species increases. However, the accelerated flow in the nozzle configuration amplifies the flame-induced vorticity sources that preserve the flame core, resulting in a more organized, symmetric, and stable flame. Ultimately, the combustion performance and operation envelope in the lean premixed flames can be increased by maintaining the flame stability and suppressing the limiting lean blowoff dynamics and emissions with the help of a strong favorable pressure gradient generated through adjusting the combustor geometry.

The effect of main stage flow velocity on thermoacoustic instability of stratified swirl burner

In this paper, two group experiments were conducted to study the effect of the main stage flow velocity on thermoacoustic instability and flame macrostructure. The experiments were carried out under atmosphere condition, and namely the pilot stage flame mode and the stratified swirl flame mode. The experimental results show that in the pilot stage flame mode, the thermoacoustic oscillation amplitude remains constant (around 1000 Pa) with the decrease of the main stage flow velocity. But when the velocity is zero (without main stage air), the thermoacoustic instability disappeared. In the stratified swirl flame mode, the amplitude of thermoacoustic instability is the largest (around 300 Pa) when the main stage flow velocity is 6.2 m/s, and the amplitude is slightly smaller in other working conditions. The time-averaged flame shape under the two flame modes is recorded and discussed. This paper highlights the effect of interactions between the pilot stage flame and main stage air or flame on thermoacoustic instability.

Control of Thermoacoustic Oscillations and NOx Emissions by Bias Jets in the Form of Corner Tangential Slots

ABSTRACT Corner tangential and bias combustion technology have significant advantages in optimizing combustion and pollutant emissions. This paper combined the corner tangential, bias combustion and jet technology for the first time to control thermoacoustic oscillations and pollutant NOx emissions synchronously. The control feasibility was examined by three variables: widths of corner tangential slots, bias jets ratios, and jet flow rates. Results indicate that the widths of corner tangential slots and bias jets ratios significantly affect the suppression effectiveness of thermoacoustic oscillations and NOx emissions. The synchronous control effectiveness with larger widths of corner tangential slots and smaller bias jets ratios is better than that with smaller widths of corner tangential slots and larger bias jets ratios. The maximum damping ratio of dynamic pressure and CH* can achieve 90.1% and 82%, respectively. The dynamic pressure drops from 39.2 Pa to 3.9 Pa and CH* reduces from 0.006 arb.units to 0.001 arb.units. A lower oscillations frequency will form by the bias jets, which avoids the occurrence of high-frequency and high-amplitude thermoacoustic self-excited oscillations. Besides, the NOx emissions drop from 26.2 ppm to 13.9 ppm reaching a maximum reduction of 12.3 ppm. Meanwhile, a slimmer flame root and flatter flame front are triggered. Two flame modes are summarized as Model A and Model B. This research realized an efficient simultaneous control of thermoacoustic oscillations and pollutant NOx emissions, which could be a helpful guideline for designing lean premixed flames.

Flame characteristics of backward-inclined pulsating combusting jet in crossflow

The flame appearances, thermal characteristics, and combustion product concentrations of a backward-inclined pulsating combusting jet in crossflow were experimentally studied. The time-averaged and streak instantaneous flame images were acquired using a still camera and a high-speed camera, respectively. The flame extinguishment processes were captured and analysed on the basis of streak instantaneous images. The flame temperatures and combustion product concentrations were measured using a homemade fine-wire thermocouple and a gas analyser, respectively. Three characteristic flame modes (crossflow dominated, transitional, and jet-dominated) were found when the backward-inclined jet flame in crossflow was subject to acoustic excitation. These characteristic flame modes were observed in different regimes of pulsation intensity and jet backward inclination angle. Discrete flame puffs with a frequency equal to the acoustic excitation frequency appeared in the jet-dominated flame. Periodic flame puffing became unstable when the pulsation intensity was increased to values near the instability limit. When the pulsation intensity was beyond the instability limit, a strong stretching effect caused the recirculation flame, which played the role of a flame ignitor, to almost vanish (misfire). The flame extinguishment then occurred from the area around the lee side of the burner tube tip. The temperature and gas concentration measurement results showed the significant improvement of combustion performance in low inclination angle even at small pulsation intensity in the regime of the crossflow dominated flame. To enhance combustion performance at a large jet backward inclination angle, the pulsation intensity must be large enough to push the flame in the regime of the jet-dominated flame.

Are the available data from laboratory spray burners suitable for CFD modelling validations? A review

A systematic compilation and evaluation of turbulent spray flame combustion data acquired from lab-scale spray-burners and CFD models validated with these are presented. Previous review studies have been limited to presentations of the progress with validation targets and modelling methods. This work expands these by presenting the details of lab-scale spray-burners and the characteristics of their datasets, the computational methodologies developed to capture the turbulent spray combustion processes acquired from the burners and their validation with the dataset. Additionally, by critically evaluating the spray and inflow boundary conditions, the reacting and non-reacting features and the main flame characteristics and data, direct correlations between the effectiveness of the dataset from the lab-scale spray burners and the outcomes from the computational methodologies using them for validation are clearly elucidated. The evaluation indicates that the turbulence modelling and validation of the swirling non-reacting flows before the application of the combustion models are important. A better representation of the multi-mode nature and the flame-turbulence-chemistry interactions in the combustion models are required. Also, a clearer experimental presentation of the flame structure and modes would help with the development of the modelling techniques. To plug these research needs, this paper reinforces the importance of developing improved numerical techniques that can be validated by selecting an appropriate lab-scale spray-burner with a robust database. Together these can be used to improve the development of and reduce emissions from combustors burning future green fuels.

A NUMERICAL STUDY ON THE TRANSITION CHARACTERISTICS OF AN INVERSE MICRO-SCALE DIFFUSION FLAME TO MICRO-SCALE PREMIXED FLAME UNDER HEAT RECIRCULATION CONDITIONS

In this investigation, the gaseous hydrogen peroxide (H2O2) is issued at the inner port, and methane (CH4) is injected at the outer port of the sub-millimeter scale burner in order to establish an inverse CH4/H2O2 micro-scale diffusion flame numerically under heat recirculation conditions. The velocity of gaseous H2O2 is considered only as a numerical parameter in this study and the velocity of CH4 is kept constant to examine the flame structure and the transition behavior of an inverse micro-scale diffusion flame. The numerical simulation has been conducted using ANSYS Fluent 14.5 based on Finite Volume Method (FVM) under normal gravity (1G) conditions with low thermally conductive burners. The semi-detailed reaction model, which consists of 58 chemical reactions and 17 chemical species, has been applied in this study. It is found that when the momentum of the gaseous H2O2 is high, then a CH4 /H2O2 inverse micro-scale diffusion flame is formed, and this flame is attached just on the top of the burner by which one can easily analysis an active flame-wall interaction phenomenon. On the other hand, when the momentum of the gaseous H2O2 has been reduced significantly, then H2 /O2 micro-scale premixed flame is established entirely inside the micro-burner. Furthermore, it is found that when the momentum of H2O2 reduces remarkably, then CH4 does not diffuse inside the micro-burner at the flame zone, and consequently only H2 /O2 micro-scale premixed flame is established there as H2O2 is playing the role of a monopropellant, where no extra oxidizer is needed for the survival of flames. This type of flame mode transition phenomenon under micro-burner systems is the first ever reported event in the literature of reacting fluid dynamics (i.e., combustion dynamics). Besides, this is the primary findings of our ongoing research, and hence, the further numerical calculation adopting different pertinent parameters is now under progress with the aim of extracting more profound physical and chemical insights that are associated with the aforesaid transition phenomena.

Computational realization of turbulent combustion in a scramjet combustor stabilized by a lobed strut

Currently, there are theoretical and technical challenges to organize stable flame in scramjet, especially for variable flight conditions. This paper applies the numerical simulation to investigate and analyze the combustion characteristics in a lobed strut-stabilized scramjet combustor under different conditions of Mach number (Ma). By comparing with the experiment date of hydrogen mole fraction, the reliability of large eddy simulation solver used in this paper is verified. On this basis, the simulations of mixing and combustion cases at Ma 1.5–3.0 are carried out separately. The numerical results show the streamwise vortices induced by the lobed strut enhance the mixing and are conducive to the subsequent stable combustion. The turbulent flame behaves lifted characteristics, and the auto-ignition provides the initial active radicals for its stabilization. There are two chemical reaction modes in the flow field, namely auto-ignition, and flame. Further comparison of the cases of different Mach numbers indicates that the lifted height of the flame depends on the Mach number. Analysis of probability density function (PDF) reveals that Mach number influent the competitive relationship between the auto-ignition mode and the flame mode. The auto-ignition and the flame coexist downstream in high ma cases, while the flame mode is the dominant in low Ma cases.

Flamelet generated manifold simulation of highly swirling spray combustion: Adoption of a mixed homogeneous reactor and inclusion of liquid-flame heat transfer

We undertake the modeling of the combustion of highly swirling fuel sprays using the Flamelet-Generated-Manifold (FGM) combustion-chemistry-reduction technique, especially the use of adiabatic tables generated with non-premixed chemical reactors. Preceding investigations indicated that tables thus generated can present uncertainties when used for predicting the finite-rate phenomena and different flame modes, and these are important for better prediction of spray flames in gas turbines. Thus, to address these, we have adopted a mixed-homogeneous chemical reactor that is applicable to both pre-mixed and non-premixed reactions and evaluated this using detailed computations of a constant-pressure mixed reactor. In addition, we have included curated levels of flame-liquid heat gain and loss in the generation of the FGM libraries and analyzed the effects on the major species formation. The methodologies were then incorporated into a Reynolds-averaged-Navier-Stokes model to analyze the data from the reacting ethanol spray flames, and the results were tested against the values of the mixture fraction at axial locations, the burner power output, the flame heat release structure, and the mean of the flame lift-off. The computed burner power output and mean flame lift-off were ∼90.4% and ∼89.6% of the reported experimental data, respectively. Compared with the newest published large-eddy-simulation data, the predictions for the mixture fraction values especially at the center of the flame in the central-recirculation-zone were not underestimated, and the spatial distribution of the flame OH captured the flame height and shape better. The inclusion of mixed homogeneous reactors and flame-liquid heat transfer in FGM can enhance their use in spray-combustion studies.

Cool flame wave propagation in high-pressure spray flames

Cool flame has an important impact on the performance and emission of advanced low-temperature combustion (LTC) internal combustion engines (ICEs) in which the liquid fuel is injected earlier at a lower ambient temperature (Tam) than that in the conventional diesel combustion engines. However, the cool flame characteristics of spray combustion under ICEs conditions are not fully understood, e.g., the effect of cool flame on the spray ignition and flame stabilization is not well studied. In this paper, the so-called cool flame wave propagation (CFWP) in the Engine Combustion Networks Spray A flames at three ambient temperatures Tam (800 K, 900 K and 1000 K) is analyzed using the data from large eddy simulation with an improved Eulerian based transported probability density function sub-grid combustion model. A good agreement between the LES results and experimental data is obtained for the spray liquid penetration length, vapor fuel penetration length, mean pressure rise profile, and flame liftoff length. It is shown that CFWP in the spray ignition process promotes the ignition of the fuel-rich and cold reactant mixture, leading to the most reactive mixture shifting toward fuel-richer locations due to the spatial stratification of temperature and concentration, and turbulent mixing. As a result, the high temperature ignition (HTI) can be shortened compared to the ignition in the homogeneous mixture. At Tam = 800 K, the HTI kernels are consistently formed following the CFWP propagating toward the spray head region. However, as Tam increases, the spatial correlation between HTI kernels and CFWP fronts is weakened. On the other hand, the turbulent mixing (quantified using local scalar dissipation rate) contributes more to the formation of HTI kernels at higher Tam. The present results indicate that CFWP is more profound at lower Tam in the spray ignition process. Finally, it is found that cool flame propagates mainly into pre-reacted fuel-rich mixture in an ignition wave propagation mode from the spray upstream region toward downstream region, whereas the ignition assisted flame mode is found in the spray upstream region where the combustion heat release is negligible.

LES/FGM investigation of ignition and flame structure in a gasoline partially premixed combustion engine

This paper presents a joint numerical and experimental study of the ignition process and flame structures in a gasoline partially premixed combustion (PPC) engine. The numerical simulation is based on a five-dimension Flamelet-Generated Manifold (5D-FGM) tabulation approach and large eddy simulation (LES). The spray and combustion process in an optical PPC engine fueled with a primary reference fuel (70% iso-octane, 30% n-heptane by volume) are investigated using the combustion model along with laser diagnostic experiments. Different combustion modes, as well as the dominant chemical species and elementary reactions involved in the PPC engines, are identified and visualized using Chemical Explosive Mode Analysis (CEMA). The results from the LES-FGM model agree well with the experiments regarding the onset of ignition, peak heat release rate and in-cylinder pressure. The LES-FGM model performs even better than a finite-rate chemistry model that integrates the full-set of chemical kinetic mechanism in the simulation, given that the FGM model is computationally more efficient. The results show that the ignition mode plays a dominant role in the entire combustion process. The diffusion flame mode is identified in a thin layer between the ultra fuel-lean unburned mixture and the hot burned gas region that contains combustion intermediates such as CO. The diffusion flame mode contributes to a maximum of 27% of the total heat release in the later stage of combustion, and it becomes vital for the oxidation of relatively fuel-lean mixtures.

On The Relationship Between H2 Addition, Local Extinction And Hydrodynamic Instability In Non-Premixed Bluff-Body Stabilized Flames

In the race for decarbonization of the power sector, H2 is found to be a better candidate as it can be readily produced in large scales from renewables (e.g. wind and solar energy). Nevertheless, the difference in the thermophysical properties of H2 in comparison to the conventional fuels possesses additional operational challenges. For instance, although the high flame speed together with a wide flammability range offer improved flame stabilization, however, it intensifies the risk of flashback. Besides, the increased strain resistance due to H2 enrichment is expected to alter the flame shape significantly. Hence, it is apparent that the widespread implementation of H2 across various practical systems needs a thorough understanding of the various physical mechanisms. In this context, we examined the effect of hydrogen (H2) enrichment to methane (CH4) in a non-premixed bluff-body stabilized burner operating under typical central jet dominated flame mode. In the chosen mode of operation, globally, the flow field and flame feature three important successive spatial regions, namely, recirculation zone, neck zone and jet-like flame zone. In such configuration, the flame is exposed to a higher stretch rate in the neck zone and eventually undergoes local extinction. Such local extinction and subsequent re-ignition of broken flame branches have strong implications over the hydrodynamic instability of the coaxial annular air shear layer. It is well known that H2 addition increases the flame extinction strain rate and thus alters the local extinction phenomenon. To understand this phenomenon, we performed experiments at 10%, 20 %, 30% and 50 % hydrogen proportion in (H2-CH4) blend. High repetition rate (5 KHz) PIV and OH PLIF measurements are performed simultaneously to perceive the quantitative insights. The results obtained from POD and 1D wavelet transform indicated the suppression of vortex shedding at the annular air shear layer for H2 addition greater than 20 %, and thus quantified the beneficial effect of H2 addition in turbulent flame stabilization.

Hydrodynamic resolved simulation of a char particle combustion by immersed boundary-lattice Boltzmann method

A Lattice-Boltzmann model coupled with boundary method is proposed for the hydrodynamic resolved simulation of a single char particle combustion. A pure lattice Boltzmann scheme with multi distribution functions is presented for this low Mach reactive flows. Real varying thermodynamic and transport properties are considered, and the fluid density can bear significant change depending on the varying temperature and species concentrations. The recently proposed boundary-thickening based direct forcing-immersed boundary method is extended to implement the boundary conditions of velocity, temperature, and species concentrations at the char particle surface. Two heterogeneous reactions at particle surface and one homogeneous reaction in fluid are adopted to describe the combustion. The Stefan flow near the particle surface caused by the heterogeneous reactions is also considered. A satisfactory agreement can be found between the present simulation results and the previous experimental and numerical results. Three flame modes of a char particle combustion and the transition mechanisms between them are investigated. Furthermore, the effect of hydrodynamic interaction and the oxygen concentration in O2/CO2 atmosphere on particle combustion behavior are explored in detail. The present work establishes the foundation for the efficient simulations of particle combustion with size change and motion.

Suppression of thermoacoustic instability and NOx emissions of unsteady swirl premixed flame with upstream microjets

AbstractPassive control methods are widely used in unsteady combustion systems due to their robustness and convenience. This paper used the upstream microjets method for the first time to control thermoacoustic instability and pollutant NOx emissions in unsteady swirl premixed flame. Three variables of CO2 microjets flow rate, upstream microjets distance, and microjets number are investigated. Results demonstrate the upstream microjets distance plays a significant role in restraining the thermoacoustic instability and NOx emissions. The smaller the upstream microjets distance, the more completely the thermoacoustic instability is suppressed while the NOx emissions are the opposite, which means a compromise choice needs to be made between them. The damping ratio of pressure amplitude in the chamber can reach the maximum of 81.3%, and the damping ratio of CH* intensity can reach the maximum of 80.2%. Besides, the frequency of pressure and CH* will transit to a lower mode. NOx emissions can realize a maximum reduction of 12.2 ppm. Meanwhile, the flame mode switches from an enlarged Mode‐A flame to a shrunk Mode‐B flame, and the flame length shows a maximum reduction of 14 mm at the largest upstream distance. However, the specific control mechanism under upstream microjets still needs further study to clarify. This research proved the feasibility of suppressing combustion instability and NOx emissions with upstream microjets in unsteady swirl premixed flame, promoting its practical application in gas turbine systems.

Smoke Release by Some Indian Woods and Wood Substitutes

This article presents the study of smoke release by some Indian woods and wood substitutes. These woods and wood substitutes are used in buildings as lining materials and furniture. The study will help to prepare fire retardant coating for interior lining materials, so the material will release nil/low smoke during fire after application of coating. Some tests were performed on Kail wood, Deodar wood, plywood, and fibre board to determine smoke release properties of these woods and boards. The SOD was determined using smoke density chamber as per ASTM E–662 which measures the amount of smoke released by a specimen in the burning (F–mode) or smoldering mode (NF mode). In smoldering mode, only radiation through a furnace of 2.5 W/cm2 was applied to the specimen surface. In flaming mode, radiation and fire from a burner were given to the specimen surface. The sample which represents the material was tested when it starts to smolder and again when a flame source was applied on the surface of specimen. The smoke density was between 0 (no smoke generated) to 800 as per standard ASTM E–662. The above said woods were tested as per standard in two modes, that is, NF and F-modes. The SOD was calculated for the above said materials to measure the amount of smoke release by materials.