共查询到19条相似文献,搜索用时 562 毫秒
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本文通过理论分析和数值模拟系统地研究了辐射热损失对球形火焰传播速度的影响。研究结果表明辐射热损失的影响分为直接影响和间接影响;直接影响指辐射热损失会降低火焰温度,从而降低火焰传播速度;间接影响指辐射热损失导致的冷却会引起逆向火焰传播的流动,从而降低火焰传播速度。对近可燃极限预混气体,直接影响起主导作用;对高辐射强度预混气体,间接影响起主导作用。本文研究的结果对球形火焰法测量火焰传播速度有着重要的指导意义。 相似文献
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地面常重力(1g)条件下,丙烷/空气预混火焰向上传播的富燃极限为9.2%C_3H_8,而向下传播时的富燃极限仅为6.3%C_3H_8,二者之间存在明显差距。利用微重力条件下的实验,对燃料浓度从6.5%到8.6%(微重力实验中测定的可燃极限)范围内的丙烷/空气预混火焰特性进行了研究。实验发现,重力对近极限丙烷/空气火焰的传播有显著影响,影响程度随着当量比的增加而增大。微重力下丙烷/空气的富燃极限为8.6%C_3H_8(φ=2.24),明显高于1g条件下向下传播火焰的可燃极限,略低于向上传播火焰的可燃极限。随着当量比的增大,根据压力变化曲线计算的火焰层流燃烧速度从8.5cm/s逐渐减小到2.7 cm/s,可燃极限处的层流燃烧速度与前人实验数据一致。 相似文献
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预混火焰拉伸和曲率效率的物理分析 总被引:1,自引:0,他引:1
湍流燃烧的基本火焰结构是拉伸的曲面涡管;拉伸流场中的管形火焰模拟了湍流燃烧的细微结构。本文对平面预混火焰、拉伸预混火焰和管形拉伸预混火焰进行了质量、能量和组分的守恒分析。通过对比这几种火焰,揭示了火焰拉伸效果是通过优先扩散改变火焰温度和熄火极限;而火焰曲率通过增强或削弱这种优先扩散效果来影响火焰温度,影响的大小和火焰厚度与火焰半径的比值呈正比。 相似文献
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Bai Han Alfonso F. Ibarreta Chih-Jen Sung James S. Tien 《Proceedings of the Combustion Institute》2005,30(1):527-535
The present study experimentally investigates the structure and instabilities associated with extremely low-stretch (1 s−1) gaseous diffusion flames. Ultra-low-stretch flames are established in normal gravity by bottom burning of a methane/nitrogen mixture discharged from a porous spherically symmetric burner of large radius of curvature. OH-PLIF and IR imaging techniques are used to characterize the reaction zone and the burner surface temperature, respectively. A flame stability diagram mapping the response of the ultra-low-stretch diffusion flame to varying fuel injection rate and nitrogen dilution is explored. In this diagram, two main boundaries are identified. These boundaries separate the stability diagram into three regions: sooting flame, non-sooting flame, and extinction. Two distinct extinction mechanisms are noted. For low fuel injection rates, flame extinction is caused by heat loss to the burner surface. For relatively high injection rates, at which the heat loss to burner surface is negligible, flame radiative heat loss is the dominant extinction mechanism. There also exists a critical inert dilution level beyond which the flame cannot be sustained. The existence of multi-dimensional flame phenomena near the extinction limits is also identified. Various multi-dimensional flame patterns are observed, and their evolutions are studied using direct chemiluminescence and OH-PLIF imaging. The results demonstrate the usefulness of the present burner configuration for the study of low-stretch gaseous diffusion flames. 相似文献
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Anil K. Kulkarni 《实验传热》2013,26(4):411-426
Measurements of radiative and total heat transfer from turbulent flames to a wall are presented for combustion of propane, methane, and natural gas. Flames were generated by a linear burner placed at the bottom of an instrumented, cooled copper wall. The radiative heat feedback from the flames to the wall was determined from measurements using a narrow-angle radiometer and by employing mean-beam-length analysis. The radiative fraction of the total heat feedback was found to be almost independent of the burner power output when plotted against scaled height (vertical distance normalized with flame length). Among the three fuels tested, radiative fraction in flame-to-wall heat transfer was the maximum, for propane and minimum for methane, which can be explained based on sooting characteristics of flames. The total radiative energy transfer as a fraction of the burner output power is also presented for the three fuels. 相似文献
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We conducted a numerical study on the fluid dynamic, thermal and chemical structures of laminar methane–air micro flames established under quiescent atmospheric conditions. The micro flame is defined as a flame on the order of one millimetre or less established at the exit of a vertically-aligned straight tube. The numerical model consists of convective–diffusive heat and mass transport with a one-step, irreversible, exothermic reaction with selected kinetics constants validated for near-extinction analyses. Calculations conducted under the burner rim temperature 300 K and the adiabatic burner wall showed that there is the minimum burner diameter for the micro flame to exist. The Damköhler number (the ratio of the diffusive transport time to the chemical time) was used to explain why a flame with a height of less than a few hundred microns is not able to exist under the adiabatic burner wall condition. We also conducted scaling analysis to explain the difference in extinction characteristics caused by different burner wall conditions. This study also discussed the difference in governing mechanisms between micro flames and microgravity flames, both of which exhibit similar spherical flame shape. 相似文献
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Numerical simulations of laminar coflow methane/air diffusion flames at atmospheric pressure and different gravity levels were conducted to gain a better understanding of the effects of gravity on soot formation by using 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 coupled with a non-grey model for the radiative properties of CO, CO2, H2O, and soot. Calculations were conducted for three coflow air velocities of 77.6, 30, and 5 cm/s to investigate how the coflowing air velocity affects the flame structure and soot formation at different levels of gravity. The coflow air velocity has a rather significant effect on the streamwise velocity and the fluid parcel residence time, especially at reduced gravity levels. The flame height and the visible flame height in general increase with decreasing the gravity level. The peak flame temperature decreases with decreasing either the coflow air stream velocity or the gravity level. The peak soot volume fraction of the flame at microgravity can either be greater or less than that of its normal gravity counterpart, depending on the coflow air velocity. At sufficiently high coflow air velocity, the peak soot volume fraction increases with decreasing the gravity level. When the coflow air velocity is low enough, soot formation is greatly suppressed at microgravity and extinguishment occurs in the upper portion of the flame with soot emission from the tip of the flame owing to incomplete oxidation. The numerical results provide further insights into the intimate coupling between flame size, residence time, thermal radiation, and soot formation at reduced gravity level. The importance of thermal radiation heat transfer and coflow air velocity to the flame structure and soot formation at microgravity is demonstrated for the first time. 相似文献
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《Proceedings of the Combustion Institute》2023,39(2):1721-1729
Whether steady-state gaseous microgravity spherical diffusion exist in the presence of radiation heat loss is an important fundamental question and has important implications for spacecraft fire safety. In this work, experiments aboard the International Space Station and a transient numerical model are used to investigate the existence of steady-state microgravity spherical diffusion flames. Gaseous spherical diffusion flames stabilized on a porous spherical burner are employed in normal (i.e., fuel flowing into an ambient oxidizer) and inverse (i.e., oxidizer flowing into an ambient fuel) flame configurations. The fuel is ethylene and the oxidizer oxygen, both diluted with nitrogen. The flow rate of the reactant gas from the burner is held constant. It is found that steady-state gaseous microgravity spherical diffusion flames can exist in the presence of radiation heat loss, provided that the steady-state flame size is less than the flame size for radiative extinction, and the flame develops fast enough that radiation heat loss does not drop the flame temperature below the critical temperature for radiative extinction (1130 K). A simple model is provided that allows for the identification of initial conditions that can lead to steady-state spherical diffusion flames. In the spherical, infinite domain configuration, the characteristic time for the diffusion-controlled system to effectively reach steady-state is found to be on the order of 100,000 s. Despite a narrow range of attainable conditions, flames that exhibit steady-state behavior are observed aboard the ISS for up to 870 s, even with the constraint of a finite boundary. Steady-state flames are simulated using the numerical model for over 100,000 s. 相似文献
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An experimental study has been conducted to find the heat transfer characteristics of methane/air flames impinging normally to a flat surface using different burner geometries. The burners used were of nozzle, tube, and orifice type each with a diameter of 10 mm. Due to different exit velocity profiles, the flame structures were different in each case. Because of nearly flat velocity profile, the flame spread was more in case of orifice and nozzle burners as compared to tube burner. Effects of varying the value of Reynolds number (600–2500), equivalence ratio (0.8–1.5) and dimensionless separation distance (0.7–8) on heat transfer characteristics on the flat plate have been investigated for the tube burner. Different flame shapes were observed for different impingement conditions. It has been observed that the heat transfer characteristics were intimately related to flame shapes. Heat transfer characteristics were discussed for the cases when the flame inner reaction cone was far away, just touched, and was intercepted by the plate. Negative heat fluxes at the stagnation point were observed when the inner reaction cone was intercepted by the plate due to impingement of cool un-burnt mixture directly on the surface. Different heat transfer characteristics were observed for different burner geometries with similar operating conditions. In case of tube burner, the maximum heat flux is around the stagnation point and decay is faster in the radial direction. In case of nozzle and orifice burner, the heat transfer distribution is more uniform over the surface. 相似文献
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Andrew J. Lock Ranjan Ganguly Ishwar K. Puri Suresh K. Aggarwal Uday Hegde 《Proceedings of the Combustion Institute》2005,30(1):511-518
While premixed and nonpremixed microgravity flames have been extensively investigated, the corresponding literature regarding partially premixed flames (PPFs) is sparse. We report the first experimental investigation of burner-stabilized microgravity PPFs. Partially premixed flames with multiple reaction zones are established in microgravity on a Wolfhard–Parker slot burner in the 2.2 s drop tower at the NASA Glenn Research Center. Microgravity measurements include flame imaging, and thermocouple and radiometer data. Detailed simulations are also used to provide further insight into the steady and transient response of these flames to variations in g. The flame topology and interactions between the various reaction zones are strongly influenced by gravity. The flames widen substantially in microgravity. During the transition from normal to microgravity, the flame structure experiences a fast change and another relatively slower transient change. The fast response is due to the altered advection as the value of g is reduced, while the slow response is due to the changes in the diffusive fluxes. The radiative heat loss from the flames increases in microgravity. A scaling analysis based on a radiation Damköhler number is able to characterize the radiation heat loss. 相似文献
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T. S. Cheng C. -P. Chen C. -S. Chen Y. -H. Li C. -Y. Wu Y. -C. Chao 《Combustion Theory and Modelling》2013,17(5):861-881
Characteristics of microjet methane diffusion flames stabilized on top of the vertically oriented, stainless-steel tubes with an inner diameter ranging from 186 to 778 μ m are investigated experimentally, theoretically and numerically. Of particular interest are the flame shape, flame length and quenching limit, as they may be related to the minimum size and power of the devices in which such flames would be used for future micro-power generation. Experimental measurements of the flame shape, flame length and quenching velocity are compared with theoretical predictions as well as detailed numerical simulations. Comparisons of the theoretical predictions with measured results show that only Roper's model can satisfactorily predict the flame height and quenching velocity of microjet methane flames. Detailed numerical simulations, using skeletal chemical kinetic mechanism, of the flames stabilized at the tip of d = 186, 324 and 529 μ m tubes are performed to investigate the flame structures and the effects of burner materials on the standoff distance near extinction limit. The computed flame shape and flame length for the d = 186 μm flame are in excellent agreement with experimental results. Numerical predictions of the flame structures strongly suggest that the flame burns in a diffusion mode near the extinction limit. The calculated OH mass fraction isopleths indicate that different tube materials have a minor effect on the standoff distance, but influence the quenching gap between the flame and the tube. 相似文献