微小颗粒横掠圆管的数值分析

利用拉格朗日粒子随机追踪模型以及El-Batsh 和Haselbacher提出的粒子沉积模型,对飞灰颗粒横掠管束时的运动轨迹和沉积特性进行了数值研究,获得了管束排布方式以及粒子直径对飞灰颗粒运动轨迹及沉积分布的影响。结果表明：当颗粒的直径小到一定程度时,管束通道内的流场和温度分布对颗粒的运动轨迹影响显著;颗粒在管壁表面的碰撞率和沉积率与管束排布和粒径有关,粒径50.0um颗粒几乎没有沉积;粒径10.0um颗粒沉积主要分布在管壁迎风面;粒径1.0um的颗粒在管壁的迎风面和背风面都有沉积。     

一种旋风燃烧器内煤颗粒燃烧及沉积特性的研究

采用数值模拟与试验相结合的方法研究了某旋风燃烧器内煤颗粒的燃烧及沉积特性．结果表明：该燃烧器在不同负荷下均能实现高温燃烧（1700K以上）,并获得较高的燃烧效率及捕渣率．煤颗粒主要在燃烧器前部沉积,且上下壁面的分布几乎一致;较小的煤颗粒跟随性好,不易沉积,60μm以上的煤颗粒几乎能在燃烧器中前部全部沉积下来并附壁燃烧;较小的过量空气系数及适中的一次风率（O．3～0．35左右）能保证煤颗粒在燃烧器内壁面上沉积分布较均匀,不至于造成低温区域燃烧通道煤粉堵塞．     

静止流体中颗粒污垢沉积模型数值模拟

为了研究静止流体中颗粒污垢沉积规律,基于固液两相流理论以及扩散理论建立了颗粒沉积模型,采取数值模拟的办法,分析研究了颗粒沉积速度、浓度及沉积量随沉积过程的变化,得到了适用于粒径大于5μm的颗粒沉积规律:颗粒沉积速度、浓度以及沉积量均呈现渐进型趋势,颗粒最终以恒定的沉降速度沉底;越靠近底端,沉积时间越短;越靠近表面浓度变化越快。分析对比模拟值与实验值,在误差允许的范围内认为两者吻合较好。     

燃气中颗粒在换热表面沉积特性

研究Stokes数对颗粒在换热器壁面捕获及沉积的影响,采用欧拉-拉格朗日方法及El-Batsh和Haselbacher提出的粒子沉积模型模拟稀疏气固两相流,讨论了颗粒横掠受热表面的沉积分布。结果表明:不管是改变粒径还是改变流速导致的St数变化,对颗粒沉积特性都有影响;St数减小,圆管周向的颗粒沉积量增加,沉积区域增大且分布均匀;St数减小到一定程度,顺叉排管束对沉积影响的区别减小。     

燃气透平静叶微颗粒碰撞和沉积特性的数值分析

基于拉格朗日颗粒追踪模型及El-Batsh和Haselbacher等提出的微颗粒沉积模型并结合燃气轮机透平静叶实际运行参数,为研究PM2.5等微颗粒在透平静叶的碰撞和沉积特性,通过计算流体CFD方法,对不同直径的微颗粒横掠透平静叶进行颗粒运动轨迹追踪和沉积情况分析。模拟结果表明:颗粒直径0.1~5.0μm时,碰撞率随颗粒直径的增大而增大,黏附率随着颗粒直径的增大反而减少,沉积率随着颗粒直径的增大先增大后减少;颗粒直径为0.1μm时,在叶片背面和压力面都有碰撞和沉积,直径为1.0和2.5μm时,沉积和碰撞大都发生在压力面后半部分和前驻点附近,直径为5.0μm时,沉积大都发生在压力面前半部分。     

飞灰复燃对炉内颗粒沉积状况的影响分析

实际工程表明,采用飞灰复燃技术对锅炉进行改造,可以减少飞灰所带走的燃料损失,提高锅炉效率,但飞灰回收复燃给壁面颗粒沉积状况也带来了影响.采用FLUENT模拟了SZL15-1.25-AⅡ型双筒链条蒸汽锅炉炉内燃烧,对比分析了采用飞灰复燃技术前后炉内壁面颗粒沉积状况.模拟结果表明,飞灰复燃对锅炉顶墙、前墙及后墙的颗粒沉积速率影响较大,其中飞灰复燃提高了顶墙和前墙的颗粒沉积速率,降低了后墙颗粒沉积速率,而对锅炉前后拱的影响很小可以忽略.减小飞灰入射质量流量或调整飞灰入射角度为水平偏下,均可以降低颗粒在水冷壁的沉积速率,有利于炉膛与水冷壁间的传热.     

超临界航空煤油RP-3沉积过程的数值研究

基于欧拉-拉格朗日方法,对冷却管道内超临界航空煤油焦炭颗粒沉积过程进行了数值模拟研究。物理模型为内径1.8mm、长度2 000.0mm的细圆管,数值计算模型中考虑了流体对颗粒的曳力、重力、Saffman力和附加质量力等。假设物理模型入口处已产生航空煤油焦炭颗粒,重点研究了航空煤油压力对焦炭颗粒沉积过程的影响。研究结果表明,压力对颗粒沉积过程的影响与颗粒大小、温度有着密切的关系。对于10、50、70μm的焦炭颗粒,煤油处于低温区时,压力对颗粒沉积过程影响并不显著,较高的压力对焦炭颗粒沉积略有抑制作用;然而随着温度的升高,压力升高对颗粒沉积过程具有明显的抑制作用。对于30μm的焦炭颗粒,高压反而有利于颗粒沉积。此外,对不同流动方式下颗粒沉积率进行了研究,结果发现竖直向上管中颗粒沉积率最低。     

喷枪结构对低温超音速火焰喷涂颗粒飞行特性的影响

建立了低温超音速火焰喷涂传热和流动模型,对喷涂过程的焰流和颗粒的运动加热历程进行了模拟分析.研究了三种不同结构的喷枪对焰流速度与温度分布、不同粒径颗粒飞行特性的影响.模拟结果表明,粒径为20μm的Cu颗粒在撞击基板时能达到临界速度,且温度低于熔点,有利于沉积并减少了颗粒氧化;枪管的扩张率对喷涂颗粒的速度影响不大,而对颗粒温度的影响较大;延长扩张段的长度代替平直枪管有利于在保证颗粒速度的同时提高颗粒的温度.     

地下水源热泵物理阻塞系统介质长度改变时沉积特性参数研究

介绍针对地下水源热泵系统阻塞机理研究开发的砂层阻塞模拟试验系统。通过对玻璃珠颗粒和砾石颗粒的对比性试验得出:砾石颗粒间的不均匀性为颗粒的沉积提供了"温床",孔隙孔道的不连续和不均匀性有利于悬浮颗粒的沉积,在不同长度的试验箱体中,流体速度与悬浮颗粒沉积率呈现出不同的变化关系。悬浮颗粒恢复率与流体速度以及试验箱体长度间的变化关系也呈现出增减不同的变化。     

超临界碳氢燃料中结焦固体颗粒的沉积规律研究

为了研究碳氢燃料中的结焦颗粒在冷却通道内的沉积规律,对超临界碳氢燃料中结焦固体颗粒在冷却管路中的积聚情况进行了数值模拟和实验研究。采用欧拉-拉格朗日离散颗粒模型,sst k-w模型对液固两相耦合下结焦颗粒的沉积进行数值模拟。模拟结果表明:结焦颗粒在圆柱形孔板附近更容易沉积;壁面越粗糙,燃料质量流量越大,燃料温度越高,结焦颗粒的积聚现象越弱;结焦颗粒直径越大,在壁面处的沉积率越大。实验结果表明结焦颗粒在圆柱形孔板中的沉积比在渐变形孔板中的沉积严重。采用渐变型孔板,减小结焦颗粒的直径有助于改善结焦颗粒的沉积现象。     

椭圆形换热单管积灰特性的数值模拟研究

椭圆形换热管作为一种强化换热元件,在抗积灰性能方面具有一定优势.本文基于ANSYS FLUENT软件平台建立了一套模拟程序,针对椭圆形换热单管的积灰特性展开了数值模拟研究.重点研究了换热管的椭圆度、烟气流速以及飞灰颗粒粒径对飞灰沉积特性的影响.研究表明,当换热管的椭圆度在1～2之间变化时,5～100μm粒径的颗粒沉积率...     

热泳力作用下柴油机微粒在冷却通道中沉降规律研究

对热泳力作用下柴油机微粒在冷却通道中的沉降规律进行了研究。推导建立了热泳沉降率数学模型,在此基础上分析了在热泳力作用下,微粒大小、排气流速、排气与壁面温差以及通道尺度等因素对柴油机微粒在冷却通道中热泳沉降率的影响规律。研究结果表明,热泳力对柴油机微粒具有一定的脱除作用,通过合理设计和控制冷却通道的结构和运行参数,可以有效地提高柴油机微粒在冷却通道中的沉降率。热泳沉降技术为柴油机微粒的净化以及柴油机微粒分布的改善提供一条有效的途经。     

Influence mechanism of dynamic and static liquid bridge forces on particle deposition behaviors in solar photovoltaic mirrors

Solar energy is one of the most promising forms of renewable energy for solving the energy crisis and environmental problems. Dust deposition on photovoltaic mirrors has a serious negative impact on the photoelectric conversion efficiency of solar power stations. In this paper, the influence mechanism of the dynamic and static liquid bridge forces on particle deposition behaviors on solar photovoltaic mirrors is investigated. In addition, the expression and physical meaning of the particle critical separation velocity are proposed. The research results show that the static liquid bridge force can be the primary deposition force causing dust particles to adhere to photovoltaic mirrors. However, the dynamic liquid bridge force can act as a resistance force for the particle motion process and even make dust particles roll along and finally stay on the mirror. The contact force is the primary separation force that causes dust particles to flow away from the mirror. Whether dust particles adhere to the mirror depends on the relative size of the deposition and separating forces. The particle critical separation velocity describes the relative size of the collision-rebound effect and mirror adhesion effect and is expressed in Eq. (16). These research findings can provide theoretical guidance for mirror cleaning methods in the operation process of photovoltaic mirrors.     

管道中亚微米颗粒热泳脱除技术的研究

通过对设计的实验系统进行测量,得到了亚微米颗粒热泳沉积效率的计算公式.由实验验证:相对于长管,短管可充分利用速度温度正在发展流,从而获得更高的热泳沉积效率.利用逐步递进原则,设计了1种将短管作为热泳除尘段,并和过渡段交替连接的除尘装置.结果表明:在采用8段热泳段的情况下,总除尘率可达到约83.24%.说明利用该原则能设计出可行的脱除亚微米颗粒的除尘设备.     

Analysis of Fouling in Refuse Waste Incinerators

Gas-side fouling of waste-heat-recovery boilers, caused mainly by the deposition of particulate matter, reduces the heat transfer in the boiler. The fouling as observed on the tube bundles in the boiler of a Dutch refuse waste incinerator varied from thin and powdery for the economizer to thick and sintered for the superheater. Analysis of process data showed that both types of layers resulted in a 27% decrease of the heat transfer coefficient of the bundles. To determine the important mechanisms in the deposition of particles, layers taken from the different bundles are analyzed using electron microscopy. The analysis revealed the existence of a melt in the thick deposit. The melt, giving rise to a liquid phase, increases the sticking efficiency of the deposit and leads to larger deposition rates. For the economizer and the superheater the actual deposition rate is calculated from the change in heat transfer. On the basis of a comparison between the calculated deposition rates and deposition rates to be expected in the case of a pure diffusion and thermophoresis process, it is shown that for both types of deposits inertia-controlled transport is the dominant transport mechanism of particles.     

Systematic experimental study of the pollution deposition impact on the energy yield of photovoltaic installations

The current study experimentally investigates the performance of two identical pairs of photovoltaic (PV) panels, the first being clean and the second being artificially polluted with three different, commonly met in urban and other environments, air pollutants (i.e. red soil, limestone and carbonaceous fly-ash particles). The PV-panels under comparison are both operating under the same environmental conditions, being nearby located and adjusted at the same inclination. The effect of pollution deposition on PVs’ power output, energy yield and conversion efficiency is examined, considering also various pollutants’ mass depositions on the PV-panels’ surfaces. According to the results obtained, a considerable reduction of PVs’ energy performance is recorded, depending both on particles’ composition and origin and on the total mass accumulated on the PV-panels’ surfaces. Based on the results, the highest reduction is caused by the deposition of red soil particles, followed by the deposition of limestone and finally by the carbon-based ash.     

Ash deposition behaviors during combustion of raw and water washed biomass fuels

Biomass-fired boilers have the tendency to suffer from severe problems of fouling and slagging due to the high potassium content of biomass fuel. The troublesome potassium, however, can be removed efficiently by water washing pretreatment. In this study, the ash deposition behaviors during combustion of raw and water washed biomass fuels were investigated by a one-dimensional furnace and a deposition probe. Two biomass fuels (corn stalk and wheat straw) were used, and deposition mass, deposition efficiency, composition and morphology of the deposit were studied. The ash deposition while firing raw biomass exhibits a “fast?slow?fast?slow” trend with the sampling time. After water washing, the deposition mass decreases dramatically, and the deposition efficiency reduces gradually as the sampling time increases. The analyses of elemental composition, morphology and chemical composition on the deposit from raw biomass imply that the condensation/thermophoresis is quite significant in the earlier deposition stage, whereas the chemical reaction is remarkable in the later stage. After water washing, the potassium content of the deposit decreases significantly. Morphology and chemical composition analyses indicate that the deposit from water washed biomass ascribes to the physical accumulation of non-viscous fly ash particles. The deposition mass can easily approach a maximum value. The ash fusion temperatures of deposits increase remarkably after water washing. In addition, ash deposition mechanisms during biomass combustion are discussed.     

Unsteady CFD simulation on ash particle deposition and removal characteristics in tube banks: Focusing on particle diameter,flow velocity,and temperature

This study establishes a numerical deposition and removal model coupled with unsteady gas–solid turbulent flow to predict transport, impaction, sticking, and removal (or rebounding) characteristics of ash particles with high alkali metals based on Ansys Fluent software extended by user–defined functions. Dynamic mesh analytical strategy is employed to adjust the grid on the interface of flue gas and deposition layer to illustrate ash deposit growth characteristics. In this study, the effects of particle size, flow velocity, and inlet gas temperature on formation and distribution of ash deposits are studied. The results indicate that ash deposit distribution has significant particle size dependence. Larger particles tend to deposit on the windward side of first–row tubes, and account for the greatest share of total deposited mass. Smaller particles may deposit on the leeward side of the tube surfaces. Variation in impaction rates with time are influenced by the thermophoretic force and enlarged deposition area. With the increase in flow velocity, total deposited mass increases gradually. The influence of flow temperature on impacting rates is mainly embodied in the effect of flow temperature on thermophoretic force. However, the increase in inlet gas temperature has little effect on the sticking efficiency and ash deposit distribution. Nevertheless, the total deposited mass shows an increasing trend with inlet flue gas temperature.     

The effect of stable stratification and thermophoresis on fine particle deposition in a bounded turbulent flow

Small inertial particle transfer and deposition in a thermally stratified turbulent channel flow is studied by large eddy simulation. The Lagrangian tracking approach is used to describe the dynamics of particles. The objective of this study is to examine the influences of thermal stratification and the thermophoresis on the preferential concentration and deposition of small particles with different particle relaxation time scales. The diameters of particle are ranged from 0.6 μm to 5 μm. Numerical results show that the thermophoresis strengthens the deposition of particles near the cold wall while it weakens the deposition near the hot wall. With the Richardson number increases, the particle mean velocity increases regularly in the core of the channel, and the particle fluctuation intensities decrease due to the re-laminarization tendency of stably stratified turbulent boundary layer. The magnitude of the thermophoretic force on particles decreases in the stable stratified flow, which results in smaller deposition rate with increasing the Richardson number.