凤凰山矿采空区自燃“三带”分布规律研究

根据凤凰山矿151309工作面现场数据,运用Fluent软件模拟了该工作面采空区在不同通风量情况下三带的分布情况;根据采空区自燃"三带"理论及其划分主指标,结合该工作面现场"三带"的观测数据,在配风量为1 300 m3/min时,分析了采空区O2浓度的变化规律,确定了采空区三带的分布情况,与模拟结果基本一致,验证了模拟结果的正确性,为后期采空区自燃的防治提供了依据。     

Influence of scalar dissipation on flame success in turbulent sprays with spark ignition

A Direct Numerical Simulation (DNS) study is performed to determine a quantitative indicator of imminent global extinction in spray flames ignited by a spark. The cases under consideration have Group Combustion numbers sufficiently small that each droplet has an individual flame form around it, which subsequently merge. The structure of the flames is examined, including identification of non-premixed behaviour in the core of the flame and premixed flame fronts except in the presence of droplets, which cause strong non-premixed behaviour. The reaction progress variable c is studied and its dissipation rate is identified as being a key indicator of whether a flame will globally extinguish after being ignited by the spark. Specifically, immediately after the spark is deactivated, the volume containing the end of the flame front and hot products is studied in detail with respect to c. For successful flames, it is observed that regions of zero dissipation of c   were predominantly restricted to the highest reaction progress variable (c>0.98)$(c>0.98)$, with zero probability within the range 0.95<c<0.98$0.95<c<0.98$ and low probability within 0.9<c<0.95$0.9<c<0.95$. In contrast, cases which subsequently extinguished had substantial probability of zero dissipation for 0.95<c<0.98$0.95<c<0.98$. This region was a secondary structure separate from the main flame kernel that was unable to evaporate sufficient liquid to create a self-sustaining flame and therefore contributed to the subsequent quenching of the flame. In the successfully-burning case under consideration, this region was part of the main flame structure. The low reaction rate contributed to a thickened flame structure near the hot core, which reduced the heat transfer to the flame front and prevented effective evaporation and preheating of the fluid ahead of the flame front. Calculation of the conditional probability of c   for its dissipation rate being zero could provide a quantitative measure to determine whether a flame is likely to extinguish within a relatively short timeframe. This is equivalent to detecting that, for every value of 0.9<c<1$0.9<c<1$, there are volumes of significant size where the value of c   is uniform. Note that a successful flame must have a volume of substantial size with c=1$c=1$. From a practical perspective, if each individual flame kernel is monitored, then extinction is imminent if secondary structures of incomplete reactions are present when the spark ceases adding energy.     

Gasification of heavy fuel oils: A thermochemical equilibrium approach

Combustion of heavy fuel oils is a major source of production of particulate emissions and ash, as well as considerable volumes of SO_x and NO_x. Gasification is a technologically advanced and environmentally friendly process of disposing heavy fuel oils by converting them into clean combustible gas products. Thermochemical equilibrium modeling is the basis of an original numerical method implemented in this study to predict the performance of a heavy fuel oil gasifier. The model combines both the chemical and thermodynamic equilibriums of the global gasification reaction in order to predict the final syngas species distribution. Having obtained the composition of the produced syngas, various characteristics of the gasification process can be determined; they include the H₂:CO ratio, process temperature, and heating value of the produced syngas, as well as the cold gas efficiency and carbon conversion efficiency of the process. The influence of the equivalence ratio, oxygen enrichment (the amount of oxygen available in the gasification agent), and pressure on the gasification characteristics is analyzed. The results of simulations are compared with reported experimental measurements through which the numerical model is validated. The detailed investigation performed in the course of this study reveals that the heavy oil gasification is a feasible process that can be utilized to generate a syngas for various industrial applications.     

ESTIMATE OF DISCRETE NONLINEARITIES IN A MAINLY LINEAR DYNAMIC SYSTEM

0 INTRODUCTIONMany mechanical systems and structures aremainly linear but contain discrete non-linearities(such as nonlinear shock absorbers, friction inmechanical joints or cracks in shafts). ln some casesthe location of the non-linearity is known, but inmany others the non-linearity develops after thesystem or structure has spent some time in service andits lOcation is not known. As a result, the research onthe location of the discrete non-linearities in mainlylinear systems and structur…     

A vortex model for Darrieus turbine using finite element techniques

Since 1970 several aerodynamic prediction models have been formulated for the Darrieus turbine. We can identify two families of models: stream-tube and vortex. The former needs much less computation time but the latter is more accurate. The purpose of this paper is to show a new option for modelling the aerodynamic behaviour of Darrieus turbines. The idea is to combine a classic free vortex model with a finite element analysis of the flow in the surroundings of the blades. This avoids some of the remaining deficiencies in classic vortex models. The agreement between analysis and experiment when predicting instantaneous blade forces and near wake flow behind the rotor is better than the one obtained in previous models.     

The effects of air and particle density difference on segregation of powder mixtures during die filling

Segregation of mono-disperse binary mixtures with different particle densities during die filling in the presence of air was numerically analysed using a coupled discrete element method (DEM) and computational fluid dynamics (CFD) approach. Die filling with powders of different particle density ratios (i.e. the ratio of the heavy particles to the light particles) at various shoe speeds was simulated, in order to explore the effects of air and particle density difference on segregation. For die filling from a stationary shoe, the air can induce significant segregation by hindering the deposition of light particles (i.e., air-sensitive particles). As the particle density ratio increases, the light particles are deposited into the die at even lower speeds compared with the heavy ones due to the effect of air drag, resulting in an increase in the degree of segregation. For die filling with a moving shoe, segregation occurs due to different post-collisional velocities resulting from different particle inertia; and the degree of segregation increases as the particle density ratio increases due to the increasing difference in particle inertia. It is found that, as the shoe velocity increases, the powder flow pattern changes from nose flow dominated to bulk flow dominated and the degree of segregation generally decreases. The effect of air is limited for nose flow dominated die filling because the air can easily evacuate through the gap between the die walls and flowing powder stream. When bulk flow dominates in die filling, the air can be entrapped in the die, which has a significant impact on the powder flow and segregation behaviours. Finally, the effect of interparticle friction on segregation was investigated.     

Computational Simulation for Transport of Priority Organic Pollutants through Nanoporous Membranes

Modeling and simulation of membrane‐based solvent extraction is conducted by computational fluid dynamics (CFD). The process is used for removal of priority organic pollutants from aqueous waste streams in nanoporous membranes. The pollutants include phenol, nitrobenzene, and acrylonitrile extracted by organic solvents. The mathematical model commonly applied to predict the performance of membrane‐based solvent extraction is the conventional resistance‐in‐series model. Here, a comprehensive mathematical model is developed to predict the transport of pollutants through nanoporous media. In order to predict the performance of the separation process, conservation equations for pollutants in the membrane module are derived and solved numerically. The model is then validated through comparing with experimental data reported in the literature. The simulation results were in good agreement with the experimental data for different values of feed flow rates.     

Evaluation of friction condition in cold forging by using T-shape compression test

Friction plays an important role in metal forming, and numerical simulation of forging processes requires precise informations about the material properties and the value of the friction factor m or coefficient μ. This paper describes the T-shape compression, a new friction testing method by combined compression and extrusion of a cylinder between a flat punch and a V-grooved die. It can realize actual cold forging condition and allows measuring the friction on the cylindrical surface of the billet during forging process. The results of experiments and simulations show that the stroke–load curve and the height of the extruded part are both sensitive to friction. In order to obtain the highest sensitivity to friction, a FE parametric study of this test has been performed: it indicates that small corner radius and V-groove angle in the die should be chosen. Two commercial FE codes, FORGE 3D and ABAQUS, were used and provided very similar results for a given friction condition. Low carbon steel drawn bar with phosphate and soap coating was chosen as specimens. Friction tests with three different lubrication conditions (solid coating, oil and oil + solid coating) were carried out, and then friction factor m and friction coefficient μ were determined by using experimental results and the calibration by numerical simulation of T-shape compression test.     

Flow streamline based Navier-Stokes Characteristic Boundary Conditions: Modeling for transverse and corner outflows

Some limitations of three dimensional Navier-Stokes Characteristic Boundary Conditions (3D-NSCBC) are discussed for flows traveling in a direction that is oblique to the boundary. To limit errors generated at boundaries with flows having any arbitrary direction, it is proposed to organize the wave decomposition in a coordinate system that is attached to the local flow streamline crossing the boundary, because some modeled expressions are not frame independent. Compared to previous 3D-NSCBC, the modified strategy accounting for oblique waves is found to improve the outflow treatment for transverse outgoing vortices, up to vortices crossing an outflow corner. The method is also applied to an expanding laminar flame.     

Thermofluids Considerations and the Dynamic Behavior of a Finger Seal Assembly

Finger seals represent a compliant seal configuration. What differentiates and makes them preferable to the brush seals is their potential hydrodynamic lifting capabilities, and thus their noncontacting nature. The fingers' compliance allows both axial and radial adjustment to rotor excursions without damage to the integrity of the seal. The work to be presented here concerns the mapping of the thermofluid and dynamic behavior of a repetitive section of the newly proposed design of a two-layer finger seal. The assembly contains four high-pressure and four low-pressure fingers arranged axially in a staggered configuration and subject to an axial pressure drop. The numerical three-dimensional temperature and pressure results were obtained using a customized Navier-Stokes–based commercial package, CFD-ACE+. The results were obtained in a parametric fashion where the high-pressure side, the speed of rotation, and the heat transfer coefficient are the controlling parameters; the gas compressibility and the viscosity are also considered in the model of the thermofluids seal behavior. The stiffness and damping characteristics of the padded/unpadded fingers and the fluid were obtained through numerical simulation and were then used to model the interaction with the motion of the shaft. It is shown that the proposed geometry provides satisfactory lifting capability for the fingers. The fingers follow the motion of the shaft and their stiffness is small when compared to that of the fluid; thus, the displacement transmissibility is in most cases close to 1. Lifting forces and seal leakages, as well as the interaction between the profiled backplate and the low-pressure fingers through Coulomb damping/friction forces, are also parametrically studied.