循环流化床内颗粒混合特性的数值模拟

为了更好地研究循环流化床内颗粒的混合行为和揭示颗粒的混合机理,将离散单元法和计算流体力学相结合,对床内颗粒的轴向混合与径向混合过程进行了数值模拟.从气体和颗粒运动速度的角度将轴向混合与径向混合结合起来,对床内颗粒的整体混合过程进行了分析,并采用颗粒特征浓度标准差对颗粒的混合程度进行了定量评价,通过数值模拟得到了床内气体的速度场、颗粒的速度场和体积分数分布.结果表明:颗粒与气体的速度分布趋势基本一致,床内呈现上部稀相下部密相;颗粒的轴向混合效果优于径向混合效果,增大流化风速有助于加速颗粒的混合,且颗粒轴向混合对流化风速的敏感性低于径向混合.     

流型对循环流化床径向气体混合影响的试验研究

在提升段净高为4.0 m、内径0.19 m的循环流化床冷态试验台上进行了快速床和气力输送两种流型中径向气体混合的试验研究,试验用物料为河沙,d_p=120 μm,真实密度ρ_s=2 400 kg/m~3.试验台颗粒循环流率(G_s)和流化风速(U_g)可独立控制,根据床层上、下部压力梯度随流化风速的变化关系判断快速流态化的存在区域.采用柱塞流模型,用CO_2作为示踪气体进行3种风速下的径向气体混合试验,得出了快速床和气力输送两种流型中气体径向扩散系数Dr随颗粒浓度变化的趋势.研究发现,在气力输送流型中Dr随颗粒浓度增加而减小,在快速床流型中Dr随颗粒浓度增加而增大.结合固体颗粒在不同流型中的存在状态解释了流型对Dr的不同影响作用.     

双孔射流流化床内颗粒混合特性的离散单元法数值模拟

将计算流体力学与离散单元法相结合,采用Fortran语言编程,根据牛顿第三定律实现气固耦合,从微观角度剖析了颗粒在流化床内的运动机制.利用Lacey混合指数对流化床内不同特性区域的颗粒混合程度进行了定量分析,并研究了颗粒混合特性的影响因素,得到颗粒轴向和径向混合序列图、气体和颗粒速度分布以及整床和三区的颗粒混合指数分布.结果表明:颗粒受到空气射流作用后,随着混合和偏析的不断进行,最后达到随机完全混合状态;相同表观气速下颗粒轴向混合进程快于径向混合进程;表观气速增大,喷泉区尺度增大,使颗粒径向混合能力得到改善,但对轴向混合能力的影响微弱.     

循环流化床提升段径向气体混合的试验研究

在3种规模的循环流化床(CFB)冷态试验台上对提升段稀相区径向气体混合规律进行了试验研究.3种试验台提升段净高均为4.Om,截面尺寸分别是内径O.19 m的圆管、O.3m×0.3m的方截面和0.25 m×0.5 m的矩形截面.以dp=120μm,ρ=2 400kg/m3的砂作为试验床料,CO2作为示踪气体,对影响气体混合的因素如表观流化速度Ug、颗粒浓度、提升段尺寸等进行了试验研究.发现颗粒的存在减缓了气体横向混合的速度,但颗粒对气体横向混合的影响不是单向的.对一定风速,存在一个颗粒浓度转折点,小于此浓度时D,随颗粒浓度的增加而减小,大于此浓度Dr随颗粒浓度的增加而增加.在本试验中此转折点浓度为8～1O kg/m3,应为气固流型的转折点.提升段直径Dt对Dr影响很大,结合前人研究结果,Dr随Dt呈线性增加,但更确切的关系确定有待进一步数据积累.与颗粒浓度和提升段直径相比,气体流化速度对Dt的影响很小.     

循环流化床中气体扩散规律的研究

二次风在循环流化床炉膛内的扩散对燃料燃烧效率有重要影响。基于塞状流 ,在截面尺寸为5 0 0mm× 2 5 0mm高为 3 0 0 0mm的冷态实验台上 ,以二氧化碳作为示踪气体 ,测量示踪气体注入点上方的示踪气体浓度的分布情况 ,利用浓度场来研究示踪气体的扩散规律 ,并尝试探索扩散系数和操作条件的关系。结果表明 ,气体在气固两相流中的扩散受风速和颗粒循环量的影响。     

半干法脱硫反应器内固体颗粒浓度分布的试验研究

在三维循环流化床脱硫反应器冷态模型上,研究了文丘里布风装置的循环流化床脱硫塔内气固两相流动中固体颗粒浓度的分布规律,以及不同操作参数:表观气速和颗粒循环流率对颗粒浓度在径向以及轴向分布的影响。试验中利用PV6D型光纤探针测量颗粒浓度,压力传感器测量反应器壁面压力。试验结果表明,在半干法循环流化床脱硫塔反应器中固体颗粒浓度的分布呈中间稀,边壁浓的趋势。截面平均颗粒浓度大体呈上稀下浓的分布。随表观气速的减小和颗粒循环速率的增大,颗粒浓度的径向分布不均性增大,截面平均颗粒浓度的轴向不均匀性增大。     

关于循环流化床锅炉的给煤问题

循环流化床燃烧技术已被运行实践证明是可靠的洁净煤燃烧技术.目前影响循环流化床锅炉运行可靠性的一个问题是给煤的堵煤问题.分析了循环流化床锅炉中燃烧过程及煤颗粒进入炉膛后与气体、床料的混合过程,探讨了系统简化、性能可靠的前墙给煤的可行性,研究了这一给煤方式对燃料在床料中的混合效果及燃烧效率的影响.提出焦炭失活和气固混合欠佳是燃烧效率的主要影响因素,认为前墙给煤不会导致燃烧效率的下降,可靠性提高,便于操作.     

循环流化床内气粒两相传热的萘升华模拟实验研究

在循环流化床试验台上对床中气体与颗粒两相间的传热特性进行了试验研究，试验中首次将萘升华热质类比技术应用于循环流化床内气粒两相间传热的研究中，考察了不同的固体颗粒循环量、一次风风速和床料平均粒径对气粒间换热的影响。试验表明：随着一次风风速的增加，循环床中气体和颗粒之间的表现热换系数变大，当固体颗粒循环量增加或颗粒平均粒径减小时，表征相间换热特性的Nu数增大。图5表1参4     

循环流化床锅炉密相区内颗粒的横向扩散研究

以热粒子作为示踪粒子,用热电偶测量示踪粒子沿径向的变化,在长900mm、宽100mm、高5200mm的循环流化床密相区进行了颗粒的横向扩散的研究。循环流化床密相区内颗粒横向扩散可用一维扩散模型来描述,模型的计算结果与实验数据吻合很好。根据实验数据拟和得到颗粒的横向扩散系数Dsr,实验表明随着流化风速的增大和静止床高的增高,横向扩散系数Dsr增大;随颗粒粒径的增大,横向扩散系数Dsr减小。最后给出了以流化风速、静止床高和颗粒粒径为影响因素的横向扩散系数Dsr的经验式。     

双循环流化床内稻壳-石英砂混合颗粒循环流率的实验研究与预测

混合颗粒循环流率是气化反应的双循环流化床系统稳定运行的关键。在自行搭建的双循环流化床冷态系统上,对气化室风速、提升管风速、初始物料质量和石英砂粒径等控制参数对不同稻壳质量比的稻壳-石英砂混合颗粒的循环流率的影响进行实验研究。研究表明:混合颗粒循环流率随着气化室和提升管风速的增加而增加;随着初始物料质量的增加,气化室侧返料管压力增加,混合颗粒循环流率增大;随着粒径增加,石英砂颗粒流化困难,循环流率减小;由于稻壳密度小,形状不规则,在一定程度上阻碍物料的流化,因此随着稻壳质量比的增加循环流率下降;基于以上各参数提出经验关联式,预测误差在-18. 04%～19. 8%间,能够很好地对双循环流化床系统中稻壳-石英砂双组份物料颗粒的循环流率进行预测。     

火电厂节能工作浅析

本文探讨了火电厂节能基本程序和工作方法,分析节能工作要从基础工作、技术创新、设备整治、燃烧管理等方面入手的论述.     

浅析磨合期汽车的使用

如何合理使用磨合期的汽车，使其摩擦机件的磨损量能够平稳、均匀地磨合而不至于过度磨损，对汽车实际使用寿命有着重要的意义。从机器磨合期的特点和磨损机理出发，对汽车磨合期如何正确使用进行了有实际意义的探讨。     

Geothermal energy in agriculture in Yugoslavia

SFR Yugoslavia has natural hot water springs practically all over its territory, indicating the presence of geothermal reservoirs underground. There is no final estimate of how much energy can be expected from this new resource, except for figures based on the results of studies and investigations.The first use of geothermal energy in Yugoslavia in numerous spas dates back to the 19th century. Some remarkable examples of central heating are known, but the most widespread use is in agriculture (in greenhouse heating, farm heating and drying of agricultural products).     

Energy conservation measures in an institutional building in sub-tropical climate in Australia

In this study, various energy conservation measures (ECMs) on heating, ventilating and air conditioning (HVAC) and lighting systems for a four-storied institutional building in sub-tropical (hot and humid climate) Queensland, Australia are evaluated using the simulation software called DesignBuilder (DB). Base case scenario of energy consumption profiles of existing systems are analysed and simulated first then, the simulated results are verified by on-site measured data. Three categories of ECMs, namely major investment ECMs (variable air volume (VAV) systems against constant air volume (CAV); and low coefficient of performance (COP) chillers against high COP chillers); minor investment ECMs (photo electric dimming control system against general lighting, and double glazed low emittance windows against single-glazed windows) and zero investment ECMs (reset heating and cooling set point temperatures) are evaluated. It is found that the building considered in this study can save up to 41.87% energy without compromising occupancies thermal comfort by implementing the above mentioned ECMs into the existing system.     

Uncertainties in decarbonising heat in the UK

Heating is arguably one of the most difficult sectors to decarbonise in the UK's energy system. Meeting the 80% greenhouse gas emission reduction target by 2050 is likely to require that heat related emissions of CO₂ from buildings are near zero by 2050, and there is a 70% reduction in emissions from industry (from 1990 levels). Though it is clear that the use of the natural gas network will reduce over time, recent modelling suggests a limited residual role for gas by 2050 to help meet peaks in heat demand. High levels of uncertainty about the way in which heat will be decarbonised present a number of challenges to policy makers. This paper will explore the risks and uncertainties associated with the transition to a low carbon heat system in the UK as outlined by the 4th carbon budget review. The potential impact of key uncertainties on the levelised costs of heat technologies and the development of energy networks are explored using a sensitivity analysis approach. Policy changes required to decarbonise the heat sector are also examined.     

Solar drying in sludge management in Turkey

Two main wastewater treatment plants in Bursa city in Turkey will start to operate and produce at least 27,000 tons of dry solids per year by the end of 2006. The purpose of this study was to investigate an economical solution to the sludge management problem that Bursa city would encounter. The general trend in Turkey is mechanical dewatering to obtain a dry solid (DS) content of 20%, and liming the mechanically dewatered sludge to reach the legal landfilling requirement, 35% DS content. This study recommends limited liming & solar drying as an alternative to only-liming the mechanically dewatered sludge. Open and covered solar sludge drying plants were constructed in pilot scale for experimental purposes. Dry solids and climatic conditions were constantly measured. Faecal coliform reduction was also monitored. The specially designed covered solar drying plant proved to be more efficient than the open plant in terms of drying and faecal coliform reduction. It was found that, if the limited liming & solar drying method was applied after mechanical dewatering instead of only-liming method, the total amount of the sludge to be disposed would be reduced by approximately 40%. This would lead to a reduction in the transportation, handling, and landfilling costs. The covered drying system would amortize itself in 4 years.     

Viscous heating in liquid flows in micro-channels

Many experimental works on forced convection through micro-channels evidenced that when the hydraulic diameter is less than 1 mm, conventional theory can no longer be considered as suitable to predict the pressure drop and convective heat transfer coefficients. This conclusion seemed valid for both gas and liquid flows. Sometimes the authors justified this claim by invoking “new” micro-effects. On the contrary, in this paper the explanation of the experimental results obtained for micro-channels in terms of friction factors will be researched inside the conventional theory (Navier-Stokes equations). In particular, this paper will focus on the role of viscous heating in fluids flowing through micro-channels. A criterion will be presented to draw the limit of significance for viscous dissipation effects in micro-channel flows. The role of the cross-sectional geometry on viscous dissipation will be highlighted and the minimum Reynolds number for which viscous dissipation effects can no longer be neglected will be calculated as a function of the hydraulic diameter and of the micro-channel geometry for different fluids. It will be demonstrated how viscous effects can explain some experimental results on the Poiseuille numbers in micro-channels, which recently appeared in the open literature.     

Electoral cycles in electricity losses in India

A third of electricity in India is lost each year, where losses refer to power that is supplied but not billed. Utilizing data from the power corporation of Uttar Pradesh, India's most populous state, we study the politics of electricity losses. Examining annual data over four decades, we document that UP's electricity losses tend to increase in periods immediately prior to state assembly elections. Drawing upon geographically disaggregated data for the period 2000–09, we observe higher line losses just prior to the 2002 and 2007 state elections. Our analysis shows that the incumbent party was more likely to retain the assembly seat as line losses in the locality increased. We interpret these results as corroboration that political parties deliberately redirect electricity to flat rate and unbilled users in a context of chronically inadequate supply. Political factors appear to affect line losses in ways that technical and economic factors alone cannot explain.     

Energy consumption in rural transportation in India

Energy consumption in rural transportation has been computed, taking into account the energy embodied in vehicles or bullocks, as well as the fuels or feeds and food (of operators). Bullock cart transport consumes over four times more energy than vehicles. However, animal transport will continue to be preferred because of its availability.