Cellular Flames Research Articles

Investigation on vented ethanol-gasoline vapor explosion characteristics initiated via single and dual sparks

Although vented explosions initiated via dual sparks have already been investigated to model accidental multi-explosions events, the vent coefficient (Kv) and vent location factors have not been analyzed together as far. Hence experiments were carried out in a self-designed vented channel contained single/dual sparks to investigate the effect of Kv and dimensionless vent location (X/D) on vented ethanol-gasoline vapor explosion overpressure and flame propagation. Results indicated that Helmholtz oscillation appeared in overpressure with small Kv, and the oscillation amplitude would be enhanced as X/D increased. Tulip flame appearance depended not only on Kv but also on X/D. The flame downstream of the vent that propagated with a low velocity oscillation and the cellular flame under small X/D, where Kv was positively correlated with cell size. The Pmax and Kv were both positively correlated with a power function (Pmax=aKvb) under the cases of single and dual sparks. The coefficient a increased as X/D rose, and index b always closed to 1. A correlation between the flame velocity (v) vs Kv following membrane burst under single and dual sparks conditions was derived based on energy equation. And the unburned gas flow velocity ratio (vdc/vsc) following membrane burst was obtained, which can explain quantitative relationship between experimental dual/single flame velocities (vd/vs≈0.5). Two ratios difference came from the spherical flame propagation.

Effects of buoyancy on the spherical flame and explosion pressure of a CH4 mixture under dilution conditions

To determine the effects of buoyancy on the spherical flame and explosion pressure under dilution conditions, a series of explosion experiments were conducted in a 20-L spherical container. The schlieren technique was used to record CH4 flames to assess the buoyancy effect. The change in the explosion pressure was recorded and correlated with the flame shape to analyze the explosion characteristics. The methane explosion occurred in two stages: initial ignition and flame development. During initial ignition, the addition of CO2 caused the flame buoyancy to increase considerably. Flames with different buoyancies had different shapes, such as spherical, ellipsoidal, and “ω” shapes. The radius of the flame at 12 mm may serve as the threshold for the onset of buoyancy effects; once the radius exceeds this value, the flame could deform due to buoyancy effects. Increasing the added CO2 concentration caused the buoyancy effect to be enhanced and appear at a very low radius of the flame. The combined effects of buoyancy and dilution reduced the flame expansion speed. The difference between the horizontal and upward expansion speeds linearly increased with the CO2 concentration. At a high CO2 concentration, the flame stagnated at speeds below 0.5 m/s. During flame development, CO2 addition made the flame surface smoother. Suppressing the cellular flame structure delayed the maximum explosion pressure rise rate and reduced the pressure peak. A coupled analysis of the flame shape and the maximum explosion pressure rise rate showed a dense cellular structure for a high explosion pressure rise rate and a smooth flame surface for a low maximum explosion pressure rise rate. The size and number of cellular structures for different mixtures were similar at a fixed maximum explosion pressure rise rate.

Quantitative three-dimensional reconstruction of cellular flame area for spherical hydrogen-air flames

The cellular flame area continues to increase with the development of cells on the spherical flame surface, which will greatly promote the flame burning rate and propagation speed. This work mainly focuses on a new three-dimensional (3D) reconstruction method of the cellular structure on the flame surface, attempting to quantitatively characterize the real flame area. Initially, the visualization investigation of hydrogen-air premixed spherical flames within a constant volume vessel was conducted using the Schlieren optical technique under room temperature and atmospheric pressure conditions. In parallel, the Cellpose 2.0 graphical user interface was used for the preliminary training of the cell segmentation model. Subsequently, this pre-trained model was applied in the image post-processing, enabling the quantitative characteristics extraction of the cellular structure, such as the cells number, area, and the flame radius, etc. Additionally, a concept of peak height h on the flame profile was proposed to characterize the fluctuation degree of flame profile. A new flame equivalent radius ru was defined by the average value of valid distance from flame centroid to flame profile pixel by pixel. Based on the comparison of cell equivalent radius r and average peak height h¯, an innovative 3D reconstruction concept was proposed for the quantitative characterization of flame area. Finally, cellularity factor ξ was introduced to evaluate the cellularization degree on spherical flames surface. Results show that the appearance of secondary cracks marks the formal onset of flame cellularization, accompanied by an increase in the h¯. In the later stages of flame development, cellularization will eventually tend to a stable value of about 0.4, indicating the occurrence of “saturated state”. After 3D reconstruction, the average cell area stable at around 26 mm2 in this stage. The results of this study provide data support for the construction of combustion models in the field of premixed hydrogen-air combustion.

A Histological Study of the Effects of Administration and Withdrawal of Anabolic Androgenic Steroid (Nandrolone Decanoate) on the Hippocampus and the Prefrontal Cortex in Adult Male Albino Rats

Abstract Background The PFC and the hippocampus are parts of the limbic systems and have a major role in emotion regulation, memory and learning. Anabolic androgenic steroids (AASs) are large groups of synthetic derivatives of testosterone, prescribed for the treatment of male hypogonadism. Common abuse of AASs has spread among the general population and Aim of the Work The aim of the current work was to study the effects of AASs on the PFC and the hippocampus histologically and statistically and to evaluate the reversibility of these effects in these regions after AASs withdrawal. Material and Methods 24 adult male albino rats (3 – 6) months old (weighing 200-225 gm) were used in the current study and divided into 3 groups; according to the procedure and treatment type: control group (I) (12 rats) and AASs treated group (II) (6 rats) and Withdrawal group (III) (6 rats). Fixation for the brain using bouin solution was done first in situ followed by its removal and transfer to a glass bottle containing bouin solution for 24 hours to ensure complete fixation. The PFC and the hippocampi were obtained through parasagittal section. Specimens were processed for preparation of paraffin blocks then stained with H&E stain and immunohistochemically stained for GFAP antibody. Image analysis &morphometry were performed on stained sections for; number of surviving pyramidal neurons and the mean area percentage of GFAP staining. Results On examination of H&E-stained sections of control groups, the PFC was formed of the six- layers, the molecular layer, outer granular layer, outer pyramidal layer, inner granular layer, inner pyramidal layer and the multiform layer. The Hippocampus was formed of two components: the dentate gyrus (DG), the hippocampus proper (Cornu Ammonis, with its four regions: CA1, CA2, CA3 and CA4). The cells forming the hippocampus proper were arranged in three layers; the molecular, the pyramidal and the polymorphic layers. In immunohistochemically stained section, the present work showed minimal apparent immune-reaction in the cytoplasm and processes of astrocytes in the PFC and CA1 region of the hippocampus in the control group. On examination of H&E-stained sections of the AASs treated groups, signs of neuronal damage were observed. The inner pyramidal layer of the PFC and the pyramidal layer of CA1 region of the hippocampus revealed abnormal shrunken pyramidal cells having deeply stained nuclei, peri-cellular spaces and flame like appearance with pointed end. Moreover, these layers demonstrated also normal pyramidal cells showing also large rounded open vesicular nuclei and prominent nucleoli. Immunohistochemically stained sections of the treated group in both regions showed apparent extensive brownish immunostaining of the astrocytes with ramifying processes along the layer. Examination of H&E-stained sections of the withdrawal group revealed the inner pyramidal layer of the PFC and the pyramidal layer of CA1 region of the hippocampus showing normal pyramidal cells having large rounded open vesicular nuclei, prominent nucleoli and apical dendrites. It also showed dark irregular cells having deeply stained nuclei, peri- cellular spaces and flame like appearance with pointed end. Immunohistochemically stained section of the treated group in both regions showed apparent minimal brownish immunostaining of the astrocytes. Conclusion Results of the present experimental study demonstrated that AASs (in the form of ND) administration affecting the PFC and the hippocampus resulted in several histopathological and morphometric harmful effects which regressed to a lesser extent through its withdrawal after 2 weeks.

Thermoacoustic parametric instability of premixed ammonia flames propagating downwards in an open-closed tube

This work investigates the self-excited thermoacoustic parametric instability of downward propagating laminar premixed ammonia flames in an open-closed combustion tube. NH3/O2/N2 flames were propagated at different laminar burning velocities at equivalence ratios of ɸ = 0.8 and ɸ = 1.2. Different unstable flame regimes were observed through pressure fluctuation measurements and synchronized high-speed imaging capturing the lateral and longitudinal perspectives of flame propagation. The observed propagation speed of quasi-planar flames showed a strong correlation with the calculated laminar burning velocities of NH3/O2/N2 mixtures. For the first time, the cellular structures of laminar premixed ammonia flames at the onset of parametric instability were clearly observed from the present experimental approach. The experimental cellular flame wavenumbers and acoustic velocity fluctuation amplitudes at the onset of parametric instability are in qualitative agreement with the theory of stability of laminar flames under acoustic excitation. Experiments have shown that NH3/O2/N2 flames are more unstable than CH4/O2/N2 flames at the same laminar burning velocity independent of Lewis number within the range of tested conditions. The influence of activation energy on parametric instability is investigated through a comparative thermoacoustic analysis between two different fuels at varied equivalence ratios. Ammonia flames are characterized by large overall activation energy and Zeldovich numbers compared to hydrocarbon flames. This work elucidates how this unique property characterizes the acoustic instability of downward propagating premixed ammonia flames in a tube based on classical theories on acoustic instability of laminar premixed flames.

Effects of H2/CO ratio and CO2 dilution on the explosion behavior and flame evolution of syngas/air mixtures

Syngas, which is used as both a fuel and a raw material, presents numerous safety risks during its utilization. In this paper, the effects of H2/CO ratio (R = 0%, 30%, 50%, 70% and 100%) and CO2 dilution ([CO2] = 0%–20%) on the explosion behaviors of syngas/air mixtures with different equivalence ratios (φ = 0.8, 1.0, 1.6, and 2.5) are examined via experiments and numerical calculations. The results show that the peaks of explosion pressure and OH* spectral intensity are significantly influenced by the H2 percentage in the syngas. The introduction of CO2 notably decreases these parameters in oxygen-rich and severe oxygen-poor states. Additionally, the flame propagation speed noticeably increases with increasing H2 proportion in the syngas. The influence of CO2 on the buoyancy instability of the spherical flame of syngas is considered negligible when compared to that of other combustible gases, such as CH4. Nevertheless, the presence of CO2 decreases the density ratio between the unburned gas and burned gas and increases the flame thickness of syngas/air mixtures. This trend reduces the cellular instability of the spherical flame, particularly in severe oxygen-poor states. Notably, there is a definite correlation between the formation of cellular flames and the rise rate of explosion pressure. The cellular structure on the flame surface becomes more pronounced when the rise rate of explosion pressure increases. Conversely, it develops later or is noticeably inhibited. Furthermore, the main reaction pathways are H2 → ∙OH → H2O2 → H2O and H2 → ∙OH → CO → CO2 during the syngas explosion process. ∙OH is critical in the entire chain reaction because it promotes the conversion of H2 to H2O and acts as a vital connection between the reactants H2 and CO in the explosion process.

Premixed flames in narrow heated channels of circular cross-section: Steady-state solutions, their linear stability analysis and the multiplicity of stable dynamic modes

Premixed flames in narrow heated circular channels subjected to a Poiseuille flow are investigated within the constant density model for various Lewis numbers using irreversible one-step Arrhenius kinetics. A global stability analysis of steady-state axisymmetric solutions is carried out, together with time-dependent direct numerical simulations. The analysis reveals the criteria for the appearance of oscillatory and three-dimensional cellular flame structures. The problem is also studied separately within the framework of the narrow-channel approximation.Among the results obtained, the following can be singled out as the main ones. First, the multiplicity of stable dynamic modes, oscillatory and steady-state, taking place for the same set of parameters for flames with Le<1 is demonstrated. The actual occurrence of one mode or another depends on the initial conditions. Second, the appearance of chaotic regimes is shown for flames with Le>1. The chaotic dynamics occurs in a narrow range of values of the flow rate, with Feigenbaum period-doubling cascades taking place both before and after this interval. The results of this study could be useful in the development and use of small-scale combustion devices.Novelty and significance statement: A systematic study of premixed flames in narrow heated circular channels in the presence of Poiseuille flow is carried out for various Lewis numbers using irreversible one-step Arrhenius kinetics. One of the novelties presented in the paper is the linear global stability analysis of steady state axisymmetric solutions of this problem, which has not been reported before. Also, for the first time, the existence of multiple stable dynamic modes, oscillatory and time-independent, which occurs at the same parameter values, is demonstrated for flames with Lewis number smaller than one. For flames with a Lewis number greater than one, cases with chaotic dynamics are found that manifest themselves in a narrow range of the flow rate. Finally, it is demonstrated that the Feigenbaum cycle-doubling cascade can appear before and after this interval of chaotic dynamics. Such analysis has not been reported before for this problem of flames in partially heated channels.

An experimental study on the flame instability of NH3/DME blends with H2 addition

It is of significance to investigate the effect of H2 on the combustion of NH3/DME blends since NH3 can partially decompose to produce hydrogen (H2) in the combustion process. However, few studies were conducted on NH3/DME/H2 mixtures, especially for flame instability. This work presents an experimental study of the flame instability of NH3/DME blends with H2 addition. Experiments were conducted using the spherical constant-volume combustion method at φ = 0.7 – 1.4, Tu = 298 K. The influences of NH3/DME blend ratio (BNH3/DME = 80/60, 60/40, 40/60, and 20/80), initial pressure (Pu = 0.1 – 0.5 MPa), and H2 fraction (XH2 = 0%, 20%, and 40%) on the flame instability were examined in detail. The results show that increasing either DME fraction or initial pressure can promote the hydrodynamic instability of NH3/DME/air flames because of the decreased flame thickness. For both XH2 = 0% and 20%, the cellular flame instability with BNH3/DME = 40/60 and 20/80 increases with increasing equivalence ratio. For XH2 = 40%, the flame instability shows a similar trend with BNH3/DME = 20/80, but the most unstable flame appears around stoichiometric combustion with BNH3/DME = 40/60. This is due to the compound effects of the diffusive-thermal imbalance and hydrodynamic instability for lean or rich fuel, while the diffusive-thermal instability can be neglected for stoichiometric combustion. Therefore, an effective control of H2 or DME addition can help enhance the combustion performance of the fuel while reducing the occurrence of dangerous events caused by potential flame instabilities. In addition, critical parameters, i.e., critical flame radius rcr, critical Peclet number Pecr, and critical Karlovitz stretch factor Kacr were analyzed to quantify the onset of flame instability. This work provides insights into the flame instabilities of NH3/DME with H2 addition.

Determination method of Markstein number based on wavenumber measurement of cellular flames at the onset of parametric instability of downward propagating flames

This work proposes a new method to estimate the Markstein number by investigating the thermoacoustic parametric instability of laminar premixed flames propagating downwards in an open-closed tube. Methane-air flames were propagated in a combustion tube to capture the flame evolution in transition to parametric instability. By synchronized high-speed imaging filming both lateral and longitudinal views of flame propagation, cellular flame wavenumbers are measured from the clear transition of flat flames to cellular flames visualized in the present approach. The Markstein number is indirectly determined from wavenumber measurements at the onset of parametric instability by employing a laminar flame model under acoustic excitation treating the Markstein number as a free parameter. Acoustic velocity fluctuation amplitudes obtained from pressure fluctuation measurements were determined to further evaluate the validity of wavenumber-derived Markstein numbers. Markstein numbers obtained in the present work are compared with Markstein numbers relative to fresh gases in the literature from indirect and computational methods.

Structure and dynamics of hexagonal cells in H2/CO2 flames

Transition from fossil fuels to sustainably produced hydrogen remains an important goal to achieve current climate targets. Hydrogen from gasification and steam reforming is produced alongside CO2. Instead of separating the CO2, it can be kept in the product gas, leading to possible reductions in NOx and improved safety aspects. However, the addition of CO2 makes hydrogen flames more prone to cellular instabilities, which can lead to problems when designing burners for hydrogen-based flames. To understand the formation and dynamics of these instabilities, fully resolved 3D simulations of H2/CO2 cellular flames on a heat flux burner are performed. The numerical configuration follows an experimental setup that observed cellular and band-like structures stabilizing on the burner plate. With the simulations employing finite rate chemistry and detailed diffusion models, hexagonal flame structures are identified and characterized. The simulations show good qualitative agreement of the flame structure with the measurements. The influence of mass flow rate, burner plate temperature and equivalence ratio on the flame structure is investigated. A transition from band-like to hexagonal structures is described in terms of cusp formation, which become the corner points of the hexagonal cells, and the stabilization mechanism is explained via the high local flame stretch above holes in the burner plate. A Markstein number-based correlation for the cell size with respect to the burner plate temperature is proposed and the simulation data is provided as a public database for further analysis.

Experimental and numerical studies on characteristics of LPG-air premixed flames in curved tube

Liquefied Petroleum Gas (LPG) is the widely used fuel worldwide from residential to industrial applications, and thus largely meets the energy demand of various systems. The present study experimentally and numerically investigates the flame propagation behavior of LPG- air mixture in straight and curved tube with inner diameter 30 mm, a length of 500 mm for straight tube and a chord length of 500 mm for the curved tube. Numerical study has been carried out using ANSYS Fluent with FGM approach for modelling of combustion. The premixed reactant in the equivalence range of 0.3 ≤ φ ≤ 2.7 is supplied through the closed end of the tube and the mixture is ignited at the open end exposed to atmosphere. Different flame regimes are observed for 30 mm diameter combustor tube, like the self- repetitive extinction and re-ignition flame, forced repetitive extinction and re-ignition flames, cellular flames, blow out flames, stretched or slanted flames. The curvature of the tube was found to enhance the instabilities occurring in the flame, but rapid changing of flame speed towards the cold inlet is not observed in curved tubes. Cellular flames were formed in the curved combustor as a result of secondary flow and thermal-diffusive instability. Curvature in the combustor tubes induces cross stream vortices in the flow due to the secondary flow, which influence the flame structure in the fuel rich regime of the mixture.

Structure and extinction of methane and propane nonpremixed counterflow flames at extremely low stretch rates in buoyant flowfields

This study experimentally investigated the structures and extinction behaviors of methane-air and propane-air counterflow nonpremixed flames influenced by individually controlled fuel injection velocity uF at extremely low stretch rates, which were induced by natural convection alone. A sooting limit fuel injection velocity (SLFIV) index was introduced, which increased with stretch rate in the low stretch rate region. Methane had higher SLFIV than propane. Quasi-one-dimensional blue flames appeared for both methane and propane within a certain uF range. The range for methane was higher than that for propane. The steady flame standoff distances yf had a relationship with uF and low stretch rate ab of yf∝uFabλ∞1/2, which differed from high-stretch flames. Different flame instabilities occurred for methane and propane by changing uF only without fuel or oxidizer dilutions. At low uF near extinction limit of methane, periodic flame holes appeared, which was due to large heat loss. This instability finally caused flame extinction by keeping retreating edges. At high uF, simultaneous cellular and wave flame instability appeared for propane, which differed from cellular flames occurred alone in highly diluted oxy-fuel flames. By modulating uF alone in this experiment, the cellular/wave flame was due to Rayleigh-Taylor and hydrodynamic instabilities. Flame extinction at the lowest uF for propane was through outer edge shrinking, where the displacement speed increased with stretch rate. Flammability maps of limit uF versus low ab were established for both methane and propane. Both maps were divided into four regions by sooting, instability and extinction limits. The three limits uF all increased with ab. This changing tendency was opposite with high-stretch flames reported previously. For propane, the instability limit uF approached infinity at higher ab. This work provided knowledge of extremely low stretch flame combustion, which can be applied to microgravity flames, because the combustion characteristics of microgravity forced low stretch flames were identical to those of low stretch flame induced by natural convection alone.

Numerical analysis and flamelet modeling of NO[formula omitted] formation in a thermodiffusively unstable hydrogen flame

In a thermodiffusively unstable premixed hydrogen flame, cellular flame structures exist due to the strong differential diffusion of hydrogen featuring wide ranges of curvature and strain rate. NOx formation pathways are directly affected by the complex cellular flame structures as different species have different sensitivities to differential diffusion effects and curvature. In this work, the NOx formation characteristics of a thermodiffusively unstable laminar premixed hydrogen flame are investigated. At first, the NOx formation characteristics in the tangential and normal directions of the flame front are analyzed, focusing on the effects of curvature. Then, reaction path analyses are conducted to identify the relevant pathways and elementary reactions that pre-dominantly contribute to the formation of NOx pollutants. To investigate the effects of curvature on NOx formation, the reaction fluxes and the elementary reaction rates are calculated by conditioning on the curvature value. Finally, the performance of a flamelet model in predicting the NOx species in the thermodiffusively unstable premixed hydrogen flame is evaluated through an a priori analysis. The results show that curvature directly affects NOx formation, particularly the dominant NNH and N2O reaction pathways while the contribution of the thermal-NO pathway is found to be overall negligible. The NOx species are mainly formed in positively-curved flame segments where the flamelet model gives good predictions, while discrepancies with respect to the model exist in the negatively-curved flame regions. The reason for this is clarified through a chemical timescale analysis and a conditional reaction path analysis.

Improvement of Municipal Solid Waste Syngas Premixed Flame with Cellular Structure on a Flat Burner

This research was conducted to study the flame instability of syngas derived from raw municipal solid waste (MSW) and its potential as a natural gas (NG) replacement in power generation. MSW syngas is a mixture of various components such as methane (CH4), nitrogen (N2), oxygen (O2), and hydrogen (H2), whereas NG is mainly composed of CH4 (&gt;70%) and CO2 (&gt;10%). The flame characteristics of these two gases are quite different thus a direct replacement of NG with MSW syngas is impossible. Improvements to MSW syngas combustion are needed through the augmentation of the gas with CH4 and H2 active additives at various ratios so that its flame characteristics are comparable to those of NG. A typical MSW syngas composed of 16.2% methane (CH4), 13.5% hydrogen (H2), 69.1% nitrogen (N2), and 0.6% oxygen (O2) (by vol.) is available in Thailand with great potential for use as an NG replacement. In this study, this gas is used as a representative fuel for improvement and is referred to as simulated Syngas 1. Its premixed flame was studied using a McKenna flat burner to understand its flame instability. Various percentages of CH4 and H2 were added to Syngas 1. Its flame characteristics were measured and compared to those of NG. These characteristics included the cellular flame, cell size, flat flame, flammability limit, and flame temperature. The results showed that the flame instability of Syngas 1 was significantly suppressed by adding minimal amounts of CH4 and H2. The new composition of Syngas 1 consisted of 19.3% methane (CH4), 19.0% hydrogen (H2), 61.2% nitrogen (N2), and 0.5% oxygen (O2) (by vol.). It yielded flame characteristics that were comparable to those of an NG flame. This study shows that MSW syngas can potentially replace NG in power generation.

Dynamics of hydrodynamically unstable premixed flames in a gravitational field – local and global bifurcation structures

The dynamics of hydrodynamically unstable premixed flames are studied using the nonlinear Michelson–Sivashinsky (MS) equation, modified appropriately to incorporate effects due to gravity. The problem depends on two parameters: the Markstein number that characterises the combustible mixture and its diffusion properties, and the gravitational parameter that represents the ratio of buoyancy to inertial forces. A comprehensive portrait of all possible equilibrium solutions are obtained for a wide range of parameters, using a continuation methodology adopted from bifurcation theory. The results heighten the distinction between upward and downward propagation. In the absence of gravity, the nonlinear development always leads to stationary solutions, namely, cellular flames propagating at a constant speed without change in shape. When decreasing the Markstein number, a modest growth in amplitude is observed with the propagation speed reaching an upper bound. For upward propagation, the equilibrium states are also stationary solutions, but their spatial structure depends on the initial conditions leading to their development. The combined Darrieus–Landau and Rayleigh–Taylor instabilities create profiles of invariably larger amplitudes and sharper crests that propagate at an increasingly faster speed when reducing the Markstein number. For downward propagation, the equilibrium states consist in addition to stationary structures time-periodic solutions, namely, pulsating flames propagating at a constant average speed. The stabilising influence of gravity dampens the nonlinear growth and leads to spatiotemporal changes in flame morphology, such as the formation of multi-crest stationary profiles or pulsating cell splitting and merging patterns, and an overall reduction in propagation speed. The transition between these states occurs at bifurcation and exchange of stability points, which becomes more prominent when reducing the Markstein number and/or increasing the influence of gravity. In addition to the local bifurcation characterisation the global bifurcation structure of the equation, obtained by tracing the continuation of the bifurcation points themselves unravels qualitative features such as the manifestation of bi-stability and hysteresis, and/or the onset and sustenance of time-periodic solutions. Overall, the results exhibit the rich and complex dynamics that occur when gravity, however small, becomes physically meaningful.

Experimental study on the instability characteristics of CO2-diluted n-butane/air cellular flames on McKenna burner

Present experimental study reports on the effects of CO2 dilution on intrinsic instabilities of non-adiabatic n-butane/air cellular flames on McKenna burner at initial 300 K temperature and 0.1 MPa pressure. The Planar Laser Induced Fluorescence (OH-PLIF and CH2O-PLIF) optical diagnostic technology was utilized to probe the structure of cellular flames. The results show that the cellular flames can be obtained under the conditions with lean, stoichiometric and rich CO2 diluted n-butane/air mixtures. At lean conditions, hydrodynamic instability was dominant and the cellular flames were formed by connected wrinkles. However, at rich conditions, diffusive-thermal instability was dominant and the cellular flames were formed by separated cells. It confirmed that the CO2 dilution can enhance flame instability significantly, as well as the enhanced effect was positive correlation with CO2 dilution ratio. Furthermore, the enhanced effects of flame instability was stronger by CO2 dilution than by N2 dilution. The chemical and thermal effects by CO2 dilution were attributed to enhance flame instability.

The effects of unburned-gas temperature and pressure on the unstable behavior of cellular-flame fronts generated by intrinsic instability in hydrogen-air lean premixed flames under adiabatic and non-adiabatic conditions: numerical simulation based on the detailed chemical reaction model

ABSTRACT The effects of unburned-gas temperature and pressure on the unstable behavior of cellular-flame fronts in hydrogen-air lean premixed flames are elucidated by numerical simulations. The detailed chemical reaction mechanism with seventeen reversible reactions of eight reactive species and a diluent is adopted for hydrogen-air combustion and heat loss is taken into account. The burning velocity of a planar flame increases as the unburned-gas temperature increases, and it decreases as the unburned-gas pressure and heat loss increase. In the dispersion relation, the growth rate increases and the unstable range widens when the unburned-gas temperature increases. The former decreases and the later narrows when the heat loss becomes larger. The normalized growth rates become large under the non-adiabatic conditions. The normalized burning velocity of a cellular flame is large when the pressure is high and heat loss is large, and it decreases as the temperature increases at high pressure. This shows that the high unburned-gas temperature at high pressure weakens the instability in hydrogen-air lean premixed flames. With increasing the domain size, the burning velocity of a cellular flame increases, and the cellular-flame front becomes more unbalance. This is an aspect of the long-wavelength disturbance to the unstable behavior of cellular-flame front.

Cellular structures of laminar lean premixed H2/CH4/air polyhedral flames

Fundamental studies on the effects of differential diffusion of hydrogen (H2) on flame structure are motivated as the high diffusivity of H2 presents challenges for the modeling and optimization of combustion systems. Polyhedral Bunsen flames are examples of cellular flames mainly induced by the thermal-diffusive and hydrodynamic instabilities, which are characterized by periodic positively curved troughs and negatively curved cusps. Stationary laminar premixed fuel-lean H2/CH4/air polyhedral flames, with 50%, 68% and 79% H2 (by volume) and Lewis number (Le) less than unity, are investigated in this study. The internal scalar structures of cellular troughs and cusps in target flames are measured with a high-spatial-resolution 1D Raman/Rayleigh scattering system, combined with planar laser-induced fluorescence of hydroxyl radicals (OH-PLIF) and chemiluminescence imaging measurements to quantify the cell number and local flame curvature. The performance of the 1D Raman/Rayleigh imaging system is first assessed by comparing measurements of temperature and major species in a laminar premixed counterflow H2/CH4/air twin flame with a corresponding simulation. The results reveal significant combined effects of differential diffusion and curvature on flame structures with differences between trough and cusp regions in the measured mole fractions, equivalence ratio, temperature, and C/H-atom ratio. The positively curved troughs have significantly higher H2 mole fraction compared to the negatively curved cusps, due to the respective focusing/defocusing effect of curvature on highly diffusive H2. Consequently, the local equivalence ratio and temperature in trough regions are higher than those of cusps. With the increase of H2 content in the reactant mixture, the scalar differences between trough and cusp regions are enlarged due to the enhanced effects of curvature and differential diffusion. Near-vertical initial trajectories in H2 mole fraction, equivalence ratio, and C/H-atom ratio plotted against temperature showed that differential diffusion of H2 alters the species mole fractions in the cold reactants (≤ 350 K).

Laminar burning characteristics of bio-aviation fuel candidate derived from lignocellulosic biomass

This study investigated the laminar burning characteristics of bio-aviation fuel candidate produced from corn stover lignin via catalytic hydrodeoxygenation (HDO). The experiments were conducted at equivalence ratios of 0.7–1.4, initial pressures of 1 bar, 2 bar, and 4 bar, and initial temperatures of 443 K and 473 K. The laminar burning velocity (LBV), explosion pressure, maximum pressure-rise rate, and combustion duration were studied. The LBV was studied using the constant volume (CVM) and constant pressure (CPM) methods. The CVM results were significantly higher than that of CPM in the presence of cellular flame. In other cases, the deviations of both were within 10 %. The analysis indicated that the LBV of the bio-aviation fuel candidate at certain equivalence ratios was lower than that of Jet A-1, RP-3, and n-decane. However, the LBV of the bio-aviation fuel candidate was greater than that of n-decane in fuel rich mixtures. A 12-term power-law correlation was determined to quantify the relationship between the LBV of the bio-aviation fuel candidate and the equivalence ratio, initial temperature, and initial pressure. The maximum deviation between the experimental data and the correlation was 11.85 %, and the average deviation was 2.95 %. Finally, the effects of the initial conditions on the explosion characteristics of the bio-aviation fuel candidate were analyzed. The explosion pressure, maximum pressure-rise rate, and deflagration index were linearly and positively correlated with the initial pressure; however, the latter two were insensitive to the initial temperature. Furthermore, a quantitative correlation between the explosion characteristics and the equivalence ratio and initial pressure was determined.

Inward-propagating cylindrical flames under different ignition conditions

Inward-propagating cylindrical flames are studied numerically by high-resolution simulations using a one-step Arrhenius kinetics. Emphasis is placed on the effect of shock waves on the flame propagation by setting initial ignition conditions with and without shock wave. It is found that without initial shock wave, the inward-propagating flame propagates initially at a constant speed, while in the later stage of the propagation, it shows a small-amplitude oscillatory motion. When the shock wave initially introduced is medium, a large-amplitude oscillatory motion is caused by the interaction of shock waves with the inward-propagating flame. Moreover, autoignition occurs at the center and develops outwardly into a cellular flame. However, as the introduced shock wave is strong, autoignition created at the center evolves outwardly a cellular detonation.