Study of the jet flow field of vacuum spray process

In this work, we study the velocity and turbulence distribution of the cone nozzle jet flow field into a vacuum chamber under different nozzle diameters and injection pressures. The simulations are carried out with using the software based on the volume of fluid method, the standard k-ε turbulence model and pressure implicit with splitting of operator arithmetic. It is found that for injection pressure lower than 10 MPa, the spray penetration increases with the increase of injection pressure. However, for injection pressure higher than 10 MPa, the spray penetration tends to reduce progressively. Moreover, the distribution of velocity and turbulence become worse than that of the spray jets with injection pressure lower than 10 MPa. The potential core, spray penetration and diffusion zone of the spray jet increase with the increase of the nozzle diameter. Furthermore, without considering the solubility of the target polymer materials, acetone solvent shows better jet flow distribution in comparison with that of chloroform and carbon tetrachloride.     

一种新型燃用煤粉工业锅炉燃烧器流场特性研究

针对一种新型的燃用煤粉工业锅炉燃烧器,利用计算流体力学软件,通过改变一、二次风速及内二次风旋流强度以及钝体流通阻塞率等参数,模拟燃烧器出口附近流场分布,研究不同参数下回流区大小及回流量的变化,分析流场的湍流强度分布.结果表明,回流区长度与内二次风速、内二次风旋流强度及钝体阻塞率成正比,与一次风速成反比.回流量总体上随着内二次风旋流强度、内二次风速、一次风速的增加而增大;改变钝体阻塞率,则有较复杂的变化.湍流强度沿径向呈先增后减的变化,沿轴向则在某个截面后不断衰减.     

微油点火燃烧器一级燃烧室点火性能试验研究

试验研究了不同一次风速和煤粉质量分数条件下微油点火燃烧器一级燃烧室的点火性能.研究结果表明,燃烧室中心温度随一次风速和煤粉质量分数的增加,呈先降低后增加的趋势;燃烧室中心线上的CO体积分数随一次风速的增加而增大,但随煤粉质量分数的增加而减小;燃烧室出口中心处的NOx体积分数随一次风速和煤粉浓度的增加则均呈现上升趋势.     

1913t／h超临界四角切圆锅炉冷态空气动力场的试验研究

为了解某电厂1913t／h超临界四角切圆锅炉大修后炉内空气动力场的分布特性,进行了锅炉冷态空气动力场试验。对6台磨煤机的一次风挡板特性及两侧二次风道挡板特性进行了测量,并对一、二次风风速进行了调平,对贴壁风速及水平烟道风速进行了测量。试验结果表明,一、二次风挡板特性较好,经调整各粉管及燃烧器喷口风速均匀,贴壁风速及水平烟道风速合理。     

Devolatilization of oil sludge in a lab-scale bubbling fluidized bed

Devolatilization of oil sludge pellets was investigated in nitrogen and air atmosphere in a lab-scale bubbling fluidized bed (BFB). Devolatilization times were measured by the degree of completion of the evolution of the volatiles for individual oil sludge pellets in the 5-15 mm diameter range. The influences of pellet size, bed temperature and superficial fluidization velocity on devolatilization time were evaluated. The variation of devolatilization time with particle diameter was expressed by the correlation, τ(d) = Ad(p)(N). The devolatilization time to pellet diameter curve shows nearly a linear increase in nitrogen, whereas an exponential increase in air. No noticeable effect of superficial fluidization velocity on devolatilization time in air atmosphere was observed. The behavior of the sludge pellets in the BFB was also focused during combustion experiments, primary fragmentation (a micro-explosive combustion phenomenon) was observed for bigger pellets (>10mm) at high bed temperatures (>700 °C), which occurred towards the end of combustion and remarkably reduce the devolatilization time of the oil sludge pellet. The size analysis of bed materials and fly ash showed that entire ash particle was entrained or elutriated out of the BFB furnace due to the fragile structure of oil sludge ash particles.     

Optimization of the process of plasma ignition of coal

Results are given of experimental and theoretical investigations of plasma ignition of coal as a result of its thermochemical preparation in application to the processes of firing up a boiler and stabilizing the flame combustion. The experimental test bed with a commercial-scale burner is used for determining the conditions of plasma ignition of low-reactivity high-ash anthracite depending on the concentration of coal in the air mixture and velocity of the latter. The calculations produce an equation (important from the standpoint of practical applications) for determining the energy expenditure for plasma ignition of coal depending on the basic process parameters. The tests reveal the difficulties arising in firing up a boiler with direct delivery of pulverized coal from the mill to furnace. A scheme is suggested, which enables one to reduce the energy expenditure for ignition of coal and improve the reliability of the process of firing up such a boiler. Results are given of calculation of plasma thermochemical preparation of coal under conditions of lower concentration of oxygen in the air mixture.     

Gas-particle turbulent flow of foursquare tangential jets in a simplified pulverized-coal firing boiler

An experimental cold-model of a simplified tangential firing boiler was established to investigate the mesoscale turbulent flow behaviors, including gas vortex structures, particle motions and interactions between two phases. A modified PIV technology, employing two pairs of lasers and cameras, was applied to measure the velocity and velocity gradient of turbulent flow in foursquare tangential jets alternatively. At a given initial gas velocity and particle mass loading, the interaction between gas and particles was studied at three different particle sizes. It was found that two main coherent vortex structures, circular eddy and hairpin eddy, distributed mainly in low speed area and heavy impingement area, respectively. The characteristics of particle motion in foursquare tangential jets correlated with gas turbulence dissipation, particle size, particle concentration and particle density. Small particles were easily entrained by gas vortex, so that they consumed more turbulence energy and attenuated the gas turbulence intensity. On the contrary, large particles had more inertia and led to heavier impingement in the chamber center, resulting in particle random distribution and complex momentum transfer between gas and particles. Moreover, large particles stretched the coherent vortex to be narrow and long, while small particles pulled down the vortices rotation intensity.     

CFD simulation and optimization of the flow field in horizontal turbo air classifiers

This paper mainly presents a numerical study of the gas flow in horizontal turbo air classifiers. The effect of the air-inlet direction on the performance of classifier was also investigated through powder classification experiments. The simulated results show that the vertical vortex is the dominant flow in conical part of the classifier and there exists the horizontal vortex in the classifying chamber. The tangential velocity profile resembles a Rankine vortex inside the rotor cage. The vertical vortex intensity increases with increasing the inlet air velocity, while the rotor cage speed has limited effect on the control of gas pathlines in the classifying chamber. Horizontal turbo air classifiers are divided into four quadrants according to the air-inlet direction. For classifiers in quadrants I and III, a double-layer flow with opposite directions generates around the rotor cage which causes a secondary vortex. The secondary vortex is eliminated and the airflow becomes uniform in the classifier that belongs to quadrant Π or IV. The experimental results with fluidized catalytic cracking catalysts and fly ash demonstrate that cut sizes of this classifier decrease averagely by 5 μm and 2.2 μm respectively, and the classification accuracy increases by 7.5–10.3%.     

Fluid and Particle Laser Doppler Velocity Measurements and Mass Transfer Predictions for the Usp Paddle Method Dissolution Apparatus

This research was motivated by the lack of experimental data concerning the complex flow fields produced by the USP Paddle Method Dissolution Test Apparatus, which influence the reproducibility and sensitivity of the resulting dissolution data.

A one-component He-Ne fiber optics laser Doppler anemometer and a conditional sampling computer data acquisition provided unique three-dimensional fluid velocity measurements in most regions of the fluid inside the vessel. Tangential velocities which are predominant in magnitude decrease as a function of distance from the paddle to the liquid surface. Low magnitude circulation patterns in the axial direction exist with little Interaction between the fluid above and below the paddle. The radial velocity component also exhibits a low magnitude. In the vicinity of the paddle, a periodic fluid motion produced by the wake formed behind the paddle exists. Close to the bottom of the vessel, tangential fluid velocities were approximately equal to solid body rotation created by the paddle, and make possible predictions of mass transfer rates from a non-disintegrating calibrator resting at the bottom of the vessel.

The motion of drug particles from a dissolving tablet was simulated by using 216 /im diameter polystyrene latex spheres. They provided the Stokes number matching an average drug particle diameter of 190 /im. Tangential particle velocity measurements taken at selected locations using two different paddle speeds (50 and 60 RPM) revealed that the particles closely followed the fluid. A comparison between the fluid and the particle tangential velocities at 50 and 60 RPM showed that for 60 RPM the particle to fluid relative velocity above the paddle increased by about 100%, with the mass transfer rate in that region predicted to be enhanced by 37%.     

灰斗抽气对旋风分离器分离性能影响数值模拟研究

 利用计算流体动力学软件FLUENT对旋风分离器内气固两相流动特性进行三维数值模拟,模拟气相流场采用雷诺应力模型,应用随机轨道模型模拟湍流流场中颗粒的运动轨迹,同时给出了不同抽气率下旋风分离器的速度、压力分布,计算出旋风器分级效率,模拟结果与文献实验数据吻合较好.结果表明,灰斗抽气可以提高锥体内旋转气流切向速度,轴向速度减少能够降低气流携带颗粒返混能力,并减小排气芯管下口短路流,提高旋风分离器分离效率.对于给定的旋风分离器,抽气率应有一最优值.     

Study of flows in a cyclone chamber

Parameters of the vortex air flow in an isothermal model of the cyclone chamber are studied. The effect of the air velocity in nozzles on the values and character of the distribution of pressure and tangential and longitudinal velocities of air in the chamber is determined. Numerical modeling of the flow is performed, and it is shown that numerical calculation by the k-w turbulence model is in agreement with experimental data.     

考虑压缩空气边界的爆炸箔起爆器飞片运动模型

为了研究爆炸箔起爆器中的飞片运动规律,对爆炸桥箔蒸气驱动飞片的过程机理进行了研究。在假设爆炸箔电爆炸后形成的蒸气均匀膨胀以及飞片进行一维刚体运动的基础上,考虑桥箔蒸气内部的压力梯度,引入了压缩空气边界条件,进行飞片运动速度的计算,得到特定发火电路中以桥箔长度、桥箔厚度、飞片厚度以及发火电压为自变量的飞片运动速度模型。根据实测飞片速度的PDV(光子多普勒测速仪)测试结果,引入能量利用率对飞片运动速度曲线进行修正,并且拟合得到了能量利用率关于上述4种自变量的经验公式。结果表明：电爆炸推动飞片运动过程中,能量利用率与桥箔厚度和飞片厚度正相关,而与桥箔长度和发火电压负相关;初期,桥箔蒸气内部具有明显的压力梯度,最大压力可达10 GPa数量级;压缩空气段长度随着时间由0逐渐增大;在桥箔长度与加速膛厚度之比为0.4¹.2、桥箔厚度与加速膛厚度之比为0.002⁰.010、飞片厚度与加速膛厚度之比为0.025⁰.160的范围内,减小桥箔长度、桥箔厚度以及飞片厚度对提高加速膛出口飞片速度、降低爆炸箔起爆器的发火能量具有积极的作用。     

The Damping Coefficient of a Pressure Wave under Conditions of Pulsating Turbulent Flow of Gas in a Channel

The available results of experimental and prediction studies of the damping coefficient and phase propagation velocity of waves under conditions of pulsating turbulent flow in a narrow channel are reviewed and analyzed. It is demonstrated that the concept of complex damping coefficient may be introduced strictly on condition of certain restrictions imposed on an oscillating and time average flow. The dependences of the complex damping coefficient on the oscillation frequency and on the Reynolds and Mach numbers of time average flow are analyzed. The calculations are performed using the data on the relative amplitude and phase of oscillation of the tangential wall stress, obtained with the aid of the turbulence model including the relaxation equations for turbulent viscosity and tangential stress. It is demonstrated that quasi-stationary models of turbulence are invalid in the region of relatively high frequencies. Numerical simulation based on the difference solution of a set of channel cross section-averaged equations of motion, continuity, and energy is performed with due regard for the experimental conditions and measuring techniques. The calculation results agree well with the experimental data.     

Transition of combustion into detonation within a channel with the diameter less than the critical diameter of the existence of stationary detonation

An experimental investigation was carried out for transition of combustion into detonation of oxygen-hydrogen and hydrogen-air stoichiometric mixtures within a cylindrical channel with a diameter of 3 mm, which is less than the critical diameter of the existence of stationary detonation in the hydrogen-air mixture. To realize the transition of combustion into detonation in the channel, the combustible mixture was ignited within a precombustion chamber with a diameter of 14.5 mm which was arranged in line with the channel on one of its faces. The expanding products of mixture combustion within the precombustion chamber accelerate the flame front at the input of the channel, thus increasing the energy amount released during mixture combustion within the narrow channel per unit time. As a result, the ratio of the burning energy to dissipative losses in channel walls increases, which makes transition into detonation realizable. The use of the precombustion chamber allowed us to obtain the transition of combustion into the detonation wave with the Chapman-Jouguet parameters at initial mixture pressures of 1 and 2 atm. The effect of the size of the turbulizing precombustion chamber on the length of the transition of combustion into detonation is analyzed, and the average velocities of flame front propagation along the channel are determined.     

A decrease in the effective cross section of a cooling stream caused by hot air penetration into the blowout channel

The phenomenon of penetration of a hot air stream into a channel of finite depth by which a cooling stream is injected was revealed and studied by numerical methods. The penetrating stream decreases the effective cross section of the cooling stream, increases the velocity of this stream, and reduces the near-wall gas screen cooling efficacy.     

The influence of initial stream turbulence on the characteristics of an axisymmetric immersed jet

Results are given of measurement of mean velocity and of velocity fluctuations in an axisymmetric immersed jet at various values of initial turbulence. It is shown that as the initial stream turbulence increases, the length of the entrance section decreases, and that the entrance section practically disappears in a strongly turbulent jet.     

Special features of heat transfer under conditions of heat protection of high-temperature firing walls of components of modern power-generating units by way of tangential injection

In order to more precisely define the characteristics of heat transfer under conditions of protection of firing wall by means of tangential injection in the case of its high temperature (in particular, higher-thanadiabatic temperature) and to assess the effect of degree of turbulence of the incoming gas flow on heat transfer, a numerical investigation is performed under conditions of parameters typical of combustors of gas-turbine plants (GTP) with high parameters of the working medium. In so doing, the heat flux distribution, the profiles of turbulence intensity, the distribution of turbulent viscosity in the injection zone region under study, and other characteristics are determined. The low-Reynolds k-ε model with wall functions and a new model of turbulent viscosity without wall functions are employed. It is found that a maximum of turbulent viscosity takes place behind the exit section of the injection slit with a shift to the main flow under conditions of tangential injection on an isothermal surface with a temperature much in excess of injection temperature (in a more general case, T _w > T _ad). This causes impairment of heat protection by injection, i.e., an increase in heat fluxes in the computational domain compared to heat fluxes calculated using integral methods.     

Reverse impact experiments against tungsten rods and results for aluminum penetration between 1.5 and 4.2 km/s

Reverse impact experiments against 0.76 mm diameter L/D = 20 tungsten rods have been conducted with a 38 mm diameter launch tube, two-stage light-gas gun using four 450 kV flash X-rays to measure penetration rates. Techniques for projectile construction, sample placement, alignment, and radiography are described. Data for penetration rate, consumption velocity, and total penetration were obtained for 28 mm diameter 6061-T651 aluminum cylinders at impact velocities between 1.5 and 4.2 km/s. It was found that penetration velocity was a linear function of impact velocity over this velocity range. Above 2 km/s impact velocity, penetration was completely hydrodynamic. There was substantial secondary penetration, and the total penetration increased linearly with impact velocity over the range 1.5 to 2.5 km/s.     

Influences of inlet height and velocity on main performances in the cyclone separator

Aiming at improving collection efficiency in the cyclone separator, the effects of inlet height and velocity on tangential velocity, static pressure and collection efficiency were studied. A three-dimensional model including gas-flow, and particle-dynamics fields was built by the Reynolds stress turbulence model, and the numerical simulation was achieved using the FLUENT software. The static pressure distribution, tangential velocity distribution, and particle trajectory of the cyclone were obtained, and the variation law of the collection efficiency with inlet height and velocity as well as particle diameter was analyzed. Numerical results indicate that both the static pressure and the tangential velocity in the cyclone basically present the axial symmetrical distribution, the static pressure shows a nonlinear increasing trend in the radius direction and the distribution of the tangential velocity is in the shape of a “hump.” The increase of inlet height in a certain range reduces the rotation numbers of particles in the cyclone and shortens the residence time, which results in the improvement of trapping performance. Furthermore, the appropriately increasing inlet velocity in a reasonable range can make the collection efficiency increased.     

Effects of Exit Tube Diameter on the Flow Field in Cyclones

A number of cyclones with different exit tube diameters have been simulated with CFD in this study. Results show that the exit tube diameter influences not only the velocity magnitude, but also the shape of the velocity profiles within cyclones. Depending on the diameter of the exit tube, the axial velocity profiles can exhibit a either maximum or a minimum on the axis. If the exit tube diameter is small, the central flow has a jet-like appearance. On the other hand, axial velocity dip in the profile can be observed near the center in a cyclone with a large gas exit tube. In addition, the well-known double vortexes, which commonly are present in a cyclone of practical design, do not exist in a cyclone with an excessively large exit tube. Quantitative comparison of velocity distribution shows that the tangential velocity increases as the exit tube diameter is reduced, giving rise to higher particle collection efficiency. Usually, the pressure drop decreases with increasing exit tube diameter. However, if the exit tube size is excessively large, the pressure drop may start to increase. Practically, cyclone with an excessively large exit tube should be avoided.