大高宽比微小宽度矩形通道内的水力特性实验研究

对高宽比c=3～15共5个矩形通道的流动进行了水力实验，实验通道的当量直径dh=1.0～1.2mm，通道宽b=0.6～0.8mm，流动雷诺数Re=50～10000，实验结果表明，大高度比微小宽度矩形通道内的层流流动计算应考虑高度比的影响，不宜采用当量圆管的公式，与普通圆管流动相比较，其层流一紊流的过渡区变窄，不出现f-Re图上阻力系数f随Re增大而增大的那一小段曲线，以及层流到紊流的最大临界雷诺数Rec随高宽比c而变，其最大值为2340。     

大管径T型三通数值模拟及局部阻力特性分析

针对供热系统管网三通分流处的局部阻力影响系统水力平衡及运行能耗的问题,通过Fluent软件模拟了400mm及以上管径T型三通内流体流动的速度场和压力场,分别考察了分流比q、雷诺数Re、管径比d及管间夹角θ对大管径三通局部阻力的影响.结果表明:在研究范围内,主管与侧支管间局部阻力系数ζ_(01)以及主管与直支管间局部阻力系数ζ_(02)均随Re的增大而减小,当Re达到4.8×105~时,基本不再变化;d0.8时,ζ_(01)和ζ_(02)均随q的增大而单调递增,而当d≥0.8时,局部阻力系数均与q呈抛物线关系;q一定时,随着d的增大,ζ_(01)明显减小,当d0.7时,减小趋势明显变缓;根据流动特征分析发现,随着θ的增大,侧支管内漩涡尺寸、流速梯度及流线弯曲程度明显增大,导致ζ_(01)明显增大,直支管内流速梯度略微增大,使得ζ_(02)略微增大.     

大管径T型三通数值模拟及局部阻力特性分析

针对供热系统管网三通分流处的局部阻力影响系统水力平衡及运行能耗的问题,通过Fluent软件模拟了400mm及以上管径T型三通内流体流动的速度场和压力场,分别考察了分流比q、雷诺数Re、管径比d及管间夹角θ对大管径三通局部阻力的影响.结果表明:在研究范围内,主管与侧支管间局部阻力系数ζ_(01)以及主管与直支管间局部阻力系数ζ_(02)均随Re的增大而减小,当Re达到4.8×10~5时,基本不再变化;d0.8时,ζ_(01)和ζ_(02)均随q的增大而单调递增,而当d≥0.8时,局部阻力系数均与q呈抛物线关系;q一定时,随着d的增大,ζ_(01)明显减小,当d0.7时,减小趋势明显变缓;根据流动特征分析发现,随着θ的增大,侧支管内漩涡尺寸、流速梯度及流线弯曲程度明显增大,导致ζ_(01)明显增大,直支管内流速梯度略微增大,使得ζ_(02)略微增大.     

基于快速流态化统一动力学模型的床层固含率分布规律模拟

快速流态化作为一种新型高效的气固接触技术,现广泛应用于各行各业。以快速流态化统一动力学模型为基础,分别探究了床料物性参数、操作运行条件以及床层直径大小对床层上部稀相区固体颗粒浓度分布规律的影响,并对相关结果进行了分析,以期为快速流态化统一动力学模型的工程推广应用提供进一步的指导。结果表明:在其他参数不变的条件下,快速床上部稀相固含率随颗粒密度的增大而减小;随颗粒直径的增大而减小;随床层直径的增大而增大;随固体循环流率的增大而增大;随操作风速的增大而减小。     

基于大涡模拟的油滴群燃烧特性研究

利用Matlab随机生成不同尺寸、位置信息的油滴群,采用一步总包反应机理对其进行大涡模拟,分析了不同来流速度、环境温度工况下不同颗粒数密度油滴群的燃烧特性及燃烧模态,统计出油滴的平均阻力特性、蒸发率以及气体湍流变动。研究表明:油滴群平均阻力系数随来流速度的增加呈现先减小后增大最后趋于稳定的趋势,约为单燃烧油滴的阻力系数的1/2,随环境温度的升高而增大,与来流速度成反比;气相湍流变动随环境温度增大而加剧,随颗粒数密度增加而减小;在油滴群燃烧特性下,蒸发率随环境温度的升高而增大,随数密度的增加而减小,随来流速度的增加而增大。     

高温鼓泡流化床的流化行为

床层温度在20－1000℃范围内,以4种粒径的煤灰为实验物料,研究了不同表观气速下最小流化速度,床层平均空隙率,压力波动标准偏差和主频的变化规律,最小流化速度随床层温度升高而减小;相同床温下,平均空隙率随表观气速升高而增大,不同床温下,压力波动偏差随流化数增加而增大。相同流化数时,B类颗粒的压力波动标准偏差受床层温度变化影响较小,而D类粒子随床层温度升高压力波动标准偏差减小,胡着流化数增加,压力波动主频减小。     

矩形波状壁面二维层流运动阻力特性的研究

采用有限体积法对雷诺数Re＜2 000的层流流态下周期性矩形筋条波状壁面阻力特性进行了数值模拟,针对不同无量鲷波幅a/h与各种雷诺数的组合,计算了壁面应力分布特性.通过实验与数值法论证结果表明,波状壁面下水流的形状阻力、摩擦阻力随雷诺数增加而增大,提出了周期性矩形波状波边界层流沿程平均水头损失系数ξ的概念,可获平均阻力系数随雷诺数和筋条几何参数间的变化规律,具有实用价值.     

卧式搅拌床内生物质颗粒的轴向混合与停留时间

采用离散单元法(DEM)模拟卧式搅拌床内非球形大颗粒流动过程并进行实验检验。对颗粒流场进行分析,讨论颗粒性质和结构参数对颗粒流动行为的影响。结果表明:轴向扩散系数随颗粒尺寸、反应器尺寸的增大以及叶片与壁面间隙的减小而增大。挡板高度增大时,轴向扩散系数先增大后减小。平均停留时间随颗粒尺寸的减小,反应器直径、挡板高度的增大以及叶片与壁面间隙的增大而增大。停留时间相对方差则随颗粒尺寸、反应器直径、挡板高度的增大以及叶片与壁面间隙的减小而增大。材料的颗粒密度对于卧式搅拌床内轴向混合及停留时间分布并无显著影响。     

高雷诺数层流管道中颗粒惯性聚集特性的力学分析

根据相对运动原理,结合CFD技术,数值研究了在圆形截面通道高Re数层流中,颗粒所受惯性升力的空间分布特征,提出了颗粒惯性聚集现象在高雷诺数层流管道中所出现的内部聚集圆环的力学成因和影响因素。研究结果表明:颗粒在高雷诺数工况下所受的惯性升力的空间分布规律与低雷诺数工况完全不同,有两个稳定的升力零点:一个靠近通道中心,是形成颗粒内环聚集区域的力学成因;另一个靠近壁面,是形成颗粒外环聚集区的力学成因。这种现象的产生与旋转诱导升力并无明显相关性,而是源于流场压力场的贡献;影响颗粒内环聚集区的主要因素为颗粒相对粒径和Re数。相同条件下,该内环聚集区的半径会随颗粒相对粒径的增大而增大,随管道Re数的增大而减小。     

设置三角形阻流件的通道内流动阻力特性数值研究

为了深入研究设置三角形阻流件的水平矩形通道内传热强化机理,采用k-ε两方程模型以及SIMPLE算法,对Re在1500～35 000范围内,流体流过该通道时的流动问题进行了数值求解.模拟计算的结果表明,当Re＞8 000时,阻力系数,随着阻流件顶角α的增大而减少;当Re≤8 000时,阻力系数,随顶角α和Re数的变化不明显;阻力系数.厂随着阻流件的高度h的增大而增加;在一定的高度范围内,与不设阻流件的光滑通道相比,设置阻流件通道的Webb性能因子田得到了改善.     

Experimental and CFD study of wall effects on orderly stacked cylindrical particles heat transfer in a tube channel

In this study, the flow and heat transfer characteristics of regularly arranged cylindrical particles in a bed with bed-to-particle diameter ratio of 2.65 (The particle and bed diameter are 25 and 66 mm respectively and the bed height is 200 mm) have been studied in two different arrangements of particles. The first layout is coaxial particles which embrace 8 layers and 3 equilateral cylindrical particles in each layer); the second arrangement is similar to the first one but the intermittent layers have been rotated 60^o. Three dimensional CFD simulation of air flow through these arrangements of particles in bed have been carried out by the standard κ-ε turbulence model with using of FEMLAB (Multiphysics in MATLAB) software version 2.3. For two configurations, comparisons between CFD results and experimental data have been drawn. Our results have been compared with prediction of empirical correlations for pressure drop of flow through the bed. The heat transfer CFD results were validated by naphthalene sublimation mass transfer experiments. The particle Nusselt number was obtained by using analogy between mass and heat transfer. A good quantitative and qualitative agreement between hydrodynamic of CFD simulation and experimental results was gained for both arrangements. The model predicts pressure drop of channel with two arrangements, coaxial and non-coaxial particles with an average error of 10% and 15%, respectively. Moreover, the CFD simulation has predicted the average particle Nusselt numbers of these two arrangements with an average quantitative error of 7% and 14%. Furthermore, the influence of wider range of Reynolds number (2500–6800) on particle Nusselt number has been investigated.     

Bed-to-wall heat transfer behavior in a pressurized circulating fluidized bed

An experimental investigation has been made to study the effect of pressure and other relevant operating parameters on bed hydrodynamics and bed-to-wall heat transfer in a pressurized circulating fluidized bed (PCFB) riser column of 37.5 mm internal diameter and 1940 mm height. The experiments have been conducted with and without bed material for the consideration of frictional pressure drop due to gas density at elevated pressures. The pressure drop measured without sand particles is assumed as the pressure drop due to gas density for the calculation of bed voidage and suspension density profiles. The specially designed heat transfer probe is used to measure the bed-to-wall heat transfer coefficient. The experimental results have been compared with the published literature and good agreement has been observed. The axial bed voidage is less in the bottom zone of the riser column and is increasing along the height of the bed. With the increase in system pressure, the bed voidage is found to be increasing in the bottom zone and decreasing in the top zone. The heat transfer coefficient increases with the increase in system pressure as well as with the gas superficial velocity. The heat transfer coefficient is also observed to be increasing with the increase in average suspension density.     

山区河流阶梯—深潭结构水力特征数值模拟

为了探究阶梯—深潭结构在不同流量工况下的消能机理,采用紊流数值模拟方法分析研究了阶梯—深潭结构紊动能、紊动能耗散率、流场结构、潭底压力强度等主要水力参数。结果表明,随着流量的增大,平均紊动能耗散率不及平均紊动能增加速率快,验证了阶梯—深潭随着流量的增加消能率会有所下降;小流量工况下深潭内环流区明显,增大了水流滞留潭区时间,能量耗散效果较优;大流量工况下流量的增加促使结构发生自适应调整,水深加大,形成水垫及漩滚耗能,水流阻力增大且维持潭底压差分布趋于平稳,削弱了主流对河床的直接冲刷,有利于河床稳定。     

Effects of freestream turbulence on bypass transition of separated boundary layer on low-pressure turbine airfoils

This paper presents experimental studies on bypass transition of separated boundary layer on low-pressure turbine airfoils,focusing on the effects of freestream turbulence on the transition process.Hot-wire probe measurements are performed on the suction side of an airfoil in the low-pressure linear turbine cascade at several Reynolds number conditions.Freestream turbulence is enhanced by use of turbulence grid located upstream of the cascade.The results of this experimental study show that the location of boundary layer separation does not strongly de-pend on the freestream turbulence level.However,as the freestream turbulence level increases,the size of separa-tion bubble becomes small and the location of turbulent transition moves upstream.The size of separation bubble becomes small as the Reynolds number increases.At low freestream turbulence intensity,the velocity fluctuation due to Kelvin-Helmholtz instability is observed clearly in the shear layer of the separation bubble.At high frees-tream turbulence intensity,the streak structures appear upstream of the separation location,indicating bypass transition of attached boundary layer occurs at high Reynolds number.     

Effects of solid particle aspect ratio on the performance of hydrogen production by electrolyzed water

Hydrogen energy has become one of the important directions of future energy development. The hydrogen produced by electrolyzed water is regarded as "green hydrogen", is clean and pollution-free, and is considered the ultimate direction of hydrogen production. If the waste heat of solid particles can be used as the energy required for the water electrolysis process, the cost of "green hydrogen" will be much lower than that of fossil fuel hydrogen production. In order to study the effect of particle structure size on hydrogen production capacity, the heat transfer model of the ellipsoidal particles packed bed with single-vacancy was constructed. Further, the temperature, the apparent thermal resistance, the average heat flux, and the vacancy affects area were studied. With the increase of the particle aspect ratio, the apparent thermal resistance decreases, the average heat flux increase, and hydrogen production increases. When the particle aspect ratio increases from 0.5 to 2.0, the average heat flux of the packed bed with single-vacancy increases from 23.13 kW/m² to 28.87 kW/m², and the apparent thermal resistance decreases from 23.33 K/W to 9.27 K/W. As the particle aspect ratio increases, the area affected by single-vacancy increases, the hindering effect of the vacancy on the heat flow increases.     

蓄热元件波纹参数对流动传热性能影响分析

为研究搪瓷蓄热元件波纹倾角和宽度对流动传热性能的影响,采用Fluent软件对某搪瓷蓄热元件进行数值模拟,得到蓄热元件不同波纹倾角和宽度时内部流场速度、温度分布及努塞尔数和阻力系数随雷诺数的变化曲线.模拟结果表明:波纹倾角由20.增大到60.时,蓄热元件阻力系数增大,努塞尔数增加,且阻力系数增幅大于努塞尔数增幅;波纹宽度...     

凹槽式叶顶流动换热的数值研究

针对叶尖间隙高度对凹槽式叶顶流动与换热的影响展开数值研究,评估4种湍流模型在叶顶换热方面的预测能力.结果表明：凹槽肩壁顶部、凹槽腔底部近前缘区域和叶顶尾缘为高换热区,凹槽腔底的中部和尾部区域为低换热区;不同湍流模型对叶尖间隙泄漏量预测差别很小,但泄漏流流动状态差异很大,这是造成不同湍流模型对叶顶换热预测存在重大差别的原因;在研究的间隙范围内,叶尖间隙泄漏量和叶顶换热强度随间隙高度的增大而增加;在所选的4种湍流模型中,k-ω模型是叶顶换热数值模拟较好的湍流模型选择.     

Mathematical model for gas–liquid two-phase flow and biodegradation of a low concentration volatile organic compound (VOC) in a trickling biofilter

A theoretical model is established for predicting the biodegradation of a low concentration volatile organic compound (VOC) in a trickling biofilter. To facilitate the analysis, the packed bed is simplified to a series of straight capillary tubes covered by the biofilm in which the liquid film flow on the surface of biofilm and the gas core flow in the center of tube. The theoretical formulas to calculate liquid film thickness in the capillary tube are obtained by simultaneously solving a set of hydrodynamic equations representing the momentum transport behaviors of the gas–liquid two-phase flow under co-current flow and counter-current flow. Subsequently, the mass transport equations are respectively established for the gas core, liquid film, and biofilm with considering the mass transport resistance in the liquid film and biofilm, the biochemical reaction in the biofilm, and the limitation of oxygen to biochemical reaction. Meanwhile, the surface area of mass transport in the capillary tube is modified by introducing the active biofilm surface area, namely the specific wetted surface area available for biofilm formation. The predicted purification efficiencies of VOC waste gas are found to be in good agreement with the experimental data for the trickling biofilters packed with ?8 mm, ?18 mm, and ?25 mm ceramic spheres under the gas–liquid co-current flow mode and counter-current flow mode. It has been revealed that for a fixed inlet concentration of toluene, the purification efficiency of VOC waste gas decreases with the increase in the gas and liquid flow rate, and increases with the increase in the specific area of packed materials and the height of packed bed. Additionally, it is found that there is an optimal porosity of packed bed corresponding to the maximal purification efficiency.     

虹吸管气液两相流过流能力影响因素分析

基于系列工况下虹吸管的过流量、压降和含气率的试验结果,分析了虹吸管内分别出现气泡流、过渡流和气团流时影响其过流能力的主要因素。结果表明,当虹吸管内为气泡流时,气体存在对流动沿程阻力系数的影响可忽略不计,不同安装高度时管道实际过流量小于计算值的原因是由于实际过水断面面积小于液相满流过水断面面积,面积减小率与流量减小率相等;当虹吸管内为气团流和过渡流时,过流断面面积减小率与流量减小率相差较大,实际过流能力减小不仅与面积减小有关,气体存在对虹吸管道沿程阻力系数的影响也不可忽略,且含气率的增大使气液两相流动阻力增大、沿程阻力系数增大、流量减小。     

Study on heating performances of steam generator for solid oxide fuel cell using waste heat from solid particles

This paper reports the heating performances of steam generator for solid oxide fuel cell using waste heat from solid particles. The model of trapezoid-fin-tube heat exchanger was set up by using FLUENT 14.0. The model has been used to investigate the effects of fin tip width (2 mm–4 mm) and fin height (34 mm–46 mm). The fin surface temperature, the particle temperature, the fin total heat flux, the heat recovery efficiency and the heat transfer coefficient were studied. The heating performance of steam generator is improved when the trapezoid-fins are placed on heat transfer tubes, which is conducive to increase the power generation efficiency of solid oxide fuel cell. When the fin height increases from 34 mm to 46 mm, the average temperature of calcined petroleum coke decrease from 414 K to 376 K, the maximum temperature decrease from 498 K to 442 K, the average heat transfer coefficient of internal and external heat exchanger increase 12.4% and 12.7% respectively, the heat recovery efficiency increases 4.3%. When the fin tip width increases from 2 mm to 3 mm, the average temperature reduce 6.7 K and the maximum temperature decrease 7.3 K, the average heat transfer coefficient of internal and external heat exchanger increase 3.8% and 3.7% respectively, the heat recovery efficiency increases 0.88%.