Flame Patterns Research Articles

Numerical investigation on the combustion characteristics of aluminum powder fuel in a supersonic cavity-based combustor

In order to explore the propulsion performance and combustion organization rules of powder fueled scramjets, research on powder fuel combustion characteristics in a supersonic cavity-based combustion chamber is conducted. The numerical simulation method based on the Euler-Lagrange framework for supersonic gas–solid two-phase turbulent flow and combustion is established, and the rationality and accuracy of the adopted method are verified by the comparison with experimental data. The structural feature and flame patterns in the combustion chamber under different air–fuel ratio conditions are compared and analyzed. The influence law of air–fuel ratio on the combustion characteristic of powder fuel is obtained. The results indicate that the powder fuel can achieve stable combustion in the supersonic airflow. The interior of the cavity and the shear layer, near the boundary layer of the expansion section are the main flame distribution areas. With the increase of the air–fuel ratio, the total pressure loss along the combustion chamber gradually decreases. While the combustion efficiency of powder particles exhibits an opposite trend. Overall, it is beneficial to have a lower air–fuel ratio in the supersonic cavity-based combustor.

Adaptive fuzzy approach for load frequency control using hybrid moth flame pattern search optimization with real time validation

Adaptive fuzzy approach for load frequency control using hybrid moth flame pattern search optimization with real time validation

Injector-injector interactions on the flow field, spray characteristics, and subsequent flame pattern in an annular combustor

In this paper, the injector-injector flow field, spray and flame patterns are investigated experimentally in an annular combustor under various mair and mfuel. The results show that the adjacent swirling jets have different velocity mangnitude on the central annular plane due to the curvature of the annular combustor. It can lead to a “deflection jet” instability and subquently switch on a periodic “wide-narrow-wide” CTRZ flow that does not evolve in time. Different bulk velocity airflow will not change the periodic CTRZ arrangement but the vortex structure inside the CTRZ. The spray and flame patterns are significantly impacted by the injector-injector flow field. The “rich fuel” region is consistent with the flame morphology. Both of their sizes rapidly decrease as the bulk velocity increases. Periodicity did not appear in spray and flame due to the weak recirculation and dispersed fuel downstream in low bulk velocity conditions. Under high bulk velocity, stronger recirculation air makes the shape of the spray and flame consistent with CTRZ. The effect of the periodic flow field on spray and flame becomes more apparent.

Flame Characteristics of Gasoline and a Water-in-Gasoline Mixture under a Controllable High-Temperature Thermal Atmosphere.

This study explores the flame characteristics of gasoline and a water-in-gasoline mixture and provides some optimization suggestions for the control strategy of water-in-gasoline mixture injection in the internal combustion Rankine cycle engines. The influences of co-flow temperature, injection pressure condition, and water content on the flame characteristics were investigated under a controllable high-temperature thermal atmosphere, including flame pattern, liftoff height, brightness, and local peak temperature distribution. Results obtained show that the water content has the most significant effect on the flame characteristics. The flame pattern gradually changes from slender to lumpy as the water content rises, while the high-temperature zone increases and becomes uniform. There is an appropriate water content range of 10-20% in which the flame characteristics are generally better. Furthermore, as the water content increases from 20 to 30%, the flame characteristics deteriorate, with a sharp decrease in the maximum flame temperature and brightness.

Numerical Study of an Improved Non-Circular MC Fuelled with H/Air for Thermo-Photovoltaic (TPV) Applications

In recent years, researchers have paid special attention to wall uniformity temperature and improvement in a micro-combustor (MC) for applications as thermos-photovoltaic (TPV) since they have a direct influence on feasibility and efficiency of the required conversion of energy. Numerous experimentation and numerical studies in the field of micro-combustion have been carried out and published in this respect. This study focuses on the flame and flow behaviour in an improved MC. Utilizing the famous approach of fluid dynamics being computational (CFD), the impact of geometrical flow parameters behaviour and temperature of wall is explored and measured. The generated model is being used to evaluate the effects of Hydrogen (H) mass flow rate (MFR) and H/air equivalency ratios on the flame pattern and outer temperature of wall of conventional and enhanced MCs. Moreover, the influence of various MC cross sections (CS), such as square, circular, and flat, on temperature of wall is examined. The findings reveal that when the MFR rises, the wall outside temperature rises as well. The MC with flat cross-sections (CSs) is found to be more effective in terms of power of emitter and efficiency of emitter.

Swirl rotation direction effects on lean direct injection pilot mixer performance: Experiments and LES modeling

In order to improve the understanding of the effects of various practical design parameters on the performance of Lean Direct Injection (LDI)-based combustors, the impact of varying the relative rotation direction of the outer air swirler (OAS) and inner air swirler (IAS) on the performance of a methane-fueled LDI-type mixer/injector is investigated under non-reacting and reacting conditions by both experiments and computational fluid dynamics (CFD) simulations. Reynolds-Averaged Navier-Stokes (RANS) and Large Eddy Simulation (LES) models are used and compared in CFD modeling of counter- and co-swirling LDI mixers, while the adoption of a nonpremixed Flamelet Generated Manifold (FGM) method to predict reacting flows is investigated for comparison with experimental measurements of the mean and fluctuating velocity fields and OH* chemiluminescence. In addition, a self-consistent and robust CFD mesh generation strategy is developed to create notionally grid-independent CFD meshes for non-reacting and reacting flow simulations. The experimental non-reacting flow fields and flame lift-off limits demonstrate that while the basic structure of the flow field does not change by altering the relative rotation direction of IAS and OAS, the details of the center recirculation zone including its width and length do, resulting in slightly reduced flame lift-off equivalence ratio for the counter-rotating configuration. Although LES-FGM is able to capture the major flow and flame patterns in both counter- and co-swirling reacting flows, some substantial disparities are apparent, particularly in the region within/near the venturi that adversely impact the predictive abilities of this LES-FGM approach.

Effect of hydrogen addition on the dynamics of premixed C1–C4 alkane-air flames in a microchannel with a wall temperature gradient

The effect of hydrogen (H2) addition on the flame dynamics of premixed C1–C4 alkane/air mixtures in a microchannel is investigated using a detailed-chemistry model through two-dimensional numerical computations. A detailed computational study have been performed in a 2 mm diameter tube with 120 mm length and a wall temperature gradient along the axial direction of the channel. The numerical simulations are carried out for various stoichiometric hydrocarbon (HC)/H2 mixtures at 0.15 m/s mixture inlet velocity. Flame repetitive extinction and ignition (FREI) flame pattern has been identified for all the fuel mixtures at these channel wall and mixture flow conditions. CH4/air mixture shows a higher HRR than C3–C4 alkane/air mixtures. Flame residence time in microchannel increases with increase in hydrogen addition percentage for all the three hydrocarbon/air mixtures considered in the present study. A non-monotonic behavior of FREI frequency is identified for CH4/air mixture, whereas it decreases monotonically for C3H8/air and C4H10/air mixtures with H2 addition. The amount of HRR and flame propagation velocity decreases with increase in H2 addition for lower-alkanes/air mixtures. The flame bifurcation effect is observed for CH4/air mixture, which disappears due to H2 addition in the mixture. The bifurcation effect is not present for other hydrocarbon/air mixtures investigated in the present study. The addition of H2 in the mixture enhances the flame stability of hydrocarbon/air mixtures in the microchannel.

Effect of small-size space porous flexible PVC material on propane deflagration

The effect of porous flexible polyvinyl chloride (PVC) barriers on the kinetic properties of propane explosions is investigated based on fluid dynamics and combustion-related theory. Furthermore, using small-sized cylindrical pipes, separate experimental studies on propane deflagration are conducted with different types of flexible PVC barriers placed in the pipeline. The results show that when the conditions of the same nature as the obstacles appear at the outlet end of the pipeline and the blocking rate is low, the presence of porous flexible PVC barriers counteracts the effect of tail-end barriers on propane deflagration. In the tail-end opening experiment, the porous flexible PVC obstacle has a significant effect on the flame pattern passing through it but has a weaker effect on the flames before they pass through it. Furthermore, it only affects the degree of “fingertip” sharpening of finger-shaped flames; the fewer the number of holes in the obstacle, the higher the sharpness of the fingertip flame. The porous flexible PVC barriers can reduce the rate of flame propagation and creasing of propane deflagration. The porous flexible PVC barriers have a lifting effect on the overpressure of the blast and a delaying effect on the onset of peak overpressure; the larger the number of holes in the barrier, the stronger the delaying effect.

The Effect of Inert Fuel Compounds on Flame Characteristics

To describe the effects of inert compounds in gaseous fuel, experiments on three different process burners (staged fuel burner, staged air burner, and low-calorific burner) were carried out. The tested burners are commercially available, but they were specially designed for experimental usage. Tests were carried out in the semi-industrial burner testing facility to investigate the influence of inert gases on the flame characteristics, emissions, and heat flux to the combustion chamber wall. Natural gas was used as a reference fuel, and, during all tests, thermal power of 500 kW was maintained. To simulate the combustion of alternative fuels with lower LHV, N2 and CO2 were used as diluents. The inert gas in the hydrocarbon fuel at certain conditions can lower NOx emissions (up to 80%) and increase heat flux (up to 5%). Once incombustible compounds are present in the fuel, the higher amount of fuel flowing through nozzles affects the flow in the combustion chamber by increasing the Reynolds number. This can change the flame pattern and temperature field, and it can be both positive and negative, depending on actual conditions.

Plasma-assisted flame stabilization by AC dielectric barrier discharge in burst mode

Compared to the continuous mode of a high-frequency alternating current (HFAC) source, the burst mode is believed to be capable of controlling flow and combustion while saving discharge energy. Thus, the effect of burst mode on plasma-assisted flame stabilization, which is driven by an HFAC source, is investigated based on a plasma manipulation diffusion flame experiment system. A specially designed injector with a coaxial dielectric barrier discharge configuration is used to generate a methane-air diffusion flame and air plasma. The influence of the actuating parameters of the burst mode on plasma flame stabilization and the discharge power consumption are studied. The results show that the plasma injector has the typical volt–ampere characteristics of dielectric barrier discharge. The discharge clearly exhibits regular unsteady operation characteristics when the source is in burst mode. Two flame stability thresholds are found with increasing discharge voltage. According to the first and second flame stability threshold values and the flame pattern, raising the control frequency and the duty cycle is favorable for flame stabilization. However, for a fixed duty cycle of 50%, because the time scale of the actuator-off duration is longer than that of combustion, when the control frequency is too low, the flame cannot be stabilized even though the applied voltage is very high. For a fixed control frequency of 100 Hz, the flame is also difficult to stabilize when the duty cycle is too small. Mainly due to the unsteady gasdynamic effects of plasma in the burst mode, this approach can achieve the same flame stabilization effect as that of the continuous mode while reducing the energy consumption. The cost-effectiveness ratios of the two optimum actuation schemes are only 6.4% and 10.1%. In addition, the burst mode is better than the continuous mode in avoiding undesirable arc discharge. To better stabilize the flame while maintaining a relatively low cost-effectiveness ratio, actuator setting schemes with high frequency and a small duty cycle should be selected.

Experimental and numerical investigation on flame propagation and transition to detonation in curved channel

The Deflagration to Detonation Transition (DDT) process in a curved channel was numerically and experimentally investigated for stoichiometric ethylene/oxygen and acetylene/oxygen/argon mixtures. The numerical method was based on the two-dimensional Navier–Stokes equations with detailed chemical mechanisms. Flame propagation through the curved channel was observed using a high-speed camera. Four distinctive flame patterns were identified, appearing in the following order: spherical flame, finger flame, tongue-like flame, and detonation. The development of the tongue-like flame occupied most of time of the DDT process and played an important role in the onset of detonation. The formation of the tongue-like flame was analyzed in detail and its interactions with boundary layers enhanced the flame surface as well as the flame velocity. The modes of detonation initiation in irregular and regular cellular detonation systems were compared. Although the local explosion always occurred at the outer wall regardless of the mixture type, the detonation was caused by the amplification of instabilities for irregular system at the outer region of the tongue-like flame, whereas shock focusing-ignition occurred at the outer wall for regular system. Furthermore, the DDT run-up distance LDDT was shorter for the irregular system than for the regular system with the same detonation sensitivity.

Numerical study on the influence of gasoline properties and thermodynamic conditions on premixed laminar flame velocity at multiple conditions

In this paper, the effects of two gasoline key properties, research octane number (RON) and fuel sensitivity S, on the laminar flame velocity (LFV) were studied. Three environmental parameters, temperature, pressure and equivalent ratio ϕ, were also taken into consideration at multiple conditions. An effective hybrid computation method, combining the flame kinetics model and the machine learning (ML) algorithm, was proposed for the parameter study. The flame kinetics model coupled with a well-validation TRF reaction mechanism to model the flame propagation characteristics; four ML algorithms, Random forest (RF), Gradient boosting decision tree (GBDT), Support vector regression (SVR), and Adaboost were adopted to establish the data mapping between investigated parameters with fuel's LFV. The result shows all ML models perform well on the test set, especially for the GBDT algorithm, which with R2 of 0.9984 and RMSE of 0.0084. Based on the power exponential formulas, the LFV correlations concerning temperature, pressure and equivalent ratio were concluded. It also found the influence mechanism of RON and S on LFV was controlled by the equivalence ratio. Besides, the tri-component diagrams of the LFV with the RON-S-ϕ under typical environments were analyzed, and the statistical information in different flame patterns was discussed in detail.

The characteristics of methane/oxygen premixed flame dynamics in a meso-scale reactor under fuel rich condition

The high surface-to-volume ratio of a micro combustor causes the flame motion to vary from that of a large-scale combustor. The effects of the length and height of the combustor on the flame behaviors are studied to revel the flame pattern. Five flame regimes are observed, namely diffusion flame, steady flame, partly RERI flame, fully RERI flame and flame flashback and extinction. The equivalence ratio range of each flame regime in several combustors is measured. The flame position of the steady flame and the corresponding wall temperature as well as the frequency and propagating distance of the RERI flame are investigated. The results show that the change of combustor length has a significant effect on the extinction area and the increase and decrease of the height of the combustor channel lead to a shift towards fuel rich side and fuel lean side respectively. Longer combustor broadens the low equivalence ratio limit of the steady flame. Higher combustor channel increases the effect of inlet Reynolds Number on the high equivalence limit of steady flame. Combustor length has little effects on the RERI flame, whereas higher channel height of combustor increases the propagating distance of the RERI flame but decreases the frequency.

Experimental Investigations on Non-premixed Methane-air Flames in Radial Microchannels with a Controlled Temperature Profile

ABSTRACT Experimental investigations were made on the stabilization and dynamics of non-premixed methane-air flame in a series of radial microchannels with a diameter of 60 mm and widths of 2.0, 2.5 and 3.0 mm with a controlled wall temperature profile. Various flame modes, termed periodically rotating flame, stable flame, broken and cellular flames, single broken flame, and periodically splitting flame were observed with high-speed imaging. Regime diagrams of all these stable and unstable flame modes were then drawn for the three channel gap widths. In addition, some critical flame characteristics, such as the mean radii of the stable flames, and the rotating frequencies of the periodically rotating flames were quantitatively assessed. Effects of the fuel/oxidizer inflow rates and the nominal equivalence ratios on these parameters were studied. Furthermore, numerical studies on the flow field were performed, revealing the inner mechanism of various flame modes to a certain extent. Disturbance of the fuel/oxidizer mixing layer for various flow conditions was considered to be greatly responsible for the formation of some flame patterns.

Transition to turbulence via flame patterns in viscoelastic Taylor-Couette flow.

Transition to inertio-elastic turbulence in Taylor-Couette flow with shear-thinning and viscoelastic polymer solutions is investigated when the rotation rate of the inner cylinder is increased and the outer cylinder is fixed. In two polymer solutions of PEO with elastic number [Formula: see text], the first instability of the circular Couette flow appears as spirals propagating in opposite directions along the axis of cylinders. Just above the onset of the spirals pattern, the localized solitons of the strong radial inflow called flame-like flow appear abruptly inside waves. The abrupt apparition of the flame-like flow is the signature of the subcritical transition to turbulence. The number of the flame-like flows follows a Gaussian distribution at given Ta number. The averaged number of the flame-like flow increases as the rotation rate is increased and it saturates in the inertio-elastic turbulence. The soliton of the strong radial inflow (flame-pattern) is created when it amplitude exceeds a critical value. The distribution of the critical amplitudes of the flame patterns follows a Gaussian law at given Ta number. The transition to turbulence is described by a mathematical model based on an error function of the probability to observe a strong inflow (flame-pattern). The statistical data of the critical amplitude and the probability to observe the flame patterns are used with the mathematical model in order to determine the stability curve of the transition to turbulence. The analysis of the transition to turbulence is completed by the characterization of the spatiotemporal properties.

Effects of starlike control discs on flow structures and combustion capability

The mixing efficiency between the air and fuel play an important role in the combustion capability. The flow structures behind a conventional nozzle were modulated using circular, three-pointed-starlike, and six-pointed-starlike control discs to investigate the characteristic flow behaviors, mixing ability, and combustion capability. The starlike tips generated flow eddies that strengthened the recirculation effect and lowered the stagnation saddle point. The smoke-streak flow structures behind the six-pointed-star control disc were classified as five characteristic flow modes: jet flow, vortex shedding, transition flow, gap flow, and turbulence flow. In the turbulence-flow mode, the turbulence intensity (T.I.) exhibited a considerable increase, and the six-pointed-starlike control disc exhibited the highest T.I. The concentration of CO2 behind the control disc was lower than that behind the conventional nozzles. In the combustion and flame investigation, the direct photograph and Schlieren flame structures show four flame patterns: jet flame, flickering flame, swirling flame and lifted flame. For six-pointed-starlike disc, the flame length is shortened about 44.5%, and the integral combustion capability is doubled with comparing that of pure-jet nozzle due to high turbulence intensity causing the high mixing rate between the air and propane; and then the combustion capability improves.

Laminar non-premixed flame patterns in compact micro disc-combustor with annular step and radial preheated channel

The study presents the laminar methane-air non-premixed flame patterns under the synergistic effect of heat and flow recirculation in a radial microchannel, as obtained through a numerical simulation. A regime diagram of the flame behaviors for various operating conditions was obtained. It was observed that the flame always remained stable until it was extinguished, which implies that the flame stability was strong. In addition, the flame patterns at small, moderate, and large Reynolds numbers (Re) were revealed. For a small Re, the flame could be well-anchored in a very upstream channel at a large global equivalence ratio (ϕ). At a moderate ϕ, the upstream part of the flame was anchored by a minor recirculation zone (RZ) near the inlet of the combustion chamber. At a small ϕ (i.e., 0.6), the flame became completely flat, and its upstream part was anchored by the major RZ behind the annular step. For a moderate Re, the flame roots were remarkably anchored by the minor and/or major RZs. For a large Re, for the first time. It was observed that the whole stable flame split into two parts in the radial channel at ϕ = 0.8 and 0.7, namely the upstream minor flame engulfed in the minor RZ and the downstream major flame anchored by the major RZ. However, when ϕ was further decreased to 0.6, the minor flame was extinguished and only the large major flame existed. The transformation mechanism of the aforementioned flame behaviors was also revealed in detail. Furthermore, this study has yielded abundant information on flame patterns in the current radial microchannel with the annular step and the radial preheated channel, which could help us gain further insights into the flame propagation characteristics in a radial channel.

Genetic Diversity of Potyviruses Associated with Tulip Breaking Syndrome.

Tulip breaking is economically the most important viral disease of modern-day tulip growing. It is characterized by irregular flame and feather-like patterns in the flowers and mosaic on the foliage. Thirty-two leaf samples were collected from cultivated tulip plants showing tulip breaking syndrome from Hungary in 2017 and 2018. Virus identification was performed by serological (ELISA) and molecular (RT-PCR) methods. All samples proved to be infected with a potyvirus and evidence was provided that three potyvirus species could be identified in the samples: Lily mottle virus (LMoV), Tulip breaking virus (TBV) and Rembrandt tulip-breaking virus (ReTBV). Recombination prediction accomplished with Recombination Detection Program (RDP) v4.98 revealed potential intraspecies recombination in the case of TBV and LMoV. Phylogenetic analyses of the coat protein (CP) regions proved the monophyletic origin of these viruses and verified them as three different species according to current International Committee on Taxonomy of Viruses (ICTV) species demarcation criteria. Based on these results, we analyzed taxonomic relations concerning potyviruses associated with tulip breaking syndrome. We propose the elevation of ReTBV to species level, and emergence of two new subgroups in ReTBV.

Flame extension area and temperature profile of horizontal jet fire impinging on a vertical plate

Impinging jet fire of high exit momentum is frequently reported to induce the domino effect in industry fires. Less literature is available for the flame extension area and temperature distribution over a vertical plate impinged by the horizontal jet flame. Thus, this paper conducts a systematic experiment to reveal the effect of nozzle exit velocity, exit diameter and exit-plate spacing on the horizontally impinging jet fire. Experimental observation shows the evolution of flame pattern and color with an increase of exit velocity and a decrease of exit-plate spacing. The flame extension area induced by the horizontal jet flame impingement was measured by a novel method that combines the picture processing technique and the thermal imaging technique. By physical analysis, a new correlation coupling the turbulent Karlovitz stretch factor and the ratio of nozzle exit diameter to exit-plate spacing, was developed for the flame extension area of both horizontally and vertically impinging jet fire. The variation of maximum temperature in the impinging zone with the exit-plate spacing still fits the classic correlation of centerline temperature in the continuous and intermittent flames. However, the horizontally impinging jet fire plume holds a shorter intermittent flame region due to the flame upward bending. It is found that the temperature profile holds a big difference in the upward and downward directions along the vertical plate. A new uniform correlation, with the plume radius as the characteristic length scale, is proposed to well collapse all the temperature data in the impinging zone.

Impact of co- and counter-swirl on flow recirculation and liftoff of non-premixed oxy-flames above coaxial injectors

Controlling the flame shape and its liftoff height is one of the main issues for oxy-flames to limit heat transfer to the solid components of the injector. An extensive experimental study is carried out to analyze the effects of co- and counter-swirl on the flow and flame patterns of non-premixed oxy-flames stabilized above a coaxial injector when both the inner fuel and the annular oxidizer streams are swirled. A swirl level greater than 0.6 in the annular oxidizer stream is shown to yield compact oxy-flames with a strong central recirculation zone that are attached to the rim of central fuel tube in absence of inner swirl. It is shown that counter-swirl in the fuel tube weakens this recirculation zone leading to more elongated flames, while co-swirl enhances it with more compact flames. These results obtained for high annular swirl levels contrast with previous observations made on gas turbine injectors operated at lower annular swirl levels in which central recirculation of the flow is mainly achieved with counter-rotating swirlers. Imparting a high inner swirl to the central fuel stream leads to lifted flames due to the partial blockage of the flow at the injector outlet by the central recirculation zone that causes high strain rates in the wake of the injector rim. This partial flow blockage is more influenced by the level of the inner swirl than its rotation direction. A global swirl number is then introduced to analyze the structure of the flow far from the burner outlet where swirl dissipation takes place when the jets mix. A model is derived for the global swirl number which well reproduces the evolution of the mass flow rate of recirculating gases measured in non-reacting conditions and the flame liftoff height when the inner and outer swirl levels and the momentum flux ratio between the two streams are varied.