煤粉低尘燃烧器内燃烧特性的数值模拟

介绍了一种用于中小型工业窑炉的新型煤粉低尘燃烧技术,利用计算机数值模拟考察了煤粉低尘旋流燃烧器的特性. 在合理选择气相流动、固相流动、煤燃烧及NO的生成等模型的同时,针对旋流燃烧场中固体颗粒在壁面附近的碰撞及熔融特性,探讨了煤粉在壁面处的运动模型,并以此为基础考察了燃烧场的两相流动特性,模拟了燃烧器内煤粉的燃烧过程及各物理量的分布. 在与实验比较的基础上,对燃烧器的结构进行了改进. 结果表明,在低化学计量比下,改进后的燃烧器性能更好,颗粒在燃烧器内充分燃尽,在保证液排渣效果的同时,NO的排放远低于常规液排渣旋风器的NO排放量.     

Continuous spin detonation of a heterogeneous kerosene–air mixture with addition of hydrogen

Regimes of continuous spin detonation of two-phase kerosene–air mixtures with small addition of hydrogen are obtained for the first time in a flow-type annular cylindrical combustor 503 mm in diameter.     

RADIATIVE TRANSFER IN A SOLAR ABSORBING PARTICLE LADEN FLOW†

Solar central receivers which utilize solid thermal carriers such as sand or small refractory particles for direct absorption of concentrated solar radiation have been investigated at several laboratories. In the central receiver concept a field of tracking mirrors (heliostats) is used to focus solar energy onto a receiver mounted atop a tower. A possible receiver configuration is a cavity in which a falling sheet of solid particles is directly irradiated by the concentrated solar flux passing through the aperture. Regardless of the particular geometry, the radiative transfer within the falling particle curtain must be studied in order to determine the net radiative heating rate for the particles

A discrete ordinate radiative transfer model has been applied to study the radiative coupling within the falling particle curtain. The model determines how much energy is absorbed by the particles, how much is transmitted to the rear wall of the receiver, and determines the effects of particle scattering and thermal emission on the net radiation absorbed by the particles. The model accounts for the directional nature of the radiation field, particle scattering, and the wavelength dependence of the optical properties

The discrete ordinate model has been used to assess the influence of the pertinent radiation transfer parameters in determining the local and overall particle heating rates. The results of this study are discussed as well as the analysis.     

Continuous spin detonation of a coal-air mixture in a flow-type plane-radial combustor

Regimes of continuous spin detonation of coal particles in an air flow in a flow-type plane-radial combustor 500 mm in diameter are studied. The tested substance is fine-grained cannel coal from Kuzbass having a particle size of 1–7 µm and containing 24.7% of volatiles, 14.2% of ashes, and 5.1% of moisture. A certain amount of hydrogen is added for coal transportation into the combustor and promotion of the chemical reaction on the surface of solid particles. To reduce air pressure losses in channels connecting the manifold and the combustor, their cross section is increased to limiting values (25 cm²), whereas the combustor exit diameter is reduced. The angle of the air flow direction and the combustor geometry are also varied. The minimum pressure difference in the air injection channels (16%) is reached with stability of continuous spin detonation in the combustor being retained. The domain of continuous spin detonation regimes in the coordinates of the fuel flow rate and specific flow rate of the mixture is constructed. The results of studying detonation burning of solid fuels can find applications in power engineering, chemical industry, and environmental science, in particular, contamination by combustion products.     

Detonation burning of anthracite and lignite particles in a flow-type radial combustor

Regimes of continuous spin detonation of anthracite and lignite particles in an air flow in a radial vortex combustor 500 mm in diameter with a constant (along the radius) cross-sectional area are studied. Ground coal with a particle size of 1–12 μm is used. For transporting coal into the combustor and promoting the chemical reaction on the surface of solid particles, hydrogen or syngas is added in the ratio CO/H₂ = 1/1, 1/2, or 1/3. Continuous spin detonation of two-phase mixtures of fine anthracite and lignite particles and air with addition of hydrogen up to 4% of the coal consumption rate is obtained for the first time. The amount of syngas added to coal increases with decreasing fraction of hydrogen in the syngas: 14, 21, and 27% for anthracite and 11, 20, and 29% for lignite at CO/H₂ = 1/3, 1/2, and 1/1, respectively. The structure of detonation waves and the flow in their vicinity are not principally different from those observed previously for long-flame bituminous coal and charcoal. Higher detonation velocities are observed for more energy-intensive coal (anthracite). A higher pressure is obtained near the cylindrical wall of the combustor in cold runs as compared to detonation in the case with identical flow rates of the coal–air mixtures.     

Kerosene Combustion in a Pseudoshock with Varied Conditions at the Supersonic Ramjet Combustor Model

Combustion, Explosion, and Shock Waves - Results of studying kerosene combustion in a pseudoshock with varied temperature and velocity in a supersonic ramjet combustor model are presented....     

Combustion of Liquid and Gaseous Fuels in a Supersonic Combustor

Results of an experimental study of a full scramjet model operating on kerosene, which was performed in an IT-302M hotshot wind tunnel based at the Institute of Theoretical and Applied Mechanics of the Siberian Division of the Russian Academy of Sciences, and an experimental study of a model operating on hydrogen, which was performed in a hotshot wind tunnel with fire heating based at the China Aerodynamic Research and Development Center, are reported. The tests were performed for Mach numbers 5 and 6 for flow parameters close to in-flight conditions. An optimal system for kerosene injection under these conditions was determined, and the thrust characteristics of the engine model were examined. The possibility of controlling kerosene combustion in tests in the short-duration wind tunnel was analyzed, and special features of fuel ignition in a short combustor were considered. Intense combustion of kerosene was achieved with upstream injection of more than 3% of hydrogen, which allowed obtaining effective thrust. The distributions of static pressure and force characteristics of the model in the case of kerosene and hydrogen combustion were compared.     

A novel combined baffle-cavity micro-combustor configuration for Micro-Thermo-Photo-Voltaic applications

The major issues of Micro-Thermo-Photo-Voltaic (MTPV) micro-combustors are flame instabilities, which narrow the operational range, and non-uniform wall temperature, which lowers the overall efficiency. The purpose of the present research is to propose a novel micro-combustor with combined baffle and cavity configuration to address these issues. For this aim, a numerical modeling approach is validated and used. The performance of the improved combustor is compared with another recent baffle-bluff configuration. It is shown that the novel design improves the average wall temperature by 36.4 K and mitigates its standard deviation by 13.4 K. Moreover, using the optimal baffle thickness, these enhancements can be augmented by 4% raise of average wall temperature, 62% increase of temperature uniformity, and 20% reduction in overall emission. The baffle length of 0.6 times the combustor length and thickness of 0.0625 times the baffle spacing result in the optimal operation due to the flame lift-off in the upstream direction. According to the sensitivity analysis, the most effective geometrical parameters are the baffle length and thickness. It is expected that using this novel micro-combustor with optimized design parameters improves the overall efficiency of MTPV systems.     

垂直上升管内超临界压力下航空煤油传热特性研究(英文)

A research on the heat transfer performance of kerosene flowing in a vertical upward tube at supercritical pres-sure is presented. In the experiments, insights are offered on the effects of the factors such as mass flux, heat flux, and pressure. It is found that increasing mass flux reduces the wall temperature and separates the experimental section into three different parts, while increasing working pressure deteriorates heat transfer. The extended corresponding-state principle can be used for evaluating density and transport properties of kerosene, including its viscosity and thermal conductivity, at different temperatures and pressures under supercritical conditions. For getting the heat capacity, a Soave–Redlich–Kwong (SRK) equation of state is used. The correlation for predicting heat transfer of kerosene at supercritical pressure is established and shows good agreement with the experimen-tal data.     

Characteristics of low vapor pressure oil ignition developed with irradiation of mega hertz level ultrasonic

In liquid fuel vaporizing type combustor for civil use, large amount of the electric power is consumed in pre-heating of fuel vaporizer during a standby period. Reduction of consumed power in pre-heating is regarded as important to develop a performance of the vaporizing type combustor from the viewpoint of energy saving. We proposed the oil combustion system using the MHz-ultrasonic atomizing method without the preheating process. In this work, we manufactured kerosene pre-vaporizing combustor with ultrasonic oscillator which had frequency of 1.7 MHz. Low CO and NO_x emission had been already achieved with manufactured combustor by authors in 2002. Aiming to investigate fundamental characteristics of the ignition process with ultrasonic atomizing, the ignition time requirement was measured and the flame luminescence was detected with spectroscopic analysis in order to consider the mixing state on pre-mixing combustor by judging differences of the flame luminescence. As the results, ultrasonic atomizing method was very effective for vaporization of kerosene. But heat release rate of only 0.54 kW was obtained with input power of 33 W because the effect of the sound absorption was not negligible. The time requirement for the ignition was influenced by an equivalence ratio and balance between primary air flow rate and secondary one. Especially, the ignition time had different tendencies between fuel rich and fuel lean condition. With flow visualization, it was clarified that probability of the ignition depended on a difference of flow pattern of the fuel aerosol.     

Modeling of the beryllia ceramics formation process

Simulation results of the formation process of the ceramic fabrications by a hot molding method are presented. Mathematical model describes motion and heat exchange of the liquid thermoplastic slurry of beryllia including the aggregate state change. Velocity and temperature fields during the formation process in the bushings with flat and circular cavities are obtained. Heat flow distribution at the wall of the form-building cavity is demonstrated. The increase of the slurry density during the transition from the liquid state into the viscous–plastic and solid–plastic states is defined. Comparison of the calculated data versus experimental data is presented.     

3‐D numerical simulations on flow and mixing behaviors in gas–liquid–solid microchannels

Three‐dimensional (3‐D) gas‐liquid–solid flow and mixing behaviors in microchannels were simulated by coupled volume of fluid and discrete phase method and simulations were validated against observations. The detachment time and length of gas slug are shortened in liquid–solid flow, compared with those in liquid flow due to higher superficial viscosity of liquid–solid mixture, which will move the bubble formation toward the dripping regime. Solid particles mainly distribute in liquid slug and particle flow shows obvious periodicity. With the increase of contact angle of the inner wall, gas slug (0–50°), stratified (77–120°), and liquid drop (160°) flows are observed. The residence time distributions of solid and liquid phases are similar because particles behave as tracers. The backmixing of solid and liquid phases in liquid drop flow is the weakest among the three flow patterns, and the backmixing of gas phase in slug flow is weaker than that in both stratified and liquid drop flows. The results can provide a theoretical basis for the design of microreactors. © 2013 American Institute of Chemical Engineers AIChE J, 59: 1934–1951, 2013     

Investigation of capillary flow in discrete cracks in cementitious materials

A series of experimental studies are presented that simulate capillary flow of water in discrete cracks in cementitious materials. A number of amendments to existing capillary flow theory are adopted which take the form of correction parameters for stick–slip behaviour of the meniscus, frictional dissipation at the meniscus wall boundary and slip between the fluid and solid wall. A benchmark study to examine capillary flow in small diameter glass capillaries is reported and this provides data to validate the amended theoretical model. Predictions made using the amended model closely match the experimental results of capillary rise in discrete cracks in cementitious materials allowing the correction parameters for capillary flow in planar cracks to be determined. Finally, capillary rise in a discrete natural crack of known aperture is considered and a relationship is proposed which predicts the capillary rise response in a natural crack in terms of an equivalent planar crack.     

Inception and termination of the core‐annular flow pattern for oil‐water downflow through a vertical pipe

Different flow patterns for lube oil–water and for kerosene‐water downflow through a vertical glass tube have been analyzed with the help of flow visualization. Core‐annular flow is the dominant flow regime, with oil forming the core, and water is forming the wall film. When the velocities are increased, transition to slug flow and transition to dispersed flow are found. The waves found during the transition to slug flow depend on oil viscosity: axisymmetric bamboo waves are seen in kerosene‐water downflow and the waves are asymmetric in case of lube oil–water flow where they have a cork‐screw shape. Based on the experimental observations, simple mathematical models have been proposed for predicting the flow pattern transition curves. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Studies on the atomization and combustion performance of a twin-fluid acoustic atomizer

A twin-fluid air-assist atomizer coupled with a Hartmann type stem and cavity air-jet acoustic generator has been used to atomize water, kerosene, diesel oil and furnace oil. The effects of atomizing air pressure, air to liquid mass ratio and liquid viscosity on the mean drop size of the spray have been investigated. From the experimental results a suitable correlation has been obtained to estimate the Sauter mean diameter (SMD) of such sprays. The influence of resonating cavity on the spray pattern and lean blow-off limits for the combustion of kerosene has also been studied. Burners incorporating this type of twin-fluid acoustic atomizer are found to be particularly suitable for highly viscous oils.     

Estimation of total aromatics in kerosene fractions by infrared spectroscopy: I

A faster and accurate infrared method for estimating total aromatics in petroleum samples boiling in the kerosene range (140–250°C) was developed using pure aromatic concentrate from kerosene as reference material. The limitations of a synthetic mixture of pure compounds used as reference are also discussed. The accuracy of results is found to be dependent on the closeness of the average absorptivity at a characteristic wave number (1600 cm^?1) of an unknown sample and kerosene used as reference. The results obtained from the proposed method on kerosene fractions of Bombay High are compared, as an illustration, with those obtained by FIA.     

Effects of liquid accelerant on the pyrolysis behaviors and kinetics of PMMA,a noncharring polymer

Thermal decomposition of solid combustibles infiltrated with combustible liquids (liquid accelerants) is one typical fire behavior in arsons. In the present study, thermogravimetric and kinetic analyses are used to investigate the pyrolysis of one typical noncharring polymer namely polymethyl methacrylate (PMMA) infiltrated with different contents of one representative liquid accelerant namely kerosene. Results indicate that the thermal decomposition process of pure PMMA and PMMA infiltrated with kerosene show a one-stage and a three-stage process, respectively. Furthermore, the first and third stages for PMMA infiltrated with kerosene can be both regarded as one-step reaction. With the increase of kerosene content, the conversion rate ranges of the first and third stages for PMMA infiltrated with kerosene are shortened. The peak and average reaction rates of the first and second stages increase, while the peak and average reaction rates of the third stage decrease. The maximum and average reaction rates for the entire decomposition process also decrease. The average activation energy values for the whole and one certain stage both increase. Moreover, the calculated kinetic parameters can be used to accurately predict the thermal decomposition behavior of PMMA infiltrated with kerosene.     

Effects of discrete source-sink arrangements on mixed convection in a square cavity filled by nanofluid

Laminar mixed convection of Al₂O₃/water nanofluid flow in a cavity in which the upper wall is moving from right to left has been studied numerically. Fifteen different arrangements of two discrete sources and four discrete sinks have been considered. This work shows when one source is located at the right side of the bottom wall and other one at the down half of the left wall, total heat transfer achieves its maximum value. The lowest heat transfer rate is achieved when more than two vortexes are created in the cavity (case 13 for Ri=1 and case 5 for Ri=100). In general, for cases with one overall vortex, the cavities which have separate sources induce better cooling and have higher Nu number.