Generalized solutions for a dead-end duct having a permeable barrier

This paper presents an analytical method for calculating the flow and pressure in A dead-end duct which has a perforated, porous barrier and an auxiliary fan or an obstacle at the end of the duct. The solutions involve two infinite series which can be applied to a general class of differential equations. It is shown that the duct length may be bounded if the barrier is porous only. No such bound exists if there are holes in the barrier. There is a minimum in the flowrate through the duct when there is a fan at the dead end. No such minimum exists when there is an obstacle     

Turbulent deflagrations, autoignitions, and detonations

Measurements of turbulent burning velocities in fan-stirred explosion bombs show an initial linear increase with the fan speed and RMS turbulent velocity. The line then bends over to form a plateau of high values around the maximum attainable burning velocity. A further increase in fan speed leads to the eventual complete quenching of the flame due to increasing localised extinctions because of the flame stretch rate. The greater the Markstein number, the more readily does flame quenching occur. Flame propagation along a duct closed at one end, with and without baffles to increase the turbulence, is subjected to a one-dimensional analysis. The flame, initiated at the closed end of the long duct, accelerates by the turbulent feedback mechanism, creating a shock wave ahead of it, until the maximum turbulent burning velocity for the mixture is attained. With the confining walls, the mixture is compressed between the flame and the shock plane up to the point where it might autoignite. This can be followed by a deflagration to detonation transition. The maximum shock intensity occurs with the maximum attainable turbulent burning velocity, and this defines the limit for autoignition of the mixture. For more reactive mixtures, autoignition can occur at turbulent burning velocities that are less than the maximum attainable one. Autoignition can be followed by quasi-detonation or fully developed detonation. The stability of ensuing detonations is discussed, along with the conditions that may lead to their extinction.     

Measurement of Turbulent Mixing in a Confined Wake Flow Using Combined PIV and PLIF

Turbulent mixing in a confined wake flow was studied by using the combined PIV/PLIF technique to measure instantaneous concentration and velocity fields. Measurements were performed at two slightly overlapping areas in the initial mixing zone and at an area at the end of the channel. The measurements revealed that there is a recirculation zone right behind the block that gives an increased mixing effect in this area. The measurement at the end of the channel shows that the concentration distribution is quite uniform, but the two streams are not completely mixed.     

主副螺纹型螺杆的改进与参数分析

对主副螺纹型螺杆进行了改进,改进措施是在副螺纹的末端加工出几条窄槽,使粘流状态的胶料顺利的向挤出段流动,其目的有两个,一是为了解决主副螺纹型螺杆的滞料问题,二是强化剪切混合效果,缩短螺杆长度。同时,对改进后的螺杆进行了参数分析,推导出了剪切混合段的长度,对实际工程设计及应用具有一定的参考价值。     

高速煤粉燃烧器内燃烧特性数值模拟及结构优化

高速煤粉燃烧器火焰喷射速度高达60~200 m/s,炉膛内火焰较长,对流换热比例提高,使得炉膛内温度分布均匀,没有传统低速煤粉燃烧器火焰短,炉膛内局部过热和结焦等缺点。笔者以14 MW高速煤粉燃烧器为研究对象,采用数值模拟的方法,研究旋流强度、二次风温度等关键参数对燃烧器内煤粉燃烧的影响,针对燃烧器内煤粉燃烧特点进行结构优化设计。对旋流强度研究结果表明,当旋流强度S=2.2、2.8、3.2及3.7时,燃烧器内回流区形状变化不大,从一次风喷口开始到旋流叶片位置结束,回流区环绕一次风管;最大回流量在一次风喷口附近,距离一次风喷口越远,回流量越小;旋流强度对一次风喷口附近最大回流量影响不大,喷口附近最大回流量均在0.45 kg/s左右,当距喷口超过一定距离(L/H<0.35)时,旋流强度对回流量的影响开始变得明显,表现为旋流强度越大,回流区末端回流量越大,回流区末端回流量最大为0.30 kg/s,最小为0.17 kg/s。研究燃烧器喷口处燃烧状态表明,喷口处火焰旋流强度为0.10~0.28,与入口旋流强度正相关,火焰喷射速度150 m/s,为中等旋流强度的高速旋流火焰;喷口中心区可燃性组分富集,缺氧,燃料和氧气分层分布。当旋流强度提高,喷口中心区可燃性组分浓度降低,CO浓度从11%降低到10%,H2浓度从1.65%降低到1.40%,焦炭浓度从0. 14%降低到0. 11%,喷口边缘O2浓度从13%降低到10%。旋流强度S=3.2和S=3.7时可燃组分和氧气浓度分布变化较小,说明旋流强度提高对燃烧的影响减弱。考察0、100和200℃下二次风温度对燃烧的影响,结果表明,当二次风温度提高,煤粉在燃烧器内的反应时间有所降低,从0.15 s降低到0.11 s,但燃烧器内的煤粉碳转化率提高20%,达到65%。对燃烧器结构进行优化,加入中心风,对比中心风直流和旋流与不加中心风3种状态,结果表明,加入旋流中心风和直流中心风后喷口中心区半径r≤75 mm范围内可燃组分浓度降低,采用直流时由于气流刚性较强,喷口中心区氧气浓度升高,采用旋流中心风对中心区氧浓度影响弱,对可燃组分浓度降低效果优于直流中心风。     

CONFINED COAXIAL JET FLOWS INTO A COLD MODEL OF CVD CHAMBER

The length of recirculation zone is an important factor in the design of CVD chambers for manufacturing monolithic infrared optical materials. The recirculation length of confined coaxial air jet flows, which consist of a central tubular flow and a surrounding annular flow, into a cold model of CVD chamber at atmospheric pressure and room temperature were studied numerically by a two-dimensional Galerkin finite element method and experimentally by observing the streamlines of methylamino chloride particles. In the experiments, the tubular Reynolds number varies from 180 to 600 and the annular Reynolds number varies from 37.0 to 165.1. The simulation and experimental results show that the length of recirculation zone increased initially with the increase of either tubular or annular Reynolds number. Nevertheless, when either one of the tubular and annular Reynolds numbers was relatively high, or both Reynolds numbers were at relatively medium values, small waves occurred, laminar flow could not be maintained, and the observed recirculation length dropped dramatically from 10.4-15.7 Dh to 3.7-4.6 D,, where Dk is the hydraulic diameter of the chamber. Suggestions of the flow conditions and chamber length for the design of CVD chambers are given to make most of the chamber length occupied by the recirculation zone so that an uniform deposition of materials could be obtained.     

Extensional stresses during polymer flow in ducts

During the flow of high molecular weight, narrow-, and broad-distribution polybutadienes and polyisoprenes rheo-optical measurements were conducted of extensional stresses acting along the flow axis in the preentrance and entrance regions of the duct and of their subsequent relaxation in the duct. The extensional stresses increase in the preentrance region, reach their maximum values at a distance of two or three tenths of the duct width from its edges, and then relax. The position of the maximum extensional stress is independent of polymer characteristics, shear stresses in the duct, and shape of the entrance and dimensions of the rectangular duct. The dependence of the maximum extensional stress on the shear stress of the duct wall can be assumed to be linear for small values. The length of the stress relaxation zone depends on the shear stress at the duct wall and the molecular mass distribution. It is independent of the molecular masses in narrow-distribution polymers. For the polymers investigated, a generalized dependence was obtained for the reduced duct length over which the extensional stresses relax to zero from the reduced deformation rate. This dependence takes into account the characteristic polymer relaxation times and the value of the molecular mass of the chain between the fluctuation entanglement. A considerable decrease in the duct's length-to-width ratio leads to an increase in the maximum values of the extensional stresses. A decreases in the duct entrance angle causes a reduction in the rate of increase of extensional stresses, the maximum values, and the acceleration of the relaxation processes in the duct. A decrease in the ratio of the width of the preentrance region to the duct width leads to a reduction in the maximum in extensional stresses. It is shown that one of the causes for the instability of the polymer flow in the ducts can be the rupture of polymers due to their extension in the preentrance and entrance regions. Calculations were done that describe satisfactorily the relationship between the values of the maximum extensional stresses and the shear rate and stresses on the duct wall.     

爆发点测试仪排风装置设计

在爆发点测试实验中,伍德合金加热、炸药爆炸后会产生有毒、有害气体,如果不及时将这些气体排出实验室,将会对实验人员的身体健康产生危害。这就需要在爆发点测试仪上设计排风装置。设计的排风装置由排风罩、通风管道和轴流风机组成,并由排风罩计算出的排风量和通风管道计算出的风压选出风机型号。     

电站锅炉引风机常见故障分析及处理方法

引风机是电站锅炉的重要辅机之一。以电站锅炉引风机常见故障(振动、噪声)为研究对象,分析了振动、噪声故障的产生原因、表征参数和处理方法等,旨在为电厂运行人员迅速判断、及时处理引风机故障提供参考。     

Optimal organization of the technological system of coke production based on the information-thermodynamic principle. 2. Cogeneration and reduction of nitrogen-oxide emissions in a coke oven heating system with gas-flux recirculation

Means of reducing nitrogen-oxide emissions are considered for the system with gas-flux recirculation in a heating duct described in the first part of this work. On the basis of the formation kinetics of thermal nitrogen oxides—predominantly nitric oxide (NO)—the NO concentration in the gas fluxes may be determined as a function of the temperature and air-fuel equivalence ratio. A heating system with cogeneration is proposed. This system combines uniform coke heating over the furnace height (by recirculation) and power generation.     

A momentum integral model for central recirculation in swirling flow in a sudden expansion

A momentum integral model is developed to predict the size and maximum recirculating mass flow rate for the central recirculation zone produced by a swirling flow in a sudden expansion as a function of expansion ratio, swirl intensity, and inlet velocity profile shape. Experimental measurements of velocity profiles in the recirculating flow are presented for three different expansion ratios and are used to validate the model. The model is shown to give good agreement with the measurements. The recirculation rate is found to increase roughly linearly with inlet swirl intensity and expansion size.     

Pseudoshock combustion regime

The combustion of fuels flowing with supersonic velocity in a duct of constant cross section is analyzed. Experimental data on the pressure at the duct wall are used to calculate the heat-release rates by a onedimensional procedure that takes into account the specific characteristics of combustion in a pseudoshock. It is shown that the heat-release rate, averaged over the length of the combustion zone and normalized to the maximum possible rate, depends on the ratio of the length of the combustion zone to the pseudoshock length for the isothermal case in flow stagnation up to a Mach number M=1.0 and not on the fuel injection technique or the length of the duct. An approach is suggested for determining the duct geometry in the pseudoshock combustion regime so as to organize the combustion process efficiently as a function of the flow parameters and the physicochemical characteristics of the field, which can be determined from specially designed experiments. Institute of Theoretical and Applied Mechanics, Siberian Branch, Russian Academy of Sciences, 630090 Novosibirsk. Translated from Fizika Goreniya i Vzryva, Vol 29, No. 6, pp. 33–38, November–December, 1993.     

Numerical Representation of the Operating Behavior of a Crossflow Friction Turbomachine

Based on the Tesla‐type turbomachinery principle of operation, a fan consisting of flat, round discs arranged in between two separate channels, which generate two air flows in opposite directions, is investigated. Simulations of a model with one disc as well as a five‐channel model at different grids were performed. With almost unthrottled operation, secondary flows could be determined at velocity magnitudes of up to 20 % of the mean main flow velocity, with secondary currents reaching up to 50 % in throttled operation. Besides high dissipation and recirculation, these secondary currencies are found to be capable of reducing the overall efficiency of the system. Thus, topic of further investigations is the potential of increasing efficiency by means of straighteners and geometric adaptions.     

水冷PEMFC热管理系统控制策略及仿真研究

针对目前燃料电池热管理系统在变载时存在温度波动较大、调节时间较长和响应速度较慢等问题，本文提出了流量同时跟随电流及功率方式和神经网络自抗扰方法两种热管理控制策略。结果表明：流量同时跟随电流及功率控制策略能够有效地削弱水泵和散热器风扇的耦合作用，明显减少电堆进出口冷却水温度及其温差的超调量和调节时间。此外，虽然神经网络自抗扰控制策略在最大功率工况下的控制效果较差，但总体控制效果比流量跟随电流控制策略好。     

Numerical analysis of the flow field inside an entrained-flow gasifier

The flow field of an entrained-flow gasifier was numerically simulated to describe coal gasification process. The standard k-ε turbulence model and SIMPLE procedure were used with the Primitive-Variable method during computation. In order to investigate the influencing factors on the flow field that may have a great effect on coal gasification process, some parametric studies were performed by changing the gas injection angle, gas inlet diameter, gas inlet velocity, extension in burner length and gasifier geometry. The calculation results showed that the basic patterns of the flow field inside the gasifier were nearly the same with a parabolic distribution irrespective of the change in parameters. There existed an obvious external recirculation zone with axial length less than 1.0 m and a narrow internal recirculation region was observed in the entrance of gasifier inlet. The geometry parameters of the burner, such as the oxygen inlet diameter and angle, influenced the flow field at the inlet region near the burner. But after a certain length along the gasifier, the flow field was nearly the same as that in the basic case.     

Wall effects in flow past a circular cylinder in a plane channel: a numerical study

This paper describes a numerical study on the steady flow of an incompressible Newtonian fluid past a circular cylinder confined in a plane rectangular channel. Using FLUENT (version 6), two-dimensional steady state computations were carried out for an uniform inlet velocity and for different values of the Reynolds numbers in the range between 0.1 and 200 and blockage ratios (ratio of the channel width to the cylinder diameter) in the range between 1.54 and 20. The flow parameters such as drag coefficient, length of the recirculation zone, and the angle of separation are presented as functions of the Reynolds number and blockage ratio. The total drag coefficient (C_D) was found to decrease with an increase in the blockage ratio (λ) for a fixed value of the Reynolds number (Re) and to decrease with increasing Reynolds number for a fixed value of λ. Similarly, for a fixed value of λ, both the angle of separation and the length of the recirculation zone increase with the increasing Reynolds number.     

Flow regimes and vertical solids conveying in a spout‐fluid bed with a draft tube

Pressure fluctuation data were obtained in a semi‐cylindrical spout‐fluid bed with draft tube, and statistical analyses of them were employed to recognise flow regimes. Also, the effects of spouting‐gas and auxiliary‐gas velocities, and length of entrainment zone on solids loading ratio in a draft tube were examined. As a result, five flow regimes appeared by changing spouting‐gas and auxiliary‐gas velocities and it was found that the simplest method to determine flow regime is to measure pressure fluctuations in a draft tube. Moreover, solids loading ratio could be controlled flexibly by auxiliary‐gas velocity and length of entrainment zone.     

Aerosol Particle Deposition in a Recirculation Region

Digital simulation results concerning aerosol particle transport and deposition in a recirculation region are presented. It is assumed that the particles are shed from sources near the back face of a block in a turbulent duct flow. The results show that a large number of particles may be captured by the block and the upper wall of the channel due to impaction and interception. The capture efficiencies increase as the source distance from the wall decreases. The gravitational effects on the particle deposition rate are also studied.     

Computational studies of turbulent premixed flame based dump combustor

The present work reports the computational studies of turbulent premixed flame based dump combustor. The effects of various flow parameters such as inlet Reynolds number, inlet turbulence intensity and expansion ratio on important flow quantities like axial velocity, turbulent kinetic energy and turbulent dissipation rate have been studied extensively. It was found out that the axial velocities and the radial velocities within the flowfield are many times higher as compared to the cold flow case due to the heat release and volumetric expansion. The reattachment length for the reacting flow case is found to be lower than that of the cold flow cases due to higher recirculation velocities. A maximum reduction of 64.6% compared to the cold flow reattachment length was observed in the case of turbulence intensity, I = 10%, equivalence ratio,  = 1.00 with an expansion ratio of 2. In contrast, minimum of 41.51% reduction in recirculation length was observed in the case of I = 5%,  = 0.5 for the same expansion ratio. This reduction was quite significant at higher inlet turbulence intensities and at higher equivalence ratios.     

Design of a two‐step pulsed pressure‐swing adsorption‐based oxygen concentrator

A two‐step pulsed pressure‐swing adsorption (PPSA) process has been modeled to assess the extent to which an oxygen concentrator might be miniaturized for medical applications. The process consists of a single bed of packed adsorbent particles that is alternately pressurized and depressurized at the feed end. An enriched oxygen product is withdrawn at ambient pressure from the product end when the bed is pressurized at the feed end. The product end remains closed during depressurization. The model development addresses the manner in which axial dispersion enters into the describing equations and the formulation of proper boundary conditions, both of which have not been handled rigorously in some prior modeling studies. The describing equations are solved using COMSOL® Multiphysics software. The effect on the performance of the adsorption time, desorption time, bed length, particle diameter, and imposed pressure drop across the bed have been investigated. An interesting novel result is that for a chosen particle size, bed length, and applied pressure drop, there is an optimum combination of adsorption and desorption times that maximizes the product purity. The results suggest that there are operating windows for both 5A and partially Ag‐exchanged Li‐substituted 13X zeolite adsorbents wherein the product oxygen purity is greater than 90%. At a given product flow rate within this operating window, the extent of miniaturization is limited by the (maximum) cycling frequency that is practically achievable. Sizing of an oxygen concentrator for personal medical applications is also discussed. A principal conclusion is that a compact oxygen concentrator capable of producing a highly oxygen‐enriched product is possible using commercially available adsorbents and implementable operating conditions. © 2009 American Institute of Chemical Engineers AIChE J, 2010