支管结构对T型管内流体混合过程热波动影响

通过CFD计算流体软件FLUENT,分别对支管无附加结构及支管有附加结构直管、渐扩管、减缩管四种结构的T型管内冷热流体混合过程进行了大涡模拟,获得了管道内部的瞬时温度。将各结构温度云图与速度矢量图、无量纲时均温度及无量纲均方根温度进行了对比。数值结果表明,附加结构的添加使管内流体流型由冲击射流变为偏转射流,显著减小了T型管壁上的温度波动;缩管结构的无量纲均方根值比其他附加结构更小,表明缩管结构更适合用以减小管壁的温度波动。     

浮升力对T型管道内冷热流体混合过程热波动的影响

利用Fluent软件对T型管道内冷热流体混合过程进行了大涡模拟,考虑了温差引起的浮升力,获得了混合过程的瞬时温度和瞬时速度,通过定义时均值和均方根值来描述温度和速度的平均大小和波动程度.数值模拟结果表明,理查德森数对冷热流体的混合有着重要的影响:当理查德森数为负值且绝对值越大,则温度和速度的波动也就越大;相反,当理查德...     

T型通道内冷热流体混合过程的热波动大涡模拟

通过大涡模拟(LES)对T型通道内冷热流体混合过程的流动与传热情况进行了数值模拟,获得了混合区域内的瞬时速度和瞬时温度,通过时均值和均方根值来描述速度和温度的平均大小和波动程度.数值结果表明,在主管下游离主管和支管交汇中心不远处区域内速度和温度波动最为剧烈.该研究揭示了混合过程热波动的流动与传热本质,获得了温度波动强度与频率,对预测热疲劳和判断易发生区域具有指导意义.     

填充有多孔介质的T型通道内冷热流体混合过程的大涡模拟

通过大涡模拟(large-eddy simulation,LES)对填充有烧结铜球多孔介质的T型通道内冷热流体混合过程的流动与传热情况进行了数值模拟,获得了混合区域内的瞬时温度和瞬时速度,通过时均值和均方根值来描述温度和速度的平均大小和波动程度。数值结果表明:在主管下游离多孔介质区域不远处的温度波动最为剧烈,在多孔介质的孔隙中流体速度波动最为剧烈。     

不同动量比对湍流穿透现象的影响

在核电工业中,T型管是一种常见的管道结构.在稳压器波动管道的T型接口处,由于主支管动量比很大,会产生湍流穿透现象.针对T型管不同动量比冷热流体混合过程中的湍流穿透现象,使用数值方法进行了研究,通过合理划分网格及设定时间步长,采用大涡模拟方法获得了管内流体的瞬时速度与温度.结果表明:当湍流穿透现象发生时,支管内速度与温度...     

多孔介质对冷热流体横向射流混合过程热波动的影响

运用流固耦合方法建模,应用FLUENT计算软件平台对填充有多孔介质的T型连接方形管道内冷热流体横向射流混合过程的流动和热传递进行大涡模拟,采用了Smagorinsky-Lilly亚格子模型,获得了瞬时速度和温度分布.结果表明,填充多孔介质能够有效减少T型连接管道中冷热流体横向射流混合的温度和速度波动.固体骨架的导热率较...     

四流T型中间包内流体流场物理模拟的研究

以某钢厂四流连铸中间包为研究对象,通过物理模拟的方式,研究了中间包设计控流装置内的流体流动行为及特点,提出了中间包的优化控流结构,即采用方形波浪底湍流控制器,其导流孔仰角为25°、中孔直径为30mm、侧孔直径为50 mm,优化控流结构后,钢液在中间包内平均停留时间增加了16%,包内死区体积减小了38%,此控流组合更有效地改善中间包流体流动,有利于非金属夹杂物的上浮,均匀钢液温度,有利于提高钢坯的质量。     

有压管道内管道车稳定运行时下游水力特性研究

管道列车水力输送作为一种高效、环保、节能的物料运输方式，具有较广阔的应用前景。当管道车在管内静止时可将其视为绕交汇式圆柱系结构，而水流绕过该结构后会产生漩涡，从而造成部分能量损失。运用数值模拟、模型试验和理论分析法研究管道车绕流流场下游的水力特性与流场稳定后的空间涡结构。结果表明，不同直径的管道车下游流场的近场区域可分为回流区与射流区，随着向管道车下游距离增加，断面平均流速呈先略微下降再升高的趋势，在1倍管道车特征长度的距离时恢复接近管道来流的平均速度；随着下游远场区域距离的增加，断面平均流速的波动幅值逐渐减小，流场逐渐趋于稳定；流场内空间涡环结构高度破碎；且涡结构在靠近环形缝隙一侧较多，靠近管轴线一侧较少。     

助燃空气旋转动量对燃气燃烧过程及NO_X生成量的影响

应用数值模拟方法,对一种新型燃气烧嘴的燃烧特性进行了研究。重点分析了助燃空气旋转动量对该新型烧嘴的燃烧特性以及NO_x生成量的影响。结果表明,随着助燃空气旋转动量的不断增加,沿燃烧器轴心线方向速度衰减速率越快,炉膛内的高温区域越向燃烧器喷口附近区域集中,炉膛出口No_x的生成量呈先增大后减小的趋势,在助燃空气旋角为30°时为最大。     

氧煤比对气流床煤气化过程的影响

为研究氧煤比对气流床煤气化炉气化过程的影响,对某厂运行的Texaco气化炉进行了数值模拟研究。利用所建立的数学模型,分析了Texaco气化炉内的气化过程,以及氧煤比对炉内温度、气相成分及炉膛出口合成气成分的影响规律。结果表明:Texaco气化炉内下行火焰的长度约占气化炉高度的1/3,炉膛上部火焰高度区域内气相温度及主要成分浓度的变化梯度最大,而在炉膛下部气相成分及温度的变化均不明显;随着氧煤比的增大(0.95~1.10),气化炉出口合成气有效成分(H2+CO)浓度逐渐降低,CO2和H2O的浓度及气化炉内气相温度逐渐升高;在保证顺利排渣和合适的出口合成气成分的条件下,存在一个最佳氧煤比。     

Effect of property variations on the mixing of laminar supercritical water streams in a T-junction

The effect of variation of supercritical water properties with temperature and pressure on the mixing characteristics of streams having different temperatures in a T shaped channel is investigated numerically by considering steady, low Reynolds number flows with the assumption of negligible bouyancy effects. The Reynolds number of the cold stream in the range 0.1 to 100, inlet temperature of hot stream varying from 300 K to 800 K, the ratio of mass flow rate of cold stream to total mass flow rate in the range of 0.1 to 0.90 and the operating pressures from 25 MPa to 45 MPa at a cold fluid inlet temperature of 293.15 K are considered as parameters controlling the mixing of hot and cold streams. The effect of property variations on the outcome of the mixing is more important for low Reynolds numbers and hot fluid inlet temperatures above the pseudo-critical temperature and result in a non-uniform temperature distribution in the outlet section.     

Numerical simulation of transient 3-D turbulent heated jet into crossflow in a thick-wall T-junction pipe

The present work is to investigate the transient three-dimensional heated turbulent jet into crossflow in a thickwall T-junction pipe using CFD package.Two cases with the jet-to-crossflow velocity ratio of 0.05 and 0.5 are computed,with a finite-volume method utilizing k-ε turbulent model.Comparison of the steady-state computations with measured data shows good qualitative agreement.Transient process of injection is simulated to examine the thermal shock on the T-junction component.Temporal temperature of the component is acquired by thermal coupling with the fluid.Via analysis of the flow and thermal characteristics,factors causing the thermal shock are studied.Optimal flow rates are discussed to reduce the thermal shock.     

锅炉过热器进口集箱三通区域静压分布的数值模拟及三通的结构改进

为解决过热器超温爆管问题,根据电站锅炉过热器带三通集箱的结构特点,采用数值模拟方法对采用径向引入方式的过热器进口集箱三通区域的静压分布和流动规律进行了研究,并在此基础上对三通区域结构进行了改进.结果表明:三通区域集箱中存在涡流区,使该区域的静压变低,导致布置在该区域的支管蒸汽流量减少;适当增大三通区域集箱内径可提高集箱涡流区的静压,但当三通区域集箱内径增大到一定程度后,继续增大三通区域集箱内径,集箱涡流区的静压已不再有明显升高.     

Design consideration for cross jet air mixing in municipal solid waste incinerators

In mass-burning municipal solid waste incinerators, overfire air injection plays a key role in the improvement of mixing and reaction between oxygen and incomplete combustion products and/or pollutants. However, the design parameters of overfire air nozzles are not well understood and sometimes confusing. In this paper, major design parameters concerning cross jet air nozzles are discussed along with flow simulation results for simplified furnace geometry. The overall performance of jet air mixing and the effects of design parameters are quantitatively evaluated. The flow simulation results are interpreted in terms of the penetration depth of the jet into the main flow, the size of the recirculation zone and the ratio of the unmixed portion of the gas flow. The momentum flux ratio J of the jet to the cross flow strongly affects the penetration depth of the jet and the mixing of two flow streams. As the inter-nozzle distance S (in non-dimensional form) decreases, the penetration depth decreases but the size of the recirculation zone increases and the resultant mixing deteriorates. The degree of mixing of the jet with the cross gas stream is evaluated in terms of the mass-averaged probability distribution of the relative concentration. Fresh air disperses more efficiently into the gas stream as J and S increase. The momentum flux ratio and the inter-nozzle distance are considered as important design parameters, and optimum values of these variables can be chosen for the given furnace conditions. This numerical evaluation also provides a basis for similarity considerations in cold flow model tests and the validity of the two-dimensional idealization. © 1997 by John Wiley & Sons, Ltd.     

A novel approach to predict the stability limits of combustion chambers with large eddy simulation

Lean premixed combustion,which allows for reducing the production of thermal NOx,is prone to
combustion instabilities.There is an extensive research to develop a reduced physical model,which
allows-without time-consuming measurements-to calculate the resonance characteristics of a
combustion system consisting of Helmholtz resonator type components （burner plenum,combustion
chamber）.For the formulation of this model numerical investigations by means of compressible Large
Eddy Simulation （LES） were carried out.In these investigations the flow in the combustion chamber is
isotherm,non-reacting and excited with a sinusoidal mass flow rate.Firstly a combustion chamber as a
single resonator subsequently a coupled system of a burner plenum and a combustion chamber were
investigated.In this paper the results of additional investigations of the single resonator are
presented.The flow in the combustion chamber was investigated without excitation at the inlet.It
was detected,that the mass flow rate at the outlet cross section is pulsating once the flow in the
chamber is turbulent.The fast Fourier transform of the signal showed that the dominant mode is at
the resonance frequency of the combustion chamber.This result sheds light on a very important
source of self-excited combustion instabilities.Furthermore the LES can provide not only the damping
ratio for the analytical model but the eigenfrequency of the resonator also.     

The effect of sheath on plasma momentum transport to an electrically biased surface

Ion and electron momentum fluxes transport to a biased workpiece surface in contact with a plasma are systematically and kinetically investigated in this study. Micro-electro-mechanical systems (MEMS), semiconductor technology, plasma etching, spray deposition, sputtering, cutting and surface treatment, etc. are strongly affected by momentum transfer from the plasma to workpiece. In this work, the plasma is composed of a collisionless presheath and sheath on an electrically negative biased surface that partially reflects or secondly emits ions and electrons. The presheath is an ionization region that continuously produces ions to maintain ion loss to the workpiece, while the sheath is a space-charge region that accelerates ions and retards electrons to the negative biased surface. The predicted total momentum flux at the wall including that of the ions and electrons as a function of negative electrically biased voltage is found to agree well with experimental data. The effects of the sheath and plasma parameters such as net current density, ion and electron reflectivities of the wall, ion-to-electron mass ratio, charge number, electron-to-ion source temperature ratio at the presheath edge, on the total momentum flux, ion and electron momentum fluxes, and their components through the sheath to the surface are obtained.