Soot,a product of insufficient combustion,is usually in the form of aggregate. The multi-scattering of soot fractal aggregates has been proved to play an important role in studying the soot radiative properties,which ...Soot,a product of insufficient combustion,is usually in the form of aggregate. The multi-scattering of soot fractal aggregates has been proved to play an important role in studying the soot radiative properties,which is rarely considered in predicting the radiative heat transfer in combustion flame. In the present study,based on the weighted sum of gray soot fractal aggregate(WSGSA) model,which is used to predict the temperature field and soot aggregates in turbulent diffusion flame,the flame temperature distribution and soot volume fraction distribution under the conditions of the model without considering radiation,the default radiation model in Fluent software and the WSGSA model are calculated respectively. The results show that the flame temperature will be seriously overestimated without considering radiation and the maximum relative discrepancy of flame centerline temperature is about 64.5%. The accuracy will be improved by the default radiation model in the Fluent software,but the flame temperature is still overestimated and the maximum relative discrepancy of flame centerline temperature is about 42.1%. However,more satisfactory results can be obtained by the WSGSA model,and the maximum relative discrepancy of flame centerline temperature is no more than 15.3%. Similar conclusions can also be obtained in studying the temperature distribution along different flame heights. Moreover,the soot volume fraction can be predicted more accurately with the application of the WSGSA model. Both without considering radiation and using the default radiation model in the Fluent software will result in the underestimating of soot volume fraction. All the results reveal that the WSGSA model can be used to predict the temperature and soot aggregates in the CH/air turbulent diffusion flame.展开更多
This paper investigates the effects of coflow O2 level and temperature on diffusion flame of a CH4/H2 jet in hot coflow （JHC） from a burner system similar to that of Dally et al. The coflow O2 mass fraction （ Yo2 ...This paper investigates the effects of coflow O2 level and temperature on diffusion flame of a CH4/H2 jet in hot coflow （JHC） from a burner system similar to that of Dally et al. The coflow O2 mass fraction （ Yo2 ） is varied from 3% to 80% and the temperature （Tcof） from 1200 K to 1700 K. The Eddy Dissipation Concept （EDC） model with detailed reaction mechanisms GRI-Mech 3.0 is used for all simulations. To validate the modeling, several JHC flames are predicted under the experimental conditions of Dally et al. [Proc. Combust. Inst., 29 （1）, 1147-1154 （2002）] and the results obtained match well with the measurements. Results demonstrate that, when Yo2 decreased, the diffusion combustion is likely to transform from traditional combustion to MILD （Moderate or Intense Low-oxygen Dilution） combustion mode. When Tcof is higher, the temperature distribution over the whole domain trends to be more uniform. Reducing yo2 or Tcof leads to less production of intermediate species OH and CO. It is worth noting that if Yo2 is high enough （ Yo2 〉80%）, increasing Yo2 does not cause obvious temperature increase.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
A combined computational and experimental investigation to examine temperature and soot volume fraction in coflow ethylene-air diffusion flames was presented.A numerical simulation was conducted by using a relatively ...A combined computational and experimental investigation to examine temperature and soot volume fraction in coflow ethylene-air diffusion flames was presented.A numerical simulation was conducted by using a relatively detailed gas-phase chemistry and complex thermal and transport properties coupled with a semi-empirical two-equation soot model.Thermal radiation was calculated using the discrete ordinates method.An image processing technique and a decoupled reconstruction method were used to simultaneously measure the distributions of temperature and soot volume fraction.The results show that the maximum error for temperature does not exceed 10% between the prediction and the measurement.And the maximum error is 6.9% for soot volume fraction between prediction and measurement.Additional simulations were performed to explore the effects of global equivalence ratio on diffusion flames and the soot formation.The results display that the soot formation increases with decreasing the coflow air velocity.And the soot formation in each case appears in the annular region,where the temperature ranges from about 1 000 K to 2 000 K and the profile becomes taller and wider when the coflow air is decreased.展开更多
The stable and crystalline phase of pure nano- structured CeO2 was directly synthesized by flame-assisted spray pyrolysis and solid state diffusion route. Different characterization techniques, including X-ray diffrac...The stable and crystalline phase of pure nano- structured CeO2 was directly synthesized by flame-assisted spray pyrolysis and solid state diffusion route. Different characterization techniques, including X-ray diffraction （XRD）, scanning electron microscopy （SEM）, Fourier trans- form infrared spectroscopy （FTIR）, ultraviolet-visible （UV- Vis）, and thermo gravimetric analysis （TGA） were employed to examine the structural, morphological, optical, and thermal properties of the final product. Similarly, the comparative carbon dioxide （CO2）-sensing response of as-synthesized CeO2 nanoparticles by both routes was also reported. The CeO2 nanoparticles synthesized by solid state diffusion method exhibit good sensitivity （3.38 %） at room temperature, low operating temperature （398 K）, fast response time （32 s）, and recovery time （36 s） along with good stability.展开更多
To study the configuration and conductivity effects on micro-scale methane-air flames by electric field and iron wind,different electric field forces and iron winds are generated by needle,circle and plate electrodes ...To study the configuration and conductivity effects on micro-scale methane-air flames by electric field and iron wind,different electric field forces and iron winds are generated by needle,circle and plate electrodes respectively in different electrodes heights under both AC and DC fields though experiments. Experimental results showed that the flame characteristics are affected by needle electrodes mainly through the action of ion wind,by plate type electrodes mainly through the action of electric field force and by annular electrodes through both the electric field force and ion wind at the same time. Under DC field 's effects of all electrodes types,the flame will consequently go down while the voltage reached to a limit value,and it will breakdown under the strong effect of the ion wind by needle electrodes. The results also showed the influence by different electrodes types to the current characteristics,resistance properties and configuration of themicro-scale flames.展开更多
In our study, we investigate the differences between the combustion of different hydrocarbon fuels CH4, C3H8, C4H10. A numerical simulation of an impinging jet diffusion flames is used. The jet injector has a 10 mm in...In our study, we investigate the differences between the combustion of different hydrocarbon fuels CH4, C3H8, C4H10. A numerical simulation of an impinging jet diffusion flames is used. The jet injector has a 10 mm in diameter and the distance between the jet flame and the vertical wall is 2 time half diameter. The fuel jet velocity was fixed for 11.8 m/s, corresponding to a Reynolds number of 6881. The flame characteristics varied from hydrocarbon to another for the same Reynolds number. The combustion products of CO, CO2, NO, OH, are depending on the methane and propane and butane flames for the same conditions. The temperature of the flame was varied from hydrocarbon to another the same as for the chemical species production rate. The concentration of the thermal and prompt NO pollutant depends on the temperature flow field and on the thermochemical characteristics of the hydrocarbon fuels.展开更多
In order to provide guideline for choosing a suitable tube-wall thickness(d)for the micro-jet methane diffusion flame,the effect of tube-wall thickness on the blow-off limit is investigated via numerical simulation in...In order to provide guideline for choosing a suitable tube-wall thickness(d)for the micro-jet methane diffusion flame,the effect of tube-wall thickness on the blow-off limit is investigated via numerical simulation in the present work.The results show that the blow-off limit of micro-jet methane diffusion flame firstly increases and then decreases with the increase of tube-wall thickness.Subsequently,the underlying mechanisms responsible for the above non-monotonic blow-off limit are discussed in terms of the flow filed,strain effect and conjugate heat exchange.The analysis indicates that the flow field is insignificant for the non-monotonic blow-off limit.A smaller strain effect can induce the increase of the blow-off limit fromd=0.1 to 0.2 mm,and a worse heat recirculation effect can induce the decrease of the blow-off limit fromd=0.2 to 0.4 mm.The non-monotonic blow-off limit is mainly determined by the heat loss of flame to the tube-wall and the performance of tube-wall on preheating unburned fuel.The smallest heat loss of flame to the tube-wall and the best performance of tube-wall on preheating unburned fuel result in the largest blow-off limit atd=0.2 mm.Therefore,a moderate tube-wall thickness is more suitable to manufacture the micro-jet burner.展开更多
An experimental study investigated the characteristics of a stretched cylindrical diffusion flame, with a convex curvature with respect to the air stream, in response to periodic air flow velocity oscillation. The fue...An experimental study investigated the characteristics of a stretched cylindrical diffusion flame, with a convex curvature with respect to the air stream, in response to periodic air flow velocity oscillation. The fuel was methane diluted with nitrogen, and the oxidizer air. The oscillation frequency was varied from 5 to 250 Hz. The results are summarized as follows. Though the fluctuation amplitude of the air stream velocity gradient was constant with respect to the frequency, the amplitude of the fuel stream increased. The fluctuation amplitude of the flame radius changed quasi-steadily from 5 to 25 Hz, and decreased with increasing frequency in the frequency range greater than 50 Hz. The flame luminosity did not respond quasi-steadily at 5 Hz, and the oscillation amplitude of flame luminosity was less than that of a steady flame, over the same velocity fluctuation range. The oscillation amplitude of luminosity peaked at 50 Hz, and was greater than that of a steady flame. It is considered that this complex change in flame luminosity with respect to frequency was closely related to the phase difference in the respective time variations in the ratio of flame thickness to radius, the velocity gradients of the air and fuel streams, and the magnitude of these values, with the ratio of flame thickness to radius related to the flame curvature effect, the velocity gradient of the air stream correlated to the flame stretch effect, and the velocity gradient of the fuel stream impacting the fuel transportation.展开更多
The operating range of the flow rate or flow velocity for the micro-jet flame is quite wide,which can be used as the heat source.In order to optimize the micro-jet tube combustor in terms of the solid material,the pre...The operating range of the flow rate or flow velocity for the micro-jet flame is quite wide,which can be used as the heat source.In order to optimize the micro-jet tube combustor in terms of the solid material,the present paper numerically investigates the impact of thermal conductivity(λs)on the operating limit of micro-jet flame.Unexpectedly,the non-monotonic blow-off limits with the increase ofλs is found,and the corresponding generation mechanisms are analyzed in terms of the thermal coupling effect,flow field,and strain effect.At first,the lower preheating temperature of the fuel and larger heat loss amount to the environment lead to a larger blow-off limit at a largerλs.After that,the smaller local flow velocity in the vicinity of flame root and smaller strain effect slightly increase the blow-off limit with the continuously increasingλs.Therefore,it is deduced that the applied performance of micro-jet combustor with a smaller thermal conductivity is better in terms of the blow-off limit.展开更多
The soot emission has been the focus of many studies due to applications in industry and the prejudicial effects caused to the environment. The presence of soot is important to the heat transfer in boilers and combust...The soot emission has been the focus of many studies due to applications in industry and the prejudicial effects caused to the environment. The presence of soot is important to the heat transfer in boilers and combustion chambers, contributing significantly to efficiency increases. In controversy, the inhaled soot may cause respiratory system damage and even cancer. Another important point is the contribution to the greenhouse effect. Therefore, the aim of this work is to analyse the soot emission in laminar diffusion flames produced from commercial fuels stabilized on a burner representative of industrial applications. Methane, vehicular natural gas and liquid petroleum gas are considered as fuels. An experimental setup was constructed to implement the technique of laser light extinction and laser-induced incandescence. These non-intrusive techniques provide instant information, in real time, about soot volume fraction. The measurements were calibrated and validated using methods and results of studies available in the literature, and also the uncertainty is analysed and suitably minimized. The results will contribute to the database for projects of combustion systems optimization, aiming to increase the efficiency and soot emissions control.展开更多
Turbulent non-premixed combustion of gaseous fuels is of importance for many technical applications,especially for the steel and refractory industry.Accurate turbulent flow and temperature fields are of major importan...Turbulent non-premixed combustion of gaseous fuels is of importance for many technical applications,especially for the steel and refractory industry.Accurate turbulent flow and temperature fields are of major importance in order to predict details on the concentration fields.The performances of the GRI-Mech 3.0 and the Jones and Lindstedt mechanisms are compared.Detailed chemistry is included with the GRI-Mech 3.0 and J-L kinetic mechanisms in combination with the laminar flamelet combustion model.The combustion system selected for this comparison is a confined non-premixed methane flame surrounded by co-flowing air The simulation results are compared with experimental data of Lewis and Smoot(2001).展开更多
Combustion is a chemical phenomenon in which a multitude of elementary chemical reactions take place, resulting in the overall process of fuel oxidation. Natural gas fuel has been explored for a few decades and extrac...Combustion is a chemical phenomenon in which a multitude of elementary chemical reactions take place, resulting in the overall process of fuel oxidation. Natural gas fuel has been explored for a few decades and extracted for a few years in the region of Paraguayan Chaco, near Bolivia border. Currently, natural gas is not very important in Paraguay＇s energy matrix, however it could be in the near future if higher volumes are extracted and transported to the most populated cities, specially to the capital. In order to improve Paraguayan natural gas combustion performance, an understanding of its fundamental properties and the combustion pathways is required. This study presents new data for Paraguayan Chaco natural gas combustion in a laminar counterflow diffusion flame configuration at atmospheric pressure. Visible chemiluminescence of excited radicals CH＊ and C2^＊ is employed experimentally. 1D numerical simulation was carried out using Paraguayan Chaco natural gas chemical composition and a standard kinetic mechanism, to which we added CH＊ and C] reactions. Typical flame structures resulting from simulation are presented and a validation of the model is realized comparing experimental and numerical CH＊ and C~ radicals profiles.展开更多
Oxygen rich combustion is a mean to increase the energy efficiency and to contribute to CO2 capture. Influence of oxygen enriched air on the stability of methane flames from non premixed laminar jets has been investig...Oxygen rich combustion is a mean to increase the energy efficiency and to contribute to CO2 capture. Influence of oxygen enriched air on the stability of methane flames from non premixed laminar jets has been investigated experimentally. The burner consists of two coaxial jets： methane flowing out of the inner, oxidizer from the outer. The flame behavior is studied according to the proportion of oxygen in the oxidizer jet, the oxidizer and the methane jets velocities. The flame is either anchored to the burner, lifted, stationary or not or blown-out. The addition of oxygen produces a decrease of the lift height, a reduction of the length of the reaction zone and an increase in the soot emission. These results have been reported into diagrams of stability where the flame configurations are connected to the competition between the dynamic effect of the injection velocity and the chemical effect of oxygen addition.展开更多
The utilization of ammonia as an alternative fuel is of great significance in the carbon neutrality strategy. However, the ammonia flame extinction mechanism induced by growing oscillations with its cramped flammabili...The utilization of ammonia as an alternative fuel is of great significance in the carbon neutrality strategy. However, the ammonia flame extinction mechanism induced by growing oscillations with its cramped flammability range, sluggish propagation speed, and poor stability is still not studied in sufficient details. In this paper, the high-fidelity simulations with efficient continuation computation and detailed models are conducted to investigate the ammonia extinction dynamics as a function of hydrogen blending ratio, and to reveal the governing sub-processes in flame extinction and oscillation development. It is found that the extinction and onset of oscillatory instability in the ammonia/hydrogen spherical diffusion flame (SDF) derive from the interaction of competing chemistry with diffusive leakage losses. Chemical oscillations occurring at the maximum temperature iso-contour are primarily responsible for the near-limit flame oscillations. In the rich-side low-temperature region, although the local heat production is inappreciable, the strong diffusive losses with governing, adverse contributions to the ignition chemistry play a leading role in flame extinction. The reactions dominating the extinction limit are also important for the oscillation frequency;the reactions that help to extend the extinction limit also tend to increase the frequency, and vice versa. The extinction limit and frequency depend mainly on the major reactant diffusivities (including NH_(3), H_(2), and O_(2)) and heat conduction, while the diffusivities of other radicals and products are fairly unimportant. Hydrogen addition could remarkably extend the steady-state and oscillatory extinction limits of ammonia SDFs, and reduce the oscillation frequency since the imaginary eigenvalue is depressed.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
Any biogas produced by the anaerobic fermentation of organic materials has the advantage of being an environmentally friendly biofuel.Nevertheless,the relatively low calorific value of such gases makes their effective...Any biogas produced by the anaerobic fermentation of organic materials has the advantage of being an environmentally friendly biofuel.Nevertheless,the relatively low calorific value of such gases makes their effective utilization in practical applications relatively difficult.The present study considers the addition of hydrogen as a potential solution to mitigate this issue.In particular,the properties of turbulent diffusion jet flames and the related pollutant emissions are investigated numerically for different operating pressures.The related numerical simulations are conducted by solving the RANS equations in the frame of the Reynolds Stress Model in combination with the flamelet approach.Radiation effects are also taken into account and the combustion kinetics are described via the GRI-Mech 3.0 reaction model.The considered hydrogen fuel enrichment spans the range from 0%to 50%in terms of volume.Pressure varies between 1 and 10 atm.The results show that both hydrogen addition and pressure increase lead to an improvement in terms of mixing quality and have a significant effect on flame temperature and height.They also reduce CO_(2)emissions but increase NOx production.Prompt NO is shown to be the predominant NO formation mechanism.展开更多
基金supported by the National Natural Science Foundation of China (No. 51806103)the Aeronautical Science Foundation of China (No.201928052002)the Fundamental Research Funds for the Central Universities(No.NT2021007)。
文摘Soot,a product of insufficient combustion,is usually in the form of aggregate. The multi-scattering of soot fractal aggregates has been proved to play an important role in studying the soot radiative properties,which is rarely considered in predicting the radiative heat transfer in combustion flame. In the present study,based on the weighted sum of gray soot fractal aggregate(WSGSA) model,which is used to predict the temperature field and soot aggregates in turbulent diffusion flame,the flame temperature distribution and soot volume fraction distribution under the conditions of the model without considering radiation,the default radiation model in Fluent software and the WSGSA model are calculated respectively. The results show that the flame temperature will be seriously overestimated without considering radiation and the maximum relative discrepancy of flame centerline temperature is about 64.5%. The accuracy will be improved by the default radiation model in the Fluent software,but the flame temperature is still overestimated and the maximum relative discrepancy of flame centerline temperature is about 42.1%. However,more satisfactory results can be obtained by the WSGSA model,and the maximum relative discrepancy of flame centerline temperature is no more than 15.3%. Similar conclusions can also be obtained in studying the temperature distribution along different flame heights. Moreover,the soot volume fraction can be predicted more accurately with the application of the WSGSA model. Both without considering radiation and using the default radiation model in the Fluent software will result in the underestimating of soot volume fraction. All the results reveal that the WSGSA model can be used to predict the temperature and soot aggregates in the CH/air turbulent diffusion flame.
基金Supported by the National Natural Science Foundation of China （51276002）, and the Specific Research Fund for the Doctoral Program of Higher Education of China （20110001130014）.
文摘This paper investigates the effects of coflow O2 level and temperature on diffusion flame of a CH4/H2 jet in hot coflow （JHC） from a burner system similar to that of Dally et al. The coflow O2 mass fraction （ Yo2 ） is varied from 3% to 80% and the temperature （Tcof） from 1200 K to 1700 K. The Eddy Dissipation Concept （EDC） model with detailed reaction mechanisms GRI-Mech 3.0 is used for all simulations. To validate the modeling, several JHC flames are predicted under the experimental conditions of Dally et al. [Proc. Combust. Inst., 29 （1）, 1147-1154 （2002）] and the results obtained match well with the measurements. Results demonstrate that, when Yo2 decreased, the diffusion combustion is likely to transform from traditional combustion to MILD （Moderate or Intense Low-oxygen Dilution） combustion mode. When Tcof is higher, the temperature distribution over the whole domain trends to be more uniform. Reducing yo2 or Tcof leads to less production of intermediate species OH and CO. It is worth noting that if Yo2 is high enough （ Yo2 〉80%）, increasing Yo2 does not cause obvious temperature increase.
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Projects(50806024,50806023 and 50806026) supported by the National Natural Science Foundation of China
文摘A combined computational and experimental investigation to examine temperature and soot volume fraction in coflow ethylene-air diffusion flames was presented.A numerical simulation was conducted by using a relatively detailed gas-phase chemistry and complex thermal and transport properties coupled with a semi-empirical two-equation soot model.Thermal radiation was calculated using the discrete ordinates method.An image processing technique and a decoupled reconstruction method were used to simultaneously measure the distributions of temperature and soot volume fraction.The results show that the maximum error for temperature does not exceed 10% between the prediction and the measurement.And the maximum error is 6.9% for soot volume fraction between prediction and measurement.Additional simulations were performed to explore the effects of global equivalence ratio on diffusion flames and the soot formation.The results display that the soot formation increases with decreasing the coflow air velocity.And the soot formation in each case appears in the annular region,where the temperature ranges from about 1 000 K to 2 000 K and the profile becomes taller and wider when the coflow air is decreased.
文摘The stable and crystalline phase of pure nano- structured CeO2 was directly synthesized by flame-assisted spray pyrolysis and solid state diffusion route. Different characterization techniques, including X-ray diffraction （XRD）, scanning electron microscopy （SEM）, Fourier trans- form infrared spectroscopy （FTIR）, ultraviolet-visible （UV- Vis）, and thermo gravimetric analysis （TGA） were employed to examine the structural, morphological, optical, and thermal properties of the final product. Similarly, the comparative carbon dioxide （CO2）-sensing response of as-synthesized CeO2 nanoparticles by both routes was also reported. The CeO2 nanoparticles synthesized by solid state diffusion method exhibit good sensitivity （3.38 %） at room temperature, low operating temperature （398 K）, fast response time （32 s）, and recovery time （36 s） along with good stability.
基金Sponsored by National Natural Science Foundation of China(Grant No.51376021)
文摘To study the configuration and conductivity effects on micro-scale methane-air flames by electric field and iron wind,different electric field forces and iron winds are generated by needle,circle and plate electrodes respectively in different electrodes heights under both AC and DC fields though experiments. Experimental results showed that the flame characteristics are affected by needle electrodes mainly through the action of ion wind,by plate type electrodes mainly through the action of electric field force and by annular electrodes through both the electric field force and ion wind at the same time. Under DC field 's effects of all electrodes types,the flame will consequently go down while the voltage reached to a limit value,and it will breakdown under the strong effect of the ion wind by needle electrodes. The results also showed the influence by different electrodes types to the current characteristics,resistance properties and configuration of themicro-scale flames.
文摘In our study, we investigate the differences between the combustion of different hydrocarbon fuels CH4, C3H8, C4H10. A numerical simulation of an impinging jet diffusion flames is used. The jet injector has a 10 mm in diameter and the distance between the jet flame and the vertical wall is 2 time half diameter. The fuel jet velocity was fixed for 11.8 m/s, corresponding to a Reynolds number of 6881. The flame characteristics varied from hydrocarbon to another for the same Reynolds number. The combustion products of CO, CO2, NO, OH, are depending on the methane and propane and butane flames for the same conditions. The temperature of the flame was varied from hydrocarbon to another the same as for the chemical species production rate. The concentration of the thermal and prompt NO pollutant depends on the temperature flow field and on the thermochemical characteristics of the hydrocarbon fuels.
基金Project(51876074)supported by the National Natural Science Foundation of China。
文摘In order to provide guideline for choosing a suitable tube-wall thickness(d)for the micro-jet methane diffusion flame,the effect of tube-wall thickness on the blow-off limit is investigated via numerical simulation in the present work.The results show that the blow-off limit of micro-jet methane diffusion flame firstly increases and then decreases with the increase of tube-wall thickness.Subsequently,the underlying mechanisms responsible for the above non-monotonic blow-off limit are discussed in terms of the flow filed,strain effect and conjugate heat exchange.The analysis indicates that the flow field is insignificant for the non-monotonic blow-off limit.A smaller strain effect can induce the increase of the blow-off limit fromd=0.1 to 0.2 mm,and a worse heat recirculation effect can induce the decrease of the blow-off limit fromd=0.2 to 0.4 mm.The non-monotonic blow-off limit is mainly determined by the heat loss of flame to the tube-wall and the performance of tube-wall on preheating unburned fuel.The smallest heat loss of flame to the tube-wall and the best performance of tube-wall on preheating unburned fuel result in the largest blow-off limit atd=0.2 mm.Therefore,a moderate tube-wall thickness is more suitable to manufacture the micro-jet burner.
文摘An experimental study investigated the characteristics of a stretched cylindrical diffusion flame, with a convex curvature with respect to the air stream, in response to periodic air flow velocity oscillation. The fuel was methane diluted with nitrogen, and the oxidizer air. The oscillation frequency was varied from 5 to 250 Hz. The results are summarized as follows. Though the fluctuation amplitude of the air stream velocity gradient was constant with respect to the frequency, the amplitude of the fuel stream increased. The fluctuation amplitude of the flame radius changed quasi-steadily from 5 to 25 Hz, and decreased with increasing frequency in the frequency range greater than 50 Hz. The flame luminosity did not respond quasi-steadily at 5 Hz, and the oscillation amplitude of flame luminosity was less than that of a steady flame, over the same velocity fluctuation range. The oscillation amplitude of luminosity peaked at 50 Hz, and was greater than that of a steady flame. It is considered that this complex change in flame luminosity with respect to frequency was closely related to the phase difference in the respective time variations in the ratio of flame thickness to radius, the velocity gradients of the air and fuel streams, and the magnitude of these values, with the ratio of flame thickness to radius related to the flame curvature effect, the velocity gradient of the air stream correlated to the flame stretch effect, and the velocity gradient of the fuel stream impacting the fuel transportation.
文摘The operating range of the flow rate or flow velocity for the micro-jet flame is quite wide,which can be used as the heat source.In order to optimize the micro-jet tube combustor in terms of the solid material,the present paper numerically investigates the impact of thermal conductivity(λs)on the operating limit of micro-jet flame.Unexpectedly,the non-monotonic blow-off limits with the increase ofλs is found,and the corresponding generation mechanisms are analyzed in terms of the thermal coupling effect,flow field,and strain effect.At first,the lower preheating temperature of the fuel and larger heat loss amount to the environment lead to a larger blow-off limit at a largerλs.After that,the smaller local flow velocity in the vicinity of flame root and smaller strain effect slightly increase the blow-off limit with the continuously increasingλs.Therefore,it is deduced that the applied performance of micro-jet combustor with a smaller thermal conductivity is better in terms of the blow-off limit.
文摘The soot emission has been the focus of many studies due to applications in industry and the prejudicial effects caused to the environment. The presence of soot is important to the heat transfer in boilers and combustion chambers, contributing significantly to efficiency increases. In controversy, the inhaled soot may cause respiratory system damage and even cancer. Another important point is the contribution to the greenhouse effect. Therefore, the aim of this work is to analyse the soot emission in laminar diffusion flames produced from commercial fuels stabilized on a burner representative of industrial applications. Methane, vehicular natural gas and liquid petroleum gas are considered as fuels. An experimental setup was constructed to implement the technique of laser light extinction and laser-induced incandescence. These non-intrusive techniques provide instant information, in real time, about soot volume fraction. The measurements were calibrated and validated using methods and results of studies available in the literature, and also the uncertainty is analysed and suitably minimized. The results will contribute to the database for projects of combustion systems optimization, aiming to increase the efficiency and soot emissions control.
文摘Turbulent non-premixed combustion of gaseous fuels is of importance for many technical applications,especially for the steel and refractory industry.Accurate turbulent flow and temperature fields are of major importance in order to predict details on the concentration fields.The performances of the GRI-Mech 3.0 and the Jones and Lindstedt mechanisms are compared.Detailed chemistry is included with the GRI-Mech 3.0 and J-L kinetic mechanisms in combination with the laminar flamelet combustion model.The combustion system selected for this comparison is a confined non-premixed methane flame surrounded by co-flowing air The simulation results are compared with experimental data of Lewis and Smoot(2001).
文摘Combustion is a chemical phenomenon in which a multitude of elementary chemical reactions take place, resulting in the overall process of fuel oxidation. Natural gas fuel has been explored for a few decades and extracted for a few years in the region of Paraguayan Chaco, near Bolivia border. Currently, natural gas is not very important in Paraguay＇s energy matrix, however it could be in the near future if higher volumes are extracted and transported to the most populated cities, specially to the capital. In order to improve Paraguayan natural gas combustion performance, an understanding of its fundamental properties and the combustion pathways is required. This study presents new data for Paraguayan Chaco natural gas combustion in a laminar counterflow diffusion flame configuration at atmospheric pressure. Visible chemiluminescence of excited radicals CH＊ and C2^＊ is employed experimentally. 1D numerical simulation was carried out using Paraguayan Chaco natural gas chemical composition and a standard kinetic mechanism, to which we added CH＊ and C] reactions. Typical flame structures resulting from simulation are presented and a validation of the model is realized comparing experimental and numerical CH＊ and C~ radicals profiles.
文摘Oxygen rich combustion is a mean to increase the energy efficiency and to contribute to CO2 capture. Influence of oxygen enriched air on the stability of methane flames from non premixed laminar jets has been investigated experimentally. The burner consists of two coaxial jets： methane flowing out of the inner, oxidizer from the outer. The flame behavior is studied according to the proportion of oxygen in the oxidizer jet, the oxidizer and the methane jets velocities. The flame is either anchored to the burner, lifted, stationary or not or blown-out. The addition of oxygen produces a decrease of the lift height, a reduction of the length of the reaction zone and an increase in the soot emission. These results have been reported into diagrams of stability where the flame configurations are connected to the competition between the dynamic effect of the injection velocity and the chemical effect of oxygen addition.
基金supported by the National Natural Science Foundation of China(Grant No.22178032)the Natural Science Foundation of Chongqing(Grant No.CSTB2023NSCQ-MSX1045)National Key Research and Development Program of China(Grant No.2022YFB4004404).
文摘The utilization of ammonia as an alternative fuel is of great significance in the carbon neutrality strategy. However, the ammonia flame extinction mechanism induced by growing oscillations with its cramped flammability range, sluggish propagation speed, and poor stability is still not studied in sufficient details. In this paper, the high-fidelity simulations with efficient continuation computation and detailed models are conducted to investigate the ammonia extinction dynamics as a function of hydrogen blending ratio, and to reveal the governing sub-processes in flame extinction and oscillation development. It is found that the extinction and onset of oscillatory instability in the ammonia/hydrogen spherical diffusion flame (SDF) derive from the interaction of competing chemistry with diffusive leakage losses. Chemical oscillations occurring at the maximum temperature iso-contour are primarily responsible for the near-limit flame oscillations. In the rich-side low-temperature region, although the local heat production is inappreciable, the strong diffusive losses with governing, adverse contributions to the ignition chemistry play a leading role in flame extinction. The reactions dominating the extinction limit are also important for the oscillation frequency;the reactions that help to extend the extinction limit also tend to increase the frequency, and vice versa. The extinction limit and frequency depend mainly on the major reactant diffusivities (including NH_(3), H_(2), and O_(2)) and heat conduction, while the diffusivities of other radicals and products are fairly unimportant. Hydrogen addition could remarkably extend the steady-state and oscillatory extinction limits of ammonia SDFs, and reduce the oscillation frequency since the imaginary eigenvalue is depressed.
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.
文摘Any biogas produced by the anaerobic fermentation of organic materials has the advantage of being an environmentally friendly biofuel.Nevertheless,the relatively low calorific value of such gases makes their effective utilization in practical applications relatively difficult.The present study considers the addition of hydrogen as a potential solution to mitigate this issue.In particular,the properties of turbulent diffusion jet flames and the related pollutant emissions are investigated numerically for different operating pressures.The related numerical simulations are conducted by solving the RANS equations in the frame of the Reynolds Stress Model in combination with the flamelet approach.Radiation effects are also taken into account and the combustion kinetics are described via the GRI-Mech 3.0 reaction model.The considered hydrogen fuel enrichment spans the range from 0%to 50%in terms of volume.Pressure varies between 1 and 10 atm.The results show that both hydrogen addition and pressure increase lead to an improvement in terms of mixing quality and have a significant effect on flame temperature and height.They also reduce CO_(2)emissions but increase NOx production.Prompt NO is shown to be the predominant NO formation mechanism.
