To compare structures of turbulent gas and spray flames is helpful for understanding the effect of evaporating droplets on turbulence and turbulent combustion.Presently some investigators did studies on the effect of ...To compare structures of turbulent gas and spray flames is helpful for understanding the effect of evaporating droplets on turbulence and turbulent combustion.Presently some investigators did studies on the effect of turbulence on droplet evaporation and the effect of droplet combustion on turbulence,and most of studies paid attention to the time-averaged results.In this paper,the specific feature is to give a review for comparative studies on instantaneous structures of turbulent methane-air jet gas flame,ethanol jet spray flame,methane-air swirling gas flame and heptane-air swirling spray flame by large-eddy simulation(LES)using a second-order moment(SOM)combustion model.The results show that evaporating droplets enhance turbulence and turbulent combustion.展开更多
Flame structures of a syngas swirl-stabilized diffusion flame in a model combustor were measured using the OH-PLIF method under different fuel and air swirl intensity.The flame operated under atmospheric pressure with...Flame structures of a syngas swirl-stabilized diffusion flame in a model combustor were measured using the OH-PLIF method under different fuel and air swirl intensity.The flame operated under atmospheric pressure with air and a typical low heating-value syngas with a composition of 28.5% CO,22.5% H2 and 49% N2 at a thermal power of 34 kW.Results indicate that increasing the air swirl intensity with the same fuel,swirl intensity flame structures showed little difference except a small reduction of flame length;but also,with the same air swirl intensity,fuel swirl intensity showed great influence on flame shape,length and reaction zone distribution.Therefore,compared with air swirl intensity,fuel swirl intensity appeared a key effect on the flame structure for the model combustor.Instantaneous OH-PLIF images showed that three distinct typical structures with an obvious difference of reaction zone distribution were found at low swirl intensity,while a much compacter flame structure with a single,stable and uniform reaction zone distribution was found at large fuel-air swirl intensity.It means that larger swirl intensity leads to efficient,stable combustion of the syngas diffusion flame.展开更多
Theoretical and practical issues concerning the multi-faceted task of mitigating the latero-torsional seismic response of a prototypal frame structure with asymmetric mass distribution are approached, Chevron braces w...Theoretical and practical issues concerning the multi-faceted task of mitigating the latero-torsional seismic response of a prototypal frame structure with asymmetric mass distribution are approached, Chevron braces with embedded magnetorheological dampers acting on the interstory drift are used to ensure additional energy dissipation. The semi-active control strategy employed to govern the modification of the damper characteristics via feedback is based on the selection of optimal forces according to a H2/LQG criterion, with respect to which the actual forces are regulated by a clipped-optimal logic. A dynamic observer is used to estimate the state through a non-collocated placement of the acceleration sensors. Several aspects to be addressed throughout the complex process including the design, modelization, and implementation phases of semi-active protection systems are discussed. Finally, experimental results obtained to mitigate the motion induced by ground excitation in a large-scale laboratory prototype, simulating the seismic response of a two-story building, are summarized.展开更多
Powder-Fueled Water Ramjet Engine(PFWRE)is of great attraction for high-speed and long-voyage underwater propulsion,as well as air–water trans-media navigation applications due to its high energy density and thrust a...Powder-Fueled Water Ramjet Engine(PFWRE)is of great attraction for high-speed and long-voyage underwater propulsion,as well as air–water trans-media navigation applications due to its high energy density and thrust adjustability.However,the complex multiphase combustion process in the combustor significantly affects engine performance.In this study,a detailed model for aluminum particle combustion in water vapor is developed and validated via literature data as well as the ground direct-connected test we conducted.Thereafter,the numerical study on the multiphase combustion process inside the aluminum-based PFWRE combustor is carried out within the Euler–Lagrange framework using the developed model.Results show that a reverse rotating vortex pair before the primary water injection causes particles to flow back towards the combustor head and leads to product deposition.Aluminum particles external to the powder jet have shorter preheating time than internal particles and burn out in advance.The analysis of the particle combustion process indicates that the flame structure inside the combustor consists of the particle preheating zone,the surface combustion heat release zone,the gas-phase combustion heat release zone,and the post-flame zone.In the present configuration,as the particle size increases from 10μm to 20μm,the preheating zone length increases from 35 mm to 85 mm.Meanwhile,heat release from gas-phase combustion decreases,and the average temperature of the combustor head first increases and then decreases.This study not only provides insight into the multiphase combustion characteristics of the aluminum-based PFWRE combustor but also offers guidance for the design of the combustion organization schemes and engine structure optimization.展开更多
To investigate the problem of ethylene jet mixing and combustion in the scramjet at high Mach number(Ma = 8), numerical simulations were carried out for different equivalent ratios at cold and combustion conditions, i...To investigate the problem of ethylene jet mixing and combustion in the scramjet at high Mach number(Ma = 8), numerical simulations were carried out for different equivalent ratios at cold and combustion conditions, in which three-dimensional steady compressible RANS and k-ω SST turbulence model were adopted. It demonstrates that as the equivalence ratio increases from 0.42 to 1.08, the combustion becomes more intensified, and the higher backpressure pushes flame to propagate upstream. The supersonic combustion region in the combustor decreases from 92% to 85% with the increase of equivalence ratio from 0.42 to 1.08, resulting in the transition of the combustor from scram-mode to dual-mode. Both mixing and combustion efficiencies decrease by 35% and 16% respectively when the equivalence ratio increases from 0.42 to 1.08, indicating that the high equivalence ratio is unfavorable to the mixing and combustion processes. Combustion mode analysis reveals that the flame in the cavity under the high Mach number is dominated by non-premixed flames, i.e., more than 95% behaves as non-premixed mode, and the heat release is also mainly contributed by non-premixed flame. Increasing the equivalence ratio is beneficial to the thrust performance. Although the viscous force hardly changes with equivalence ratio, the percentage of pressure force used to balance the viscous force increases gradually,which limits the engine performance.展开更多
In this paper, an innovative jet lifted flame with side micro-jets has been proposed and its effects on the flame structure have also been investigated. Due to the changes of the initial combustion conditions, mixing ...In this paper, an innovative jet lifted flame with side micro-jets has been proposed and its effects on the flame structure have also been investigated. Due to the changes of the initial combustion conditions, mixing and aerodynamics which resulted from the perturbation of the side micro-jets, such a lifted jet flame has different flame structure compared with the common premixed flame. Results demonstrate that use of the micro-jets can control, to a certain extent, the flame structure, including the flame length, lift-off distance and blow-off limit. With the same fuel and air flow rate, the flame length with the side micro-jets will decrease about 5%-40% as the air volume ratio a increases from 58%-76%. Compared with the common diffusion flame, the jet flame with the side micro-jets demonstrates to be easier to be a momentum-dominated flame. The flame length with 2 micro-jets is about 5% less than with 6 micro-jets under the same fuel and air flow rate. With the same a, the fewer number of the controlled jets lead to the flame with relatively shorter length, not easier to be blown off and higher NOx emission. With certain fuel flow rate, the critical air volume ratio is largest for the flame with 3 micro-jets, which is more difficult to be blown off than the cases with 2, 4 or 6 micro-jets.展开更多
A scramjet combustor with double cavitybased flameholders was experimentally studied in a directconnected test bed with the inflow conditions of M = 2.64,Pt = 1.84 MPa,Tt = 1 300 K.Successful ignition and selfsustaine...A scramjet combustor with double cavitybased flameholders was experimentally studied in a directconnected test bed with the inflow conditions of M = 2.64,Pt = 1.84 MPa,Tt = 1 300 K.Successful ignition and selfsustained combustion with room temperature kerosene was achieved using pilot hydrogen,and kerosene was vertically injected into the combustor through 4×φ 0.5 mm holes mounted on the wall.For different equivalence ratios and different injection schemes with both tandem cavities and parallel cavities,flow fields were obtained and compared using a high speed camera and a Schlieren system.Results revealed that the combustor inside the flow field was greatly influenced by the cavity installation scheme,cavities in tandem easily to form a single side flame distribution,and cavities in parallel are more likely to form a joint flame,forming a choked combustion mode.The supersonic combustion flame was a kind of diffusion flame and there were two kinds of combustion modes.In the unchoked combustion mode,both subsonic and supersonic combustion regions existed.While in the choked mode,the combustion region was fully subsonic with strong shock propagating upstream.Results also showed that there was a balance point between the boundary separation and shock enhanced combustion,depending on the intensity of heat release.展开更多
The mechanism of plasma-assisted combustion at increasing discharge voltage is investigated in detail at two distinctive system schemes（pretreatment of reactants and direct in situ discharge）.OH-planar laser-induced...The mechanism of plasma-assisted combustion at increasing discharge voltage is investigated in detail at two distinctive system schemes（pretreatment of reactants and direct in situ discharge）.OH-planar laser-induced fluorescence（PLIF） technique is used to diagnose the turbulent structure methane-air flame,and the experimental apparatus consists of dump burner,plasma-generating system,gas supply system and OH-PLIF system.Results have shown that the effect of pretreatment of reactants on flame can be categorized into three regimes：regime I for voltage lower than 6.6 k V;regime II for voltage between 6.6 and 11.1 k V;and regime III for voltage between11.1 and 12.5 k V.In regime I,aerodynamic effect and slower oxidation of higher hydrocarbons generated around the inner electrode tip plays a dominate role,while in regime III,the temperature rising effect will probably superimpose on the chemical effect and amplify it.For wire-cylinder dielectric barrier discharge reactor with spatially uneven electric field,the amount of radicals and hydrocarbons are decreased monotonically in radial direction which affects the flame shape.With regard to in situ plasma discharge in flames,the discharge pattern changes from streamer type to glow type.Compared with the case of reactants pretreatment,the flame propagates further in the upstream direction.In the discharge region,the OH intensity is highest for in situ plasma assisted combustion,indicating that the plasma energy is coupled into flame reaction zone.展开更多
The buoyancy effect on micro hydrogen jet flames in still air was numerially studied.The results show that when the jet velocity is relatively large(V≥0.2 m/s),the flame height,width and temperature decrease,whereas ...The buoyancy effect on micro hydrogen jet flames in still air was numerially studied.The results show that when the jet velocity is relatively large(V≥0.2 m/s),the flame height,width and temperature decrease,whereas the peak OH mass fraction increases significantly under normal gravity(g=9.8 m/s^2).For a very low jet velocity(e.g.,V=0.1 m/s),both the peak OH mass fraction and flame temperature under g=9.8 m/s^2 are lower than the counterparts under g=0 m/s^2.Analysis reveals that when V≥0.2 m/s,fuel/air mixing will be promoted and combustion will be intensified due to radial flow caused by the buoyancy effect.However,the flame temperature will be slightly decreased owing to the large amount of entrainment of cold air into the reaction zone.For V=0.1 m/s,since the heat release rate is very low,the entrainment of cold air and fuel leakage from the rim of tube exit lead to a significant drop of flame temperature.Meanwhile,the heat loss rate from fuel to inner tube wall is larger under g=9.8 m/s^2 compared to that under g=0 m/s^2.Therefore,the buoyancy effect is overall negative at very low jet velocities.展开更多
The effects of inlet gas parameters and sloping sidewall angle on the flame structure and combustion limit with and without sidewall were experimentally investigated.Flame height and impact angle were obtained by chem...The effects of inlet gas parameters and sloping sidewall angle on the flame structure and combustion limit with and without sidewall were experimentally investigated.Flame height and impact angle were obtained by chemiluminescence intensity analysis of CH*distribution.First,the combustion characteristics of flame with and without sidewall at different equivalence ratios were explored;then,the influence of Reynolds number and inlet gas temperature on flame structure and combustion limit of v-shaped flame with sidewall were analyzed,and the results with sidewall were compared with those without sidewall.Finally,the variation trend of flame parameters with different sloping sidewall angles was analyzed.The experimental results show that the existence of sidewall makes flame shape change from“M-shaped”to“inverted N-shaped”,and conical shape to trapezoidal shape.The inhibition effect of sidewall on flame stretching downstream is strengthened with the increase in Reynolds number;but as the temperature of the inlet gas increases,the inhibitory effect is obviously weakened.When sloping sidewall angle decreases from 90°to 55°at 5°intervals,flame height and impact angle of v-shaped flame reach the extreme value whenβ=80°.Compared with the case without sidewall,the range of v-shaped flame with sidewall has no obvious trend of broadening or shrinking when inlet gas temperature is increased;however,as sloping sidewall angle decreases,the range of the v-shaped flame shrinks obviously and flammability limit increases significantly.展开更多
In order to obtain the combustion characteristics of the CH4/Air premixed flame under the action of the wall interaction,a study on the impact of the jet flame on the wall at different separation distances was carried...In order to obtain the combustion characteristics of the CH4/Air premixed flame under the action of the wall interaction,a study on the impact of the jet flame on the wall at different separation distances was carried out.The separation distance from the burner outlet to the lower surface of the wall is changed and the flame structure is obtained through experiments.The temperature,velocity and reaction rate are obtained through numerical simulation,and the law of flame characteristics change is obtained through analysis.The results show that as the separation distance increases,the premixing cone inside the flame gradually changes from a horn shape to a complete cone shape and the length of the premixing cone profile increases.Also,the peak temperature and velocity of the mixture in the axial direction gradually increase,and the temperature and velocity in the radial direction first increase and then decrease.The temperature gradient and velocity reach the maximum when the separation distance is 11 mm.The peaks of reactants(CH_(4))net reaction rate intermediate products(CO)and products(CO_(2),H_(2)O)on the axis and the axial distance corresponding to the peaks increase accordingly.The chemical reaction rate near the wall also gradually decreases with the increase of the separation distance.展开更多
The resistance characteristics of a corrugated flame arrester in hydrogen explosions were investigated using a visual explosion-retardant system.By varying the structural parameters of the corrugated flame-retardant s...The resistance characteristics of a corrugated flame arrester in hydrogen explosions were investigated using a visual explosion-retardant system.By varying the structural parameters of the corrugated flame-retardant system and the concentration of hydrogen,the flame resistance characteristics and the mechanisms affecting hydrogen explosions were summarized.The results indicate that there was a clear correlation between the flame propagation and pressure rise during explosion resistance.When the flame resistance failed,a significant enhancement effect was observed at the backend of the flame-retardant system.Additionally,a noticeable oscillation occurred in the initial stage of flame propagation due to the obstruction caused by the flame-retardant system.The thickness and porosity of the flame-retardant system,along with the hydrogen concentration,influenced the intensities of shock waves and flames entering the narrow channel,as well as the physical and chemical effects during internal propagation,thereby affecting the overall flame resistance.展开更多
This study quantitatively examines the impact of magnetic fields on methane flame characteristics,specifically analyzing changes in flame height,width,and velocity.Using flame photography and Particle Image Velocimetr...This study quantitatively examines the impact of magnetic fields on methane flame characteristics,specifically analyzing changes in flame height,width,and velocity.Using flame photography and Particle Image Velocimetry(PIV),the effects of varying magnetic field strengths(ranging from 25 to 45 mT)on flame behavior were measured across equivalence ratios(φ)from 0.8 to 2.0.The results reveal that applying a magnetic field increases flame height by up to 6.75%while reducing flame width by approximately 6%under a field strength of 45 mT atφ=2.0.Additionally,PIV data demonstrate a significant increase in upward flame velocity,with an observed enhancement of 20%at higher magnetic field intensities.The gradient magnetic field was found to reduce flame distortion,leading to a smoother flame profile.Compared to the control group(M0,with no magnetic field),these findings confirm that magnetic fields can effectively adjust flame properties.This study underscores the potential of magnetic fields in optimizing combustion processes.展开更多
Soot nanoparticles produced during combustion exhibit diverse nanostructures,which are affected by different combustion parameters such as flame stoichiometry and temperature.This work focuses on characterizing RP-3 j...Soot nanoparticles produced during combustion exhibit diverse nanostructures,which are affected by different combustion parameters such as flame stoichiometry and temperature.This work focuses on characterizing RP-3 jet flame properties and exploring the intricate relationship between the effect of temperature and carbon formation.The observed flame length displayed a notable increase in proportion to the equivalence ratio's growth.The flame color underwent a great transformation,evolving from pale blue in fuel-lean conditions to bright green at stoichiometric levels,and to brilliant yellow under fuel-rich conditions.Through systematic sampling and thorough observation of soot morphology at different flame heights,there is a clear correlation between the height of the flame and the acceleration of carbon agglomerate growth.Furthermore,an insightful observation is presented wherein the rise in flame height leads to a gradual reduction in the contribution of surface growth to the overall soot particle size.These findings contribute significantly to the understanding of the complex interplay between combustion conditions and soot nanostructures.The trends in flame characteristics,coupled with insights into soot morphology,provide a foundation for comprehending the underlying mechanisms governing soot formation in RP-3 flames.These results contribute to the understanding of combustion dynamics,offering valuable perspectives for optimizing combustion processes and elucidating the environmental implications of flame-formed soot.展开更多
Combining swirl and micro-mixing diffusion combustion is a new approach to hydrogen gas turbine combustion.For swirl micro-mixing diffusion combustion,swirl intensity variation impacts the flow field,flame structure a...Combining swirl and micro-mixing diffusion combustion is a new approach to hydrogen gas turbine combustion.For swirl micro-mixing diffusion combustion,swirl intensity variation impacts the flow field,flame structure and NO_(x) emissions.In this study,four micro-mixing diffusion burners with the swirl number(Sn)of 0.62/0.45/0.3/0 are designed for the experiments.The effects of swirl intensity on micro-mixing diffusion combustion are investigated experimentally using OH^(*) chemiluminescence and Particle Image Velocimetry(PIV).In addition,CFD calculations are used to clarify the mechanism of swirl intensity’s effect on NO_(x) emissions.The results indicate that the weakening of swirl intensity leads to the evolution of the swirl recirculation vortex to the dual recirculation vortex and finally to the bluff body recirculation vortex,which causes the radial contraction of the flame and induces combustion oscillation.When Sn decreases from 0.62 to 0.45,the flame spread angle θ decreases by 10.7%;the unit flame rotation angleψdecreases by 9.0%,and the unit flame length LF increases by 8.0%.The increase in LF causes an increase in residence time,ultimately leading to a rise in NO_(x) emissions.Meanwhile,the reduced swirl intensity leads to increased mixing time scale and spatial mixing deficiency,which is another contributor to the deterioration of NO_(x) emission performance.展开更多
Hybrid deflagration/auto-ignition flame structures coexist in the combustion of advanced engines.Decoupling exergy destruction caused by different irreversible processes under varied flame regimes is thus important fo...Hybrid deflagration/auto-ignition flame structures coexist in the combustion of advanced engines.Decoupling exergy destruction caused by different irreversible processes under varied flame regimes is thus important for understanding engine thermodynamics.In this study,the flame propagation modes for the premixed DME/air mixtures are numerically investigated under engine-relevant conditions.Local entropy generation and exergy destruction characteristics are compared under different flame structures.Results reveal that as the typical premixed flame transition towards auto-ignition front,the exergy destruction from heat conduction and species mass diffusion gradually vanish and are dominated by chemical reaction.The distributions of temperature and species mole fraction in the flame domain are analyzed to clarify the exergy destruction behaviors caused by heat conduction and mass diffusion.Furthermore,by dividing the DME oxidation process into four stages,the main reaction channels that contribute to the increase in exergy destruction from chemical reaction have been identified.It is found that the production and consumption of CH_(2)O and HO_(2) radical dominate the exergy destruction behavior during DME oxidation.On this basis,the kinetic mechanism of low-temperature chemistry causing greater exergy destruction is elucidated.Specifically,low-temperature chemistry leads to significant exergy destruction due to(a)the large irreversibility of itself and(b)(mainly)the enhancement of H_(2)O_(2)loop reactions by low-temperature reaction intermediates.Thus the reduction of combustion irreversibility is promising to be achieved by reasonably regulating the fuel oxidation path.展开更多
Compact flame-holders for afterburners are an increasing requirement for modern aero engines.However,flame-holder design is non-trivial since high inlet temperatures,velocities,and elaborate structures induce complex ...Compact flame-holders for afterburners are an increasing requirement for modern aero engines.However,flame-holder design is non-trivial since high inlet temperatures,velocities,and elaborate structures induce complex turbulence,combustion,and spray coupling in modern afterburners.In this work,the LES-pdf and stochastic fields-Lagrangian particle spray methods are used to investigate methane and aviation kerosene combustion structures formed by new-type concave flame-holders.The flow pattern,combustion mode,and flame structure of gaseous and liquid fuel around a concave flame-holder are analyzed,discussed,and compared with experimental results.Results reveal that the flame stability of a concave flame-holder is better than that of the non-concave one.Furthermore,when using liquid fuel,the concave flame-holder forms a stable and compact flame.These results suggest concave flame-holders are a promising design for compact afterburners.展开更多
Metal particles such as aluminum（ Al）,magnesium（ Mg）,boron（ B） and nickel（ Ni）,as well as Mg/Al alloy（ Mg/Al = 3/4） are currently the most widely used ingredients in modified doublebase propellants. In this ...Metal particles such as aluminum（ Al）,magnesium（ Mg）,boron（ B） and nickel（ Ni）,as well as Mg/Al alloy（ Mg/Al = 3/4） are currently the most widely used ingredients in modified doublebase propellants. In this contribution,the combustion properties of the metal species are studied by means of the high-speed photography technique and the non-contact wavelet-based measurement of flame temperature distribution. The combustion process of the Al,Mg and Mg/Al samples shows both gas phase reaction and surface oxidation,which yield volatile and nonvolatile products,corresponding to the oxide and suboxide respectively. However,the combustion of B and Ni shows only gas phase reaction,due to their high melting point as well as high enthalpy of vaporization. In addition to the experiments,a hypothetical combustion model has been proposed to clarify the combustion characteristics of metal species in modified double-base propellants.展开更多
Experimental and numerical investigations have been carried out on the effects of multi-swirl interaction patterns on self-excited unstable combustion characteristics based on a five-nozzle can combustor.The multi-swi...Experimental and numerical investigations have been carried out on the effects of multi-swirl interaction patterns on self-excited unstable combustion characteristics based on a five-nozzle can combustor.The multi-swirl interaction patterns include equal swirl intensity interaction and strong-weak swirl interaction.The thermo-acoustic instability characteristics indicate that increasing the central nozzle swirl intensity transforms the interaction pattern from equal swirl intensity interaction to strong-weak swirl interaction,which can significantly weaken the thermo-acoustic coupling effect under low equivalence ratio conditions,and substantially reduce the dynamic pressure amplitude during unstable combustion.The instantaneous flame structures show that the multi-swirl flames exhibit chaotic oscillations under low equivalence ratio conditions.With equivalence ratios greater than 0.71,a clear flame interaction boundary appears,and the flames can exhibit periodic oscillations in a regular structure.However,different interaction patterns result in the completely different phase oscillations in the central and outer flames.The time-averaged flame structures also indicate that strong-weak swirl interaction leads to an increase in the flame angle and a decrease in the flame length for both the central and outer flames,and the variations in the flame angle and length have great impacts on the thermo-acoustic instability mode.The fuel-staging combustion characteristics demonstrate that the instability combustion conditions with a dominant frequency of 100 Hz are greatly broadened by the strong-weak swirl interaction pattern,and the overlapping operating conditions between this mode and other modes are greatly increased.This implies that it is more flexible to adjust the thermo-acoustic unstable mode,which is conducive to the passive suppression of thermo-acoustic instability.展开更多
The influences of DC and AC electric fields,at frequencies up to 1.48 MHz and the maximum strength of about 6 kV/cm,on soot formation and flame structure were investigated using a counterflow type acetylene diffusion ...The influences of DC and AC electric fields,at frequencies up to 1.48 MHz and the maximum strength of about 6 kV/cm,on soot formation and flame structure were investigated using a counterflow type acetylene diffusion flame.The distributions of flame luminosity,soot volume fraction,flame temperature and OH concentration in flame were measured by non-invasive detection methods. Under the influence of electric fields,the changes in distribution of the soot volume fraction were confirmed. Electric fields of high frequency and high intensity reduced the soot volume fraction,whereas other electric fields increased it.The maximum values of flame temperature and OH concentration decreased. In the relationship between the maximum value of the soot volume fraction and the maximum temperature,the maximum soot volume fraction showed both increase and decrease with maximum temperatures depending on the frequencies and intensities of the electric fields,and both of them occurred at temperatures lower than 1900 K.The production of the incipient particles seemed to be the dominant process controlling the soot volume fraction due to the electric fields.The luminosity of a sooting diffusion flame was found to depend on the volume fraction and temperature of the soot particles in the flame.As for the behavior of the flame in the electric fields,the ionic wind effect was not found to be dominant in the present work,and the result of the previous simulation based on the ionic wind theory was not consistent with the present experimental results.展开更多
基金sponsored by the Project of National Natural Science Foundation of China under the Grant51390493。
文摘To compare structures of turbulent gas and spray flames is helpful for understanding the effect of evaporating droplets on turbulence and turbulent combustion.Presently some investigators did studies on the effect of turbulence on droplet evaporation and the effect of droplet combustion on turbulence,and most of studies paid attention to the time-averaged results.In this paper,the specific feature is to give a review for comparative studies on instantaneous structures of turbulent methane-air jet gas flame,ethanol jet spray flame,methane-air swirling gas flame and heptane-air swirling spray flame by large-eddy simulation(LES)using a second-order moment(SOM)combustion model.The results show that evaporating droplets enhance turbulence and turbulent combustion.
基金support by the National High Technology R&D Project of China (No. 2006AA05A104)National Natural Science Foundation of China (No. 50806076,50876110)to the research work
文摘Flame structures of a syngas swirl-stabilized diffusion flame in a model combustor were measured using the OH-PLIF method under different fuel and air swirl intensity.The flame operated under atmospheric pressure with air and a typical low heating-value syngas with a composition of 28.5% CO,22.5% H2 and 49% N2 at a thermal power of 34 kW.Results indicate that increasing the air swirl intensity with the same fuel,swirl intensity flame structures showed little difference except a small reduction of flame length;but also,with the same air swirl intensity,fuel swirl intensity showed great influence on flame shape,length and reaction zone distribution.Therefore,compared with air swirl intensity,fuel swirl intensity appeared a key effect on the flame structure for the model combustor.Instantaneous OH-PLIF images showed that three distinct typical structures with an obvious difference of reaction zone distribution were found at low swirl intensity,while a much compacter flame structure with a single,stable and uniform reaction zone distribution was found at large fuel-air swirl intensity.It means that larger swirl intensity leads to efficient,stable combustion of the syngas diffusion flame.
基金Project DPC-ReLUIS 2005-2008, RL n.7 "Technologies for the isolation and control of structures and infrastructures"
文摘Theoretical and practical issues concerning the multi-faceted task of mitigating the latero-torsional seismic response of a prototypal frame structure with asymmetric mass distribution are approached, Chevron braces with embedded magnetorheological dampers acting on the interstory drift are used to ensure additional energy dissipation. The semi-active control strategy employed to govern the modification of the damper characteristics via feedback is based on the selection of optimal forces according to a H2/LQG criterion, with respect to which the actual forces are regulated by a clipped-optimal logic. A dynamic observer is used to estimate the state through a non-collocated placement of the acceleration sensors. Several aspects to be addressed throughout the complex process including the design, modelization, and implementation phases of semi-active protection systems are discussed. Finally, experimental results obtained to mitigate the motion induced by ground excitation in a large-scale laboratory prototype, simulating the seismic response of a two-story building, are summarized.
基金supported by the National Natural Science Foundation of China(No.22305053).
文摘Powder-Fueled Water Ramjet Engine(PFWRE)is of great attraction for high-speed and long-voyage underwater propulsion,as well as air–water trans-media navigation applications due to its high energy density and thrust adjustability.However,the complex multiphase combustion process in the combustor significantly affects engine performance.In this study,a detailed model for aluminum particle combustion in water vapor is developed and validated via literature data as well as the ground direct-connected test we conducted.Thereafter,the numerical study on the multiphase combustion process inside the aluminum-based PFWRE combustor is carried out within the Euler–Lagrange framework using the developed model.Results show that a reverse rotating vortex pair before the primary water injection causes particles to flow back towards the combustor head and leads to product deposition.Aluminum particles external to the powder jet have shorter preheating time than internal particles and burn out in advance.The analysis of the particle combustion process indicates that the flame structure inside the combustor consists of the particle preheating zone,the surface combustion heat release zone,the gas-phase combustion heat release zone,and the post-flame zone.In the present configuration,as the particle size increases from 10μm to 20μm,the preheating zone length increases from 35 mm to 85 mm.Meanwhile,heat release from gas-phase combustion decreases,and the average temperature of the combustor head first increases and then decreases.This study not only provides insight into the multiphase combustion characteristics of the aluminum-based PFWRE combustor but also offers guidance for the design of the combustion organization schemes and engine structure optimization.
文摘To investigate the problem of ethylene jet mixing and combustion in the scramjet at high Mach number(Ma = 8), numerical simulations were carried out for different equivalent ratios at cold and combustion conditions, in which three-dimensional steady compressible RANS and k-ω SST turbulence model were adopted. It demonstrates that as the equivalence ratio increases from 0.42 to 1.08, the combustion becomes more intensified, and the higher backpressure pushes flame to propagate upstream. The supersonic combustion region in the combustor decreases from 92% to 85% with the increase of equivalence ratio from 0.42 to 1.08, resulting in the transition of the combustor from scram-mode to dual-mode. Both mixing and combustion efficiencies decrease by 35% and 16% respectively when the equivalence ratio increases from 0.42 to 1.08, indicating that the high equivalence ratio is unfavorable to the mixing and combustion processes. Combustion mode analysis reveals that the flame in the cavity under the high Mach number is dominated by non-premixed flames, i.e., more than 95% behaves as non-premixed mode, and the heat release is also mainly contributed by non-premixed flame. Increasing the equivalence ratio is beneficial to the thrust performance. Although the viscous force hardly changes with equivalence ratio, the percentage of pressure force used to balance the viscous force increases gradually,which limits the engine performance.
基金Supported by the Natural Science Foundation of Henan Province （20074800060）.
文摘In this paper, an innovative jet lifted flame with side micro-jets has been proposed and its effects on the flame structure have also been investigated. Due to the changes of the initial combustion conditions, mixing and aerodynamics which resulted from the perturbation of the side micro-jets, such a lifted jet flame has different flame structure compared with the common premixed flame. Results demonstrate that use of the micro-jets can control, to a certain extent, the flame structure, including the flame length, lift-off distance and blow-off limit. With the same fuel and air flow rate, the flame length with the side micro-jets will decrease about 5%-40% as the air volume ratio a increases from 58%-76%. Compared with the common diffusion flame, the jet flame with the side micro-jets demonstrates to be easier to be a momentum-dominated flame. The flame length with 2 micro-jets is about 5% less than with 6 micro-jets under the same fuel and air flow rate. With the same a, the fewer number of the controlled jets lead to the flame with relatively shorter length, not easier to be blown off and higher NOx emission. With certain fuel flow rate, the critical air volume ratio is largest for the flame with 3 micro-jets, which is more difficult to be blown off than the cases with 2, 4 or 6 micro-jets.
文摘A scramjet combustor with double cavitybased flameholders was experimentally studied in a directconnected test bed with the inflow conditions of M = 2.64,Pt = 1.84 MPa,Tt = 1 300 K.Successful ignition and selfsustained combustion with room temperature kerosene was achieved using pilot hydrogen,and kerosene was vertically injected into the combustor through 4×φ 0.5 mm holes mounted on the wall.For different equivalence ratios and different injection schemes with both tandem cavities and parallel cavities,flow fields were obtained and compared using a high speed camera and a Schlieren system.Results revealed that the combustor inside the flow field was greatly influenced by the cavity installation scheme,cavities in tandem easily to form a single side flame distribution,and cavities in parallel are more likely to form a joint flame,forming a choked combustion mode.The supersonic combustion flame was a kind of diffusion flame and there were two kinds of combustion modes.In the unchoked combustion mode,both subsonic and supersonic combustion regions existed.While in the choked mode,the combustion region was fully subsonic with strong shock propagating upstream.Results also showed that there was a balance point between the boundary separation and shock enhanced combustion,depending on the intensity of heat release.
基金financed by National Natural Science Foundation of China(No.51436008)
文摘The mechanism of plasma-assisted combustion at increasing discharge voltage is investigated in detail at two distinctive system schemes（pretreatment of reactants and direct in situ discharge）.OH-planar laser-induced fluorescence（PLIF） technique is used to diagnose the turbulent structure methane-air flame,and the experimental apparatus consists of dump burner,plasma-generating system,gas supply system and OH-PLIF system.Results have shown that the effect of pretreatment of reactants on flame can be categorized into three regimes：regime I for voltage lower than 6.6 k V;regime II for voltage between 6.6 and 11.1 k V;and regime III for voltage between11.1 and 12.5 k V.In regime I,aerodynamic effect and slower oxidation of higher hydrocarbons generated around the inner electrode tip plays a dominate role,while in regime III,the temperature rising effect will probably superimpose on the chemical effect and amplify it.For wire-cylinder dielectric barrier discharge reactor with spatially uneven electric field,the amount of radicals and hydrocarbons are decreased monotonically in radial direction which affects the flame shape.With regard to in situ plasma discharge in flames,the discharge pattern changes from streamer type to glow type.Compared with the case of reactants pretreatment,the flame propagates further in the upstream direction.In the discharge region,the OH intensity is highest for in situ plasma assisted combustion,indicating that the plasma energy is coupled into flame reaction zone.
基金Project(51576084)supported by the National Natural Science Foundation of China。
文摘The buoyancy effect on micro hydrogen jet flames in still air was numerially studied.The results show that when the jet velocity is relatively large(V≥0.2 m/s),the flame height,width and temperature decrease,whereas the peak OH mass fraction increases significantly under normal gravity(g=9.8 m/s^2).For a very low jet velocity(e.g.,V=0.1 m/s),both the peak OH mass fraction and flame temperature under g=9.8 m/s^2 are lower than the counterparts under g=0 m/s^2.Analysis reveals that when V≥0.2 m/s,fuel/air mixing will be promoted and combustion will be intensified due to radial flow caused by the buoyancy effect.However,the flame temperature will be slightly decreased owing to the large amount of entrainment of cold air into the reaction zone.For V=0.1 m/s,since the heat release rate is very low,the entrainment of cold air and fuel leakage from the rim of tube exit lead to a significant drop of flame temperature.Meanwhile,the heat loss rate from fuel to inner tube wall is larger under g=9.8 m/s^2 compared to that under g=0 m/s^2.Therefore,the buoyancy effect is overall negative at very low jet velocities.
基金This work was supported by the National Natural Science Foundation of China(Grant No.51976082)Qing Lan Project.
文摘The effects of inlet gas parameters and sloping sidewall angle on the flame structure and combustion limit with and without sidewall were experimentally investigated.Flame height and impact angle were obtained by chemiluminescence intensity analysis of CH*distribution.First,the combustion characteristics of flame with and without sidewall at different equivalence ratios were explored;then,the influence of Reynolds number and inlet gas temperature on flame structure and combustion limit of v-shaped flame with sidewall were analyzed,and the results with sidewall were compared with those without sidewall.Finally,the variation trend of flame parameters with different sloping sidewall angles was analyzed.The experimental results show that the existence of sidewall makes flame shape change from“M-shaped”to“inverted N-shaped”,and conical shape to trapezoidal shape.The inhibition effect of sidewall on flame stretching downstream is strengthened with the increase in Reynolds number;but as the temperature of the inlet gas increases,the inhibitory effect is obviously weakened.When sloping sidewall angle decreases from 90°to 55°at 5°intervals,flame height and impact angle of v-shaped flame reach the extreme value whenβ=80°.Compared with the case without sidewall,the range of v-shaped flame with sidewall has no obvious trend of broadening or shrinking when inlet gas temperature is increased;however,as sloping sidewall angle decreases,the range of the v-shaped flame shrinks obviously and flammability limit increases significantly.
基金supported by the National Natural Science Foundation of China(Grant No.51976082)and Qing Lan project.
文摘In order to obtain the combustion characteristics of the CH4/Air premixed flame under the action of the wall interaction,a study on the impact of the jet flame on the wall at different separation distances was carried out.The separation distance from the burner outlet to the lower surface of the wall is changed and the flame structure is obtained through experiments.The temperature,velocity and reaction rate are obtained through numerical simulation,and the law of flame characteristics change is obtained through analysis.The results show that as the separation distance increases,the premixing cone inside the flame gradually changes from a horn shape to a complete cone shape and the length of the premixing cone profile increases.Also,the peak temperature and velocity of the mixture in the axial direction gradually increase,and the temperature and velocity in the radial direction first increase and then decrease.The temperature gradient and velocity reach the maximum when the separation distance is 11 mm.The peaks of reactants(CH_(4))net reaction rate intermediate products(CO)and products(CO_(2),H_(2)O)on the axis and the axial distance corresponding to the peaks increase accordingly.The chemical reaction rate near the wall also gradually decreases with the increase of the separation distance.
文摘The resistance characteristics of a corrugated flame arrester in hydrogen explosions were investigated using a visual explosion-retardant system.By varying the structural parameters of the corrugated flame-retardant system and the concentration of hydrogen,the flame resistance characteristics and the mechanisms affecting hydrogen explosions were summarized.The results indicate that there was a clear correlation between the flame propagation and pressure rise during explosion resistance.When the flame resistance failed,a significant enhancement effect was observed at the backend of the flame-retardant system.Additionally,a noticeable oscillation occurred in the initial stage of flame propagation due to the obstruction caused by the flame-retardant system.The thickness and porosity of the flame-retardant system,along with the hydrogen concentration,influenced the intensities of shock waves and flames entering the narrow channel,as well as the physical and chemical effects during internal propagation,thereby affecting the overall flame resistance.
基金financial support from the National Natural Science Foundation of China(No.52325604)National NSFC Ordered Synthesis and Low-Carbon Clean Combustion of Sustainable Aviation Fuels(SAF)(No.W2412101)+2 种基金MOST(2022YFB4003900/2021YFA0716200)National Science and Technology Major Project(J2019-Ⅲ-0005-0048)the Space Application System of China Manned Space Program。
文摘This study quantitatively examines the impact of magnetic fields on methane flame characteristics,specifically analyzing changes in flame height,width,and velocity.Using flame photography and Particle Image Velocimetry(PIV),the effects of varying magnetic field strengths(ranging from 25 to 45 mT)on flame behavior were measured across equivalence ratios(φ)from 0.8 to 2.0.The results reveal that applying a magnetic field increases flame height by up to 6.75%while reducing flame width by approximately 6%under a field strength of 45 mT atφ=2.0.Additionally,PIV data demonstrate a significant increase in upward flame velocity,with an observed enhancement of 20%at higher magnetic field intensities.The gradient magnetic field was found to reduce flame distortion,leading to a smoother flame profile.Compared to the control group(M0,with no magnetic field),these findings confirm that magnetic fields can effectively adjust flame properties.This study underscores the potential of magnetic fields in optimizing combustion processes.
基金financially supported by the National Key R&D Program (2021YFA0716200/ 2022YFB4003900)National Natural Science Foundation of China (No.52325604)the financial support from the ANSO scholarship。
文摘Soot nanoparticles produced during combustion exhibit diverse nanostructures,which are affected by different combustion parameters such as flame stoichiometry and temperature.This work focuses on characterizing RP-3 jet flame properties and exploring the intricate relationship between the effect of temperature and carbon formation.The observed flame length displayed a notable increase in proportion to the equivalence ratio's growth.The flame color underwent a great transformation,evolving from pale blue in fuel-lean conditions to bright green at stoichiometric levels,and to brilliant yellow under fuel-rich conditions.Through systematic sampling and thorough observation of soot morphology at different flame heights,there is a clear correlation between the height of the flame and the acceleration of carbon agglomerate growth.Furthermore,an insightful observation is presented wherein the rise in flame height leads to a gradual reduction in the contribution of surface growth to the overall soot particle size.These findings contribute significantly to the understanding of the complex interplay between combustion conditions and soot nanostructures.The trends in flame characteristics,coupled with insights into soot morphology,provide a foundation for comprehending the underlying mechanisms governing soot formation in RP-3 flames.These results contribute to the understanding of combustion dynamics,offering valuable perspectives for optimizing combustion processes and elucidating the environmental implications of flame-formed soot.
文摘Combining swirl and micro-mixing diffusion combustion is a new approach to hydrogen gas turbine combustion.For swirl micro-mixing diffusion combustion,swirl intensity variation impacts the flow field,flame structure and NO_(x) emissions.In this study,four micro-mixing diffusion burners with the swirl number(Sn)of 0.62/0.45/0.3/0 are designed for the experiments.The effects of swirl intensity on micro-mixing diffusion combustion are investigated experimentally using OH^(*) chemiluminescence and Particle Image Velocimetry(PIV).In addition,CFD calculations are used to clarify the mechanism of swirl intensity’s effect on NO_(x) emissions.The results indicate that the weakening of swirl intensity leads to the evolution of the swirl recirculation vortex to the dual recirculation vortex and finally to the bluff body recirculation vortex,which causes the radial contraction of the flame and induces combustion oscillation.When Sn decreases from 0.62 to 0.45,the flame spread angle θ decreases by 10.7%;the unit flame rotation angleψdecreases by 9.0%,and the unit flame length LF increases by 8.0%.The increase in LF causes an increase in residence time,ultimately leading to a rise in NO_(x) emissions.Meanwhile,the reduced swirl intensity leads to increased mixing time scale and spatial mixing deficiency,which is another contributor to the deterioration of NO_(x) emission performance.
基金supported by the National Natural Science Foundation of China(51888103 and U2141203)the National Science and Technology Major Project(Grants No.J2019-Ⅲ-0004-0047 and 2021-JCJQ-ZD-062-12)。
文摘Hybrid deflagration/auto-ignition flame structures coexist in the combustion of advanced engines.Decoupling exergy destruction caused by different irreversible processes under varied flame regimes is thus important for understanding engine thermodynamics.In this study,the flame propagation modes for the premixed DME/air mixtures are numerically investigated under engine-relevant conditions.Local entropy generation and exergy destruction characteristics are compared under different flame structures.Results reveal that as the typical premixed flame transition towards auto-ignition front,the exergy destruction from heat conduction and species mass diffusion gradually vanish and are dominated by chemical reaction.The distributions of temperature and species mole fraction in the flame domain are analyzed to clarify the exergy destruction behaviors caused by heat conduction and mass diffusion.Furthermore,by dividing the DME oxidation process into four stages,the main reaction channels that contribute to the increase in exergy destruction from chemical reaction have been identified.It is found that the production and consumption of CH_(2)O and HO_(2) radical dominate the exergy destruction behavior during DME oxidation.On this basis,the kinetic mechanism of low-temperature chemistry causing greater exergy destruction is elucidated.Specifically,low-temperature chemistry leads to significant exergy destruction due to(a)the large irreversibility of itself and(b)(mainly)the enhancement of H_(2)O_(2)loop reactions by low-temperature reaction intermediates.Thus the reduction of combustion irreversibility is promising to be achieved by reasonably regulating the fuel oxidation path.
基金National Science and Technology Major Project (2017-Ⅰ-0004-0005)National Natural Science Foundation of China (91741125)。
文摘Compact flame-holders for afterburners are an increasing requirement for modern aero engines.However,flame-holder design is non-trivial since high inlet temperatures,velocities,and elaborate structures induce complex turbulence,combustion,and spray coupling in modern afterburners.In this work,the LES-pdf and stochastic fields-Lagrangian particle spray methods are used to investigate methane and aviation kerosene combustion structures formed by new-type concave flame-holders.The flow pattern,combustion mode,and flame structure of gaseous and liquid fuel around a concave flame-holder are analyzed,discussed,and compared with experimental results.Results reveal that the flame stability of a concave flame-holder is better than that of the non-concave one.Furthermore,when using liquid fuel,the concave flame-holder forms a stable and compact flame.These results suggest concave flame-holders are a promising design for compact afterburners.
基金Supported by the Science and Technology on Combustion and Explosion Laboratory Foundation(9140C350319140C35161)
文摘Metal particles such as aluminum（ Al）,magnesium（ Mg）,boron（ B） and nickel（ Ni）,as well as Mg/Al alloy（ Mg/Al = 3/4） are currently the most widely used ingredients in modified doublebase propellants. In this contribution,the combustion properties of the metal species are studied by means of the high-speed photography technique and the non-contact wavelet-based measurement of flame temperature distribution. The combustion process of the Al,Mg and Mg/Al samples shows both gas phase reaction and surface oxidation,which yield volatile and nonvolatile products,corresponding to the oxide and suboxide respectively. However,the combustion of B and Ni shows only gas phase reaction,due to their high melting point as well as high enthalpy of vaporization. In addition to the experiments,a hypothetical combustion model has been proposed to clarify the combustion characteristics of metal species in modified double-base propellants.
基金the National Science and Technology Major Project(HT-J2019-II-0018-0039)of China for financial support。
文摘Experimental and numerical investigations have been carried out on the effects of multi-swirl interaction patterns on self-excited unstable combustion characteristics based on a five-nozzle can combustor.The multi-swirl interaction patterns include equal swirl intensity interaction and strong-weak swirl interaction.The thermo-acoustic instability characteristics indicate that increasing the central nozzle swirl intensity transforms the interaction pattern from equal swirl intensity interaction to strong-weak swirl interaction,which can significantly weaken the thermo-acoustic coupling effect under low equivalence ratio conditions,and substantially reduce the dynamic pressure amplitude during unstable combustion.The instantaneous flame structures show that the multi-swirl flames exhibit chaotic oscillations under low equivalence ratio conditions.With equivalence ratios greater than 0.71,a clear flame interaction boundary appears,and the flames can exhibit periodic oscillations in a regular structure.However,different interaction patterns result in the completely different phase oscillations in the central and outer flames.The time-averaged flame structures also indicate that strong-weak swirl interaction leads to an increase in the flame angle and a decrease in the flame length for both the central and outer flames,and the variations in the flame angle and length have great impacts on the thermo-acoustic instability mode.The fuel-staging combustion characteristics demonstrate that the instability combustion conditions with a dominant frequency of 100 Hz are greatly broadened by the strong-weak swirl interaction pattern,and the overlapping operating conditions between this mode and other modes are greatly increased.This implies that it is more flexible to adjust the thermo-acoustic unstable mode,which is conducive to the passive suppression of thermo-acoustic instability.
基金supported by Grant-in-Aid for Scientific Research on Priority Area“Exploration of Combustion Mechanism”
文摘The influences of DC and AC electric fields,at frequencies up to 1.48 MHz and the maximum strength of about 6 kV/cm,on soot formation and flame structure were investigated using a counterflow type acetylene diffusion flame.The distributions of flame luminosity,soot volume fraction,flame temperature and OH concentration in flame were measured by non-invasive detection methods. Under the influence of electric fields,the changes in distribution of the soot volume fraction were confirmed. Electric fields of high frequency and high intensity reduced the soot volume fraction,whereas other electric fields increased it.The maximum values of flame temperature and OH concentration decreased. In the relationship between the maximum value of the soot volume fraction and the maximum temperature,the maximum soot volume fraction showed both increase and decrease with maximum temperatures depending on the frequencies and intensities of the electric fields,and both of them occurred at temperatures lower than 1900 K.The production of the incipient particles seemed to be the dominant process controlling the soot volume fraction due to the electric fields.The luminosity of a sooting diffusion flame was found to depend on the volume fraction and temperature of the soot particles in the flame.As for the behavior of the flame in the electric fields,the ionic wind effect was not found to be dominant in the present work,and the result of the previous simulation based on the ionic wind theory was not consistent with the present experimental results.
