过滤速度对DPF内颗粒沉积特性影响的试验研究

基于柴油机颗粒过滤器(DPF)沉积过程可视化台架,在不同过滤速度下研究颗粒物在DPF载体切片上的堆积密度、颗粒层渗透系数及孔隙率。研究结果表明:在自然堆积状态或过滤速度0.15m/s时,碳黑的堆积密度分布在90kg/m3左右;当过滤速度0.15m/s时,ZS1100与DK4A柴油机颗粒物的堆积密度大于碳黑堆积密度,分布在100kg/m3左右;随过滤速度的增大、颗粒粒径减小及SOF含量的增加,颗粒的堆积密度先快速增大,后逐渐趋于稳定值;随颗粒层厚度的增加,过滤压降呈现典型的三阶段变化;随过滤速度的增加,颗粒层渗透系数和孔隙率呈先快速降低,后逐渐趋于稳定值的趋势。     

模拟碳烟在DPF过滤壁面上沉积特性的试验

基于可视化单通道试验台架,采用固体颗粒发生器产生来流颗粒使颗粒均匀沉积到柴油机颗粒捕集器(DPF)过滤壁面上,使用激光位移传感器在线测量过滤壁面上颗粒层厚度和电镜离线观测颗粒层形貌与结构,对碳黑颗粒特性和灰沉积量在DPF过滤壁面上的沉积过程开展研究.结果表明:针对颗粒层厚度曲线,沉积过程可分为深床期、长树期、搭桥期及颗粒层期;而针对过滤压降曲线分为深床期、过渡期和颗粒层期.随壁面过滤速度增大,过滤压降增大,颗粒层厚度增大,形成的颗粒层越致密,且厚度曲线进入颗粒层期的厚度从15μm增加至约30μm.在固定的壁面过滤速度工况下,由于碳黑颗粒特性存在差异,颗粒自身团聚程度越高(SB4AFW200PU),对应的最终过滤压降和堆积密度越大.在有灰沉积的工况下,随着灰沉积量从0 g/L增加至6 g/L,沉积碳黑颗粒时,DPF的初始压降增大,但最终过滤压降和碳黑颗粒层的堆积密度呈先减小后增大的趋势,尤其在灰沉积量为2 g/L时同时达到最低.     

荷电颗粒可压缩性颗粒层模型

研究发现荷电颗粒有趋向于沉积在颗粒链尖端的趋势,据此在已建立的中和可压缩性颗粒层模型中加入荷电颗粒的沉积性质,建立了荷电颗粒可压缩性颗粒层模型,从而研究滤料过滤过程中的颗粒层过滤阶段的机理。通过模型发现,不可压缩时,荷电颗粒形成的颗粒层的高度与中和颗粒层相似,但空隙分布均匀;由于下滑角较小,其不易被压缩,当压降增大到一定程度时,呈周期性压缩,所以在较高过滤风速下其压降显著低于中和工况。模型结果成功地解释了实验现象。     

颗粒帘换热器中固体颗粒流动特性实验研究

提出了一种基于气体-固体颗粒快速热平衡的颗粒帘换热器。为揭示该换热器中气粒两相间的动力学特性,在颗粒帘换热器实验平台上开展了颗粒帘换热器中固体颗粒流动特性的冷态实验。实验结果表明:颗粒帘沿程厚度随进气速度的增加而减小,颗粒帘后沿水平偏移量随进气速度的增加而增大;颗粒帘沿程厚度及颗粒帘后沿水平偏移量随颗粒帘初始厚度的增加而增大,随颗粒粒径及颗粒质量流量的增加而减小;颗粒帘沿程厚度和颗粒帘后沿水平偏移量随着颗粒帘下落高度的增加而增大;颗粒帘落点水平偏移量随进气速度的增加而增大,随颗粒帘初始厚度及颗粒粒径及颗粒质量流量的增加而减小。这些结果为颗粒帘换热器结构的优化设计和气固两相流动、换热及积灰特性的研究奠定了良好的基础。     

堆石料三轴固结排水剪切试验全过程颗粒破碎特性研究

为研究灰岩堆石料三轴固结排水剪切试验全过程的颗粒破碎特性,通过对不同初始孔隙率下制备试样、不同围压下固结试样、不同围压下三轴固结排水(CD)剪切破坏的试样进行筛分试验,研究了三轴CD剪切试验过程中制样、固结和剪切三个试验阶段的颗粒破碎特性。结果表明,剪切过程中的颗粒相对破碎率最大,固结过程的次之,制样过程的最小;制样过程中的颗粒相对破碎率随试样初始孔隙率的增大基本呈线性减小变化;固结过程中的颗粒相对破碎率随固结压力的增大呈线性增大变化,随试样初始孔隙率的增大基本呈线性减小变化;剪切过程中的颗粒相对破碎率随剪切围压的增大呈线性增大变化,随试样初始孔隙率的增大基本呈线性减小变化。     

颗粒帘换热器中颗粒空隙率的计算方法与实验研究

运用流动过程能量守恒原理与Ergun公式,推导出基于颗粒帘工况条件与运行参数的颗粒空隙率计算公式,并根据实验测试结果计算得到各工况条件对颗粒空隙率的影响规律.结果表明:颗粒空隙率沿下落行程呈现先减小后增大的规律;颗粒空隙率随进气速度及颗粒帘初始厚度的增大而增大,随颗粒粒径及颗粒质量流量的增大而减小;进气速度、颗粒帘初始厚度对颗粒空隙率的影响大于颗粒粒径、颗粒质量流量对颗粒空隙率的影响.     

柴油机颗粒过滤器再生时颗粒层氧化燃烧特性探究

基于可视化单通道台架,采用激光位移传感器在线测量再生时过滤壁面上颗粒层厚度,采用电镜离线观测颗粒层形貌,探索已沉积的炭黑颗粒层在柴油机颗粒过滤器(DPF)过滤壁面上的再生氧化过程。结果表明,基于颗粒层厚度变化曲线,再生过程分为3个阶段:第Ⅰ阶段,颗粒层厚度缓慢降低;第Ⅱ阶段,颗粒层厚度快速降低,氧化反应主要发生在DPF孔隙气流处,颗粒层表面出现凹坑形貌;第Ⅲ阶段,颗粒层厚度再次缓慢下降。同时,炭黑颗粒微观形貌由均匀堆积形貌向链状和环状形貌变化,颗粒层随氧化的进行呈现凹坑结构。     

局部流化清灰的颗粒层除尘器研究

提出了一种组合流态化清灰的颗粒层除尘器,该除尘器采用固定床过滤、流化床清灰,实现了颗粒层除尘、清灰一体化,同时也克服了移动床清灰过程中颗粒间隙增大和颗粒错位等因素对细微灰尘除尘效率的影响;分析了颗粒层的清灰周期对除尘器性能的影响.实验研究了颗粒层清灰时间与流化速度的关系以及颗粒层除尘器的性能.理论和实验表明,新型颗粒层除尘器除尘效率高,清灰方便、及时.     

声场作用下颗粒绕流液滴迁移行为研究

脱硫协同除尘的物理基础是单一液滴对颗粒的捕集,引入新的物理机制强化液滴捕集可以进一步提高脱硫协同除尘的效果.本文以声场作为外加物理场,采用数值模拟方法研究了声场作用下颗粒绕流液滴时的迁移行为和在液滴表面的沉积分布.研究结果表明,声场使颗粒在声波传播方向上发生振荡运动,振荡振幅与速度随颗粒粒径减小和声压级提高显著增加;声场改变颗粒绕流时的迁移行为,部分颗粒的迁移结果(沉积、逃逸)和沉积方式(分离前沉积、回流区沉积)发生改变;声场引起的颗粒速度偏移使其沉积位置偏离无声场时的沉积区域,使液滴表面的沉积分布特征由离散转变为弥散.     

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。     

灰分对柴油机颗粒物捕集器性能的影响

为了降低灰分对柴油机颗粒物捕集器(DPF)性能的影响,建立了DPF灰分和碳烟的数学模型,研究了灰分量、碳载量和灰分分布形态等多个因素对多种DPF性能的影响.结果表明:非对称孔结构(ACT)DPF增加了进/出口孔径比例有利于降低压降,但不利于捕集效率的提高;碳烟层的捕集效率高于灰分层,对称结构捕集效率高于ACT结构,低孔隙率捕集效率高于高孔隙率;灰分分布系数增加,DPF压降和捕集效率均上升,灰分分布系数对ACT结构DPF的影响小于对称结构;ACT结构有利于提升DPF容灰能力,延长清灰周期,提高经济性.     

Effects of ash composition and ash stack heights on soot deposition and oxidation processes in catalytic diesel particulate filter

In order to provide the theorical basis for regeneration strategy and solid catalytic reaction of catalytic diesel particulate filter (CDPF), the effects of ash composition and ash stack heights on soot deposition and oxidation processes in CDPF are investigated. The MgO ash nanoparticles have an inert effect on soot oxidation process through a series of thermogravimetric experiments. Based on the visualized single-channel bench, the influence of ash stack height on the soot deposition and regeneration processes is studied. The deposition process of soot particles on the CDPF slice without ash is divided into three stages, including depth filtration, transitional filtration and soot cake formation stages. When the ash stack height is 15 μm, there are only transitional filtration and soot cake formation stages. Then, only soot cake formation stage remains when the height increases to 30 μm. During the regeneration process, the pressure drop mainly has three linear decline stages, the oxidation rates of which are 0.77, 1.78 and 0.54 Pa/s, respectively. When the ash stack height increases from 0 to 30 μm, the oxidation rate of soot layer during Ⅱ-regeneration stage decreases from 1.78 to 0.51 Pa/s. The oxidation rate is 0.51 Pa/s at 30 μm ash stack height, which is close to the 0.55 Pa/s oxidation rate of the clean DPF sample. Consequently, around 30 μm ash stack height is the limit distance of back-diffusion of active oxygen molecules, which leads to the failure of the CDPF catalytic layer.     

Particulate fouling in waste incinerators as influenced by the critical sticking velocity and layer porosity

Fouling of heat transfer surfaces introduces a major uncertainty into the design and operation of heat exchange equipment. Fouling layers as observed on the tube bundles of the economizer in a Dutch waste incinerator were thin and powdery. The fouling layer showed an asymptotic growth rate with a levelling off increase of the thickness. In this study, the influence of the critical sticking velocity on the growth rate of particulate fouling layers is described. The critical sticking velocity of an incident particle hitting a powdery layer is defined as the maximum impact speed at which the particle will stick to the layer. Since the critical sticking velocity is a key parameter in the deposition mechanism, a well-defined experimental set-up has been built to assign it. Experimental results showed that the critical sticking velocity increases with the porosity of the fouling layer. Literature shows that the porosity of a thin sintered powdery layer changes with the layer thickness. Based on the experimental results and the variation of porosity with thickness for a thin sintered powdery fouling layers, a correlation is developed which shows that the sticking velocity decreases exponentially as the fouling layer thickness increases. Therefore, fewer particles are likely to stick as the fouling layer builds up and consequently the deposition rate decreases. The change in the critical sticking velocity as the fouling layer builds up contributes to the explanation of the asymptotic growth of particulate fouling layers on the tube bundle of waste incinerators.     

地下水源热泵运行时渗透系数对含水层的影响

为减少地下水源热泵运行对地下空间的影响,运用颗粒迁移理论研究了地下水源热泵长期运行时渗透系数对含水层参数变化特性的影响。结果表明,随着地下水源热泵的长期运行,含水层孔隙率和渗透系数的变化特性与渗透系数的初始值无关,而承压水头的变化特性则受渗透系数的影响,渗透速度为影响含水层参数变化的重要因素之一,较小的渗透速度可有效减少地下水源热泵运行对地下空间的影响。     

Thermophoresis particle deposition on unsteady two-dimensional forced convective heat and mass transfer flow along a wedge with variable viscosity and variable Prandtl number

In this paper, the effects of thermophoresis particle deposition on an unsteady two dimensional forced convective heat and mass transfer flow past a wedge taking into account the variation of fluid viscosity and fluid Prandtl number with temperature are studied. The local similarity equations are derived and solved numerically using Nachtsheim–Swigert shooting iteration technique along with the sixth order Runge–Kutta integration scheme. Comparisons with previously published work are performed, and the results are found to be in excellent agreement. Results for the non-dimensional velocity, temperature, concentration, Prandtl number and thermophoretic velocity are displayed graphically whereas thermophoretic deposition velocity is shown in the tabulated form for various values of the pertinent parameters. The obtained numerical results show that in modeling the thermal boundary-layer flow with a temperature-dependent viscosity, consideration of the Prandtl number as a constant within the boundary layer produces unrealistic results, and therefore, it must be treated as a variable rather than a constant within the boundary layer. The results also show that the thermophoretic particle deposition velocity decreases as the thermophoretic coefficient increases.     

Experimental investigation of soot morphology and primary particle size along axial and radial direction of an ethylene diffusion flame via electron microscopy

The effect of oxygen concentrations on the formation and evolution of soot particles was investigated by analyzing soot morphology using SiC fiber deposition technique and thermophoretic sampling method in a co-flow diffusion ethylene flame. Soot particles were examined via transmission electron microscopy at different heights along the flame centerline. Results show that the flame temperature exhibits a bimodal distribution. As the flame height increases, the flame distribution gradually changes from bimodal to unimodal, and the increase in oxygen concentration not only causes the flame height to decrease, but also causes the bimodal distribution of the flame more pronounced. The morphological evolution of soot deposits are strongly dependent on the oxygen concentration, radial and axial position, and flame temperature. Soot deposits along the flame centerline begin to be oxidized into dense flocculent and fibrous mesh structures with the flame temperature increases. In front of the flame, the oxidation is enhanced with temperature rise at the same height, resulting in more dense morphology of the soot deposits and the decrease in primary particle size. The results of thermophoretic sampling show that soot growth undergoes various stages of nucleation, growth, coagulation, agglomeration and oxidation, and the average particle size distributions of soot increase first and then decrease. The increase in oxygen concentration leads to advances in all stages of soot formation, including surface growth, agglomeration and oxidation. Additionally, the flame temperatures increase sharply as the increase of flame heights, leading to the soot aggregates to be oxidized to loose chain-like agglomerates.     

Modeling of soot deposition in wavy-fin exhaust gas recirculator coolers

This work presents one of the first CFD studies carried out to understand the fouling of exhaust gas recirculator (EGR) cooler surfaces. The deposition of soot particles in wavy-fin EGR coolers is studied by way of simulations carried out in a periodic framework. In the presence of very high temperature gradients, usually prevalent in EGR flows, the particle deposition process is dominated by the thermophoretic force. Calculations are performed for 10 and 100 nm particles at various Reynolds numbers and wall temperature gradients ranging from 1.0 to 9.45 × 10⁶ K/m. It is seen that for the sub-micron particle sizes considered, the deposition process is independent of the particle size. Simulations in the wavy-fin geometry indicate the presence of preferential deposition patterns, corresponding to the regions of higher heat transfer. At lower Reynolds numbers, the amount of deposition increases considerably due to the higher particle residence times. Also, the amount of deposition exhibits a linear relationship with the applied wall temperature gradient, thus confirming the importance of thermophoresis in the soot deposition process.     

灰分分布系数对柴油机性能的影响研究

通过构建柴油机耦合柴油机颗粒捕集器(diesel particulate filter,DPF)的一维热力学仿真模型,研究了灰分分布系数对柴油机性能的影响,并重点分析了灰分分布系数对柴油机系统热效率的影响.结果表明:炭载量为6 g/L,灰分量为33 g/L时,DPF压降和捕集效率随灰分分布系数增加而上升;随着灰分分布系...     

Flow-field effects on soot formation in normal and inverse methane–air diffusion flames

We investigate the effects of the flow-field configuration on the sooting characteristics of normal and inverse coflowing diffusion flames. The numerical model solves the time-dependent, compressible, reactive-flow, Navier-Stokes equations, coupled with submodels for soot formation and thermal radiation transfer. A benchmark calculation is conducted and compared with experimental data, and shows that computed peak temperatures and species concentrations differ from the experimental values by less than 10%, while the computed peak soot volume fraction differs from the experimental values by 10–40%, depending on height. Simulations are conducted for three normal diffusion flames in which the fuel/air velocities (cm/s) are 5/10, 10/10, and 10/5, and for an inverse diffusion flame (where the fuel and air ports have been reversed) with a fuel/air velocity of 10/10. The results show significant differences in the sooting characteristics of normal and inverse diffusion flames. This work supports previous conclusions from the experimental work of others. However, in addition, we use the ability of the simulations to numerically track soot parcels along pathlines to further explain the experimentally observed phenomena. In normal diffusion flames, both the peak soot volume fraction and the total mass of soot generated is several orders of magnitude greater than for inverse diffusion flames with the same fuel and air velocities. In normal diffusion flames, soot forms in the annular region on the fuel-rich side of the flame sheet, while in inverse flames, the soot forms in a fuel-rich region on top of the flame sheet. Surface growth is the dominant soot formation mechanism (compared to nucleation) for both types of flames; however, surface growth rates are much faster for normal diffusion flames compared to inverse flames. Soot oxidation rates are also much faster in normal flames, where the dominant soot-oxidizing species is OH, compared to inverse flames, where the dominant soot-oxidizing species is O₂. In the inverse flames, surface growth continues after oxidation has ceased, causing the peak soot volume fraction to be sustained for a long period of time, and causing the emission of soot, even though the quantity of soot is small. Comparison of soot formation among the three normal diffusion flames shows that the peak soot volume fraction and total mass of soot generated increases as the fuel-to-air velocity ratio increases. A larger fuel–air velocity ratio results in a longer residence time from the nucleation to the oxidation stage, allowing for more soot particle growth. When the fuel-to-oxidizer ratio decreases, there is less time for surface growth, and the particles cross the flame sheet (where they are oxidized) earlier, resulting in decreased soot volume fraction.