Types Of Flames Research Articles

Experimental DMD Analysis of Combustion Modes and Instabilities in a Scramjet Combustor

The combustion characteristics of a gaseous hydrogen fueled scramjet combustor were experimentally investigated through single-shot micro-pulse detonation engine (μPDE) operations across a range of fuel injection pressures and corresponding equivalence ratios. This study utilized a lab-scale direct-connect scramjet combustor integrated with a μPDE, designed to simulate flight conditions at Mach numbers between 4.0 and 5.0 at altitudes of 20 to 25 km. Static wall pressure was measured using 16 pressure probes from the isolator to the scramjet combustor at a sampling rate of 500 Hz. Additionally, Z-fold type Schlieren and flame luminosity were captured using a high-speed camera in a test section equipped with a quartz visualization window. Experiments were conducted under five conditions with equivalence ratios ranging from 0.10 to 0.55, each performed three times to ensure repeatability. Fast Fourier transform (FFT) and dynamic mode decomposition (DMD) analyses were applied to the pressure and image data obtained. Although auto-ignition was not achieved in all cases, combustion persisted while fuel was injected following the single-shot μPDE ignition. This allowed for the identification of distinct combustion modes according to equivalence ratio, by observing the dominant flame types and the shock structure within the flow of the combustor. These combustion modes showed different types of instability.

Thermal characteristics of ghosting flame in coal mine dead-end roadway pool fires affected by fire locations

As an important phenomenon in the evolution of fire, the ghosting flame exhibits dynamic and irregular behavior in environments with limited ventilation. By ghosting flame, we mean the phenomenon where the flame separates from the fuel surface under certain conditions. This study investigates the combustion characteristics of the ghosting flame in pool fires within a coal mine dead-end roadway by small-scale experiments. Results indicate that the ghosting flame appears more readily and earlier when the fire source is closer to the heading face or when the oil pan size is larger. Additionally, the fire source's location significantly influences the type of ghosting flame observed. When the fire source is near the heading face, the flame predominantly exhibits horizontal-type ghosting flame; however, when the fire source is at the entrance of the roadway, only the edge-type ghosting flame occurs. Furthermore, dead-end roadway fires easily transition to ventilation-controlled conditions, leading to a continuous increase in the mass loss rate. The occurrence of a ghosting flame does not result in a sudden change in the mass loss rate. Moreover, the transient oxygen volume fraction when the ghosting flame occurs is related to the ghosting flame type and is also affected by ventilation conditions, fuel types, roadway structure, and the location of measurement points. Importantly, the root cause for the occurrence of the ghosting flame lies in the pressure differential generated by the temperature disparity between the two sides of the fire source.

Accurately measuring slowly propagating flame speeds: Application to ammonia/air flames

Environmental concerns have driven the development of alternative fuels and refrigerant working fluids with low global warming potential. Ammonia (NH3) is a potential zero-carbon fuel, while hydrofluorocarbons (HFCs) like R-32 and R-1234yf are being adopted as refrigerants. When mixed with air, these compounds can sustain slowly propagating flames with laminar flame speeds less than 10 cm/s. Unlike typical hydrocarbon-fueled flames, these slow flames are influenced by buoyancy-induced flow and radiation heat loss. In this study, we experimentally investigate the flame speeds of NH3/air mixtures using the constant-pressure spherically expanding flame method, while circumventing gravity-induced natural convection, and account for radiation-induced inward flow. To mitigate buoyant convection, a low-cost drop tower was built and used to study slow spherically expanding flames in free fall. A computational model (SRADIF) is utilized that combines thermodynamic equilibrium and finite rate optically thin limit radiation heat loss calculations to estimate the inward flow. The developed methodology is utilized to investigate slowly propagating NH3/air flames over a range of equivalence ratios. A systematic approach was undertaken to understand and quantify the errors that could arise when deriving the laminar flame speed. It was found that attempting to study slowly propagating flames in a static configuration, as opposed to in free fall, results in large differences in flame dynamics and subsequently all derived quantities. It is necessary to study slowly propagating flames in free-fall. Additionally, using experimental data that has not been corrected for radiation-induced flow leads to large errors in all derived quantities. Furthermore, direct comparisons of experimental measurements and detailed flame simulations are found to be necessary to determine if existing extrapolation approaches are applicable to these slowly propagating flames, which are challenging to study.

Experimental Determination of a Mixture Composed of Camisea Natural Gas and CO2 Laminar Burning Velocity

The aim of this work is to provide new experimental data on laminar burning velocities for a new synthetic mixture composed of Camisea natural gas and CO2. It was found that the relevant published experimental background data are limited to mixtures composed of methane and CO2; considering the fact that Camisea natural gas is widely used in Peru, this experimental research will serve as a supportive resource for further experimental and industrial implementations in this country, such as the design and modeling of new engines or industrial burners that are designed to be fueled by this mixture. An experimental setup for analyzing three types of flame geometry, which is feasible to implement for a wide range of conditions, was built in PUCP PI0735 laboratory and all the measurements were obtained for a range of mixtures (0%, 21.2%, 28.5%, 38.9%, 50% CO2) and ratios from around 0.55 to 0.95 at atmospheric conditions. The laminar burning velocities results obtained were analyzed in groups based on %CO2. In addition, the experimental margin error was determined by considering all the sources. The following conclusions were reached: (1) The laminar burning velocity decreases with the increase in CO2 percentage in the mixture due to the CO2 decreasing the flame temperature effect. (2) The flat flame type provided the highest value of burning velocity for each group of CO2 percentage in which it appears. (3) The highest obtained laminar burning velocity value was 22.64 ± 0.15 cm/s, for a flat flame with a ratio of 0.72 and 29.98% of CO2, while the lowest obtained value was 6.78 ± 0.15 cm/s for a conical trunk flame with a ratio of 0.59 and 49.83% of CO2. (4) The highest evaluated CO2 percentage was 50.97% for a conical trunk flame with a ratio of 0.69 and a burning velocity value of 11.04.

Early fire detection using wavelet based features

In recent years, millions of hectares of vegetation worldwide have been destroyed by wildfires and forest fires. Computer vision-based fire classification, which separates pixels from image or video datasets into fire and non-fire categories, has gained more attention recently due to technological innovations. Fire pixels in an image or video can be classified using either a deep learning strategy or a traditional machine learning approach. Deep learning algorithms could process enormous volumes of data, but their training model performance is constrained because they fail to consider the differences in complexity between training samples. Moreover, it is not obvious to train a deep learning model without a dedicated GPU unit.Similarly, deep learning techniques that have a scarcity of training data and insufficient features exhibit poor performance in intricate real-world fire situations. Consequently, to categorize fire and non-fire pixels from the processed photos from the publicly accessible datasets, Corsican and FLAME, as well as the aerial private dataset Firefront_Gestosa, the current study uses a lightweight technique based on SVM and a refined set of features.The present research implemented a novel framework for fire detection and classification from a variety of RGB and Infra-red images acquired during real missions, addressing the significant requirement for swift and accurate recognition of diverse types of flames, ranging from wildfires to industrial and domestic fires. The framework employs wavelet decomposition-based features, including wavelet length, standard deviation, variance, energy, and Shannon’s entropy, extracted through a sliding window sampling method within a machine learning approach. It should be noted that managing multidimensional data to train a model is difficult in machine learning applications. This issue is solved adopting a feature selection approach, which eliminates redundant or unnecessary data that affects the functionality of the model. Thus, to enhance model performance, feature selection using ranking algorithms based on theoretical mutual information is applied in combination to the Support Vector Machine (SVM) with Radial Basis Function (RBF) kernel.Extensive experiments demonstrate exceptional results, notably with Haar wavelets, achieving an impressive overall accuracy of 99.43% and remarkable performance across specificity, precision, recall, F-measure, and G-mean metrics. Thus, the present study showcases the potential of advanced image processing techniques to significantly advance fire detection and classification, thereby contributing to fire prevention, management, and research in various contexts.

A simple and accurate method for the determination of Rh, Pd, and Pt in e-waste and spent automotive catalysts using HR-CS FAAS for assessing the value of secondary raw materials

This work presents a simple and accurate method for the fast sequential determination of Rh, Pd, and Pt in spent automotive catalysts and e-wastes using high-resolution continuum source flame atomic absorption spectrometry (HR-CS FAAS). Extensive research was carried out in model systems on the impact of potential interfering substances on analyte's signals measured in two types of flame (air-C2H2 and N2O–C2H2). Mutual analyte interactions were also taken into account. Different background corrections offered by the HR-CS AAS spectrometer were tested to obtain interference-free analyte signals and the best detectability. Using an air-C2H2 flame and 1 % La solution as a spectrochemical buffer provided good sensitivity and accurate determinations of Rh, Pd, and Pt using a simple calibration graph. Microwave-assisted leaching of PGE from waste samples with aqua regia at 240 °C for 60 min efficiently leached all target metals, which significantly simplified and shortened the sample preparation step. The detectability of the method (detection limit of 0.4, 0.6, and 5 mg kg−1 for Rh, Pd, and Pt, respectively) and precision (< 7 %) were satisfactory. The accuracy of the method was confirmed by analysis of certified reference materials (spent automotive catalyst (ERM-EB504), electronic scrap (BAM-M505a)), and calculated zeta score values. The recoveries for Rh, Pd, and Pt in ERM-EB504 were 93, 101, and 96 %, respectively, and for Pd in BAM-M505a, 97 %. The developed method can be used to assess the value of secondary raw materials, such as various types of spent catalysts and e-waste containing Rh, Pd, and Pt.

Dynamics of slowly propagating flames: Role of the Rayleigh-Taylor instability

Environmental concerns have motivated the development of alternate fuels and refrigerant working fluids that have low global warming potential. Ammonia (NH3) is a candidate zero-carbon fuel and hydrofluorocarbons (HFCs) such as R-32 and R-1234yf are being adopted as refrigerants. Upon mixing with air, these compounds can sustain flames that are slowly propagating i.e., with laminar flame speeds less than 10 cm/s. These flames are referred to as slow as they are affected by buoyancy-induced flow and radiation heat loss, in contrast to typical hydrocarbon-fueled flames. In this study, we investigate the flame dynamics of slowly propagating refrigerant/air flames, with a focus on the effect of the Rayleigh-Taylor (RT) instability. Dispersion relations, that characterize the range of unstable wavelengths and their growth rates during early stages of instability growth, are derived from two-dimensional direct numerical simulations (DNS, including detailed chemistry and transport) of RT-unstable R-32/air flames. These results are compared to predictions from reduced-order theoretical models. DNS are also performed to study the long-term evolution of such flames and to quantify flame speed enhancements. Results showed that RT-unstable flames can propagate up to 6–7 times the laminar flame speed. Although slow from a flamelet point of view, these flames propagate much faster at larger scales. These results have implications on developing metrics to assess explosion risk, and minimize it while storing, transporting, and utilizing these compounds. (225 words)

Learning-Based Super-Resolution Imaging of Turbulent Flames in Both Time and 3D Space Using Double GAN Architectures

This article presents a spatiotemporal super-resolution (SR) reconstruction model for two common flame types, a swirling and then a jet flame, using double generative adversarial network (GAN) architectures. The approach develops two sets of generator and discriminator networks to learn topographic and temporal features and infer high spatiotemporal resolution turbulent flame structure from supplied low-resolution counterparts at two time points. In this work, numerically simulated 3D turbulent swirling and jet flame structures were used as training data to update the model parameters of the GAN networks. The effectiveness of our model was then thoroughly evaluated in comparison to other traditional interpolation methods. An upscaling factor of 2 in space, which corresponded to an 8-fold increase in the total voxel number and a double time frame acceleration, was used to verify the model’s ability on a swirling flame. The results demonstrate that the assessment metrics, peak signal-to-noise ratio (PSNR), overall error (ER), and structural similarity index (SSIM), with average values of 35.27 dB, 1.7%, and 0.985, respectively, in the spatiotemporal SR results, can reach acceptable accuracy. As a second verification to highlight the present model’s potential universal applicability to flame data of diverse types and shapes, we applied the model to a turbulent jet flame and had equal success. This work provides a different method for acquiring high-resolution 3D structure and further boosting repeat rate, demonstrating the potential of deep learning technology for combustion diagnosis.

Evaluation of ground water quality using heavy metal pollution indices and estimation of health risk

ABSTRACT This paper presents the monitoring of heavy metals, including cadmium (Cd), chromium (Cr), lead (Pb), copper (Cu), and zinc (Zn), in the groundwater of the Byrnihat Industrial Area and Guwahati City, which is located in close proximity to the Byrnihat Industrial Area. A total of 20 water samples were collected at distances approximately 1–2 km apart from each other. The pH and the TDS of the collected samples were analysed immediately after collecting samples using a digital pH and TDS meters. The collected samples were analysed using a Flame type Atomic Absorption Spectrophotometer. The highest observed values of Cd, Cr, Pb, Cu and Zn in mg/L were recorded to be 0.0526, 0.1438, 0.5175, 0.1060 and 0.0925, respectively. The average values of Cd, Cr, Pb, Cu and Zn were 0.0246 ± 0.0168 mg/L, 0.0282 ± 0.0421 mg/L, 0.4805 ± 0.0206 mg/L, 0.0653 ± 0.0276 mg/L, and 0.0642 ± 0.0098 mg/L, respectively. Cd and Pb concentrations were above the guideline values set by Bureau of Indian Standards (BIS) and the World Health Organisation (WHO) [WHO recommended values, Cd: 0.003 mg/L, Pb: 0.01 mg/L]. Though the average concentration of Cr was below the allowable limit, five samples within the Byrnihat industrial area were found to be above the limit. High values of pollution indices was observed for all the samples. The cumulative hazard index (HI) values for adults and children were found to be 8.56 and 9.37, respectively. The total cancer risk (TCR) was observed to be 7.87E-03 for adults and 8.54E-03 for children. The result obtained from the present investigation show significant contamination of ground water by heavy metals, particularly Pb, Cd, and Cr concentrations. The observed high values of hazard index and cancer risks indicate significant health risks, necessitating the need for immediate regulatory action.

Combustion condition predictions for C2-C4 alkane and alkene fuels via machine learning methods

The accurate and rapid prediction of hydrocarbon type was a precondition for the utilization of fossil fuels with high efficiency and safety. In this study, machine learning based techniques were used to predict the type and equivalence ratio of flames of C2-C4 alkane and alkene fuels based on the differences in flame morphology between various combustion conditions. The test results of different machine learning algorithms, including ANN, SVM, SVR, KNN, MLR, and RF were compared in detail using statistical methods. Results indicated that ANN, SVM, KNN, and RF all exhibited an outstanding performance in predicting the types of C2-C4 alkane and alkene flames, achieving accuracies of 95.7 %, 96.3 %, 93.8 %, and 96.5 %, respectively. For the prediction of the equivalence ratio among these fuels, the mean absolute percentage errors of the ANN, SVR, MLR, and RF were only 5.6 %, 3.8 %, 8.2 %, and 3.8 %, respectively. The performance of SVM, SVR, and RF algorithms was significantly superior to that of ANN, MLR, and KNN algorithms for flame prediction. Moreover, the data of feature analysis revealed that the importance level of designed features exhibited a significant distinction between different prediction targets. For predicting the type of C2-C4 alkane and alkene fuels, the features associated with blue region showed a stronger importance level. However, the yellow region related features played a more significant role for the prediction of the equivalence ratio.

Advanced Fire &amp; Gas Detection System

Abstract: This paper presents a cost-effective, user-friendly, and dependable fire and gas detection system leveraging the capabilities of an Arduino microcontroller. The system employs an MQ2 gas sensor module for gas detection and a flame sensor module for fire detection. Upon the detection of either gas or fire, the system initiates an alarm, activates LED indicators, and communicates the situation through a 16x2 LCD display. Furthermore, the system can be configured to issue SMS alerts to specific recipients. Designed for versatile applications, it finds utility in diverse settings, including residential, commercial, and educational environments. It exhibits proficiency in detecting an extensive array of gasses, encompassing methane, propane, and carbon monoxide, while also displaying sensitivity to various flame types, including those generated by wood, paper, and plastic. Installation and upkeep are straightforward, with the Arduino microcontroller and sensor modules easily adaptable to a small breadboard or circuit board. The strategically positioned LCD display and alarm ensure swift notifications to occupants in case of potential hazards. Notably, this system is budget-friendly, with the Arduino microcontroller and sensor modules available at reasonable prices from numerous online and retail suppliers. Rigorous testing has validated its effectiveness in gas and fire detection, establishing it as an invaluable asset for safeguarding lives and property against fire and gas-related risks

The kinetics and warm flame chemistry associated with radiative extinction of spherical diffusion flames

Several studies have found that microgravity diffusion flames with sufficient heat loss cool until they extinguish at a critical temperature near 1130 K. In this work, the associated chemical kinetics are explored for spherical diffusion flames burning ethylene. The flames are simulated with a transient numerical model with detailed chemistry, transport, and radiation. This incorporates the UCSD mechanism with 57 species and 270 reactions. Species concentrations, reaction rates, and heat release rates are examined. Upon ignition, the peak temperature is above 2000 K, but this decreases until extinction due to radiative losses. This allows the chemistry to be studied over a wide range of peak temperatures for the same fuel and oxidiser. When the peak temperature is high, the dominant chemistry is similar to that for typical normal-gravity ethylene diffusion flames. There are two distinct zones: an ethylene pyrolysis zone and an oxidation zone, and negligible reactant leakage. The ethylene mainly reacts with H and OH. The concentration of OH in the ethylene pyrolysis zone is high due to the long residence times and reactions of CO2 and H2O with H. As the flame cools and the peak temperature approaches the critical extinction temperature, there is increased reactant leakage leading to higher O, OH, and HO2 concentrations on the fuel side. Most reactions shift towards the oxidiser side and there is large overlap between the two zones. Reactions involving HO2 become more significant and the large consumption rates of H by HO2 increase the concentrations of OH and O on the fuel side. The appearance of warm flame chemistry delays extinction but is not sufficiently exothermic to prevent it.

A Review on Micro-Combustion Flame Dynamics and Micro-Propulsion Systems

This work presents a state-of-the-art review of micro-combustion flame dynamics and micro propulsion systems. In the initial section, we focus in on the different challenges of micro-combustion, investigating the typical length and time scales involved in micro-combustion and some critical phenomena such as flammability limits and the quenching diameter.We present an extensive collection of studies on the principal types of micro-flame dynamics, including flashback, blow-off, steady versus non-steady flames, mild combustion, stable flames, flames with repetitive extinction, and ignition and pulsatory flame burst. In the final part of this review, we focus on micropropulsion systems, their performance metrics, conventional manufacturing methods, and the advancements in Micro-Electro-Mechanical Systems manufacturing.

Post-Wear Surface Morphology Assessment of Selective Laser Melting (SLM) AlSi10Mg Specimens after Heat Exposure to Different Gas Flames

The wear surface morphology of AlSi10Mg specimens, originally manufactured using selective laser melting (SLM), has been analyzed in the context of exposure to heat from gas flames. The first stage of the experimental work included the performance of surface heat-exposure on SLM-prepared specimens through oxyacetylene gas welding. Gas welding was utilized with three different flames, namely; reducing, neutral, and oxidizing on the as-built specimens of SLM. The post-surface-treated specimens were subjected to pin-on-disk wear testing against fixed parameters. After the performance of wear testing at two different radii, the mass loss of each of the four types of specimens was calculated including the three specimens exposed to heat along with the as-built specimens. The results showed that the maximum amount of mass losses at 24 mm and 30 mm radii belongs to the neutral flame specimens and the least was for the as-built condition specimens. Upon analysis, the heat-exposure specimens through all three types of gas flames resulted in an increase in the amount of mass in contrast to the as-built specimens. Moreover, the morphologies of the developed wear tracks at surfaces were examined using the scanning electron microscope (SEM) for the understating of the mechanism.

Experimental Investigation on the Symmetry and Stabilization of Ethanol Spray Swirling Flames Utilizing Simultaneous PIV/OH-PLIF Measurements

A detailed experimental study of ethanol spray swirling flames was performed in an axial bluff body stabilized burner. The characteristics of the non-reacting and reacting sprays were recorded by particle imaging velocimetry (PIV) and planar laser-induced fluorescence (PLIF) of the OH radical. A few typical flames with different structures (outer-side-flame-lifting, stable, and near-blow-off) were compared and analyzed. The parameters of the spray, including the spray half-angle (α) and droplet number density (nd), are quantified, and it has been found the flame structure and stability were strongly correlated with the droplet distribution. Several parameters of the flow field, such as velocity magnitude (|U| vorticity (ωz), and turbulent kinetic energy (TKE), are quantitively analyzed, and it is observed that the local strain rate rose as the air flow rate increased, which is not conducive to local flame stability. Regarding the flame, quantities such as progress variable (&lt;c&gt;), flame height (Lf), lift–off height (hlf), and symmetry factor (Snd and S&lt;c&gt;) are calculated, and it can be observed that the flame symmetry keeps worsening when approaching blow–off, and the inner flame branch exhibits a worse stabilization than the outer one. Our comprehensive investigations offer a deeper understanding of stable combustion in such two–phase flames.

Numerical Study of the Effect of Primary Nozzle Geometry on Supersonic Gas-Solid Jet of Bypass Injected Dry Powder Fire Extinguishing Device

A two-way coupled model between polydisperse particle phases with compressible gases and a density-based coupling implicit solution method, combining the third-order MUSCL with QUICK spatial discretization scheme and the second-order temporal discretization scheme, are constructed based on the discrete-phase model (DPM) and the stochastic wander model (DRWM) in the Eulerian–Lagrangian framework in conjunction with a unitary particulate source (PSIC) approach and the SST k-ω turbulence model. The accuracy of the numerical prediction method is verified using previous supersonic nozzle gas-solid two-phase flow experiments. Numerical simulation of a two-phase jet of dry powder extinguishing agent gas with pilot-type supersonic nozzle was performed to analyze the influence of geometrical parameters, such as the length ratio rL and the area ratio rA of the main nozzle on the two-phase flow field, as well as on the jet performance indexes, such as the particle mean velocity vp,a, velocity inhomogeneity Φvp, particle dispersion Ψp, particle mean acceleration ap,a, etc. By analyzing the parameters, we indicate the requirements for the combination of jet performance metrics for different flame types such as penetrating, spreading, and dispersing.

Effects of reactants stratification and pre-heating on the stabilization and emissions of partially cracked ammonia swirl flames

Ammonia is an easy solution for the transportation and storage of hydrogen. To achieve combustion properties similar to those of methane, ammonia can be partially cracked on site to introduce hydrogen and nitrogen in the fuel mixture. In this work, an atmospheric pressure dual-swirl ammonia-hydrogen burner is used to study different configurations of stratified flames of ammonia cracked at 28 %. First, flame stabilization is evaluated in terms of the overall equivalence ratio and the distribution of air between the hydrogen and ammonia streams. This is done for both cases where either hydrogen or ammonia occupies the central part of the burner or its periphery. The configuration of the burner is adjusted in such a way that several stratification levels are scrutinized, depending on where the two streams meet. Seven types of flames are identified and described. A stability map is measured. The results show that the configuration with ammonia flowing centrally and hydrogen occupying the periphery enhances stability. Second, measurements of NOx, N2O, unburnt ammonia, and unburnt hydrogen in the exhaust gases are performed. Full stratification reduces NOx emissions, but both lean overall equivalence ratio and lean ammonia equivalence ratio increase N2O emissions. The flame with ammonia in the center and hydrogen at the periphery with an overall equivalence ratio of 0.55 gives the best results in terms of stability and low pollutant emissions. This condition is further investigated by changing the reactants temperature. The reactants preheating is beneficial for N2O emissions but comes with a strong NOx penalty.

Laminar burning velocities of rich NH3+N2+O2 flames: Comparing the effects of elevated temperatures and oxygen ratios on model validation

Ammonia (NH3) is a promising carbon-free alternative fuel. One of its fundamental combustion properties, the laminar burning velocity (SL), is widely used to validate the combustion kinetic models. This study investigates the SL across varying unburned mixture temperature Tu and oxygen ratio xO2, particularly regarding their impact on the model validation. The focus is on the fuel-rich conditions that exhibit noticeable discrepancies among different kinetic model predictions. To provide accurate experimental data for the analyses, atmospheric measurements were carried out using the heat flux method, covering equivalence ratios ranging from 1.2 to 1.9, Tu from 298 K to 448 K, and xO2 from 0.30 to 0.38. Notably, many of these conditions have not been thoroughly explored in existing literature. Seven kinetic models were used to compare with the experimental data and assess the simulation discrepancies, where the increment parameters were used to compare the effect of SL vs. Tu and SL vs. xO2 dependences on the model validations. Through these analyses, the α coefficient, describing the SL vs. Tu dependence, is found too difficult to distinguish the performances of the various models and serve as validation targets. Sensitivity analyses reveal that the reactions governing the dependences of SL on both Tu and xO2 are fundamentally the same. Similar flame conditions with nearly identical sensitivity values were identified, suggesting that a model accurately reproducing the SL vs. Tu trend is promised to replicate the SL vs. xO2 trend, and vice versa. Consequently, the SL measurement with elevated Tu, which is difficult in experiments and in distinguishing the performance of different models, is suggested to be substituted by the much easier measurement with elevated xO2. This conclusion, alongside the methodology, holds potential for application in other flame types and help reduce the experimental efforts for model updating.

Forced ignition of cool, warm and hot flames in a laminar non-premixed counterflow of DME versus air

Recently, there has been great interest in the ignition of cool and warm flames (controlled by low-temperature chemistry, LTC, and intermediate-temperature chemistry, ITC, respectively) as they may have significant influence on the ignition and subsequent propagation of hot flame. However, it is still unclear how the flow affects the ignition of the cool/warm flame and their transition to the hot flame in a non-premixed fuel/oxidizer system. In this study, we conduct 2D simulations on the forced ignition of cool, warm and hot flames in a laminar, axisymmetric, non-premixed counterflow of dimethyl ether (DME) versus air. The objective is to assess the effects of flow on the ignition of cool, warm, and/or hot flames and the transition among them. Different ignition energies and strain rates are considered. It is found that the transient ignition process is mainly controlled by the extinction/transition limits and the minimum ignition energies of cool, warm and hot flames. When the strain rate is lower than the transition limits of cool and warm flames, all three flame types can be directly initiated but they eventually evolve into a hot flame. At intermediate strain rate between the transient limit and the extinction limit, quasi-steady cool and warm flames can be obtained without further transition to a hot flame. When the strain rate exceeds the extinction limit of cool and warm flames, only the hot flame can be ignited. An ignition regime diagram in terms of ignition energy and strain rate is proposed, in which different regimes for cool, warm and hot flames as well as transition among them are identified. Furthermore, the ignition of cool and warm flames in a non-premixed counterflow is shown to be very sensitive to ignition position as well as strain rate and ignition energy.

Flame stability characteristics of a flame spray pyrolysis burner

The stability characteristics of an established flame spray pyrolysis (FSP) burner are quantified in order to determine viable operating conditions and map the structure of FSP flames. The primary novelty of this work lies in exploring a large parameter space which is necessary to explain the dependence of visible flame emissions on input variables. Using high-speed flame luminescence imaging (1–120 kHz), we demonstrate that the input pilot heat release and the flowrates of liquid and dispersion gases are primary control variables that dictate combustion quality. Indeed, variation in these input conditions allows the qualitative identification of four flame types: (i) stable, (ii) lifted, (iii) unstable and (iv) low intensity. Quantification of these flame types is derived via statistical analysis of the temporal luminosity fluctuations. Key metrics such as the mean and standard deviation effectively describe the behaviour of the aforementioned flame types and act as classifying parameters for FSP flames. High normalised mean intensities and coefficients of variation are characteristic of stable flames. However, flame extinction events in the ignition region and throughout the entire flame are characteristic of unstable flame configurations. In this case, poor combustion efficiency and significant intensity fluctuations are quantified by significantly lower mean intensities and higher coefficients of variation. Further insight is given via the implementation of phase-Doppler anemometry (PDA). It is demonstrated how variation in the input flowrates - and therefore variation in gas and droplet-phase velocities - correlate with the identified flame structures. This analysis demonstrates how input parameters can be varied in order to configure the combustion mode and control combustion stability during FSP. Both of which are relevant for understanding the boundary conditions for nanoparticle growth using FSP.