循环流化床锅炉的热流密度分布

循环流化床燃烧技术的发展方向是超临界。超临界循环流化床锅炉的主要技术关键是炉膛受热面的横向热流分布。在管内工质温度不同的3台实际运行的循环流化床锅炉上，在水冷壁不同高度上测量了膜式壁特征金属壁温，将有限元用于水冷壁的换热分析，得到了循环流化床水冷壁的换热系数分布。该结果为超临界锅炉的设计提供了基础。     

循环流化床锅炉水冷壁的热流密度分布

超临界循环流化床锅炉的一个主要技术关键是炉膛受热面的横向热流分布.在管内工质温度不同、容量不同的3台循环流化床锅炉上,测量了不同高度上膜式水冷壁的金属壁温.将有限元算法用于水冷壁的换热分析,得到了循环流化床锅炉炉膛内烟气向水冷壁的换热系数分布.该结果为超临界CFB锅炉的设计提供了依据.     

附加鳍片对CFB锅炉膜式水冷壁管屏热变形影响的数值分析

基于世界首台600 MW超临界循环流化床(CFB)锅炉实炉实验数据,采用ANSYS软件建立高度×宽度为4.6 m×2 m的膜式水冷壁管屏计算模型,开展了100%BMCR(锅炉最大连续蒸发量)工况下水冷壁管屏热变形的数值模拟,分析了背火侧附加鳍片及其尺寸对水冷壁管屏热变形的影响。结果表明：由于炉内热负荷分布不同,随着炉膛高度的增加,水冷壁管屏向火侧热变形先增大后减小,在炉膛高度39.5 m处达到最大值,最大热变形量为1.43 mm;在水冷壁管背火侧焊接一块附加鳍片,可有效减小水冷壁管屏热变形,当附加鳍片尺寸为6 mm×4 mm时,水冷壁管屏向火侧热变形量减至0.57 mm。     

循环流化床锅炉水冷壁壁温与热流的计算研究

目前对于循环流化床锅炉水冷壁的壁温和热流的了解尚不能满足工程设计的需求。该文采用有限元法计算鳍片尺寸、床温和床的壁面换热系数对壁温的影响，探讨在工程上通过测量水冷壁背面两点的温度确定水冷壁吸收热量的方法，为循环流化床锅炉的设计和运行提供参考。     

膜式壁鳍片温度与宽度关系的有限元分析

膜式水冷壁鳍片温度一直是影响锅炉安全的重要因素之一,采用有限元法对某台SZS 10—2.5—Q锅炉膜式水冷壁温度分布特性进行数值分析,得出了在一定条件下膜式水冷壁鳍片的温度分布与鳍片宽度的关系。结果表明:随着鳍片宽度的增加,鳍片上特定点的温度更高,并且最大温度梯度更高;当鳍片超过70 mm时鳍片上的最高温度超过材料许用温度。     

600MW超临界锅炉炉膛膜式水冷壁的热行为研究

建立了超临界锅炉膜式水冷壁温度场的有限元计算模型,系统地分析了管壁、鳍片、水垢和积灰厚度对水冷壁温度分布的影响.结果表明:在锅炉炉膛膜式水冷壁向火侧中心和鳍片中心的温度最高,且向火侧内、外壁面温差最大,为29 K;管壁厚度对B、C、D点温度影响较小,但对向火侧A点的温度影响较大,而鳍片厚度对管壁内温度影响较小;随着水垢厚度的增加、积灰厚度的减小,水冷壁向火侧温度基本上呈线性升高的变化趋势.     

锅炉鳍片管的应力分析——有限单元法的应用

随着锅炉机组容量增大,近年来膜式水冷壁炉墙显示了很大优越性,被广泛采用。为保证膜式水冷壁管安全可靠运行,需要了解组成膜式水冷壁的鳍片管的应力状态,以选取合理的结构和支撑。本文选择了垂直布置的单根鳍片管为计算、研究对象,应用有限单元法作为研究鳍片管应力状态的计算手段。有限单元法在锅炉行业用以计算鳍片管应力尚属首次,为此,本文介绍了二维有限     

富氧条件下循环流化床锅炉炉内传热特性

在富氧气氛下对循环流化床锅炉炉内的传热特性研究,考虑锅炉产生的烟气成分变化,在传热系数的计算中考虑了气体的辐射特性,在密相区采用以前的鼓泡床传热传质模型,稀相区膜式水冷壁颗采用粒团更新模型,对模型求解,研究并分析了运行参数(如床料的直径、空隙率、烟气成分以及烟气温度等)在富氧燃烧条件下对传热的影响。     

膜式水冷壁管管壁温度计算方法

引言随着我国电站锅炉工业的迅速发展,将愈来愈多地采用膜式水冷壁管子。所谓膜式水冷壁管就是带有矩形或梯形鳍片的水冷壁管(见图1)。在运行中膜式水冷壁一面受到炉膛火焰的强烈热辐射,另一面则涂以绝热材料。在一般运行工况下,鳍片的端部(图1中A 点)壁温最高(有时壁温最高点在管子正面B 点)。为了保证膜式水冷壁的安全运行,除了核算管子本体材料的强度(按向火侧正面的内外壁平均温     

燃煤电厂锅炉辐射受热面温度及热应力分析

建立了超临界锅炉膜式水冷壁有限元数学模型,结合炉膛分区热力计算的相关理论,求得管内对流换热系数和管外炉侧平均热流密度。应用ANSYS WORKBENCH软件,模拟得出各区段水冷壁截面温度及热应力分布。结果表明:各区段温度及热应力的分布规律基本相同,最高温度集中在鳍片向火侧的顶点附近;最大热应力集中在管内向火侧的壁面附近;针对区段4研究了管壁厚度变化、鳍片厚度变化、鳍片长度变化,得出结论:温度和热应力具有同等的变化属性,随着管壁厚度和鳍片长度的变化呈正比变化趋势;随着鳍片厚度的变化呈反比趋势。     

An experimental and theoretical investigation into the heat transfer of a finned water wall tube in a circulating fluidized bed boiler

Heat transfer improvement in a water wall tube with fins was investigated in a circulating fluidized bed (CFB) boiler. Experiments were first conducted in a 6 MWth CFB boiler then a model was developed to analyse and interpolate the results. Temperatures at some discrete points within the wall cross‐section of the tube were measured by burying 0.8 mm thermocouples within a tube. Experimental data showed an increase in heat absorption up to 45 per cent. A good agreement between measured and predicted values was noted. The distribution of temperature in the metal wall and of heat flux around the outer wall of a tube with longitudinal and lateral fins was analysed by numerical solution of a two‐dimensional heat conduction equation. Effects of bed‐to‐wall heat transfer coefficient, water‐to‐tube inside heat transfer coefficient, bed temperature, water temperature and thermal conductivity of the tube material on the heat flux around the water tube are discussed. The present work also examines the influence of the length of the longitudinal fin and the water tube thickness. Heat flux was highest at the tip of the longitudinal fin. It dropped, but increased again near the root of the lateral fin. Copyright © 2000 John Wiley & Sons, Ltd.     

The impact of bed temperature on heat transfer characteristic between fluidized bed and vertical rifled tubes

In the present work, the heat transfer study focuses on assessment of the impact of bed temperature on the local heat transfer characteristic between a fluidized bed and vertical rifled tubes (38mm-O.D.) in a commercial circulating fluidized bed (CFB) boiler. Heat transfer behavior in a 1296t/h supercritical CFB furnace has been analyzed for Geldart B particle with Sauter mean diameter of 0.219 and 0.246mm. The heat transfer experiments were conducted for the active heat transfer surface in the form of membrane tube with a longitudinal fin at the tube crest under the normal operating conditions of CFB boiler. A heat transfer analysis of CFB boiler with detailed consideration of the bed-to-wall heat transfer coefficient and the contribution of heat transfer mechanisms inside furnace chamber were investigated using mechanistic heat transfer model based on cluster renewal approach. The predicted values of heat transfer coefficient are compared with empirical correlation for CFB units in large-scale.     

Heat flux distribution on circulating fluidized bed boiler water wall

The future of circulating fluidized bed (CFB) combustion technology is in raising the steam parameters to supercritical levels. Understanding the heat flux distribution on the water wall is one of the most important issues in the design and operation of supercritical pressure CFB boilers. In the present paper, the finite element analysis (FEA) method is adopted to predict the heat transfer coefficient as well as the heat flux of the membrane wall and the results are validated by direct measurement of the temperature around the tube. Studies on the horizontal heat flux distribution were conducted in three CFB boilers with different furnace size, tube dimension and water temperature. The results are useful in supercritical pressure CFB boiler design.     

A study of heat transfer effectiveness of circular tubes with internal longitudinal fins having tapered lateral profiles

This paper presents a finite-difference based numerical simulation of steady, laminar heat transfer in circular tubes fitted with four identical longitudinal fins having tapered lateral profiles. The lateral view of the tip of each fin is a circular arc. The tube is exposed to constant heat flux. The temperature dependence of thermal conductivity and viscosity has been taken into account. The flow is assumed to be locally fully developed but thermally undeveloped. The momentum equation for the fluid and energy equations for the fluid and the tube wall with/without fins are solved iteratively and simultaneously. At each axial location, bulk velocity, bulk temperature of the fluid and Effectiveness which is an indicator of the enhancement of heat transfer due to addition of fins are calculated. A parametric study of effectiveness vs. axial distance for various combinations of fin materials and coolants reveals interesting results. The velocity profiles, friction factors and comparisons with respect to constant property solutions have also been discussed.     

Longitudinal heat conduction study of a uniform power density apparatus for the experimental determination of the heat transfer coefficient

A combined experimental and numerical analysis was performed to accurately evaluate the effect of longitudinal heat conduction along an aluminium test section on the local heat transfer coefficient (HTC) experimental determination. The test section contains a rectangular channel with fins typical of compact heat exchangers. The hydraulic diameter of the channel is around 2 mm. The fluid stream is heated by flat electrical resistances clamped to the outer walls of the test section. The test section contains four zones with different fin pads each, in order to be able to generate data for several geometries at the same time at exactly the same mass flow rate.It was found that longitudinal conduction can lead to a quite uneven distribution of the heat flux to the fluid at the beginning and end of each zone of the test section. Three phenomena were found to be the cause for the observed uneven heat distribution, i.e. the longitudinal gradient of temperature along the wall induced by the fluid temperature rise, the presence of heated and non-heated zones, and the stepwise non-uniformity of the HTC along the test section induced where the fin pad changes.The study has shown that the elementary data reduction methodology, assuming that the surface power density generated by the electrical resistance and the heat flux density reaching the fluid are equal and uniform along the channel, is too far from the reality at the beginning and end of each zone of the test section, especially at low mass flow rates.     

Heat flux distribution on circulating fluidized bed boiler water wall

The future of circulating fluidized bed (CFB) combustion technology is in raising the steam parameters to supercritical levels. Understanding the heat flux distribution on the water wall is one of the most important issues in the design and operation of supercritical pressure CFB boilers. In the present paper, the finite element analysis (FEA) method is adopted to predict the heat transfer coefficient as well as the heat flux of the membrane wall and the results are validated by direct measurement of the temperature around the tube. Studies on the horizontal heat flux distribution were conducted in three CFB boilers with different furnace size, tube dimension and water temperature. The results are useful in supercritical pressure CFB boiler design. __________ Translated from Journal of Power Engineering, 2007, 27(3): 336–340 [译自: 动力工程]     

Optimum Profiles for Asymmetrical Longitudinal Fins in Annular Ducts

In the present work the geometry of annular ducts with asymmetrical longitudinal fins is optimized in order to enhance the heat transfer under laminar coolant flow conditions. The heat transferred is also maximized for a given amount of material or hydraulic resistance. Polynomial profiles are assigned to the two lateral fin surfaces. Velocity and temperature distributions on the annular duct cross section are determined with the help of a finite-element model. A global heat transfer coefficient and an equivalent Nusselt number are then calculated. Lastly, optimum asymmetrical fins obtained by means of a genetic algorithm are shown for different situations and their performance is compared with those of optimum symmetrical fins.     

Effect of fins on forced convection heat transfer around a tube

Effect of fins on heat transfer around a tube was investigated experimentally. A test tube of 30 mm diameter was installed in a test section of an open‐type wind tunnel as a single tube, or as a center tube in a single tube row and in a tube bundle of staggered layout. Fins made of paper were put on the test tube having certain fin spacing. It was clarified from the experiment that the local heat transfer coefficient around the tube degrades with decreasing fin spacing, especially on the downstream side of the tube, and the minimum fin spacing where the effect of the fin begins to appear is the largest for the single tube and the smallest for the tube bundle. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(5): 445–454, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10098     

Exact,analytical heat transfer from longitudinal radiating fins of rectangular profile in a tube/fin ensemble

In this technical note, the problem concerning the quantification of heat transfer rates from an array of longitudinal radiating fins of rectangular profile in a tube/fin ensemble to a nonzero temperature sink is investigated. Radiating fins constitute essential elements in the thermal control of spacecrafts and satellites. We consider quasi one-dimensional heat conduction in the longitudinal radiating fins and neglect radiative exchange between the fins and the tubes carrying a hot fluid. It is demonstrated that the governing nonlinear differential equation of second order with constant coefficients and nonhomogeneous can be solved in exact, analytical implicit form. The pertinent temperature distributions eventually provide the magnitudes of heat transfer rates and fin efficiencies influenced by the radiation–conduction parameter and the sink temperature.     

Heat transfer from a horizontal fin array by natural convection and radiation—A conjugate analysis

The problem of natural convection heat transfer from a horizontal fin array is theoretically formulated by treating the adjacent internal fins as two-fin enclosures. A conjugate analysis is carried out in which the mass, momentum and energy balance equations for the fluid in the two-fin enclosure are solved together with the heat conduction equations in both the fins. The numerical solutions by using alternating direction implicit (ADI) method yield steady state temperature and velocity fields in the fluid, and temperatures along the fins. Each end fin of the array is exposed to limited enclosure on one side and to infinite fluid medium on the other side. Hence a separate analysis is carried out for the problem of end fin exposed to infinite fluid medium with appropriate boundary conditions. From the numerical results, the heat fluxes from the fins and the base of the two-fin enclosure, and the heat flux from the end fin are calculated. Making use of the heat fluxes the total heat transfer rate and average heat transfer coefficient for a fin array are estimated. Heat transfer by radiation is also considered in the analysis. The results obtained for a four-fin array are compared with the experimental data available in literature, which show good agreement. Numerical results are obtained to study the effectiveness for different values of fin heights, emissivities, number of fins in a fixed base, fin base temperature and fin spacing. The numerical results are subjected to non-linear regression and equations are obtained for heat fluxes from the two-fin enclosure and single fin as functions of Rayleigh number, aspect ratio and fin emissivity. Also regression equations are obtained to readily calculate the average Nusselt number, heat transfer rate and effectiveness for a fin array.